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Multistage andesite genesis in the Garibaldi Lake area, southwestern British Columbia Green, Nathan Louis 1977

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MULTISTAGE ANDESITE GENESIS IN THE GARIBALDI LAKE AREA, SOUTHWESTERN BRITISH COLUMBIA by NATHAN LOUIS GREEN B.Sc. (Hons.)> U n i v e r s i t y o f Manitoba, 1970 M . S c , U n i v e r s i t y o f Manitoba, 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department o f G e o l o g i c a l S c i e n c e s We a c c e p t t h i s t h e s i s as c o n f o r m i n g to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA August, 1977 © Nathan.Louis Green, 1977 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requ i rement s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Co lumb ia , I a g ree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s tudy . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d tha t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i thout my w r i t t e n p e r m i s s i o n . Department o f G e o l o g i c a l S c i e n c e s The U n i v e r s i t y o f B r i t i s h Co lumbia 2075 Wesbrook P l a c e V a n c o u v e r , C a n a d a V6T 1W5 D a t e August 12, 1977 i MULTISTAGE ANDESITE. GENESIS IN THE GARIBALDI LAKE AREA,  SOUTHWESTERN BRITISH COLUMBIA ABSTRACT The p r o d u c t s of P l e i s t o c e n e - R e c e n t v o l c a n l s m i n the a r e a j u s t n o r t h o f Mount G a r i b a l d i a r e h o r n b l e n d e - a n d e s i t e , h o r n b l e n d e -b l o t i t e a n d e s i t e , and l e s s voluminous two-pyroxene a n d e s i t e , o l i v i n e - b a s a l t , b a s a l t i c - a n d e s i t e and h o r n b l e n d e - d a c l t e . The b a s i c l a v a s a r e p r e d o m i n a n t l y a l k a l i - b a s a l t , hy-normative h a w a i i t e s and m u g e a r i t e s . A n d e s i t e s and a s s o c i a t e d i n t e r m e d i a t e r o c k s have a c a l c - a l k a l i n e a f f i n i t y , and show s t r o n g enrichment i n Sr (700-1400 ppm) and Ba (400-800 ppm), f u r t h e r emphasizing the major c o m p o s i t i o n a l gap between t h e s e l a v a s and contemporaneous b a s a l t s . 8 7 86 A l l l a v a s d i s p l a y d e p l e t i o n i n Rb (4-25 ppm), low Sr /Sr r a t i o s (0.7028-0.7036) and h i g h K/Rb r a t i o s (500-1100) r e l a t i v e to t y p i c a l o r o g e n i c v o l c a n i c s e r i e s . Cheakamus V a l l e y b a s a l t s c o n t a i n p h e n o c r y s t s of o l i v i n e , c l i n o p y r o x e n e and p l a g i o c l a s e . G a r i b a l d i Lake b a s a l t i c - a n d e s i t e s have s i m i l a r p h e n o c r y s t s , but a l s o c o n t a i n amphibole and t i t a n o m a g n e t i t e . In the Sphinx Moraine b a s a l t i c - a n d e s i t e , c r y s t a l c l o t s o f d i o p s i d i c a u g i t e and h o r n b l e n d e , and amphibole megacrysts e n c l o s i n g ragged g r a i n s of o l i v i n e (Fo Q-, o r.) suggest t h a t amphibole and c l i n o p y r o x e n e ol — oU c o - p r e c i p i t a t e d from more m a f i c p a r e n t a l magma, w i t h e a r l y - f o r m e d o l i v i n e i n r e a c t i o n r e l a t i o n s h i p w i t h the l i q u i d . C a l c u l a t e d p h e n o c r y s t s e q u i l i b r a t i o n p r e s s u r e s range from 4.5 to 11 kb and average 7.5 kb (^25 km), i n d i c a t i n g lower c r u s t a l c r y s t a l l i z a t i o n . E s t i m a t e d f n f o r h o r n b l e n d e - b e a r i n g l a v a s suggest p h e n o c r y s t e q u i l i b r a t i o n i i under P. < P, w i t h X, X i q ^ 0.09-0.15 (3-5 wt. % ) . The p h e n o c r y s t T o t a l ' m i n e r a l o g y o f The T a b l e (PL + AMPB + OPX + MT), Mount P r i c e (PL + AMPB + BO + OPX + MT), and The B l a c k Tusk (PL + CPX + OPX + MT) s i l i c i c -a n d e s i t e s demonstrates, t h a t c h e m i c a l l y d i s t i n c t magma ba t c h e s c r y s t a l l i z e d under d i f f e r e n t P (2-5 kb) , T (825-975°C) and P R Q c o n d i t i o n s . The low K/Na and Fe/Mg r a t i o s , s t r o n g enrichment o f Sr and^Ba, and marked d e p l e t i o n - i n Rb and V argue a g a i n s t e q u i l i b r i u m m e l t i n g o f q u a r t z e c l o g i t e w i t h i n subducted o c e a n i c c r u s t . The a n d e s i t e s have 2+ Mg/l(Mg + Fe ) v a l u e s and N i and Cr c o n t e n t s too low f o r t h e s e l a v a s t o r e p r e s e n t u n m o d i f i e d p a r t i a l m e l t s o f mantle p e r i d o t i t e . The r e s u l t s o f e q u i l i b r i u m c a l c u l a t i o n s , however, suggest t h a t the p a r e n t a l magmas of the a n d e s i t e s c o u l d have e q u i l i b r a t e d w i t h p e r i d o t i t i c residuum a t 1150-1235°C and 15-20 kb. e x p l a i n e d by a m u l t i s t a g e model. I t i n v o l v e s : (1) v a r y i n g degrees o f p a r t i a l m e l t i n g of wet p e r i d o t i t e above the B e n i o f f zone t o produce w a t e r - u n d e r s a t u r a t e d t h o l e i i t e magmas; (2) h i g h - p r e s s u r e (>5 kb) f r a c t i o n a t i o n o f o l i v i n e ± c l i n o p y r o x e n e ± amphibole ± C r - s p i n e l from the b a s i c m e l t s a t o r near the base o f the c r u s t ; and (3) lo w - p r e s s u r e (2-5 kb) s e g r e g a t i o n of p l a g i o c l a s e ± amphibole ± b i o t i t e + pyroxene + m a g n e t i t e from more i n t e r m e d i a t e c o m p o s i t i o n s . The e v o l u t i o n o f G a r i b a l d i Lake a n d e s i t e magmas can o n l y be i i i TABLE OF CONTENTS PAGE ABSTRACT i LIST OF TABLES v i i i L IST OF FIGURES x i ACKNOWLEDGEMENTS x i v I. INTRODUCTION 1 G e n e r a l Statement 1 P r e v i o u s work 4 Method o f i n v e s t i g a t i o n 5 T e c t o n i c s e t t i n g o f southwestern B r i t i s h Columbia 6 G e o l o g i c s e t t i n g o f the G a r i b a l d i Lake a r e a 8 I I . GEOLOGY AND GE0CHR0N0LQGY OF GARIBALDI LAKE VOLCANIC SUITES 10 I n t r o d u c t i o n 10 Cheakamus V a l l e y b a s a l t s 10 The B l a c k Tusk 15 A n c e s t r a l Mountain Stage 15 P l u g Dome Stage 18 The C i n d e r Cone 22 Mount P r i c e 25 T a b l e Bay U n i t 25 Mount P r i c e U n i t 28 Summit E r u p t i o n U n i t 29 P r i c e Bay U n i t 30 C l i n k e r Peak U n i t 30 The T a b l e 31 i v TABLE OF CONTENTS (Continued) PAGE Enostu c k Meadows 33 Sphinx Moraine 36 Age o f v o l c a n i s m 37 I I I . GEOCHEMISTRY OF GARIBALDI LAKE LAVAS 40 G e n e r a l statement 40 Rock c l a s s i f i c a t i o n 40 B a s a l t i c r o c k s 50 Cheakamus V a l l e y b a s a l t s 50 Helm Creek f l o w 52 D i f f e r e n t i a t e d r o c k s 55 Comparison w i t h o t h e r v o l c a n i c s u i t e s 62 S t r o n t i u m I s o t o p e d a t a 65 A n d e s i t e p e t r o g e h e s i s 66 A s s i m i l a t i o n and magma m i x i n g 66 P a r t i a l m e l t i n g o f subducted o c e a n i c c r u s t 67 P a r t i a l m e l t i n g o f mantle p e r i d o t i t e 68 F r a c t i o n a l c r y s t a l l i z a t i o n o f a b a s a l t i c magma 69 D i s c u s s i o n 79 IV. MINERALOGY OF THE GARIBALDI LAKE LAVAS 81 I n t r o d u c t i o n 81 O l i v i n e 81 Pyroxene 83 Cheakamus V a l l e y b a s a l t s 84 The C i n d e r Cone 85 Sphinx Moraine 89 V TABLE OF CONTENTS (Continued) PAGE En o s t u c k Meadows 92 The T a b l e 92 The B l a c k Tusk 93 Mount P r i c e 94 D i s c u s s i o n 94 P l a g i o c l a s e 95 Cheakamus V a l l e y b a s a l t s 102 The C i n d e r Cone 103 Sphinx Moraine 106 En o s t u c k Meadows 106 The Table' 107 The Black. Tusk 108 Mount P r i c e 109 D i s c u s s i o n 110 Amphibole 111 M i c a 119 I r o n - t i t a n i u m o x i d e 123 A p a t i t e 134 P y r r h o t i t e 134 Other a c c e s s o r y m i n e r a l s 135 R e s i d u a l g l a s s e s 135 C r y s t a l l i z a t i o n temperatures 136 D i s c u s s i o n 138 v i TABLE OF CONTENTS (Continued) PAGE V. P-T PATH OF ASCENDING GARIBALDI LAKE MAGMAS 141 I n t r o d u c t i o n 141 Method o f c a l c u l a t i o n 141 E q u i l i b r a t i o n o f groundmass l i q u i d and p h e n o c r y s t s 145 S i l i c a and alumina a c t i v i t i e s i n p r e - e r u p t i v e G a r i b a l d i Lake m e l t s 152 Water c o n t e n t o f G a r i b a l d i Lake a n d e s i t e magmas 155 Water a n d . s i l i c a a c t i v i t y 156 Water c o n t e n t o f h o r n b l e n d e - b e a r i n g b a s a l t i c -a n d e s i t e s 157 Water c o n t e n t o f h o r n b l e h d e - b i o t i t e a n d e s i t e s 161 P o s s i b l e s o u r c e s o f G a r i b a l d i Lake a n d e s i t e s 163 E q u i l i b r a t i o n o f a n d e s i t e w i t h t r a n s f o r m e d o c e a n i c c r u s t 163 E q u i l i b r a t i o n o f a n d e s i t e w i t h g a r n e t - p y r o x e n i t e 167 E q u i l i b r a t i o n o f a n d e s i t e w i t h mantle p e r i d o t i t e 168 E q u i l i b r a t i o n o f b a s a l t i c l a v a s w i t h mantle p e r i d o t i t e 169 D i s c u s s i o n 173 V I . PETROGENETIC IMPLICATIONS 177 M u l t i s t a g e f r a c t i o n a t i o n o f hydrous b a s a l t magma -a p r o b a b l e model 179 Stage I : W a t e r - u n d e r s a t u r a t e d t h o l e i i t e magmas from Hydrous mantle p e r i d o t i t e above the B e n i o f f zone 179 v i i TABLE OF CONTENTS (Continued) PAGE Stage I I : B a s a l t i c - a n d e s i t e by h i g h - p r e s s u r e f r a c t i o n a t i o n o f w a t e r - u n d e r s a t u r a t e d t h o l e i i t e magmas 183 Stage I I I : S i l i c i c - a n d e s i t e s by l o w - p r e s s u r e f r a c t i o n a t i o n i n v o l v i n g s e p a r a t i o n of p h e n o c r y s t s and m i c r o p h e n o c r y s t s 184 C o n c l u s i o n s 185 REFERENCES 187 APPENDIX I . MAJOR AND TRACE ELEMENT DATA FOR GARIBALDI LAKE LAVAS 209 APPENDIX I I . MAJOR ELEMENT PARTIAL MELTING CALCULATIONS FOR GARIBALDI LAKE ANDESITES 221 APPENDIX I I I . REPRESENTATIVE M1CR0PR0BE ANALYSES OF CONSTITUENT MINERALS IN GARIBALDI LAKE LAVAS 229 APPENDIX IV. ACTIVITY - COMPOSITION MODELS FOR SOLID PHASES 240 APPENDIX V. ESTIMATION OF THE THERMODYNAMIC PROPRTIES OF PARGASITE 243 v i i i L IST OF TABLES TABLE PAGE I Modal a n a l y s e s o f the Helm Creek l a v a 23 I I P otassium-argon a n a l y t i c a l d a t a f o r v o l c a n i c r o c k s from the G a r i b a l d i Lake a r e a 38 I I I Averages o f Cheakamus V a l l e y and o t h e r b a s a l t i c s u i t e s 53 IV Average G a r i b a l d i Lake, Cascade' and Andean a n d e s i t e s u i t e s 63 V. E m p i r i c a l c a l c u l a t i o n of b a s a l t i c p a r e n t to the Sphinx Moraine l a v a 72 T e s t of f r a c t i o n a t i o n scheme b e t w e e n . b a s a l t i c - a n d e s i t e and a n d e s i t e 74 T e s t o f f r a c t i o n a t i o n scheme f o r The B l a c k Tusk l a v a s 76 T e s t of- f r a c t i o n a t i o n scheme f o r Mount P r i c e l a v a s 77 T e s t o f f r a c t i o n a t i o n scheme f o r The T a b l e l a v a s 78 C r y s t a l l i z a t i o n temperatures c a l c u l a t e d f o r v a r i o u s G a r i b a l d i Group l a v a s 137 C o e f f i c i e n t s A, B, and C f o r the e q u a t i o n l o g a^ = AG°/2.303RT = A/T + B + C/T(P - 1) f o r l i s t e d r e a c t i o n s , where " i " r e f e r s to Si02> A ^ O ^ a n d K^O Thermodynamic parameters used i n c a l c u l a t i o n o f a n d e s i t i c e q u i l i b r a t i o n p r e s s u r e s and temperatures C a l c u l a t e d temperatures and p r e s s u r e s o f e q u i l i b r a t i o n of a n d e s i t i c l a v a s w i t h p o s s i b l e r e f r a c t o r y m i n e r a l assemblages 153 XIV E s t i m a t e d P_ ^ .. and P T T - i n G a r i b a l d i Group T o t a l H^O r h o r n b l e n d e - a n d e s i t e s 160 XV C a l c u l a t e d temperatures and p r e s s u r e s o f e q u i l i b r a t i o n of b a s i c l a v a s w i t h s p i n e l - and g a r n e t - p e r i d o t i t e - 171 VI V I I V I I I IX X XI X I I X I I I 146 149 i x LIST OF TABLES (Continued) TABLE PAGE XVI L o c a t i o n and modal a n a l y s e s o f i n v e s t i g a t e d Q u aternary v o l c a n i c r o c k s , G a r i b a l d i - C a l l a g h a n L a k e s a r e a 211 XVII Chemical a n a l y s e s and CIPW no r m a t i v e m i n e r a l s o f the Cheakamus V a l l e y b a s a l t s 213 XVI I I Chemical a n a l y s e s and CIPW no r m a t i v e m i n e r a l s o f the Helm Creek l a v a , The C i n d e r Cone 215 XIX Chemical a n a l y s e s and CIPW no r m a t i v e m i n e r a l s o f the D e s o l a t i o n V a l l e y (The C i n d e r Cone), Sphinx Moraine and Enostuck b a s a l t i c - a n d e s i t e s 216 XX Chemical a n a l y s e s and CIPW n o r m a t i v e m i n e r a l s o f The T a b l e l a v a s 217 XXI C h e m i c a l a n a l y s e s and CIPW n o r m a t i v e m i n e r a l s o f The B l a c k Tusk l a v a s 218 XXII Chemical a n a l y s e s and CIPW no r m a t i v e m i n e r a l s o f Mount P r i c e l a v a s 219 X X I I I P a r e n t a l c o m p o s i t i o n s used i n major element m e l t i n g models 224 XXIV M i n e r a l c o m p o s i t i o n s used i n major element models 225 XXV R e p r e s e n t a t i v e m e l t i n g models o f a Juan de Fuca e c l o g i t e p a r e n t 226 XXVI R e p r e s e n t a t i v e m e l t i n g models o f a mantle p e r i d o t i t e 227 XXVII R e p r e s e n t a t i v e m e l t i n g models o f a r h y o d a c i t e - e n r i c h e d p e r i d o t i t e 228 XXVIII R e p r e s e n t a t i v e e l e c t r o n m i c r o p r o b e a n a l y s e s o f o l i v i n e 230 XXIX R e p r e s e n t a t i v e e l e c t r o n m icroprobe a n a l y s e s o f c l i n o p y r o x e n e s 231 XXX R e p r e s e n t a t i v e e l e c t r o n m i c r o p r o b e a n a l y s e s o f orthopyroxenes 232 X LIST OF TABLES (Continued) TABLE PAGE XXXI R e p r e s e n t a t i v e e l e c t r o n m i c r o p r o b e a n a l y s e s o f f e l d s p a r s 233 XXXII R e p r e s e n t a t i v e e l e c t r o n m icroprobe a n a l y s e s o f amphiboles 234 XXXIII R e p r e s e n t a t i v e e l e c t r o n m i c r o p r o b e a n a l y s e s o f b i o t i t e 235 XXXIV R e p r e s e n t a t i v e e l e c t r o n m icroprobe a n a l y s e s of i r o n - t i t a n i u m o x i d e s 236 XXXV R e p r e s e n t a t i v e e l e c t r o n m icroprobe a n a l y s e s o f p y r r h o t i t e 237 XXXVI P a r t i a l e l e c t r o n m i c r o p r o b e a n a l y s e s o f a p a t i t e 238 XXXVII E l e c t r o n m i c r o p r o b e a n a l y s e s o f groundmass and r e s i d u a l g l a s s e s i n t h e G a r i b a l d i - C a l l a g h a n Lakes l a v a s 239 XXXVIII Thermodynamic d a t a f o r m i n e r a l phases a t 298.15°K and 1 atm. used t o c a l c u l a t e e q u i l i b r i u m c u r v e s a t e l e v a t e d temperatures and p r e s s u r e s 246' x i TABLE OF FIGURES FIGURE PAGE 1 D i s t r i b u t i o n and t e c t o n i c s e t t i n g o f Quaternary v o l c a n i c r o c k s i n s outhwestern B r i t i s h Columbia 2 G e o l o g y o f Western G a r i b a l d i Park 3 Schematic s t r a t i g r a p h i c column o f the Cheakamus V a l l e y b a s a l t s 12 4 P l e i s t o c e n e - R e c e n t v o l c a n i c r o c k s i n the G a r i b a l d i Lake a r e a 16 5 The geology o f The B l a c k Tusk 20 6 Photomicrographs o f . G a r i b a l d i Lake l a v a s 26 7 The geology of Mount P r i c e and The T a b l e 34 8 M a j o r element SiO^ v a r i a t i o n diagrams f o r v o l c a n i c r o c k s from the G a r i b a l d i Lake a r e a 42 9 T r a c e element SiO^ v a r i a t i o n diagrams f o r v o l c a n i c r o c k s from the G a r i b a l d i Lake a r e a 44 10 S i 0 2 v e r s u s Mg/(Mg ^ F e ) , Ca/Sr, K/Ba, K/Sr, K/Rb, and Rb/Sr f o r v o l c a n i c r o c k s from the G a r i b a l d i Lake a r e a 46 11 (Na 20 + K 20) v e r s u s S i 0 2 diagram 48 12 AMF p l o t o f c h e m i c a l d a t a from the G a r i b a l d i Lake a r e a 56 13 MgO v e r s u s (FeO + F e ^ ) diagram 58 14 P r o j e c t i o n o f Cheakamus V a l l e y b a s a l t c o m p o s i t i o n s i n the CMAS system 60 15 Compositions o f p y r o x e n e s , o l i v i n e s and amphiboles i n G a r i b a l d i Lake l a v a s , p l o t t e d i n terms o f Ca, Mg, and Fe (atom %) 86 16 T i v e r s u s A l on the b a s i s o f 6 oxygens f o r pyroxenes o f b a s a l t i c l a v a s 90 (in., pooko.fcfr x i i TABLE OF FIGURES (Continued) FIGURE PAGE 17 P l a g i o c l a s e . c o m p o s i t i o n s (mole %) o f G a r i b a l d i Lake l a v a s 98 18 Weight p e r c e n t t o t a l i r o n (as FeO) i n p l a g i o c l a s e v e r s u s mol. p e r c e n t a n o r t h i t e 100 19 P l a g i o c l a s e i n the G a r i b a l d i Lake l a v a s 104 20 Amphibole i n the G a r i b a l d i Lake l a v a s 112 21 C o mpositions o f a n a l y z e d amphiboles i n terms o f : (a) S i " ^ and (Na + Ca + K) atoms p e r f o r m u l a u n i t ( H a l l i m o n d Diagram), and (b) and ( c ) , the number o f ( A 1 V I + F e 3 + + T i ) , (Na + K ) , and ( A 1 I V ) atoms p e r f o r m u l a u n i t ( a f t e r Deer and o t h e r s , 1966) 116 22 R e a c t i o n r e l a t i o n s h i p s i n G a r i b a l d i Lake l a v a s 120 23 P h l o g o p i t e - b i o t i t e c o m p o s i t i o n a l r e l a t i o n s of a n a l y z e d micas 124 24 C o mpositions of t i t a n o m a g n e t i t e s and i l m e n i t e s i n m o l e c u l a r p e r c e n t o f FeO, ^&2°3' and T i 0 2 128 25 Log £ -T diagram i n which a r e p l o t t e d maximum 2 v a l u e s o f temperature and oxygen f u g a c i t y f o r e&uilib^ateiQra (us,irig oury;fes-:o;frsBi5d#irigtl.dn;-and .A-L i r i d s l e y , 1964) 132 26 C a l c u l a t e d P-T i n t e r s e c t i o n s f o r e q u i l i b r a t i o n between:{(1) the groundmass l i q u i d o f b a s a l t i c -a n d e s i t e (598) and i t s a s s o c i a t e d p h e n o c r y s t assemblage; (2) the p h e n o c r y s t - e q u i l i b r a t e d l i q u i d o f the b a s a l t i c - a n d e s i t e and a s p i n e l - l h e r z o l i t e r e f r a c t o r y assemblage; and (3) the p h e n o c r y s t -e q u i l i b r a t e d l i q u i d and a q u a r t z e c l o g i t e m i n e r a l assemblage. 150 x i i i TABLE OF FIGURES (Continued) FIGURE PAGE 27 Comparison of calculated melt-lherzolite equilibrium conditions with experimentally-determined l i q u i d i for andesite (Stern and others, 1975), and olivine tholeiite and SiO^-saturated tholeiite (Nicholls and Ringwood, 1973) under conditions of P R < p T o t a 1 for a range of water contents. ^ 174 28 Multistage evolution of andesite magmas 180 x i v ACKNOWLEDGEMENTS The w r i t e r takes g r e a t p l e a s u r e i n acknowledging h i s i n d e b t e d n e s s to P r o f e s s o r H.R. Wynne-Edwards, who m a i n t a i n e d an i n s p i r i n g i n t e r e s t , and gave guidance and encouragement d u r i n g a l l phases o f the study. The w r i t e r b e n e f i t e d g r e a t l y from d i s c u s s i o n s w i t h P r o f e s s o r s R.L. Armstrong, W.H. Mathews and T.H. Brown, f e l l o w g r aduate s t u d e n t s I . J . Duncan, G.T. Nixon and L.C. P i g a g e , and Dr. J.G. Souther o f the G e o l o g i c a l Survey o f Canada. Dr. Souther k i n d l y s u p p l i e d base maps and h e l i c o p t e r s u p p o r t d u r i n g the f i e l d w o r k ; much a p p r e c i a t i o n i s extended to C A . G i o v a n e l l a , who p r o v i d e d the w r i t e r w i t h s e v e r a l e x c e l l e n t photographs o f The B l a c k Tusk, and P r o f e s s o r R.L. Armstrong, who a l l o w e d use o f h i s photograph o f Mount P r i c e as a f r o n t i s p i e c e . The w r i t e r wishes to e x p r e s s h i s g r a t i t u d e to the B r i t i s h Columbia Parks Branch and i t s employees, w i t h o u t whose a s s i s t a n c e t h i s s t u d y would not have been p o s s i b l e . George Georgakopoulos k i n d l y a i d e d the a u t h o r i n the o p e r a t i o n o f the m i c r o p r o b e ; numerous d i s c u s s i o n s w i t h , and the a s s i s t a n c e of Dr. B.R. Watters a i d e d the w r i t e r d u r i n g the X-ray f l u o r e s c e n c e a n a l y s e s . Dr. D.W. F i e s i n g e r k i n d l y p r o v i d e d a copy o f h i s t h e s i s , which p r o v i d e d i n s i g h t i n t o problems t h a t remain u n s o l v e d c o n c e r n i n g G a r i b a l d i Group magma g e n e s i s ; Dr. J.W. N i c h o l l s of the U n i v e r s i t y of C a l g a r y k i n d l y s e n t the w r i t e r c o p i e s of h i s computer programs, XALFRAC and PANDT. G r a t i t u d e i s e x p r e s s e d to J . H a r a k a l who h e l p e d the a u t h o r ru n th e mass s p e c t r o m e t e r d u r i n g the p otassium-argon d a t i n g p r o j e c t . The a u t h o r would a l s o l i k e to e x p r e s s h i s a p p r e c i a t i o n to h i s w i f e , Brenda, and son, Benjamin, who were e x t r e m e l y p a t i e n t XV d u r i n g t h e c o u r s e o f t h i s s t u d y , and w i t h o u t whose s u p p o r t t h i s s t u d y would n o t have been completed. N a t i o n a l R e s e a r c h C o u n c i l o f Canada Grant 67-8302 ( P r o f . H.R. Wynne-Edwards) p r o v i d e d s u p p o r t f o r b o t h f i e l d w o r k and p r e p a r a t i o n o f the m a n u s c r i p t . xvi 1 MULTISTAGE ANDESITE GENESIS IN THE GARIBALDI LAKE AREA,  SOUTHWESTERN BRITISH COLUMBIA I . INTRODUCTION G e n e r a l Statement Zones o f l i t h o s p h e r i c s u b d u c t i o n a r e c l o s e l y a s s o c i a t e d w i t h a c t i v e c a l c - a l k a l i n e v o l c a n i s m . T h i s r e l a t i o n s h i p has suggested to many p e t r o l o g i s t s t h a t the o r i g i n o f the c a l c - a l k a l i n e r o c k s e r i e s i s connected w i t h p r o c e s s e s o c c u r r i n g i n o r above the s u b d u c t i o n zone. P e t r o l o g i c a l s t u d i e s o f t h e s e v o l c a n i c r o c k s p r o v i d e i n f o r m a t i o n r e g a r d i n g the e a r t h ' s t h e r m a l regime, and the mechanisms of magma g e n e r a t i o n and t r a n s f e r i n r e g i o n s o f p l a t e convergence. In southwestern B r i t i s h Columbia, L a t e C e n o z o i c u n d e r t h r u s t i n g of the American P l a t e by the Juan de F u c a P l a t e was accompanied by e r u p t i o n o f b a s a l t and c a l c - a l k a l i n e a n d e s i t e from a l i n e o f v o l c a n i c c e n t e r s p a r a l l e l t o and a p p r o x i m a t e l y 200 km i n l a n d from the American P l a t e m argin. Major a n d e s i t i c v o l c a n o e s , Mount G a r i b a l d i (2678 m above sea l e v e l ) , Mount C a y l e y (2393 m), Meager Mountain (2691 m) and P l i n t h Peak (2708 m), r i s e f i v e hundred to one thousand meters above the g l a c i a l l y eroded s u r f a c e o f the s o u t h e r n Coast Mountains. S e v e r a l l e s s -prominent a n d e s i t i c c e n t e r s (Mount P r i c e , The T a b l e , The B l a c k Tusk and The C i n d e r Cone) and a s s o c i a t e d v a l l e y b a s a l t s a r e l o c a t e d i n the a r e a of s t u d y , the G a r i b a l d i Lake a r e a about 5 km n o r t h o f Mount G a r i b a l d i ( F i g . 1 ) . T h i s study documents the h i s t o r y o f P l e i s t o c e n e and Recent v o l c a n i c c e n t e r s i n the G a r i b a l d i Lake a r e a , and determines the 2 F i g u r e 1. D i s t r i b u t i o n and t e c t o n i c s e t t i n g of Quaternary v o l c a n i c r o c k s i n s outhwestern B r i t i s h Columbia. Toothed l i n e (^tfkk^d^k) i n d i c a t e s approximate p o s i t i o n o f the consuming margin between the Juan de Fuca, E x p l o r e r and American p l a t e s . Arrows r e p r e s e n t the L a t e C e n o z o i c d i r e c t i o n o f Juan de Fuca and E x p l o r e r p l a t e motion r e l a t i v e to the American P l a t e . Trend of earthquake e p i c e n t e r s may r e p r e s e n t edge of downgoing Juan de Fuca P l a t e . 4 p r o c e s s e s o f d i f f e r e n t i a t i o n and magma g e n e s i s which most a d e q u a t e l y e x p l a i n o bserved c h e m i c a l and m i n e r a l o g i c a l v a r i a t i o n s w i t h i n each c e n t e r . Mechanisms i n v o l v i n g a n a t e x i s o f s i a l i c c r u s t , p a r t i a l m e l t i n g o f e c l o g i t e w i t h i n subducted o c e a n i c l i t h o s p h e r e , p a r t i a l m e l t i n g o f p e r i d o t i t i c m a n t l e , and c r y s t a l f r a c t i o n a t i o n o f b a s a l t i c and a n d e s i t i c magmas a r e a s s e s s e d i n the l i g h t o f major element, t r a c e element and s t r o n t i u m i s o t o p e d a t a . P h e n o c r y s t - c o m p o s i t i o n s and m i n e r a l e q u i l i b r i a a r e e v a l u a t e d i n terms o f c r y s t a l f r a c t i o n a t i o n p r o c e s s e s . Thermodynamic c a l c u l a t i o n s , u s i n g a n a l y t i c a l d a t a on the m i n e r a l o g y and c o m p o s i t i o n o f the l a v a s , y i e l d e s t i m a t e s o f the P, T and P c o n d i t i o n s o f p h e n o c r y s t c r y s t a l l i z a t i o n f o r the a n d e s i t e H 20 magmas, and p r o v i d e a t e s t o f p o s s i b l e e q u i l i b r a t i o n o f the l a v a s w i t h e c l o g i t i c , p y r o x e n i t i c o r p e r i d o t i t i c r e s i d u e . F i n a l l y , a m u l t i s t a g e c r y s t a l f r a c t i o n a t i o n model i s e v o l v e d t h a t s a t i s f a c t o r i l y accounts f o r t h e n a t u r e and v a r i a t i o n o f c a l c - a l k a l i n e a n d e s i t e magmas i n the G a r i b a l d i Lake a r e a . P r e v i o u s Work The g e o l o g y o f the G a r i b a l d i Lake a r e a was f i r s t d e s c r i b e d by Burwash (1914a). Burwash (1914b) c o n c l u d e d t h a t most o f the v o l c a n i c r o c k s i n the r e g i o n a r e P l e i s t o c e n e i n age, and proposed the name G a r i b a l d i Group f o r "the p r o d u c t s o f P l e i s t o c e n e v u l c a n i s m " . Subsequent mapping by Mathews (1948, 1951, 1952a, 1952b, 1958b) r e v e a l e d t h a t many o f th e s e v o l c a n i c p r o d u c t s p o s s e s s unusual m o r p h o l o g i c a l f e a t u r e s which can o n l y be a t t r i b u t e d t o the i n t e r a c t i o n o f m o l t e n l a v a s w i t h the P l e i s t o c e n e i c e - s h e e t . Mathews (1957) f u r t h e r demonstrated t h a t G a r i b a l d i Group l a v a s a r e c h e m i c a l l y s i m i l a r t o Qua t e r n a r y v o l c a n i c 5 r o c k s of the H i g h Cascades i n n o r t h e r n C a l i f o r n i a , Oregon and Washington. Method o f I n v e s t i g a t i o n The p r e s e n t r e p o r t i s based on a t o t a l o f about 4 month's f i e l d w o r k d u r i n g the summers o f 1974 and 1975. R e c o n n a i s s a n c e mapping of exposed Q u a t e r n a r y v o l c a n i c r o c k s between Squamish and C a l l a g h a n Lakek(10i?kmcnorth ofHmapfarea) was done on 1:31,680 b l a c k - a n d - w h i t e a e r i a l photographs w i t h f i n a l c o m p i l a t i o n on the B r i t i s h Columbia Department o f Lands, F o r e s t s and Water Resources 1:63,360 map of Western G a r i b a l d i Park. V o l c a n i c s u i t e s i n the G a r i b a l d i Lake a r e a were mapped u s i n g a 1:15,840 s c a l e enlargement of N a t i o n a l T o p o g r a p h i c S e r i e s Map No. 92G/14E. A s s i s t a n c e i n the f i e l d w o r k was a b l y p r o v i d e d by R. S c a g e l . Whole-rock c h e m i c a l a n a l y s e s o f s e v e n t y - f o u r s e l e c t e d samples were determined by X-ray f l u o r e s c e n c e s p e c t r o m e t r y . S i , A l , T i , t o t a l Fe, Mmy Mg, Ca, Na, K, and P were measured u s i n g the method of N o r r i s h and Hutton (1969). In a d d i t i o n , S r , Ba, Rb, Y, Nb, Z r , Cu, N i , Zn, Cr, and V were determined on compressed p e l l e t s from powdered samples and the a b s o r p t i o n c o e f f i c i e n t s were measured d i r e c t l y ( N o r r i s h and C h a p p e l l , 1967) or c a l c u l a t e d . E i g h t a d d i t i o n a l a n a l y s e s of v o l c a n i c r o c k s from the s t u d y a r e a from Mathews (1957) and s i x u n p u b l i s h e d c h e m i c a l a n a l y s e s by Dr. B.M. Gunn o f the U n i v e r s i t y o f M o n t r e a l were i n c o r p o r a t e d i n t h i s s t u d y . A p p r o x i m a t e l y 150 t h i n s e c t i o n s were examined; modal c o m p o s i t i o n s , based on a minimum o f 1,000 p o i n t s , were determined f o r 42 of t h e s e specimens (Appendix I ) . 6 M i n e r a l a n a l y s e s were made w i t h an ARL-SEMQ e l e c t r o n probe X-ray a n a l y z e r u s i n g an a c c e l e r a t i n g v o l t a g e o f 15 kV, a specimen c u r r e n t o f about 25 nanoamperes, and e l e c t r o n beam di a m e t e r s o f 2-10 m i c r o n s . The c o r r e c t i o n p r o c e d u r e was t h a t o f Bence and A l b e e (1968) and A l b e e and Ray (1970). N a t u r a l and s y n t h e t i c m i n e r a l s o f known c o m p o s i t i o n were used as s t a n d a r d s f o r the a n a l y s e s . S u l p h i d e a n a l y s e s were c o r r e c t e d u s i n g the computer program EMPADR V I I ( R u c k l i d g e and G a s p a r r i n i , 1969). T e c t o n i c S e t t i n g o f Southwestern B r i t i s h Columbia The major r e g i o n a l s t r u c t u r e s o f the southwestern B r i t i s h Columbia c o n t i n e n t a l margin a r e i l l u s t r a t e d i n F i g . 1. The g e n e r a l c o n f i g u r a t i o n o f p l a t e b o u n d a r i e s a l o n g the southwestern B r i t i s h Columbia c o n t i n e n t a l margin can be d e f i n e d as a t r i p l e j u n c t i o n between a t r a n s f o r m f a u l t west o f the Queen C h a r l o t t e I s l a n d s (Queen C h a r l o t t e f a u l t system, a s p r e a d i n g r i d g e system (Juan de Fuca Ridge -E x p l o r e r Ridge - Dellwood K n o l l s ) , and a s u b d u c t i o n zone a l o n g the c o n t i n e n t a l margin s o u t h o f Queen C h a r l o t t e Sound (Chase and o t h e r s , 1975). A l t h o u g h McKenzie and P a r k e r (1967) su g g e s t e d t h a t the Juan de Fuca P l a t e i s b e i n g u n d e r t h r u s t beneath N o r t h America (American P l a t e ) , the absence of a major b a t h y m e t r i c t r e n c h a t the f o o t o f the c o n t i n e n t a l s l o p e o f f Vancouver I s l a n d , the absence o f a c l e a r l y d e f i n e d eastward d i p p i n g earthquake zone beneath t h e c o n t i n e n t , and the q u i e s c e n t n a t u r e o f v o l c a n i s m on the mainland have c o n t r i b u t e d t o u n c e r t a i n t y as to whether s u b d u c t i o n has r e c e n t l y t a ken p l a c e ( S t a c e y , 1973). R i d d i h o u g h and Hyndman (1976), however, r e v i e w e v i d e n c e which 7 s u p p o r t s c o n t i n u i n g convergence o f the p l a t e s . T h e i r e v i d e n c e i n c l u d e s : (1) A zone o f low heat f l o w which l i e s on the c o n t i n e n t a l s i d e o f the " i n f e r r e d " t r e n c h . The heat f l o w p a t t e r n i s c o n s i s t e n t w i t h c o o l e r subducted o c e a n i c l i t h o s p h e r e a c t i n g as a heat s i n k beneath the c o n t i n e n t (Hyndman, 1976a). (2) A p l a t e a u o f n e r o - z e r o Bouguer g r a v i t y anomaly o v e r Vancouver I s l a n d between an o c e a n i c " h i g h " and a n ' i n l a n d "low" ( S t a c e y , 1973). The g r a v i t y anomalies a r e l e s s pronounced but co m p a t i b l e w i t h f e a t u r e s e x p e c t e d i n a s u b d u c t i o n zone. The lower magnitude o f the g r a v i t y anomalies may be due to a slower convergence r a t e and to a t r e n c h f i l l e d w i t h sediments (Hyndman, 1976b). (3) The s t y l e o f d e f o r m a t i o n t h a t c h a r a c t e r i z e s the c o n t i n e n t a l s l o p e s o u t h o f Nootka I s l a n d o f f the c o a s t o f Vancouver I s l a n d . D i s h a r m o n i c f o l d s and b l o c k f a u l t s i n c r e a s e i n i n t e n s i t y southward. S e i s m i c r e f l e c t i o n d a t a r e v e a l t h a t the o c e a n i c basement d i p s beneath the s l o p e sediments ( T i f f i n and o t h e r s , 1972; Murray and T i f f i n , 1974). A t t e n u a t i o n of magnetic anomalies ( B a r r , 1974) and g r a v i t y measurements ( S r i v a s t a v a , 1973). a r e a l s o c o m p a t i b l e w i t h c'bntinuedo'convgrgencefof; t h e v J u a r i e d e u E u c a s a n d u . M e r i c a n l p l a t e s . r-e -3 iTheOabsencelofeaiwe'llBdefined' B e n i o f f k e a r t h q u a k e zone beneath the-t'c'ontirfenl; in&yelre1.expfLaihieds.by^.a^saibwe'rrgonver.getice -.'rate (2 cm/yr o r l e s s ) c o u p l e d w i t h the p r o b a b l e h i g h temperature o f the y o u t h f u l Juan de F u c a P l a t e ( B a r r , 1974). R i d d i h o u g h and Hyndman (1976) c a l c u l a t e d the maximum depth o f earthquakes which c o u l d be expe c t e d i n southwestern B r i t i s h Columbia; t h e i r p r e d i c t i o n (70 km) agrees r e a s o n a b l y w e l l w i t h the observed maximum (60 km). Thus, i t seems r e a s o n a b l e t h a t s u b d u c t i o n c o n t i n u e s a l o n g the c o a s t o f B r i t i s h Columbia. 8 A zone o f earthquake e p i c e n t e r s extends a p p r o x i m a t e l y N40°E from the n o r t h e r n end o f the Juan de Fuca Ridge to Nootka I s l a n d a l o n g the west c o a s t of Vancouver I s l a n d ( F i g . 1 ) . T h i s earthquake zone c o i n c i d e s w i t h the boundary between i n t e n s e l y f o l d e d and f a u l t e d , and r e l a t i v e l y undeformed s h e l f sediments ( T i f f i n and o t h e r s , 1972). E x t r a p o l a t i o n o f t h i s t r e n d onto the c o n t i n e n t f u r t h e r demarks the n o r t h e r n l i m i t o f Q u a t e r n a r y c a l c - a l k a l i n e v o l c a n i s m and i n t e n s e s e i s m i c a c t i v i t y ( F i g . 1 ) . The combined p r e s e n c e o f t h e se f e a t u r e s s u g g e s t s a g e n e t i c r e l a t i o n s h i p to the downgoing p l a t e . The l i n e a r f e a t u r e , p o s s i b l y marking the edge o f the Juan de Fuca P l a t e , p a r a l l e l s y e t l i e s somewhat s o u t h o f a p r e d i c t e d p l a t e edge u s i n g c a l c u l a t e d t r i p l e p o i n t p o s i t i o n s and the E x p l o r e r - American p l a t e v e c t o r s ( R i d d i h o u g h , 1976). Geologic- S e t t i n g o f t h e . G a r i b a l d i Lake A r e a The. Q u a ternary v o l c a n i c r o c k s o f the a r e a are u n d e r l a i n by m e t a v o l c a n i c , metasedimentary and g r a n i t i c r o c k s o f C r e t a c e o u s and p o s s i b l e J u r a s s i c age ( F i g . 2 ) . The g e o l o g y of the r e g i o n has been d e s c r i b e d by Mathews (1958a) and R oddick and Woodsworth (1975). P r e - C r e t a c e o u s g r e e n s t o n e , c h e r t , and a r g i l l i t e , i n t e r c a l a t e d w i t h minor conglomerate, greywacke and l i m e s t o n e , o c c u r as l a r g e pendants w i t h i n the q u a r t z d i o r i t e s o f the Coast P l u t o n i c Complex (Roddick and H u t c h i s o n , 1974). These h i g h l y s h e a r e d sequences, which are w i d e l y d i s t r i b u t e d a l o n g the s l o p e s of the Cheakamus R i v e r and C a l l a g h a n Creek v a l l e y s , may be c o r r e l a t i v e w i t h p a r t s o f James' (1929) Goat Mountain F o r m a t i o n i n the B r i t a n n i a Beach a r e a (Mathews, 1958a). 9 C r e t a c e o u s metasediments, c o m p r i s i n g the Cheakamus F o r m a t i o n , r e s t unconformably upon the h i g h l y sheared heterogeneous assemblage o f E a r l y C r e t a c e o u s q u a r t z d i o r i t e s and d i o r i t e s ( C l o u d b u r s t q u a r t z d i o r i t e s ; Mathews, 1958a) w i t h i n Rubble Creek V a l l e y ( F i g . 2 ) . The b l o c k - f a u l t e d C r e t a c e o u s s u c c e s s i o n of p o o r l y s t r a t i f i e d arenaceous greywackes, q u a r t z d i o r i t e c o b b l e conglomerates and minor a r g i l l i t e s u n d e r l i e s the a r e a northwest of G a r i b a l d i Lake. The Cheakamus Fo r m a t i o n i s i n p o s s i b l e f a u l t c o n t a c t to the n o r t h e a s t w i t h younger C r e t a c e o u s a r g i l l i t e s , q u a r t z i t e s and greywackes of the Helm F o r m a t i o n ( F i g . 2 ) . 'Pie .metamorphic grgde •©€. (the Helm Foranaf-ion seclimengs -increases towards the n o r t h e a s t , and i s p o s s i b l y r e l a t e d t o the emplacement of a n e i g h b o u r i n g b a t h o l i t h t o the n o r t h . Mathews ( 1 9 5 8 a ) . a l s o mapped as C r e t a c e o u s i n age a sequence o f m a s s i v e g r e e n s t o n e s and agglomerates (Empetrum F o r m a t i o n ) , which o u t c r o p on a r i d g e 3 km n o r t h o f G a r i b a l d i Lake. However, the h i g h e r metamorphic grade of these r o c k s and the p r e s e n c e of i n t e r b e d d e d d o l o m i t i c l i m e s t o n e s w i t h i n the g r e e n s t o n e sequence s u g g e s t s , i n s t e a d , t h a t t h i s s m a l l s l i c e of Empetrum F o r m a t i o n r o c k s r e p r e s e n t s an u p l i f t e d b l o c k o f P r e - C r e t a c e o u s s t r a t a . Younger q u a r t z d i o r i t e s o c c u r as d i s c r e t e b o d i e s which i n t r u d e the C l o u d b u r s t q u a r t z d i o r i t e s , P r e - C r e t a c e o u s pendants, and a l l C r e t a c e o u s sediments n o r t h o f G a r i b a l d i Lake. The l a c k of d e f o r m a t i o n a l f e a t u r e s , except near t h e i r m a r g i n s , l e d Mathews (1958a) to c o n c l u d e t h a t t h e s e q u a r t z d i o r i t e s were emplaced d u r i n g a p e r i o d o f f a u l t i n g i n l a t e C r e t a c e o u s t i m e s . The C r e t a c e o u s o r younger C a s t l e Towers b a t h o l i t h o u t c r o p s e a s t and n o r t h e a s t o f G a r i b a l d i Lake; a narrow zone of m i g m a t i t e s e p a r a t e s . t h e g r a n i t i c body from e n c l o s i n g c o u n t r y r o c k s . 10 I I , GEOLOGY AND GEOCHRONOLOGY OF GARIBALDI LAKE VOLCANIC SUITES I n t r o d u c t i o n Burwash (1914b) f i r s t r e c o g n i z e d the e x i s t e n c e of s e v e r a l s m a l l a n d e s i t i c v o l c a n o e s i n the G a r i b a l d i Lake a r e a of southwestern B r i t i s h Columbia, With the e x c e p t i o n of r e c o n n a i s s a n c e o b s e r v a t i o n s (Burwash, 1918; Brock, 1928), t h e s e v o l c a n i c f e a t u r e s remain v i r t u a l l y u n d e s c r i b e d u n t i l Mathews (1948) mapped the a r e a . Mathews (1951, 1952a, 1958b) was a b l e t o c o n c l u d e t h a t many o f the p r o d u c t s of v o l c a n i c a c t i v i t y a r e s u b g l a c i a l o r i n t r a g l a c i a l i n o r i g i n , y e t the s t r a t i g r a p h y of the i n d i v i d u a l v o l c a n o e s was not worked out i n d e t a i l . In t h i s c h a p t e r , the g e n e r a l p e t r o l o g i c a l f e a t u r e s and the c h r o n o l o g i c a l r e l a t i o n s h i p s o f P l e i s t o c e n e - R e c e n t v o l c a n i c r o c k s i n the G a r i b a l d i Lake a r e a a r e d i s c u s s e d m a i n l y on the b a s i s o f the w r i t e r ' s o b s e r v a t i o n s . In t h i s a r e a v o l c a n i s m has produced r o c k s of c a l c - a l k a l i n e a f f i n i t y w i t h minor a s s o c i a t e d b a s i c l a v a s . F i v e major v o l c a n i c sequences of P l e i s t o c e n e and Recent age o c c u r i n the r e g i o n , but l a c k o f p y r o c l a s t i c marker h o r i z o n s and absence of v i s i b l e v o l c a n i c c e n t e r s l e a v e some a s p e c t s of the h i s t o r y o f the o l d e r e r u p t i v e c e n t e r s u n c e r t a i n . I t has been p o s s i b l e , however, to e s t a b l i s h sequences o f events a t the younger c e n t e r s . Cheakamus V a l l e y B a s a l t s P l e i s t o c e n e to Recent o l i v i n e - b a s a l t s occupy a 1 to 2 km wide b e l t w i t h i n the Cheakamus R i v e r and C a l l a g h a n Creek v a l l e y s , e x t e n d i n g 22 km n o r t h from G a r i b a l d i s t a t i o n to C a l l a g h a n Lake ( F i g . 2 ) . 11 At l e a s t f o u r e p i s o d e s of b a s a l t i c v o l c a n i s m have c o n t r i b u t e d to the a p p r o x i m a t e l y 80 m t h i c k sequence; the d i p o f a l l u n i t s v a r i e s from h o r i z o n t a l to 30°. The two lowermost b a s a l t u n i t s ( A l p i n e Lodge phases; F i g . 3 ) , exposed i n r a i l r o a d c u t s 1 km n o r t h o f G a r i b a l d i S t a t i o n , : a r e - s e p a r a t e d from each o t h e r and from the o v e r l y i n g l a v a s by e r o s i o n s u r f a c e s . Stream c h a n n e l s , up to 5 m deep and 6 m i n w i d t h , o c c u r a t the top o f each u n i t , b e i n g s u b s e q u e n t l y f i l l e d by younger l a v a s . At i t s t e r m i n u s , the o l d e s t f l o w r e s t s on g r e e n i s h - g r e y sands and g r a v e l s . The t h i r d o f Cheakamus Dam phase ( F i g . 3) appears to r e p r e s e n t the most e x t e n s i v e p e r i o d o f e x t r u s i o n w i t h i n the b e l t , adding a p p r o x i m a t e l y 20 m of i n t e r f i n g e r i n g f l o w - u n i t s to the s e c t i o n . G l a c i a l s t r i a t i o n s on the exposed upper s u r f a c e of the Cheakamus Dam b a s a l t s i n d i c a t e t h a t t h e se l a v a s p r e d a t e the l a t e s t g l a c i a t i o n . The uppermost f l o w o f the t h i r d phase i s r e p r e s e n t e d by a v a r i a b l e t h i c k n e s s o f f l a t - l y i n g pahoehoe o l i v i n e b a s a l t , which o v e r l i e s a l l o l d e r u n i t s n o r t h of Brandywine F a l l s ( F i g . 2 ) . A r e c e s s i o n o f the c o n t i n e n t a l i c e - s h e e t p r i o r to the f i n a l phase of v o l c a n i s m i s i n d i c a t e d by a 1 to 5 m t h i c k h o r i z o n of baked t i l l s and sandy c l a y s which u n d e r l i e s the younger b a s a l t s w i t h i n a q u a r r y , 7 km n o r t h of Brandywine F a l l s . P i e c e s of c a r b o n i z e d wood, 14 which y i e l d a C date of 34,200 y r B.P. (W. B l a k e , p e r s o n a l communi-c a t i o n ) , a r e found w i t h i n the c l a y s ; t h i s date c o r r e s p o n d s w e l l to the Olympia I n t e r s t a d e , the n o n - g l a c i a l i n t e r v a l p r e c e d i n g the l a s t major g l a c i a t i o n (Hansen and E a s t e r b r o o k , 1974). The f i n a l o r Brandywine F a l l s phase o f b a s a l t i c e x t r u s i o n i s 12 F i g u r e 3. Schematic s t r a t i g r a p h i c column of the Cheakamus V a l l e y b a s a l t s . 13 FEET METERS UNITS DESCRIPTION 2 2 C H 180 A h 6 0 140 H h-40 100 H 6 0 -4 0 H h 2 0 O - h O BRANDYWINE FALLS PHASE: DARK GREY TO BLACK BASALT, BLOCKY FLOW TOPSJ SERIES OF ANASTOMOSING FLOW UNITS; RESTRICTED LATERAL EXTENT; WELL-DEVELOPED COLUMNAR JOINTING; FANNED ENTABLATURE JO I NT-COLUMNS;ABUNDANT SPIRACLES AND PIPE VESICLES; SCORIACEOUS FLOW BASE UP TO 2 M THICK; FLOWS PORPHYRITIC WITH PHENOCRYSTS OF CLEAR WHITE PLAGIOCLASE AND MINOR OLIVINE TILL AND GLACIAL FLUVIAL SAND AND SILT; CARBONACEOUS MATERIAL; C 1 4 DATE 3<l,200 YR B.P. CHEAKAMUS DAM PHASE: LIGHT TO DARK GREY BASALT; SERIES OF INTERFINGERING FLOW UNITS; PROMINENT COLUMNAR JOINTING; ENTABLATURE:COLONADE THICKNESS RATIO BETWEEN 1:1 AND 1-10; OCCASIONAL BASAL ENTABLATURE JOINT SET; RARE ROPY °=LOW TO°S OTHERWISE HIGHLY GLACIATED WHERE EXPOSED; SCORIACEOUS PLOW BASE UP TO 1 M THJCK, WEAKLY VESICULAR FLOW TOPS; VESICLE CYLINDERS UP AND CLEAR WHITE PLAG CM; SPARSELY PORPHYRITIC WITH GIOCLASE; K-AR DATE 0.05 M.Y. OLIVINE ALPINE LODGE PHASE II: MEDIUM GREY TO STEEL GREY BASALT; HOLOCRYSTALLINE; SERIES OF INTERFINGERING FLOW UNITS; WELL-DEVELOPED COLUMNAR JOINTING; ENTABLATURE:COLONADE THICKNESS RATIO BETWEEN 1:1 AND 3:2; YELLOW TO RUSTY BROWN WEATHERING; HIGHLY SCORIACEOUS NEAR FLOW BASE; FLOW UNITS BETWEEN 2 AND 10 M THICK; MINOR PIPE VESICLES; PHENOCRYSTS OF OLIVINE AND VERY MINOR PLAGIOCLASE ALPINE LODGE PHASE I: STEEL GREY BASALT; YELLOWISH-BROWN WEATHERING; PROMINENT COLUMNAR JOINTING; SERIES OF OVERLAPPING FLOW TONGUES; ROSETTES OF COLUMNS NEAR BASE OF FLOWS; SCORIACEOUS FLOW BASE UP TO I M THICK; PORPHYRITIC WITH SMALL PHENOCRYSTS OF OLIVINE GLACIAL-FLUVIAL GRAVELS AND CLAYS CRETACEOUS QUARTZ DIORITE WITH MINOR PENDANTS OF GREENSTONE S L E N D E R J O I N T S E T S T O U T J O I N T S E T B A S A L T I C S C O R I A G L A C I A L D E P O S I T F L U V I A L D E P O S I T G R A N I T I C R O C K S ) 14 c h a r a c t e r i z e d by anastomosing f l o w - u n i t s , h e m i - c y l i n d r i c a l i n shape; the l a v a s have a r e s t r i c t e d l a t e r a l e x t e n t (50 t o 100 m) and fanned e n t a b l a t u r e columns ( F i g . 4 a ) . These " e s k e r - l i k e " b a s a l t s r e s t on a 2 m t h i c k bed o f g r e e n i s h - g r e y h y a l o c l a s t i t e b r e c c i a near Brandywine F a l l s ; the p r e s e n c e o f s p i r a c l e s , by which steam c o u l d have p e r c o l a t e d t hrough the l a v a s , a l s o a t t e s t s t o the i n t e r a c t i o n o f water, p o s s i b l y g l a c i a l , w i t h the adv a n c i n g b a s a l t s . Immediately e a s t o f Brandywine F a l l s , the b a s a l t s l e f t the o l d e r p l a t e a u and flowed down a g l a c i a l c h a n n e l cut i n t o the e a r l i e r l a v a s a l o n g the e a s t e r n s i d e o f the Cheakamus V a l l e y . The b a s a l t s now occupy the same s t r a t i g r a p h i c l e v e l as t h e Cheakamus Dam l a v a s on the o p p o s i t e s i d e o f D a i s y Lake ( F i g . 2 ) . Mathews (1958b) suggested t h a t t h e s e f l o w s d e v e l o p e d d u r i n g the waning s t a g e s o f the ( F r a s e r G l a c i a t i o n ) i c e - s h e e t p o s s i b l y by passage of l a v a a l o n g t u n n e l s o r t r e n c h e s thawed by he a t e d meltwater. An a u g i t e - b a s a l t , c h a r a c t e r i z e d by 1 m bulbous j o i n t columns, i s exposed n e a r C a l l a g h a n Lake; t h i s f l o w may be s t r a t i g r a p h i c a l l y e q u i v a l e n t t o the upper Brandywine F a l l s -lavas but e x t e n s i v e d r i f t c o v e r makes c o r r e l a t i o n i m p o s s i b l e . A l t h o u g h c h e m i c a l l y s i m i l a r (Chapter I I I ) , the Cheakamus V a l l e y b a s a l t s a r e p o r p h y r i t i c w i t h p h e n o c r y s t s o f e i t h e r o l i v i n e ( A l p i n e Lodge and Cheakamus Dam p h a s e s ) , p l a g i o c l a s e (Brandywine F a l l s phase) or a u g i t e ( C a l l a g h a n flow) predominant. P l a g i o c l a s e pheno.Gry.stsgar.erzoned750generaMydnormaltzohingHOV.er c >• zx -range An^,. to A n ^ • The pyroxene i s c h a r a c t e r i s t i c a l l y g r e e n i s h or brownish, t i t a n i u m - r i c h d i o p s i d i c a u g i t e ; o r t h o p y r o x e n e i s ab s e n t . O l i v i n e p h e n o c r y s t s show no r e a c t i o n w i t h the l i q u i d and have a c o m p o s i t i o n o f about E o 7 9 , w i t h s l i g h t i r o n - e n r i c h m e n t a t the rims 15 ( t o F o , 0 ) . The groundmass of the b a s a l t s i s composed of p l a g i o c l a s e DO l a t h s (ATI^Q_2Q), o l i v i n e ( F o ^ ^ ) , equant a u g i t e g r a n u l e s , i r o n -t i t a n i u m o x i d e s and i n t e r s t i t i a l brown g l a s s . The B l a c k Tusk The o l d e s t and, perhaps, most s t r i k i n g f e a t u r e o f the r e g i o n i s The B l a c k Tusk, a d i s e c t e d l a v a cone s i t u a t e d 2.5 km n o r t h of D r i f t w o o d Bay on G a r i b a l d i Lake. Only the southwestern quadrant o f the v o l c a n o , which r i s e s to an a l t i t u d e o f 2316 m, has s u r v i v e d P l e i s t o c e n e and subsequent a l p i n e g l a c i a t i o n s . The mountain i s markedly e l o n g a t e p a r a l l e l t o the l o c a l d i r e c t i o n o f W i s c o n s i n i c e movement. The geology o f The B l a c k Tusk, shown i n F i g . 5, may be d i v i d e d i n t o two p r i n c i p a l magmatic c y c l e s o r " s t a g e s " : the a n c e s t r a l mountain s t a g e and the p l u g dome s t a g e . A n c e s t r a l Mountain Stage P r o d u c t s o f the i n i t i a l phase of e r u p t i o n a t The B l a c k Tusk crop out on the b l u f f s n orthwest (Microwave B l u f f ) , southwest (West B l u f f ) and s o u t h e a s t ( E a s t B l u f f ) o f the v o l c a n i c s p i r e ( F i g . 5 ) . These r o c k s c o n s i s t o f grey h o r n b l e n d e - d a c i t e l a v a f lows andciiiinor a s h - f l o w t u f f s e r u p t e d from a v e n t now e i t h e r removed by the g l a c i a t i o n or b u r i e d beneath the l a v a s o f s u c c e e d i n g u n i t s . . The base of the sequence i s n o t exposed, but i s everywhere c o v e r e d by s c r e e and reworked m a t e r i a l ; i n t e r n a l s t r a t i g r a p h i c r e l a t i o n s h i p s a r e u n c e r t a i n . P a l e - g r e y h y p o c r y s t a l l i n e p l a t y a n d e s i t e s , which y i e l d K/Ar whole r o c k d a t e s between 1.1 and 1.3 m.y. (see below) are exposed a l o n g the base of the t h r e e b l u f f s . P i n k i s h - g r e y l i t h i c t u f f w i t h 1 to 3 cm 16 F i g u r e 4. P l e i s t o c e n e - R e c e n t v o l c a n i c r o c k s i n the G a r i b a l d i Lake a r e a . (a) C r o s s - s e c t i o n o f b a s a l t f l o w i n Cheakamus R i v e r v a l l e y , 7 km n o r t h o f Brandywine F a l l s . I n c l i n e d columns on r i g h t mark the o r i g i n a l margin o f f l o w . (b) The B l a c k Tusk from E a s t B l u f f l o o k i n g n o r t h w e s t . Note p l u g dome capped by younger l a v a f l o w shown i n s e c t i o n on the steep f a c e o f the mountain. (c) C o n t a c t zone of. the p l u g dome a t The B l a c k Tusk. Dark f i n e - g r a i n e d h y p e r s t h e n e a n d e s i t e c o r e s o u t e r r i m o f p a l e g r e y columnar a n d e s i t e . (d) The C i n d e r Cone from The B l a c k Tusk l o o k i n g . e a s t . O l d e r t u f f r i n g ( f o r e g r o u n d ) b u r i e d by younger S t r o m b o l i a n c i n d e r cone. (e) The T a b l e from Mount G a r i b a l d i l o o k i n g n o r t h . F l a t - l y i n g l a v a s form an e l o n g a t e p l a t e a u t h a t p a r a l l e l s the l o c a l d i r e c t i o n o f P l e i s t o c e n e ice-movement. ( f ) H o r n b l e n d e - a n d e s i t e b l o c k i n a Mount P r i c e t u f f -b r e c c i a showing b r e a d c r u s t t e x t u r e . (Photographs 4b, 4c and 4d by C.A. G i o v a n e l l a ) . 17 18 h o r i z o n t a l p a r t i n g s form a s m a l l k n o l l near the headwaters of Marble Creek ( F i g . 2 ) , and o v e r l i e s the p l a t y a n d e s i t e s on the West B l u f f . Columnar d a c i t e s w i t h d i s t i n c i t v e c o n t o r t e d f l o w - b a n d i n g caused by a l t e r n a t i o n of v i t r i c and v e s i c u l a r bands, however, c o n s t i t u t e the b u l k o f the Microwave B l u f f and E a s t B l u f f l a v a s ; h o r i z o n t a l l y -j o i n t e d dykes o f s i m i l a r d a c i t e t r e n d N30°W a c r o s s the l a v a s o f Microwave B l u f f . The l a v a s o f the a n c e s t r a l mountain a r e w i t h o u t e x c e p t i o n p o r p h y r i t i c . A c i c u l a r b l a d e s o f p a r t i a l l y r e s o r b e d greenish-brown h o r n b l e n d e a r e the predominant p h e n o c r y s t i n a l l l a v a s except the p l a t y a n d e s i t e s . P l a g i o c l a s e i s u s u a l l y found as murky, c o r r o d e d p h e n o c r y s t s w i t h a t h i n s e i v e - t e x t u r e d zone between t h e i r weakly zoned c o r e and r i m . Orthopyroxene and r a r e c l l n o p y r o x e n e a a r e a l s o p r e s e n t as c o r r o d e d m i c r o p h e n o c r y s t s. The p i l o t a x i t i c groundmass c o n s i s t s o f twinned p l a g i o c l a s e m i E r o l i t e s , spongy a u g i t e , equant orth o p y r o x e n e g r a i n s , t i t a n i f e r o u s m a g n e t i t e and a p a l e g r e y i n t e r s t i t i a l g l a s s . P l u g Dome Stage A f t e r a p e r i o d o f dormancy, p o s s i b l y s e v e r a l hundred thousand y e a r s i n d u r a t i o n , a c t i v i t y began from a reopened c e n t r a l v e n t . D u r i n g subsequent cone b u i l d i n g , the a l r e a d y d e e p l y eroded a n c e s t r a l mountain was b u r i e d by e r u p t i o n s o f b l a c k h y p e r s t h e n e - a n d e s i t e . The low e s t exposures a l o n g the r i d g e on the n o r t h s i d e o f The B l a c k Tusk a r e p r o b a b l y the o l d e s t flows o f the second c y c l e ( F i g . 5 ) . These l a v a s , termed the Moraine A n d e s i t e s , are d a r k - g r e y h o l o c r y s t a l l i n e a n d e s i t e s e r u p t e d when the main vent, was l e s s than 2100 m i n e l e v a t i o n . The upper pa-cc o f - t h e West B l u f f c o n s i s t s o f a s i m i l a r h y p a r s t h e n e -19 The upper p a r t o f the West B l u f f c o n s i s t s o f a s i m i l a r h y p e r s t h e n e -a n d e s i t e f l o w t h a t came t o r e s t w i t h a p r e c i p i t o u s 100 m n o r t h w e s t e r n margin; Mathews (1958b) su g g e s t e d ponding by i c e t o e x p l a i n t h e s t e e p f a c e of the l a v a . The p r e s e n c e o f g l a c i a l e r r a t i c s on the upper s u r f a c e of the West B l u f f a n d e s i t e i n d i c a t e s t h a t the l a v a was c o v e r e d by i c e e i t h e r d u r i n g o r a f t e r i t s e r u p t i o n , but the absence of e r o s i o n a l f e a t u r e s a t t r i b u t a b l e t o g l a c i a t i o n s u g g e s t s t h a t an i c e - s h e e t was n o t p r e s e n t f o r an extended p e r i o d of time (Mathews, 1948). The c u l m i n a t i n g event o f The B l a c k Tusk v o l c a n i s m was the e x t r u s i o n of a mushroom-shaped p l u g dome near the summit o f the second s t a g e cone. The p l u g , exposed i n s e c t i o n on the steep e a s t e r n f a c e o f the mountain by Recent c i r q u e - c u t t i n g ( F i g . 4b), i s composite i n c h a r a c t e r , a p a l e , f i n e - g r a i n e d f a c i e s , l o c a l i z e d p r i n c i p a l l y a l o n g t h e m a r g i n s , h a v i n g been i n t r u d e d by a d a r k e r grey but e q u a l l y f i n e -g r a i n e d h y p e r s t h e n e - a n d e s i t e . A c o m p l i c a t e d p a t t e r n of r a d i a t i n g j o i n t columns o u t l i n e s the margins o f the dome ( F i g . 4b). An a n d e s i t i c f l o w mantles the p l u g , andmmight r e p r e s e n t e x t r u s i o n of l a v a from the summit o f an endogenous dome as d e s c r i b e d by Rittmann (1962). The uppermost f l o w - u n i t of the a n d e s i t e d i p s 25° southwest and t e r m i n a t e s 50 m below the summit. The absence of g l a c i a l e r r a t i c s and s t r i a t i o n s on i t s exposed s u r f a c e s i n d i c a t e s t h a t the l a v a s t o o d above the upper l i m i t o f the W i s c o n s i n i c e - s h e e t (Mathews, 1948). The a n d e s i t e s of the second s t a g e are p o r p h y r i t i c w i t h p h e n o c r y s t s o f p l a g i o c l a s e (^n5o-35^' o r t n o P y r o x e n e ^ E n79_72^ a n < ^ minor i r o n - t i t a n i u m o x i d e s e n c l o s e d i n a f i n e - g r a i n e d m a t r i x of p l a g i o c l a s e , o r t h o p y r o x e n e , t i t a n i f e r o u s m a g n e t i t e , r a r e a u g i t e and p a l e brown i n t e r s t i t i a l g l a s s . Two types of p l a g i o c l a s e p h e n o c r y s t s are cha 20 Figure 5. The geology of The Black Tusk. THE GEOLOGY kilometers THE BLACK TUSK LEGEND i -z LU o LU CC Q Z < LU z LU o o r-(/} LU -J CL ZD o LU o < r— LU CC O o C/> if) ^ < c-QBs item Talus G lac ia l moraine T e r r a c e gravels Plug Dome, Summit hypersthene -andesite West Bluff hypersthene- andesi te Moraine hypersthene - andesites Hornblende - dacite, minor andesite P la ty - andes i te , minor ash - f low tuff P L U G DOME S T A G E Quartz diorite W^///////// Cheakamus Format ion: greywacke, conglomerate , argillite Greenstone, agglomerate , l imestone A N C E S T R A L MOUNTAIN S T A G E S Y M B O L S # 1.2 K - A r date in millions of years s - " G e o l o g i c a l contact (approximate , defined WVAVWW Fault (assumed) G e o l o g y by N. G r e e n (1974 - 75) m o d i f i e d after Mathews (1958). 22 a r e c h a r a c t e r i s t i c : s e i v e - t e x t r u e d , weakly zoned c r y s t a l s , and o s c i l l a t o r y - z o n e d t o homogeneous uncorroded c r y s t a l s . P a l e green a u g i t e o c c u r s as minor m i c r o p h e n o c r y s t s . P a r t i a l l y r e s o r b e d amphibole and o r t h o p y r o x e n e x e n o c r y s t s a r e l o c a l l y p r e s e n t , as a r e s m a l l c l o t s o f p l a g i o c l a s e , pyroxene and m a g n e t i t e . The p l a g i o c l a s e o f the groundmass i s d i s t i n c t i v e , c o n s i s t i n g o f Bb.elebbji'ekle/'ywas^^ tailed";iriandlgiass.;7CO.reaG &bj)iis±milar to r a p i d l y quenched c r y s t a l s d e s c r i b e d by Bryan (1972) i n o c e a n i c t h o l e i i t e s . The C i n d e r Cone A p a i r o f i n t e r s e c t i n g p y r o c l a s t i c cones, known l o c a l l y as The C i n d e r Cone, o c c u r a t the head of D e s o l a t i o n V a l l e y , immediately n o r t h o f Helm G l a c i e r ( F i g . 2 and 4d). The horse-shoe shaped e a s t e r n cone i s younger because i t s f l a n k s p a r t l y f i l l the c r a t e r o f the o l d e r s t r u c t u r e . The i n t e r n a l form of the cones i s exposed i n m e l t - w a t e r channels c u t i n t o t h e i r s o u t h e r n and e a s t e r n m a r g i n s . The b a s a l u n i t exposed w i t h i n t h e s e channels c o n s i s t s o f h i g h l y -i n d u r a t e d , t h i n l y bedded p a l a g o n i t i c ash and l a p i l l i ; l o c a l a n g u l a r u n c o n f o r m i t i e s and s m a l l - s c a l e c r o s s - b e d d i n g a r e c o n s p i c u o u s . O c c a s i o n a l l a y e r s o f e q u i d i m e n s i o n a l , f r a c t u r e - b o u n d b l o c k s and " c a u l i f l o w e r " bombs are i n t e r b e d d e d w i t h the f i n e - g r a i n e d t u f f s . M i n o r beddi n g sags o b s e r v e d beneath some b l o c k s i n d i c a t e the p l a s t i c , c o h e s i v e n a t u r e o f the t e p h r a . A two- t o t h r e e - m e t e r - t h i c k bed of g r e e n i s h - g r e y , equant l a p i l l i r e s t s immediately upon the m a f i c t u f f u n i t . Upward w i t h i n the sequence, the massive l a p i l l i t u f f s a r e succeeded by a l t e r n a t i n g beds o f ash and l a p i l l i . I n d i v i d u a l l a p i l l i u n i t s a r e T A B L E I . M O D A L A N A L Y S E S O F T H E H E L M C R E E K L A V A S a m p l e Number 453 455 404 411 412 451 P h e n o c r y s t A s s e m b l a g e P l a g i o c l a s e 18.9 18.2 5.2 6.2 0.8 2.9 01i v i ne 8.8 9-4 3-3 5-9 - 2.8 3.8 Aug i t e 0.5 0.06 9.6 2.4 7-4 5-9 Amph i b o l e - - - 0.6 1.9 -Q u a r t z - - X — Groundmass P l a g i o c l a s e 30.4 31 -3 33-7 5-5 9-2 60.5 Aug i t e 19-6 21 .6 3.0 6.3 0.4 12.3 G l a s s " 21.8 19-0 45-1 73.1 77.6 14.6 T o t a l Groundmass 71.8 71 - 9 81.8 85-4 87-2 87.4 N o t e s : Modal a n a l y s e s b a s e d on c o u n t i n g o f 1000 p o i n t s ; s t a n d a r d d e v i a t i o n i s a p p r o x i m a t e l y 5 p e r c e n t . X r e p r e s e n t s x e n o c r y s t i c m i n e r a l . * U n d i f f e r e n t i a t e d g r o u n d m a s s c o n s i s t i n g p r e d o m i n a n t l y o f g l a s s w i t h m i n o r m a g n e t i t e , a u g i t e and p l a g i o c l a s e . 24 t h i c k e r than 5 cm; ash l a y e r s a r e u s u a l l y l e s s than 2 cm t h i c k . An e j e c t a b l a n k e t c o n s i s t i n g o f fragments o f s c o r i a c e o u s l a v a , and r a r e almond-, pendant- and r i b b o n - s h a p e d bombs, mantles the f l a n k s o f the younger a g g l u t i n a t e b r e c c i a cone and p a r t s o f the o l d e r s t r u c t u r e . These f e a t u r e s a r e i n t e r p r e t e d t o r e p r e s e n t a t r a n s i t i o n between S u r t s e y a n and S t r o m b o l i a n types o f a c t i v i t y (Green, 1975). At l e a s t two l a v a flows e r u p t e d from The C i n d e r Cone. The o l d e s t l a v a , termed the D e s o l a t i o n V a l l e y f l o w , i s o n l y exposed a t the f o o t o f D e s o l a t i o n V a l l e y ( F i g . 2 ) , and as t e p h r a i n the o l d e r t u f f r i n g . The b a s a l t i c - a n d e s i t e i s c h a r a c t e r i z e d by e u h e d r a l to s u b h e d r a l o l i v i n e ( F o Q -,.-.), d i o p s i d i c a u g i t e , h y p e r s t h e n e , p l a g i o c l a s e ( A n , c „ c ) o l — / U D_> — J J and chromium-rich t i t a n o m a g n e t i t e c o n t a i n e d w i t h i n a b l a c k v i t r o p h y r i c groundmass ( F i g . 6 a ) . A b a s a l t i c l a v a , termed the Helm Creek f l o w , b u r i e s the b a s a l t i c - a n d e s i t e n o r t h o f The C i n d e r Cone, and i s a l s o exposed as f l o w and p y r o c l a s t i c m a t e r i a l w i t h i n t h e S t r o m b o l i a n c i n d e r cone ( F i g . 2 ) . The samples o f the Helm Creek f l o w examined are p o r p h y r i t i c v e s i c u l a r l a v a s which may be d i v i d e d i n t o two groups on the b a s i s o f the n a t u r e and abundance o f p h e n o c r y s t s p r e s e n t ( T a b l e I ) . Type I b a s a l t (453,455) i s d i s t i n g u i s h e d by p l a g i o c l a s e (An^Q_^^) and o l i v i n e (FOg^_g 2) p h e n o c r y s t s r a n g i n g from 1 t o 3 mm i n l e n g t h , and i s r e s t r i c t e d to a s m a l l segment o f the f l o w near the n o r t h e r n end o f Helm Lake ( F i g 2 ) . Type I I b a s a l t (404, 411, 412, 451) makes up the remainder o f the f l o w , and i s c h a r a c t e r i z e d by the absence o f l a r g e p l a g i o c l a s e p h e n o c r y s t s , and by the p r e s e n c e o f s e c t o r - z o n e d c l i n o p y r o x e n e and r e s o r b e d amphibole p h e n o c r y s t s ( F i g . 6c and 6d). At p r e s e n t , i t i s i m p o s s i b l e to determine i f the h e t e r o g e n e i t y o f the f l o w r e p r e s e n t s two d i s t i n c t i v e p u l s e s o f magma. 25 F i e l d e v i d e n c e , however, was not found which c o u l d s u b s t a n t i a t e t h i s p o s s i b i l i t y . Mount P r i c e Mount P r i c e i s a s m a l l composite v o l c a n o s i t u a t e d on the s outhwestern s h o r e o f G a r i b a l d i Lake. The n e a r l y s y m m e t r i c a l cone, h e a v i l y v e g e t a t e d on i t s lower s l o p e s , r i s e s to an a l t i t u d e of 2050 m. The r o c k s o f themmountain, shown i n F i g . 7, a r e d i v i s i b l e i n t o f i v e i n f o r m a l u n i t s on the b a s i s o f l i t h o l o g y and apparent time r e l a t i o n s h i p s : T a b l e Bay, Mount P r i c e , summit e r u p t i o n , P r i c e Bay, and C l i n k e r Peak u n i t s . T a b l e Bay U n i t T a b l e Bay v o l c a n i s m commenced w i t h i n an e a s t - f a c i n g c i r q u e -l i k e d e p r e s s i o n c a r v e d i n t o the C l o u d b u r s t q u a r t z d i o r i t e s o f C l i n k e r Ridge, E a r l y e r u p t i o n s c o n s t r u c t e d a s m a l l cone o f i n t e r b e d d e d columnar h o r n b l e n d e - d a c i t e f l o w s and a s s o c i a t e d t u f f b r e c c i a s on the e a s t e r n edge o f the b a s i n ( F i g . 7 ) ; the l a v a s o f t h i s " o l d e r cone" are i n g r a d a t i o n a l c o n t a c t w i t h g l a c i a l outwash. H i g h l y i n d u r a t e d sediment, c o m p r i s i n g 3 mm t o 12 cm a a r i g u l a r t o rounded c l a s t s of t o n a l i t e , g r e e n s t o n e and r e d - o x i d i z e d v e s i c u l a r l a v a s e t i n a b r o wnish-grey c l a y m a t r i x , i s i n t e r c a l a t e d w i t h a n d o o v e r l a i n by c r u d e l y j o i n t e d p a l a g o n i t i c l i t h i c t u f f s . A whole r o c k K/Ar date of 1.2 m.y. B.P. determined f o r an o v e r l y i n g T T a b l e Bay d a c i t e s u g g e s t s t h a t the sediment may be r e l a t e d to e i t h e r a m i d d l e P l e i s t o c e n e i c e - s h e e t , o r a l o c a l mountain g l a c i e r . ( The T a b l e Bay l a v a s a r e u s u a l l y p o r p h y r i t i c w i t h p h e n o c r y s t s o f 26 F i g u r e 6. Photomicrographs o f G a r i b a l d i Lake l a v a s . (a) S k e l e t a l o l i v i n e (OL) p h e n o c r y s t s and m i c r o p h e n o c r y s t s of p l a g i o c l a s e i n v i t r o p h y r i c groundmass of the D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e . Approximate f i e l d o f view i s 2 mm by 1 mm. (b) Hollow, s e c t o r - z o n e d p l a g i o c l a s e (PL) m i c r o l i t e i n groundmass of the West B l u f f a n d e s i t e , The B l a c k Tusk. Approximate f i e l d o f view i s 0.4 mm by 0.4 mm. (c) S e c t o r - z o n e d c l i n o p y r o x e n e (CPX) p h e n o c r y s t w i t h s u b h e d r a l i n c l u s i o n s o f m a g n e t i t e (MT) i n upper p a r t o f the Helm Creek l a v a , The C i n d e r Cone. Approximate f i e l d of view i s 2 mm by 1 mm. (d) P a r t l y r e s o r b e d p h e n o c r y s t o f T i - p o o r k a e r s u t i t e (HB) i n the upper p a r t o f the Helm Creek Lava, The C i n d e r Cone. Approximate f i e l d o f view i s 1 mm by 1 mm. (e) P a r t l y r e s o r b e d megacryst of p a r g a s i t i c amphibole (HB) c o n t a i n i n g i n c l u s i o n s o f f o r s t e r i t i c o l i v i n e . Sphinx Moraine b a s a l t i c - a n d e s i t e (598). Approximate f i e l d o f view i s 2 mm by 3 mm. ( f ) Corroded p h e n o c r y s t o f b i o t i t e i n t e r g r o w n .with p l a g i o c l a s e and rimmed by m a g n e t i t e , p l a g i o c l a s e , o r t h o p y r o x e n e and amphibole. Approximate f i e l d o f view i s 1 mm by 1 mm. 27 28 p l a g i o c l a s e , amphibole, o r t h o p y r o x e n e (En^^_g^) and minor i r o n -t i t a n i u m o x i d e . P l a g i o c l a s e p h e n o c r y s t s are weakly zoned - normal z o n i n g over the range An,. . c - and show a s e i v e - t e x t u r e d zone near oil—4O t h e i r r i m s . A c i c u l a r b l a d e s of amphibole, p a r t l y pseudomorphed by opaques and pyroxene, e x h i b i t s t r o n g p l e o c h r o i s m from pale-brown to r e d - or green brown. The groundmass i s t y p i c a l l y grey d e v i t r i f i e d g l a s s i n c l u d i n g m i c r o l i t e s o f f e l d s p a r , o r t h o p y r o x e n e , t i t a n o m a g n e t i t e and minor a u g i t e . Mount P r i c e U n i t A second e r u p t i v e phase o c c u r r e d w i t h i n the c i r q u e - l i k e b a s i n subsequent t o a q u i e s c e n t p e r i o d o f unknown d u r a t i o n . The f o c u s o f v o l c a n i c a c t i v i t y then seems to have s h i f t e d westward, where e r u p t i o n o f a n d e s i t i c l a v a s and p y r o c l a s t i c m a t e r i a l formed the n e a r l y - s y m m e t r i c a l c e n t r a l mass o f Mount P r i c e , and p a r t l y b u r i e d the " o l d e r cone". Fragmental u n i t s a r e m o s t l y P e l e a n - t y p e p y r o c l a s t f l o w s . These d e p o s i t s , 2 t o 15 m t h i c k , a r e composed o f an a s h - r i c h m a t r i x c o n t a i n i n g many j o i n t e d b l o c k s , commonly 7 cm to 1.5 m i n s i z e , o f l i g h t grey p l a t y a n d e s i t e ; many b l o c k s have r a d i a l c o n t r a c t i o n c r a c k s ( F i g . 4 f ) . Weakly banded welded-ash o u t c r o p s n e a r the n o r t h e r n base o f the cone ( F i g . 7 ) , but i n g e n e r a l , ash d e p o s i t s a r e r a r e . A s s o c i a t e d l a v a f l o w s , predominant o n l y i n the upper p a r t s of the mountain ( F i g . 7 ) , i n c l u d e h y p e r s t h e n e - h o r n b l e n d e andesitesaaridnminor h o r n b l e n d e - b i o t i t e a n d e s i t e s , and v a r y i n t h i c k n e s s from 3 t o 30 m. The a n d e s i t e s o f Mount P r i c e c o n t a i n p h e n o c r y s t s o f o r t h o p y r o x e n e , r e d brown amphibole, p a l e green a u g i t e , p l a g i o c l a s e and r a r e b i o t i t e e n c l o s e d i n a p i l o t a x i t i c groundmass. S m a l l c l o t s o f 29 a u g i t e , o r t h o p y r o x e n e , p l a g i o c l a s e and t i t a n o m a g n e t i t e a r e c o n s p i c u o u s . The p l a g i o c l a s e p h e n o c r y s t s have homogeneous c o r e s and zoned rims w i t h c o r r o d e d m argins; o n l y p l a g i o c l a s e m egacrysts d i s p l a y s e i v e - t e x t u r e d c o r e s . In c o n t r a s t , the welded ash c o n t a i n s p l a g i o c l a s e , amphibole, h y p e r s t h e n e , t i t a n o m a g n e t i t e and i l m e n i t e p h e n o c r y s t s . The zoned p l a g i o c l a s e p h e n o c r y s t s a r e h i g h l y fragmented and rounded c r y s t a l s w i t h "m6'th=eaten"ocores .PPf i'smati'cbb0ia.deso6f^^ttwinnedhhorhbl'endeaar-es s t r o n g l y p l e o c h r o i c from yellow-brown t o r e d brown. The groundmass o f the ash e x h i b i t s a weak l a y e r i n g c o n s i s t i n g o f dark brown g l a s s and g r a n u l a t e d fragments o f i n c l u d e d c r y s t a l s . Summit E r u p t i o n U n i t The l a s t e r u p t i o n from Mount P r i c e produced two a n d e s i t e f lows which o r i g i n a t e d from a c r a t e r a t t h e summit ( F i g . 7 ) . The l a r g e r o f these l a v a s f l o w e d down the n o r t h e r n f l a n k o f the mountain. Subsequent d e g r a d a t i o n l e f t a 30 m c l i f f o f r u s t y brown columnar a n d e s i t e on the shor e o f G a r i b a l d i Lake. The s m a l l e r l a v a f l o w e d down the southwestern s l o p e and was l a t e r b u r i e d , except n e a r the v e n t , by the younger C u l l i t o n Creek f l o w . A s s o c i a t e d a g g l u t i n a t e b r e c c i a s c o n s i s t i n g o f b l o c k s up t o 3 m i n s i z e f i l l e d the summit c r a t e r . P o s t - e r u p t i v e encroachment o f a c o n t i n e n t a l i c e - s h e e t i s e v i d e n t by the numerous g l a c i a l e r r a t i c s found on the d e p o s i t s . The summit e r u p t i o n a n d e s i t e s a r e p e t r o g r a p h i c a l l y s i m i l a r to the e a r l i e r l a v a s o f Mount P r i c e except t h a t b i o t i t e i s more abundant as a p h e n o c r y s t phase. I n d i v i d u a l b i o t i t e g r a i n s may be h i g h l y r e s o r b e d , and e i t h e r p a r t l y pseudomorphed o r rimmed by o r t h o -pyroxene, p l a g i o c l a s e arid m a g n e t i t e ( F i g . 6 f ) . By c o n t r a s t , p r i s m a t i c , opaque-rimmed r e d brown h o r n b l e n d e c r y s t a l s surrounded by a p a l e - y e l l o w n u r l a s s cn~of?~-3f f i b e r o u s h y p e r s • 21'-"T<-- " 30 t o c o l o u r l e s s corona o f f i b r o u s - h y p e r s t h e n e . G l o m e r o p o r p h y r i t i c c l o t s of amphibole, o r t h o p y r o x e n e , p l a g i o c l a s e , m a g n e t i t e and b i o t i t e a r e common. P r i c e Bay U n i t L a t e s a t e l l i t e e r u p t i o n s formed a broad, n e a r l y - s y m m e t r i c a l advent cone o f a g g l u t i n a t e b r e c c i a and agglomerate on the n o r t h e r n f l a n k o f Mount P r i c e . The p y r o c l a s t s c o n s i s t o f 10 cm to 3 m fragments o f r e d - o x i d i z e d a n d e s i t e ; many o f the s c o r i a c e o u s bombs a r e s p i n o s e i n shape w i t h sharp r e - e n t r a n t a n g l e s i n d i c a t i n g e j e c t i o n w h i l e d u c t i l e and c o o l i n g b e f o r e d e p o s i t i o n . No l a v a f l o w s a r e a s s o c i a t e d w i t h the cone, a l t h o u g h a 10 m dyke i s exposed i n a g u l l y a l o n g i t s n o r t h e r n s l o p e . P o s t - g l a c i a l o r l a t e g l a c i a l e x t r u s i o n i s i n d i c a t e d by the absence o f e r r a t i c s on the s l o p e s o f the cone. The f r a g m e n t a l r o c k s o f the P r i c e Bay cone a r e i d e n t i c a l t o ' the h o r n b l e n d e - b i o t i t e a n d e s i t e s o f the antece d a n t v o l c a n o . Some m i c r o p h e n o c r y s t s o f p l a g i o c l a s e , however, p o s s e s s a "beltT-buckle" form i n d i c a t i n g r a p i d quenching. S m a l l x e n o c r y s t s o f q u a r t z surrounded by r e a c t i o n rims o f a u g i t e a r e a l s o p r e s e n t . C l i n k e r Peak U n i t The l a t e s t a c t i v i t y w i t h i n the v o l c a n i c f i e l d o c c u r r e d a t C l i n k e r Peak, a bre a c h e d l a v a r i n g on the western s h o u l d e r o f Mount P r i c e . Two l a v a s , the B a r r i e r and C u l l i t o n Creek f l o w s , emanated from the C l i n k e r Peak summit. The Bar r i e r ( a l s o C l i n k e r Mountain or Rubble Creek) f l o w swept down the n o r t h w e s t e r n s l o p e o f Mount P r i c e , s p r e a d out a t i t s f o o t and f i l l e d the v a l l e y o f Rubble Creek t o a depth o f 250 m. 31 The C u l l l t o n Creek l a v a flowed down the s o u t h e r n s l o p e of the cone i n t o the C u l l i t o n Creek v a l l e y and s p r e a d westward f o r 1.5 km. The u n u s u a l morphology o f these flows i s d i s c u s s e d by Mathews (1952). The two l a v a f l o w s may be contemporaneous, but each l e v e e of the B a r r i e r l a v a has one to t h r e e c r e s t s ( F i g . 7) s u g g e s t i n g up t o t h r e e d i s t i n c t e r u p t i v e s u r g e s . I t i s p o s s i b l e t h a t the C u l l i t o n Creek f l o w breached the r i n g p r i o r t o the f i n a l p u l s e o f the B a r r i e r l a v a , as a l a v a dam on the s o u t h e r n r i m o f the cone s t a n d s h i g h e r than the l a v a g u t t e r o f the l a t t e r f l o w . The approximate age of the C l i n k e r Peak l a v a s can n o t be l e s s than s e v e r a l thousand y e a r s ; i t p o s t d a t e s the d i s a p p e a r a n c e of the C o r d i l l e r a n i c e - s h e e t from h i g h e r a l t i t u d e s i n l a t e W i s c o n s i n t i m e s , but p r e d a t e s the d i s a p p e a r a n c e of the i c e - s h e e t i n the Cheakamus R i v e r V a l l e y (Mathews, 1952a). Both C l i n k e r Peak a n d e s i t e s c o n t a i n s u b h e d r a l to a n h e d r a l p l a t e s of b i o t i t e , a e i c u l a r b l a d e s o f r e s o r b e d amphibole, s e i v e -t e x t u r e d p 3 i a g i o c l a s e and rounded i r o n - t i t a n i u m o x i d e s as p h e n o c r y s t phases; i l m e n i t e o c c u r s o n l y i n the B a r r i e r f l o w . Orthopyroxene (En^^_^^), a p a t i t e and r a r e a u g i t e a r e p r e s e n t as m i c r o p h e n o c r y s t s . Two d i s t i n c t t ypes of q u a r t z a r e found i n the l a v a s : h i g h l y r e s o r b e d p h e n o c r y s t s and x e n o c r y s t s rimmed by a u g i t e . The groundmass o f the l a v a s c o n s i s t s of b l a c k t o grey g l a s s , p l a g i o c l a s e m i c r o l i t e s , equant a u g i t e and h y p e r s t h e n e g r a i n s , and abundant t i t a n i f e r o u s m a g n e t i t e anhedra. The T a b l e The T a b l e i s a f l a t - t o p p e d v o l c a n o s i t u a t e d a p p r o x i m a t e l y 1 km s o u t h o f Mount P r i c e ( F i g . 4 e ) . Mathews (1951) p o s t u l a t e d t h a t the 32 mountain d e v e l o p e d by " r e p e a t e d f l o o d i n g o f l a v a i n t o a more or l e s s c y l i n d r i c a l v e n t thawed through the ( W i s c o n s i n ) i c e - s h e e t " . A s m a l l , g l a c i a l l y - s c o u r e d p l a t e a u forms the basement t o the mountain. T h i s p l a t e a u , which s e p a r a t e s the s u b - a l p i n e T a b l e Meadows from T a b l e Bay on G a r i b a l d i Lake ( F i g . 7 ) , c o n s i s t s of o l d e r h o r n b l e n d e -a n d e s i t e f l o w s , termed the T a b l e Meadows A n d e s i t e s . These a n d e s i t e s , cut by dykes of s i m i l a r a s p e c t n e a r the base of The T a b l e , a r e c h a r a c t e r i z e d by p h e n o c r y s t s of s e i v e - t e x t u r e d , o s c i l l a t o r y - z o n e d p l a g i o c l a s e , opaque-rimmed green amphibole, o r t h o p y r o x e n e ( E n _ c £ c ) , / J - D J r a r e a u g i t e and minor t i t a n o m a g n e t i t e s e t i n a p i l o t a x i t i c m a t r i x of p l a g i o c l a s e , a u g i t e , h y p e r s t h e n e , m a g n e t i t e and p a l e brown g l a s s . Both the l a v a s and a s s o c i a t e d dykes c o n t a i n 4 t o 15 mm cognate i n c l u s i o n s o f p l a g i o c l a s e , h o r n b l e n d e , o r t h o p y r o x e n e and m a g n e t i t e ; l o c a l l y , the i n c l u s i o n s c o n t i a n p h l o g o p i t e , which a l s o o c c u r s i n t h e a n d e s i t e as a x e n o c r y s t i c m i n e r a l rimmed by o r t h o p y r o x e n e and m a g n e t i t e . The i n t e r n a l and e x t e r n a l form o f The T a b l e i s d e s c r i b e d by Mathews (1951) as: "a p i l l a r o f b l a c k d a c i t e ( a n d e s i t e ) r i s i n g i n s h e e r c l i f f s , s e v e r a l hundred t o almost 1000 f e e t (305 m) h i g h t o a n e a r l y h o r i z o n t a l summit s u r f a c e 1000 f e e t l o n g and about 750 f e e t (230 m) wide....The c e n t r a l p a r t of t h e . . . . p i l l a r i s composed o f a s u c c e s s i o n o f f l a t - l y i n g l a y e r s a p p r o x i m a t e l y 10 f e e t (3iiim) t h i c k , c h a r a c t e r i z e d by s t o u t v e r t i c a l j o i n t columns ( c o l o n a d e ) , a l t e r n a t i n g w i t h l a y e r s 100 t o 200 f e e t (30 t o 75 m) t h i c k c h a r a c t e r i z e d by s l e n d e r j o i n t columns ( e n t a b l a t u r e ) . . . . A t s e v e r a l p l a c e s around the p e r i p h e r y of the mass, however, the l a y e r s a r e bowed s h a r p l y downward and i n d i v i d u a l f l o w - u n i t s assume a v e r t i c a l a t t i t u d e . . . . A t the s o u t h w e s t e r n end of the mountain a s t e e p f l o w d e r i v e d d i r e c t l y from the upper p a r t of The T a b l e i s s s e p a r a t e d from the u n d e r l y i n g q u a r t z d i o r i t e s by an u n d i s t u r b e d mantle o f g l a c i a l t i l l " . 33 The a n d e s i t e s o f The T a b l e a r e r e a d i l y grouped, w i t h i n c r e a s i n g fc. L i . ' s t r a t i g r a p h i c h e i g h t , a c c o r d i n g t o t h e i r f e r r o m a g n e s i a n p h e n o c r y s t assemblages, as f o l l o w s : (1) amphibole + a u g i t e ± o l i v i n e ± m a g n e t i t e ; (2) amphibole + o r t h o p y r o x e n e ± o l i v i n e ± m a g n e t i t e ; and (3) amphibole + m a g n e t i t e . The groundmass o f the a n d e s i t e s i s composed of d i s o r i e n t e d p l a g i o c l a s e m i c r o l i t e s , equant a u g i t e and h y p e r s t h e n e g r a i n s , m a g n e t i t e anhedra and v a r i a b l e amounts of p a l e - g r e y g l a s s . A c c e s s o r y m i n e r a l s i n c l u d e c r i s t o b a l i t e which o c c u r s b o t h i n c a v i t i e s and i n t e r s t i t i a l l y i n the groundmass, and minute n e e d l e s o f a p a t i t e ; f i b r o u s amphibole i s p r e s e n t i n t h e groundmass of the m i d d l e f l o w . Enostuck Meadows The Enostuck ( o r Eenostuck) Meadows b a s a l t i c - a n d e s i t e i s s i t u a t e d on the upper Skookum Creek, 1 km s o u t h of Mamquam Lake ( F i g . 2 ) . The l a v a forms a " d r u m l i n o i d " r i d g e almost 180 m wide and 90 m h i g h f o r the g r e a t e r p a r t o f i t s 800 m l e n g t h . J o i n t columns, w e l l d e v e l o p e d throughout the f l o w , a r e g e n e r a l l y p e r p e n d i c u l a r to the p r e s e n t s u r f a c e except n e a r - t h e summit where v e r t i c a l columns predominate (Mathews, I;-' 1958b). The a n d e s i t e c o n t a i n s numerous p h e n o c r y s t s o f o l i v i n e ( t o FOg^) and p l a g i o c l a s e ( A n ^ ^ _ ^ ^ ) , w i t h s m a l l e r amounts o f a u g i t e , up to 1 mm i n l e n g t h , and r a r e o r t h o p y r o x e n e (En^^_^^) m i c r o p h e n o c r y s t s . The f e l d s p a r p h e n o c r y s t s a r e s t r o n g l y zoned - normal, r e v e r s e and o s c i l l a t o r y - and commonly show an i n c l u s i o n - r i c h , rounded c o r e . Amphibole o c c u r s as x e n o c r y s t i c g r a i n s o r i n 0.2 to 0.4 mm c l o t s w i t h a u g i t e , p l a g i o c l a s e , and minor o r t h o p y r o x e n e . The groundmass of the l rava i s h y p o c r y s t a l l i n e w i t h p l a g i o c l a s e , a u g i t e , t i t a n i f e r o u s m a g n e t i t e and minor h y p e r s t h e n e e n c l o s e d i n a p a l e brown m e s o s t a s i s 34 F i g u r e 7. The geology o f Mount P r i c e and The T a b l e . THE GEOLOGY OF MOUNT PRICE AND THE TABLE 123°08 ' 49°57 ' 123°05' 123°00' 4 9 ° 5 7 ' -i 4 9 ° 5 5 ' 49°53' 123°05 ' 123°00 ' 2 3 kilometers O UJ CC Q Z < ui z ui O o r~ w o LU o < UJ cc o LEGEND < o u. cc -z-=-=Qs T a l u s - t ; slide m a t e r i a l - s M K a Q o j G lac ia l mora ine - m; o u l w a s h - o >1.2 MOUNT PRICE Qc CLINKER PEAK:Bar rier andesite - a ; C u l l i t o n Creek andesite - b I££nrn PRICE BAY UNIT: Agglutinate IlT^Jxtj breccia, minor hornblende-biotite andesite flows SUMMIT ERUPTION UNIT >:;,Q.Pvb>: Agglutinate brecc ia , tuff breccia QPs ^ Q . P w £ •V.QPb'V Hornblende-biotite andesite MOUNT PRICE UNIT Welded- ash Hornblende - biotite andesite I^^SSJ Andesitic tuff breccia; minor hornblende-biotite andesite flows TABLE BAY UNIT Hornblende-andesite flows and tuff breccia Basal agglomerate, baked glacial till, minor palagonitic tuff Quarlz diorite Helm Formation: slaty argilite, quartzite, minor conglomerate Cheakamus Formation: argilite, greywacke, conglomerate Cloudburst quartz diorites © L SYMBOLS K - A r date in millions of years Geological contact (approximate, defined) Levee Pressure ridge Vent Lake THE TABLE Table hornblende-andesites Agglutinate breccia -UIMIHIjhl Table Meadows trtrTl'I'iTri hornblende-andesite Geology by N. Green (1974 - 75) modif ied after Mathews (1958). 36 g l a s s . No d i r e c t e v i d e n c e f o r c o - e x i s t e n c e o f g l a c i a l - i c e and molten l a v a i s p r e s e n t , as g l a c i a l e r r a t i c s and s t r i a e a r e absent from the s u r f a c e s o f the fl o w . Mathews (1948)> however, p o s t u l a t e d t h a t the l a v a , l i k e The T a b l e , was e x t r u d e d beneath the C o r d i l l e r a n i c e -sheet from a ve n t l o c a t e d a t the n o r t h e a s t e r n end o f the r i d g e . Sphinx Moraine A g l a c i a t e d b l u f f o f o l i v i n e - b e a r i n g a n d e s i t e p r o t r u d e s 30 m above the moraine o f the Sphinx G l a c i e r , about 330 m i n l a n d from the e a s t e r n s h o r e o f G a r i b a l d i Lake ( F i g . 2).. S t o u t j o i n t columns bowed towards the s o u t h grade upward i n t o s l e n d e r i r r e g u l a r columns. The n o r t h e r n f a c e o f the l a v a e x h i b i t s t h r e e columnar arches which p o s s i b l y , formed when r a f t e d i c e was i n c l u d e d i n the advancing f l o w . The l a v a , termed the Sphinx Moraine ( o r F i r e E x t i n g u i s h e r ) b a s a l t i c -a n d e s i t e , i s composed o f c a l c i c p l a g i o c l a s e ( An ^  _,.,-) , a u g i t e , o r t h o p y r o x e n e ( E n ^ ^ _ ^ g ) , and magnesian o l i v i n e (Fogg_3o^ p h e n o c r y s t s s e t i n a weakly p i l o t a x i t i c m a t r i x of p l a g i o c l a s e m i c r o l i t e s , equant a u g i t e and or t h o p y r o x e n e g r a n u l e s , t i t a n o m a g n e t i t e anhedra, and g r e y -brown i n t e r s t i t i a l g l a s s . Some p l a g i o c l a s e m i c r o l i t e s show " b e l t -b u c k l e " and h o u r - g l a s s t e x t u r e s . Amphibole o c c u r s as megacrysts up to 1 cm i n l e n g t h ( F i g . 6 e ) , or i n s m a l l c l o t s w i t h a u g i t e , o l i v i n e , m a g n e t i t e and p l a g i o c l a s e ; l a r g e amphibole x e n o c r y s t s c o n t a i n i n c l u s i o n s of s u b h e d r a l o l i v i n e (FOg^_g^). The l a v a c o n t a i n s numerous c l o t s o f unfused g n e i s s i c q u a r t z d i o r i t e , and i s cut by a 10 cm a n d e s i t i c v e i n c a r r y i n g 40 p e r c e n t h i g h l y d i s i n t e g r a t e d g r a n i t i c m a t e r i a l . The p r e s e n c e o f g l a c i a l s t r i a e on the upper s u r f a c e o f the 37 ,14 f l o w s u g g e s t s t h a t e r u p t i o n p r e d a t e d the l a s t g l a c i a t i o n . A C J date of 5,690 y r B.P. determined on c a r b o n i z e d hemlock wood exposed w i t h i n the moraine (Lowdon and B l a k e , 1975) p r o v i d e s a minimum age f o r t h i s g l a c i a t i o n . No v e n t a r e a i s found w i t h i n the b l u f f , and i t i s p o s s i b l e t h a t the l a v a r e p r e s e n t s a s o u t h e r n e x t e n s i o n o f the b a s a l t i c - a n d e s i t e exposed on the w e s t e r n s l o p e s of G e n t i a n Peak ( F i g 2 ) . Age- o f V o l c a n i s m As p a r t of a program to determine the l a t e Q u a ternary c h r o n o l o g y of the G a r i b a l d i Group v o l c a n i c a c t i v i t y , seven samples from the G a r i b a l d i Lake a r e a have been dated by the p o t a s s i u m - a r g o n method. Samples i n c l u d e a b a s a l t from the Cheakamus V a l l e y ( A l p i n e Lodge p h a s e ) , t h r e e a n d e s i t e s . f r o m The B l a c k Tusk, an a n d e s i t i c l a v a from t h e T a b l e Bay cone o f Mount P r i c e , and the B a s a l t i c - a n d e s i t e and a l k a l i b a s a l t f l o w s from The C i n d e r Cone. L o c a t i o n o f the samples and t h e i r c a l c u l a t e d ages a r e g i v e n i n T a b l e I I . The r e l a t i v e s t r a t i g r a p h i c . p o s i t i o n o f the two a n d e s i t e s from the a n c e s t r a l mountain s t a g e o f The B l a c k Tusk i s not known, but the sample s i t e s a r e a t the n o r t h e r n (610) and s o u t h e r n (612) base of the West' B l u f f . The o t h e r sample (634) from The. B l a c k Tusk i s r e p r e s e n t a t i v e of the f e e d e r dyke t o the l a t e s t a g e p l u g dome. The c a l c u l a t e d K/Ar dates i n d i c a t e t h a t most o f the e r u p t i v e c e n t e r s i n the G a r i b a l d i Lake a r e a a r e v e r y young. Four of the l a v a s have c a l c u l a t e d ages l e s s than 0.1 m.y.; t h r e e l a v a s have ages r a n g i n g from 1.0 t o 1.3 m.y. A l t h o u g h t h e s e dates : W g g e l s t two d i s t i n c t e p i s o d e s of v o l c a n i s m a r e p r e s e n t , t h i s may n o t be the c a s e . Potassium-argon ages determined f o r l a v a s from Columnar TABLE I I . POTASSIUM-ARGON ANALYTICAL DATA FOR VOLCANIC ROCKS FROM THE GARIBALDI LAKE AREA Sample Location Unit Rock-type % K±Ra A r R a d b ( x l 0 " 1 2 m o l / g ) 40 f l 100 A rRad 4 0 A r H r T o t a l C a l c u l a t e d age (m.y.) 36-4 49°58.3'N 123°09'W Cheakamus V a l l e y b a s a l t 0. .40+0.002 0.038 1..1 0.05±0.05 610 49°58.4'N 123°02.9'W The Black Tusk (West B l u f f ) andesi te 1. .00±0.009 1 .902 32.3 1 .U0.06 612 49°57.9'N 123°02.7'W The Black Tusk (West B l u f f ) andesite 0. ,93±0.040 2.123 14.8 1.3+0.08 634 49°58.5'N 123°02.6'W The Black Tusk (Plug Dome) andesi te 1 , 03±0.007 0.157 1 .9 0.09±0.08 208 50°02.1'N 122°59.1'W The Cinder Cone (Desolation V a l l e y flow) andesite 0, ,89±0.018 0.169 3.6 0.11±0.03 458 49°58.9'N 123°01'W The Cinder Cone (Helm Creek flow) b a s a l t 0. .58±0.013 0.039 0.5 0.04±0.04 522 49°55'N 123°00.9'W Mount P r i c e (Table Bay cone) andesite 9, .93±0.008 1 .883 18.8 1 .1±0.08 a. P o t a s s i u m a n a l y s e s by. K. S c o t t u s i n g atomic a b s o r p t i o n t e c h n i q u e s ; R i s the range of t r i p l i c a t e a n a l y s e s . b. Argon a n a l y s e s by J . H a r a k a l and N. "Green u s i n g MS10 s p e c t r o m e t e r ; c o n s t a n t s used i n model age c a l c u l a t i o n s a r e : Ae = 0.585 x 1 0 ~ 1 0 / y e a r ; A3 = 4.72 x 1 0 ~ 1 0 ; 4 ° K / K = 1.19 x ~ 1 0 ~ 4 . Sample wei g h t s between 9 and 10 grams. 39 Peak (10 km south) and o f Mount C a y l e y (10 km n o r t h ) suggest t h a t v o l c a n i s m p e r s i s t e d i n the r e g i o n between 0.25 and 0.75 m.y. ( J , H a r a k a l , p e r s o n a l communication). Mathews (1958b) q u e s t i o n e d whether the G a r i b a l d i Group v o l c a n i s m ' c o u l d be g e n e t i c a l l y r e l a t e d to i s o s t a t i c u p l i f t d u r i n g the waning s t a g e s o f the P l e i s t o c e n e i c e - s h e e t . A l t h o u g h the younger dates agree r e a s o n a b l y w e l l w i t h the p e r i o d o f L a t e P l e i s t o c e n e g l a c i a t i o n , l i t t l e i s known about e a r l i e r g l a c i a t i o n s . 40 I I I . GEOCHEMISTRY OF GARIBALDI LAKE LAVAS G e n e r a l Statement The common a s s o c i a t i o n o f a n d e s i t e v o l c a n i s m w i t h zones o f l i t h o s p h e r e s u b d u c t i o n s u g g e s t s t h a t the o r i g i n o f the c a l c - a l k a l i n e v o l c a n i c s e r i e s i s connected w i t h p r o c e s s e s o c c u r r i n g i n o r above the B e n i o f f zone. The g e o c h e m i s t r y of such v o l c a n i c r o c k s p r o v i d e s i n f o r m a t i o n r e g a r d i n g t h e i r s o u r c e r o c k s , and the r o l e o f v a r i o u s p r o c e s s e s o f magma g e n s i s o r d i f f e r e n t i a t i o n . In t h i s c h a p t e r , major and t r a c e element d a t a f o r P l e i s t o c e n e - R e c e n t l a v a s from the G a r i b a l d i Lake a r e a a r e used to p l a c e c o n s t r a i n t s on the o r i g i n o f a n d e s i t e magmas ge n e r a t e d beneath southwestern B r i t i s h Columbia. Rock C l a s s i f i c a t i o n M a j or and t r a c e element a n a l y s e s o f e i g h t y - s e v e n Quaternary v o l c a n i c r o c k s from the G a r i b a l d i Lake a r e a a r e p r e s e n t e d i n Appendix I , and summarized d i a g r a m a t i c a l l y i n F i g s . 8 r t o 10. The nomenclature used to c l a s s i f y r o c k c o m p o s i t i o n s i s based on the system o f Wise (1969): 1. O l i v i n e - b a s a l t : Modal o l i v i n e , but absence o f Ca-poor pyroxene as p h e n o c r y s t , m i c r o p h e n o c r y s t , or r e a c t i o n phase ( p r e - e r u p t i v e ) on o l i v i n e ; c o l o u r i n d e x (normative o l + hy + d i + mt + i l + ap) > - 3 0 " " p e r c e n t ; - D i f f e r e n t i a t i o n Index (normative qz + ab + o r + ne) (Thornton and T u t t l e , 1960) < 40 p e r c e n t ; SiO^ < 54 p e r c e n t . 2. B a s a l t i c - a n d e s i t e : Modal o l i v i n e w i t h Ca-poor pyroxene as p h e n o c r y s t , m i c r o p h e n o c r y s t , or r e a c t i o n phase on o l i v i n e ; c o l o u r 41 i n d e x , 30 t o 22.5 p e r c e n t ; D i f f e r e n t i a t i o n Index, 40 to 53 p e r c e n t ; s i l i c a , 54 t o 58 p e r c e n t . 3. A n d e s i t e : P r e s e n c e of Ca-poor pyroxene w i t h o r w i t h o u t p h e n o c r y s t s of amphibole o r b i o t i t e ; c o l o u r i n d e x , 22.5 t o 15 p e r c e n t ; D i f f e r e n t i a t i o n Index, 53 to 63 p e r c e n t ; s i l i c a , 58 t o 62.5 p e r c e n t . 4. D a c i t e : P h e n o c r y s t s o f amphibole and/or b i o t i t e w i t h sub-o r d i n a t e Ca-poor pyroxene; c o l o u r i n d e x < 15 p e r c e n t ; D i f f e r e n t i a t i o n Index > 63 p e r c e n t ; s i l i c a > 62.5 p e r c e n t . T h i s c l a s s i f i c a t i o n scheme d i f f e r s from t h a t of Mathews (1957) i n t h a t the a n d e s i t e - d a c i t e boundary i s s e t a t 62.5 r a t h e r than 57 weight p e r c e n t SiO^. "The b o u n d a r i e s d e f i n e d in. t h i s manner c o r r e s p o n d a p p r o x i m a t e l y to i m p o r t a n t changes i n p h e n o c r y s t and groundmass m i n e r a l assemblages i n the G a r i b a l d i Lake s e r i e s , and make comparison e a s i e r w i t h o t h e r v o l c a n i c p r o v i n c e s . D i s t r i b u t i o n of l a v a types i n the G a r i b a l d i Lake a r e a based on volume e s t i m a t e s o f Mathews (1958.b) and the proposed c l a s s i f i c a t i o n scheme a r e : b a s a l t , 23 p e r c e n t ; b a s a l t i c - a n d e s i t e , 9 p e r c e n t ; a n d e s i t e , 67 p e r c e n t ; and d a c i t e , 1 p e r c e n t . In terms of s i l i c a c o n t e n t , l a v a s o f the G a r i b a l d i Lake a r e a f a l l i n t o two groups, one c o m p r i s i n g b a s i c r o c k s w i t h Si02 between 48.93 and 52.13 p e r c e n t , and the o t h e r c o m p r i s i n g l a v a s o f i n t e r m e d i a t e c o m p o s i t i o n w i t h SiO^ v a r y i n g from 54.37 to 65.26 p e r c e n t ( F i g s . 8-10). Rocks w i t h s i l i c a c o m p o s i t i o n s between these two groups are n o t a b l y a b s e n t ; a s i m i l a r gap i s r e c o g n i z e d w i t h i n the G a r i b a l d i Group l a v a s and p y r o c l a s t i c r o c k s o f P a u l Ridge, Mount Ca y l e y and Meager Mountain (Green and W a t t e r s , 1977). 42 F i g u r e 8. Major element SiO^ v a r i a t i o n diagrams f o r v o l c a n i c r o c k s from the G a r i b a l d i Lake a r e a . Symbols r e p r e s e n t : s o l i d c i r c l e s , Cheakamus V a l l e y b a s a l t s ; open s q u a r e s , Helm Creek l a v a from The C i n d e r Cone; c i r c l e d d o t s , D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e from The C i n d e r Cone; s o l i d s q u a r e s , Sphinx Moraine b a s a l t i c - a n d e s i t e ; h a l f - f i l l e d s q u a r e s , E n o s t u c k b a s a l t i c - a n d e s i t e ; open c i r c l e s , The T a b l e ; open t r i a n g l e s , Mount P r i c e ; s o l i d t r i a n g l e s , The B l a c k Tusk. C o m p o s i t i o n a l f i e l d s f o r Mount C a y l e y (dot-dashed l i n e ) , P a u l Ridge ( s o l i d l i n e ) , and Meager Mountain (dashed l i n e ) s u i t e s a r e from Green and Watters (1977). Oxides i n weight p e r c e n t . 44 F i g u r e 9. T r a c e element SiO^ v a r i a t i o n diagrams f o r v o l c a n i c r o c k s from the G a r i b a l d i Lake a r e a . Symbols as g i v e n i n F i g . 8. C o m p o s i t i o n a l f i e l d s f o r Mount C a y l e y (dot-dashed l i n e ) , P a u l Ridge ( s o l i d l i n e ) and Meager Mountain s u i t e s from Green and M a t t e r s (1977). T r a c e element c o n c e n t r a t i o n s i i n p p a r t s p p e r M m i M i o n . 2 0 0 1 0 0 • 1 0 0 L Cu 5 0 0 • 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0 2 5 1 5 5 T— i — i — i — i — i —I— i — i —I—I—I— i — i — i — i — r 4». • Cr • ® • ° Zn A • <s£> • Ni • • • 3' o; Sr ;47°\., Ba (.fir A . v I I I I I I 1 I I I 1 1 1 1 1 1 L 45 2 0 0 1 0 0 1 5 0 1 0 0 5 0 0 1 5 0 1 0 0 5 0 0 6 0 0 4 0 0 2 0 0 0 5 0 5 5 6 0 Si0 2 6 5 46 F i g u r e 10. Si0 2 v e r s u s Mg/(Mg + F e ) , Ca/Sr, K/Ba, K/Sr, K/Rb, and Rb/Sr r a t i o s f o r v o l c a n i c r o c k s from the G a r i b a l d i Lake a r e a . Symbols as g i v e n i n F i g . 8. !2 2 K T T o 22 5 2 co 1 1 1 1 1 1 1 1 1 1 1 1 1 1 o o Lu < _L ID • < O ® • • O CD \ J I I L 0,1* 0) v 9 ® H ® B • • 4: i i i i i a & CO \ CC <3 ® O lO CM o o CM O O O O O O CM 00 s|-48 F i g u r e 11. (Na^O + K^O) v e r s u s SiO^ diagram. G a r i b a l d i Lake a r e a . Symbols as g i v e n i n F i g . 8. Broken l i n e s i n d i c a t e Kuno's ( 1 9 6 8 ) . f i e l d b o u n d a r i e s between the t h o l e i i t i c , h i g h - a l u m i n a b a s a l t , and a l k a l i b a s a l t r o c k s e r i e s . 10 T— i—i—i—i—i—i—i—i—i—I—i—i—i—i i r A L K A L I B A S A L T 'iT ^ A A A T H O L E / ITE J I I I I I I i i i I I I I I I L 50 55 60 65 Si0 2 V O 50 B a s a l t i c Rocks O l i v i n e - b a s a l t s o n l y o c c u r as p l a t e a u - f o r m i n g l a v a f l o w s w i t h i n the Cheakamus R i v e r and C a l l a g h a n Creek v a l l e y s , and as a s i n g l e f l o w e r u p t e d from The C i n d e r Cone. These r o c k s a r e grouped as (1) the Cheakamus V a l l e y b a s a l t s , and (2) the Helm Creek f l o w , r e s p e c t i v e l y . Cheakamus V a l l e y B a s a l t s The m a f i c m i n e r a l o g y of the Cheakamus V a l l e y b a s a l t s , which c o n s i s t s o f a h i g h - c a l c i u m t i t a n i u m - r i c h pyroxene and o l i v i n e showing no r e a c t i o n r e l a t i o n s h i p w i t h the l i q u i d , s u g g e s t s c l o s e a f f i n i t i e s t o a l k a l i - o l i v i n e b a s a l t s ( W i l k i n s o n , 1967). The " m i l d l y a l k a l i n e " c h a r a c t e r o f the Cheakamus V a l l e y l a v a s i s a l s o i l l u s t r a t e d i n F i g . 11, where the b a s a l t i c c o m p o s i t i o n s l i e a s t r i d e Kuno's (1968) s u b - a l k a l i n e ( t h o l e i i t i c p l u s h i g h - a l u m i n a b a s a l t ) and a l k a l i b a s a l t f i e l d boundary. The Cheakamus V a l l e y b a s a l t s , however, a r e p r e d o m i n a n t l y h y p e r s t h e n e - n o r m a t i v e r o c k s c o n t a i n i n g v a r y i n g amounts o f n o r m a t i v e o l i v i n e o r q u a r t z . In the system 01-Cpx-Ne-Qz (Yoder and T i l l e y , 1962, p. 350), t h e s e a n a l y s e s s t r a d d l e the r e g i o n between the p l a n e of s i l i c a - s a t u r a t i o n (Cpx-Opx-Ab) and the c r i t i c a l p l a n e of s i l i c a -u n d e r s a t u r a t i o n (Cpx-01-Ab), l y i n g m a i n l y w i t h i n the f i e l d o f o l i v i n e t h o l e i i t e s . Many o f the l a v a s c o n t a i n l e s s than 10 p e r c e n t n o r m a t i v e h y p e r s t h e n e (hy) and p o s s e s s low hy/(hy+di) i n the norm (<; 0.45) so t h a t t h e i r a n a l y s e s p l o t c l o s e t o the i n t e r m e d i a t e p l a n e Cpx-01-Ab. Hypersthene-normative o l i v i n e - b a s a l t s w i t h a l k a l i b a s a l t 51 m i n e r a l o g y a r e w i d e l y r e c o g n i z e d and d e s c r i b e d as " t r a n s i t i o n a l b a s a l t s " (Hooper, 1974). The f r e q u e n c y w i t h which the name " t r a n s i t i o n a l b a s a l t " has been used, however, c o n f l i c t s w i t h the usage o f the name " o l i v i n e t h o l e i i t e " ; the l a t t e r term i s w e l l d e f i n e d as a b a s a l t c o n t a i n i n g n o r m a t i v e hy and o l (Yoder and T i l l e y , 1962). B e s t and B r i m h a l l (1974) sug g e s t e d t h a t (1) many " t r a n s i t i o n a l b a s a l t s " , c h a r a c t e r i z e d by h i g h e r SiO^ and a l k a l i e s but lower CaO t h a n t y p i c a l o l i v i n e t h o l e i i t e s (so t h a t n o r m a t i v e p l a g i o c l a s e i s a n d e s i n e ) , a r e i n r e a l i t y h a w a i i t e s as d e f i n e d by Macdonald (1960), > and (2) " t r a n s i t i o n a l b a s a l t s " p o s s e s s i n g a p p r o p r i a t e c o n t e n t s o f Si02 and Al^O^ combined w i t h low Na 20 and K^O (so t h a t n o r m a t i v e an > 50. p e r c e n t and hy > 0) s h o u l d be d e f i n e d as o l i v i n e t h o l e i i t e s . The Cheakamus V a l l e y b a s a l t s , when c l a s s i f i e d f o l l o w i n g t h e recomm-e n d a t i o n s o f Best and B r i m h a l l (1974), a r e p r e d o m i n a n t l y hy-n o r m a t i v e h a w a i i t e s . The c h e m i c a l v a r i a t i o n o f t h e Cheakamus V a l l e y b a s a l t s i s dominated by d e p l e t i o n i n MgO and enrichment i n Na^O, K^O, ¥^0^, and Ba as a f u n c t i o n o f i n c r e a s i n g s i l i c a c o n t e n t , accompanied by minor i n c r e a s e s i n Al^O^, S r , and Rb, and d e c r e a s e s i n CaO, TiO,,, t o t a l i r o n , Gr, V, Cu, Zn, and N i ( F i g . 8 and 9 ) . Compared w i t h i s l a n d - a r c t h o l e i i t e s (Jakes and White, 1972a) o r o c e a n i c t h o l e i i t e s ( E n g e l and o t h e r s , 1965), the Cheakamus V a l l e y b a s a l t s a r e h i g h e r i n Na^O, K 20, S r , Ba, and Rb, but lower i n alumina and c c a i c i u m . The major and t r a c e element c o m p o s i t i o n s b e a r an o v e r a l l c h e m i c a l resemblance t o some b a s a l t s from the Columbia P l a t e a u and the Snake R i v e r P l a i n , a l t h o u g h the l a t t e r l a v a s a r e c h a r a c t e r i z e d by s l i g h t l y 52 lower MgO, Na20, Al^O^, and h i g h e r T i C ^ , CaO, t o t a l i r o n , Rb, and Ba ( T a b l e I I I ) . When p l o t t e d on c o n v e n t i o n a l AMF ( F i g . 12) and MgO-(FeO-H^O^) ( F i g . 13) diagrams, the Cheakamus V a l l e y l a v a s d i s p l a y a lower degree o f i r o n - e n r i c h m e n t than the t h o l e i i t i c b a s a l t s o f T h i n g m u l i v o l c a n o , I c e l a n d ( C a r m i c h a e l , 1964). The younger b a s a l t s o f the Cheakamus V a l l e y sequence show p r o g r e s s i v e enrichments i n Na20, 1^0, A^O^, Si02, Rb, Ba, C r / N i , K/Sr, Rb/Sr, and Ba/Sr r e l a t i v e t o the o l d e r l a v a s . These e n r i c h m e n t s , and the accompanying d e p l e t i o n s i n CaO, MgO, t o t a l i r o n , T i 0 2 , N i , Cr, K/Rb, K/Ba, Ca/Sr, a r e i n d i c a t i v e o f a f r a c t i o n a t i o n p r o c e s s i n v o l v i n g removal o f o l i v i n e , c l i n o p y r o x e n e , and p l a g i o c l a s e ( P h i l p o t t s and S c h n e t z l e r , 1970; Leeman, 1976). These t h r e e phases a r e the major p h e n o c r y s t s i n a l l Cheakamus V a l l e y l a v a s , and the same m i n e r a l s r e p r e s e n t e d i n the CMAS t e t r a h e d r o n d e s i g n e d by O'Hara (1968; see a l s o Jamieson, 1970) to demonstrate p o s s i b l e p o l y b a r i c f r a c t i o n a t i o n t r e n d s i n magnesian b a s a l t i c systems. The v a r i a t i o n o f the Cheakamus V a l l e y b a s a l t s when p r o j e c t e d w i t h i n t h i s t e t r a h e d r o n ( F i g . 14) s u p p o r t s the s u g g e s t i o n the the l a v a s have undergone l i m i t e d low to moderate p r e s s u r e (1 atm to a p p r o x i m a t e l y 10 kb) f r a c t i o n a t i o n . Helm Creek Flow The Helm Creek o l i v i n e - b a s a l t i s a composite l a v a f l o w which c o n s i s t s o f an upper p a r t o f ne-normative m u g e a r i t e (MacDonald, 1968) u n d e r l a i n by a lower p a r t of weaky hy-normative a l k a l i b a s a l t . The p r e v a l e n c e o f t i t a n i u m - r i c h d i o p s i d i c a u g i t e as the s o l e p y r o x e n i c phase, the low hy/(hy+di) i n the norm (< 0.45), and the 53 TABLE H I . AVERAGES OF CHEAKAMUS VALLEY AND OTHER BASALT SUITES Sample No. 1 2 3 4 5 6 7 8 9 S i 0 2 49.61 49.65 50.19 51 .09 50.47 50.12 47.20 50.10 50.10 T i 0 2 1.53 1 .52 1 .48 1 .45 1.72 3.21 2.09 0.76 0.86 A 1 2 ° 3 15.73 15.66 15.74 16.31 15.00 13.04 15.80 18.90 19.80 F e 2 0 3 11 .79 11 .75 11.26 10.02 11 .75 14.44 11 .90 8.82 8.46 MgO 8.68 8.37 7.93 6.42 5.54 4.39 7.88 6.00 5.71 CaO 8.94 8.79 8.66 8.51 9.59 8.12 10.26 11 .43 11 .20 Na 2 0 3.37 3.02 3.54 4.14 3.03 2.70 2.81 2.96 3.25 K 2 0 0.48 0.49 0.53 0.60 0.76 1 .27 0.69 0.31 0.44 Ni 140 118 113 69 44 20 105 34 7 Cr 172 180 174 141 79 36 235 60 20 Rb 5.8 6 5.7 9 11 33 12 - 5 Sr 460 472 475 559 241 271 307 270 245 Ba 85 121 135 163 440 480 380 90 75 V 216 190 193 182 420 480 - 250 265 Cr/Ni 1 .23 1 .53 1 .54 2.04 1 .80 1 .80 2.24 1 .76 2.86 K/Rb 687 678 772 553 573 319 477 - 730 K/Ba 47 34 33 31 14 22 15 29 49 K/Sr 9 9 9 9 26 39 19 10 15 Rb/Sr 0.013 0.013 0.012 0.016 0.046 0.122 0.039 - 0.020 Ba/Sr 0.19 0.26 0.28 0.29 1 .83 1 .77 1 .24 1 .08 0.93 Ca/Sr 139 132 129 108 310 234 261 330 357 A. To ta l i r o n as F e , ^ . B. Key. Averages o f : 1. A l p i n e Lodge Phase b a s a l t s ; 2. Cheakamus Dam Phase b a s a l t s ; 3. Brandywine F a l l s Phase b a s a l t s ; 4. Ca l laghan b a s a l t ; 5. P i c t u r e Gorge b a s a l t , Columbia R i ve r P l a teau (McDougal l , 1976); 6. M idd le Yakima b a s a l t , Columbia R i ve r P la teau (McDougal l , 1976); 7. Western Snake R i ve r P l a i n o l i v i n e t h o l e i i t e (Leeman, 1974, Table 41 , A n a l y s i s SNS-1); 8. Mount M i se r y , S t . K i t t s (Baker, 1968); 9. Mont se r ra t b a s a l t (Rea, 1974, Table 5, A n a l y s i s 2 ) . 54 h i g h a l k a l i / s i l i c a r a t i o s ( F i g . 11) d i s t i n g u i s h the b a s a l t i c c o m p o s i t i o n s from u n d e r s a t u r a t e d t h o l e i i t e s (Coombs and W i l k i n s o n , 1969). From a l k a l i b a s a l t through to muge a r i t e c o m p o s i t i o n s i n the lava* there; i s . an i r r e g u l a r i n c r e a s e i n . the Na^O, K^O, ^ 2^5' Sr , Ba, and Rb abundances, but d e c r e a s e s i n CaO, MgO, t o t a l i r o n , T i 0 2 , A 1 2 0 3 , Y, V, K/Rb, K/Ba, K/Sr, and Ca/Sr ( F i g s . 8-10). T h e s e - v a r i a t i o n s a r e best- e x p l a i n e d by 4^fettonal..'crys,ta4'll.z!a*$pn o f amphibole, o l i v i n e and c l i n o p y r o x e n e . The o c c u r r e n c e o f r e s o r b e d T i -poor k a e r s u t i t e x e n o c r y s t s (3.75 p e r c e n t T i O ^ ) , and c r y s t a l c l o t s o f magnesian o l i v i n e (Fo_, „„) and aluminous d i o p s i d i c a u g i t e (5.03 o o - o j p e r c e n t A l 0'^ , W o ^ E n ^ ) i n t n e l a v a s u p p o r t s t h i s s u g g e s t i o n . D.H. Green and o t h e r s (1974) s u g g e s t e d t h a t some h a w a i i t e s , m u g e a r i t e s and benmoreites may o r i g i n a t e by f r a c t i o n a t i o n of o l i v i n e , k a e r s u t i t i c h o r n b l e n d e and c l i n o p y r o x e n e from hydrous or more p r o b a b l y w a t e r - u n d e r s a t u r a t e d a l k a l i b a s a l t o r b a s a n i t e magmas w i t h i n t h e upper mantle (Pjj ^ >8 kb) . An i n d i c a t i o n o f the thermochemical c o n d i t i o n s under which the Helm Creek magma was ge n e r a t e d can be o b t a i n e d from r e c e n t e x p e r i m e n t a l s t u d i e s on b a s a l t i c c o m p o s i t i o n s . By analogy w i t h amphiboles produced i n e x p e r i m e n t a l m e l t i n g runs on b a s a l t i c c o m p o s i t i o n s under hydrous c o n d i t i o n s (T.H. Green and Ringwood, 1968; Burnham and Holloway, 1972; He l z 1973; Cawthorn' and o t h e r s , 1973), the p r e s s u r e s and temperatures a t which t h e Helm Creek amphiboles c r y s t a l l i z e d were p r o b a b l y between 5 and 10 kb at about 1050°C. E x p e r i m e n t a l s t u d i e s on w a t e r - u n d e r s a t u r a t e d h a w a i i t e (Knutson and T.H. Green, 1975) a l s o c o n s t r a i n the amount of water which c o u l d have been p r e s e n t i n the Helm Creek magma. The e x p e r i m e n t a l l y - o b t a i n e d l i q u i d u s assemblage o f h a w a i i t e w i t h 5 55 p e r c e n t IL^O a t 5 and 10 kb i s amphibole w i t h o l i v i n e and/or c l i n o -pyroxene, but o n l y the l a t t e r two phases a r e p r e s e n t i n m e l t i n g runs w i t h 2 p e r c e n t H^O. The above d a t a s u g g e s t s t h a t the Helm Creek magma may have c o n t a i n e d more than 2 p e r c e n t water b e f o r e e r u p t i o n . D i f f e r e n t i a t e d Rocks The major and t r a c e element d a t a f o r G a r i b a l d i Lake v o l c a n i c s u i t e s r e v e a l t h a t t h e r e i s a complete g r a d a t i o n from b a s a l t i c - a n d e s i t e to d a c i t e w i t h i n the r e g i o n as a whole, but t h i s g r a d a t i o n does not oc c u r w i t h i n e r u p t i v e p r o d u c t s o f any s i n g l e v o l c a n i c c e n t e r . B a s a l t i c - a n d e s i t e s o c c u r o n l y as i s o l a t e d f l o w s w i t h i n D e s o l a t i o n V a l l e y , Enostuck Meadows and the Sphinx G l a c i e r moraine, and as a b a s a l u n i t i n The T a b l e ; d a c i t i c r o c k s a r e r e s t r i c t e d t o The B l a c k Tusk and Mount P r i c e s u c c e s s i o n s . With i n c r e a s i n g s i l i c a c o n t e n t , the G a r i b a l d i Lake l a v a s and p y r o c l a s t i c r o c k s show an o v e r a l l enrichment i n K^O, Rb, and Ba, and d e p l e t i o n i n MgO, t o t a l i r o n , CaO, T i 0 2 , V, C r , N i , and Sr ( F i g s . 8 and 9 ) . Ba/Sr, K/Sr, and Rb/Sr r a t i o s i n c r e a s e , Ca/Sr and K/Rb r a t i o s d e c r e a s e s l i g h t l y , and a l k a l i / s i l i c a r a t i o s remain n e a r l y c o n s t a n t or d e c r e a s e w i t h i n c r e a s i n g d i f f e r e n t i a t i o n ( F i g s . 10 and 11). The s i l i c a v a r i a t i o n diagrams, however, a r e n e i t h e r smooth nor , i n g e n e r a l , c o n t i n u o u s f o r any v o l c a n o . T h i s i s p a r t l y e x p l a i n e d by the c o - e x i s t e n c e of s e v e r a l d i s t i n c t magma ba t c h e s d u r i n g the e r u p t i v e h i s t o r y , and thus s e v e r a l i n d i v i d u a l l y c o h e r e n t e r u p t i v e groups w i t h i n each v o l c a n o (Condie and Swenson, 1973). The G a r i b a l d i Lake a n d e s i t i c s u i t e s d i s p l a y a t y p i c a l c a l c - a l k a l i n e t r e n d o f p r o g r e s s i v e d e p l e t i o n i n MgO and t o t a l i r o n c o n t e n t s , and i n (Fe0+Fe20^) r e l a t i v e t o MgO ( F i g s . 12 and 13). 56 F i g u r e 12. AMF p l o t o f c h e m i c a l d a t a from the G a r i b a l d i Lake a r e a . Symbols as g i v e n i n F i g . 8. The s o l i d c urved l i n e (Th) r e p r e s e n t s the T h i n g m u l i t h o l e i i t i c t r e n d ( C a r m i c h a e l , 1964). Open curve l i n e (C) r e p r e s e n t s the average Cascades c o m p o s i t i o n a l t r e n d ( C a r m i c h a e l , 1964). A = Na 20 + K 20; M = MgO; F = FeO + 0 . 8 9 9 8 ( F e 2 0 3 ) . Weight P e r c e n t . 57 58 F i g u r e 13. MgO v e r s u s (FeO + Fe 0„) diagram. G a r i b a l d i Lake a r e a . 2 3 Symbols as g i v e n i n F i g . 8. Trends f o r Skaergaard (Sk; Wager, 1960), T h i n g m u l i (Th; C a r m i c h a e l , 1964), T a l a s e a (Ta; Lowder and C a r m i c h a e l , 1970), and average Cascades (C; C a r m i c h a e l , 1964) igneous s u i t e s a r e g i v e n f o r comparison. 59 J I I I I I I I I u 00 ID ^ (VI O 06W 60 F i g u r e 14. P r o j e c t i o n of Cheakamus V a l l e y b a s a l t c o m p o s i t i o n s i n the CMAS t e t r a h e d r o n (O'Hara, 1968). D i o p s i d e (CMS^) p r o j e c t i o n onto C^A-M-S. Compositions o f Cheakamus V a l l e y b a s a l t s l i e a l o n g the OL-PL c o t e c t i c . B. E n s t a t i t e (MS) p r o j e c t i o n onto ^2^2 _^2^~^'2^3' D a t a P ° i n t s d e f i n e a weak t r e n d towards o l i v i n e . C. O l i v i n e (M^S) p r o j e c t i o n onto CS-MS-A. The w e l l - d e f i n e d trend' towards the MS c o r n e r i s erroneous (a p r o j e c t i o n e f f e c t ) , as the p o i n t s c l u s t e r on the o l i v i n e p r o j e c t i o n onto C-MS-A where the l i n e j o i n i n g the group.of p o i n t s i n the t e t r a h e d r o n t o o l i v i n e i s more n e a r l y p e r p e n d i c u l a r to the p l a n e o f p r o j e c t i o n . s (b) Enstatite Projection 62 The s t e a d y a b s o l u t e d e p l e t i o n i n t o t a l i r o n i n these l a v a s may be c o n t r a s t e d w i t h the pronounced i r o n - e n r i c h m e n t c h a r a c t e r i z i n g the t h o l e i i t i c s e r i e s of T h i n g m u l i v o l c a n o , I c e l a n d ( C a r m i c h a e l , 1964) o r the more moderate enrichment t r e n d o f the T a l a s e a i s l a n d - a r c t h o l e i i t e s u i t e (Lowder and C a r m i c h a e l , 1970). The s i l i c a gap (52.16-54.37 p e r c e n t ) between the G a r i b a l d i Lake a n d e s i t e s and t h e contemporaneous Cheakamus V a l l e y b a s a l t s i s emphasized by the s t r o n g enrichment of Sr and Ba i n the a n d e s i t i c l a v a s . The average c o n t e n t s o f Sr and Ba i n the b a s i c r o c k s a r e 475 and 130 ppm, r e s p e c t i v e l y , whereas the average a n d e s i t e c o n t a i n s 960 ppm Sr and 450 ppm Ba. These d i f f e r e n c e s r e p r e s e n t enrichment f a c t o r s o f 2.0 f o r Sr and 3.4 f o r Ba. The enrichments o c c u r not o n l y i n a b s o l u t e terms ( i n c r e a s e i n c o n c e n t r a t i o n of Sr and Ba p r e s e n t ) , but a l s o i n r e l a t i v e terms, s i n c e Sr i s e n r i c h e d r e l a t i v e to Ca, and Ba i s e n r i c h e d r e l a t i v e t o K. The average Ca/Sr and K/Ba r a t i o s i n the Cheakamus V a l l e y b a s a l t s a r e 139 and 47, r e s p e c t i v e l y , whereas the c o r r e s p o n d i n g r a t i o s i n ' t h e a n d e s i t e s a r e , r e s p e c t i v e l y , 42 and 22 ( F i g . 10). These r a t i o s i n d i c a t e t h a t Sr i n c r e a s e s i n amount w h i l e Ca d e c r e a s e s , and t h a t b o t h K and Ba i n c r e a s e i n amount but a t d i f f e r e n t r a t e s . T h i s i n f o r m a t i o n p l a c e s l i m i t a t i o n s on p o s s i b l e p e t r o g e n e t i c p r o c e s s e s which may have been i n f l u e n t i a l i n the e v o l u t i o n o f the G a r i b a l d i Lake a n d e s i t e s as d i s c u s s e d below. Comparison With Other V o l c a n i c S u i t e s The a n d e s i t e s o f the G a r i b a l d i Lake a r e a show many s i m i l a r i t i e s t o l a v a s from c i r c u m - o c e a n i c v o l c a n i c - a r c s . 63 TABLE IV .. AVERAGE GARIBALDI LAKE, CASCADE AND ANDEAN ANDESITE SUITES. Sample No. 1 2 3 4 5 6 7 8 S i 0 2 58.48 60.33 60.99 58.44 59.75 57.19 58.31 58.93 T i 0 2 0.68 0.71 0.65 0.94 0.83 1 .32 1 .02 0.92 A 12°3 18.54 18.38 17.98 17.17 17.03 16.98 16.91 16.82 F e 2 0 3 6.03 5.68 5.36 7.26 6.19 7.66 6.50 6.72 MgO 3.57 2.64 2.87 2.25 2.45 3.78 3.44 3.93 CaO 6.37 5.83 5.67 7.39 6.74 6.29 6.83 6.64 Na 20 4.36 4.05 4.27 4.14 3.66 4.22 3.95 3.57 K 20 1 .28 1 .27 1 .32 1 .00 1 .81 1.31 1 .26 1 .87 Ni 32 32 28 18 19 24 21 31 Cr 29 25 27 66 106 52 79 127 Zn 73 68 57 82 88 73 80 65 Cu 36 33 21 56 39 63 47 99 Rb 13 14 14 14 44 29 15 61 Sr 1186 885 961 619 495 342 523 472 Ba 486 420 523 408 318 443 289 450 K/Rb 817 759 783 592 341 375 697 273 K/Ba 22 25 21 20 47 25 36 35 K/Sr 9 12 11 13 30 32 20 37 Rb/Sr 0.011 0.016 0.015 0.023 0.089 0.085 0.029 0.129 Ba/Sr 0.41 0.48 0.54 0.66 0.64 1 .30 0.55 0.95 Cr/Ni 0.9 0.8 1 .0 3.8 5.6 2.2 3.8 4.1 Zn/Cu 2.0 2.1 2.7 1 .6 2.2 1 .2 1 .7 0.65 A. Total i r o n as F e ^ . B. Average andesites from: 1. The Table; 2. The Black Tusk; 3. Mount P r i c e ; 4. Mount J e f f e r s o n (Condie and Swenson, 1973); 5. Mount Rainer (Condie and Swenson, 1973); 6. South S i s t e r , Oregon (St e i n b o r n , 1972); 7. Mount Hood (S t e i n b o r n , 1972); 8. Andean provinces (Gunn and o t h e r s , 1974). Average values f o r some t r a c e elements taken from p a r t i a l analyses i n Church and T i l t o n (1973). 64 They a r e q u a r t z n o r m a t i v e , p o s s e s s a s i l i c a mode o f 59 p e r c e n t , and e x h i b i t low Mg/(Mg+EFe) r a t i o s (< 0.55). The K/Na r a t i o s o f the a n d e s i t e s remain c o n s t a n t o r i n c r e a s e s l i g h t l y as s i l i c a i n c r e a s e s . The T a b l e , D e s o l a t i o n V a l l e y (The C i n d e r Cone), Sphinx Moraine and E n o s t u c k b a s a l t i c - a n d e s i t e s b e a r a c l o s e c h e m i c a l resemblance t o the .'. E v e r i t t H i l l l a v a from Mount S h a s t a , C a l i f o r n i a (Gunn and P a y e t t e , ms.), e x c e p t t h a t t h e l a t t e r f l o w i s lower i n Na 20, K^O, S r , and Ba, and h i g h e r i n r u b i d i u m . The a n d e s i t e s o f The T a b l e , Mount P r i c e and The B l a c k Tusk, however, show s m a l l , but c o n s i s t e n t l y lower con-c e n t r a t i o n s o f T i O ^ , t o t a l i r o n , C r , V, Cu, and Rb, and h i g h e r A l ^ O ^ , N i , S r , and Ba than most l a v a s w i t h s i m i l a r SiO^ c o n t e n t s from o t h e r Quaternary v o l c a n i c p r o v i n c e s (Cascade, V a l l e y o f Mexico, C e n t r a l American, Andean) o f the w e s t e r n American C o r d i l l e r a ( T a b l e I V ) . Compared to average c a l c - a l k a l i n e a n d e s i t e s o f the L e s s e r A n t i l l e s (Gunn and o t h e r s , 1974) and A l e u t i a n I s l a n d s (Marsh, 1974), the G a r i b a l d i Lake r o c k s a r e d i s t i n c t l y p o o r e r i n CaO, t o t a l i r o n , and Rb, C r / N i and Rb/Sr r a t i o s , and r i c h e r i n N i , Ba, S r , and have a h i g h e r K/Rb r a t i o . Gunn and o t h e r s (1974) observed s i g n i f i c a n t v a r i a t i o n s i n the g e o c h e m i c a l f e a t u r e s o f d i f f e r e n t v o l c a n i c c e n t e r s on the i s l a n d of M a r t i n i q u e . S i m i l a r d i f f e r e n c e s a r e e v i d e n t i n the G a r i b a l d i Lake l a v a s , where The B l a c k Tusk h y p e r s t h e n e - a n d e s t i e s a r e e n r i c h e d i n A 1 2 0 3 , CaO, and Zn, but d e p l e t e d i n K 2 0 , Rb, S r , Ba, and Mg/(Mg + EFe) r a t i o s r e l a t i v e t o the h o r n b l e n d e - a n d e s i t e s o f Mount P r i c e and The T a b l e . Moreover, the l a v a s o f the G a r i b a l d i Lake v o l c a n o e s c o n t a i n more A^O^, CaO, t o t a l i r o n , N i , and Cr, and l e s s K 20, Sr, Ba, and Rb than the G a r i b a l d i Group a n d e s i t e s o f Mount C a y l e y and 65 Meager Mountain ( F i g . 8 and 9; Green and W a t t e r s , 1977). S t r o n t i u m I s o t o p e Data 87 86 Sr /Sr r a t i o s f o r e l e v e n G a r i b a l d i Lake a n d e s i t e s (Armstrong, Green and W a t t e r s , 1977) i n d i c a t e a range from 0.7028 to 0.7036 w i t h a mean of 0.7032 ± 0.0002 (1 sigma). The.mean 8 7 86 Sr /Sr v a l u e o f the G a r i b a l d i Lake r o c k s i s lower than the average r a t i o s i n c a l c - a l k a l i c l a v a s from the Cascades (0.7037, Church and T i l t o n , 1973), o r the w e s t e r n and southwestern c i r c u m -P a c i f i c v o l c a n i c p r o v i n c e s : (1) Rabaul, 0.7038 (Peterman and Heming, 1974), (2) T a l a s e a , 017035 (Peterman and o t h e r s , 1970), (3) Sunda a r c , I n d o n e s i a , 0.7048 ( W h i t f o r d , 1975), (4) Bismarck a r c , 0.7036 (Page and Johnson, 1974), (5) V i t i Levu, F i j i , 0.7040 ( G i l l , 1970), (6) Eua, Tongan I s l a n d s , 0.7036 (Ewart and Bryan, 1972), and (7) M a r i a n a I s l a n d s , 0.7038 (Hedge, 1966). The c o n s t a n c y o f Sr /Sr i n the G a r i b a l d i Lake l a v a s , and the l a c k of s i g n i f i c a n t d i f f e r e n c e s i n the i s o t o p i c c o m p o s i t i o n o f the contemporaneous Cheakamus V a l l e y b a s a l t s (mean 0.7032 ± 0.0002) i s c o m p a t i b l e w i t h d e r i v a t i o n o f the a n d e s i t e s by c r y s t a l f r a c t i o n a t i o n o f a b a s a l t i c magma. The i s o t o p i c d a t a , however, do not p r e c l u d e d i f f e r e n t degrees o f p a r t i a l m e l t i n g o f an i s o t o p i c a l l y u n i f o r m s o u r c e i n the m a n t l e , a l t h o u g h the l i m i t e d range i n s t r o n t i u m - i s o t o p e c o n t e n t , t o g e t h e r w i t h a wide v a r i a t i o n i n Sr c o n c e n t r a t i o n s (459-1810 ppm), does n o t f a v o u r d e r i v a t i o n from the b a s a l t - s e d i m e n t p r o t i o n o f a subducted p l a t e (Church and T i l t o n , 1973). 66 A n d e s i t e P e t r o g e n e s l s P e t r o l o g i c a l and e x p e r i m e n t a l s t u d i e s suggest s e v e r a l mechanisms which may p l a y a r o l e i n the g e n e r a t i o n o f c a l c - a l k a l i n e a n d e s i t e magmas. These i n c l u d e : (1) C o n t a m i n a t i o n o f b a s a l t i c magma by c r u s t a l m a t e r i a l (Kuno, 1950), o r m i x i n g o f b a s a l t i c o r r h y o l i t i c magma w i t h e a r l i e r e r u p t i v e p r o d u c t s i n the v o l c a n i c p i l e ( E i c h e l b e r g e r , 1975; Anderson, 1976); (2) P a r t i a l m e l t i n g o f h i g h - p r e s s u r e e c l o g i t i c e q u i v a l e n t s o f o c e a n i c t h o l e i i t e (Green and Ringwood, 1968), o r f r a c t i o n a t i o n o f a b a s a l t i c magma c o n t r o l l e d by h i g h - p r e s s u r e c r y s t a l l i z a t i o n o f g a r n e t and pyroxene (Green, 1972; Ringwood, 1974); (3) P a r t i a l m e l t i n g o f mantle p e r i d o t i t e under hydrous c o n d i t i o n s ( K u s h i r o , 1974; Mysen and B o e t t c h e r , 1975a, 1975b); and, (4) F r a c t i o n a l c r y s t a l l i z a t i o n o f a b a s a l t i c magma a t s h a l l o w l e v e l s c o n t r o l l e d by o l i v i n e , pyroxene, amphibole and/or m a g n e t i t e (Bowen, 1928; Osborn, 1969; Ewart and Bryan, 1972; Cawthorn and O'Hara, 1976) . These p r o c e s s e s can be e v a l u a t e d i n the l i g h t o f the major and t r a c e element d a t a f o r the G a r i b a l d i Lake v o l c a n i c r o c k s . A s s i m i l a t i o n and Magma M i x i n g Mathews (1957) proposed t h a t a n d e s i t e s o f the G a r i b a l d i Lake a r e a o r i g i n a t e d by d i f f e r e n t i a t i o n o f a b a s a l t i c magma m o d i f i e d by s y n t e x i s , b u t p e t r o g r a p h i c and i s o t o p i c e v i d e n c e argues a g a i n s t such a mechanism. X e n o l i t h i c c r u s t a l m a t e r i a l i s r a r e , and where p r e s e n t , the i n c l u s i o n s r e t a i n sharp c o n t a c t s w i t h the v o l c a n i c 67 groundmass. Moreover, t h e h i g h Sr and low Rb c o n t e n t s , the h i g h K/Rb 87 86 r a t i o s and e x c e p t i o n a l l y low Sr / S r r a t i o s o f the a n d e s i t e s p r e c l u d e e x t e n s i v e i n v o l v e m e n t o f s i a l i c c r u s t i n any a c c e p t a b l e p e t r o g e n e t i c model (Church and T i l t o n , 1973; Condie and Swenson, 1973). The absence o f r h y o l i t i c l a v a s o r x e n o l i t h s , the r a r i t y o f r e v e r s e l y zoned p h e n o c r y s t s , the r e l a t i v e l y u n i f o r m c o m p o s i t i o n o f the a n d e s i t e s , and t h e i r n o n - l i n e a r v a r i a t i o n i n b u l k c o m p o s i t i o n do n o t p r o v i d e s u p p o r t f o r a l t e r n a t i v e models o f magma m i x i n g ( E i c h e l b e r g e r , 1975; Anderson, 1976). P a r t i a l M e l t i n g o f Subducted Oceanic C r u s t T.H. Green (1972) proposed t h a t a n d e s i t e magmas o r i g i n a t e by p a r t i a l m e l t i n g o f d r y q u a r t z e c l o g i t e a l o n g the top o f an under-t h r u s t l i t h o s p h e r i c s l a b a t depths between 100 and 150 km. Magmatic l i q u i d s r e s e m b l i n g a n d e s i t e s from the G a r i b a l d i Lake a r e a c o u l d be ge n e r a t e d by 13 t o 45 p e r c e n t p a r t i a l m e l t i n g o f t r a n s f o r m e d o c e a n i c c r u s t o f the Juan de Fuca p l a t e , but th e s e m e l t s would be e n r i c h e d i n Fe and d e p l e t e d i n Mg r e l a t i v e t o the observed c o m p o s i t i o n s (Appendix I I ) . F u r t h e r m o r e , as the l i q u i d u s pyroxenes o f e c l o g i t e p o s s e s s low K/Na r a t i o s , m e l t s produced by l e s s t h a n 20 p e r c e n t m e l t i n g a r e c h a r a c t e r i z e d by h i g h e r K/Na r a t i o s and h i g h e r K c o n t e n t s than the a n a l y z e d l a v a s (Thorpe and o t h e r s , 1976). The c o n c e n t r a t i o n of S r , Ba, and, to a l e s s e r e x t e n t , Rb i n t h e a n d e s i t e s , on the o t h e r hand, c o u l d o n l y be gen e r a t e d by l e s s than 15 p e r c e n t m e l t i n g of an e c l o g i t i c m i n e r a l assemblage ( c . f . D e l o n g , 1974). R e f r a c t o r y k y a n i t e , a p a t i t e , r u t i l e , p y r r h o t i t e and z i r c o n c o u l d e x p l a i n the low abundance o f V, N i , C r , and Zr i n th e s e l a v a s , but i t i s u n l i k e l y t h a t t h ese phases would be r e f r a c t o r y a f t e r 15 t o 45 p e r c e n t m e l t i n g 68 ( G i l l , 1974). Thus, w h i l e i t i s d i f f i c u l t to demonstrate c o n c l u s i v e l y t h a t the G a r i b a l d i Lake a n d e s i t e s d i d n o t o r i g i n a t e by p a r t i a l f u s i o n of t r a n s f o r m e d o c e a n i c c r u s t , i t seems improbable t h a t the l a v a s were g e n e r a t e d by such a p r o c e s s u n l e s s t h e i r c h e m i s t r y was m o d i f i e d c o n s i d e r a b l y d u r i n g t h e i r a s c e n t to the E a r t h ' s s u r f a c e . P a r t i a l M e l t i n g o f M a n t l e P e r i d o t i t e C o n t r o v e r s y s t i l l s u r rounds i n t e r p r e t a t i o n o f the h i g h -p r e s s u r e m e l t i n g b e h a v i o r o f p e r i d o t i t i c c o m p o s i t i o n s . In p a r t i c u l a r , t h e r e a r e d i f f e r e n t o p i n i o n s among e x p e r i m e n t a l p e t r o l o g i s t s as to whether a n d e s i t e magmas can be p a r t i a l m e l t i n g p r o d u c t s o f wet mantle p e r i d o t i t e a t p r e s s u r e s g r e a t e r than 10 kb (Mysen and o t h e r s , 1974; N i c h o l l s , 1974). I n s p i t e o f t h i s disagreement, l e a s t - s q u a r e s m i x i n g c a l c u l a t i o n s , which e v a l u a t e the weighted p r o p o r t i o n s o f magma and r e f r a c t o r y m i n e r a l s h a v i n g a b u l k c o m p o s i t i o n e q u i v a l e n t t o p o s s i b l e s o u r c e p e r i d o t i t e s (Appendix I I ) , i n d i c a t e t h a t l i q u i d s produced by 2 t o 6 p e r c e n t m e l t i n g o f a hydrous (am p h i b o l e - b e a r i n g ) p e r i d o t i t i c m i n e r a l assemblage c o u l d p o s s e s s major element abundances r e s e m b l i n g the G a r i b a i d i Lake a n d e s i t e s . The Rb, Sr and Ba c o n t e n t s o f the l a v a s a r e a l s o c o m p a t i b l e w i t h a model i n v o l v i n g l e s s than 6 p e r c e n t m e l t i n g o f an u n d e p l e t e d mantle c o m p o s i t i o n ( i . e . 0.03-0.06 ppm Rb, 30-60 ppm S r , and 10-20 ppm Ba; G a s t , 1968). However, D.H. Green (1976) n o t e d t h a t the sou r c e p e r i d o t i t e i n i s l a n d - a r c (and presumably c o n t i n e n t a l - m a r g i n ) r e g i o n s i s p r o b a b l y d e p l e t e d i n i t s K^O, Na^O, T i O ^ , and g e o c h e m i c a l l y r e l a t e d t r a c e element ( e . g . S r , Rb, Ba, Ree, Z r , e t c . ) c o n t e n t s by e a r l i e r l o s s o f a magmatic component. V a r i a b l e and l o c a l enrichment 69 i n i n c o m p a t i b l e t r a c e elements, c o n t r o l l e d by e i t h e r i n t r o d u c t i o n of a s m a l l melt f r a c t i o n ( r h y o d a c i t e ; N i c h o l l s , 1974) or m i g r a t i o n through a fL ^ O - r i c h vapor phase ( B e s t , 1975) r i s i n g from t h e . s u b d u c t i o n zone c o u l d m o d i f y such a " d e p l e t e d " s o u r c e c o m p o s i t i o n s u f f i c i e n t l y to a l l o w g e n e r a t i o n o f the G a r i b a l d i Lake major and t r a c e element d i s t r i b u t i o n s , y e t i t remains q u e s t i o n a b l e whether the a n d e s i t e s r e p r e s e n t d i r e c t m e l t i n g p r o d u c t s o f an upper mantle p e r i d o t i t e . 87 86 A v e r y weak p o s i t i v e c o r r e l a t i o n between Sr /Sr r a t i o s w i t h SiO^ and Ba, but n o t w i t h S r , Rb, o r Rb/Sr c o n t e n t s does not suggest s i g n i f i c a n t subducted b a s a l t - s e d i m e n t c o n t r i b u t i o n t o the m e l t s (Armstrong, Green and W a t t e r s , 1977). L i q u i d s i n e q u i l i b r i u m 2+ w i t h mantle o l i v i n e (Mg/(Mg + Fe ) = 0.84-0.94) must be h i g h l y 2+ magnesian (Mg/(Mg + Fe )>0.61) (Roeder and E m s l i e , 1970; N i c h o l l s and W h i t f o r d , 1976). A l l the a n a l y z e d l a v a s , however, have Mg-r a t i o s too low (<0.55) f o r them t o r e p r e s e n t m e l t s o r i g i n a l l y i n e q u i l i b r i u m w i t h a r e s i d u a l p e r i d o t i t i c m i n e r a l o g y . The a n d e s i t e s a r e a l s o r e l a t i v e l y d e p l e t e d i n N i (12-100 ppm-, mean 39 ppm) and Cr (8-147 ppm, mean 42 ppm). These low t r a n s i t i o n m e t a l abundances argue s t r o n g l y a g a i n s t a d i r e c t mantle d e r i v a t i o n ( T a y l o r and o t h e r s , 1969). F r a c t i o n a l C r y s t a l l i z a t i o n o f a B a s a l t i c Magma Low-pressure f r a c t i o n a l c r y s t a l l i z a t i o n i n v o l v i n g o l i v i n e , p yroxene, p l a g i o c l a s e , m a g n e t i t e and/or amphibole has been suggested as an im p o r t a n t mechanism i n d e r i v i n g a n d e s i t e s from a b a s a l t i c m e l t . Where magma i n a h i g h - l e v e l chamber, p e r i o d i c a l l y f e d w i t h new 70 ba t c h e s o f p a r e n t a l magma from depth, undergoes c o n t i n u o u s f r a c t i o n a t i o n , l a v a c o m p o s i t i o n s e x t r a c t e d from time t o time w i l l e x h i b i t : (1) l a r g e c o m p o s i t i o n a l d i f f e r e n c e s b e t w e e n . p a r e n t a l magma and daughter l i q u i d s ; (2) s t r o n g c o n t r o l o f l i q u i d c o m p o s i t i o n s by-lo w - p r e s s u r e phase e q u i l i b r i a ; (3) l a r g e v a r i a t i o n s i n the concen-t r a t i o n s o f e x c l u d e d elements r e l a t i v e t o more co m p a t i b l e major elements; and (4) s i g n i f i c a n t v a r i a t i o n s i n the r a t i o o f two i n c o m p a t i b l e elements (O'Hara, 1976a, 1976b). I f a s i m i l a r p r o c e s s was o p e r a t i v e w i t h i n the G a r i b a l d i Lake magma r e s e r v o i r s , then c r y s t a l f r a c t i o n a t i o n c o u l d account f o r : 2+ (1) the r e l a t i v e l y u n i f o r m Mg/(Mg + Fe ) r a t i o s y e t w i d e l y v a r y i n g i n c o m p a t i b l e e l e m e n t a l abundances i n the a n d e s i t e s , and, (2) the h i g h c o n c e n t r a t i o n l e v e l s of Sr and Ba, and t h e i r enrichment i n the a n d e s i t e s r e l a t i v e t o the c o - e x i s t i n g b a s a l t i c r o c k s . As mentioned e a r l i e r , the Cheakamus V a l l e y b a s a l t s have p r o b a b l y undergone some f r a c t i o n a t i o n o f o l i v i n e , c l i n o p y r o x e n e and p l a g i o c l a s e . F u r t h e r s e g r e g a t i o n o f these phases y i e l d s major element c o m p o s i t i o n s r e s e m b l i n g the G a r i b a l d i Lake a n d e s i t e s ( F i e s i n g e r , 1975), but such a p r o c e s s seems i n c a p a b l e of g e n e r a t i n g the t r a c e element abundances i n t h e s e l a v a s . I n p a r t i c u l a r , c r y s t a l l i z a t i o n and removal o f o l i v i n e , c l i n o p y r o x e n e and p l a g i o c l a s e from the b a s a l t i c magmas ( i n the p r o p o r t i o n s r e q u i r e d by the major element models; F i e s i n g e r , op. c i t . ) can n o t rep r o d u c e the n e a r l y t w o - f o l d enrichment i n Rb, Sr and Ba co n t e n t s o f the a n d e s i t e s . The r e l a t i v e l y h i g h N i c o n t e n t o f t h e b a s a l t i c -a n d e s i t e s (67-100 ppm, mean 87 ppm) a l s o p r e c l u d e s an o r i g i n i n v o l v i n g e x t e n s i v e f r a c t i o n a t i o n of o l i v i n e ( T a y l o r , 1969). The Cheakamus V a l l e y b a s a l t s t h e r e f o r e can n o t r e p r e s e n t the p a r e n t a l 71 l i q u i d s of the G a r i b a l d i Lake a n d e s i t i c s u i t e s . The Sphinx Moraine and Enostu c k b a s a l t i c - a n d e s i t e s c o n t a i n c r y s t a l c l o t s c o n s i s t i n g e s s e n t i a l l y o f amphibole and c l i n o p y r o x e n e . The average mode o f the c l o t s i s as f o l l o w s : 32 p e r c e n t amphibole, 46 p e r c e n t c l i n o p y r o x e n e , 2 p e r c e n t o l i v i n e , 17 p e r c e n t p l a g i o c l a s e , 3 p e r c e n t m a g n e t i t e . Random a c c u m u l a t i o n can not account f o r the d i f f e r e n c e i n r e l a t i v e abundance of the m i n e r a l s c o m p r i s i n g the c l o t s compared w i t h the r e l a t i v e abundance o f these m i n e r a l s as p h e n o c r y s t s i n the l a v a , e.g. p l a g i o c l a s e a c counts f o r 17 p e r c e n t of the c l o t s , but 36 p e r c e n t o f the p h e n o c r y s t s . T h e r e f o r e , the i n c l u s i o n s may r e p r e s e n t cognate m a t e r i a l which c r y s t a l l i z e d w i t h i n a magma chamber, o r d u r i n g the asc e n t of v a r i o u s magma b a t c h e s . Slow and u n i n t e r r u p t e d s e g r e g a t i o n o f c r y s t a l s and l i q u i d (such t h a t c r y s t a l - l i q u i d e q u i l i b r i u m and a degree of s u b - s o l i d u s h o m o g e n i z a t i o n c o u l d be m a i n t a i n e d ) c o u l d e x p l a i n the remarkably u n i f o r m c o m p o s i t i o n s and absence o f z o n i n g observed i n t h e c l o t m i n e r a l s (except the p l a g i o c l a s In the p r e s e n t s t u d y , a model based on the c r y s t a l l i z a t i o n and removal o f t h e c l o t m i n e r a l s under c o n d i t i o n s o f b u l k e q u i l i b r i u m has been t e s t e d . In s i m u l a t i n g c r y s t a l / l i q u i d f r a c t i o n a t i o n , a c a l c u l a t e d cumulate ( b u l k ) c o m p o s i t i o n was added t o the b a s a l t i c -a n d e s i t e l i q u i d s u n t i l " p a r e n t a l " magmas i n e q u i l i b r i u m w i t h the observed o l i v i n e c o m p o s i t i o n (K^ = 0.3; Roeder and E m s l i e , 1970) were produced. T a b l e V g i v e s an example o f the r e s u l t s o b t a i n e d , and the com p o s i t i o n s o f e x t r a c t e d m i n e r a l phases. The c a l c u l a t e d p a r e n t a l magma has h i g h e r MgO, CaO, and K^O, but lower T102, A ^O^, t o t a l i r o n , and Na 00 than the average Cheakamus V a l l e y b a s a l t . TABLE V . EMPIRICAL CALCULATION OF BASALTIC PARENT TO THE SPHINX MORAINE LAVA. Cumulate M i n e r a l s Bulk Lava C a l c . Ave.* Amph. O l i v . Cpx. P l a g . Mt. Cumulate (588) Parent B a s a l t S i 0 2 43.67 39.97 49.33 49.87 - 45. .70 55.29 50.52 50.35 T i 0 2 2.93 - 1 .26 - 6.68 1 . .72 0.76 1 .29 1.51 A 1 9 0 9 12.73 - 4.96 31.65 4.26 12. .47 17.54 15.01 15.84 FeO 10.33 13.68 10.60 0.42 86.35 11 , .29 6.81 9.07 11 .50 MgO 15.30 46.23 14.35 - 2.70 12, .00 6.37 9.20 8.13 CaO 11 .72 0.11 19.18 15.60 - 15. .12 8.32 11 .70 8.78 Na 90 2.80 - 0.32 2.45 - 1 .54 3.72 2.61 3.37 C. K 20 0.47 - - - - 0. .16 1.19 0.61 0.51 Rb 13 7* 6 Sr 1305 847* 477 Ba 462 263* 130 Ni 99 155* 115 Prop. 32. + 2. 1 46. + 17. + 3. + 51 .25 § 48.75 § - -$ Trace element abundances c a l c u l a t e d using average p a r t i t i o n c o e f f i c i e n t s f o r b a s a l t i c rocks ( A r t h , 1976; Leeman, 1976). <t> Average Cheakamus V a l l e y b a s a l t , t P r o p o r t i o n of mineral phase i n average c r y s t a l c l o t . § P r o p o r t i o n s o f bulk cumulate and daughter compositions which must be combined to y i e l d a l i q u i d i n e q u i l i b r i u m w i t h the observed o l i v i n e composition ( K n = 0.3, Roeder and Emslie, 1970). 73 I t i s a l s o p o s s i b l e t o p r e d i c t the e f f e c t o f f r a c t i o n a l c r y s t a l l i z a t i o n o f the cumulate phases on the abundance o f Rb, Sr, Ba, and N i , assuming c a l c u l a t e d p a r t i t i o n c o e f f i c i e n t s ( A r t h , 1976; Leeman, 1976). Almost a t w o - f o l d enrichment i n the c o n c e n t r a t i o n o f Rb, S r , and Ba may r e s u l t from the f r a c t i o n a t i o n o f amphibole, c l i n o p y r o x e n e , o l i v i n e , p l a g i o c l a s e and m a g n e t i t e i n the p r o p o r t i o n s g i v e n i n T a b l e V; e x c l u s i o n of p l a g i o c l a s e (which o c c u r s m a i n l y as overgrowths on the c l o t s ) from the f r a c t i o n a t e d m i n e r a l assemblage f u r t h e r i n c r e a s e s the enrichment f a c t o r s . Thus, i t i s p o s s i b l e t h a t the h i g h abundance l e v e l s o f c e r t a i n i n c o m p a t i b l e o r e x c l u d e d elements ( i . e . Sr and Ba) i n t h e G a r i b a l d i Lake a n d e s i t e s a r e i n h e r i t e d i n i t i a l l y by e x t e n s i v e f r a c t i o n a t i o n o f v a r i a b l e p r o p o r t i o n s of amphibole, c l i n o p y r o x e n e and o l i v i n e from a hydrous " p a r e n t a l " b a s a l t i c magma. The m a t h e m a t i c a l s o l u t i o n s f o r d e r i v a t i o n of s i l i c i c - a n d e s i t e s from, the b a s a l t i c - a n d e s i t e l i q u i d s g i v e r e a s o n a b l e major element c o n t e n t s i n the d e r i v e d l a v a s ( T a b l e V I ) , but computed e x t r a c t s o f c l o t and p h e n o c r y s t m i n e r a l s d e p a r t somewhat from the observed modes. However, as l a v a s s i m i l a r i n c o m p o s i t i o n t o the Sphinx M o r a i n e , E n o s t u c k or D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e s do n o t o c c u r w i t h i n The T a b l e , Mount P r i c e o r The B l a c k Tusk, i t i s p o s s i b l e t h a t the s i l i c i c - a n d e s i t e s a r e d i f f e r e n t i a t e s o f magma b a t c h e s w i t h s l i g h t l y d i f f e r e n t p a r e n t a l c o m p o s i t i o n s . H i g h - l e v e l s e g r e g a t i o n o f c l o t o r p h e n o c r y s t m i n e r a l s p r o b a b l y produced f u r t h e r s i l i c a - e n r i c h m e n t i n the a n d e s i t e s . Many l a v a s from the G a r i b a l d i Lake v o l c a n o e s c o n t a i n f i n e - g r a i n e d b a s i c x e n o l i t h s , which can be grouped i n o r d e r of d e c r e a s i n g abundance as f o l l o w s : TABLE VI. TEST OF FRACTIONATION SCHEME BETWEEN BASALTIC-ANDESITE AND ANDESITE. Extract Step Plag. Amph. O l i v . Cpx. Mt. z r2 588 to 533 -14.49 g. -12.74 g. -2.89 g. -6.63 g. -0.62 g. 0 .1562 A n 5 g A b 4 0 § F o 85 W o 4 1 F s 1 8 6. .67 % T i 0 2 588 to 639 -7.07 g. -4.18 g- -3.71 g. -6.85 g. -0.18 g. 0 .0991 A n 5 9 A b 4 Q § F o 85 W o 4 1 F s 1 8 6, .67 % T i 0 2 588 to 623 -14.36 g. -15.97 g- -3.12 g. -4.04 g. -0.14 g. 0 .2058 A n 5 g A b 4 0 § F o 85 W o 4 1 F s 1 8 6, .67 % T i 0 2 A. See Tables V, VII , VIII , and IX for detai ls of parents and der ivat ives . B. Figures beneath mineral names refer to number of grams of that phase which has to be removed from each 100 grams of start ing composition in order to obtain the composition of the derived lava. § Amphibole composition: 43.27 % S i 0 2 , 2.93 % T i 0 2 , 12.78 % k\ftv 10.33 % FeO, 15.30 % MgO, 11.72 % CaO, 2.80 % Na 20, 0.47 % K 2 0 . 75 (a) Amphibole ± orth o p y r o x e n e + p l a g i o c l a s e + m a g n e t i t e , (b) Orthopyroxene ± c l i n o p y r o x e n e + p l a g i o c l a s e + m a g n e t i t e , (c) B i o t i t e ± amphibole ± ort h o p y r o x e n e + p l a g i o c l a s e + m a g n e t i t e , and p r o v i d e the b e s t e v i d e n c e o f a f r a c t i o n a t i o n p r o c e s s . On t h e assumption t h a t c o n t i n u o u s major and t r a c e element v a r i a t i o n i n d i c a t e s a g e n e t i c r e l a t i o n s h i p between l a v a s of a p a r t i c u l a r v o l c a n i c c e n t e r (Mount P r i c e , The T a b l e o r The B l a c k T u s k ) , an attempt has been made to match the o b s e r v e d t r e n d s by removal o f one o r more o f the phases (amphibole, b i o t i t e , o r t h o p y r o x e n e , c l i n o p y r o x e n e , p l a g i o c l a s e and m a g n e t i t e ) commonly p r e s e n t i n the l a v a s from the l a v a c o m p o s i t i o n s p o s s e s s i n g the h i g h e s t Mg/(Mg + EFe) v a l u e i n each v o l c a n o . The r e s u l t s of t h e se c a l c u l a t i o n s show t h a t the d e r i v a t i o n o f v a r i o u s l a v a s by s u b t r a c t i o n of t h e cumulate and p h e n o c r y s t m i n e r a l s i s c h e m i c a l l y f e a s i b l e , and t h a t the computed e x t r a c t s agree f a i r l y w e l l w i t h t h e observed modes ( T a b l e s V I I t o I X ) . T r a c e element abundances i n the c a l c u l a t e d d e r i v a t i v e s (computed u s i n g e s t i m a t e d modes f o r the e x t r a c t s and p u b l i s h e d p a r t i t i o n c o e f f i c i e n t s ; A r t h , 1976) a l s o compare c l o s e l y w i t h the ob s e r v e d t r a c e element c o n t e n t s . The d i s t i n c t i v e c h e m i c a l f e a t u r e s o f each v o l c a n i c s u i t e can be e x p l a i n e d by s e g r e g a t i o n o f d i f f e r e n t m i n e r a l phases, and t h e r e f o r e p r o b a b l y r e f l e c t s d i f f e r e n c e s i n the p h y s i c a l c o n d i t i o n s (T, P L o a c j > P H Q and c o m p o s i t i o n o f the p a r e n t a l magmas) under which f r a c t i o n a t i o n p r o c e e d e d . An i m p o r t a n t f e a t u r e o f the c a l c u l a t i o n s , however, i s the n e c e s s i t y o f i n c l u d i n g m a g n e t i t e i n t h e f r a c t i o n a t e d m i n e r a l assemblage, as o n l y i n c l u s i o n of t h i s phase can produce the s t r o n g d e p l e t i o n i n V t h a t c h a r a c t e r i z e s the l a v a s . The i n c r e a s i n g dominance o f p l a g i o c l a s e c r y s t a l l i z a t i o n TABLE VII. TEST OF FRACTIONATION SCHEME FOR THE BLACK TUSK LAVAS. Parent Derivatives Extracts Derivation Ave. 1 2 3 1-2 1-3 cumulate Obsd. Calc. Obsd. Calc. Calc. Calc. mode S i 0 2 59.75 60.19 60.20 61 .14 61.11 % Plag. 58.82 58.73 67. T i 0 2 0.72 0.80 0.74 0.63 0.64 % Cpx. 21.84 20.06 12. Al 2 0 3 18.17 18.36 18.42 18.63 18.60 % Opx. 15.45 10.56 15. FeO 6.57 6.43 6.45 5.64 5.62 % Mt. 3.89 10.65 6. MgO 3.11 2.64 2.66 2.58 2.57 CaO 6.38 6.07 6.10 6.00 5.98 Total wt. 8.78 13.99 Na20 4.16 4.33 4.20 4.15 4.21 i n g. K 20 1 .14 1.16 1 .23 1.23 1 .29 (100 g. Rb 13 11 14 § 14 15 parent) Sr 856 867 847 934 847 Ba 555 - - 403 455 i r 2 0.0304 0.0101 Ni 35 27 29 31 25 V 135 106 120 111 86 A. The computer method used ( J . N i c h o l l s , pers. comm.) i s based on the method of Bryan and others (1969). § Trace element abundances c a l c u l a t e d using measured p a r t i t i o n c o e f f i c i e n t s f o r rocks of s i m i l a r composition (Ewart and Taylor, 1969; Ewart and others, 1973; A r t h , 1976), and assuming bulk e q u i l i b r i u m . B. Key. 1. Two-pyroxene andesite (623). 2. Hypersthene-andesite (631). 3. Hypersthene-andesite (611). TABLE V I I I . TEST OF FRACTIONATION SCHEME FOR MOUNT PRICE LAVAS. Parent D e r i v a t i v e s E x t r a c t s D e r i v a t i o n 1 2 3 1-2 1-3 Obsd. Cal c. Obsd. C a l c . Obsd. + C a l c . Obsd. 5 C a l c . S i 0 9 59.74 61.05 61.07 63.03 63.02 % P l a g . 63. 71.02 47. 52.80 T i 0 2 0.76 0.71 0.80 0.55 0.62 % B i o t . 13. 4.61 13. 17.01 A 12°3 18.46 18.18 18.23 17.82 17.81 % Opx. 18. 20.08 -FeO 6.01 5.49 5.50 4.74 4.72 % Mt. 6. 4.29 7. 7.69 MgO 3.43 2.90 2.91 2.83 2.82 % Amph. - 33. 22.50 CaO 6.13 5.89 5.87 5.23 5.22 Na 20 4.19 4.30 4.19 4.56 4.57 To t a l wt. 17.40 17.16 K 20 1.28 1.48 1 .44 1 .23 1.22 i n g. Rb 15 18 18 14 17 (100 g. Sr 939 892 898 865 930 parent) Ba 459 532 522 545 509 Ni 25 25 22 23 22 z r 2 0.0258 0.0051 V 109 103 90 60 79 A. See Table VII f o r d i s c u s s i o n of method of c a l c u l a t i n g d e r i v a t i v e c o m p o s i t i o n s , t Mode o f cognate i n c l u s i o n s i n daughter l i q u i d . § Mode o f phenocryst assemblage i n parent. B. Key. 1. H o r n b l e n d e - b i o t i t e a n d e s i t e (533). 2. H o r n b l e n d e - b i o t i t e a n d e s i t e (581). 3. Hornblende-a n d e s i t e (535). TABLE IX. TEST OF FRACTIONATION SCHEME FOR THE TABLE LAVAS. Parent D e r i v a t i v e s E x t r a c t s D e r i v a t i o n 1 2 3 1-2 1-3 Obsd. Calc. Obsd. Calc. Obsd. f C a l c . Obsd.1" Calc. S i 0 2 57.82 58.44 58.45 58.30 58.47 % Plag. 72. 65.34 21. 20.26. T i 0 2 0.71 0.65 0.68 0.63 0.62 % Amph. 12. 22.87 77. 75.45 A 1 2 0 3 18.51 18.27 18.27 18.76 18.72 % Opx. 12. 9.52 -FeO 6.40 6.34 6.35 5.82 5.95 % Mt. 4. 2.27 2. 4.29 MgO 3.92 3.61 3.61 3.68 3.59 CaO 6.97 6.74 6.74 6.57 6.81 Total wt. 14.46 5.02 Na 20 4.53 4.68 4.61 4.91 4.65 i n g. K 20 1 .14 1 .27 1 .29 1.33 1.19 (100 g. Rb 13 13 15 13 14 parent) Sr 1260 1310 1210 1287 1273 Ba 471 482 528 482 485 E r 2 0.0058 0.1928 Ni 37 29 35 33 34 V 123 121 105 121 100 A. See Table VII f o r d i s c u s s i o n of method of c a l c u l a t i n g d e r i v a t i v e compositions, f Average mode of cognate i n c l u s i o n s i n daughter l a v a . B. Key. 1. Hornblende-pyroxene andesite (639). 2. Hornblende-pyroxene andesite (638). 3. Hornblende-andesite (667). 79 i n more s i l i c e o u s c o m p o s i t i o n s produces the observed d e c r e a s e i n Sr c o n t e n t s , and c o n t i n u e d enrichment i n Rb and Ba. Depending on the p r o p o r t i o n s of amphibole and/or pyroxene e x t r a c t e d , N i and Cr behave as e i t h e r c o m p a t i b l e or e x c l u d e d elements. D i s c u s s i o n The b a s i c l a v a s of the G a r i b a l d i Lake a r e a are p r e d o m i n a n t l y a l k a l i - o l i v i n e b a s a l t s , h a w a i i t e s and m u g e a r i t e s . The i n t e r m e d i a t e r o c k s , however, have a c a l c - a l k a l i n e a f f i n i t y , r a n g i n g i n c o m p o s i t i o n from b a s a l t i c - a n d e s i t e to d a c i t e . A l l l a v a s d i s p l a y d e p l e t i o n i n Rb (4-20 ppm), h i g h K/Rb r a t i o s (500^-1000) and low S r 8 7 / S r 8 6 r a t i o s (0.7028-0.7036). The a n d e s i t e s show s t r o n g enrichment i n Sr (700-^1400 ppm) and Ba (400-800 ppm), w h i c h - f u r t h e r emphasizes the major c o m p o s i t i o n a l . g a p between.these l a v a s and the contemporaneous Cheakamus V a l l e y b a s a l t s . No s i m p l e model i n v o l v i n g d i r e c t p a r t i a l m e l t i n g , e i t h e r o f subducted o c e a n i c c r u s t o r o f hydrous p e r i d o t i t e w i t h i n the o v e r l y i n g mantle wedge, can e x p l a i n the major and t r a c e element g e o c h e m i s t r y o f the G a r i b a l d i Lake a n d e s i t i c r o c k s . The low K/Na ( l e s s t h a n 0.6) and Fe/Mg v a l u e s ( l e s s than 2.0), t h e marked d e p l e t i o n i n Rb, and V, and s t r o n g enrichment of Sr and Ba i n the a n d e s i t e s argue a g a i n s t e q u i l i b r i u m m e l t i n g o f a g a r n e t - b e a r i n g ( e c l o g i t i c ) assemblage i n subducted o c e a n i c 2+ l i t h o s p h e r e . The a n d e s i t e s , however, have Mg/(Mg + Fe ) v a l u e s ( l e s s t h a n 0.6) and N i (12-100 ppm) and Cr (8-147 ppm) c o n t e n t s too low f o r t h e s e l a v a s to r e p r e s e n t u n m o d i f i e d p a r t i a l m e l t s o f mantle p e r i d o t i t e . Thus, i t seems improbable t h a t t h e s e l a v a s were d e r i v e d by p a r t i a l m e l t i n g o f e i t h e r o f t h e s e s o u r c e r o c k s u n l e s s t h e i r c h e m i s t r y was 80 m o d i f i e d c o n s i d e r a b l y d u r i n g a s c e n t t o the s u r f a c e . I n s t e a d , the g e o c h e m i c a l e v o l u t i o n o f the a n d e s i t e magmas can be i n t e r p r e t e d w i t h a m u l t i s t a g e model i n v o l v i n g : (1) p a r t i a l m e l t i n g o f hydrous mantle p e r i d o t i t e t o y i e l d s i l i c a - o r o l i v i n e -s a t u r a t e d b a s a l t i c magmas ( N i c h o l l s and Ringwood, 1973), (2) s e g r e g a t i o n of e a r l y formed o l i v i n e , c l i n o p y r o x e n e and amphibole from the hydrous ( w a t e r - u n d e r s a t u r a t e d ) b a s a l t i c m e l t s a t h i g h p r e s s u r e s (>5 k b ) , and (3) f r a c t i o n a t i o n o f amphibole + orth o p y r o x e n e + p l a g i o c l a s e + m a g n e t i t e (Cawthom? and O'Hara, 1976; Cawthornr, 1976), o r p l a g i o c l a s e + c l i n o p y r o x e n e + or t h o p y r o x e n e + m a g n e t i t e ( E g g l e r , 1974) from magmas t h a t a r e w a t e r - u n d e r s a t u r a t e d , n e a r l y anhydrous, o r w a t e r - s a t u r a t e d at upper c r u s t a l l e v e l s . 81 IV. MINERALOGY OF THE GARIBALDI LAKE LAVAS I n t r o d u c t i o n B a s a l t s o f the G a r i b a l d i Lake a r e a t y p i c a l l y c o n t a i n p h e n o c r y s t s of o l i v i n e , p l a g i o c l a s e and c l i n o p y r o x e n e i n a groundmass of the same m i n e r a l s p l u s t i t a n o m a g n e t i t e , i l m e n i t e and p a l e brown g l a s s . The b a s a l t i c - a n d e s i t e s have a s i m i l a r m i n e r a l o g y , but a l s o c o n t a i n amphibole and t i t a n o m a g n e t i t e p h e n o c r y s t s ; i l m e n i t e i s absent. P e t r o g r a p h i c a l l y , t h e r e a r e t h r e e types of a n d e s i t e s : l a v a s of The T a b l e c o n t a i n p h e n o c r y s t s o f amphibole, o r t h o p y r o x e n e , p l a g i o c l a s e , t i t a n o m a g n e t i t e , and r a r e c l i n o p y r o x e n e and o l i v i n e , whereas the p h e n o c r y s t assemblage of The B l a c k Tusk l a v a s c o n s i s t s o n l y o f p l a g i o c l a s e , c l i n o p y r o x e n e , orthopyroxene and t i t a n o m a g n e t i t e . The Mount P r i c e a n d e s i t e s p o s s e s s a s i m i l a r m i n e r a l o g y to t h a t of The T a b l e l a v a s , but a l s o c o n t a i n b i o t i t e and i l m e n i t e . T h i s c h a p t e r p r e s e n t s the d e t a i l s o f the m i n e r a l o g y o f the l a v a s . P h e n o c r y s t c o m p o s i t i o n s a r e e v a l u a t e d i n terms o f e x p e r i m e n t a l phase e q u i l i b r i a and suggested c r y s t a l f r a c t i o n a t i o n p r o c e s s e s . O l i v i n e Magnesian o l i v i n e s (Fo 0-, -.„) a r e found as p h e n o c r y s t s i n a l l o / — / U b a s a l t s and b a s a l t i c - a n d e s i t e s , and as a groundmass c o n s t i t u e n t i n some b a s a l t s . The p h e n o c r y s t s , u s u a l l y equant o r s l i g h t l y e l o n g a t e p a r a l l e l to the c - a x i s , a r e commonly e u h e d r a l but l o c a l l y rounded i n o u t l i n e due to magmatic c o r r o s i o n . S k e l e t a l c r y s t a l s , s i m i l a r to those d e s c r i b e d by Kuno (1950) from the l a v a s of Hakone v o l c a n o , a r e common ( F i g . 6 a ) . The o l i v i n e s f r e q u e n t l y c o n t a i n s u b h e d r a l to a n h e d r a l i n c l u s i o n s o f brown chromium-rich m a g n e t i t e ; some o l i v i n e s a r e m a r g i n a l l y a l t e r e d t o 82 i d d i n g s i t e . The D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e i s c h a r a c t e r i z e d by o l i v i n e p h e n o c r y s t s j a c k e t e d by a d i s t i n c t i v e c orona o f h y p e r s t h e n e p r i s m s , s u g g e s t i n g a r e a c t i o n r e l a t i o n between the o l i v i n e and t h e e n c l o s i n g l i q u i d . R e p r e s e n t a t i v e o l i v i n e a n a l y s e s a r e l i s t e d i n Appendix I I I and p l o t t e d i n F i g . 15 i n terms o f atomic p e r c e n t Ca, Fe, and Mg t o g e t h e r w i t h the c o - e x i s t i n g f e r r o m a g n e s i a n phases. Both p h e n o c r y s t i c and groundmass c r y s t a l s e x h i b i t "normal" z o n i n g of a s i m i l a r magnitude , t h e groundmass c r y s t a l s b e i n g r i c h e r i n i r o n ( F o , c c £ ) . No i n s t a n c e o f DJ-JO r e v e r s e z o n i n g i s known. The o l i v i n e s from b a s a l t i c - a n d e s i t e s (F°gy 77) a r e c o n s i s t e n t l y more magnesian than those o f t h e Cheakamus V a l l e y b a s a l t s 55)• I n v a r i a b l y , t h e more magnesian o l i v i n e s c o ^ e x i s t w i t h an i f o n - . r i c h . .amphibole; '.the;.most magnesian c r y s t a l s (Fo 0., Q / ) o/ —OH i d e n t i f i e d o c c u r as i n c l u s i o n s i n a p a r g a s i t i c h o r n b l e n d e megacryst from the Sphinx Moraine b a s a l t i c - a n d e s i t e ( F i g . 22b). The c o m p o s i t i o n o f t h e o l i v i n e i s c o n s i s t e n t w i t h an o r i g i n as a r e l i c t o r x e n o c r y s t i c phase from a more m a f i c i p a r e n t a l magma. Comparison w i t h o l i v i n e s o f S a n t o r i n i v o l c a n o ( N i c h o l l s , 1971), Mount S h a s t a (Anderson, 1974; Smith and C a r m i c h a e l , 1968), and t h e A l e u t i a n I s l a n d lavas. -(Marsh, 1974) s h o w s f s i m i l a r . c b n c e n t r a t i b n s f o f NiO, MnO, and CaO i n the G a r i b a l d i Lake o l i v i n e s ; NiO tends to be zoned i n a s i m i l a r manner to magnesium. Both CaO and MnO become p r o g r e s s i v e l y e n r i c h e d i n t h e more f a y a l i t i c zones. The Ca2SiO^ component has been shown to d e c r e a s e w i t h p r e s s u r e and i n c r e a s e w i t h temperature (Stormer, 1973). The r e l a t i v e l y s m a l l v a r i a t i o n i n the CaO c o n c e n t r a t i o n o f o l i v i n e s from the Helm Creek l a v a and t h e Enostuck b a s a l t i c - a n d e s i t e may i n d i c a t e i n i t i a l o l i v i n e c r y s t a l l i z a t i o n a t v e r y s h a l l o w l e v e l s , whereas the s l i g h t l y i n c r e a s i n g c a l c i u m c o n t e n t o f o l i v i n e from the Cheakamus 83 V a l l e y b a s a l t s , and The T a b l e , D e s o l a t i o n V a l l e y and Sphinx Moraine b a s a l t i c - a n d e s i t e s may i n d i c a t e t h e dominance o f a p r e s s u r e - d r o p w i t h t h e commencement o f o l i v i n e c r y s t a l l i z a t i o n a t a deeper l e v e l d u r i n g magmatic a s c e n t . The p o s i t i v e c o r r e l a t i o n between i r o n and manganese c o n t e n t , on the o t h e r hand, may be e x p l a i n e d as a s t r u c t u r a l e f f e c t where i n c r e a s i n g f e r r o u s i r o n i n the o l i v i n e a l l o w s g r e a t e r manganese s u b s t i t u t i o n ( S i m k i n and Smith, 1970). However, as i r o n - r i c h s i l i c a t e l i q u i d s i n n a t u r e a l s o tend to be e n r i c h e d i n manganese, i t seems more l i k e l y t h a t t h e s i m i l a r c h e m i c a l c h a r a c t e r i s t i c s o f t h e s e elements d u r i n g d i f f e r e n t i a t i o n may account f o r the o b s e r v e d v a r i a t i o n (Roeder, 1974). Pyroxene P a l e green to p a l e b r o w n i s h - g r e e n p h e n o c r y s t s o f a d i o p s i d i c a u g i t e , o f t e n w i t h a weak p l e o c h r o i s m , a r e p r e s e n t i n t h e b a s a l t s and b a s a l t i c - a n d e s i t e s , and i n most o f .the ; a n d e s i t e s o f t h e G a r i b a l d i Lake a r e a . Orthopyroxene p h e n o c r y s t s , f r e q u e n t l y found as p r i s m a t i c c r y s t a l s bounded by b r o a d p i n a c o i d a l and comparably narrow p r i s m a t i c f a c e s , a r e p r e s e n t i n a l l b a s a l t i c - a n d e s i t e s and a n d e s i t e s . Both a u g i t e and o r t h o p y r o x e n e o c c u r as a c o n s t i t u e n t o f the groundmass i n a l l l a v a s except some h o r n b l e n d e - a n d e s i t e s from Mount P r i c e and h y p e r s t h e n e -a n d e s i t e s from The B l a c k Tusk, where h y p e r s t h e n e i s the o n l y groundmass p y r o x e n i c phase. R e p r e s e n t a t i v e pyroxene a n a l y s e s a r e p r e s e n t e d i n Appendix I I I ; e s t i m a t e d s i t e o c c u p a n c i e s d e r i v e d a c c o r d i n g to the method of Wood and Banno (1973), and pyroxene end-member c o m p o s i t i o n s c a l c u l a t e d a c c o r d i n g to the p r o c e d u r e s o u t l i n e d by Cawthorn and C o l l e r s o n (1974) a r e a l s o l i s t e d . C ompositions o f a n a l y z e d pyroxenes i n terms o f atomic p e r c e n t 84 Ca, Fe and Mg a r e shown i n F i g . 15; i n some c a s e s , d a t a from s i m i l a r r o c k s have been combined. In g e n e r a l , the c l i n o p y r o x e n e s show a l i m i t e d c o m p o s i t i o n a l range ( d i o p s i d e , e n d i o p s i d e , s a l i t e and a u g i t e i n the c l a s s i f i c a t i o n o f Deer and o t h e r s , 1966). The orth o p y r o x e n e s ( b r o n z i t e s and h y p e r s t h e n e s ) show z o n i n g o f a g r e a t e r magnitude than the c o - e x i s t i n g c l i n o p y r o x e n e s . Both normal and r e v e r s e z o n i n g o f ortho p y r o x e n e may be p r e s e n t . Cheakamus V a l l e y B a s a l t s B a s a l t s o f the Cheakamus R i v e r v a l l e y c o n t a i n o n l y a c a l c i u m -r i c h pyroxene. The a u g i t e s a r e d i o p s i d i c , p l o t t i n g h e a r the magnesian end o f the Skaergaard t r e n d (Brown, 1957). The dominant c a t i o n s u b s t i t u t i o n i n the pyroxenes i s i r o n f o r c a l c i u m , a l t h o u g h on the s c a l e o f a s i n g l e c r y s t a l Fe-Ca, Fe-Mg, o r Ca-Mg s u b s t i t u t i o n may be p r e s e n t , as i n o t h e r o r o g e n i c s u i t e s (Smith and C a r m i c h a e l , 1968; Lowder, 1970). A u g i t e s from the Brandywine F a l l s b a s a l t (71) g e n e r a l l y e x h i b i t g r e a t e r Fe-Mg s u b s t i t u t i o n than the d i o p s i d i c a u g i t e s df e i t h e r the Cheakamus Dam (36) o r C a l l a g h a n (435) l a v a s . The most s t r i k i n g f e a t u r e s of the pyroxene a n a l y s e s a r e t h e i r moderately, h i g h T i 0 2 (1.34-2.47 p e r c e n t ) and Na 20 (0.32-0.89 p e r c e n t ) , and v a r i a b l e Al^O^ (2.55-5.56 p e r c e n t ) c o n t e n t s . The T i c o n t e n t s o f the pyroxenes tends t o be i n v e r s e l y p r o p o r t i o n a l t o the t i t a n i u m c o n c e n t r a t i o n i n the b a s a l t s . A l t h o u g h somewhat v a r i a b l e , A l g e n e r a l l y d e c r e a s e s and T i i n c r e a s e s w i t h i n c r e a s i n g i r o n c o n t e n t i n the pyroxenes from the Cheakamus Dam and Brandywine F a l l s b a s a l t s . By c o n t r a s t , pyroxenes from the C a l l a g h a n b a s a l t show i n c r e a s i n g A l and T i w i t h i n c r e a s i n g i r o n c o n t e n t . 85 Coarse, p a l e brown s u b h e d r a l c l i n o p y r o x e n e s a r e p r e s e n t i n a t i t a n o m a g n e t i t e - r i c h s e g r e g a t i o n v e i n o f a Cheakamus Dam b a s a l t . These a u g i t e s , l i k e the p h e n o c r y s t s o f the b a s a l t , show h i g h T i O ^ and Al^ O ^ c o n t e n t s , and t h e i r T i / A l r a t i o s approach 1:2 ( F i g . 16) The amount o f A l and T i e n t e r i n g c l i n o p y r o x e n e i s p r i m a r i l y a r e f l e c t i o n of the s i l i c a a c t i v i t y o f the magma (Verhoogen, 1962) as shown by the r e a c t i o n s : C a A l o S i o 0 o = C a A l o S i 0 ' + S i 0 o , Z Z O Z D . Z and, . CaAl„SiCV'+ T i 0 o = CaTiAl„0- + SiO.. 2 6 2 2 6 2 The c o n d i t i o n s under which the s e g r e g a t i o n v e i n pyroxenes c r y s t a l l i z e d , however, may be more im p o r t a n t than magma c o m p o s i t i o n i n d e t e r m i n i n g whether o r n o t a T i - r i c h pyroxene w i l l be p r e c i p i t a t e d . B a r b e r i and o t h e r s (1971) o b s e r v e d t h a t a t low p r e s s u r e s Al^O^ may be p r e f e r e n t i a l l y removed from an u n d e r s a t u r a t e d l i q u i d by f r a c t i o n a t i o n o f p l a g i o c l a s e ; the pyroxenes would not be t i t a n i f e r o u s as they would c o n t a i n l i t t l e a l u m i n a . On the o t h e r hand, a h i g h P would a l l o w pyroxene t o c r y s t a l l i z e b e f o r e p l a g i o c l a s e , and make Al^O^ a v a i l a b l e f o r s u b s t i t u t i o n i n t o the pyroxene. The h i g h m o l e c u l a r p r o p o r t i o n of C a A l ^ S i O ^ and CaTiAl„0 £ (up to 10 p e r c e n t ) i n the s e g r e g a t i o n v e i n pyroxenes 2 O and the r e l e g a t i o n o f p l a g i o c l a s e t o the l a t t e r s t a g e s o f c r y s t a l l i z a t i o n may i n d i c a t e t h a t the pyroxenes c r y s t a l l i z e d under h i g h P c o n d i t i o n s . 2 The C i n d e r Cone Both l a v a s o f The C i n d e r Cone c o n t a i n a h i g h - c a l c i u m pyroxene; o n l y the D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e c o n t a i n s a 86 F i g u r e 15. Compositions o f p y r o x e n e s , o l i v i n e s and amphiboles i n G a r i b a l d i Lake l a v a s , p l o t t e d i n terms o f Ca, Mg, and Fe (atom % ) . F i l l e d and h a l f - f i l l e d c i r c l e s , c l i n o p y r o x e n e s ; f i l l e d t r i a n g l e s , amphiboles; open c i r c l e s , o r t h o p y r o x e n e ; open s q u a r e s , o l i v i n e . Samples r e p r e s e n t i n g Cheakamus V a l l e y b a s a l t s a r e : 36-7, Cheakamus Dam b a s a l t and i t s s e g r e g a t i o n v e i n (28-6); 71-1, Brandywine F a l l s b a s a l t ; 435, C a l l a g h a n b a s a l t . U n s p e c i f i e d Mount P r i c e l a v a s a r e : 536, P r i c e Bay Cone; 565, Summit Lava; 380, C u l l i t o n Creek a n d e s i t e . S o l i d l i n e s r e p r e s e n t the pyroxene t r e n d s from the Skaergaard i n t r u s i o n (Brown, 1957). 88 c o - e x i s t i n g o r t h o p y r o x e n e . P h e n o c r y s t i c d i o p s i d i c a u g i t e from the D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e show l i t t l e v a r i a t i o n i n Fe c o n t e n t , d e s p i t e a dominant Fe-Mg s u b s t i t u t i o n . The maximum v a r i a t i o n i n f e r r o s i l i t e c o n t e n t i s about 5 p e r c e n t , u n l i k e t h e t h o l e i i t i c h i g h - c a l c i u m pyroxene c r y s t a l l i z a t i o n t r e n d o f T h i n g m u l i v o l c a n o , I c e l a n d ( C a r m i c h a e l , 1967a); t h e a u g i t e o f the groundmass has a c o m p o s i t i o n a l range s i m i l a r to t h a t o f t h e p h e n o c r y s t s . Orthopyroxene (En 0 , c ) , b o t h as m i c r o p h e n o c r y s t s and as / O - D J r e a c t i o n rims about o l i v i n e , shows a sy m p a t h e t i c i n c r e a s e i n c a l c i u m w i t h i r o n . C l i n o p y r o x e n e o c c u r s as p h e n o c r y s t s and as equant g r a i n s i n t h e m e s o s t a s i s g l a s s o f the Helm Creek l a v a (412, 451). The c a l c i u m -r i c h pyroxenes t r e n d towards h e d e n b e r g i t e i n the pyroxene q u a d r i l a t e r a l ( F i g . 15) i n c o n t r a s t to t r e n d s o f d e c r e a s i n g c a l c i u m c o n t e n t i n a u g i t e s from t h o l e i i t i c r o c k s o f T h i n g m u l i v o l c a n o , I c e l a n d ( C a r m i c h a e l , 1967a) and the Skaergaard i n t r u s i o n (Brown, 1957). S e c t i o n s c u t p a r a l l e l t o [010] o f the p h e n o c r y s t s show s e c t o r - z o n i n g ( F i g . 6 c ) , whereas o n l y c o n c e n t r i c z o n i n g i s ob s e r v e d i n s e c t i o n s p a r a l l e l to [100]. The [100] s e c t o r s o f a l l c r y s t a l s c o n t a i n h i g h e r T i , A l ^ , and lower S i and Mg than the [ i l l ] s e c t o r s ; Ca and Na show no c o n s i s t e n t v a r i a t i o n . Such z o n a t i o n i s common i n pyroxenes o f a l k a l i n e l a v a s ( S t r o n g , 1969), and i n d i r e c t c o n t r a s t to t h o l e i i t i c pyroxenes which show s e c t o r - z o n i n g i n Ca/(Mg + Fe) (Nakamura and Coombs, 3+ 3+ 1973). Verhoogen (1962) has shown t h a t Fe and T i a r e more s t a b l e O | ^ j than Fe and T i i n t h e pyroxene l a t t i c e . Thus, t h e [lOO] s e c t o r s a r e l i k e l y to c o n t a i n s i g n i f i c a n t l y h i g h e r c o n c e n t r a t i o n s o f 89 3+ 3+ T i and Fe . Both A l and T i i n c r e a s e w i t h i n c r e a s i n g Fe c o n t e n t 2+ s u g g e s t i n g t h a t the dominant s u b s t i t u t i o n s a r e : R + S i = A l + A l , 2+ and R + 2 S i = T i + 2A1. F i g . 16 demonstrates t h a t A ^ O ^ has e n t e r e d the c l i n o p y r o x e n e s as the CaAl„SiCi m o l e c u l e , u n l i k e the z b t i t a n i f e r o u s pyroxenes o f the Cheakamus V a l l e y b a s a l t s . The s e c t o r - z o n e d a u g i t e s r e p r e s e n t quenched p r o d u c t s of a r a p i d d i s e q u i l i b r i u m c r y s t a l l i z a t i o n a t h i g h temperature and r e l a t i v e l y low p r e s s u r e . The dominance o f Ca-Tschermaks-type s u b s t i t u t i o n s i n t h e s e pyroxenes must i n d i c a t e t h a t e i t h e r the s i l i c a a c t i v i t y o f the magma remained n e a r l y c o n s t a n t d e s p i t e the l a r g e - s c a l e v a r i a t i o n i n major element c h e m i s t r y e x h i b i t e d by the Helm Creek l a v a (Chapter I I I ) , o r t h a t h i g h P c o n d i t i o n s p r e v a i l e d d u r i n g c r y s t a l l i z a t i o n . Sphinx Moraine The Sphinx Moraine b a s a l t i c - a n d e s i t e (598) c o n t a i n s b o t h a c a l c i u m - r i c h pyroxene and a n . o r t h o p y r o x e n e . The d i o p s i d i c a u g i t e s a r e s i m i l a r to those of the D e s o l a t i o n V a l l e y l a v a , except t h a t they show g r e a t e r Ca-Mg s u b s t i t u t i o n . Orthopyroxene (En-,„ ,-„)> /y-oo b o t h as a p h e n o c r y s t and as a groundmass phase, shows i n c r e a s i n g c a l c i u m w i t h i r o n . The r a r e o r t h o p y r o x e n e o c c u r r i n g i n the groundmass i s m o s t l y low-alumina h y p e r s t h e n e , a l t h o u g h the c o m p o s i t i o n s d e t e r m i n e d by m i c r o p r o b e a n a l y s i s i n d i c a t e the p r e s e n c e o f p i g e o n i t e . Because of t h i s pyroxene's s m a l l g r a i n - s i z e , however, i t i s not p o s s i b l e t o c o n f i r m o p t i c a l l y whether or not p i g e o n i t e i s p r e s e n t . Aluminous b r o n z i t e (En^,., 4.74 p e r c e n t A^O^) c o m p o s i t i o n s are r e c o r d e d i n the c o r e o f some p h e n o c r y s t s ; Duggan and W i l k i n s o n (1973) 90 F i g u r e 16. T i v e r s u s A l on the b a s i s o f 6 oxygens f o r pyroxenes of b a s a l t i c l a v a s . S o l i d c i r c l e s , Cheakamus Dam s e g r e g a t i o n v e i n ; open s q u a r e s , Helm Creek l a v a . S o l i d l i n e s r e p r e s e n t v a r i o u s r a t i o s o f T i : A l ; dashed l i n e s a r e r e g r e s s i o n l i n e s f o r a n a l y t i c a l d a t a . 9 1 i 1 1 1 r i i i i • 0.10 0.20 0.30 Al on the basis of 6 O 92 d e s c r i b e d s i m i l a r aluminous b r o n z i t e s i n a h i g h - a l u m i n a t h o l e i i t i c a n d e s i t e to which they a s c r i b e d a h i g h - p r e s s u r e o r i g i n . E nostuck Meadows The Enostuck b a s a l t i c - a n d e s i t e c o n t a i n s two pyroxenes, d i o p s i d i c a u g i t e and r a r e b r o n z i t e . The c l i n o p y r o x e n e c o m p o s i t i o n s a r e i n f l u e n c e d by Fe-Mg and Ca-Mg s u b s t i t u t i o n s , and p r e s e n t an i l l - d e f i n e d t r e n d towards the h e d e n b e r g i t e c o r n e r o f the pyroxene q u a d r i l a t e r a l ( F i g . 15). The Ca-poor pyroxene i s r e s t r i c t e d t o the groundmass and r e a c t i o n rims about o l i v i n e ; i t appears to show a l i m i t e d c o m p o s i t i o n a l range ( E n o n -,,-), but the a v a i l a b l e oU— / J d a t a i s i n s u f f i c i e n t t o c o n f i r m such a t r e n d . The T a b l e The m a j o r i t y of l a v a s from The T a b l e c o n t a i n b o t h c l i n o -pyroxene and o r t h o p y r o x e n e . The a u g i t e o c c u r s p r i m a r i l y as equant g r a i n s i n the groundmass; t h e i r s m a l l g r a i n - s i z e p r e c l u d e s m i c r o -probe a n a l y s i s . The a u g i t e m i c r o p h e n o c r y s t s , however, a r e remarkably homogeneous i n t h e i r c o m p o s i t i o n , p l o t t i n g c o n s i s t e n t l y a t one d i s c r e t e p o i n t i n the pyroxene q u a d r i l a t e r a l . A u g i t e i s a l s o p r e s e n t as r e a c t i o n rims about rounded q u a r t z x e n o c r y s t s ; the pyroxene of the r e a c t i o n coronas i s d i s t i n c t l y more c a l c i c than the c o - e x i s t i n g m i c r o p h e n o c r y s t s . P h e n o c r y s t s and groundmass g r a i n s of orthopyroxene (En^g_^^) i n The T a b l e a n d e s i t e s (637, 667) a r e c h a r a c t e r i z e d by low c a l c i u m c o n t e n t s (0.39-1.00 p e r c e n t ) ; o n l y the i r o n - r i c h c o m p o s i t i o n s approach the Skaergaard t r e n d . The o r t h o p y r o x e n e s (En^,._^) of the T a b l e Meadows a n d e s i t e s (644, 649, 650) a r e c o m p o s i t i o n a l l y s i m i l a r t o those o f The 93 T a b l e a n d e s i t e s , except t h a t they show h i g h e r Ca (0.88-1.70 p e r c e n t ) and Mn (0.70-1.20 p e r c e n t ) c o n c e n t r a t i o n s . Ewart (1971) observed t h a t h y p e r s t h e n e s c o - e x i s t i n g w i t h amphibole have lower c a l c i u m c o n t e n t s than h y p e r s t h e n e s c o - e x i s t i n g w i t h a u g i t e . As the o r t h o p y r o x e n e of b o t h l a v a sequences c o - e x i s t w i t h amphibole, i t i s more p r o b a b l e t h a t t h e i r d i f f e r e n t Ca c o n t e n t s r e f l e c t d i f f e r e n c e s i n c r y s t a l l i z a t i o n temperatures (Ewart and o t h e r s , 1976). TheeBlack Tusk Most o f The B l a c k Tusk l a v a s c o n t a i n o n l y a c a l c i u m - p o o r pyroxene; o n l y the Microwave B l u f f a n d e s i t e s and the o l d e r p l a t y a n d e s i t e (610) of the West B l u f f c o n t a i n b o t h a u g i t e and o r t h o p y r o x e n e . The c l i n o p y r o x e n e s o f The B l a c k Tusk a n d e s i t e s a r e s i m i l a r t o those o f the D e s o l a t i o n V a l l e y l a v a ; the maximum v a r i a t i o n i n f e r r o s i l i t e i s 7 p e r c e n t . The range of o r t h o p y r o x e n e c o m p o s i t i o n s i s from En^g t o En^^; the h y p e r s t h e n e of the groundmass has a c o m p o s i t i o n s i m i l a r t o t h a t o f the p h e n o c r y s t s . B r o n z i t e s o f the West B l u f f a n d e s i t e s (613, 620, 624, 628) a r e c h a r a c t e r i z e d by lower CaO (0.53-1.53 p e r c e n t ) and h i g h e r MnO (up t o 0.86 p e r c e n t ) than the c o r r e s p o n d i n g pyroxenes of the Summit Lava (602, 608) or the p l a t y a n d e s i t e s . S e v e r a l o f the West B l u f f a n d e s i t e s c o n t a i n a pyroxene w i t h s u b - c a l c i c c o m p o s i t i o n . I t was not p o s s i b l e t o determine whether these c r y s t a l s r e p r e s e n t a homogeneous c a l c i u m - p o o r c l i n o p y r o x e n e o r an aggregate of f i n e a u g i t e - o r t h o p y r o x e n e i n t e r g r o w t h s . F i e s i n g e r (1975) d e s c r i b e d s i m i l a r " f i b r o u s " pyroxenes from the R i n g Creek l a v a (30 km to south) f o r which G a n d o l f i photographs f a i l e d to i n d i c a t e the p r e s e n c e o f a m o n o c l i n i c pyroxene. 94 Mount P r i c e A l l l a v a s o f Mount P r i c e c o n t a i n two pyroxenes, a l t h o u g h i n many the c a l c i u m - r i c h phase i s r e s t r i c t e d t o the groundmass; a welded-ash (542) c o n t a i n s m i c r o p h e n o c r y s t s o f o r t h o p y r o x e n e , but no groundmass p y r o x e n i c phase. Only the B a r r i e r a n d e s i t e (531, 584) c o n t a i n s c l i n o p y r o x e n e l a r g e enough f o r m i c r o p r o b e a n a l y s i s . The s a l i t e s show the g r e a t e s t v a r i a t i o n i n f e r r o s i l i t e c o n t e n t (about 10 p e r c e n t ) i n any a n d e s i t i c l a v a of the G a r i b a l d i Lake a r e a ; these pyroxenes p r e s e n t a p o o r l y d e f i n e d t r e n d r o u g h l y p a r a l l e l to the magnesian end o f the Skaergaard t r e n d ( F i g . 15), but a r e more c a l c i c i n c o m p o s i t i o n (Wo^g_^). The c o m p o s i t i o n a l range of the Mount P r i c e o r t h o p y r o x e n e s ( E n ^ _ g ^ ) i s r e p r e s e n t e d i n a l l l a v a s . A u g i t e , h y p e r s t h e n e (En^g) and p l a g i o c l a s e r e p r e s e n t breakdown p r o d u c t s o f amphibole i n most l a v a s ; h y p e r s t h e n e f r e q u e n t l y o c c u r s as r e a c t i o n rims around amphibole ( F i g . 22a) and b i o t i t e ( F i g . 2 2 f ) , s u g g e s t i n g a r e a t i o n r e l a t i o n s h i p between amphibole o r b i o t i t e and the e n c l o s i n g l i q u i d . In g e n e r a l , the c l i n o p y r o x e n e of the breakdown •. p r o d u c t s e x h i b i t h i g h e r MgO and Al^O^, and the o r t h o p y r o x e n e s , lower CaO and A^O^, and h i g h e r FeO and MnO c o n t e n t s than the c o - e x i s t i n g groundmass and m i c r o p h e n o c r y s t phases. D i s c u s s i o n The pyroxenes o f the G a r i b a l d i Lake l a v a s a r e m a i n l y d i o p s i d i c a u g i t e and b r o n z i t e - h y p e r s t h e n e . Both pyroxenes a r e p r e s e n t i n the b a s a l t i c - a n d e s i t e s and a n d e s i t e s , but c l i n o p y r o x e n e i s the o n l y p y r o x e n i c phase i n the Cheakamus V a l l e y b a s a l t s and the Helm Creek l a v a . P i g e o n i t e and s u b - c a l c i c a u g i t e o f the "quench" t r e n d (Muir and T i l l e y , 1967) have 95 not been p o s i t i v e l y i d e n t i f i e d . The c o r e s o f c l i n o p y r o x e n e p h e n o c r y s t s i n the b a s a l t i c - a n d e s i t e s and b a s a l t s have h i g h A l c o n t e n t s (up t o 5.5 p e r c e n t ) , s u g g e s t i n g t h a t they c r y s t a l l i z e d under d i f f e r e n t c o n d i t i o n s than the c o - e x i s t i n g m i c r o p h e n o c r y s t s . The a n a l y z e d c l i n o p y r o x e n e s o f the a n d e s i t e s show v e r y l i m i t e d range of F e - c o n t e n t , i n c o n t r a s t t o the wide range o f F e - c o n t e n t i n the h i g h - c a l c i u m pyroxenes of t h o l e i i t i c magmas. L i m i t e d range o f Fe i n pyroxenes o f c a l c - a l k a l i n e a n d e s i t e s i s c o n s i d e r e d to i n d i c a t e f a c t o r s c o n t r o l l i n g i r o n - e n r i c h m e n t i n c a l c - a l k a l i n e magmas, such as h i g h oxygen f u g a c i t y (Fodor, 1971), or c r y s t a l l i z a t i o n under hydrous c o n d i t i o n s . Best and Mercy (1967) sug g e s t e d t h a t the l i m i t e d F e-enrichment i n c a l c i u m - r i c h pyroxenes from hydrous, d i f f e r e n t i a t e d m a f i c magmas r e s u l t s when the magmas l e a v e the pyroxene s t a b i l i t y f i e l d w h i l e o n l y m o d e r a t e l y e n r i c h e d i n Fe, and amphibole or b i o t i t e become the dominant f e r r o m a g n e s i a n phases. Aluminous b r o n z i t e (4.7 p e r c e n t Al^O^) has been r e c o r d e d i n the cor e s o f some ortho p y r o x e n e p h e n o c r y s t s i n the b a s a l t i c - a n d e s i t e s . D.H. Green and Ringwood's (1967) e x p e r i m e n t a l s t u d i e s suggest t h a t c r y s t a l s o f t h ese c o m p o s i t i o n s c o u l d have e q u i l i b r a t e d w i t h b a s a l t i c l i q u i d s a t about 9 kb. I f t h i s i s c o r r e c t , the aluminous orthopyroxenes i n the G a r i b a l d i Lake l a v a s a r e x e n o c r y s t s . P l a g i o c l a s e P l a g i o c l a s e i s u b i q u i t o u s as a groundmass, p h e n o c r y s t , and x e n o c r y s t phase i n most l a v a s from the G a r i b a l d i Lake a r e a . Groundmass f e l d s p a r o c c u r s as s u b h e d r a l l a t h s o r d i s t i n c t m i c r o l i t e s w i t h i n a p a l e brown i n t e r s t i t i a l g l a s s . P l a g i o c l a s e p h e n o c r y s t s range up to 5 mm i n 96v s i z e , and p o s s e s s s u b h e d r a l . t o e u h e d r a l o u t l i n e s . Some l a v a s c o n t a i n both, c o m p o s i t i o n a l l y homogeneous (unzoned) and zoned p h e n o c r y s t s ; the zoned c r y s t a l s f r e q u e n t l y show a rounded i n c l u s i o n - r i c h o r s e i v e - t e x t u r e d core surrounded by actnarrowcconcenttiGally7:ZQn6drr.lm-.-t--0ther'plagioclase p h e n o c r y s t s e x h i b i t a homogeneous c o r e s e p a r a t e d from t h e i r zoned margin by e i t h e r a c l o u d y zone o f g l a s s i n c l u s i o n s o r a sharp boundary. The g l a s s i n c l u s i o n s a r e abundant i n p l a n a r a r r a y s o r as s m a l l c l u s t e r s . The i n c l u s i o n s range from 5 t o 150 micr o n s i n l e n g t h , and a r e e i t h e r e l o n g a t e o r s p h e r i c a l i n form; most o f the i n c l u s i o n s c o n t a i n a vapor bubble w i t h a d i a m e t e r o f about 1/3 t h a t o f the e n c l o s i n g m e l t i n c l u s i o n . A ggregates o f s e i v e - t e x t u r e d p h e n o c r y s t s a l s o e n c l o s e p o c k e t s o f g l a s s which now appear as i n t e r s t i t i a l a r e a s i n the g l o m e r o c r y s t s , and were p r o b a b l y formed by t r a p p i n g of c o - e x i s t i n g m e l t between c o a l e s c i n g g r a i n s . L a r g e e u h e d r a l t o s u b h e d r a l c r y s t a l s (up to 1 cm) o f p l a g i o c l a s e a r e t h e major c o n s t i t u e n t of cognate i n c l u s i o n s i n the a n d e s i t i c l a v a s ; t h e s e f e l d s p a r s , i n c o n t r a s t t o the c o - e x i s t i n g p h e n o c r y s t s r a r e l y show e x t e n s i v e z o n i n g or c o n t a i n i n c l u s i o n - r i c h zones. S e i v e - t e x t u r e d p l a g i o c l a s e p h e n o c r y s t s a r e c h a r a c t e r i s t i c of l a v a s from o r o g e n i c r e g i o n s . I t has been su g g e s t e d t h a t the i n c l u s i o n - r i c h zones o r c o r e s r e p r e s e n t i n c o m p l e t e r e s o r p t i o n o f e a r l y - f o r m e d p h e n o c r y s t s , which broke down i n t o a more c a l c i c f e l d s p a r and g l a s s ; r e s o r p t i o n p r o b a b l y r e s u l t s from c r y s t a l - m e l t r e - e q u i l i b r a t i o n as the magma r i s e s i n the c r u s t (Macdonald and K a t s u r a , 1965; Vance, 1965). Mathews (1957) su g g e s t e d t h a t the co r e s o f i n c l u s i o n - c h a r g e d p h e n o c r y s t s might r e p r e s e n t f e l d s p a r s d e r i v e d from p a r t l y a s s i m i l a t e d fragments o f q u a r t z d i o r i t e , but where s a t i s f a c t o r y a n a l y s i s o f the i n c l u s i o n - r i c h c o r e s i s p o s s i b l e , t h e i r c o m p o s i t i o n s do not d i f f e r 97 s i g n i f i c a n t l y from those o f . p l a g i o c l a s e i n the cognate i n c l u s i o n s . R e s u l t s o f m i c r o p r o b e a n a l y s e s a r e p r e s e n t e d i n F i g . 17; r e p r e s e n t a t i v e a n a l y s e s are g i v e n i n Appendix I I I . The p h e n o c r y s t s a r e p l a g i o c l a s e w i t h c o m p o s i t i o n s v a r y i n g from A n ^ to An^,.. Zoning i s g e n e r a l l y normal.and s t r o n g e s t near the c r y s t a l margins; the outermost r i m of a l l p h e n o c r y s t s c o r r e s p o n d s i n c o m p o s i t i o n w i t h the a s s o c i a t e d groundmass f e l d s p a r s , s u g g e s t i n g t h a t the zoned margin o f the p h e n o c r y s t s c r y s t a l l i z e d near o r a f t e r e x t r u s i o n . The most a n o r t h i t i c c o m p o s i t i o n s do not o c c u r i n the c o r e s of the i n c l u s i o n - r i c h p h e n o c r y s t s , but w i t h i n . o n e of the zones o u t s i d e the c o r e s . P h e n o c r y s t -c o r e s a r e r a t h e r homogeneous (about An,.,.) , and surrounded by a t h i n s t r o n g l y r e v e r s e d zone (up to 15 p e r c e n t An i n c r e a s e ) ; the r e v e r s e d zone commonly e n c l o s e s the i n c l u s i o n band. Groundmass p l a g i p e l a s e e x h i b i t s a wide c o m p o s i t i o n a l range (up to 50 p e r c e n t An), and as a consequence, t h e r e i s a l a r g e o v e r l a p i n c o m p o s i t i o n between p h e n o c r y s t and groundmass f e l d s p a r s . The groundmass p l a g i o c l a s e s , however, are g e n e r a l l y more s o d i c and p o t a s s i c i n c o m p o s i t i o n . Of the minor elements i n the f e l d s p a r s , o n l y i r o n and magnesium are p r e s e n t i n s i g n i f i c a n t .amounts. T o t a l i r o n ( r e p o r t e d as FeO) ranges from 0.1 to 1.6 p e r c e n t ; magnesium con t e n t v a r y s between 0.05 and 0.66 geesxum p e r c e n t , but shows no c o n s i s t e n t c o r r e l a t i o n w i t h i r o n c o n t e n t . The i r o n c o n c e n t r a t i o n s , a l t h o u g h h i g h e r than average, are;..within the range of p u b l i s h e d a n a l y s e s (Deer and o t h e r s , 1966). S i m i l a r Mg-contents have been r e p o r t e d i n l a b r a d o r i t e p h e n o c r y s t s from Snake R i v e r P l a i n l a v a s (Thompson, 1972) and i n s e c t o r - z o n e d microphenocrysts- from submarine b a s a l t s (Bryan, 1974). 98 F i g u r e 17. P l a g i o c l a s e c o m p o s i t i o n s (mole %) o f G a r i b a l d i Lake l a v a s . Cheakamus V a l l e y b a s a l t samples as i n F i g . 15; Mount P r i c e l a v a s a r e : 542, welded-ash; 565, Summit Lava. P r i c e Bay l a v a (536) i n c l u d e d w i t h C l i n k e r Peak l a v a s - 380, C u l l i t o n Creek a n d e s i t e ; 531 and 584, B a r r i e r a n d e s i t e . The s o l i d l i n e r e p r e s e n t s the l i m i t of t e r n a r y f e l d s p a r s o l i d s o l u t i o n (Smith and Mackenzie, 1958). 100 F i g u r e 18. Weight p e r c e n t t o t a l i r o n (as FeO) i n p l a g i o c l a s e v e r s u s mol. p e r c e n t a n o r t h i t e . Symbols r e p r e s e n t : s o l i d c i r c l e s , Cheakamus V a l l e y b a s a l t s ; open s q u a r e s , Helm Creek l a v a ; s o l i d s q u a r e s , Sphinx Moraine b a s a l t i c - a n d e s i t e ; h a l f - f i l l e d s q u a r e s , E n o s t u c k b a s a l t i c - a n d e s i t e ; c i r c l e d d o t s , D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e ; open c i r c l e s , The T a b l e ; open t r i a n g l e s , Mount P r i c e ; s o l i d t r i a n g l e s , The B l a c k Tusk. 101 Jo a> oo += o g o O CM 102 Most p l a g i o c l a s e c o m p o s i t i o n s i n The T a b l e , Mount P r i c e and The B l a c k Tusk a n d e s i t e s a r e d i s t i n c t l y p o o r e r i n i r o n than those o f the Cheakamus V a l l e y and Helm Creek b a s a l t s . The v a r i a t i o n o f t o t a l i r o n c o n t e n t w i t h a n o r t h i t e . c o n t e n t i n the f e l d s p a r s i s p r e s e n t e d i n F i g . 18. In a l l s u i t e s , the amount o f i r o n i n p l a g i o c l a s e i n c r e a s e s or remains n e a r l y c o n s t a n t as the a n o r t h i t e component d e c r e a s e s i n the i n t e r v a l An.,, to A n r c , and de c r e a s e s as a n o r t h i t e d e c r e a s e s below An C I,. 76 55 55 Ribbe and Smith (1966) a t t r i b u t e d the d e c r e a s e i n the i r o n c o n t e n t o b s e r v e d w i t h d e c r e a s i n g An c o n t e n t to d e c r e a s i n g temperature and the IV 3+ 2+ 2+ a v a i l a b i l i t y o f A l (Fe ) and.Ca (Fe. ) s i t e s i n the p l a g i o c l a s e . Charge b a l a n c e and s i t e occupancy c o n s i d e r a t i o n s r e q u i r e 2+ 3+ s u b s t i t u t i o n o f the type R !(R S).Si> 0 o i n the c a l c i c end-members, z o 2+ 2+ + 3+ and o f t h e typ e R R S i „ 0 o o r R R S i o 0 o i n the s o d i c end-members 2 o o o (Smith, 1974; Smith and S t e e l e , 1974). When Ca i s p l o t t e d as a f u n c t i o n o f A l , (Al+Fe+2Mg), (Si-Mg), (Si-Fe-Mg) and S i (Bryan, 1974), the a n a l y t i c a l d a t a i n d i c a t e "that the iron-magnesium s u b s t i t u t i o n i n most G a r i b a l d i Lake p l a g i o c l a s e s i s b e s t e x p l a i n e d i n terms o f d i v a l e n t Fe and Mg s u b s t i t u t i n g f o r A l i n a f o r m u l a u n i t o f the 2"L. 2+ 3+ type R' R S i o 0 o , a l t h o u g h some i r o n as Fe may a l s o s u b s t i t u t e 2 o d i r e c t l y f o r A l . The d i f f e r e n t amounts o f i r o n i n the p l a g i o c l a s e s , however, p r o b a b l y r e f l e c t the b u l k F e - c o n t e n t and the o x i d a t i o n s t a t e o f the p a r e n t a l magmas. Cheakamus V a l l e y B a s a l t s P l a g i o c l a s e p h e n o c r y s t s are s u b h e d r a l , 2 to 13 mm i n l e n g t h , and g e n e r a l l y have normal o s c i l l a t o r y z o n i n g . Some c r y s t a l s 103 e x h i b i t a s u b h e d r a l , s e i v e - t e x t u r e d core ( F i g . 19A) surrounded by a c o n t i n u o u s s u b h e d r a l to e u h e d r a l s o d i c r i m s ; c l o u d y zones are formed by minute i n c l u s i o n s o f o l i v i n e , a u g i t e , t i t a n o m a g n e t i t e and brown g l a s s . P l a g i o c l a s e i n the groundmass seldom exceed 0.2 mm i n s i z e and show the same c o m p o s i t i o n a l range as the rims of the p h e n o c r y s t s . The f e l d s p a r , c o m p o s i t i o n s i n the Cheakamus Dam b a s a l t (36-7) range from l a b r a d o r i t e (An ) i n the c o r e s o f p h e n o c r y s t s to c a l c i c - o l i g i o c l a s e ( A n O Q ) i n a s e g r e g a t i o n v e i n (28-6). Compositions as c a l c i c as A n ^ a r e found i n the Brandywine F a l l s (71-1) and C a l l a g h a n (435) b a s a l t s ( F i g . 17); z o n i n g i s normal, w i t h some r e v e r s a l s . There i s u s u a l l y a mantle of more s o d i c p l a g i o c l a s e ^n^^_25^ around the p h e n o c r y s t s . The groundmass f e l d s p a r s of the Brandywine F a l l s and C a l l a g h a n b a s a l t s tend to be more p o t a s s i c than those of the Cheakamus Dam b a s a l t s . The C i n d e r Cone P l a g i o c l a s e i s a major p h e n o c r y s t phase i n the D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e , and i n the lower p a r t s of the Helm Creek composite f l o w . P h e n o c r y s t s i n the D e s o l a t i o n V a l l e y l a v a (208,4410> '405, 557) tend to be e u h e d r a l , p o l y s y n t h e t i c a l l y twinned, and weakly zoned. The f e l d s p a r s e x h i b i t a c o m p o s i t i o n a l range from l a b r a d o r i t e (An,„) i n the p h e n o c r y s t s t o s o d i c - a n d e s i n e (An_,) i n r a r e groundmass Dj JO m i c r o l i t e s . The p l a g i o c l a s e s a r e v e r y low i n the o r t h o c l a s e component even though the c o m p o s i t i o n s of n o r m a t i v e f e l d s p a r components f o r a n a l y z e d r o c k s p l o t j u s t o u t s i d e the t w o - f e l d s p a r f i e l d i n the system 104 F i g u r e 19. P l a g i o c l a s e i n the G a r i b a l d i Lake l a v a s . (a) S e i v e - t e x t u r e d p l a g i o c l a s e p h e n o c r y s t w i t h narrow s o d i c r i m c o n t a i n e d i n groundmass o f p l a g i o c l a s e , o l i v i n e , a u g i t e , t i t a n o m a g n e t i t e and p a l e brown g l a s s ; Brandywine F a l l s b a s a l t . Approximate f i e l d o f view i s 2 mm by 3 mm. (b) E u h e d r a l , patchy-zoned p l a g i o c l a s e p h e n o c r y s t i n Sphinx Moraine b a s a l t i c - a n d e s i t e ; note c o r r o d e d r i m on t h i c k c o n c e n t r i c a l l y zoned margin. Approximate f i e l d of view i s 2 mm by 3 mm. (c) Cloudy p l a g i o c l a s e p h e n o c r y s t surrounded by t h i n c o n c e n t r i c a l l y zoned''rim, Mount-'Pfice^aridesite. '.Clouded c o r e c o n s i s t s o f minute i n c l u s i o n s o f b i o t i t e , amphibole, pyroxene and t i t a n o m a g n e t i t e . Approximate f i e l d o f view i s 2 mm by 3 mm. (d) C o n c e n t r i c a l l y - z o n e d p l a g i o c l a s e m i c r o p h e n o c r y s t ( c e n t e r ) and s e i v e - t e x t u r e d p l a g i o c l a s e p h e n o c r y s t , Mount P r i c e a n d e s i t e . S e i v e - t e x t u r e d p h e n o c r y s t i s rimmed by a t h i n s o d i c zone. Approximate f i e l d o f view i s 2 mm by 3 mm. (e) Quench groundmass p l a g i o c l a s e i n West B l u f f a n d e s i t e o f The B l a c k Tusk. Note n e a r l y complete " b e l t - b u c k l e " growth form ( c e n t e r ) . Approximate f i e l d o f view i s 0.4 mm by 0.6 mm. ( f ) Quench groundmass p l a g i o c l a s e as ( e ) . Note s m a l l m i c r o p h e n o c r y s t ( c e n t e r ) c o n s i s t i n g o f aggregate of " b e l t - b u c k l e " m i c r o l i t e s , and i s o l a t e d " h o u r - g l a s s " g r a i n s . Approximate f i e l d o f view i s 0.4 mm by 0.6 mm. 105 106 An-Ab-Or The 0.5 to 2 mm p l a g i o c l a s e p h e n o c r y s t s o f t h e Helm Creek f l o w (412, 451) have e i t h e r s i n g l e o r m u l t i p l e l a m e l a r t w i n s . P e n e t r a t i o n o r " c r u c i f o r m " twins o f p l a g i o c l a s e a r e common i n th e lower p a r t o f the l a v a . Most f e l d s p a r s a r e s u b h e d r a l to e u h e d r a l , t a b u l a r c r y s t a l s which show normal z o n i n g . The p h e n o c r y s t c o m p o s i t i o n s range from A n ^ t o An^^, and show o n l y a s l i g h t o v e r l a p w i t h those of the groundmass m i c r o l i t e s ( A n^^_^g). Sphinx Moraine The Sphinx Moraine b a s a l t i c - a n d e s i t e (598) c o n t a i n s p l a g i o c l a s e p h e n o c r y s t s which show a s e i v e - o r honeycomb-textured (Kuno, 1950) c o r e (An,.,) surrounded by a s u b h e d r a l o s c i l l a t o r y -6 / normal zoned r i m ( P i g . 19b). These p l a g i o c l a s e s c o n t a i n the most c a l c i c c o m p o s i t i o n s (An^^) y e t found i n a l a v a from the G a r i b a l d i Lake a r e a ( F i g . 17). Su b h e d r a l to e u h e d r a l , a l b i t e - t w i n n e d m i c r o p h e n o c r y s t s show a d i s c o n t i n u o u s o u t e r zone o f N a - r i c h c o m p o s i t i o n ( A n ^ ) ; t h i s zone has a c i c u l a r p r o j e c t i o n s t h a t t y p i c a l l y o c c u r a t the ends o f the e l o n g a t e , t a b u l a r c r y s t a l s . L o f g r e n (1974) produced s i m i l a r p r o j e c t i o n s e x p e r i m e n t a l l y by r a p i d l y quenching p l a g i o c l a s e l i q u i d s (a 100°C change i n t e m p e r a t u r e ) . The r a p i d quenching o f the Sphinx Moraine l a v a i s a l s o i n d i c a t e d by groundmass l a t h s showing " b e l t - b u c k l e " and " h o u r - g l a s s " growth forms. Enostuck Meadows P l a g i o c l a s e i s found as b o t h a p h e n o c r y s t and a groundmass 107 c o n s t i t u e n t o f the b a s a l t i c - a n d e s i t e ; b o t h p h e n o c r y s t and groundmass f e l d s p a r s e x h i b i t a p p r e c i a b l e z o n i n g (up to 30 mole p e r c e n t An). The c l o u d y p h e n o c r y s t s and g l o m e r o c r y s t s o f p l a g i o c l a s e show rounded to i r r e g u l a r , weakly zoned c o r e s (An^^_^^) which a r e i n t e r p r e t e d as c r y s t a l s formed a t depth; r e s o r p t i o n o f t h e s e f e l d s p a r s produced p a t c h y z o n i n g s i m i l a r t o t h a t d e s c r i b e d by Vance (1965). Some p h e n o c r y s t s have a s m a l l r e v e r s a l i n z o n i n g , c r y s t a l l i z i n g A n ^ to An^ 3> f o l l o w i n g r e s o r p t i o n . The groundmass p l a g i o c l a s e c o m p o s i t i o n s ( A n ^ , ^ ^ ) o v e r l a p the p h e n o c r y s t - r i m c o m p o s i t i o n s . The T a b l e F e l d s p a r i n The T a b l e l a v a s c o n s i s t o f : (1) l a r g e glomero-p o r p h y r i t i c c l o t s , welded t o g e t h e r by a common r i m zone, (2) i s o l a t e d , s u b h e d r a l p h e n o c r y s t s showing i n c l u s i o n - r i c h c o r e s o r o u t e r zones, and (3) s u n h e d r a l l a t h s and m i c r o l i t e s i n t h e groundmass. Twinning o f p l a g i o c l a s e f o l l o w s a l b i t e and C a r l s b a d laws; o s c i l l a t o r y z o n i n g i s u s u a l but the o v e r a l l c o m p o s i t i o n a l d i f f e r e n c e s between a d j a c e n t zones i s s m a l l . The c r y s t a l s f r e q u e n t l y have a l a r g e unzoned o r s l i g h t l y zoned c o r e w i t h a c o m p o s i t i o n between An^,_ and An,.,., and a more c o n s p i c u o u s l y zoned b o r d e r r a n g i n g to a c o m p o s i t i o n o f about An^Q a t the margin. Groundmass f e l d s p a r s a r e a n d e s i n e to a n o r t h o c l a s e i n c o m p o s i t i o n ( F i g . 17); a l k a l i f e l d s p a r i s p r e s e n t i n t h e groundmass o f some o f The T a b l e a n d e s i t e s (637, 667). In g e n e r a l , p l a g i o c l a s e o f The T a b l e a n d e s i t e s show a narrower c o m p o s i t i o n a l range, and a r e d i s t i n c t l y more p o t a s s i c than the u n d e r l y i n g T a b l e Meadows a n d e s i t e s (644, 649, 650). 108 The B l a c k Tusk A l l l a v a s o f The B l a c k Tusk c o n t a i n minor p l a g i o c l a s e phenocrysts-. Some p h e n o c r y s t s show e v i d e n c e o f undulose e x t i n c t i o n ; d u s t i n c l u s i o n s i n the p l a g i o c l a s e c o n s i s t o f f i n e l y d i v i d e d p a r t i c l e s o f a u g i t e , o r t h o p y r o x e n e , t i t a n o m a g n e t i t e and g l a s s , o f t e n e l o n g a t e p a r a l l e l t o the c l e a v a g e t r a c e s o f the c r y s t a l s . . I n c l u s i o n s a r e p r e s e n t i n the co r e o f the h o s t f e l d s p a r o r i n a d e f i n i t e zone i n i t s o u t e r p a r t . I n the l a t t e r case, the o u t e r margin o f the i n c l u s i o n - r i c h zone p a r a l l e l s the o u t l i n e o f " t h e f e l d s p a r , w h i l e the i n n e r p a r t f o l l o w s f r a c t u r e s c u t t i n g - t h e p h e n o c r y s t - c o r e . These p a t c h y zoned c r y s t a l s r e p r e s e n t e a r l y -formed f e l d s p a r , whereas e u h e d r a l , twinned m i c r o p h e n o c r y s t s which show no r e s o r p t i o n a r e i n t e r p r e t e d as f e l d s p a r c r y s t a l l i z e d c o m p l e t e l y a t upper c r u s t a l l e v e l s . Groundmass p l a g i o c l a s e o f the " a n c e s t r a l mountain" l a v a s (610) are p r e s e n t as s u b h e d r a l l a t h s a s s o c i a t e d w i t h i n t e r s t i t i a l m a g n e t i t e anhedra and p a l e brown g l a s s . On the o t h e r hand, groundmass p l a g i o c l a s e of the younger h y p e r s t h e n e - a n d e s i t e s (613, 620, 628, 602, 608) show complex growth forms. The c r y s t a l s a r e c h a r a c t e r i s t i c a l l y e l o n g a t e i n the a - a x i s d i r e c t i o n , w i t h h o l l o w c o r e s p a r a l l e l t o " a " f i l l e d by brown g l a s s ( F i g . 19e) ; these f e l d s p a r s p o s s e s s " h o u r - g l a s s " and " s w a l l o w - t a i l e d " forms ( F i g . 1 9 f ) . P l a g i o c l a s e c r y s t a l s viewed i h s e c t i o n c u t p e r p e n d i c u a l r t o the a - a x i s show a c h a r a c t e r i s t i c h o l l o w , r e c t a n g u l a r " b e l t - b u c k l e " form. Bryan (1972, 109 1974) s u g g e s t e d t h a t the development of s i m i l a r s k e l e t a l forms i n submarine b a s a l t s i s r e s t r i c t e d to s u p e r c o o l e d , h i g h l y v i s c o u s magma i n which r e l a t i v e l y r a p i d c r y s t a l growth i s accompanied by low r a t e s o f d i f f u s i o n . The range i n p h e n o c r y s t c o m p o s i t i o n s i s c o n t i n u o u s from l a b r a d o r i t e (An ) to Na-andesine((An ) . Groundmass p l a g i o c l a s e DO 3D began c r y s t a l l i z i n g An^,-_^, but some l a t h s a r e zoned t o An^^; m i c r o l i t e s of a l k a l i - f e l d s p a r may be p r e s e n t i n the groundmass of the Summit L a v a (602, 608). Mount P r i c e The l a v a s o f Mount P r i c e commonly c o n t a i n two types of p h e n o c r y s t s : (1) p l a g i o c l a s e w i t h rounded, p a t c h y zoned c o r e s surrounded by e u h e d r a l to s u b h e d r a l o s c i l l a t o r y zoned rims ( F i g . 1 9 c ) , and (2) e u h e d r a l o s c i l l a t o r y and normal zoned c r y s t a l s which do n o t show r e s o r p t i o n t e x t u r e s ( F i g . 19d). E u h e d r a l , zoned t o weakly zoned, p o l y s y n t h e t i c a l l y twinned m i c r o p h e n o c r y s t s and groundmass l a t h s sometimes e x h i b i t a h o l l o w c o r e s u g g e s t i n g s u p e r c o o l i n g of the l a v a s . The Mount P r i c e p l a g i o c l a s e s show a wide c o m p o s i t i o n a l range ( F i g . 17), but o n l y the p h e n o c r y s t s of the B a r r i e r a n d e s i t e (531, 584) a r e c h a r a c t e r i z e d by c o m p o s i t i o n s more c a l c i c than A n ^ . The absence of c a l c i c - p l a g i o c l a s e s (>:' An^^~) i n the C u l l i t o n Creek l a v a (380), which e r u p t e d contemporaneously w i t h the B a r r i e r f l o w from C l i n k e r Peak (Chapter I I ) , s u g g e s t s t h a t the former l a v a r e p r e s e n t s a l i q u i d tapped a t a d i f f e r e n t l e v e l i n the d i f f e r e n t i a t e d magma column. '.jiss 110 D i s c u s s i o n The modal dominance o f p l a g i o c l a s e p h e n o c r y s t s i n most l a v a s i n d i c a t e s t h a t the f e l d s p a r i s a l i q u i d u s phase at low p r e s s u r e s . The predominant c o m p o s i t i o n a l range o f the p h e n o c r y s t s i s l a b r a d o r i t e t o a n d e s i n e ; the a n d e s i t e s , however, tend to c o n t a i n more a n o r t h i t i c p l a g i o c l a s e c o m p o s i t i o n s t h a t the b a s a l t s . A l t h o u g h some c r y s t a l s show o s c i l l a t o r y - n o r m a l z o n i n g , most p l a g i o c l a s e s i n the a n d e s i t i c l a v a s e x h i b i t c o n t i n u o u s normal z o n i n g p a t t e r n s . Marsh (1974) n o t e d t h a t under w a t e r - s a t u r a t e d c o n d i t i o n s , a n e a r l y i s o t h e r m a l d e c r e a s e o f p r e s s u r e d u r i n g c r y s t a l l i z a t i o n o f p l a g i o c l a s e s h o u l d produce a c o n t i n u o u s l y normal zoned c r y s t a l . A s e m i - q . u a n t i t a t i v e method of. e s t i m a t i n g p r e - e r u p t i o n water p r e s s u r e s p r e v a i l i n g i n the G a r i b a l d i Lake a n d e s i t e magmas i s p r o v i d e d by the p l a g i o c l a s e geothermometer (Kudo and W e i l l , 1970) which i s s e n s i t i v e t o water c o n c e n t r a t i o n . By comparing the p l a g i o c l a s e p h e n o c r y s t c r y s t a l l i z a t i o n temperatures c a l c u l a t e d u s i n g the 0.5, 1.0, and 5.0 kb P u e q u a t i o n s of Kudo and W e i l l w i t h the c o r r e s p o n d i n g pyroxene e q u i l i b r a t i o n temperatures ( T a b l e X ) , agreement i s o b t a i n e d at water p r e s s u r e s i n the range 3.1-4.1 kb f o r the a n d e s i t e s o f The T a b l e and Mount P r i c e , and 3.5-5.7 kb f o r the b a s a l t i c - a n d e s i t e s . There i s a c o m p o s i t i o n a l d i f f e r e n c e between the average p h e n o c r y s t and the average groundmass f e l d s p a r s , but the a n a l y t i c a l d a t a i n d i c a t e a l a r g e o v e r l a p between p h e n o c r y s t s and groundmass g r a i n s . As the p h e n o c r y s t c o r e s have a r e s t r i c t e d range i n c o m p o s i t i o n (An,-Q_gi-), the c o m p o s i t i o n a l o v e r l a p has presumably r e s u l t e d from the c r y s t a l l i z a t i o n of b o t h groundmass f e l d s p a r and p h e n o c r y s t - r i m s a t I l l t he s u r f a c e . Amphibole P a l e brown to greenish-brown p l e o c h r o i c h o r n b l e n d e i s u b i q u i t o u s as p h e n o c r y s t s i n a l l l a v a s from the G a r i b a l d i Lake a r e a except the Cheakamus V a l l e y b a s a l t s , the D e s o l a t i o n V a l l e y b a s a l t i c -a n d e s i t e , and some h y p e r s t h e n e - a n d e s i t e s o f the second e r u p t i v e c y c l e a t The B l a c k Tusk; s i m i l a r amphibole i s a c o n s t i t u e n t o f s p o r a d i c a l l y d i s t r i b u t e d cognate x e n o l i t h s i n the a n d e s i t e s of Mount P r i c e and The T a b l e , and the Sphinx Moraine b a s a l t i c - a n d e s i t e . Rare c o r r o d e d g r a i n s o f i n t e n s e l y p l e o c h r o i c h o r n b l e n d e w i t h X = p a l e y e l l o w , Y = orange, and Z = r u s s e t brown a r e p r e s e n t i n the upper p a r t o f the Helm Creek composite f l o w . The p h e n o c r y s t s , g e n e r a l l y p o s s e s s i n g a s u b h e d r a l to e u h e d r a l a c i c u l a r or b l a d e form, may r e a c h up to 15 mm but average about 2 mm i n l e n g t h . Hornblende i n d l k t y t a x i t i c and h y a l o - o p h i t i c i n c l u s i o n s o c c u r s as d i s c r e t e p r isms i n an open mesh w i t h p l a g i o c l a s e , pyroxene and m a g n e t i t e ( ( F i g . 20d), o r as s l e n d e r n e e d l e s t h a t cut h a p h a z a r d l y a c r o s s a l l o t h e r c o n s t i t u e n t s ; l o c a l l y , i t i s p r e s e n t i n medium to c o a r s e e q u i g r a n u l a r aggregates w i t h p l a g i o c l a s e , pyroxene and ma g n e t i t e ( F i g . 20e and f ) . The amphiboles i n The T a b l e x e n o l i t h s c o n t a i n minute i n c l u s i o n s o f c l i n o p y r o x e n e ; p r i s m a t i c megacrysts i n the Sphinx Moraine b a s a l t i c - a n d e s i t e p o i k -i l i t i c a l l y e n c l o s e ragged g r a i n s o f c l i n o p y r o x e n e and f o r s t e r i t i c o l i v i n e ( F i g . 22b). The i r r e g u l a r c o n t a c t s o f the i n c l u d e d g r a i n s s u g g e s t s t h a t they r e p r e s e n t p r i m a r y c r y s t a l s p a r t l y r e p l a c e d by amphibole through r e a c t i o n o f the e a r l y - f o r m e d m i n e r a l s w i t h the l i q u i d . Without e x c e p t i o n , the p h e n o c r y s t i c and x e n o l i t h i c h o r n -112 F i g u r e 20. Amphibole i n the G a r i b a l d i Lake l a v a s . (a) M i c r o p h e n o c r y s t o f r u s s e t brown b a s a l t i c h o r n b l e n d e surrounded by a narrow r i m of f i n e m a g n e t i t e and g l a s s , Mount P r i c e . Approximate f i e l d o f view i s 1 mm by 2 mm. (b) Twinned, e u h e d r a l m i c r o p h e n o c r y s t of p a l e brown h o r n b l e n d e ( c e n t e r ) i n welded-ash, Mount P r i c e . Approximate f i e l d o f view i s 1 mm by 2 mm. (c) Magnetite-pseudomorphed p h e n o c r y s t o f p a l e brown hor n b l e n d e c o - e x i s t i n g w i t h a c i c u l a r b a s a l t i c h o r n b l e n d e ( r i g h t ) ; b a s a l t i c h o r n b l e n d e shows o n l y p e r i p h e r a l r e s o r p t i o n e f f e c t s . Approximate f i e l d o f view i s 1 mm by 2 mm. (d) H y a l o - o p h i t i c c l o t o f magnetite-pseudomorphed p a l e brown h o r n b l e n d e , p l a g i o c l a s e , t i t a n o m a g n e t i t e , o r t h o p y r o x e n e and g l a s s i n The T a b l e a n d e s i t e . Approximate f i e l d o f view i s 2 mm by 3 mm. (e) C o a r se, e q u i g r a n u l a r aggregate of amphibole, p l a g i o c l a s e t i t a n o m a g n e t i t e , o r t h o p y r o x e n e and g l a s s i n T a b l e Meadows a n d e s i t e . Note development o f opaque r i m on h o r n b l e n d e , where i t i s i n c o n t a c t w i t h groundmass. Approximate f i e l d o f view i s 2 mm by 3 mm. ( f ) E q u i g r a n u l a r a g g r e g a t e o f h o r n b l e n d e , a u g i t e , o l i v i n e , p l a g i o c l a s e and m a g n e t i t e . Sphinx Moraine b a s a l t i c -a n d e s i t e . Approximate f i e l d of view i s 2 mm by 3 mm. 113 114 b l e n d e s d i s p l a y some degree o f r e s o r p t i o n and r e c r y s t a l l i z a t i o n . L a r g e c r y s t a l s a r e u s u a l l y surrounded by a 0.1 to 0.5 mm opaque margin; the opaque r i m i s always t h i c k e r where the hor n b l e n d e i s i n c o n t a c t w i t h the i n t e r s t i t i a l g l a s s than where the m i n e r a l abuts a g a i n s t o t h e r c r y s t a l s . I n the advance s t a g e s o f r e a c t i o n , the amphiboles a r e e i t h e r pseudomorphed by m a g n e t i t e ( F i g . 2 0 c ) , sometimes rimmed by a r e a c t i o n corona o f a c i c u l a r h y p e r s t h e n e ( F i g . 22a), or r e p l a c e d by a f i n e i n t e r g r o w t h o f magnesian a u g i t e , h y p e r s t h e n e , p l a g i o c l a s e and t i t a n o m a g n e t i t e , The l a t t e r r e l a t i o n s h i p has o f t e n been i n t e r p r e t e d as r e c r y s t a l l i z a t i o n due t o l o w e r i n g o f P upon e x t r u s i o n or h i g h -l e v e l i n t r u s i o n . A l t e r n a t i v e l y , Boyd (1959) has shown t h a t i n the pr e s e n c e o f excess s i l i c a a t temperatures and water p r e s s u r e s above 900°C and 1 kb, p a r g a s i t e breaks down to an assemblage of d i o p s i d e , e n s t a t i t e and l a b r a d o r i t e . M i c r o p h e n o c r y s t s of deep r e d to y e l l o w brown b a s a l t i c h o r n b l e n d e a r e a l s o common i n the l a v a s o f Mount P r i c e and The T a b l e . The a c i c u l a r o r b l a d e - f o r m c r y s t a l s f r e q u e n t l y show s i m p l e twins p a r a l l e l to (100) ( F i g . 20b), and weak z o n i n g i n d i c a t e d by s m a l l changes i n e x t i n c t i o n a n g l e . The m i c r o p h e n o c r y s t s , u n l i k e c o -e x i s t i n g p h e n o c r y s t s , e x h i b i t o n l y p e r i p h e r a l r e s o r p t i o n e f f e c t s ( F i g . 20b and c ) . In some l a v a s , s l e n d e r n e e d l e s o f b a s a l t i c h o r n b l e n d e have a l s o d e v e l o p e d a l o n g the margin o f p h e n o c r y s t s pseudomorphed by m a g n e t i t e . These r e l a t i o n s h i p s suggest t h a t two g e n e r a t i o n s o f amphibole a r e p r e s e n t , and t h a t the m i c r o p h e n o c r y s t s c r y s t a l l i z e d as i n c o n g r u e n t breakdown p r o d u c t s o f the p h e n o c r y s t s . The c r y s t a l l i z a t i o n o f i n t r a t e l l u r i c h o rnblende can o c c u r when water i s t r a p p e d by r a p i d c r y s t a l l i z a t i o n c r e a t i n g h i g h e r than normal 115 water p r e s s u r e ( F i s k e and o t h e r s , 1963); the r e l e a s e o f vapor p r e s s u r e a f t e r e x t r u s i o n would e x p l a i n the absence o f hornblende m i c r o l i t e s . The c h e m i s t r y of s e l e c t e d amphiboles i s g i v e n i n Appendix I I I w i t h the r e l a t i v e s t r u c t u r a l formulae c a l c u l a t e d on a b a s i s of 24 oxygens. As n e i t h e r the o x i d a t i o n s t a t e o f the i r o n , n or the amount of water i n these amphiboles can be determined by m i c r o p r o b e , the 3+ a n a l y s e s a r e n e c e s s a r i l y i n c o m p l e t e , and Fe and water c o n t e n t s , r e s p e c t i v e l y , have been c a l c u l a t e d by charge b a l a n c e c o n s i d e r a t i o n s and by d i f f e r e n c e . In view of the d i v e r s i t y i n c h e m i s t r y o f the e n c l o s i n g l a v a s (Chapter I I I ) , the amphiboles a r e remarkably u n i f o r m i n c o m p o s i t i o n . The s i m i l a r i t y i n t h e i r Ca, Mg, and Fe c o n t e n t s i s demonstrated by p l o t t i n g t h e i r c o m p o s i t i o n s i n the pyroxene q u a d r i l a t e r a l ( F i g . 15). 3+ G e n e r a l l y , the hor n b l e n d e s a r e aluminous w i t h c l o s e t o 2A1 s u b s t i t u t i n g i n the t e t r a h e d r a l s i t e ; most a r e s u b s i l i c i c , w i t h 2 to 10 p e r c e n t n o r m a t i v e n e p h e l i n e . Some z o n i n g of the c o n v e n t i o n a l Fe-Mg type i s p r e s e n t ( F i g . 15), but s t r o n g z o n i n g o f S i , A l , and T i predominates. The co r e s o f the p a l e brown ho r n b l e n d e s have h i g h e r Mg, T i , and A l , whereas the rims have h i g h e r Fe and S i c o n t e n t s . On the b a s i s o f Leake's (1968) c l a s s i f i c a t i o n , the amphibol a r e c a l c i f e r o u s (> 1.50 Ca per 24 oxygens) and f a l l i n the f e r r o a n p a r g a s i t e - t s c h e r m a k i t e - magnesio ho r n b l e n d e range ( F i g . 21a). Some a n a l y s e s c o n t a i n s u f f i c i e n t t i t a n i u m (> 0.25 T i p e r 24 oxygens) to j u s t i f y the p r e f i x t i t a n i f e r o u s . The a n a l y s e s r e v e a l i n c r e a s i n g s u b s t i t u t i o n o f Na + K but not t r i v a l e n t c a t i o n s w i t h i n c r e a s i n g [ A 1 I V ] c o n t e n t ( F i g . 21b and 22c) ; the r e s t r i c t e d range o f ( [ A 1 V I ] + 3+ T i + Fe ) atoms shown i n F i g . 22c, however, may be m o d i f i e d i f t h e r e 116 F i g u r e 21. Compositions o f a n a l y z e d amphiboles i n terms o f : (a) ; S i ^ and (Na + Ca + K) atoms per f o r m u l a u n i t ( H a l l i m o n d Diagram), and (b) and ( c ) , the number of ( A 1 V I + F e 3 + + T i ) , (Na + K ) , and ( A 1 I V ) atoms per f o r m u l a u n i t ( a f t e r Deer and o t h e r s , 1966). Dashed l i n e i n (a) e n c l o s e s the f i e l d o f amphiboles produced exp e rimen t a l l y f r om lme'llt sof) fa Jiy.dro u sp >p"er.M'o.t^t e (Mysen and B o e t t c h e r , 1975b). Symbols r e p r e s e n t : f i l l e d s q u a r e s , Sphinx Moraine b a s a l t i c - a n d e s i t e ; h a l f - f i l l e d c i r c l e s , L o g g i n g Road a n d e s i t e (2 km n o r t h of The B l a c k T u s k ) ; h a l f - f i l l e d s q u a r e s , Enostuck b a s a l t i c - a n d e s i t e ; open t r i a n g l e s , Mount P r i c e ; open c i r c l e s , The T a b l e ; open s q u a r e s , Helm Creek l a v a . 117 - i — i — i — 7 - ] — i — i i — i — | — i — i r T S C H E R M A K I T E P A R G A S I T E 0 . 4 k l - T R E M O L I T E J I I I I I I (W J I I I 0.0 0 . 4 0.8 1.2 (Na+K) atoms T — r ~ i — i — I — i — i — i — " — | — i — i -I- P A R G A S I T E T S C H E R M A K I T E 0 . 0 0 .8 C O 6." 2.00 (Alvl+Fe3++Ti) atoms (a) NO +C0 118 has been more e x t e n s i v e o x i d a t i o n of f e r r o u s i r o n than e s t i m a t e d . Jakes and White (1972) c h a r a c t e r i z e d h o r n b l e n d e s i n c a l c -a l k a l i n e a n d e s i t e s from i s l a n d - a r c and c o n t i n e n t a l - m a r g i n r e g i o n s . They found t h a t h o r n b l e n d e s i n i s l a n d - a r c a n d e s i t e s have lower Fe/Mg r a t i o s , s l i g h t l y h i g h e r Na + K v a l u e s and h i g h e r t o t a l aluminum c o n t e n t s ( A l ^ + A l ^ ) , and a r e a p p r e c i a b l y more aluminous ( A l ^ > 1*5) tham those of c o n t i n e n t a l l a v a s ( A l ^ = 1.0-1.5). A l t h o u g h the c o m p o s i t i o n o f p h e n o c r y s t s i n the G a r i b a l d i Lake a n d e s i t e s conforms to the c r i t e r i a of Jakes and White f o r i s l a n d -a r c amphiboles, the c o - e x i s t i n g m i c r o p h e n o c r y s t s o f b a s a l t i c h ornblende i n the Mount P r i c e l a v a s p o s s e s s c h a r a c t e r i s t i c s o f c o n t i n e n t a l -type h o r n b l e n d e s . T h i s d i f f e r e n c e i n c o m p o s i t i o n between p h e n o c r y s t and m i c r o p h e n o c r y s t h o r n b l e n d e s s t r o n g l y s u p p o r t s the p e t r o g r a p h i c e v i d e n c e f o r two e p i s o d e s o f amphibole c r y s t a l l i z a t i o n . The appearance of c l i n o p y r o x e n e b e f o r e amphibole on the l i q u i d u s o f i s l a n d - a r c a n d e s i t e s l e d Jakes and White (1972) to suggest t h a t i s l a n d - a r c type h o r n b l e n d e s p r e c i p i t a t e from magmas t h a t have h i g h e r l i q u i d u s temperatures and lower i n i t i a l water c o n t e n t s than t h e i r c o n t i n e n t a l c o u n t e r p a r t s . In an analogous f a s h i o n , the d i f f e r e n c e s between p h e n o c r y s t and m i c r o p h e n o c r y s t c o m p o s i t i o n s i n the Mount P r i c e l a v a s may r e p r e s e n t d i f f e r e n t c o n d i t i o n s of c r y s t a l l i z a t i o n . The p h e n o c r y s t s p r o b a b l y c r y s t a l l i z e d under h i g h e r temperatures and lower P _ H 2 ° c o n d i t i o n s than the c o - e x i s t i n g m i c r o p h e n o c r y s t s . E x p e r i m e n t a l m e l t i n g s t u d i e s on hydrous b a s a l t i c c o m p o s i t i o n s a t 5 and 10 kb have produced c a l c i f e r o u s h o r n b l e n d e s s i m i l a r to p h e n o c r y s t s i n the G a r i b a l d i Lake l a v a s . The e x p e r i m e n t a l l y produced amphiboles, however, tend to be lower i n S i and T i but h i g h e r i n 119 Ca and A l , r e f l e c t i n g s l i g h t l y v a r y i n g c o n d i t i o n s o f temperature, p r e s s u r e , water a c t i v i t y and oxygen f u g a c i t y d u r i n g c r y s t a l l i z a t i o n . Holloway. and F o r d (1973) have shown t h a t s u b s t i t u t i o n o f F f o r OH i n the amphibole s t r u c t u r e i n c r e a s e s b o t h i t s temperature and p r e s s u r e s t a b i l i t y . The s m a l l c o n c e n t r a t i o n o f f l u o r i n e (0.3-0.4 p e r c e n t ) i n the G a r i b a l d i Lake h o r n b l e n d e s i s p o s s i b l y s u f f i c i e n t to s t a b i l i z e them to temperatures 20-30°C above f l u o r i n e - f r e e amphiboles (950-1020°C). M i c a M i c a i s t y p i c a l l y found i n Mount P r i c e a n d e s i t e s as a n h e d r a l p l a t e s up to 4 mm i n d i a m e t e r , o f t e n p o i k i l i t i c a l l y e n c l o s i n g s m a l l g r a i n s o f i l m e n i t e and p l a g i o c l a s e ( F i g . 2 2 c ) ; the mica shows i n t e n s e p l e o c h r o i s m from p a l e y e l l o w t o r e d brown. I s o l a t e d p h e n o c r y s t s a r e p a r t l y r e s o r b e d and rimmed by a c o a r s e r e a c t i o n corona ( F i g . 22d and e) which shows the r e l a t i o n : M i c a + m e l t -> h o r n b l e n d e + F e - T i o x i d e + p l a g i o c l a s e + o r t h o p y r o x e n e . In c o n t r a s t , o t h e r p l a t e s show a r e a c t i o n r i m o f c o l o u r l e s s o r t h o p y r o x e n e . Rare m i c a anhedra a l s o o c c u r w i t h i n fhe T a b l e Meadows a n d e s i t e s . Some h i g h l y r e s o r b e d c r y s t a l s a r e p o i k i l i t i c a l l y e n c l o s e d by p l a g i o c l a s e , whereas o t h e r g r a i n s e x h i b i t a c o r r o d e d m a r g i n overgrown by s u b h e d r a l o r t h o p y r o x e n e , p l a g i o c l a s e and t i t a n o m a g n e t i t e ( F i g . 2 2 f ) . P a r t i a l d e c o m p o s i t i o n of the m i c a p r o b a b l y took p l a c e above the a l k a l i - f e l d s p a r l i q u i d u s so t h a t the r e l e a s e d K A l S i o 0 o component r e a c t e d w i t h the s u r r o u n d i n g g l a s s t o y i e l d p l a g i o c l a s e ( E u g s t e r and Wones, 1962). R e p r e s e n t a t i v e m i c r o p r o b e a n a l y s e s f o r the Mount P r i c e and The T a b l e micas a r e g i v e n i n Appendix I I I and p l o t t e d i n F i g . 23. The 120 F i g u r e 22. R e a c t i o n r e l a t i o n s h i p s i n G a r i b a l d i Lake l a v a s . (a) Magnetite-pseudomorphed hornblende rimmed by a f i n e c o r o n a o f a c i c u l a r o r t h o p y r o x e n e , T a b l e Meadows a n d e s i t e . Groundmass c o n s i s t s o f p l a g i o c l a s e m i c r o l i t e s , equant pyroxene, m a g n e t i t e anhedra, and p a l e brown g l a s s . Approximate f i e l d of view i s 2 mm by 3 mm. (b) Amphibole megacryst rimmed by t h i n opaque margin. Note i n c l u d e d g r a i n s o f f o r s t e r i t i c o l i v i n e i n i n t e r i o r o f amphibole, and s u b h e d r a l o r t h o p y r o x e n e i n a l t e r e d r i m . „Sphinx Moraine b a s a l t i c - a n d e s i t e . Approximate f i e l d o f view i s 2 mm by 3 mm. (c) A n h e d r a l p l a t e o f r e d brown b i o t i t e i n B a r r i e r a n d e s i t e , Mount P r i c e . B l a c k i n c l u s i o n s are i l m e n i t e . Approximate f i e l d o f view i s 1 mm by 2 mm. (d) A n h e d r a l b i o t i t e s u rrounded by c o a r s e r e a c t i o n r i m o f amphibole, p l a g i o c l a s e , o r t h o p y r o x e n e and t i t a n o m a g n e t i t e . Note rounded o u t l i n e p o s s i b l y due to magmatic c o r r o s i o n . B a r r i e r a n d e s i t e , Mount P r i c e . Approximate f i e l d of view i s 1 mm by 1.5 mm. (e) Corroded b i o t i t e ( b l a c k ) as ( d ) , but l a c k i n g rounded o u t l i n e ; Summit Lava , Mount P r i c e . Approximate f i e l d o f view i s 1 mm by 1.5 mm. ( f ) Corroded p h l o g o p i t e ( b l a c k , c e n t e r ) w i t h overgrowth o f s u b h e d r a l o r t h o p y r o x e n e w i t h minor p l a g i o c l a s e and t i t a n o m a g n e t i t e , The T a b l e . Approximate f i e l d o f view i s 1 mm by 1.5 mm. 121 122 s t r u c t u r a l formulae are c a l c u l a t e d on the anhydrous b a s i s o f 22 oxygens i n the absence of v a l u e s f o r the r e s p e c t i v e f l u o r o h y d r o x y l groups. The Y-groups (5.69-5.84) and X-groups (1.88-2.01) a r e s l i g h t l y lower than e x p e c t e d a f t e r the Z-groups a r e made up to 8.00. These d e f i c i e n c i e s would be compensated f o r , or a t l e a s t reduced, 3+ by the c o n t r i b u t i o n o f undetermined elements, p a r t i c u l a r i l y Fe and Ba. A l t h o u g h b o t h groups of micas a r e c h a r a c t e r i z e d by S i / A l r a t i o s o f about 5.69:2.31 i n t h e i r t e t r a h e d r a l . . s i t e s , the Mg/Fe r a t i o s o f The T a b l e micas, g r e a t e r than 4:1, i n d i c a t e p h l o g o p i t e c o m p o s i t i o n s , whereas the Mount P r i c e m i c a s , w i t h Mg/Fe about 2.2:1, i s b i o t i t e r a t h e r than p h l o g o p i t e (Fig.7.23). The T a b l e p h l o g o p i t e s a r e low i n T i and Na, and the Mount P r i c e b i o t i t e s a r e low i n T i but h i g h i n Na when compared w i t h b i o t i t e from Mt. L a s s e n V o l c a n i c N a t i o n a l Park, C a l i f o r n i a (Carmichael-, 1967b). U n l i k e the Mt. L a s s e n b i o t i t e s , the G a r i b a l d i Lake micas c o n t a i n s u f f i c i e n t A l to f i l l the t e t r a h e d r a l s i t e s to e i g h t w i t h S i . The Mount P r i c e b i o t i t e s e x h i b i t some degree of z o n i n g , becoming T i - p o o r and i r o n - r i c h towards the margin, but the range i n c o m p o s i t i o n i s not g r e a t . In c o n t r a s t , The T a b l e p h l o g o p i t e s show weak r e v e r s e z o n i n g w i t h magnesium i n c r e a s i n g towards the r i m . Wones and E u g s t e r (1965) found t h a t the Fe/Mg r a t i o o f mica f a l l s s l i g h t l y d u r i n g c r y s t a l l i z a t i o n i n the system b i o t i t e -s a n a d i n e - m a g n e t i t e - l i q u i d i f the p a r t i a l p r e s s u r e of oxygen remains c o n s t a n t . The T a b l e p h l o g o p i t e s a r e e n r i c h e d i n T i , Al,j.and K, but d e p l e t e d i n Na when compared w i t h the Mount P r i c e b i o t i t e s . Carman (1974) showed e x p e r i m e n t a l l y t h a t the p r e s e n c e o f sodium i n 123 p h l o g o p i t e w i l l d e c r e a s e i t s s t a b i l i t y . R u t h e r f o r d (1969) a l s o demonstrated t h a t 19 mole p e r c e n t sodium-annite reduces the maximum s t a b i l i t y of a n n i t e i n e q u i l i b r i u m w i t h a l k a l i - f e l d s p a r and q u a r t z by 85°C a t 2 kb. In c o n t r a s t , the s u b s t i t u t i o n o f T i i n t o p h l o g o p i t e i n c r e a s e s i t s temperature s t a b i l i t y (Forbes and Flower, 1974). The c h e m i s t r y o f The T a b l e p h l o g o p i t e s , c o u p l e d w i t h these e x p e r i m e n t a l s t u d i e s , s u g g e s t s t h a t these micas c r y s t a l l i z e d a t h i g h e r temperatures than the Mount P r i c e b i o t i t e s . I r o n - T i t a n i u m Oxides P r e c i p i t a t i o n of an o x i d e phase has o c c u r r e d a t an i n t e r m e d i a t e o r l a t e s t a g e i n the c r y s t a l l i z a t i o n h i s t o r y o f n e a r l y a l l l a v a s s t u d i e d . E u h e d r a l t o s u b h e d r a l t i t a n o m a g n e t i t e o c c u r s as i n c l u d e d g r a i n s i n s i l i c a t e p h e n o c r y s t s , d i s c r e t e p h e n o c r y s t s o r m i c r o p h e n o c r y s t s , and groundmass g r a n u l e s i n the Helm Creek composite f l o w , a l l b a s a l t i c - a n d e s i t e s and most o f the a n d e s i t e s ; the g-phase i s a minor c o n s t i t u e n t o f cognate x e n o l i t h s i n the Sphinx Moraine b a s a l t i c - a n d e s i t e , and some l a v a s o f The T a b l e , Mount P r i c e and The B l a c k Tusk. I n c o n t r a s t , t i t a n o m a g n e t i t e i s r e s t r i c t e d t o the l a t e s t a g e s o f c r y s t a l l i z a t i o n i n 'the Cheakamus V a l l e y b a s a l t s , where i t i s found as equant g r a i n s i n t h e groundmass o r as d e n d r i t i c t o s k e l e t a l c r y s t a l s i n t h e m e s o s t a s i s o f c o a r s e s e g r e g a t i o n v i e n s . E x s o l u t i o n l a m a l l a e a r e r a r e l y o b s e r v e d i n t h e t i t a n o m a g n e t i t e s , but m a r t i t - i z a t i o n o r h e m a t i t l z a t i o n a l o n g the margins o f the c r y s t a l s i s common i n the a n d e s i t i c l a v a s . I l m e n i t e c o - e x i s t s w i t h the t i t a n o m a g n e t i t e 124 F i g u r e 23. P h l o g o p i t e - b i o t i t e c o m p o s i t i o n a l r e l a t i o n s o f a n a l y z e d m i c as; the d i v i s i o n between p h l o g o p i t e and b i o t i t e i s a r b i t r a r i l y chosen t o be where Mg:Fe = 2:1 (Deer and o t h e r s , 1966). Open t r i a n g l e s , Mount P r i c e ; open c i r c l e s , The T a b l e . ANNITE SIDEROPHYLLITE F e 6 ( S i 6 A I 2 0 2 0 K 0 H ) 4 K 2 F e 5 A I ( S i 5 A I 5 0 2 0 M 0 H ) 4 1—I 1 1 1 T l—I—r B I O T I T E CD P H L O G O P I T E J I I I I I I I L PHLOGOPITE EASTONITE Mg 6(Si 6AI 20 2 0)(OH> 4 K 2 Mg 5 AI (S i 5 AI 50 2 0 ) ( 0 H ) 4 126 i n some Cheakamus V a l l e y b a s a l t s , a few Mount P r i c e a n d e s i t e s and the West B l u f f a n d e s i t e of The B l a c k Tusk. In the b a s a l t s , the rhombohedral phase i s p r e s e n t as 0.2 mm e l o n g a t e rods o r n e e d l e s , commonly w i t h i n d e n t e d m a r g i n s , i n the groundmass, and as a c i c u l a r c r y s t a l s c o r e d by f a y a l i t i c o l i v i n e i n the p a l e brown g l a s s o f s e g r e g a t i o n v e i n s . A n h e d r a l m i c r o p h e n o c r y s t s of i l m e n i t e , sometimes e n c l o s e d by c o r r o d e d p l a t e s o f b i o t i t e , a r e common i n some Mount P r i c e l a v a s , whereas the West B l u f f a n d e s i t e c o n t a i n s o n l y r a r e p h e n o c r y s t s of c o r r o d e d i l m e n i t e surrounded by c o l o u r l e s s g l a s s and equant g r a i n s o f o r t h o p y r o x e n e . R e p r e s e n t a t i v e a n a l y s e s of the o x i d e phases are p r e s e n t e d i n Appendix I I I , and p l o t t e d infcterms o f FeO, Fe^O^, and T i O ^ i n F i g . 24. The a n a l y s e s have been r e c a l c u l a t e d a c c o r d i n g to the method d e s c r i b e d by C a r m i c h a e l (1967) t o check t h e i r q u a l i t y . The u l v o s p i n e l c o n t e n t s (mole p e r c e n t ) of the t i t a n o m a g n e t i t e s range from 14.77 to 70.23 p e r c e n t , and t h e c o - e x i s t i n g i l m e n i t e s c o n t a i n between 10.0 and 37.8 p e r c e n t R 2 ° 3 ' P r i n c i P a i ± y F e 2 ^ 3 ^ n s o ± i d s o l u t i o n . A l t h o u g h the a n a l y z e d o x i d e c y r s t a l s a r e i n d i v i d u a l l y homogeneous, some v a r i a t i o n i n c o m p o s i t i o n has been n o t e d between d i f f e r e n t c r y s t a l s i n the same r o c k . I n the Mount P r i c e l a v a s , t i t a n o m a g n e t i t e o f the p o s t - e r u p t i v e c r y s t a l l i z a t i o n s t a g e i s d e p l e t e d i n minor elements and q u i t e r i c h i n t i t a n i u m (near 30 p e r c e n t T i O ^ ) . P r e v o t and M e r g o i l (1972) suggested t h a t such h i g h t i t a n i u m c o n t e n t i s p r o b a b l y a consequence of maghematization. Because they a r e p o o r e r i n A l and Mg, and r i c h e r i n Mn than c o -e x i s t i n g l o w - T i (9-12 p e r c e n t TiO^) m a g n e t i t e s , the m i c r o c r y s t s may be r e l a t e d t o t h e v e r y d i f f e r e n t i a t e d c h a r a c t e r o f the r e s i d u a l 127 l i q u i d . The T i O ^ c o n t e n t (6-13 p e r c e n t ) of most G a r i b a l d i Lake m a g n e t i t e s i s comparable t o t h a t of o x i d e s i n e j e c t e d p l u t o n i c b l o c k s from S o u f r i e r e v o l c a n o , S t . V i n c e n t (Lewis, 1970); t i t a n o m a g n e t i t e s i n the Sphinx Moraine and Enostuck b a s a l t i c -a n d e s i t e s a r e r i c h e r i n T i O ^ (10-13 p e r c e n t ) , MgO (3-5 percent)', A l ^ O ^ (3-4 p e r c e n t ) , and Cr^O^^CO.5-1.5 p e r c e n t ) than those o f the a n d e s i t i c l a v a s . In the D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e , e u h e d r a l t i t a n o -m a g n e t i t e t h a t o c c u r s as i n c l u s i o n s i n o l i v i n e p h e n o c r y s t s , i s weakly zoned and c o n t a i n s s u b s t a n t i a l C^O^ (up t o 13.37 p e r c e n t ) , A l ^ O ^ (up to 6.29 p e r c e n t ) , and MgO (up to 4.36 p e r c e n t ) , as do c o r e s of g r a i n s i n c o n t a c t w i t h the groundmass g l a s s . The m i c r o p h e n o c r y s t s i n the l a v a show s i m i l a r z o n i n g w i t h rims d e p l e t e d i n C^O^, MgO, and Al^O^, and e n r i c h e d i n T i O ^ (12,6 p e r c e n t ) and t o t a l i r o n . There appears to be s y s t e m a t i c and c o n t i n u o u s v a r i a t i o n i n c o m p o s i t i o n between i n c l u d e d g r a i n s and chromium-poor m i c r o p h e n o c r y s t s i n the r e s i d u a l g l a s s . The s i m i l a r i t y between o x i d e s e n c l o s e d by o l i v i n e and the c o r e s of m i c r o p h e n o c r y s t s s u g g e s t s t h a t the (3-phase i s r e p r e s e n t a t i v e o f p r e - e r u p t i v e s p i n e l s , arid t h a t t h e i r chromium-rich n a t u r e i s p r i m a r y and not the r e s u l t o f i n t e r a c t i o n w i t h the magma. The chromium c o n t e n t o f the t i t a n o m a g n e t i t e s f u r t h e r i m p l i e s t h a t they c r y s t a l l i z e d from a m e l t more m a f i c than the D e s o l a t i o n V a l l e y l a v a (60-80 ppm C r ) . The temperature and l o g f of the e q u i l i b r a t i o n o f the 2 c o - e x i s t i n g i l m e n i t e s and t i t a n o m a g n e t i t e s ( F i g . 25) can be o b t a i n e d u s i n g the c u r v e s o f Buddington and L i n d s l e y (1964). In cases where 128 F i g u r e 24. Compositions o f t i t a n o m a g n e t i t e s and i l m e n i t e s i n m o l e c u l a r p e r c e n t o f FeO, Fe^O^* a n c * T i ° 2 ' T i e - l i n e s j o i n c o - e x i s t i n g t i t a n o m a g n e t i t e and i l m e n i t e c o m p o s i t i o n s , (a) Cheakamus V a l l e y b a s a l t s : s o l i d c i r c l e s , l a v a s ; o p e n . c i r c l e s , s e g r e g a t i o n v e i n , (b) s o l i d s q u a r e , Sphinx Moraine b a s a l t i c - a n d e s i t e ; h a l f - f i l l e d s q u a r e s , E n o s t u c k b a s a l t i c - a n d e s i t e ; c i r c l e d d o t s , D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e ; open s q u a r e s , Helm Creek l a v a , (c) Mount P r i c e , (d) The B l a c k Tusk, (e) The T a b l e . Fe ,0 4 FeO F e 3 0 4 FeO 130 t i t a n i f e r o u s m a g n e t i t e i s n o t accompanied by d i s c r e t e i l m e n i t e , the maximum u l v o s p i n e l c o n t e n t of such m a g n e t i t e s has been used t o e s t i m a t e the oxygen f u g a c i t y f o r the l a v a by assuming a c r y s t a l l i z a t i o n temperature i n c o n j u n c t i o n w i t h Buddington and L i n d s l e y ' s f -T 2 c u r v e s . E q u i l i b r a t i o n c o n d i t i o n s deduced i n t h i s manner g i v e maximum f a t any g i v e n temperature ( N i c h o l l s , 1971, p. 103). Temperatures 2 assumed a r e the two-pyroxene e q u i l i b r a t i o n temperatures c a l c u l a t e d w i t h Wood and Banno's (1973) e m p i r i c a l geothermometer. C a r m i c h a e l (1967b) det e r m i n e d f -T e q u i l i b r a t i o n c u r v e s f o r 2 v a r i o u s l a v a s i n which t i t a n o m a g n e t i t e and i l m e n i t e c o - e x i s t w i t h d i f f e r e n t f e r r o m a g n e s i a n s i l i c a t e p h e n o c r y s t s ; t h e s e curves have been r e p r o d u c e d i n F i g . 25, and i n d i c a t e t h a t when t i t a n i f e r o u s m a g n e t i t e s which c o - e x i s t w i t h s i m i l a r f e r r o m a g n e s i a n s i l i c a t e s a r e compared, t h e i r u l v o s p i n e l c o n t e n t s v a r y w i t h i n r e s t r i c t e d l i m i t s . The e s t i m a t e d f and 2 T f o r G a r i b a l d i Lake l a v a s i n which o x i d e s c o - e x i s t w i t h o l i v i n e f a l l a l o n g the q u a r t z - f a y a l i t e - m a g n e t i t e (QFM) c u r v e , whereas the r e s u l t f o r l a v a (624) i n which i r o n - t i t a n i u m o x i d e s c o - e x i s t w i t h o r t h o p y r o x e n e i n d i c a t e s f and T c l o s e t o C a r m i c h a e l ' s (1967b) o x i d e - o r t h o p y r o x e n e 2 curv e and the n i c k e l - n i c k e l o x i d e (NiNiO) b u f f e r ( F i g . 25). The f and T v a l u e s d e f i n e d f o r the Mount P r i c e a n d e s i t e s , i n which 2 the i r o n t i t a n i u m o x i d e s a r e accompanied by amphibole and b i o t i t e , f a l l i n an i n t e r m e d i a t e p o s i t i o n between the NiNiO and m a g n e t i t e -h e m a t i t e (HM) b u f f e r s ( F i g . 25). The i n d i c a t e d f f o r t h e s e l a v a s 2 are h i g h e r than t h a t of The B l a c k Tusk a n d e s i t e (624) , even though X. T i t a n o m a g n e t i t e Orthopyroxene feoth g u i t e s & r e v i r t u a l l y F e 3 0 4 F e S x 0 3 i d e n t i c a l . The d i f f e r e n c e s i n f , however, may r e f l e c t the 2 131 s y m p a t h e t i c v a r i a t i o n o f s i l i c a a c t i v i t y and oxygen f u g a c i t y as r e p r e s e n t e d by: ^ F e o 0 . + SiO_ = F e S i O . + ^ 0 , . 3 3 4 2 3 6 2 t i t a n o m a g n e t i t e l i q u i d o r thopyroxene A lower s i l i c a a c t i v i t y i n the Mount P r i c e a n d e s i t e s r e l a t i v e t o t h a t o f The B l a c k Tusk l a v a c o u l d e x p l a i n the s l i g h t l y h i g h e r f v a l u e s . 2 Osborne ( 1962, 1969) proposed t h a t d i f f e r e n t i a t i o n of b a s a l t i c magmas f o l l o w e d e i t h e r o f two t r e n d s depending upon the oxygen f u g a c i t y c o n d i t i o n s under which the magma f r a c t i o n a t e s . In p a r t i c u l a r , h i s e x p e r i m e n t a l d a t a showed t h a t i n o r d e r f o r a b a s a l t t o undergo s i l i c a - e n r i c h m e n t w i t h o u t e x t e n s i v e i r o n - e n r i c h m e n t , i t must be b u f f e r e d a t moderate to h i g h oxygen f u g a c i t i e s , and as a consequence, p r e c i p i t a t e m a g n e t i t e i n s u f f i c i e n t amounts d u r i n g the e a r l y and m i d d l e s t a g e s o f c r y s t a l l i z a t i o n . F i g u r e 25 i n d i c a t e s t h a t the f u g a c i t y o f oxygen i n l a v a s from the G a r i b a l d i Lake a r e a remained more o r l e s s c o n s t a n t w i t h f a l l i n g temperature. Such a constancy o f f i s the 2 r e q u i r e d c o n d i t i o n of the Osborne model f o r the g e n e r a t i o n o f c a l c - a l k a l i n e a n d e s i t e s . However, i n a l l l a v a s the o x i d e i s o n l y p r e s e n t i n minor amounts (<2 p e r c e n t ) , and i t i s u n l i k e l y t h a t t h e r e has been s i g n i f i c a n t f r a c t i o n a t i o n o f t h i s phase. E g g l e r and Burnham (1973) n o t e d t h a t n e i t h e r m a g n e t i t e n o r i l m e n i t e a r e l i q u i d u s phases i n a n d e s i t i c l i q u i d s . N e v e r t h e l e s s , t h e p r e c i p i t a t i o n o f t i t a n o m a g n e t i t e must be c o n s i d e r e d as an i m p o r t a n t though minor p a r t i c i p a n t i n any f r a c t i o n a t i o n scheme e n v i s a g e d f o r the e v o l u t i o n o f the G a r i b a l d i n l i a k e t s i l i c i u e T - a n d e s i t e s (Chapter I I I ) . 132 25. Log f -T diagram i n which a r e p l o t t e d maximum v a l u e s of 2 temperature and oxygen f u g a c i t y f o r e q u i l i b r a t i o n ( u s i n g c urves of Buddington and L i h d s l e y , 1964) o f o x i d e phases i n the G a r i b a l d i Lake l a v a s . HM i s h e m a t i t e - m a g n e t i t e b u f f e r , NiNiO i s n i c k e l - n i c k e l o x i d e b u f f e r , and FMQ i s f a y a l i t e - m a g n e t i t e - q u a r t z b u f f e r . Orthopyroxene, b i o t i t e and/or amphibole, and b i o t i t e c u r v e s a r e from C a r m i c h a e l (1967b), and r e p r e s e n t l o g f -T c o n d i t i o n s 2 d e f i n e d by t i t a n o m a g n e t i t e and i l m e n i t e c o - e x i s t i h g w i t h d i f f e r e n t f e r r o m a g n e s i a n s i l i c a t e s . U n d e r l i n e d sample numbers i n d i c a t e d e t e r m i n a t i o n based on maximum u l v o s p i n e l c o n t e n t o f t i t a n o m a g n e t i t e and assumed c r y s t a l l i z a t i o n temperature i n the absence o f c o - e x i s t i n g i l m e n i t e . S o l i d c i r c l e , Cheakamus V a l l e y b a s a l t ; s o l i d s q u are, Sphinx M o r a i n e ; h a l f - f i l l e d s q u a r e , Enostuck; c i r c l e d d o t , D e s o l a t i o n V a l l e y ; open c i r c l e , The T a b l e ; s o l i d t r i a n g l e , The B l a c k Tusk; open t r i a n g l e s , Mount P r i c e . Log oxygen fugacity 134 A p a t i t e A p a t i t e i s commonly p r e s e n t as minute n e e d l e s i n the groundmass and as s t o u t prisms i n c l u d e d i n p l a g i o c l a s e p h e n o c r y s t s o f The T a b l e , Mount P r i c e and The B l a c k Tusk a n d e s i t e s . Somewhat l a r g e r stubby p r i s m a t i c m i c r o p h e n o c r y s t s (up t o 0.8 mm i n l e n g t h ) o c c u r i n the C u l l i t o n Creek a n d e s i t e of C l i n k e r Peak on Mount P r i c e . P a r t i a l a n a l y s e s o f two e u h e d r a l c r y s t a l s from the C u l l i t o n Creek l a v a a r e g i v e n i n Appendix I I I . The a p a t i t e s a r e lower i n Fe and Mg, but h i g h e r i n Na and Mn when compared w i t h a n a l y s e s o f a p a t i t e s i n a n d e s i t e s from Rabaul C a l d e r a , Papua, New Guinea (Heming and C a r m i c h a e l , 1973). The l a t t e r d i f f e r e n c e s may be e x p l a i n e d by s u b s t i t u t i o n o f Na f o r Ca, and c o u p l e d s u b s t i t u t i o n of S 6 + f o r P 5 + t o g e t h e r w i t h N a + f o r C a 2 + ( C r u f t , 1966). P y r r h o t i t e P y r r h o t i t e i s found as s m a l l b l e b s e n c l o s e d by micropheno-c r y s t s o f t i t a n o m a g n e t i t e , and more r a r e l y w i t h i n g l a s s i n c l u s i o n s i n o l i v i n e p h e n o c r y s t s of the Sphinx Moraine b a s a l t i c - a n d e s i t e . A n a l y s e s of t h e s e p y r r h o t i t e s a r e g i v e n i n Appendix I I I , t o g e t h e r w i t h a n a l y s e s of s u l p h i d e g l o b u l e s i n Juan de Fuca Ridge b a s a l t s (Mathez, 1976) and a n d e s i t e s from Rabual C a l d e r a , New Guinea (Heming and C a r m i c h a e l , 1973). The Sphinx Mo-raine p y r r h o t i t e s c o n t a i n s i g n i f i c a n t l y more N i (4-7 p e r c e n t ) than e i t h e r the Rabaul or Juan de Fuca s u l p h i d e s ; p y r r h o t i t e b l e b s e n c l o s e d by t i t a n o m a g n e t i t e c o n t a i n l e s s N i than those found i n o l i v i n e s . P r e l i m i n a r y c a l c u l a t i o n o f S^, SO^, and SO^ f u g a c i t i e s u s i n g the method d e s c r i b e d by Heming and C a r m i c h a e l (1973) i n d i c a t e a 135 -1 -4 -4 range o f l o g f between 10 and 10 , l o g f between 10 and 2 3 -7 -2 10 , and l o g f g 0 g r e a t e r than 10 f o r the Sphinx Moraine l a v a ; S 0 2 i s the dominant s u l p h u r gas s p e c i e s . Other A c c e s s o r y M i n e r a l s Rounded q u a r t z x e n o c r y s t s , surrounded by r e a c t i o n rims of a c i c u l a r a u g i t e , a r e p r e s e n t i n many a n d e s i t e s o f The T a b l e , Mount P r i c e and The B l a c k Tusk, and i n the upper p a r t o f the Helm Creek composite f l o w . S m a l l c o r r o d e d g r a i n s o f a l u m i n o - s i l i c a t e ( a n d a l u s i t e ? ) a r e found i n one l a v a o f The B l a c k Tusk, and p o s s i b l y r e p r e s e n t t h e r m a l metamorphism a l o n g the margins o f the magma c o n d u i t . C r i s t o b a l i t e o c c u r s b o t h i n c a v i t i e s and i n the groundmass o f some h o l o c r y s t a l l i n e d a c i t e s . R e s i d u a l G l a s s P a l e brown t o c o l o u r l e s s g l a s s i s p r e s e n t i n most o f the r o c k s s t u d i e d , but due to the pr e s e n c e o f m i c r o l i t e s , many g l a s s e s a r e too v a r i a b l e t o be a n a l y z e d s a t i s f a c t o r i l y w i t h the mi c r o p r o b e . The c o m p o s i t i o n o f the g l a s s e s (Appendix I I I ) ranges from h i g h - s i l i c a a n d e s i t e to r h y o l i t e ; the r e s i d u a l g l a s s e s from i n d i v i d u a l v o l c a n i c c e n t e r s form c l o s e l y r e l a t e d groups. The g l a s s e s a r e e n r i c h e d i n the nor m a t i v e s a l i c c o n s t i t u e n t s , but d e p l e t e d i n Mg, Fe, Ca and T i i n r e l a t i o n t o the b u l k r o c k c o m p o s i t i o n s . Decreases i n Al^O^ and CaO r e s u l t m a i n l y from s e p a r a t i o n o f p l a g i o c l a s e , a u g i t e or ho r n b l e n d e ; T i O ^ , t o t a l i r o n , and MgO d e c r e a s e s r e f l e c t c r y s t a l l i z a t i o n o f f e r r o m a g n e s i a n m i n e r a l s and i r o n - t i t a n i u m o x i d e s . In the system 136 Qz-Ab-Or-H 20 ( T u t t l e and Bowen, 1958), the a n a l y s e s a l l l i e on the Ab s i d e o f the t e r n a r y minima, and d e f i n e a t r e n d , s u b - p a r a l l e l to the Ab-Qz j o i n , t h a t converges towards the t e r n a r y minimum a t 500 b a r s . C r y s t a l l i z a t i o n Temperatures The r e s u l t s o f a p p l i c a t i o n o f v a r i o u s geothermometry methods to l a v a s from the G a r i b a l d i Lake a r e a a r e summarized i n T a b l e X. S i n c e p l a g i o c l a s e i s a l o w - p r e s s u r e l i q u i d u s or n e a r - l i q u i d u s phase i n these l a v a s , b u l k p l a g i o c l a s e and groundmass c o m p o s i t i o n s used w i t h the p l a g i o c l a s e geothermometer of Kudo and W e i l l (1970), as m o d i f i e d by Mathez (1973), may p r o v i d e an e s t i m a t e o f the l o w - p r e s s u r e l i q u i d u s t e m p e r a t u r e s . Temperatures c a l c u l a t e d f o r the a n d e s i t i c l a v a s range from 952-1183°C a t P = 1 b a r , 975-1222°C a t P D = 0.5 kb, 891-1183°C a t P u _ = 1.0 kb, and 842-1168°C a t P„ _ = 5 kb; the low temperatures i n d i c a t e d a t P = 1 bar r e s u l t from m o d i f i c a t i o n s made by Mathez (1973) t o account f o r the n o n - i d e a l b e h a v i o r o f p l a g i o c l a s e s o l i d s o l u t i o n . P l a g i o c l a s e c r y s t a l l i z a t i o n temperatures (1053-1211°C) i n the b a s a l t i c - a n d e s i t e s a r e i n e x c e l l e n t agreement w i t h temperatures (1050-1177°C) determined f o r o l i v i n e - l i q u i d e q u i l i b r a t i o n u s i n g magnesium, i r o n , c a l c i u m and manganese p a r t i t i o n r e l a t i o n s o f Roeder (1974) and Leeman (1974). The p r e d i c t e d 1 b a r l i q u i d u s temperatures a r e g e n e r a l l y lower than those (1207-1307 C) c a l c u l a t e d f o r a n d e s i t i c l a v a s o f the younger v o l c a n i c i s l a n d s o f Tonga (Ewart and o t h e r s , 1973; Ewart, 1976), or the e x p e r i m e n t a l l y -determined 1 bar l i q u i d u s temperatures (1197-1227°C) o f P a r i c u t i n b a s a l t s and a n d e s i t e s ( T i l l e y and o t h e r s , 1968). The lower temperatures a r e c o n s i s t e n t w i t h c r y s t a l l i z a t i o n o f the a n d e s i t i c l a v a s under TABLE X. CRYSTALLIZATION TEMPERATURES CALCULATED FOR VARIOUS GARIBALDI GROUP LAVAS. P I a g i o c l a s e I r o n - O l i v i n e / O r t h o -p y r o x e n e / Geothe rmomete r P H 2 0 1 b a r 3 0.5 ., b kb 1 .0 kb 5.0 kb 1 i L d 11 1 U III Oxi d e s c c I i n u - ^ p y r o x e n e C l i n o -p y r o x e n e B a s a l t s I Cheakamus V a l l e y gmds . 1035-1060 1 050-1078 1 020 -1060 944 - 990 1025 1010-1022 -v e i n 1004-1035 1004- 1039 969 - 1020 9 0 3 - 944 1017 - -I I Helm Creek gmds . - - - - - 987 -1023 -B a s a l t i c -Andes i te s I I I S ph i n x M o r a i n e gdms . 1075-1084 - - - - 997-1030 1023 phen. 1152-1164 1 184-1197 11 54- 1 1 64 11 3 0 - 1142 - - -IV E n o s t u c k gdms . 1005-1038 - - 987-1027 -phen. 1120-1174 1140- 1211 1125- 1 175 1092 - 1 1 58 - - 1 083 V D e s o l a t i o n V a l l e y gdms . 1001-1039 - - 993-1011 993 phen. 1090-1114 1 107- 11 34 1101- 1119 1 0 5 3 -1081 - - -Andes i t e s VI The T a b l e gdms . 9 43 - 993 phen. 1 051 - 1123 VI I The B l a c k Tusk gdms . 906 - 945 phen. 1100- 1183 V I I I Mount P r i c e gdms . 830 - 960 phen. 952- 1061 1060-1147 1063-1138 990 -1098 1116-1222 1107-1183 1059-1168 975-1079 891-1063 842-982 924 825 -875 1005-1017 915 -964 937 965 N o t e s : gmds. = groundmass m i n e r a l o r m i c r o p h e n o c r y s t - d e f i n e d t e m p e r a t u r e s , phen . = p h e n o c r y s t - d e f i n e d t e m p e r a t u r e s . Tempera tu re s i n °C. (a) M a t h e z , 1973; (b) Kudo and W e i l l , 1970; ( c ) B u d d i n g t o n and L i n d s l e y , 1964; (d) P o w e l l and P o w e l l , 1974; (e) Wood and Banno , 1973. See t e x t f o r f u r t h e r d i s c u s s i o n . 138 e l e v a t e d water p r e s s u r e s . Another s e t o f p l a g i o c l a s e geothermometer c a l c u l a t i o n s u t i l i z i n g the p h e n o c r y s t r i m p l u s groundmass c o m p o s i t i o n s s h o u l d approximate the i n i t i a l quenching temperatures o f the l a v a s a t one atmosphere. The c a l c u l a t e d 1 b a r temperatures (830-1075°C) a r e i n good:.agreement..with e s t i m a t e s based on c o - e x i s t i n g i r o n - t i t a n i u m o x i d e s (825-1025°C) and o r t h o p y r o x e n e - c l i n o p y r o x e n e (915-1023°C) c r y s t a l l i z a t i o n t e mperatures. The o l i v i n e - c l i n o p y r o x e n e geothermometer o f P o w e l l and P o w e l l (1974) has a l s o been a p p l i e d , u s i n g p h e n o c r y s t -r i m c o m p o s i t i o n s i n the b a s a l t i c and b a s a l t i c - a n d e s i t e l a v a s . Wood (1976) q u e s t i o n e d the v a l i d i t y of t h i s geothermometer and suggested t h a t a p p l i c a t i o n i n i t s p r e s e n t form i s u n l i k e l y t o produce r e l i a b l e r e s u l t s . The c a l c u l a t e d temperatures ( T a b l e X ) , however, agree r e a s o n a b l y w e l l w i t h the o t h e r e s t i m a t e s . D i s c u s s i o n The m i n e r a l o g y o f G a r i b l a d i Lake a n d e s i t i c l a v a s s u g g e s t s t h a t the m a j o r i t y o f these magmas have been m o d i f i e d by c r y s t a l f r a c t i o n a t i o n ( i n c l u d i n g c r y s t a l a c c u l m u l a t i o n ) processes^: dominated by e i t h e r amphibole o r p l a g i o c l a s e . The p h e n o c r y s t assemblage of the b a s a l t i c -and s i l i c i c - a n d e s i t e s i n d i c a t e s t h a t a t l e a s t two s t a g e s o f f r a c t i o n a t i o n a r e i n v o l v e d i n the e v o l u t i o n o f these magmas. The p r e s e n c e of magnesian o l i v i n e ( F o D Q „ ) and p h l o g o p i t e , o /—o(J p a r g a s i t i c - a m p h i b o l e , chromium-rich (up t o 13.5% Cr^O^) t i t a n o m a g n e t i t e , and n i c k e l - r i c h (up t o 7% N i ) s u l p h i d e s i n the b a s a l t i c - a n d e s i t e s i s c o m p a t i b l e w i t h c r y s t a l l i z a t i o n from a more m a f i c p a r e n t a l magma. 139 A l l a n and o t h e r s (1975) and Stewart (1975) argued t h a t f r a c t i o n a t i o n o f e a r l y - f o r m e d amphibole a t h i g h - p r e s s u r e s (10-18 kb) i s a v i a b l e mechanism by which c a l c - a l k a l i n e a n d e s i t e s can be formed from b a s a l t i c m e l t s . In the G a r i b a l d i Lake l a v a s , however, the p e t r o g r a p h i c e v i d e n c e i n d i c a t e s t h a t c l i n o p y r o x e n e and/or o l i v i n e were the l i q u i d u s o r n e a r - l i q u i d u s phases b e f o r e amphibole became s t a b l e . C r y s t a l c l o t s c o n s i s t i n g e s s e n t i a l l y o f p a r g a s i t i c - h o r n b l e n d e and d i o p s i d i c a u g i t e , and amphibole megacrysts e n c l o s i n g ragged g r a i n s o f f o r s t e r i t i c o l i v i n e i n the Sphinx M oraine b a s a l t i c - a n d e s i t e suggest t h a t amphibole and c l i n o p y r o x e n e c o - p r e c i p i t a t e d , w i t h e a r l y - f o r m e d o l i v i n e i n r e a c t i o n r e l a t i o n s h i p w i t h the l i q u i d . By analogy w i t h e x p e r i m e n t a l l y -o b t a i n e d l i q u i d u s and n e a r - l i q u i d u s phases o f n a t u r a l and s y n t h e t i c systems, the aluminous n a t u r e o f c l i n o p y r o x e n e and amphibole i n the b a s a l t i c - a n d e s i t e s s u g g e s t s t h a t c r y s t a l l i z a t i o n o c c u r r e d a t p r e s s u r e s between 5 and 10 kb, and temperatures i n the range o f 975-1050°C. H i g h - l e v e l f r a c t i o n a t i o n i n v o l v i n g s e p a r a t i o n o f p h e n o c r y s t s and m i c r o p h e n o c r y s t s i s i n d i c a t e d by f i n e - g r a i n e d b a s i c x e n o l i t h s i n the s i l i c i c - a n d e s i t e s . The p h e n o c r y s t and x e n o l i t h assemblage of The T a b l e and Mount P r i c e a n d e s i t e s , AMPB + PL + OPX + MT, sugges t s t h a t t h ese phases c o - e x i s t e d s t a b l y ; t h i s c o - e x i s t e n c e r e q u i r e s c r y s t a l l i z a t i o n a t p r e s s u r e s l e s s than 3 kb under w a t e r - s a t u r a t e d c o n d i t i o n s ( E g g l e r , 1973). The p r e c i p i t a t i o n o f p l a g i o c l a s e b e f o r e c l i n o p y r o x e n e , and the modal dominance o f p l a g i o c l a s e p h e n o c r y s t s i n the a n d e s i t e s , however, i n d i c a t e s t h a t the magmas e q u i l i b r a t e d a t w a t e r - u n d e r s a t u r a t e d c o n d i t i o n s . The absence o f amphibole i n the D e s o l a t i o n V a l l e y b a s a l t i c -a n d e s i t e and the younger s i l i c i c - a n d e s i t e s o f The B l a c k Tusk i n d i c a t e s ". i-< ^ . i -.'si- I;j44te<J ua r ? - ' ItsooJ: va::?: • 140 t h a t t h ese l a v a s c r y s t a l l i z e d under lower water p r e s s u r e s than the l a v a s of the o t h e r a n d e s i t i c v o l c a n o e s . The p h e n o c r y s t and x e n o l i t h assemblage of The B l a c k Tusk a n d e s i t e s , PL + CPX + OPX + MT, i m p l i e s c r y s t a l l i z a t i o n a t h i g h e r t e m p e r a t u r e s , and p o s s i b l y lower p r e s s u r e s (about 2 k b ) , than The T a b l e and Mount P r i c e l a v a s . A l t h o u g h the m i n e r a l o g y o f the a n d e s i t e s i s c o n s i s t e n t w i t h an o r i g i n i n v o l v i n g c r y s t a l f r a c t i o n a t i o n o f hydrous b a s a l t i c magmas, the contemporaneous Cheakamus V a l l e y b a s a l t s c o n t a i n a p h e n o c r y s t assemblage, OL + PL + CPX, which i n d i c a t e s t h a t they c r y s t a l l i z e d under e s s e n t i a l l y anhydrous c o n d i t i o n s . I f the a n d e s i t e s a r e f r a c t i o n a t i o n p r o d u c t s o f a hydrous b a s a l t i c m e l t , the anhydrous c h a r a c t e r o f the Cheakamus V a l l e y b a s a l t s r e q u i r e s the g e n e r a t i o n , beneath southwestern B r i t i s h Columbia, of b a s a l t i c magmas c o n t a i n i n g v a r y i n g c o n c e n t r a t i o n s o f water. T h i s s u g g e s t i o n i s co m p a t i b l e w i t h the o b s e r v e d p h e n o c r y s t assemblages o f The T a b l e (PL + AMPB + OPX + MT), Mount P r i c e (PL +-AMPB + BO + OPX + MT) and The B l a c k Tusk (PL + CPX + OPX + MT) a n d e s i t e s which i n d i c a t e c r y s t a l l i z a t i o n of c h e m i c a l l y d i s t i n c t magma ba t c h e s under d i f f e r e n t c o n d i t i o n s o f p r e s s u r e , temperature and v o l a t i l e c o n t e n t . 141 V. P-T PATH OF ASCENDING GARIBALDI LAKE MAGMAS I n t r o d u c t i o n C r y s t a l f r a c t i o n a t i o n has p l a y e d an im p o r t a n t r o l e i n the e v o l u t i o n o f the G a r i b a l d i Lake v o l c a n i c s e r i e s . A knowledge o f the p r e s s u r e s (P) and temperatures (T) i n the s o u r c e r e g i o n o f the magmas, o r i n r e s e r v o i r s where l e n g t h y r e s i d e n c e time a l l o w e d magma c o m p o s i t i o n o r c r y s t a l l i z a t i o n p a t h t o become s u b s t a n t i a l l y m o d i f i e d , would p e r m i t f u r t h e r s p e c i f i c a t i o n o f the p e t r o g e n e s i s o f the v o l c a n i c s u i t e . N i c h o l l s and o t h e r s (1971), N i c h o l l s and C a r m i c h a e l (1972) and Bacon and C a r m i c h a e l (1973) have develo p e d a method o f c a l c u l a t i n g temperatures and p r e s s u r e s a t which magmatic l i q u i d s may have been i n e q u i l i b r i u m w i t h e n c l o s e d p h e n o c r y s t s , o r w i t h p o s t u l a t e d s o u r c e m a t e r i a l . T h i s p e t r o l o g i c a l - t h e r m o d y n a m i c approach y i e l d s e s t i m a t e s o f the P-T c o n d i t i o n s a t two s t a g e s i n the a s c e n t o f G a r i b a l d i Lake magmas. The f i r s t s t a g e i n v o l v e s the thermochemical c o n d i t i o n s a t the time o f p h e n o c r y s t c r y s t a l l i z a t i o n . The second s t a g e uses thermodynamic d a t a f o r the p h e n o c r y s t - e q u i l i b r a t e d magmas to determine c o m p a t i b l e s o u r c e r o c k and P-T m e l t i n g c o n d i t i o n s . Method o f C a l c u l a t i o n The c a l c u l a t i o n s a r e based on the premise t h a t t h e a c t i v i t y o f a component i n a magmatic l i q u i d w i l l be d e f i n e d by any m i n e r a l assemblage i n e q u i l i b r i u m w i t h t h a t l i q u i d . I f the groundmass m i n e r a l s o f a G a r i b a l d i Lake l a v a , f o r example, a r e assumed to r e p r e s e n t e q u i l i b r i u m c r y s t a l l i z a t i o n o f the magma a t th e s u r f a c e , the a c t i v i t y o f 142 s i l i c a i n the p r e s e n c e o f a m i n e r a l b u f f e r system, such as M g 2 S i 0 4 (OLIV ) + S i O . ( g l a s s ) = 2MgSiO. (OPX ) , i s g i v e n i n g e n e r a l form by: SS 2 3 s s „ m «„o P = 1 atm A I To P T AG AV ( l o g a ' ) ' = R e a c t i o n + R e a c t i o n (P - 1) 2 2.303RT 2.303RT Assuming t h a t AC^ i n e q u a t i o n (A) i s z e r o , a l l s o l i d s mix i d e a l l y w i t h r e s p e c t t o volume, and the a c t i v i t y c o e f f i c i e n t s f o r m i n e r a l phases a r e independent o f p r e s s u r e and temperature, c a l c u l a t i o n o f s i l i c a a c t i v i t y i s s i m p l i f i e d , as Eq. (A) reduces t o : <loS a S i o / ' T = I +B + f ( P - l ) + ( 2 1 ° S a M g S i 0 3 " 1 0 8 a M g 2 S i 0 4 ) - ( B ) where A and B a r e c o n s t a n t s d e r i v e d by l i n e a r l e a s t - s q u a r e s a n a l y s i s o f AG° P = . 1 a t m / 2 . 3 0 3 R T v e r s u s 1/T f o r the b u f f e r r e a c t i o n , C i s the R e a c t i o n volume change ( c a l / b a r ) o f the r e a c t i o n f o r pure components a t P = 1 atm and 298.15°K d i v i d e d by 2.303R, T i s the a b s o l u t e temperature, P i s ba r s p r e s s u r e , and the a c t i v i t y term r e f e r s to m i n e r a l o g i c a l - c o m p o s i t i o n a l d a t a f o r the groundmass m i n e r a l s . s The s i l i c a a c t i v i t y i n the l a v a , c a l c u l a t e d a t one atmosphere p r e s s u r e and an e s t i m a t e d s u r f a c e quench temperature, can be r e d e f i n e d as a f u n c t i o n o f P, T and magma c o m p o s i t i o n ( i n t h e absence o f the m i n e r a l b u f f e r ) as f o l l o w s ( N i c h o l l s and C a r m i c h a e l , 1972): d> V V° / - i s P , T v S i 0 „ S i 0 0 - S i O . ( T t ,s ( l o g a ) = l o g X + 2 + 2 2 (P - 1) (C) B 1 U 2 2 T 2.303RT where X i s the mole f r a c t i o n o f S i 0 9 i n the l i q u i d , V i s the o X L ^ ^ 0XU2 143 p a r t i a l molar volume o f s i l i c a i n the l i q u i d , V ° i s t h e molar volume o f pure s i l i c a , and § Q - n > a f u n c t i o n o f c o n s t a n t c o m p o s i t i o n , i s d e f i n e d by the r e g u l a r s o l u t i o n f o r m u l a t i o n : d o g a s ± 0 2 - l o g X s i 0 2 ) = l o g y s . 0 2 = ^ 2 (D) at P = 1 atm. With t h e a d d i t i o n o f v o l a t i l e s , the b u l k c o m p o s i t i o n o f the l a v a may be a c l o s e a p p r o x i m a t i o n to the c o m p o s i t i o n o f the magma a t depth where the p h e n o c r y s t s would be i n s o l u t i o n and some h i g h e r c o n c e n t r a t i o n o f v o l a t i l e components might be p r e s e n t . S i l i c a a c t i v i t i e s , computed from Eq. ( C ) , can be c o n s i d e r e d r e p r e s e n t a t i v e o f the magma a t h i g h e r p r e s s u r e s and temperatures, i f the l a v a a t the s u r f a c e i s assumed to e x i s t as a melt o f i d e n t i c a l c o m p o s i t i o n a t deeper l e v e l s i n the c r u s t o r mantle (Bacon and C a r m i c h a e l , 1973). The s i l i c a a c t i v i t y , c a l c u l a t e d u s i n g the method j u s t d e s c r i b e d , r e f e r s to a magmatic l i q u i d which i s c h e m i c a l l y d e f i n e d by the c o m p o s i t i o n o f the groundmass m i n e r a l s . Recent e x p e r i m e n t a l s t u d i e s , however, i n d i c a t e t h a t the a c t i v i t y o f components i n a l a v a can be a l t e r e d s i g n i f i c a n t l y by t h e i n t r o d u c t i o n o f s m a l l amounts o f water to t h e magma ( E g g l e r , 1972; N i c h o l l s and Ringwood, 1973), o r by s m a l l c o m p o s i t i o n a l changes i n the s i l i c a t e melt ( R e i n and Chipman, 1965). I f the magma e x s o l v e s v o l a t i l e s o r undergoes some degree o f c r y s t a l f r a c t i o n a t i o n as i t r i s e s to the s u r f a c e , then n e i t h e r t h e groundmass m i n e r a l s nor the b u l k magma c h e m i s t r y w i l l r e f l e c t ag^Q^ b e f o r e e r u p t i o n ; a t a g i v e n P and T, s i l i c a a c t i v i t y d e f i n e d by groundmass m i n e r a l c o m p o s i t i o n s w i l l be e q u i v a l e n t to the p r e - e r u p t i v e v a l u e s o n l y i f the mel t c r y s t a l l i z e s under e q u i l i b r i u m c o n d i t i o n s d u r i n g i t s a s c e n t . The c a l c u l a t i o n s have t h e r e f o r e been m o d i f i e d to e v a l u a t e t h e e f f e c t o f 144 c o m p o s i t i o n a l v a r i a t i o n on the s i l i c a a c t i v i t y o f magmas. The c o m p o s i t i o n a l dependence o f the s i l i c a a c t i v i t y i n a magmatic l i q u i d i s g i v e n i n g e n e r a l form by: a S i O „ 8(AH + TAS ) RT l n 2 a s i o 2 8 n s i o 2 where n„._ i s the number o f moles o f s i l i c a , and AH and AS , SiC> 2 m m r e s p e c t i v e l y , a r e the e n t h a l p y and e n t r o p y change t h a t o c c u r when the l i q u i d i s formed from components i n which a = a° . The a c t i v i t y o i u 2 o i U 2 o f s i l i c a i n p r e - e r u p t i v e G a r i b a l d i Lake l i q u i d s , t h e r e f o r e , can be c a l c u l a t e d i f we know the multi-component c o m p o s i t i o n a l v a r i a t i o n o f the e n t h a l p y ..and e n t r o p y o f m i x i n g , t h e e x t e n t to which the l a v a ' s c o m p o s i t i o n has changed s i n c e the p h e n o c r y s t s f i r s t p r e c i p i t a t e d , and the a c t i v i t y o f s i l i c a i n the groundmass l i q u i d a t some temperature. We u n f o r t u n a t e l y have n e i t h e r the thermodynamic d a t a nor the d e t a i l e d s o l u t i o n model t h a t a r e r e q u i r e d f o r such complex l i q u i d s , and a d i f f e r e n t approach must be taken. The n e c e s s a r y assumption i s t h a t component a c t i v i t i e s c a l c u l a t e d from p h e n o c r y s t - c o r e c o m p o s i t i o n s p r o v i d e a c l o s e a p p r o x i m a t i o n to t h o s e o f the magma b e f o r e p h e n o c r y s t c r y s t a l l i z a t i o n . Two e q u a t i o n s o f type ( C ) , where ( j ^ . . - . r e f e r s to e i t h e r b l U 2 groundmass- o r p h e n o c r y s t - d e f i n e d a c t i v i t i e s , can then be w r i t t e n to b r a c k e t t h e range o f p o s s i b l e a v a l u e s i n t h e magmatic l i q u i d . oi(J 2 M i n e r a l c o m p o s i t i o n s r e p r e s e n t a t i v e o f the p h e n o c r y s t or megacryst assemblage c o n t a i n e d i n the l a v a , o r o f the p o s t u l a t e d s o u r c e r e g i o n (e.g. lower c r u s t , upper mantle, o r subducted o c e a n i c l i t h o s p h e r e ) a r e used to w r i t e another e q u a t i o n o f type ( B ) . ( M i n e r a l b u f f e r s c o n s i d e r e d i n the c a l c u l a t i o n s d e s c r i b e d h e r e , t o g e t h e r w i t h 145 t h e i r c o n s t a n t s A, B and C a r e p r e s e n t e d i n T a b l e XI; a c t i v i t y -c o m p o s i t i o n models used f o r t h e s o l i d phases a r e g i v e n i n Appendix I V ) . P T The l o g a ' e x p r e s s i o n , thus c a l c u l a t e d , i s combined w i t h t h e bxu 2 c o r r e s p o n d i n g e x p r e s s i o n f o r the l a v a to y i e l d an e q u a t i o n i n two v a r i a b l e s , P and T. I f the p r o c e d u r e i s r e p e a t e d f o r a second l i q u i d component (e.g. A l ^ O ^ o r K^O), t h e two P-T e q u a t i o n s can be s o l v e d f o r a u n i q u e p r e s s u r e and temperature where the G a r i b a l d i Lake l a v a c o u l d have e q u i l i b r a t e d w i t h t h e o b s e r v e d o r s t i p u l a t e d m i n e r a l assemblage. A l t h o u g h the groundmass m i n e r a l c o m p o s i t i o n s r e f l e c t t h e a c t i v i t y o f magmatic components a t one atmosphere and the s u r f a c e quench temperature, t h e p h e n o c r y s t - d e r i v e d a c t i v i t i e s a r e d e f i n e d o n l y at t h e p r e s s u r e and temperature o f p h e n o c r y s t - m e l t e q u i l i b r a t i o n . The f i r s t s t e p o f the c a l c u l a t i o n s , t h e r e f o r e , i s to e s t i m a t e t h e p r e s s u r e and temperature o f i n i t i a l p h e n o c r y s t c r y s t a l l i z a t i o n . E q u i l i b r a t i o n o f Groundmass L i q u i d and P h e n o c r y s t s B a s a l t i c - a n d e s i t e s o f the G a r i b a l d i Lake a r e a a r e commonly p o r p h y r i t i c w i t h p h e n o c r y s t s o f p l a g i o c l a s e ( A n ^ ^ _ ^ ^ ) , d i o p s i d i c a u g i t e , aluminous b r o n z i t e (4.7 p e r c e n t Al^O^, E n ^ ) ; some l a v a s a l s o c o n t a i n megacrysts and r a r e p h e n o c r y s t s o f magnesian o l i v i n e ^°Q-J_QQ^ AN<^ p a r g a s i t i c amphibole. Core c o m p o s i t i o n s o f t h e anhydrous p h e n o c r y s t s c l o s e l y resemble the h i g h - p r e s s u r e m i n e r a l assemblage r e p o r t e d i n a h i g h - a l u m i n a t h o l e i i t i c a n d e s i t e from New South Wales (Duggan and W i l k i n s o n , 1973). As the groundmass o f the l a v a s c o n t a i n s t i t a n o m a g n e t i t e , a u g i t e , h y p e r s t h e n e , p l a g i o c l a s e , r a r e o l i v i n e and i n t e r s t i t i a l g l a s s , i t i s p o s s i b l e to e s t i m a t e t h e p r e s s u r e and temperature a t which the p h e n o c r y s t s c o u l d have been i n e q u i l i b r i u m w i t h the groundmass. Assuming 146 TABLE X I . COEFFICIENTS A, B AND C FOR THE EQUATION log a, = AG°/2.303RT = A/T + B + C(P-1)/T FOR LISTED REACTIONS, WHERE " i " REFERS TO S i 0 2 > A l j O j or ^ 0 . React ion A B C Source o f Data (1) C a A l 2 S i 0 6 + S i 0 2 ( g l a s s ) = C a A ^ S ^ O g - 1 4 1 0 -0.532 0 .0523 (4) (2) C a M g S i 2 0 & + A l 2 0 3 ( l i q u i d ) = C a A ^ S i O g + MgSiO^ -5844 1.647 0.1421 (4) (3) Mg 2SiO^ + S i 0 2 ( g l a s s ) = 2MgSi0 3 -1034 0 . 5 9 7 -0.0424 (2) CO CaMgSi.O, + SiO. ( g l a s s ) + A l . 0 . ( l i q u i d ) = Z b Z 2 3 C a A l 2 S i 2 0 g + MgSiO ? - 7 2 5 4 1.115 0 . 1 9 4 5 (4) (5) 2 CaMgSi 20 & + 3 M g S i 0 3 + S i O j ( g l a s s ) + HjO = Ca 2Mg 5Sig0 2 2(OH) 2 -7084 9-083 0.1011 (2),(8) (6) N a C a j M g ^ A I ( A l 2 S i 6 ) 0 2 2 ( 0 H ) 2 + 2 . 5 S i 0 2 ( g l a s s ) = NaAISijOg + C a A I 2 S i 2 0 g + CaMgSijO^ + L S M g ^ i O ^ + H 2 0 690 -5.660 0.0400 (IMS) (7) NaCa 2M g ; (Al ( A l 2 S i g ) 0 2 2 ( 0 H ) 2 + C a M g $ i 2 0 6 + lOMgSiOj =.., C a 2 M g 5 S i g 0 2 2 ( O H ) 2 + C a A l j S ^ O g + NaAlSi Og + SMgjSiO^ 1698 -0 .0930 0.1501 ( 0,(5) (8) NaCa 2M g i )Al ( A l 2 S i 6 ) 0 2 2 ( O H ) 2 + 4S i 0 2 ( g l a s s ) = NaAl S i j O g + C a A I 2 S i 2 0 g + 3MgSi(>3 + CaMgS^Og + HjO -1415 -3 - 9 9 4 - 0 . 1 0 3 6 (D,(5) (9) K F e 3 A l S i 3 0 ] Q ( O H ) 2 + 3 S i 0 2 ( g l a s s ) = K A I S i , 0 o + 3FeSiO, + H.O 3 0 3 2 6919 -9.689 -0 .0910 (7) (10) S i 0 2 ( g l a s s ) = S i 0 2 (Qz) -309 0.183 -0.0239 (2) ( M ) C a 3 A I 2 S i 3 O u + 3MgSi0 3 + 6 S i 0 2 (Qz) + K 20 ( l i q u i d ) = 3CaMgSi 20 6 + 2KA1S i -27170 0 - 3131 0.0883 (0,(6) (12) HMgSiOj + C a A l 2 S i 2 0 g + KjO ( l i q u i d ) = 2 K A l S i 3 0 g + 5Mg 2SiO, ) + C a M g S i ^ - 2 7 9 4 1 6.246 0.0748 ( 0,(6) ( 1 3 ) M g S i 0 3 + C a A l 2 S i 2 O g + 5 S i 0 2 (Qz) + K 2Q ( l i q u i d ) = 2 K A I S i 3 0 g + C a M g S i 2 0 6 -31853 8.125 -0 .1392 ( 0,(6) (14) 3MgSi0 3 + C a 3 A l 2 S i 3 0 ] 2 + S i 0 2 ( g l a s s ) = 3CaMgSi_0, + A l . S i O . ZD Z 3 -1331 -1.290 0.0242 (0 (15) 15CaMgSi,0, + 6Al„SiO, + K.,0 ( l i q u i d ) = Z 0 z s z 2KAlSi Og + 5 C a 3 A l 2 S i 3 0 | 2 + 15MgSi0 3 - 1 9 1 8 5 8.030 0.0969 ( 0,(6) (16) MgjSiO^ + MgSiOj + S i 0 2 ( g l a s s ) + A 1 2 0 3 ( l i q u i d ) = M g 3 A l 2 S i 3 0 ] 2 -9709 4 .556 0.0475 (3) (17) C a M g S i 2 0 6 + MgSi0 3 + A l j O ( l i q u i d ) = C a A I 2 S i 2 0 g + Mg2SiOI) -6428 0 .718 0.2369 (4) f i m Mg2SiOi) + A l 2 0 3 ( l i q u i d ) = M g A l ^ + MgSi0 3 . -8131 2.632 0 .1344 (4) (19) C a M g S i 2 0 & + 1/2 S i 0 2 ( g l a s s ) + A1 20 ( l i q u i d ) = C a A I 2 S i 2 0 g + l / 2 M g 2 S i 0 1 ) -6776 0 . 8 5 9 0.2157 (4) : (0 Robie and Waldbaum (1968) ; (2) N i c h o l l s and o t h e r s (1970; (3) N i c h o l l s and Carmichael (1972); (4) Bacon and Carmichael (1973); (5) Boyd (1959); (6) Marsh and Carmichael (1974)• (7) F e i s inger (1975) ; (8) Ghent and Co1eman (1973) 147 t h a t t h e anhydrous p h e n o c r y s t s once r e p r e s e n t e d an e q u i l i b r i u m assemblage a t some unknown P and T, s i l i c a and alumina a c t i v i t i e s would be g i v e n by r e a c t i o n s (1) and (2) ( T a b l e X I ) : C a A l 2 S i 0 6 + S i 0 2 = C a A l ^ i ^ g (1) Ca-tschermaks g l a s s a n o r t h i t e CaMgSi.O, + A l o 0 „ = C a A l o S i 0 , + MgSiC" . (2) Z O Z j Z D J d i o p s i d e l i q u i d Ca-tschermaks e n s t a t i t e C o r r e s p o n d i n g a c t i v i t i e s d e f i n e d by the groundmass phases a r e g i v e n by r e a c t i o n s (3) and (4) ( T a b l e X I ) : M g 2 S i 0 4 + S i 0 2 = 2MgSi0 3 (3) f o r s t . e r i t e g l a s s e n s t a t i t e C a M g S i 2 0 6 + S i 0 2 + A 1 2 0 3 = C a A l ^ ^ O g + M g S i 0 3 . (4) d i o p s i d e g l a s s l i q u i d a n o r t h i t e e n s t a t i t e These b u f f e r r e a c t i o n s can be used to e s t i m a t e t h e P-T c o n d i t i o n s where the groundmass l i q u i d o f s e v e r a l G a r i b a l d i Lake l a v a s e q u i l i b r a t e d w i t h t h e i r a s s o c i a t e d p h e n o c r y s t assemblage. There i s an u n c e r t a i n t y , however, i n knowing which m i n e r a l c o m p o s i t i o n s ( a l l o f which show a range) c o r r e l a t e w i t h each o t h e r . In t h e c a l c u l a t i o n s t h a t f o l l o w , t h e average groundmass m i n e r a l and p h e n o c r y s t - c o r e c o m p o s i t i o n s a r e assumed to r e p r e s e n t e q u i l i b r i u m a t d i f f e r e n t s t a g e s o f c r y s t a l l i z a t i o n . The Sphinx Moraine b a s a l t i c - a n d e s i t e (598), f o r example, e x h i b i t s the f o l l o w i n g groundmass m i n e r a l (GM) a c t i v i t i e s : aP,P^._ = 0.65, sP^^i.^ = 0.62, MgSiO., ^Ig„SiO, CPX PLAG a„ w r.. « =0.72, and a„ _. n =0.60. At P = 1 atm and an e s t i m a t e d C a M g S i 2 0 6 C a A l 2 S i 2 0 g (two-pyroxene geothermometer; Wood and Banno, 1973) s u r f a c e quench temperature o f 1017°C (1290°K), l o g = -0.3863 and l o g aj*f _ b i u 2 A X 2 U 3 148 GM GM GM GM = -4.2738. From ^ = T ( l o g - l o g X ± ) , where XjT r e f e r s to the mole f r a c t i o n o f the i t h component i n the groundmass determined by e i t h e r . s u b t r a c t i n g the a p p r o p r i a t e p r o p o r t i o n s o f p h e n o c r y s t m i n e r a l s from the b u l k r o c k c o m p o s i t i o n , o r d i r e c t w i d e - s c a n m i c r o p r o b e a n a l y s i s GM GM o f the groundmass, = -205.378 and <$> = -4498.639 ( T a b l e X I I ) . D X U ^ 2 3 The v a r i a t i o n o f s i l i c a and a l u m i n a a c t i v i t i e s i n the absence of a m i n e r a l b u f f e r can then be e x p r e s s e d as: M GM .P,T i f n r Q f t n \ , ,-205.378, ( l o g a s ± 0 ) = l o g (0.5928) + ( ) + (1.344 x 1 0 " 6 - 0^5247)( P - 1 ) , and, GM .P,T . , ,-4498.639, ( l o g a M o ) = l o § (0.1625) + ( ) + ( 0 , 1 f 6 1 - 6.7002 x 10" 5) (P - 1 ) . CPX U s i n g the a p p r o p r i a t e p h e n o c r y s t - c o r e c o m p o s i t i o n s ( a Q a ^ g^Q = 0.072, < W ? e- n = ° - 6 6 8 > arP5 Q- n =0.76, and a ° P * = 0.73), two CaAl2Sx20g CaMgSx 20^ MgSxO^ p h e n o c r y s t - b u f f e r (PH) e q u a t i o n s can a l s o be w r i t t e n : i, PH .P,T -1410 , 0.0523 ,„ n , n ,„ ( l o g a s i o 2 ) = — f ~ + — f ~ ( p - 1 } + °' 4 3 5' and, M P H ^ > T -5844 ,0.1421 / T 1 , N . N / Q - , ( 1 ° g a A l 2 0 3 ) = ~T ( p ~ 1) + 0.487. The f o u r e x p r e s s i o n s can be s o l v e d s i m u l t a n e o u s l y to determine the approximate p r e s s u r e and temperature of phenocryst-groundmass e q u i l i b r a t i o n ( F i g . 26). The c a l c u l a t e d groundmass-phenocryst e q u i l i b r i u m c o n d i t i o n s s u g g e s t t h a t t h e G a r i b a l d i Lake b a s a l t i c - a n d e s i t e magmas began TABLE X I I . THERMODYNAMIC PARAMETERS USED IN CALCULATION OF ANDESITIC EQUILIBRATION PRESSURES AND TEMPERATURES. Sample C r y s t a l 1 i z a t i o n Temperature (°C) T, 1 bar log a ^ ^ T, 1 bar l o g a A l 2 0 3 T, 1 bar log a ^ *s,o2 X A 1 2 0 3 X K 2 0 S i 0 2 598 gm 1017 -0.3863 -4.2738 -14.4840 0.5928 0. 1635 0.0134 -205.4 -4498.6 -16268.0 ph* 1052 -0.6285 -3.9231 -16.1890 0.5925 0. 1100 0.0076 -570.2 -3927.9 -18645.4 637 gm 1017 -0.2776 -4.3710 -14.8700 0.6269 0. 1625 0.0093 -96.1 -4620.6 -16561.3 ph" 1036 -0.6090 -4.0964 -15.5785 0.6167 0. 1169 0.0081 -456.8 -4140.9 -17649.3 668 gm 1009 -0.3214 -4.4465 -14.4998 0.6135 0. 1748 0.0134 -140.0 -4729.4 -16187-7 ph" 1028 -0.6573 -4.1153 -15-7478 0.5983 0. 1028 0.0108 -564.9 -4064.3 -17297-4 208 gm 1011 -0.2779 -4.4280 -15-4079 0.6055 0. 1532 0.0160 -77.1 -4639.4 -17450.6 ph" 1016 -0.6470 -4.1027 -16.3023 0.5922 0. .1098 0.0078 -540.7 -4026.5 -18297.1 624 gm 931 -0.4105 -4.7131 -16.2970 0.7039 0. .1328 0.0135 -310.6 -4618.8 -17370.7 Ph* 984 -0.7336 -4.2138 -17.6028 0.6433 0. .1128 0.0084 -686.3 -4135-7 -19506.2 584 gm 965 -0.3316 -4.5922 0.7322 0, .1123 -242.9 -4509.5 Phenocryst parameters c a l c u l a t e d as i n d i c a t e d in t e x t . Gm = groundmass; Ph = phenoc ry s t s . .0-150 F i g u r e 26. C a l c u l a t e d P-T i n t e r s e c t i o n s f o r e q u i l i b r a t i o n between: (1) the groundmass l i q u i d o f b a s a l t i c - a n d e s i t e (598) and i t s a s s o e i a t e d p p h e n o c r y s t a a s s e m b l a g e ; (2) the p h e n o c r y s t - e q u i l i b r a t e d l i q u i d o f the b a s a l t i c - a n d e s i t e and a s p i n e l - l h e r z o l i t e r e f r a c t o r y assemblage; and (3) the p h e n o c r y s t - e q u i l i b r a t e d l i q u i d and a q u a r t z e c l o g i t e m i n e r a l assemblage. P a r a l l e l o g r a m s i n d i c a t e e s t i m a t e d e r r o r s i n the c a l c u l a t e d i n t e r s e c t i o n s . The o b l i q u e , l i n e s r e p r e s e n t a n d e s i t e l i q u i d i w i t h v a r i o u s amounts of water (weight p e r c e n t ; T.H. Green, 1972). Shaded r e g i o n s i n d i c a t e t r a n s i t i o n zones between p l a g i o c l a s e - p e r i d o t i t e (PP), s p i n e l - p e r i d o t i t e (SP) and g a r n e t - p e r i d o t i t e (GP) Approximate s l a b - m a n t l e i n t e r f a c e geotherms (dashed l i n e s a f t e r Marsh and C a r m i c h a e l , 1974) a r e taken from (a) Schatz and Simmons (1972), and (b) Oxburgh and T u r c o t t e (1970) . S o l i d hexagon i n d i c a t e s the e s t i m a t e d s u r f a c e quench temperature o f the l a v a . 151 DEPTH (km) PRESSURE ( k b a r ) 152 c r y s t a l l i z i n g a t 4.5 to 11 kb"*" ( T a b l e X I I I ) . S i n c e t h e Moho d i s c o n t i n u i t y now l i e s about 30 km beneath s o u t h w e s t e r n B r i t i s h Columbia (Souther, 1970; S t a c e y , 1974), t h e c a l c u l a t e d P and T imply t h a t t h e magmas c o u l d have commenced t h e i r r e c o r d e d c r y s t a l l i z a t i o n h i s t o r y s h o r t l y a f t e r e n c o u n t e r i n g the base o f t h e c r u s t (see St a c e y , 1974). The s m a l l d i f f e r e n c e between s u r f a c e quench and p h e n o c r y s t e q u i l i b r a t i o n t emperatures e s t i m a t e d f o r some l a v a s ( l e s s t h a n 25 degrees; T a b l e X I I I ) , however, i s c l e a r l y anomalous (Marsh, 1976a), and i m p l i e s e i t h e r t h a t t h e e s t i m a t e d s u r f a c e quench temperatures a r e too h i g h , o r t h a t the p h e n o c r y s t - c o r e c o m p o s i t i o n s c r y s t a l l i z e d from a l i q u i d l e s s f r a c t i o n a t e d t h a n the groundmass. As b o t h p o s s i b i l i t i e s a r e l i k e l y , t h e c a l c u l a t e d P and T c o n d i t i o n s a r e i n t e r p r e t e d to r e p r e s e n t t y p i c a l e q u i l i b r a t i o n p r e s s u r e s and te m p e r a t u r e s . Marsh (1976b) sug g e s t e d t h a t A l e u t i a n a n d e s i t i c l i q u i d s do not b e g i n c r y s t a l l i z i n g u n t i l they r e a c h depths o f l e s s t h a n 15 km (^4.5 k b ) . S i l i c a and Alumina A c t i v i t i e s i n P r e - E r u p t i v e G a r i b a l d i Lake M e l t s The normal, r e v e r s e and o s c i l l a t o r y z o n i n g o f p h e n o c r y s t s i n the G a r i b a l d i Lake b a s a l t i c - a n d e s i t e s i n d i c a t e s a c o n t i n u o u s r e a c t i o n 1. The u n c e r t a i n t i e s a s s o c i a t e d w i t h the c a l c u l a t e d e q u i l i b r i u m c o n d i t i o n s i n v o l v e t h e r e l i a b i l i t y o f the a c t i v i t y - c o m p o s i t i o n models used f o r the s o l i d s , the u n c e r t a i n t i e s i n t h e a n a l y t i c a l d a t a , the e s t i m a t e d s u r f a c e quench temperature and t h e thermodynamic c o n s t a n t s , and t h e assumption t h a t e q u i l i b r i u m i s a t t a i n e d between l i q u i d and c r y s t a l l i n e p h ases. These f a c t o r s a r e d i f f i c u l t to e v a l u a t e q u a n t i t a t i v e l y . The c a l c u l a t i o n s , however, have been found to y i e l d e r r o r s o f up to ±40°C and ±6 kb when the r e s u l t s a r e compared t o h i g h - p r e s s u r e (>10 kb) e x p e r i m e n t a l e q u i l i b r i a ( C a r m i c h a e l and o t h e r s , 1976). TABLE X I I I . CALCULATED TEMPERATURES AND PRESSURES OF EQUILIBRATION OF ANDESITIC LAVAS WITH POSSIBLE REFRACTORY MINERAL ASSEMBLAGES ( 1 ) C a l c u l a t e d T e m p e r a t u r e s and P r e s s u r e s B a s e d on G r o u n d m a s s - D e f i n e d Component A c t i v i t i e s Samp]e P h e n o c r y s t s S p i n e l - P e r i d o t i t e G a r n e t - P e r i d o t i t e Q u a r t z E c l o g i t e T (°C) P (kb) T (°C) P (kb) T (°C) P (kb) T (°C) P ( k b ) 5 9 8 6 3 7 6 6 8 2 0 8 6 2 4 5 2 3 1 0 5 3 1 0 3 6 1 0 2 8 1 0 1 6 9 8 3 9 4 9 6 . 0 6 . 4 7 - 9 8 . 6 9 - 9 9 - 8 8 6 6 7 3 1 7 2 7 7 0 8 7 8 4 7 5 2 1 . 4 • 4 . 7 • 3 . 4 - 5 . 2 0 . 4 - 2 . 3 5 8 7 4 1 5 4 1 5 3 9 3 5 5 2 5 0 0 - 4 . 2 • 1 0 . 9 - 9 . 6 • 1 1 . 6 - 3 - 7 - 6 . 8 2 1 1 0 2 2 2 2 2 2 5 3 2 2 3 3 2 0 2 8 40.2 34.4 38.1 31-9 35.4 ( 2 ) C a l c u l a t e d T e m p e r a t u r e s and P r e s s u r e s Based on P h e n o c r y s t - D e f i n e d Component A c t i v i t i e s Sample Wt. P e t . S p i n e l - P e r i d o t i t e H 2 0 H 2 0 T (°C) P (kb) G a r n e t - P y r o x e n i t e T (°C) P (kb) Q u a r t z E c l o g i t e T (°C) P ( k b ) 5 9 8 0 . 0 9 9 2 . 8 7 1 2 3 5 1 7 . . 8 1 1 5 5 1 3 . . 6 1 1 8 0 40. . 6 6 3 7 0 . . 1 1 9 3 . 4 4 1 0 7 8 1 1 . • 3 1 1 3 7 9 . . 8 1 5 6 9 40. .3 6 6 8 0 . 1 1 7 3 - 3 9 11 52 1 5 . • 9 1 0 6 1 1 1 . . 7 1 3 4 6 42. , 8 2 0 8 0 . 1 2 7 3 . 6 6 1 1 7 5 1 5 . . 9 1 0 7 3 1 1 . . 4 1 2 7 4 40. • 1 6 2 4 0 . , 1 2 9 3 - 7 3 1 1 9 0 1 8 . • 5 1 1 3 2 1 4 . . 8 9 5 4 40. , 1 154 between c r y s t a l s and l i q u i d d u r i n g magmatic d i f f e r e n t i a t i o n . Whether such d i s e q u i l i b r i u m r e f l e c t s a r a p i d c o o l i n g r a t e , a change i n melt c o m p o s i t i o n , o r a l o s s o f v o l a t i l e c o n s t i t u e n t s can not be determined from the a v a i l a b l e a n a l y t i c a l d a t a , but the s y s t e m a t i c v a r i a t i o n i n the p h e n o c r y s t s ' c o m p o s i t i o n i m p l i e s some m o d i f i c a t i o n , b e f o r e e r u p t i o n , of the a c t i v i t y o f magmatic components. Assuming t h a t s u b - s o l i d u s r e - e q u i l i b r a t i o n has n o t o c c u r r e d , s i l i c a and alumina a c t i v i t i e s b u f f e r e d by the p h e n o c r y s t - c o r e c o m p o s i t i o n s w i l l p r o b a b l y p r o v i d e a c l o s e r a p p r o x i m a t i o n t o those o f t h e magma b e f o r e e x t r u s i o n t h a n v a l u e s d e r i v e d from the groundmass m i n e r a l c o m p o s i t i o n s . The r e g u l a r s o l u t i o n e x p r e s s i o n (D) used t o f o r m u l a t e the a c t i v i t y c o e f f i c i e n t y_^, however, r e q u i r e s the d e f i n i t i o n o f l o g a^ at P = 1 atm, and the p h e n o c r y s t - d e f i v e d a c t i v i t i e s a r e o n l y d e f i n e d a t the p r e s s u r e s and temperatures where the p h e n o c r y s t s e q u i l i b r a t e d w i t h the m e l t . These a c t i v i t i e s must be e x t r a p o l a t e d t o the s u r f a c e b e f o r e Eq. (C) can be used t o c a l c u l a t e the e f f e c t o f temperature and p r e s s u r e on the p r e - e r u p t i v e a c t i v i t y o f magmatic components, and f o d tes:tt eqiui-libra;tioar.of f thee magmas wit h u e a r l i e r - - r e f r a c t o r y assemblages. The problem i s t o e s t i m a t e the a c t i v i t y o f components i n a magmatic l i q u i d t h a t undergoes e q u i l i b r i u m r a t h e r than d i s e q u i l i b r i u m c r y s t a l l i z a t i o n d u r i n g i t s a s c e n t ; a t P = 1 atm, however, the l i q u i d i s quenched a t an unknown temperature. As t h i s temperature depends on a b u l k magma c o m p o s i t i o n which i s not n e c e s s a r i l y e q u i v a l e n t t o t h a t o f the quenched l a v a , the magma must be assumed t o r i s e i n s t a n t a n e o u s l y t o the E a r t h ' s s u r f a c e f o l l o w i n g i n i t i a l p r e c i p i t a t i o n o f the p h e n o c r y s t phases. I t s a d i a b a t i c a s c e n t then can be t r e a t e d LAVA as n e a r l y i s o t h e r m a l (Lang, 1972), and t h e v a r i a t i o n o f a„.„ , Si C ^ and 155 LAVA Al^O^, b u f f e r e d by s o l i d s o f c o n s t a n t c o m p o s i t i o n , i s a f u n c t i o n o f p r e s s u r e . The s i l i c a and alumina a c t i v i t y i n the l i q u i d a t the s u r f a c e can be approximated by: "Pc c 1 LAVA.P = 1 b a r , T PR\P,T ? ( l o g a. ) - ( l o g a. ) - A V S 0 L I D S I dP where A V S 0 L I D S } 'pc AV SOLIDS dP = 2 3Q3RT ( P c ~ ^ ' P ° i S t h e P h e n o c r y s t _ groundmass e q u i l i b r a t i o n p r e s s u r e ( b a r s ) , A V C A T T i s t h e volume change o f the b u f f e r r e a c t i o n f o r pure components a t one atmosphere p r e s s u r e , and T i s t h e e q u i l i b r a t i o n temperature (°K). V a l u e s o f s i l i c a and alu m i n a a c t i v i t y c a l c u l a t e d a t one atmosphere p r e s s u r e f o r p h e n o c r y s t - e q u i l i b r a t e d G a r i b a l d i Lake l i q u i d s ( T a b l e X I I I ) a r e lower than those d e f i n e d by the groundmass m i n e r a l c o m p o s i t i o n s , i m p l y i n g a b a s a l t i c p r e c u r s o r t o the a n d e s i t e l a v a s (compare N i c h o l l s and C a r m i c h a e l , 1972, T a b l e 3 ) . Water Content o f G a r i b a l d i Lake A n d e s i t e Magmas Water a c t s as a d e p o l y m e r i z e r o f s i l i c a t e m e l t s ; i t s p r e s e n c e a l t e r s the a c t i v i t y of components i n a magma, and t h e r e b y s t a b i l i z e s l i q u i d u s phases l e s s p o l y m e r i z e d than t h o s e o f the anhydrous l i q u i d . E x p e r i m e n t a l s t u d i e s i n d i c a t e t h a t i f a n d e s i t i c l i q u i d s a r e d e r i v e d as e q u i l i b r i u m p a r t i a l m e l t s of mantle p e r i d o t i t e , they must be s a t u r a t e d or n e a r l y s a t u r a t e d w i t h water (10-20 p e r c e n t d i s s o l v e d water depending on the depth o f o r i g i n ; Mysen and B o e t t c h e r , 1975a, 1975b). Under w a t e r - d e f i c i e n t c o n d i t i o n s , p a r t i a l m e l t i n g of a p e r i d o t i t i c m i n e r a l assemblage must y i e l d a b a s a l t i c o r p i c r i t i c l i q u i d (D.H. Green, 1970, 1973). An e s t i m a t e o f the c o n c e n t r a t i o n o f water i n G a r i b a l d i Lake a n d e s i t e s may i n d i c a t e the degree o f w a t e r - s a t u r a t i o n 156 under which these magmas were generated. Water and S i l i c a A c t i v i t y I f the G a r i b a l d i Lake andesites ascended towards the surface without a p p r e c i a b l y changing t h e i r bulk composition, the d i f f e r e n c e between groundmass- and phenocryst-defined s i l i c a a c t i v i t i e s may be explained by the presence of v o l a t i l e c o n s t i t u e n t s at depth. The e f f e c t of water on the a c t i v i t y of s i l i c a can be c a l c u l a t e d using an expression derived by J.W. N i c h o l l s (Marsh and Carmichael, 1974): n ^ > T i v I *SiO_ , ... -6 0.0047, , n ( l o g a s ± 0 ) = l o g X S 1 Q + 2 + (1.344 x 10 ) (P - 1) - { t ( | + b + f-)} + 1/2 l o g (1 - ) (E) where a, b and c are constants derived by least-squares a n a l y s i s of Eggler's (1972) experimental data on a P a r i c u t i n andesite (a = -81951.5, b = 63.6892 and c = 1.0743), <f> i s a f u n c t i o n of 2 composition defined by Eq. (D), t = (3^ Q ) (1 - (X^ Q + XS±Q ) ) / ( l -2 2' 2 2 X„ ) w i t h X„ and X..,. , the mole f r a c t i o n s of water and s i l i c a , H^u 2 •3l*-,2 r e s p e c t i v e l y , i n the magma. Re w r i t i n g Eq. (E) as: WR GM 1 v T O I -Si0_ . GM , 'siO- / w a , , , cP. , lo g X + :l = l o g X s. + 2 - { t ( - + b + — ) } + 1/2 lo g (1 - XJJ Q ) (F) WR GM where X_ and X r e f e r to the bulk (WR) and groundmass (GM) c -i j ,WR , GM . , compositions of the l a v a , and, q>c.n and <})„.„ , r e s p e c t i v e l y , are c a l c u l a t e d w i t h the phenocryst- and groundmass-defined s i l i c a a c t i v i t i e s , allows c a l c u l a t i o n of the mole f r a c t i o n of water i n the magma which produce the observed d i f f e r e n c e i n s i l i c a a c t i v i t y at any pressure 157 and temperature. C a l c u l a t e d v a l u e s o f f o r the G a r i b a l d i Lake b a s a l t i c -a n d e s i t e s are p r e s e n t e d i n T a b l e X I I I . The mole f r a c t i o n o f water i n the l i q u i d i s c o n v e r t e d t o weight p e r c e n t u s i n g the r e l a t i o n ( C a r m i c h a e l and o t h e r s , 1976): Wt % H 20 = 1800 X j ^ / ( g f w + x ] ^ ( 1 8 - gfw)) (G) where gfw i s the gram f o r m u l a weight o f the l a v a . The r e s u l t s suggest t h a t the a n d e s i t e magmas c o u l d have c o n t a i n e d between 2 and 4 weight p e r c e n t water a t the P and T of phenocryst-groundmass e q u i l i b r a t i o n . W a t e r - s a t u r a t e d a n d e s i t i c m e l t s c o n t a i n 8 t o 12 p e r c e n t water under s i m i l a r p r e s s u r e and temperature c o n d i t i o n s (Hamilton and o t h e r s , 1964), Water Content o f H o r n b l e n d e - B e a r i n g B a s a l t i c - A n d e s i t e s P h e n o c r y s t s o f o l i v i n e , o r t h o p y r o x e n e , c l i n o p y r o x e n e , p l a g i o c l a s e and p a r g a s i t i c h o r n b l e n d e c o - e x i s t i n s e v e r a l b a s a l t i c -a n d e s i t e s o f the G a r i b a l d i Lake a r e a . Assuming-that these p h e n o c r y s t s once r e p r e s e n t e d an e q u i l i b r i u m assemblage a t some P and T, the approximate P r p o t a ^ and P^ ^ under which they c r y s t a l l i z e d can be c a l c u l a t e d . A t - e q u i l i b r i u m ^ P^ Q w i l l be d e f i n e d by a l l b u f f e r r e a c t i o n s which i n v o l v e a component o f the amphibole s o l i d s o l u t i o n . F o r example, as the f l u o r i n e c o n t e n t o f the amphiboles i s s m a l l , the p h e n o c r y s t assemblage can be r e p r e s e n t e d by the d i v a r i a n t r e a c t i o n s (5) and (6) :; ( T a b l e - XI) : * : 2 C a M g S i 2 0 6 + 3MgSi0 3 + S i 0 2 + H 20 = Ca 2Mg S i g Q ^ C O H ^ (5) d i o p s i d e e n s t a t i t e g l a s s steam t r e m o l i t e 158 N a A l S i 3 0 g + C a A l 2 S i 2 O g + C a M g S i ^ + 1 . 5 M g 2 Si0 4 a l b i t e a n o r t h i t e d i o p s i d e f o r s t e r i t e i n p l a g i o c l a s e + H 20 = N a C a 2 M g 4 A l ( A l 2 S i 6 ) 0 2 2 ( O H ) 2 steam p a r g a s i t e + 2 . 5 S i 0 2 (6) g l a s s N o t i n g t h a t r e a c t i o n (3) ( T a b l e XI) d e f i n e s the s i l i c a a c t i v i t y f o r the p h e n o c r y s t assemblage, ( 3 ) , (X5) and (.(6) can be r e c a s t as a u n i v a r i a n t r e a c t i o n i n T and P m ^ .. : T o t a l C a M g S i 2 0 6 + 10MgSiO 3 + N a C a 2 M g 4 A l ( A l 2 S i 6 ) 0 2 2 ( O H ) 2 = d i o p s i d e e n s t a t i t e p a r g a s i t e C a 2 M g 5 S i g 0 2 2 ( O H ) 2 + 5 M g 2 S i 0 4 + N a A l S i ^ g + C a A l ^ i ^ g (7) t r e m o l i t e f o r s t e r i t e a l b i t e a n o r t h i t e i n p l a g i o c l a s e T h i s r e a c t i o n b a l a n c e s the h y d r o x y l group so t h a t the c a l c u l a t i o n s do n o t depend on o b t a i n i n g a v a l u e f o r the f u g a c i t y of water. The a c t i v i t y o f t r e m o l i t e and p a r g a s i t e components i n h o r n b l e n d e , however, must be e s t i m a t e d ; t h i s may be a c c o m p l i s h e d by f o r m u l a t i n g the a c t i v i t y -c o m p o s i t i o n r e l a t i o n s i n terms of s i t e o c c u p a n c i e s on s t a t i s t i c a l thermodynamic grounds. As a f i r s t a p p r o x i m a t i o n , the a c t i v i t y o f t r e m o l i t e can be w r i t t e n a s : HORNBLENDE _ . A . M4 2 M13.3, M2.2 V 2 a C a 2 M g 5 S i g 0 2 2 ( 0 H ) 2 " ( 1 " X N a , K ) ( X C a ) ( XMg } < V ( X 0 H } where X r e p r e s e n t s the f r a c t i o n o f the s u p e r s c r i p t e d s i t e o c c u p i e d by 159 a c e r t a i n c a t i o n . S i m i l a r i l y , the a c t i v i t y o f p a r g a s i t e i n hornblende may be approximated by the e x p r e s s i o n : HORNBLENDE = A M4 2 M13 3 M2 M2 V 2 a N a C a 2 M g 4 A l ( A l 2 S i 6 ) 0 2 2 ( O H ) 2 ^ N a , K M X C a ; {\g > C X M g H A 1 M V to account f o r c o u p l e d s u b s t i t u t i o n s on the A, M2 and t e t r a h e d r a l s i t e s ; s i t e o c c u p a n c i e s can be e s t i m a t e d u s i n g the method o f P o w e l l (1975). In T a b l e XIV, p h e n o c r y s t c r y s t a l l i z a t i o n p r e s u r e s ( p r n o t a i ) f o r t h r e e G a r i b a l d i Lake b a s a l t i c - a n d e s i t e s , c a l c u l a t e d from'Eq. ( 7 ) , a r e g i v e n t o g e t h e r w i t h a summary of r e l e v a n t a c t i v i t y - c o m p o s i t i o n a l d a t a . Three s e t s o f c a l c u l a t e d P m ^ .. a r e p r e s e n t e d , each based on the t o t a l average o r t h o p y r o x e n e - c l i n o p y r o x e n e e q u i l i b r a t i o n temperature (1010-1026°C; Wood and Banno, 1973), but u s i n g the mean and extreme anhydrous m i n e r a l c o m p o s i t i o n s . (As the amphiboles show l i t t l e o r no c o m p o s i t i o n a l z o n i n g , a s i n g l e p a r g a s i t e o r t r e m o l i t e a c t i v i t y term i s d e f i n e d f o r each l a v a ) . The c a l c u l a t i o n s emphasize the s e n s i t i v i t y o f the d e r i v e d p r e s s u r e s to v a r i a t i o n s i n m i n e r a l c o m p o s i t i o n s . The u n c e r t a i n t y i n the a c t i v i t y - c o m p o s i t i o n r e l a t i o n s of the amphibole components i s p r o b a b l y b a l a n c e d i n Eq. (7-)*;; g r e a t e r u n c e r t a i n t y r e s u l t s because of the l a r g e s t o i c h i o m e t r i c c o e f f i c i e n t s f o r MgSiO^ and Mg 2SiO^, and the r e l i a b i l i t y o f the e s t i m a t e d thermodynamic c o n s t a n t s f o r p a r g a s i t e (Appendix V ) . A change o f ±20 degrees i n temperature r e s u l t s i n a d i f f e r e n c e o f a p p r o x i m a t e l y 500 b a r s i n e s t i m a t e d p T o t a 1 * Water f u g a c i t i e s , c a l c u l a t e d from Eq. (5) o r ( 6 ) , at the mean p T o t a - ^ a n d average two-pyroxene c r y s t a l l i z a t i o n temperature a r e a l s o p r e s e n t e d i n T a b l e XIV. Lower and h i g h e r v a l u e s o f f , H 20 r e s p e c t i v e l y , a r e e s t i m a t e d a t the maximum and minimum e q u i l i b r a t i o n 160 TABLE XIV. ESTIMATION OF P T o M , AM IN GARIBAIOI GROUP KOWttLEKDE-AKDESITES (1) Component A c t i v i t i e s Uied In Calculation of P T o „ , and P ^ . «2 Sampl. 'NgSIOj - >| (xJJJ)1 (X^) 'caMcjSijOj " C"on X ^ o n V 'KOJSIO,, " "HSJSIO,, Maximum Mean Minimum Maximum Mean Minimum Maximum Mean Minimum 598 0.730 0.700 0.658 0.759 0.653 0.608 0.719 0.700 0.642 637 0.721 0.700 0.668 0.762 0.625 0.564 0.563 0.543 0.528 6*8 0.750 0.720 0.701 0.694 0.661 0.625 0.659 0.640 0.634 Sample "CaAljSljOg " XCaAI 2SI 20 g 1 0 9 "MaCajMg^AKAIjSijJOjj (0H) 2 l o 9 aCa 2Mg 5S1 g0 J 2(0H) 2 Maximum Mean 1 Nlnlmum 598 0.723 0.668 0.608 -2.4627 -1.9784 637 0.652 0.621 0.559 -2.8158 -2.1159 668 0.650 0.608 0.560 -2.6708 -2.0507 (2) Calculted P T o t a | and P^ 0" Sample "Total <t a"> < 0 f u „ ( b a r s )1 0 P H . ( b a r s ) ( 2 ) Liquid Wt. Pet. H2° H2° Vo HO Maximum Mean Minimum Mean Mean 598 7307 5298 1.086 2511 2544 0.154 ( 3 ) 4.5 637 9972 8675 6096 3487 3234 0.132 3 8 668 882<i 7491 6335 2889 2820 0.137 4.0 Notes: (1).Maximum, minimum and mean pressures calculated using m ean pargasite and tremolite a c t i v i t i e s at o l i v i n e - clinopyroxene c r y s t a l l i z a t i o n temperatures (Powell and Powell, 1974). (2) Fugaclty coefficients after Hoiloway and others (1971). (3) Calculated from relation (Ewart and others, 1975): In f u n - 2 ln X u - -2854/T • 12.631 • f V dP, where / P  M2° H20 fy "RT jy . p-[( 0-'0??8 t ^ 3 2 x , 0-5 + K k c M x | 0-7 . 2.3935 x to"nTJ ) + p ( 7 ? 3 6 5 t X I 0 ' B . K 1 M t „ | 0-8 . J 5 „ , n-l3 T J + 161 p r e s s u r e s . The mole f r a c t i o n o f water i n the a n d e s i t i c l i q u i d ( T a b l e XIV) i s c a l c u l a t e d w i t h an e x p r e s s i o n d e r i v e d by F. Spera (Ewart and o t h e r s , 1975): ln \0 -1 ^ C-^ f^  12-631 +l ifdP <» where j ~ dP = P { ( Q , 1 ° 9 8 8 + 4.432 x 10 5 + 1.4048 x 10 7 T ^ —8 - 2.3935 x 1 0 _ 1 1 T 2 ) + P ( 7 ' 3 3 6 ^ K 1 0 1.196 x I O - 8 - 9.5 x 1 0 ~ 1 3 T ) + P 2 ( 1 , 8 7 6 * 1 0 + 4.5863 x 10~ 3) + p 3 ( - 1 . 1 9 1 T X 1 0 - 1 4 ) } > P i s the mean ^r£ota^y ^ ^ s t n e e q u i l i b r a t i o n temperature, and f ^ ^ i s the mean water f u g a c i t y ; mole f r a c t i o n of water i s c o n v e r t e d to weight p e r c e n t u s i n g Eq. (G). The c a l c u l a t e d P_ . and P„ ^ a r e p e t r o l o g i c a l l y s i g n i f i c a n t T o t a l H^O i n the f o l l o w i n g r e s p e c t s . They suggest t h a t : (1) The G a r i b a l d i Lake magmatic l i q u i d s s t a r t e d c r y s t a l l i z i n g under c o n d i t i o n s o f P„ _ < P m IL^O T o t a l (2) The p h e n o c r y s t assemblage c o u l d have e q u i l i b r a t e d a t p r e s s u r e s ^ between '.AaaridllOKkb. (3) The p h e n o c r y s t - e q u i l i b r a t e d magmas c o n t a i n e d between 3 and 5 p e r c e n t water at the P and T o f p h e n o c r y s t e q u i l i b r a t i o n . Water Contents of H o r n b l e n d e - B i o t i t e A n d e s i t e s A n d e s i t e s of Mount P r i c e and C l i n k e r Peak c o n t a i n p h e n o c r y s t s of b i o t i t e , amphibole, o r t h o p y r o x e n e , p l a g i o c l a s e and minor c l i n o p y r o x e n e which d e f i n e the f u g a c i t y of water a c c o r d i n g to 162 r e a c t i o n s . (8) o r (9) ( T a b l e XI) : N a C a 2 M g 4 A l ( A l 2 S i 6 ) 0 2 2 ( O H ) 2 + 4SiC> 2 = 3MgSiC<3 p a r g a s i t e g l a s s e n s t a t i t e + N a A l S i 3 O g + C a A l 2 S i 2 O g + C a M g S i 2 0 6 + H 20 (8) a l b i t e a n o r t h i t e d i o p s i d e steam K F e 3 A l S i 3 0 1 ( ) ( O H ) 2 + 3 S i 0 2 = a n n i t e g l a s s K A l S i 3 O g + 3FeSiC> 3 + H 20 ( 9 ) s a n i d i n e f e r r o s i l i t e steam As the b i o t i t e f r e q u e n t l y shows an overgrowth o f amphibole, the s i l i c a a c t i v i t y i n the l a v a s can be d e s c r i b e d by a combination o f ( 8 ) % a n d ( 9 ) . A n d e s i t e s o f the P r i c e Bay s a t e l l i t e cone o f Mount P r i c e a l s o c o n t a i n m i c r o p h e n o c r y s t s o f t i t a n o m a g n e t i t e , i l m e n i t e and r e s o r b e d q u a r t z , which a r e p l a u s i b l y i n t e r p r e t e d as c o - e v a l w i t h the p h e n o c r y s t assemblage. I n c o r p o r a t i n g the a p p r o p r i a t e a c t i v i t y - c o m p o s i t i o n models (Appendix I V ) , the e q u i l i b r i u m p r e s s u r e can be e s t i m a t e d at the F e - T i o x i d e quench temperature u s i n g -(8), (9) and r e a c t i o n ( 1 0 ) : S i 0 2 = S i 0 2 (10) g l a s s q u a r t z The f u g a c i t y o f water i s then d e f i n e d a t T and p T o t a 2 by r e a c t i o n (8) o r ((?'<)).. The c a l c u l a t i o n s a r e e x t r e m e l y s e n s i t i v e to the a c t i v i t y -c o m p o s i t i o n models used. Because the o x i d a t i o n s t a t e o f i r o n , and the c o m p o s i t i o n o f the h y d r o x y l s i t e i n the b i o t i t e i s unknown, i t i s 2+ V n e c e s s a r y t o assume t h a t a l l i r o n i s p r e s e n t as Fe , and X = 1.0 OH 163 v * i i - - A C BIOTITE b e f o r e the c a l c u l a t i o n s p r o c e e d ( i . e . aT.„ n ,,_.„.. K F e 3 A l S i 3 0 1 Q ( 0 H ) 2 ( F e 2 + / ( M g + F e 2 + ) ) 3 ( X ^ „ ) 2 ; Wones, 1972). C a l c u l a t e d f„ . f o r V one l a v a (536) i s 5869 b a r s a t 8.95 kb t o t a l p r e s s u r e . I f X = 0.75, (Jn a v a l u e common to v o l c a n i c b i o t i t e s ( C a r m i c h a e l , 1967b; Deer and o t h e r s , 1966), i s used, f„ n would be 3296 b a r s a t the same P_ . H^O T o t a l The e s t i m a t e d water f u g a c i t i e s , however, are c o n s i s t e n t w i t h the p r e v i o u s s u g g e s t i o n t h a t p h e n o c r y s t s i n the G a r i b a l d i Lake a n d e s i t e s e q u i l i b r a t e d under c o n d i t i o n s o f P T T . < P m . H^O T o t a l P o s s i b l e Sources o f G a r i b a l d i Lake A n d e s i t e s D i f f e r e n t p e t r o l o g i s t s have s u g g e s t e d t h a t the s o u r c e r e g i o n of c a l c - a l k a l i n e a n d e s i t e magmas may be r e p r e s e n t e d by the h i g h - p r e s s u r e e c l o g i t i c e q u i v a l e n t o f subducted o c e a n i c l i t h o s p h e r e , by the p e r i d o t i t i c m i n e r a l o g y o f the mantle wedge o v e r l y i n g the B e n i o f f zone, o r by a p y r o x e n i t i c m i n e r a l assemblage produced through r e a c t i o n o f n e a r - p r i m i t i v e p e r i d o t i t i c mantle w i t h r h y o d a c i t i c m e l t s r i s i n g from the s u b d u c t i o n zone. The p e t r o l o g i c a l - t h e r m o d y n a m i c approach, d e s c r i b e d above, can be used t o t e s t t h e d e r i v a t i o n o f G a r i b a l d i Lake magmas from these p o s t u l a t e d s o u r c e s . I f t h e l a v a s r e p r e s e n t p a r t i a l m e l t i n g p r o d u c t s o f e c l o g i t e , p e r i d o t i t e o r p y r o x e n i t e , then t h e s e c a l c u l a t i o n s may i n d i c a t e l i q u i d u s temperatures and p r e s s u r e s which a r e c o n s i s t e n t w i t h the e x p e r i m e n t a l o b s e r v a t i o n s . E q u i l i b r a t i o n o f A n d e s i t e With Transformed O c e a n i c C r u s t The G a r i b a l d i Lake a n d e s i t e s f r e q u e n t l y c o n t a i n round t o o v o i d , pyroxene-rimmed g r a i n s o f q u a r t z , which can not be a t t r i b u t e d t o 1 6 4 m e c h a n i c a l d i s a g g r e g a t i o n of i n c l u d e d q u a r t z d i o r i t e . I n c l u s i o n s o f q u a r t z d i o r i t e a r e r a r e , and where p r e s e n t , show no n o t a b l e s i g n s of a s s i m i l a t i o n . As T.H. Green (1972) has shown t h a t q u a r t z p r e c i p i t a t e s o v e r a wide range o f p r e s s u r e s and temperatures i n hydrous a n d e s i t e li'qfai'd'S;, t h e s e ^xrenO,cry<s'ts<!>If o-ftiq^uart^za^ay * a c t u a l l y - be h i g h - p r e s s u r e p h e n o c r y s t s ( N i c h o l l s and o t h e r s , 1971). A l t e r n a t i v e l y , the p r e s e n c e o f q u a r t z i n e c l o g i t e o f t r a n s f o r m e d o c e a n i c t h o l e i i t e (T.H. Green and Rihgwood, 1968) s u g g e s t s t h a t the x e n o c r y s t s may i n d i c a t e p a s t e q u i l i b r i u m w i t h e c l o g i t e . Marsh and C a r m i c h a e l (1974) t e s t e d t h e h y p o t h e s i s t h a t c a l c - a l k a l i n e a n d e s i t e s o r i g i n a t e by p a r t i a l f u s i o n o f q u a r t z e c l o g i t e a l o n g the s u b d u c t i o n zone. They proceeded on the b a s i s t h a t s a n i d i n e , r a t h e r than p h l o g o p i t e , i s the p o t a s s i u m - b e a r i n g phase i n e c l o g i t e . I f w i t h i n c r e a s i n g temperature q u a r t z e c l o g i t e b e g i n s to m e l t , the a c t i v i t y o f K^O i n the l i q u i d would be g i v e n by: 3 C a M g S i 2 0 6 +.2KAlS ± 3 0 g = C a 3 A l 2 S i 3 0 1 2 + 3MgSi0 3 d i o p s i d e s a n i d i n e g r o s s u l a r e n s t a t i t e + 6 S i 0 2 + K 20 (11) q u a r t z l i q u i d L i k e w i s e , i n s i l i c a - u n d e r s a t u r a t e d a n d e s i t e s the a c t i v i t y o f K^O would be d e f i n e d by r e a c t i o n (12) ( T a b l e X I ) : 2 K A l S i o 0 o + 5Mg„SiO. + CaMgSi o0, = s a n i d i n e f o r s t e r i t e d i o p s i d e H M g S i 0 3 + C a A l 2 S i 2 O g + K^O (12) e n s t a t i t e a n o r t h i t e l i q u i d 165 o r i n s i l i c a - s a t u r a t e d a n d e s i t e s by r e a c t i o n (13) ( T a b l e X I ) : M g S i 0 3 + C a A l 2 S i 2 O g + 5 S i 0 2 + K^O = e n s t a t i t e a n o r t h i t e q u a r t z l i q u i d 2 K A l S i o 0 Q + CaMgSi o0,, (13) J O Z D s a n i d i n e d i o p s i d e where K A I S i 0 Q r e p r e s e n t s the s a n i d i n e component i n p l a g i o c l a s e . 3 o Combining (11) w i t h (12) o r (13) a l l o w s c o n s t r u c t i o n o f a u n i v a r i a n t c u r v e f o r the e q u i l i b r a t i o n o f an a n d e s i t i c l i q u i d w i t h q u a r t z e c l o g i t e ( F i g . 2 6). To d e f i n e the unique e q u i l i b r i u m c o n d i t i o n s , another independent r e l a t i o n , a l s o dependent on p r e s s u r e and temperature, must be used. Marsh and C a r m i c h a e l (1974) chose the c o i n c i d e n c e o f the e c l o g i t e ( o c e a n i c t h o l e i i t e ) s o l i d u s and the a n d e s i t e l i q u i d u s as the n e c e s s a r y c o n d i t i o n . However, the P-T p o s i t i o n o f the a n d e s i t e l i q u i d u s i s e x t r e m e l y s e n s i t i v e t o the c o m p o s i t i o n and water c o n t e n t o f the magma. A l t e r n a t i v e l y , i f an a n d e s i t e magma was i n e q u i l i b r i u m w i t h q u a r t z e c l o g i t e , then the l i q u i d must have been i n e q u i l i b r i u m w i t h q u a r t z a t i t s p o i n t of o r i g i n , o r LAVA,P,T QUARTZ, P ,T ( l o g a ^ ) = ( l o g ) C a l c u l a t i o n o f the s i l i c a u n i v a r i a n t c u r v e , t h e r e f o r e , g e n e r a t e s a unique P-T i n t e r s e c t i o n f o r a n d e s i t e - q u a r t z e c l o g i t e e q u i l i b r a t i o n ( F i g . 2 6). S i m i l a r c a l c u l a t i o n s can be performed t o t e s t e q u i l i b r i u m between an a n d e s i t i c l i q u i d and k y a n i t e - b e a r i n g e c l o g i t e , where s i l i c a and p o t a s s i u m a c t i v i t i e s , r e s p e c t i v e l y , a r e d e f i n e d by r e a c t i o n s (14) 166 and (15) ( T a b l e X I ) : 3MgS10 3 + C a 3 A l 2 S i 3 0 1 2 + S10 2 = 3CaMgSi 2C> 6 + A 1 2 S 1 0 5 (14) e n s t a t i t e g r o s s u l a r g l a s s d i o p s i d e k y a n i t e 15CaMgSi„0, + 6 A l o S i 0 c + K.O = d i o p s i d e k y a n i t e l i q u i d 2 K A l S i 3 0 g + C a 3 A l 2 S i 2 0 1 2 + 15MgSi0 3 s a n i d i n e g r o s s u l a r e n s t a t i t e Approximate c o m p o s i t i o n s o f m i n e r a l phases i n q u a r t z o r k y a n i t e , .„ , VCPX . . vOPX . . , GARNET n _ e c l o g x t e ( X ^ s , ^ - 0.4, X ^ . ^ = 0.3, and X ^ ^ . ^ = 0.3, where q u a r t z , k y a n i t e and s a n i d i n e a r e assumed to be pure) can be used t o e s t i m a t e a ^ Q and ^ i n the r e f r a c t o r y assemblages. The volume term o f r e a c t i o n ( 1 1 ) , however, changes s i g n w i t h i n c r e a s i n g p r e s s u r e , and n ( 1 1 K P . T A _ _ R f-o n ( l o g ^ ) = •— + B + I T 3 0 3 ^ ( P " 1) must be e v a l u a t e d w i t h (Marsh and C a r m i c h a e l , 1974): AV° = a + bT + c T 2 + dP + e P 2 K where a = 0.4041, b = -24.415 x 1 0 " 5 , c = 11.166 x 1 0 _ 8 , d = -8.922 —6 —12 x 10 , and e = 26.85 x 10 . C a l c u l a t e d P-T c o n d i t i o n s o f a n d e s i t e -q u a r t z e c l o g i t e e q u i l i b r a t i o n f o r s e v e r a l G a r i b a l d i Lake magmas a r e g i v e n i n T a b l e X I I I . R e s u l t s o f c a l c u l a t i o n s i n v o l v i n g groundmass-defined a c t i v i t i e s i n d i c a t e g e o l o g i c a l l y r e a l i s t i c e q u i l i b r a t i o n p r e s s u r e s (32-40 k b ) , b u t anomalous l i q u i d u s temperatures (2028-2253°C); o n l y the e s t i m a t e d l i q u i d u s temperature o f the p h e n o c r y s t - e q u i l i b r a t e d 167 E n o s t u c k (668) l i q u i d ( d e f i n e d by e x t r a p o l a t e d a c t i v i t i e s based on p h e n o c r y s t - c o r e c o m p o s i t i o n s ) agrees w i t h p r e d i c t e d m a n t l e - s l a b i n t e r f a c e geotherms (Oxburgh and T u r c o t t e , 1970; Schatz and Simmons, 1 9 7 2 ) T h e c o n s t a n c y o f c a l c u l a t e d e q u i l i b r a t i o n p r e s s u r e s r e f l e c t s the s t e e p dT/dP s l o p e o f the 1^0 u n i v a r i a n t curve ( F i g . 26). E q u i l i b r a t i o n o f G a r i b a l d i Lake a n d e s i t e s and k y a n i t e - b e a r i n g e c l o g i t e o n l y o c c u r s a t g e o l o g i c a l l y u n r e a l i s t i c p r e s s u r e s and temperatures. The c a l c u l a t e d e q u i l i b r i u m c o n d i t i o n s suggest t h a t the G a r i b a l d i Lake l a v a s were n e v e r i n e q u i l i b r i u m w i t h an e c l o g i t i c m i n e r a l assemblage, or i f i r t h e y were, t h a t the l i q u i d s s u b s e q u e n t l y underwent a c o n s i d e r a b l e amount o f m o d i f i c a t i o n . E q u i l i b r a t i o n o f A n d e s i t e W i t h G a r n e t - P y r o x e n i t e Ringwood (1974) proposed t h a t the p e r i d o t i t i c mantle o v e r l y i n g a B e n i o f f zone r e a c t s w i t h hydrous s i l i c i c ( r h y o d a c i t i c ? ) l i q u i d g e n e r a t e d by m e l t i n g o f w a t e r - s a t u r a t e d q u a r t z e c l o g i t e to produce d i a p i r s o f hydrous g a r n e t - p y r o x e n i t e . I f as the d i a p i r s r i s e and the g a r n e t - p y r o x e n i t e b e g i n s t o m e l t , the s i l i c a a c t i v i t y i n the a n d e s i t i c l i q u i d would be d e f i n e d by r e a c t i o n ( 3 ) . S i m i l a r i l y , the a c t i v i t y o f alum i n a i n the m e l t would be d e f i n e d by r e a c t i o n (16) ( T a b l e X I ) : M g 2 S i 0 4 + M g S i 0 3 + S i 0 2 + A l ^ = M g 3 A l 2 S i 3 0 1 2 (16) f o r s t e r i t e e n s t a t i t e g l a s s l i q u i d pyrope The p r e s s u r e s and temperatures c a l c u l a t e d f o r e q u i l i b r a t i o n o f the GARNET G a r i b a l d i Lake magmatic l i q u i d s w i t h g a r n e t - p y r o x e n i t e (X^ A T c- n OPX OLIV = 0.49, 2^ s ± 0 =0.80 and X M s ± 0 = 0.85) l i e o u t s i d e the ga r n e t s t a b i l i t y f i e l d (1061-1155°C, 9-15 kb; T a b l e X I I I ) . Thus, e q u i l i b r i u m between the G a r i b a l d i Lake a n d e s i t e s and a p y r o x e n i t i c r e f r a c t o r y 168 assemblage of the assumed m i n e r a l c o m p o s i t i o n s can n o t be e s t a b l i s h e d . E q u i l i b r a t i o n o f A n d e s i t e With Mantle P e r i d o t i t e The s u g g e s t i o n by M cBirney (1969) t h a t d e h y d r a t i o n of downgoing o c e a n i c l i t h o s p h e r e , f o l l o w e d by i n t r o d u c t i o n o f water i n t o the o v e r l y i n g m a n t l e , might l e a d to the p r o d u c t i o n of c a l c - a l k a l i n e a n d e s i t e s has s t i r r e d c o n s i d e r a b l e d i s c u s s i o n amongst e x p e r i m e n t a l p e t r o l o g i s t s (Mysen and o t h e r s , 1974). A l t h o u g h the G a r i b a l d i Lake b a s a l t i c - a n d e s i t e s do not c o n t a i n m a n t l e - d e r i v e d u l t r a m a f i c n o d u l e s , v a r i o u s l a v a s do c o n t a i n p i c o t i t e , s u b - s i l i c i c t s o h e r m a k i t i c -p a r g a s i t i c h o r n b l e n d e , N i - r i c h s u l p h i d e s , magnesian o l i v i n e and p h l o g o p i t e (Mg/(Mg + Fe) > 0.8) which a r e s u g g e s t i v e of a ( m a n t l e - d e r i v e d ) b a s a l t i c o r p i c r i t i c p a r e n t a g e (Chapter IV) I f the magmatic l i q u i d s e q u i l i b r a t e d w i t h a m antle p e r i d o t i t e assemblage a t depth, then the s i l i c a a c t i v i t y i n the l i q u i d phase would be d e f i n e d by r e a c t i o n ( 3 ) . In an analogous manner, the alumina a c t i v i t y would be g i v e n i n p l a g i o c l a s e - p e r i d o t i t e by r e a c t i o n ( 1 7 ) : C a M g S i 2 0 6 + M g S i 0 3 + A l j O = C a A l ^ ^ O g + M g 2 S i 0 4 , (17) d i o p s i d e e n s t a t i t e l i q u i d a n o r t h i t e f o r s t e r i t e i n s p i n e l - p e r i d o t i t e by r e a c t i o n ( 1 8 ) : M g 2Si0 4 + A 1 2 0 3 = M g S i 0 3 + M g A l ^ , (18) f o r s t e r i t e l i q u i d e n s t a t i t e s p i n e l o r i n g a r n e t - p e r i d o t i t e by r e a c t i o n (16). T y p i c a l m i n e r a l c o m p o s i t i o n s e - i A • A i - . i i , VSPINEL _ 7 R GARNET f o r m a n t l e - d e r i v e d u l t r a m a f i c n o d u l e s (X., ., _ = 0.75, X, ,, „. „ M g A l ^ T t f g 3 A l 2 S i 3 0 1 2 = 0.65, X ° L I ^ = 0.90, X ° P J . n = 0.85, and X P L A ( T _. . = 0.40, 0.60 M g 2 S i 0 4 M g S i 0 3 C a A l 2 S i 2 0 g or 0.80) can be used t o e s t i m a t e a„.„ and a A 1 d e f i n e d by the S i 0 2 A 1 2 0 3 169 d i f f e r e n t p e r i d o t i t e assemblages. S i l i c a and alumina u n i v a r i a n t c urves can then be c a l c u l a t e d by e q u a t i n g ( 3 ) , and (16) , (17) o r (18), r e s p e c t i v e l y , w i t h e q u i v a l e n t e q u a t i o n s o f type (C) f o r a l a v a ; the i n t e r s e c t i o n o f t h e s e c u r v e s g i v e s a unique P and T f o r l a v a - p e r i d o t i t e e q u i l i b r a t i o n ( F i g . 26). R e s u l t s o f c a l c u l a t i o n s based on the groundmass-defined a c t i v i t i e s g i v e temperatures 100-600°C below quenching temperatures and n e g a t i v e p r e s s u r e s (-2 t o -12 k b ) . These v a l u e s a r e u n r e a l i s t i c and s uggest t h a t the G a r i b a l d i Lake a n d e s i t e s c o u l d n ot have e q u i l i b r a t e d w i t h a p e r i d o t i t i c m i n e r a l assemblage. The c a l c u l a t i o n s based on p h e n o c r y s t - d e f i n e d s i l i c a and alu m i n a a c t i v i t i e s , however, i n d i c a t e t h a t the p h e n o c r y s t - e q u i l i b r a t e d magmas c o u l d have been g e n e r a t e d by p a r t i a l m e l t i n g o f a s p i n e l - l h e r z o l i t e mantle ( T a b l e X I I I ) . The p h e n o c r y s t r ^ e q u i l i b r a t r e d a S p h i n x l M o f a i n e v (5&8)el'iquid, f o r example, c o u l d have been i n e q u i l i b r i u m w i t h s p i n e l - p e r i d o t i t e a t 1235°C and 17.8 kb; the p r e s s u r e - t e m p e r a t u r e i n t e r s e c t i o n i s c o m p a t i b l e w i t h the p e t r o l o g i c c r i t e r i o n t h a t i t must l i e w i t h i n the s t a b i l i t y f i e l d o f the s p i n e l - p e r i d o t i t e m i n e r a l assemblage ( F i g . 26). The p r e s s u r e s and temperatures o f m a g m a - l h e r z o l i t e e q u i l i b r a t i o n a r e i n t e r p r e t e d as the c o n d i t i o n s under which t h e l i q u i d s and l h e r z o l i t e l a s t c o - e x i s t e d i n e q u i l i b r i u m , o r the p o i n t a t which the magma s e g r e g a t e d , l e a v i n g a l h e r z o l i t e residuum. Thus, the r e s u l t s o f the e q u i l i b r i u m c a l c u l a t i o n s s t r o n g l y suggest t h a t the G a r i b a l d i Lake b a s a l t i c - a n d e s i t e magmas ar e o r i g i n a l l y mantle ( p e r i d o t i t e ) d e r i v e d . E q u i l i b r a t i o n o f B a s a l t i c Lavas With M a n t l e P e r i d o t i t e M i l d l y a l k a l i n e b a s a l t i c l a v a s o c c u r w i t h i n the Cheakamus R i v e r v a l l e y , which f l a n k s the a n d e s i t i c s t r a t o - v o l c a n o e s o f the 170 G a r i b a l d i Lake a r e a , and as a Recent a l k a l i - o l i v i n e b a s a l t f l o w e r u p t e d from The C i n d e r Cone. The Cheakamus V a l l e y b a s a l t s c o n t a i n a v i t r o p h y r i c groundmass w i t h s p a r c e c l i n o p y r o x e n e , o l i v i n e , p l a g i o c l a s e and i r o n - t i t a n i u m o x i d e s . A complete o v e r l a p o f groundmass and p h e n o c r y s t c o m p o s i t i o n s e x i s t s f o r a l l phases except o l i v i n e . Comparison of the b a s a l t i c c o m p o s i t i o n s w i t h r e l a t i o n s i n the s i m p l e system CaO-MgO-Al20.j-Si02 (O'Hara, 1968) s u g g e s t s t h a t t h e s e l a v a s commenced f r a c t i o n a t i n g o l i v i n e , c l i n o p y r o x e n e and p l a g i o c l a s e between 5 and 10 kb (Chapter I I I ) . T h e r e f o r e , as t h e l i q u i d s e v o l v e d , s i l i c a a c t i v i t y would be d e s c r i b e d by r e a c t i o n (1) ( T a b l e X I ) . S i m i l a r i l y , a lumina a c t i v i t y i n t h e l a v a s would be d e f i n e d by r e a c t i o n (19) ( T a b l e X I ) : C a M g S i 2 0 6 + 1/2 S i 0 2 + A l ^ = C a A l 2 S i 2 0 8 + 1/2 l ^ S i O ^ (19) d i o p s i d e g l a s s l i q u i d a n o r t h i t e f o r s t e r i t e V a l u e s of l o g a, X and ((> f o r s i l i c a and a l u m i n a i n t h r e e Cheakamus V a l l e y b a s a l t s a r e l i s t e d i n T a b l e XV, where X and X A 1 r e f e r o i L ^ AJ-2^^ to the b u l k c o m p o s i t i o n o f the l a v a s . C a l c u l a t e d p T o t a ^ where t h e s e l a v a s c o u l d have been i n e q u i l i b r i u m w i t h s p i n e l - o r g a r n e t -p e r i d o t i t e a r e a l s o g i v e n i n T a b l e XV. F i e s i n g e r (1975) found t h a t s i m i l a r c a l c u l a t i o n s , u s i n g a c t i v i t i e s e s t i m a t e d from the groundmass m i n e r a l o g y , y i e l d r e s u l t s which i n d i c a t e t h a t the b a s a l t s c o u l d n o t have o r i g i n a t e d i n the m a n t l e , o r i f they had t h a t they have undergone c o n s i d e r a b l e m o d i f i c a t i o n . The r e s u l t s ,'ofsealeula'tionsrbased on p h e n o c r y s t c o m p o s i t i o n s , i n s t e a d . o f the groundmass c o m p o s i t i o n s , suggest t h a t the l a v a s c o u l d have been g e n e r a t e d i n the m a n t l e . The c a l c u l a t e d TABLE XV. CALCULATED TEMPERATURES AND PRESSURES OF EQUILIBRATION OF BASIC LAVAS WITH SPINEL- AND GARNET-PERI DOT ITE 35 1021 -1.0672 -3-9929 0.5241 0.0975 -1017-9 -3S58.6 1556 37.3 1604 38.2 435 1017 -0.8872 -4.2176 0.5466 0.1028 -806.1 -4166.2 1284 26.0 1254 25-0 71 1025 -0.7462 -4.2018 0.5284 0.0960 -60S-0 -4132.9 1174 19.4 1078 17-4 404 1007 -0.9702 -4.1041 0.5819 0.1079 -940.6 -4015-5 1422 30.8 1422 30.8 453 1007 -0.9702 -4.1041 0.5374 0.1084 -896.6 -4018.1 1432 31.4 1431 31-3 172 p r e s s u r e s and temperatures a r e c o n s i s t e n t w i t h a model i n v o l v i n g p a r t i a l m e l t i n g o f mantle l h e r z o l i t e ; o n l y c a l c u l a t i o n s i n v o l v i n g f e l d s p a t h i c p e r i d o t i t e g i v e u n r e a l i s t i c p r e s s u r e s and t e m p e r a t u r e s . A g a r n e t - p e r i d o t i t e m e l t i n g model can p o s s i b l y be d i s c a r d e d f o r l a v a 71 ( T a b l e XV) as the c a l c u l a t e d P-T i n t e r s e c t i o n l i e s o u t s i d e the g a r n e t s t a b i l i t y f i e l d . The P-T i n t e r s e c t i o n determined f o r s p i n e l - l h e r z o l i t e - b a s a l t e q u i l i b r a t i o n (1174°C; 19.4 k b ) , however, i s c o m p a t i b l e w i t h the p e t r o g e n e t i c c r i t e r i o n t h a t i t must l i e w i t h i n the s p i n e l - l h e r z o l i t e s t a b i l i t y f i e l d . T h e r e f o r e , e q u i l i b r a t i o n o f the Brandywine F a l l s b a s a l t (71) w i t h a s p i n e l - l h e r z o l i t e r e f r a c t o r y assemblage seems p l a u s i b l e . In c o n t r a s t , c a l c u l a t i o n o f I?Total where the Cheakamus Dam (35) and C a l l a g h a n (435) b a s a l t s c o u l d have been i n e q u i l i b r i u m w i t h g a r n e t - l h e r z o l i t e g i v e s temperatures o f 1604°C and 1254°C and p r e s s u r e s o f 38.2 and 25 kb ( T a b l e XV); the c a l c u l a t e d l i q u i d u s temperature 41604rG;)sand p;t?essureC;(38Y2" kb) f o r the eheakamusoDamebasalt ,uhowever^iseem-.,uhfeasonably h i g h . The Helm Creek a l k a l i - b a s a l t / m u g e a r i t e l a v a o f The C i n d e r Cone shows c o n s i d e r a b l e c h e m i c a l h e t e r o g e n e i t y , which may be a t t r i b u t e d t o c r y s t a l f r a c t i o n a t i o n o f k a e r s u t i t i c h o r n b l e n d e , accompanied by o l i v i n e and c l i n o p y r o x e n e , from a p a r e n t a l a l k a l i p i c r i t e w i t h i n the upper mantle (Chapter I I I ) . C a l c u l a t i o n of p ^ o t a ^ ( u s i n S p h e n o c r y s t -d e f i n e d a c t i v i t i e s ) where the magmatic l i q u i d c o u l d have been i n e q u i l i b r i u m w i t h mantle p e r i d o t i t e y i e l d s temperatures between 1422°C and 1432°C and p r e s s u r e s from 30.8 to 31.4 kb ( T a b l e XV). D.H. Green (1970) proposed t h a t a l k a l i p i c r i t e magma may be the p a r t i a l m e l t o f upper mantle p y r o l i t e w i t h about 0.5 p e r c e n t water at 'v-1400°C and 25 kb p r e s s u r e . 173 D i s c u s s i o n The thermodynamic c a l c u l a t i o n s d e s c r i b e d h e r e p r o v i d e a s e m i - q u a n t i t a t i v e assessment o f p o s s i b l e p e t r o g e n e t i c models f o r the G a r i b a l d i Lake v o l c a n i c s u i t e . A l t h o u g h the f o r m u l a t i o n s a r e s u b j e c t to c o n s i d e r a b l e u n c e r t a i n t y when the r e s u l t s a r e compared to e x p e r i m e n t a l e q u i l i b r i a , t h e r e appears to be a remarkable i n t e r n a l c o n s i s t e n c y i n the e s t i m a t e d e q u i l i b r i u m c o n d i t i o n s . C a l c u l a t e d p h e n o c r y s t e q u i l i b r a t i o n p r e s s u r e s ( P T O T A T ) f ° r the a n d e s i t i c l a v a s range between 4.5 and 11 kb and average 7.5 kb (^25 km), i n d i c a t i n g lower c r u s t a l c r y s t a l l i z a t i o n . Water f u g a c i t i e s e s t i m a t e d f o r the h o r n b l e n d e - and b i o t i t e - b e a r i n g a n d e s i t e s i n d i c a t e p h e n o c r y s t e q u i l i b r a t i o n under P^ Q < p T o t a 1 » w i t h X^ 1^ ^ 0.09-0.15. C a l c u l a t e d water c o n t e n t s o f the b a s a l t i c - a n d e s i t e s range from 3 to 5 p e r c e n t , i n e x c e l l e n t agreement w i t h 3 to 7 p e r c e n t water c o n t a i n e d w i t h i n v a p o r - s a t u r a t e d g l a s s i n c l u s i o n s i n t h e p h e n o c r y s t s (determined i n a few cases by d i f f e r e n c e by m i c r o p r o b e ) . The e s t i m a t e d water c o n t e n t s suggest t h a t t h e G a r i b a l d i Lake magmas were water-u n d e r s a t u r a t e d , o r a t b e s t , w a t e r - s a t u r a t e d a t p r e s s u r e s l e s s than 5 kb. C a l c u l a t i o n s based on groundmass-defined a c t i v i t i e s ( aV^^ ) s u g g e s t t h a t n e i t h e r a n d e s i t i c nor b a s a l t i c magmas o f the G a r i b a l d i Lake a r e a c o u l d be d i r e c t p a r t i a l m e l t s o f any assumed m i n e r a l assemblage i n the upper m a n t l e . I t i s l i k e l y , however, t h a t t h e s e magmatic l i q u i d s have undergone c r y s t a l f r a c t i o n a t i o n on t h e i r a s c e n t to the s u r f a c e , and t h a t o n l y the p h e n o c r y s t c o m p o s i t i o n s r e f l e c t the a c t i v i t y o f components i n the magmatic l i q u i d b e f o r e e r u p t i o n . The LAVA p h e n o c r y s t - e q u i l i b r a t e d l i q u i d s o f the a n d e s i t e s ( a ^ d e f i n e d by 174 F i g u r e 27. Comparison of c a l c u l a t e d m e l t - l h e r z o l i t e e q u i l i b r i u m conditions>'with(=experimentaliyridetermined l i q u i d i f o r a n d e s i t e ( S t e r n and o t h e r s , 1975), and o l i v i n e t h o l e i i t e and S i O ^ - s a t u r a t e d t h o l e i i t e ( N i c h o l l s and Ringwood, 1973) under c o n d i t i o n s o f P 7 7 . < P m n^O T o t a l f o r a range of water c o n t e n t s . Shaded r e g i o n i n d i c a t e s the f i e l d o f o l i v i n e c r y s t a l l i z a t i o n on o r n e a r the l i q u i d u s o f hydrous o l i v i n e t h o l e i i t e c o m p o s i t i o n s (Ringwood, 1974). The f i v e m e l t -l h e r z o l i t e e q u i l i b r a t i o n p o i n t s f o r G a r i b a l d i Lake b a s a l t i c - a n d e s i t e s ( T a b l e X I I I ) are, p l o t t e d .(stars) andiindica,teewaterteonteh'tswbetweeni,5 andwllDgweight p e r c e n t . (Hatched r e g i o n r e p r e s e n t s p o s s i b l e P-T c o n d i t i o n s f o r e q u i l i b r a t i o n between p a r e n t a l l i q u i d s and a mantle p e r i d o t i t e ; n o t e t h a t the c a l c u l a t e d p o i n t s l i e w e l l above the v a r i o u s w a t e r - u n d e r s a t u r a t e d a n d e s i t e l i q u i d i ) . E r r o r b a r shows e s t i m a t e d e r r o r i n c a l c u l a t e d p o i n t s . P e r i d o t i t e s o l i d i a r e taken from B o e t t c h e r (1973). 175 1000 1200 1400 TEMPERATURE (PC) 176 p h e n o c r y s t - c o r e c o m p o s i t i o n s ) c o u l d have e q u i l i b r a t e d w i t h r e f r a c t o r y s p i n e l - l h e r z o l i t e a t temperatures from 1078°C to 1235°C and p r e s s u r e s between 11.3 kb and 18.5 kb; t h e s e magmatic l i q u i d s c o u l d n o t have been i n e q u i l i b r i u m w i t h q u a r t z e c l o g i t e a t any g e o l o g i c a l l y r e a s o n a b l e P and T. The a s s o c i a t e d ( m i l d l y a l k a l i c ) b a s a l t i c magmas c o u l d have e q u i l i b r a t e d w i t h a p e r i d o t i t i c m i n e r a l assemblage a t h i g h e r p r e s s u r e s and t e m p e r a t u r e s . Mysen and B o e t t c h e r (1975a, 1975b) co n c l u d e d t h a t m a n t l e - d e r i v e d a n d e s i t e l i q u i d s must be s a t u r a t e d o r n e a r l y s a t u r a t e d w i t h water. The w a t e r - u n d e r s a t u r a t e d c h a r a c t e r o f the G a r i b a l d i Lake a n d e s i t e magmas c o n s e q u e n t l y s u g g e s t s t h a t the i n i t i a l p a r t i a l m e l t s c o u l d n ot have been a n d e s i t i c i n c o m p o s i t i o n . The c a l c u l a t e d P-T i n t e r s e c t i o n s o f m e l t -l h e r z o l i t e e q u i l i b r a t i o n , however, a r e co m p a t i b l e w i t h e x p e r i m e n t a l l y -determined phase e q u i l i b r i a f o r w a t e r - u n d e r s a t u r a t e d t h o l e i i t e ( F i g . 27). I t i s s a t i s f y i n g t o f i n d t h a t the i n f e r r e d p a r e n t a l l i q u i d s p l o t w i t h i n the s t a b i l i t y f i e l d o f o l i v i n e i n hydrous o l i v i n e t h o l e i i t e c o m p o s i t i o n s , and between the l i q u i d u s c u r v e s o f o l i v i n e t h o l e i i t e and S i C ^ - s a t u r a t e d t h o l e i i t e f o r 5 and 10 p e r c e n t water c o n t e n t ( N i c h o l l s and Ringwood, 1973). Such magmatic l i q u i d s c o u l d f r a c t i o n a t e o l i v i n e ± c l i n o p y r o x e n e ± C r - s p i n e l d u r i n g the e a r l y s t a g e s o f a s c e n t (Jakes and G i l l , 1970); s e g r e g a t i o n o f amphibole, p l a g i o c l a s e , pyroxene and ma g n e t i t e a t s h a l l o w e r l e v e l s (2-10 kb) would a l l o w e v o l u t i o n o f the more s i l i c e o u s a n d e s i t e s . 177 V I . PETROGENETIC IMPLICATIONS Any model o f a n d e s i t e g e n e s i s i n t h e G a r i b a l d i Lake a r e a must s a t i f y a number o f c o n t r a i n t s imposed by the geoc h e m i s t r y and m i n e r a l o g y of the b a s a l t i c and a n d e s i t i c r o c k s . 87 86 (1) A l l a n a l y z e d l a v a s show d e p l e t i o n i n Rb, low Sr /Sr r a t i o s and h i g h K/Rb r a t i o s r e l a t i v e t o t y p i c a l c a l c - a l k a l i n e v o l c a n i c 87 86 s e r i e s ; t h e u n i f o r m i t y o f Sr /Sr i n the whole range o f G a r i b a l d i Lake r o c k s s u g g e s t s t h a t b a s a l t s and a n d e s i t e s a r e d e r i v e d from a common s o u r c e . 2+ (2) The a n d e s i t e s have Mg/(Mg + Fe ) v a l u e s and N i and Cr co n t e n t s too low f o r t h e s e l a v a s t o r e p r e s e n t u n m o d i f i e d p a r t i a l m e l t s o f mantle p e r i d o t i t e . Low K/Na and Fe/Mg r a t i o s , marked d e p l e t i o n o f Rb, N i , Cr and V, and s t r o n g enrichment o f Sr and Ba i n the a n d e s i t e s , however, argue a g a i n s t e q u i l i b r i u m m e l t i n g o f a g a r n e t - b e a r i n g ( e c l o g i t i c ) assemblage i n subducted o c e a n i c c r u s t . (3) F r a c t i o n a l c r y s t a l l i z a t i o n o f Cheakamus V a l l e y b a s a l t s , i n v o l v i n g t h e i r o b s e r v e d p h e n o c r y s t m i n e r a l o g y (OL + CPX + P L ) , i s i n c a p a b l e o f g e n e r a t i n g t h e n e a r l y t w o - f o l d enrichment o f Sr and Ba i n the a n d e s i t e s r e l a t i v e t o the b a s a l t s . Thus, t h e s e b a s a l t s can not r e p r e s e n t the p r i m a r y magma from which a n d e s i t e was d e r i v e d d i r e c t l y . C r y s t a l f r a c t i o n a t i o n o f the p h e n o c r y s t assemblages i n The T a b l e (PL + AMPB + OPX + MT), Mount P r i c e (PL + AMPB + BO + OPX + MT), and The B l a c k Tusk (PL + CPX + OPX + MT) s i l i c i c - a n d e s i t e s , however, can s a t i s f a c t o r i l y account f o r the major and t r a c e element v a r i a t i o n o b s e r v e d w i t h i n t h e s e s u i t e s . (4) The m i n e r a l o g y o f t h e a n d e s i t e s e x h i b i t s numerous 178 d i s e q u i l i b r i u m f e a t u r e s ( e.g. o s c i l l a t o r y z o n i n g and r e s o r p t i o n phenomena). F o r s t e r i t i c o l i v i n e , p a r g a s i t i c h o r n b l e n d e , aluminous pyroxene, and C r - r i c h t i t a n o m a g n e t i t e i n some a n d e s i t i c l a v a s a r e p r o b a b l y r e l i c t o r x e n o c r y s t i c phases from more m a f i c p a r e n t a l magmas. C r y s t a l c l o t s c o n s i s t i n g e s s e n t i a l l y o f ho r n b l e n d e and d i o p s i d i c a u g i t e , and amphibole megacrysts e n c l o s i n g ragged g r a i n s o f f o r s t e r i t i c o l i v i n e i n t h e Sphinx Moraine b a s a l t i c - a n d e s i t e i n d i c a t e t h a t amphibole and c l i n o p y r o x e n e c o - p r e c i p i t a t e d from a b a s a l t i c magma at h i g h - p r e s s u r e s (>5 k b ) , w i t h e a r l y - f o r m e d o l i v i n e i n r e a c t i o n r e l a t i o n s h i p w i t h the l i q u i d . (5) C a l c u l a t e d p h e n o c r y s t e q u i l i b r a t i o n p r e s s u r e s f o r G a r i b a l d i Lake b a s a l t i c - a n d e s i t e s range from 4.5 to 11 kb and average 7.5 kb (^25 km), i n d i c a t i n g lower c r u s t a l c r y s t a l l i z a t i o n . C a l c u l a t e d f H 2 ° f o r h o r n b l e n d e - and b i o t i t e - b e a r i n g a n d e s i t e s suggest p h e n o c r y s t e q u i l i b r a t i o n under P^ < P r p o t a ^ » w i t h minimum X ^ 1 ^ ^ 0.09-0.15 (3-5 wt. % ) . (6) The r e s u l t s o f e q u i l i b r i u m c a l c u l a t i o n s i n d i c a t e t h a t n e i t h e r b a s a l t i c - a n d e s i t e nor a n d e s i t i c l i q u i d s c o u l d have e q u i l i b r a t e d w i t h e c l o g i t i c o r p y r o x e n i t i c m i n e r a l assemblages a t any g e o l o g i c a l l y r e a s o n a b l e P and T. The p a r e n t a l l i q u i d s o f t h e b a s a l t i c - a n d e s i t e s , however, c o u l d have been i n e q u i l i b r i u m w i t h s p i n e l - l h e r z o l i t e r e s i d u e a t 1150-1235°C and 15-20 kb. The P and T o f m e l t - l h e r z o l i t e e q u i l i b r i u m l i e w i t h i n t h e o l i v i n e s t a b i l i t y f i e l d i n hydrous o l i v i n e t h o l e i i t e c o m p o s i t i o n s , and between the l i q u i d u s c u r v e s o f o l i v i n e t h o l e i i t e and S i 0 2 ~ s a t u r a t e d t h o l e i i t e f o r 5% and 10% water c o n t e n t . The G a r i b a l d i Lake a n d e s i t e s c o u l d not have been d e r i v e d d i r e c t l y as p r i m a r y magmas by m e l t i n g o f near-anhydrous subducted o c e a n i c 179 c r u s t , by m e l t i n g o f hydrous p e r i d o t i t e i n t h e o v e r l y i n g mantle wedge, o r by m e l t i n g o f p e r i d o t i t i c m a t e r i a l which had been m o d i f i e d by hydrous s i l i c a t e melt o r i g i n a t i n g i n the subducted l i t h o s p h e r e . Thus, t h e s e a n d e s i t e s do not have a s i n g l e s t a g e o r i g i n , b u t must be e x p l a i n e d by a m u l t i s t a g e model. M u l t i s t a g e F r a c t i o n a t i o n o f Hydrous B a s a l t Magma - A P r o b a b l e Model The e v o l u t i o n a r y model e n v i s i o n e d h e r e shows t h a t the c h e m i c a l and m i n e r a l o g i c a l v a r i a t i o n s o b s e r v e d i n G a r i b a l d i Lake v o l c a n i c s u i t e s can be e x p l a i n e d by m u l t i s t a g e f r a c t i o n a t i o n o f hydrous b a s a l t magma. The i d e a l i z e d s t a g e s , d e p i c t e d i n F i g . 28, r e p r e s e n t a s i m p l i f i c a t i o n o f the p r o c e s s e s which produced t h e wide range o f l a v a types found i n the G a r i b a l d i Lake a r e a . Stage I : W a t e r - U n d e r s a t u r a t e d T h o l e i i t e Magmas From Hydrous M a n t l e P e r i d o t i t e Above The B e n i o f f Zone As t h e Juan de Fuca P l a t e went beneath southwestern B r i t i s h Columbia, water was i n t r o d u c e d a l o n g t h e B e n i o f f Zone as h y d r a t e d m i n e r a l s o c c u r r i n g i n i t s b a s a l t i c c r u s t . Through t h e depth i n t e r v a l c o r r e s p o n d i n g to t h e a m p h i b o l i t e - e c l o g i t e t r a n s i t i o n (^80-100 km), w a t e r - r i c h f l u i d s were r e l e a s e d by m i n e r a l r e a c t i o n s i n v o l v i n g the breakdown o f amphibole, z o i s i t e a n d s e r p e n t i n e . The amount o f water r e l e a s e d v a r i e d i n time and l o c a t i o n , a c c o r d i n g t o t h e volume o f w a t e r - r i c h o c e a n i c f l o o r m a t e r i a l t r a n s p o r t e d down t h e s u b d u c t i o n zone. The s t r o n g l y endothermic d e h y d r a t i o n r e a c t i o n s p r o b a b l y absorbed enough h e a t to negate any s h e a r - s t r a i n h e a t i n g ( or o t h e r t h e r m a l e f f e c t s ) 180 F i g u r e 28. M u l t i s t a g e e v o l u t i o n o f a n d e s i t e magmas. Schematic c r o s s - s e c t i o n shows c o n f i g u r a t i o n o f Juan de Fuca and American p l a t e s beneath southwestern B r i t i s h Columbia, and v a r i o u s s t a g e s i n the a s c e n t o f magmas g e n e r a t e d above the B e n i o f f zone. Depth o f Moho d i s c o n t i n u i t y a f t e r S t a c e y (1973, 1974). INSULAR BELT V A N C O U V E R I S L A N D COAST PLUTONIC COMPLEX S T R A I T O P G E O R G I A G A R I B A L D I L A K E A R E A AMERICAN PLATE co cc u i r-UJ S o =!50 x H QL UJ o K>00oc 100 ASTHENOSPHERE STAGE III: Low - Pressure Fractionation of Intermediate Melt Compositions S E G R E G A T I O N O F PL + A M P S + O P X + M T , P l + C P X • O P X + M T , O R PL + O P X + MT F R O M B A S A L T I C -A N D E S I T E C O M P O S I T I O N S T O P R O D U C E S I L I C I C - A N D E S I T E A N D D A C I T I C L I Q U I D S AT 8 2 5 - 9 7 5 ° C A N D 2 - 5 KB 2b STAGE II: High - Pressure Fractionation of Water-undersaturated Tholeiite Magmas Q U A R T Z T H O L E I I T E M A G M A S F R A C T I O N A T E A M P B ( P h 2 0 > ° - 5 ' T O T A L ' - A M P B + C P X < P h 2 0 < P T O T A l ) , O R O L • C P X + C r - S P I N E L ( A N H Y D R O U S ) TO P R O D U C E B A S A L T I C - A N D E S I T E S AT 9 7 5 - l 0 5 0 ° C A N D 5 - 1 0 KB 2a 1c 1 0 - 2 0 KB S E G R E G A T I O N O F < 5 P E R C E N T N E A R -L I Q U I D U S O L + C P X * C r - S P I N E L T O Y I E L D Q U A R T Z T H O L E I I T E L I Q U I D S STAGE I: Partial Melting of Hydrous Mantle Peridotite at 60-85 km Depths 1 0 - 3 0 P E R C E N T M E L T I N G W H E R E W E T P E R I D O T I T E S O L I D U S I N T E R S E C T S P R E V A I L I N G M A N T L E T E M P E R A T U R E l>1100 C ) S E G R E G A T I O N O F W A T E R - U N D E R S A T U R A T E D ( 3 - 1 5 P E R C E N T H j O ) T H O L E I I T E M A G M A S 1b 1a U P W A R D - M I G R A T I N G S U B S O L I D U S A Q U E O U S S O L U T I O N S S C A V E N G E S M A L L A M O U N T S O F P O T A S S I U M A N D O T H E R I N C O M P A T I B L E C O N S T I T U E N T S F R O M P E R I D O T I T E D E H Y D R A T I O N O F S U B D U C T E D O C E A N I C C R U S T B E L O W B A S A L T - A M P H I B O L I T E - E C L O G I T E S O L I D U S ( T < 7 5 0 ° C ) I—1 0 0 MULTISTAGE EVOLUTION OF ANDESITE MAGMAS 182, g e n e r a t e d a l o n g the B e n i o f f zone. In the upper p a r t o f the downgoing s l a b temperatures (650-750°C) remained below the w a t e r - s a t u r a t e d b a s a l t - a m p h i b o l i t e - e c l o g i t e s o l i d u s . Thus,, l i t t l e o r no s i l i c a t e m e l t c o u l d o r i g i n a t e i n the subducted o c e a n i c c r u s t . The s u b s o l i d u s aqueous s o l u t i o n s m i g r a t e d upward i n t o the o v e r l y i n g mantle wedge, sc a v e n g i n g K and o t h e r i n c o m p a t i b l e components from p e r i d o t i t e d u r i n g a s c e n t . U n d e r t h r u s t i n g of the o c e a n i c p l a t e d e p r e s s e d i s o t h e r m s i n t h e a d j a c e n t m a n t l e , i n v e r t i n g the geothermal g r a d i e n t so t h a t temperature i n c r e a s e d upward i n the mantle wedge. R i s i n g hydrous f l u i d s c o u l d n ot lower t h e m e l t i n g temperature o f p e r i d o t i t e s u f f i c i e n t l y t o i n d u c e p a r t i a l m e l t i n g u n t i l w e l l above the a c t u a l B e n i o f f zone. The i n i t i a l p r o d u c t from the B e n i o f f zone was t h e r e f o r e aqueous not magmatic. At s h a l l o w e r depths (60-80 km), the s o l i d u s o f p e r i d o t i t e i n the p r e s e n c e o f h i g h water v a p o r p r e s s u r e was de p r e s s e d below the a c t u a l temperature, and m e l t i n g l e d to the development o f water-u n d e r s a t u r a t e d (3%-15% H^O) o l i v i n e t h o l e i i t e and S i O ^ - s a t u r a t e d t h o l e i i t e magmas. With l i t t l e o r no a d d i t i o n of h e a t , s u b s t a n t i a l m e l t i n g (10%-30%) was a c h i e v e d ; the s m a l l amount o f K and g e o c h e m i c a l l y r e l a t e d t r a c e elements c a r r i e d i n t h e va p o r phase b e i n g overwhelmingly d i l u t e d by the l a r g e mass o f r e s u l t i n g magma. The depth a t which p a r t i a l m e l t i n g took p l a c e depended upon the s u p p l y o f upward-migrating hydrous f l u i d s o r l o c a l h e t e r o g e n e i t y i n the mantle wedge. Such h e t e r o g e n e i t y c o u l d have d e v e l o p e d as a r e s u l t o f i n t e r a c t i o n o f p e r i d o t i t e and w a t e r - r i c h f l u i d s d u r i n g e a r l i e r e p i s o d e s o f magma g e n e r a t i o n . Both c o n d i t i o n s c o u l d have c o n c e i v a b l y o p e r a t e d s i m u l t a n e o u s l y . Thus, p a r t i a l m e l t i n g o f mantle c o m p o s i t i o n s proceeded 1-83 under d i f f e r e n t P, T and P c o n d i t i o n s , and the b a s a l t i c magmas s e g r e g a t e d above the B e n i o f f zone c o n t a i n e d v a r i a b l e c o n c e n t r a t i o n s of water. Stage I I : B a s a l t i c - A n d e s i t e s by H i g h - P r e s s u r e F r a c t i o n a t i o n o f Wa t e r - U n d e r s a t u r a t e d T h o l e i i t e Magmas The 20-25 kb l i q u i d u s temperatures o f w a t e r - u n d e r s a t u r a t e d t h o l e i i t e magmas a r e g e n e r a l l y lower than the 1-atm l i q u i d u s t e m p e r a t u r e s . Thus, the t h o l e i i t i c l i q u i d s were f o r c e d t o c r y s t a l l i z e as p r e s s u r e f e l l d u r i n g a s c e n t . In the 10-20 kb i n t e r v a l , c r y s t a l l i z a t i o n o f the r i s i n g b a s a l t i c magmas was c o n t r o l l e d by n e a r -l i q u i d u s o l i v i n e , c l i n o p y r o x e n e and C r - s p i n e l . At t h i s s t a g e , f r a c t i o n a t i o n . y i e l d e d c o m p o s i t i o n s no more s i l i c e o u s t h a n q u a r t z t h o l e i i t e . The r i s i n g magma ba t c h e s encountered the base o f the c r u s t a t 30-35 km depths. The c o m p a r a t i v e l y low d e n s i t y o f the c r u s t a l r o c k s a c t e d as a b a r r i e r t o the denser m a f i c magmas, which p r o b a b l y h a l t e d and began c r y s t a l l i z i n g w i t h i n l o p o l i t h i c magma chambers. Heat l o s s e s from the r e l a t i v e l y deep-seated (20-30 km) chambers was l a r g e because of the low temperature o f the s u r r o u n d i n g r o c k s , and the magmas c o u l d c o o l enough to undergo e x t e n s i v e f r a c t i o n a t i o n . The c o u r s e o f c r y s t a l l i z a t i o n towards b a s a l t i c - a n d e s i t e and a n d e s i t e , and c o n s e q u e n t l y any change i n c o m p o s i t i o n o f r e s i d u a l l i q u i d s depended upon the water c o n t e n t o f the p a r t l y - c r y s t a l l i z e d m e l t s . When water c o n t e n t was more than 5%, the magma c o o l i n g paths e n t e r e d the amphibole s t a b i l i t y f i e l d a t 5-10 kb and temperatures o f a p p r o x i m a t e l y 975-1050°C. I f P _ > 0.5 P n i n the magma chamber, 184 amphibole c o u l d s e p a r a t e a l o n e , w i t h e a r l y - f o r m e d o l i v i n e and c l i n o p y r o x e n e i n r e a c t i o n r e l a t i o n s h i p w i t h the l i q u i d . However, the ev i d e n c e i s t h a t the magmas o f the G a r i b a l d i Lake a r e a e q u i l i b r a t e d under c o n d i t i o n s o f P^ ^ < 0.5 ^ j j o t a i > amphibole and c l i n o p y r o x e n e c r y s t a l l i z e d s i m u l t a n e o u s l y , and o l i v i n e was consumed i n r e a c t i o n w i t h the e n c l o s i n g l i q u i d . 50% c r y s t a l l i z a t i o n and consequent removal o f amphibole + c l i n o p y r o x e n e from q u a r t z t h o l e i i t e c o m p o s i t i o n s produced s t r o n g enrichment o f Sr and Ba i n the r e s i d u a l b a s a l t i c -a n d e s i t e and a n d e s i t e m e l t s , but o n l y moderate d e p l e t i o n of N i and Cr. T h i s e v o l u t i o n a r y p a t h c o r r e s p o n d s t o t h a t a l o n g which t h e Sphinx Moraine b a s a l t i c - a n d e s i t e o f the G a r i b a l d i Lake a r e a c r y s t a l l i z e d . A l t e r n a t i v e l y , q u a r t z t h o l e i i t e magmas t h a t c o n t a i n e d i n s u f f i c i e n t water to s t a b i l i z e amphibole, f r a c t i o n a t e d 10%-15% o l i v i n e ± c l i n o p y r o x e n e ± C r - s p i n e l , and t h e r e b y approached c o m p o s i t i o n s s i m i l a r t o the D e s o l a t i o n V a l l e y b a s a l t i c - a n d e s i t e o f the G a r i b a l d i Lake a r e a . The l i q u i d u s temperatures o f n e a r l y - a n h y d r o u s t h o l e i i t e magmas were f a r enough above the 1-atm l i q u i d u s t e m p e r a t u r e s , t h a t the m e l t s were not f o r c e d t o undergo s i g n i f i c a n t amounts o f f r a c t i o n a t i o n . Thus, t h e s e b a s a l t s reached the s u r f a c e c a r r y i n g p h e n o c r y s t s (one o r more o f o l i v i n e , c l i n o p y r o x e n e and p l a g i o c l a s e ) , but o n l y s l i g h t l y m o d i f i e d i n b u l k c o m p o s i t i o n ( e . g . Cheakamus V a l l e y b a s a l t s ) . Stage I I I : S i l i c i c - A n d e s i t e s by Low-Pressure F r a c t i o n a t i o n I n v o l v i n g S e p a r a t i o n o f P h e n o c r y s t s and M i c r o p h e n o c r y s t s As p r e s s u r e b u i l t up w i t h the magma chambers, r e s i d u a l l i q u i d s were e x p e l l e d t o r i s e h i g h e r i n the c r u s t . With c o n t i n u e d f r a c t i o n a t i o n i n the 2-5 kb i n t e r v a l , c l i n o p y r o x e n e and/or amphibole became u n s t a b l e , 185 b e i n g r e p l a c e d o r j o i n e d by p l a g i o c l a s e , o r t h o p y r o x e n e and m a g n e t i t e . Most o f the a s c e n d i n g b a s a l t i c - a n d e s i t e s and a n d e s i t e s were s t o r e d i n s h a l l o w (2-15 km) magma chambers b e f o r e they e r u p t e d on the s u r f a c e . Thus, they c o u l d undergo c o n s i d e r a b l e d i f f e r e n t i a t i o n . At t h i s s t a g e , minor a s s i m i l a t i o n o f q u a r t z d i o r i t e may a l s o have o c c u r r e d . Depending upon c o n d i t i o n s o f p r e s s u r e , temperature, v o l a t i l e c o n t e n t and c o m p o s i t i o n o f the m e l t s , f r a c t i o n a t i o n was governed by s e p a r a t i o n o f p l a g i o c l a s e + amphibole + o r t h o p y r o x e n e + magn e t i t e , p l a g i o c l a s e + c l i n o p y r o x e n e + or t h o p y r o x e n e + m a g n e t i t e , o r p l a g i o c l a s e + ort h o p y r o x e n e + m a g n e t i t e . R e s i d u a l l i q u i d s were d e p l e t e d ( r e l a t i v e t o t h e i r p a r e n t ) i n S r , N i , Cr and V, and e n r i c h e d i n K, Rb and Ba. H i g h - l e v e l d i f f e r e n t i a t i o n t h e r e f o r e produced m e l t s r a n g i n g from s i l i c i c - a n d e s i t e t o d a c i t e i n c o m p o s i t i o n . C o n c l u s i o n I f the m u l t i s t a g e model p r e s e n t e d h e r e i s a c c e p t e d as g e n e r a l l y a p p l i c a b l e t o the G a r i b a l d i Lake v o l c a n i c s u i t e s , t h e r e a r e c o n c l u s i o n s which b e a r on the e v o l u t i o n o f a n d e s i t e magmas ge n e r a t e d beneath southwestern B r i t i s h Columbia, and v o l c a n i c - a r c r e g i o n s i n g e n e r a l . (1) The t r a n s f e r o f magma from the s i t e o f p a r t i a l m e l t i n g t o the s u r f a c e i s n o t u n i n t e r r u p t e d a s c e n t , as sugg e s t e d by Marsh (1976a), but r a t h e r , movement must be i r r e g u l a r w i t h s u f f i c i e n t time between surges f o r m o d i f i c a t i o n o f m e l t c o m p o s i t i o n s by c r y s t a l f r a c t i o n a t i o n p r o c e s s e s , perhaps a t s e v e r a l d i f f e r e n t d e p ths. (2) T r a c e element abundances i n l a v a s e r u p t e d on the s u r f a c e a r e c o n t r o l l e d by c r y s t a l / m e l t p a r t i t i o n c o e f f i c i e n t s o f l i q u i d u s phases a t s u c c e s s i v e s t a g e s o f c r y s t a l l i z a t i o n . The c o n c e n t r a t i o n s o f 186 S r , Ba, C r , N i and V i n a n d e s i t i c l a v a s p r o v i d e i n f o r m a t i o n r e g a r d i n g the e x t e n t t o which m e l t c o m p o s i t i o n s have been m o d i f i e d d u r i n g a s c e n t , and the n a t u r e o f m i n e r a l assemblages which f r a c t i o n a t e d from the l i q u i d d u r i n g s u c c e s s i v e e p i s o d e s o f d i f f e r e n t i a t i o n . (3) Each magma p u l s e i n v o l c a n i c - a r c r e g i o n s can e v o l v e u n i q u e l y under d i f f e r e n t c o n d i t i o n s o f p r e s s u r e , temperature, v o l a t i l e c o n t e n t and c o m p o s i t i o n o f s o u r c e r o c k , and by d i f f e r e n t s t a g e s o f a s c e n t . Thus, an u n d e r s t a n d i n g o f p r o c e s s e s r e l a t e d t o the g e n e r a t i o n and t r a n s f e r o f magmas i n r e g i o n s o f p l a t e convergence can o n l y be o b t a i n e d by d e t a i l e d s t u d y o f e r u p t i v e c e n t e r s a l o n g the e n t i r e l e n g t h of the v o l c a n i c a x i s . The G a r i b a l d i Lake a r e a i s o n l y one such v o l c a n i c f i e l d , but the w e a l t h o f d a t a a v a i l a b l e has en a b l e d i t s magmatic h i s t o r y t o be t r a c e d i n c o n s i d e r a b l e d e t a i l . 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E a r t h S c i . , 10, pp. 615-628. , 1974, P l a t e t e c t o n i c s , v o l c a n i s m and the l i t h o s p h e r e i n B r i t i s h Columbia. N a t u r e , 250, pp. 133-134. S t e i n b o r n , T.L., T r a c e element geo c h e m i s t r y o f s e v e r a l v o l c a n i c c e n t e r s i n the High Cascades. U n p u b l i s h e d M.Sc. t h e s i s , U n i v e r s i t y o f Oregon, 115 p. S t e r n , C.R., Huang, W.L., and W y l l i e , P . J., 1975, B a s a l t - a n d e s i t e -r h y o l i t e - ^ O : C r y s t a l l i z a t i o n i n t e r v a l s w i t h excess ^ 0 and ^ 0 -u n d e r s a t u r a t e d l i q u i d u s s u r f a c e s to 35 k i l o b a r s , w i t h i m p l i c a t i o n s f o r magma g e n e s i s . E a r t h P l a n e t . S c i . L e t t . , 28, pp. 189-196. Stewart, D.S., 1975, C r y s t a l c l o t s i n c a l c - a l k a l i n e a n d e s i t e s as breakdown p r o d u c t s o f h i g h - A l amphiboles. C o n t r i b . M i n e r a l . P e t r o l . , 53, pp. 195-204. Stormer, J.C., 1973, C a l c i u m z o n i n g i n o l i v i n e and i t s r e l a t i o n s h i p to s i l i c a a c t i v i t y and p r e s s u r e . Geochim. Cosmochim. A c t a , 57, pp." 1815-1821. S t r o n g , D.F., 1969, F o r m a t i o n o f the h o u r - g l a s s s t r u c t u r e i n a u g i t e . M i n e r a l o g . Mag., _37_, pp. 472-479. T a y l o r , S.R., 1969, T r a c e element c h e m i s t r y o f a n d e s i t e s and a s s o c i a t e d c a l c - a l k a l i n e r o c k s , i n P r o c e e d i n g s o f the A n d e s i t e C o n f e r e n c e , A.R. McBirney, ed. , Oreg. Dept. G e o l . M i n e r . Ind. B u l l . , 65_, pp. 43-63. , Kaye, M., White, A.R., Duncan, A.R., and Ewart, A., 1969, G e n e t i c s i g n i f i c a n c e o f Co, C r , N i , Sc, and V c o n t e n t o f a n d e s i t e s . Geochim. Cosmochim. A c t a . , ^3_, pp. 275-286. .207. Thompson, R.N., 1972, M e l t i n g b e h a v i o r o f two Snake R i v e r l a v a s a t p r e s s u r e s up to 35 kb. C a r n e g i e I n s t . Wash. Yearb., _71_, pp. 406-410. Thornton, C P . , and T u t t l e , O.F., 1960, Chemistry of igneous r o c k s : I . D i f f e r e n t i a t i o n Index. Amer. J . S c i . , 258, pp. 664-684. Thorpe, R.S., P o t t s , P.J., and F r a n c i s , P.W., 1976, Rare e a r t h d a t a and p e t r o g e n e s i s o f a n d e s i t e from the North C h i l e a n Andes. C o n t r i b . M i n e r a l . P e t r o l . , j54_, pp. 65-78. T i f f i n , D.L., Cameron, B.E.B., and Murray, J.W., 1972, T e c t o n i c s and d e p o s i t i o n a l h i s t o r y o f t h e c o n t i n e n t a l margin o f f Vancouver I s l a n d , B r i t i s h Columbia. Can. J . E a r t h S c i . , 9_, pp. 280-296. T i l l e y , C E . , Yoder, H.S., and S c h a i r e r , J . F . , 1968, M e l t i n g r e l a t i o n s o f igneous r o c k s e r i e s . C a r n e g i e I n s t . Wash. Yearb., 66^, pp. 450-457. T u t t l e , O.F., and Bowen, N.L., 1958, O r i g i n o f g r a n i t e i n t h e l i g h t o f e x p e r i m e n t a l s t u d i e s i n the system N a A l S i o 0 o - K A l S i „ 0 o - S i 0 o - H „ 0 . J O J O Z Z G e o l . Soc. Amer. Mem., 74. Verhoogen, J . , 1962, D i s t r i b u t i o n o f t i t a n i u m between s i l i c a t e s and o x i d e s i n igneous r o c k s e r i e s . Amer. J . S c i . , 260, pp. 211-220. Wager, L.R., 1960, The major element v a r i a t i o n o f the l a y e r e d s e r i e s o f the Skaergaard i n t r u s i o n . J . P e t r o l . , 1, pp. 217-248. W h i t f o r d , D.J., 1975, S t r o n t i u m i s o t o p i c s t u d i e s o f the v o l c a n i c r o c k s o f t h e Sunda a r c , I n d o n e s i a , and t h e i r p e t r o g e n e t i c i m p l i c a t i o n s . Geochim. Cosmochim. A c t a , _39_, pp. 1287-1302. W i l k i n s o n , J.F.G., 1967, The p e t r o g r a p h y o f b a s a l t i c r o c k s , pp. 163-214, i n : The P o l d e r v a a r t T r e a t i s e on Rocks o f B a s a l t i c C o m p o s i t i o n , H.H. Hess, ed., I n t e r s c i e n c e , New York. 208 Wise, W.S., 1969, Geology and p e t r o l o g y o f the Mount Hood a r e a : a study o f High Cascade v o l c a n i s m . G e o l . Soc. Amer. B u l l . , 80_, pp. 969-1006. Wones, D.R., 1972, S t a b i l i t y o f b i o t i t e : a r e p l y . Amer. M i n e r a l o g i s t , 57, pp. 316-317. _ , and E u g s t e r , H.P., 1965, S t a b i l i t y o f b i o t i t e : e xperiment, t h e o r y and a p p l i c a t i o n . Amer. M i n e r a l o g i s t , 50, p..1228. Wood, B . J . , 1976, An o l i v i n e - c l i n o p y r o x e n e geothermometer: a d i s c u s s i o n . C o n t r i b . M i n e r a l . P e t r o l . , 56^, pp. 297-303. , and Banno, S., 1973, G a r n e t - o r t h o p y r o x e n e and o r t h o p y r o x e n e -c l i n o p y r o x e n e r e l a t i o n s h i p s i n s i m p l e and complex systems. C o n t r i b . M i n e r a l . P e t r o l . , 42, pp. 109-124. , and C a r m i c h a e l , I.S.E., 1973, p T o t a l > P H 0 a n d t h e o c c u r r e n c e o f cummingtonite i n v o l c a n i c r o c k s . C o n t r i b . M i n e r a l . P e t r o l . , 40, pp. 149-158. Yoder, H.S., and T i l l e y , C.E., 1962, O r i g i n o f b a s a l t i c magmas: an e x p e r i m e n t a l study o f n a t u r a l and s y n t h e t i c systems. J . P e t r o l . , 3, pp. 342-532. 209 APPENDIX I . MAJOR AND TRACE ELEMENT DATA FOR GARIBALDI LAKE LAVAS The f o l l o w i n g t a b l e s c o n t a i n the l o c a t i o n s , modal a n a l y s e s , and major and t r a c e element c o m p o s i t i o n s o f r e p r e s e n t a t i v e samples from v a r i o u s v o l c a n i c s u i t e s i n the G a r i b a l d i Lake a r e a , t o g e t h e r w i t h t h e i r CIPW no r m a t i v e m i n e r a l s , n o r m a t i v e p l a g i o c l a s e c o m p o s i t i o n (An %) , and D i f f e r e n t i a t i o n Index (D.I.; Thornton and T u t t l e , 1960). CIPW norms a r e c a l c u l a t e d a f t e r the a n a l y s e s a r e r e c a s t t o 100 p e r c e n t v o l a t i l e - f r e e . The Fe^O^/ (FeO + r a t i o o f the v o l c a n i c r o c k s has not been determined a n a l y t i c a l l y , and i t i s t h e r e f o r e n e c e s s a r y t o s e t a r b i t r a r i l y a v a l u e f o r t h i s r a t i o . b e f o r e the n o r m a t i v e m i n e r a l c o m p o s i t i o n s a r e c a l c u l a t e d . O x i d a t i o n o f a l a v a , f o r example, can a l t e r the n o r m a t i v e c o m p o s i t i o n o f the r o c k g i v i n g r i s e t o q u a r t z and m a g n e t i t e i n p l a c e of o l i v i n e and h y p e r s t h e n e . Such v a r i a n c e c o u l d a f f e c t the c a l c u l a t i o n o f An % and D.I. f o r the l a v a , and the c l a s s i f i c a t i o n of the r o c k ' s b u l k c o m p o s i t i o n . The F e 2 0 3 / ( F e 0 + Fe^O^) r a t i o has been det e r m i n e d f o r some G a r i b a l d i Group v o l c a n i c r o c k s (Mathews, 1957; F i e s i n g e r , 1975; Anderson, 1975). C o m p i l a t i o n o f t h i s d a t a i n d i c a t e s t h a t t h e i n t e r m e d i a t e l a v a s p o s s e s s s i g n i f i c a n t l y h i g h e r Ye^O^I(FeO + Fe^O^) r a t i o s than the c o - e x i s t i n g b a s a l t s , presumably r e f l e c t i n g the c r y s t a l l i z a t i o n o f the i n t e r m e d i a t e l a v a s under h i g h e r water p r e s s u r e and oxygen f u g a c i t y c o n d i t i o n s . The r a t i o has the f o l l o w i n g v a l u e s : ( i ) i f S i 0 2 i s l e s s than 56 p e r c e n t , F e 2 0 3 / ( F e O + F e 2 0 3 ) e q u a l s 0.15 ± 0.01 (1 0); o r , ( i i ) i f S i 0 0 i s g r e a t e r than or e q u a l t o 56 p e r c e n t , 210 Fe 20 3/(.Fe0 + F e 2 0 3 ) e q u a l s 0.32 ± 0.11 (1 a) . The mean Fe^O^/(FeO + F e 2 0 3 ) r a t i o s g i v e n above a r e c o n s i d e r e d t o p r o v i d e a c l o s e a p p r o x i m a t i o n t o the degree o f o x i d a t i o n t h a t e x i s t e d i n the G a r i b a l d i Lake b a s i c and i n t e r m e d i a t e r o c k s b e f o r e e r u p t i o n . The mean v a l u e s have t h e r e f o r e been used i n the c a l c u l a t i o n o f no r m a t i v e m i n e r a l c o m p o s i t i o n s . 211 T A B L E X V I . L O C A T I O N A N D M O D A L A N A L Y S E S O F I N V E S T I G A T E D Q U A T E R N A R Y V O L C A N I C R O C K S , G A R I B A L D I - C A L L A G H A N L A K E S A R E A N u m b e r S a m p l e R o c k - t y p e L a t i t u d e L o n g i t u d e 0 1 i v . P l a g . C p x . O p x . A m p b . M i c a O x i d e G d m s . C h e a k a m u s V a l l e y b a s a l t s ( T a b l e X V I I ) 1 4 3 5 1 1 o l i v i n e - b a s a l t 4 9 ° 5 8 3 ' 1 2 3 ° 0 8 3 1 2 7 4 3 6 3 o l i v i n e - b a s a l t 4 9 ° 5 8 3 1 1 2 3 ° 0 9 0 1 3 7 4 1 0 3 o l i v i n e - b a s a l t 4 9 ° 5 8 6 1 1 2 3 ° 0 8 9 ' 4 7 4 1 1 8 o l i v i n e - b a s a l t 4 9 ° 5 4 3 1 1 2 3 ° 1 0 4 1 1 5 . 7 1 0 1 1 3 t r . 7 2 6 5 7 4 2 8 4 o l i v i n e - b a s a l t 4 9 ° 5 8 9 1 1 2 3 ° 0 8 3 ' 6 7 4 3 5 5 o l i v i n e - b a s a l t 4 9 ° 5 8 3 ' 1 2 3 ° 0 8 9 1 7 7 4 3 5 7 o l i v i n e - b a s a l t 4 9 ° 5 8 3 ' 1 2 3 ° 0 8 9 ' 1 1 . 4 1 4 3 3 2 - - 0 . 5 7 0 6 8 7 4 4 3 2 o l i v i n e - b a s a l t 4 9 ° 5 9 6 ' 1 2 3 ° 0 8 5 ' 9 7 4 4 9 1 o l i v i n e - b a s a l t 5 0 ° 0 0 2 ' 1 2 3 ° 0 8 0 ' 1 0 7 4 5 0 1 o l i v i n e - b a s a l t 4 9 ° 5 9 9 ' 1 2 3 ° 0 8 1 ' 11 7 4 4 4 8 o l i v i n e - b a s a l t 5 0 ° 0 2 3 1 1 2 3 ° 0 7 4 1 8 . 5 3 7 3 0 t r . 8 4 5 1 2 5 8 0 1 5 o l i v i n e - b a s a l t 1 3 7 4 8 0 1 o l i v i n e - b a s a l t 4 9 ° 5 8 7 1 1 2 3 ° 0 8 3 1 4 . 0 1 6 0 5 7 1 . 5 7 2 3 1 4 7 4 8 3 1 o l i v i n e - b a s a l t 5 0 ° 0 2 3 1 1 2 3 ° 0 6 6 ' 1 5 2 7 0 1 6 o l i v i n e - b a s a l t 1 6 7 4 5 3 1 o l i v i n e - b a s a l t 4 9 ° 5 9 6 ' 1 2 3 ° 0 8 5 ' 1 7 7 4 5 7 1 o l i v i n e - b a s a l t 5 0 ° 0 0 5 ' 1 2 3 ° 0 8 6 ' 1 8 7 4 7 1 1 o l i v i n e - b a s a l t 5 0 ° 0 2 2 ' 1 2 3 ° 0 6 9 ' 8 . 3 1 6 0 1 0 7 1 . 0 6 4 0 1 9 7 4 7 2 1 o l i v i n e - b a s a l t 5 0 ° 0 2 0 ' 1 2 3 ° 0 6 8 ' 2 0 7 4 7 4 1 o l i v i n e - b a s a l t 5 0 ° 0 2 0 1 1 2 3 ° 0 6 7 ' 2 1 2 7 0 1 5 o l i v i n e - b a s a l t 2 2 2 7 0 1 7 o l i v i n e - b a s a l t 2 3 7 4 1 1 9 o l i v i n e - b a s a l t 5 0 ° 0 1 8 ' 1 2 3 ° 0 6 1 ' 2 4 7 4 1 2 6 o l i v i n e - b a s a l t 5 0 ° 0 4 2 ' 1 2 3 ° 0 5 4 ' 1 0 . 0 1 2 6 5 7 0 . 4 71 3 2 5 7 4 1 9 2 o l i v i n e - b a s a l t 5 0 ° 0 2 4 ' 1 2 3 ° 0 7 1 ' 2 6 7 4 1 9 7 o l i v i n e - b a s a l t 5 0 ° 0 4 4 ' 1 2 3 ° 0 5 5 ' 2 7 7 4 1 9 8 o l i v i n e - b a s a l t 5 0 ° 0 2 2 ' 1 2 3 ° 0 6 7 1 7 . 9 1 3 4 1 7 6 1 . 0 6 0 1 2 8 7 4 2 0 6 o l i v i n e - b a s a l t 5 0 ° 0 2 8 ' 1 2 3 ° 0 7 3 ' 6 . 7 1 1 2 2 0 5 t r . 61 4 2 9 7 4 4 3 5 o l i v i n e - b a s a l t 5 0 ° 1 2 8 ' 1 2 3 ° 1 0 0 ' 8 . 0 7 0 2 4 3 0 . 7 6 0 1 S a m p l e 5 8 0 1 5 t a k e n f r o m M a t h e w s ( 1 9 5 8 b , T a b l e I I , N o . 1 5 ) ; S a m p l e s 2 7 0 1 5 , 2 7 0 1 6 a n d 2 7 0 1 7 ( B . M . G u n n , p e r s o n a l c o m m u n i c a t i o n ) . H e l m C r e e k L a v a , T h e C i n d e r C o n e ( T a b l e X V I I I ) o l i v i n e - b a s a l t o l i v i n e - b a s a l t m u g e a r i t e -m u g e a r i t e m u g e a r i t e S a m p l e 5 8 0 1 6 t a k e n f r o m M a t h e w s ( 1 9 5 8 b 5 8 0 1 6 7 4 4 5 3 7 4 4 0 4 7 4 4 1 1 7 4 4 5 1 4 9 " 5 8 . 9 ' 4 9 ° 5 8 . 2 ' 4 9 ° 5 8 . 2 1 5 0 ° 0 8 . 1 ' 1 2 3 0 0 . 8 ' 1 2 3 ° 0 0 . 4 ' 1 2 3 ° 0 0 . 5 ' 1 2 2 ° 5 9 . 8 1 3 . 3 5 . 9 3 . 8 1 8 . 9 5 . 2 6 . 2 2 . 9 0 . 5 9 . 6 2 . 4 5 . 9 0 . 6 t r . 1 . 0 1 . 0 0 . 5 7 0 . 8 8 0 . 8 8 4 . 9 8 7 . 4 T a b l e I I , N o . 1 6 ) . D e s o l a t i o n V a l l e . y , S p h i n x M o r a i n e a n d E n o s t u c k M e a d o w s B a s a l t i c - a n d e s i t e s ( T a b l e X I X ) 1 5 8 0 1 3 b a s a l t i c - a n d e s i t e 2 7 4 2 0 8 b a s a l t i c - a n d e s i t e 5 0 ° 0 2 2 ' 1 2 2 ° 5 8 3 ' 4 0 1 9 6 1 7 - - t r . 7 4 5 3 7 4 4 5 0 b a s a l t i c - a n d e s i t e 5 0 ° 0 2 0 ' 1 2 2 ° 5 8 4 1 4 7 4 5 5 7 b a s a l t i c - a n d e s i t e 5 0 ° 5 7 9 1 1 2 3 ° 0 0 8 ' 4 2 1 4 2 2 7 - - 1 . 0 7 7 9 5 7 4 4 1 0 b a s a l t i c - a n d e s i t e 4 9 ° 5 8 3 1 1 2 3 ° 0 0 7 1 6 2 2 3 7 1 3 - - 0 . 5 6 8 3 6 7 4 5 9 1 b a s a l t i c - a n d e s i t e 4 9 ° 5 5 7 1 1 2 3 ° 0 0 8 ' 7 8 7 4 5 9 8 5 8 0 1 2 b a s a l t i c - a n d e s i t e b a s a l t i c - a n d e s i t e 4 9 ° 5 5 7 ' 1 2 3 ° 0 0 8 ' 4 1 6 8 6 0 0 . 2 1 . 9 1 . 0 8 0 0 9 7 4 6 6 8 b a s a l t i c - a n d e s i t e 4 9 ° 4 8 1 ' 1 2 2 ° 5 5 6 1 3 2 1 3 7 6 0 . 6 0 . 6 0 . 7 8 6 0 S a m p l e s 5 8 0 1 2 a n d 5 8 0 1 3 t a k e n f r o m M a t h e w s ( 1 9 5 8 b , T a b l e I I , N o . 1 2 a n d 1 3 ) . T H E T A B L E ( T A B L E X X ) 1 7 4 6 4 6 h b - a n d e s i t e 4 9 ° 5 3 8 ' 1 2 3 ° 0 0 5 1 2 7 4 6 4 8 h b - a n d e s i t e 4 9 ° 5 3 9 ' 1 2 3 ° 0 0 3 ' 3 7 4 6 5 1 h b - a n d e s i t e 4 9 ° 5 4 1 ' 1 2 3 ° 0 0 8 ' 4 7 4 6 5 3 h b - a n d e s i t e 4 9 ° 5 4 2 1 1 2 3 ° 0 1 0 1 1 7 . 5 1 . 0 6 0 5 6 1 . 9 5 9 0 5 7 4 6 3 6 h b - a n d e s i t e 4 9 ° 5 3 8 ' 1 2 3 ° 0 0 8 ' 6 7 4 6 3 7 h b - a n d e s i t e 4 9 ° 5 3 8 ' 1 2 3 ° 0 0 9 ' 0 . 5 1 7 . 8 1 . 2 3 4 6 6 t r . 6 9 9 7 7 4 6 3 8 h b - a n d e s i t e 4 9 ° 5 3 8 ' 1 2 3 ° 0 0 8 ' 2 0 . 0 t r . 2 2 1 9 7 1 . 0 5 6 9 8 7 4 6 4 1 h b - a n d e s i t e 4 9 ° 5 3 7 1 1 2 3 ° 0 0 7 1 9 7 4 6 4 2 h b - a n d e s i t e 4 9 ° 5 3 7 ' 1 2 3 ° 0 0 6 1 1 3 . 8 t r . 2 3 6 6 t r . 7 5 7 1 0 7 4 6 6 7 h b - a n d e s i t e 4 9 ° 5 3 4 1 1 2 3 ° 0 0 7 1 ' 2 1 2 N u m b e r S a m p l e R o c k - t y p e L a t i t u d e L o n g i t u d e 0 1 i v . P l a g . C p x . O p x . flmpb. M i c a O x i d e G d m s . 5 8 0 1 1 h b - a n d e s i t e 7 4 6 5 0 h b - a n d e s i t e 4 9 ° 5 4 . 1 ' 1 2 3 ° 0 0 6 ' 1 9 0 0 . 6 6 . 0 3 0 - 1 0 6 9 5 7 4 6 5 2 h b - a n d e s i t e 4 9 ° 5 4 . 2 ' 1 2 3 ° 0 0 9 ' 1 7 5 - 1 0 . 5 5 0 - 2 0 6 5 0 7 4 6 4 4 h b - a n d e s i t e 4 9 ° 5 3 . 8 ' 1 2 3 ° 0 0 4 1 2 0 5 2 . 3 6 . 0 1 0 1 t r . 1 7 5 9 7 7 4 6 4 5 h b - a n d e s i t e 4 9 ° 5 3 . 8 ' 1 2 3 ° 0 0 5 ' t r . 2 0 0 3 . 5 0 . 5 8 5 t r . 1 0 6 6 0 7 4 6 3 9 h b - a n d e s i t e 4 9 ° 5 3 . 8 ' 1 2 3 ° 0 0 7 1 1 5 0 t r . 5 . 5 1 9 4 - 2 0 5 7 8 7 4 6 4 0 h b - a n d e s i t e 4 9 ° 5 3 . 7 1 1 2 3 ° 0 0 9 ' 1 5 4 - t r . 5 8 - 2 0 5 7 7 x e n o l i t h 6 2 5 - 1 1 . 9 1 0 2 - 3 1 * 1 2 3 S a m p l e 5 8 0 1 1 t a k e n f r o m M a t h e w s ( 1 9 5 8 b , T a b l e I I , N o . 1 1 ) . T h e B l a c k T u s k ( T a b l e X X I ) 1 7 4 6 1 0 h y - a u g - a n d e s i t e 4 9 ° 5 8 5 ' 1 2 3 ° 0 2 7 ' 1 5 2 1 7 . 0 5 7 _ t r . 6 2 0 2 7 4 6 1 1 h y - a u g - a n d e s i t e 4 9 ° 5 8 3 ' 1 2 3 ° 0 2 8 ' 3 7 4 4 1 9 h b - d a c i t e 4 9 ° 5 8 9 ' 1 2 3 ° 0 3 0 1 5 8 - 1 5 1 4 . 3 0 . 5 7 7 9 4 7 4 4 2 1 h b - d a c i t e 4 9 ° 5 8 9 ' 1 2 3 ° 0 3 1 ' 5 7 4 6 2 5 a u g - h y - a n d e s i t e 4 9 ° 5 8 2 1 1 2 3 ° 0 3 5 ' 2 0 0 6 . 0 8 8 1 . 2 2 . 0 6 2 0 6 7 4 6 2 8 a u g - h y - a n d e s i t e 4 9 ° 5 8 1 ' 1 2 3 ° 0 3 1 1 7 7 4 6 3 1 a u g - h y - a n d e s i t e 4 9 ° 5 8 6 ' 1 2 3 ° 0 2 4 1 8 7 4 6 1 3 h y - a n d e s i t e 4 9 ° 5 8 1 1 1 2 3 ° 0 3 1 1 9 7 4 6 1 8 h y - a n d e s i t e 4 9 ° 5 8 3 1 1 2 3 ° 0 3 6 ' 1 0 7 4 6 2 2 h y - a n d e s i t e 4 9 ° 5 8 3 1 1 2 3 ° 0 3 3 1 11 7 4 6 2 4 h y - a n d e s i t e 4 9 ° 5 8 2 ' 1 2 3 ° 0 3 4 1 1 2 7 4 6 3 4 h y - a n d e s i t e 4 9 ° 5 8 5 1 1 2 3 ° 0 2 4 1 1 3 7 4 6 3 5 h y - a n d e s i t e 4 9 ° 5 8 3 1 1 2 3 ° 0 2 4 1 8 3 0 . 8 5 3 - t r . 8 4 8 1 4 7 4 6 0 5 h y - a n d e s i t e 4 9 ° 5 8 3 1 1 2 3 ° 0 2 4 ' 5 5 t r . 1 0 3 - 0 . 3 8 2 6 1 5 5 8 0 0 9 h y - a n d e s i t e 7 4 6 0 0 h y - a n d e s i t e 4 9 ° 5 8 5 ' 1 2 3 ° 0 2 5 ' 6 0 - 9 0 t r . t r . 8 3 0 7 4 6 0 3 h y - a n d e s i t e 4 9 ° 5 8 4 ' 1 2 3 ° 0 2 5 ' 9 0 0 . 9 6 8 - t r . 8 2 6 7 4 6 2 9 a u g - h y - a n d e s i t e 4 9 ° 5 8 2 1 1 2 3 ° 0 3 2 ' 0 2 2 . 2 2 6 - t r . 9 5 0 7 4 4 1 6 h b - d a c i t e 4 9 ° 5 8 9 ' 1 2 3 ° 0 3 0 1 3 3 - 2 1 1 7 . 5 1 . 8 7 5 3 c r y s t a l c l o t 6 0 0 1 0 . 0 2 4 7 - 5 . 3 S a m p l e 5 8 0 0 9 t a k e n f r o m M a t h e w s ( 1 9 5 8 b , T a b l e I I , N o . 9 ) . M o u n t P r i c e ( T a b l e X X I I ) 1 7 4 5 2 3 h b - a n d e s i t e 4 9 ° 5 5 6 ' 1 2 3 ° 0 1 5 ' 5 5 t r . 2 . 5 1 5 0 - 1 . 0 7 7 0 2 7 4 5 2 2 h b - b o - a n d e s i t e 4 9 ° 5 5 2 1 1 2 3 ° 0 0 8 ' 11 5 t r . 1 . 3 3 0 0 . 3 1 . 0 8 2 9 3 7 4 5 1 9 h b - b o - d a c i t e 4 9 ° 5 4 9 ' 1 2 3 ° 0 1 2 ' 7 9 1 . 2 2 . 7 1 1 4 2 . 0 t r . 7 3 5 4 7 4 5 4 3 h b - a n d e s i t e 4 9 ° 5 5 2 1 1 2 3 ° 0 2 1 ' 1 6 8 t r . 1 . 3 5 7 - 1 . 4 5 4 7 5 7 4 5 6 5 h b - b o - a n d e s i t e 4 9 ° 5 5 1 ' 1 2 3 ° 0 1 9 1 2 0 5 t r . 6 . 2 11 4 3 . 0 2 . 0 + 5 6 0 6 7 4 5 4 2 h b - a n d e s i t e 4 9 ° 5 5 6 ' 1 2 3 ° 0 2 3 1 2 3 5 - 2 . 0 1 8 4 - 0 . 2 5 5 9 7 8 7 4 5 3 5 7 4 5 7 9 h b - a n d e s i t e h b - b o - a n d e s i t e 4 9 ° 5 5 4 9 ° 5 5 7 • 0 1 1 2 3 ° 0 1 1 2 3 ° 0 2 7 ' 2 1 1 0 0 t r . 3 . 4 4 2 t r . t r . + 7 9 0 9 7 4 5 8 1 h b - b o - a n d e s i t e 4 9 ° 5 4 8 ' 1 2 3 ° 0 2 4 1 1 0 7 4 5 3 1 h b - a n d e s i t e 4 9 ° 5 6 2 ' 1 2 3 ° 0 3 2 ' 2 0 0 2 . 0 1 . 0 1 3 0 - t r . + 6 4 0 11 7 4 5 3 3 h b - a n d e s i t e 4 9 ° 5 6 3 1 1 2 3 ° 0 3 2 ' 1 0 0 t r . 2 . 0 5 0 - 1 . 0 + 8 2 0 1 2 7 4 5 3 4 h b - a n d e s i t e 4 9 ° 5 6 4 1 1 2 3 ° 0 3 3 ' 1 3 7 4 5 4 7 h b - b o - a n d e s i t e 4 9 ° 5 5 3 1 1 2 3 ° 0 2 6 ' 1 0 0 t r . 1 . 5 3 0 7 . 0 2 . 0 7 6 0 1 4 7 4 5 8 4 h b - b o - a n d e s i t e 4 9 ° 5 5 1 1 1 2 3 ° 0 2 4 ' 1 5 5 8 0 0 6 h b - b o - a n d e s i t e 1 6 2 7 0 2 0 h b - b o - a n d e s i t e V 1 7 1 8 2 7 0 1 9 2 7 0 1 8 h b - b o - a n d e s i t e h b - b o - a n d e s i t e 7 4 3 8 0 h b - b o - a n d e s i t e 4 9 ° 5 4 6 ' 1 2 3 ° 0 6 1 1 9 8 1 . 3 3 . 0 3 5 2 . 5 t r . + 7 9 5 7 4 5 3 6 h b - b o - a n d e s i t e 4 9 ° 5 5 7 1 1 2 3 ° 0 1 8 ' 7 0 t r . t r . 5 5 2 . 4 0 . 2 8 2 8 S a m p l e 5 8 0 0 6 t a k e n f r o m M a t h e w s ( 1 9 5 8 b , T a b l e I I , N o . 6 ) ; S a m p l e s 2 7 0 1 8 , 2 7 0 1 9 a n d 2 7 0 2 0 r e p r e s e n t l a v a d e b r i s i n t h e R u b b l e C r e e k s l i d e ( B . M . G u n n , p e r s o n a l c o m m u n i c a t i o n ) . M o d a l a n a l y s e s b a s e d o n c o u n t i n g a p p r o x i m a t e l y 1 0 0 0 p o i n t s ; d e v i a t i o n i s a b o u t 5 p e r c e n t b a s e d o n d u p l i c a t e a n a l y s e s h y : h y p e r s t h e n e ; b o : b i o t i t e ; a u g : a u g i t e . + i n c l u d e s 0 . 1 t o 0 . 2 p e r c e n t q u a r t z x e n o c r y s t s ; * i n t e r s t i t i a l g l a s s . T A B L E X V I I . C H E M I C A L A N A L Y S E S A N D C I P W N O R M A T I V E M I N E R A L S O F T H E C H E A K A M U S V A L L E Y B A S A L T S U n i t : A l p i n e L o d g e C h e a k a m u s Dam B r a n d y w i n e F a l l s N u m b e r : 1 2 3 4 5 6 7 8 9 1 0 11 1 2 1 3 1 4 1 5 S a m p l e : 4 3 5 1 1 7 4 3 6 4 7 4 1 0 3 7 4 1 1 8 7 4 2 8 4 7 4 3 5 5 7 4 3 5 7 7 4 4 3 2 7 4 4 9 1 7 4 5 0 1 7 4 4 4 8 5 8 0 1 5 7 4 8 0 1 7 4 8 3 1 2 7 0 1 6 S i 0 2 4 9 . 8 3 4 9 . 3 8 4 9 . 3 3 4 9 . 9 1 4 9 . 9 4 4 8 . 5 7 5 0 : 2 5 4 9 . 8 6 5 0 . 3 1 4 8 . 6 9 4 9 . 9 1 4 9 . 6 8 5 0 . 5 9 4 8 . 8 9 51 . 3 6 T i 0 2 1 . 5 3 1 . 5 2 1 . 5 1 1 . 5 1 1 . 5 2 1 . 5 4 1 . 5 8 1 . 5 6 1 . 4 7 1 . 5 4 1 . 5 9 1 . 3 4 1 . 4 7 1 . 5 3 1 . 4 3 A 1 2 0 3 1 5 . 8 9 1 5 . 5 7 1 5 . 3 2 1 5 . 6 2 1 5 . 7 1 1 5 . 0 8 1 5 . 8 6 1 5 . 4 4 1 5 . 8 3 1 5 . 4 3 1 5 . 9 5 1 6 . 3 9 1 5 . 6 7 1 5 . 9 3 1 5 . 4 6 F e 2 0 3 1 1 . 7 8 11 . 8 0 11 . 6 1 11 . 3 8 11 . 5 7 11 . 9 0 1 2 . 3 2 11 . 3 1 1 0 . 2 3 1 1 . 8 6 1 2 . 6 0 1 2 . 6 9 1 0 . 9 8 1 2 . 1 1 11 . 3 9 MnO 0 . 1 4 0 . 1 4 0 . 1 6 0 . 1 4 0 . 1 5 0 . 1 5 0 . 1 6 0 . 1 4 0 . 1 5 0 . 1 5 0 . 1 5 0 . 2 6 0 . 1 4 0 . 1 5 0 . 1 6 MgO 8 . 4 5 8 . 5 7 8 . 2 9 8 . 8 6 8 . 2 3 8 . 1 6 8 . 2 7 8 . 5 0 8 . 8 1 8 . 8 1 8 . 1 8 7 . 6 2 7 . 5 1 8 . 7 9 7 . 1 8 C a O 8 . 9 6 8 . 9 1 8 . 6 4 8 . 8 5 8 . 6 3 8 . 8 0 8 . 8 9 8 . 5 9 8 . 9 6 8 . 8 2 8 . 8 9 8 . 8 4 8 . 7 3 9 . 0 0 8 . 4 9 N a 2 0 3 . 2 6 3 . 4 7 3 . 5 1 2 . 3 4 3 . 4 7 3 . 7 5 2 . 5 2 3 . 7 0 3 . 0 6 3 . 3 5 1 . 7 7 2 . 7 2 3 . 6 4 3 . 2 6 3 . 5 1 K j O 0 . 4 6 0 . 5 0 0 . 4 7 0 . 5 6 0 . 4 7 0 . 4 7 0 . 4 3 0 . 5 8 0 . 5 2 0 . 4 3 0 . 3 8 0 . 5 4 0 . 5 9 0 . 3 9 0 . 5 5 P 2 ° 5 0 . 3 0 0 . 2 9 0 . 2 8 0 . 2 9 0 . 2 7 0 . 2 7 0 . 2 7 0 . 3 3 0 . 3 1 0 . 3 0 0 . 2 5 0 . 2 3 0 . 2 8 0 . 2 8 0 . 4 8 H 2 0 + 0 . 2 4 0 . 2 2 0 . 1 7 0 . 2 8 0 . 2 1 0 . 1 6 0 . 2 0 0 . 1 7 0 . 2 4 0 . 2 8 0 . 2 5 0 . 5 2 0 . 2 0 0 . 3 0 _ H 2 0 " - - - - - - - - - - - 0 . 3 8 - - -T o t a l 1 0 0 . 8 4 1 0 0 . 3 7 9 9 . 2 9 9 9 . 7 4 1 0 0 . 1 7 9 8 . 8 5 1 0 0 . 7 5 1 0 0 . 1 8 9 9 . 8 9 9 9 . 6 6 9 9 . 9 2 1 0 1 . 2 1 9 9 . 8 0 1 0 0 . 6 3 1 0 0 . 0 0 T r a c e e l e m e n t s ( P P M ) Nb - 9 1 2 _ 11 11 _ _ Y - 2 3 2 0 - 2 2 2 0 _ _ _ _ Z r - 1 0 9 9 5 - 1 1 2 9 3 _ _ _ _ Rb 6 6 8 6 4 7 - 6 - 5 4 _ _ 6 S r 4 5 9 4 6 0 4 8 0 4 8 3 4 6 7 4 5 9 - 4 7 6 _ 4 8 0 4 6 2 _ _ 4 6 6 B a - 8 5 1 1 0 - 1 0 4 91 - - - _ 1 8 0 _ _ _ 1 3 6 C u 5 9 5 8 6 0 5 4 5 5 6 4 - 5 0 - 5 9 _ _ _ Zn 1 0 7 1 1 2 1 0 9 101 1 0 4 1 1 5 - 9 9 _ _ 1 0 8 _ _ N i 1 3 3 1 4 2 1 3 1 1 1 0 1 1 8 1 4 7 - 1 1 9 _ _ 7 8 _ _ _ 1 1 6 C r 1 7 8 1 6 6 1 8 6 1 9 0 1 7 2 1 9 2 - 1 7 9 _ _ 1 6 3 _ _ V 2 2 8 2 0 4 1 8 9 1 9 4 1 9 6 1 9 8 - 1 8 4 - - 1 7 6 - - - -K / R b 6 8 2 6 9 1 4 7 0 7 8 7 9 5 2 5 2 7 _ 8 4 5 6 6 1 7 1 7 7 4 2 K / S r 8 . 3 9 . 0 8 . 1 9 . 6 8 . 4 8 . 5 - 1 0 . 1 _ 7 . 4 6 . 8 _ 9 . 7 K / B a - 4 8 . 8 3 5 . 5 - 3 7 . 5 4 2 . 9 _ _ _ 1 7 . 5 _ 3 3 . 3 C a / S r 1 3 9 . 5 1 3 8 . 4 1 2 8 . 6 131 . 0 1 3 2 . 1 1 3 7 . 0 _ 1 2 9 . 0 _ 131 . 3 1 3 7 . 5 _ 1 3 0 . 2 R b / S r 0 . 0 1 2 2 0 . 0 1 3 0 0 . 0 1 7 3 0 . 0 1 2 2 0 . 0 0 8 8 0 . 0 1 6 1 - 0 . 0 1 2 0 0 . 0 1 1 2 0 . 0 0 9 5 _ 0 . 0 1 3 1 C r / V 0 . 8 0 . 8 1 . 0 1 . 0 0 . 9 ' 1 . 0 - 1 . 0 - - 0 . 9 - - -R e c a l c u l a t e d V o l a t i 1 e F r e e S i 0 2 5 0 . 0 2 4 9 . 7 9 5 0 . 2 6 5 0 . 6 6 5 0 . 4 4 4 9 . 7 1 5 0 . 4 9 5 0 . 3 3 5 0 . 9 2 4 9 . 4 8 5 0 . 6 1 5 0 . 0 5 51 . 2 6 4 9 . 2 2 51 . 8 5 T i 0 2 1 . 5 4 1 . 5 3 1 . 5 4 1 . 5 3 1 . 5 4 1 . 5 8 1 . 5 9 1 . 5 7 1 . 4 9 1 . 5 7 1 . 6 1 1 . 3 5 1 . 4 9 1 . 5 4 1 . 4 4 A 1 2 0 3 1 5 . 9 5 1 5 . 7 0 1 7 . 6 1 1 5 . 8 6 1 5 . 8 7 1 5 . 4 3 1 5 . 9 4 1 5 . 5 8 1 6 . 0 2 1 5 . 6 8 1 6 . 1 7 1 6 . 5 1 1 5 . 8 8 1 6 . 0 4 1 5 . 6 1 F e 2 0 3 * 2 . 0 3 2 . 0 4 2 . 0 3 1 . 9 8 2 . 0 0 2 . 0 9 2 . 1 2 1 . 9 6 1 . 7 7 2 . 0 6 2 . 1 9 2 . 1 9 1 . 9 1 2 . 0 9 1 . 9 7 F e O * 8 . 8 2 8 . 8 7 8 . 8 2 8 . 6 1 8 . 7 1 9 . 0 3 9 . 2 3 8 . 5 1 7 . 7 2 8 . 9 9 9 . 5 3 9 . 5 3 8 . 3 0 9 . 0 9 8 . 5 7 MnO 0 . 1 4 0 . 1 4 0 . 1 6 0 . 1 4 0 . 1 5 0 . 1 5 0 . 1 6 0 . 1 4 0 . 1 5 0 . 1 5 0 . 1 5 0 . 2 6 0 . 1 4 0 . 1 5 0 . 1 6 MgO 8 . 4 8 8 . 6 4 8 . 4 5 8 . 9 9 8 . 3 1 8 . 3 5 8 . 3 1 8 . 5 8 8 . 9 2 8 . 9 5 8 . 2 9 7 . 6 8 7 . 6 1 8 . 8 5 7 . 2 5 C a O 8 . 9 9 8 . 9 8 8 . 8 0 8 . 9 8 8 . 7 2 9 . 0 1 8 . 9 3 8 . 6 7 9 . 0 7 8 . 9 6 9 . 0 1 8 . 9 1 8 . 8 5 9 . 0 6 8 . 5 7 N a 2 0 3 . 2 7 3 . 5 0 3 . 5 8 2 . 3 8 3 . 5 1 3 . 8 4 2 . 5 3 3 . 7 3 3 . 1 0 3 . 4 0 1 . 7 9 2 . 7 4 3 . 6 9 3 . 2 8 3 . 5 4 tLfi 0 . 4 6 0 . 5 0 0 . 4 8 0 . 5 7 0 . 4 7 0 . 4 8 0 . 4 3 0 . 5 9 0 . 5 3 0 . 4 4 0 . 3 9 0 . 5 4 0 . 6 0 0 . 3 9 0 . 5 5 P 2 ° 5 0 . 3 0 0 . 2 9 0 . 2 9 0 . 2 9 0 . 2 7 0 . 2 8 0 . 2 7 0 . 3 3 0 . 3 1 0 . 3 0 0 . 2 5 0 . 2 3 0 . 2 8 0 . 2 8 0 . 4 8 N o r m a t i v e M i n e r a l s Oz 0 . 0 1 - 3 . . 4 3 O r 2 . 7 3 2 . 9 8 2 . 8 3 3 . . 3 6 2 . 8 1 2 . 8 4 2 . 5 5 3 . 4 6 3 . 1 1 2 . 5 8 2 . 2 8 3 . 2 1 3 . 5 3 2 . 3 2 3 . 2 5 A b 2 7 . 6 9 2 9 . 6 1 3 0 . . 2 6 2 0 . 1 0 2 9 . 6 6 3 2 . . 4 3 21 . 4 2 31 . 6 0 2 6 . 2 1 2 8 . 8 1 1 5 . 1 9 2 3 . 1 9 31 . 5 1 2 7 . 7 7 2 9 . 9 8 A n 2 7 . 4 7 2 5 . 6 5 2 5 . 1 2 3 0 . . 9 2 2 6 . 1 6 2 3 . 4 7 3 0 . 8 4 2 4 . 0 3 2 8 . 2 6 2 6 . 2 2 3 4 . . 9 5 31 . 1 5 2 5 . 0 0 2 7 . 8 7 2 5 . 0 6 D i 1 2 . . 3 4 1 3 . 8 3 1 3 . 5 7 9 . . 5 1 1 2 . 4 4 1 5 . 8 1 9 . 5 3 1 3 . . 6 4 11 . 8 7 1 3 . 2 1 6 . 6 3 9 . . 4 3 1 3 . 8 6 1 2 . 3 9 11 . 6 8 E n 7 . . 7 1 3 . 8 9 5 . 2 7 1 9 . 2 7 7 . 0 2 0 . 0 5 1 7 . 8 3 3 . . 5 1 11 . 9 9 4 . 4 5 1 8 . . 6 9 1 2 . . 7 7 6 . 6 9 5 . 0 3 1 2 . 2 7 F s 4 . 4 7 2 . 2 3 3 . 0 8 1 0 . . 2 5 4 . 1 0 0 . . 0 3 11 . 0 9 1 . 9 2 5 . 6 9 2 . 4 9 1 2 . 0 4 9 . . 3 2 4 . 0 5 2 . 9 0 5 . 0 3 F o 6 . . 7 4 9 . 3 6 8 . . 1 3 0 . . 0 8 6 . 9 1 11 . 1 9 9 . . 5 1 4 . 4 3 9 . . 6 3 2 . . 5 8 5 . 6 5 9 . 2 4 1 . 6 5 F a 4 . . 3 0 5 . 9 2 5 . 2 3 0 . 0 5 4 . 4 5 7 . . 4 9 5 . 7 3 2 . 3 2 5 . 9 5 2 . . 0 7 3 . 7 6 5 . 8 7 1 . 2 1 M t 2 . . 9 4 2 . . 9 5 2 . 9 4 2 . 8 7 2 . . 9 0 3 . 0 2 3 . . 0 7 2 . 8 4 2 . 5 7 2 . . 9 9 3 . 17 3 . 1 8 2 . 7 6 3 . . 0 3 2 . 8 6 11 2 . . 9 2 2 . . 9 1 2 . 9 2 2 . 91 2 . . 9 2 2 . . 9 9 3 . 0 2 2 . 9 9 2 . 8 3 2 . . 9 7 3 . 0 6 2 . 5 6 2 . 8 3 2 . 9 3 2 . 7 3 A p 0 . 7 0 0 . . 6 8 0 . 6 6 0 . 6 8 0 . 6 3 0 . 6 4 0 . . 6 3 0 . 7 7 0 . 7 3 0 . . 7 1 0 . 5 9 0 . 5 4 0 . 6 6 0 . . 6 5 1 . 1 2 D . I . 3 0 . 4 2 3 2 . 5 9 3 3 . 0 9 2 3 . 4 6 3 2 . 4 6 3 5 . 3 2 2 3 . . 9 9 3 5 . . 0 6 2 9 . 3 2 31 . . 3 9 2 0 . 8 9 2 6 . 4 0 3 4 . 7 4 3 0 . . 0 9 3 3 . 2 4 A n % 4 9 . . 8 0 4 6 . 6 2 4 5 . 3 6 6 0 . 61 4 6 . 8 7 41 . . 9 5 5 9 . 01 4 3 . 2 0 51 . 8 9 4 7 . 6 5 6 9 . 7 0 5 7 . 3 3 4 4 . 4 8 5 0 . 0 9 4 5 . 5 3 * F e 2 0 3 a n d F e O c o n t e n t s i n a n a l y s e s r e c a l c u l a t e d t o 1 0 0 p e r c e n t v o l a t i l e f r e e d e t e r m i n e d u s i n g r a t i o s g i v e n i n t h e p r o c e e d i n g t e x t . 21 T A B L E X V I I . C O N T I N U E D U n i t : Number: S a m p l e : 16 74531 17 74571 18 74711 19 74721 20 74741 B r a n d y w i n e F a l l s 21 22 27015 27017 23 74119 24 74126 25 74192 26 74197 27 74198 28 74206 C a l l a g h i 29 7 4 4 3 5 S i 0 2 5 0 . 9 5 4 9 . . 4 3 . 4 9 . . 7 0 4 9 , , 4 5 4 8 . 6 0 5 1 . ,19 5 1 . 64 5 0 . 13 5 0 , , 7 9 5 0 . , 5 8 4 9 , , 0 9 5 0 . 46 5 0 . , 2 4 5 1 . 0 9 T i 0 2 1 . 4 8 1 . 5 3 1 , , 4 9 1 . 5 4 1 . 5 9 1 . , 4 4 1 . 42 1 . ,47 1 , . 4 5 1. . 4 8 1 , .47 1 . 44 1. .47 1 . 4 5 A 1 2 0 3 1 5 . 7 3 1 5 . . 7 6 1 5 . , 3 2 1 5 , , 7 3 1 6 . 8 1 1 5 . 63 1 5 , 67 1 6 . 0 6 1 5 , , 3 6 1 5 . , 4 3 1 5 , . 9 4 1 5 . , 6 5 1 5 , . 6 4 1 6 . 3 1 F e 2 0 3 1 0 . 7 9 11 .47 11. . 2 6 11 , .81 1 1 . 6 2 1 1 . 38 1 1 . 29 1 0 . 83 1 0 , , 7 6 11. .17 11 , . 2 9 1 1 . , 0 6 10. , 9 8 1 0 . 0 2 MnO 0 . 1 4 0 . . 1 5 0 . , 14 0 , , 1 5 0 . 1 4 0 . 16 0 . 16 0 . 14 0 , , 14 0 . .14 0 . , 14 0 . 13 0 , .14 0 . 1 4 MgO 8 . 4 7 8 . .77 7. 62 8 . . 6 4 8 . 4 5 7 . 29 7 . 0 8 8 . , 0 8 7. , 9 2 7. , 8 5 7. , 8 2 7 . 91 7. . 4 6 6 . 4 2 CaO 8 . 7 4 8 . . 9 0 8 . . 5 5 8 , .91 8 . 6 8 8 . 55 8 . , 5 5 8 . ,47 8 , . 5 7 8 . . 6 0 8 . .73 8 . 4 8 8 , . 6 8 8 . 5 1 N a 2 0 2 . 7 7 3 , , 5 2 3 . 83 2 . . 7 8 3 . 6 3 3 . 60 3 . 44 3 . 60 3 . , 9 8 3 . , 7 0 3 . 87 3 . 59 3 . , 9 9 4 . 1 4 KjO 0 . 4 9 0 . .47 0 . 53 0 , , 4 6 0 . 5 1 0 . 50 0 . 51 0 . 58 0 , , 6 3 0 . , 5 4 0 . , 5 5 0 . 57 0 . , 6 0 0 . 6 0 P 2 ° 5 0 . 2 9 0 . . 3 0 0 . , 2 9 0 . . 2 9 0 . 2 7 0 . 25 0 . 24 0 . 27 0 . , 2 9 0 . , 2 9 0 , , 2 9 0 . 29 0 . 29 0 . 3 1 H 2 0 + H , 0 ~ 0 . 1 8 0 . .14 0 . , 14 0 , , 14 - 0 , 16 0 . , 2 0 0 . ,17 0 , , 2 4 0 . 20 0 . ,17 0 . 1 7 2 T o t a l 1 0 0 . 0 3 1 0 0 . . 4 4 9 8 . .87 9 9 , . 9 0 1 0 0 . 3 0 9 9 . 99 1 0 0 . 00 9 9 . 79 1 0 0 , . 0 9 9 9 , , 9 5 9 9 . , 4 3 9 9 . 78 9 9 , , 6 6 9 9 . 1 6 T r a c e e l e m e n t s ( P P M ) Nb - 12 - - - - 17 8 - 10 9 13 Y - 19 - - - - 21 22 - 23 22 18 Z r - 90 - - - - 106 105 - 109 108 9 8 Rb 5 8 5 5 6 6 5 6 5 6 6 9 S r 4 4 8 4 8 6 453 469 472 500 4 7 5 477 491 470 491 559 Ba - 113 - 166 167 - 131 117 117 121 144 163 Cu 57 55 - - - 57 47 53 53 47 49 4 8 Zn 101 107 - - - 101 9 6 101 104 93 96 93 Ni 128 117 - 121 113 102 103 116 106 117 102 69 C r 157 181 - - - 164 176 178 175 177 179 141 V 193 191 - - - 193 198 209 189 189 186 182 K/Rb 813 530 849 854 750 803 987 723 846 860 859 579 K / S r 8 . 7 9 . 1 8 . 4 8 . 9 8 . 9 9 . 6 1 1 . 0 9 . 4 9 . 3 1 0 . 1 1 0 . 1 8 . 9 K/Ba - 3 8 . 9 - 2 5 . 2 2 5 . 2 - 3 9 . 9 3 9 . 3 3 9 . 0 3 9 . 1 3 4 . 6 3 0 . 6 C a / S r 1 4 2 . 7 1 2 5 . 7 1 4 0 . 6 1 3 0 . 3 1 2 9 . 5 121 .1 1 2 8 . 9 1 2 8 . 9 1 2 7 . 1 1 2 9 . 0 1 2 6 . 3 1 0 8 . 8 R b / S r 0 . 0 1 0 7 0 . 0 1 7 1 0 . 0 0 9 9 0 . 0 1 0 4 0 . 0 1 1 9 0 . 0 1 2 0 0 . 0 1 1 2 0 . 0 1 3 0 0 . 0 1 1 0 0 . 0 1 1 7 0 . 0 1 1 8 0 . 0 1 5 4 C r / V 0 . 8 0 . 9 - - - 0 . 8 0 . 9 0 . 9 0 . 9 0 . 9 1 . 0 0 . 8 R e c a l c u l a t e d V o l a t i l e F r e e S i 0 2 51 . 4 9 49 . 7 5 50 . 8 2 50 . 0 6 48 . 9 3 51 . 6 8 52 . 1 3 50 . 7 7 51 . 3 0 51 .17 49 . 9 6 51 . 1 4 50 . 9 6 5 2 . 0 5 T i 0 2 1 . 5 0 1 . 5 4 1 . 5 2 1 . 5 6 1 . 6 0 1 . 4 5 1 . 4 4 1 . 4 9 1 . 4 6 1 . 5 0 1 . 5 0 1 . 4 6 1 . 4 9 1 . 4 8 A 1 2 0 3 1 5 . 9 0 15 . 8 6 15 . 6 7 15 . 9 2 1 6 , . 9 2 15 . 7 8 15 . 8 2 16 . 2 7 15 . 5 2 15 .61 16 . 2 2 15 . 8 6 15 . 8 7 1 6 . 6 2 F e 2 0 3 * 1 . 8 7 1 . 9 8 1 . 9 7 2 . 0 5 2 . 0 0 1 . 9 7 1 . 9 5 1 . 8 8 1 . 8 6 1 . 9 4 1 . 9 7 1 . 9 2 1 .91 1 . 7 5 FeO* 8 . 1 3 8 .61 8 . 5 9 8 . 9 2 8 .72 8 . 5 7 8 . 5 0 8 . 1 8 8 .10 8 . 4 3 8 . 5 7 8 . 3 6 8 .31 7 . 6 1 MnO 0 . 1 4 0 . 1 5 0 . 1 4 0 . 1 5 0 , . 1 4 0 . 1 6 0 . 1 6 0 . 1 4 0 , .14 0 . 1 4 0 . 1 4 0 . 1 3 0 . 1 4 0 . 1 4 MgO 8 . 5 6 8 . 8 3 7 . 7 9 8 . 7 5 8 .51 7 . 3 6 7 . 1 5 8 . 1 8 8 . 0 0 7 . 9 4 7 . 9 6 8 . 0 2 7 . 5 7 6 . 5 4 CaO 8 . 8 3 8 . 9 6 8 . 7 4 9 . 0 2 8 .74 8 . 6 3 8 . 6 3 8 . 5 8 8 . 6 6 8 . 7 0 8 . 8 9 8 . 6 0 8 .81 8 . 6 7 N a 2 0 2 . 8 0 3 . 5 4 3 . 9 2 2 .81 3 , , 6 5 3 . 6 3 3 . 4 7 3 . 6 5 4 . 0 2 3 . 7 4 3 . 9 4 3 . 6 4 4 . 0 5 4 . 2 2 KjO 0 . 5 0 0 . 4 7 0 . 5 4 0 .47 0 , ,51 0 . 5 1 0 .51 0 . 5 9 0 . , 6 4 0 . 5 5 0 . 5 6 0 . 5 8 0 .61 0 . 6 1 P 2 ° 5 0 . 2 9 0 . 3 0 0 , , 3 0 0 . 2 9 0 , .27 0 . 2 5 0 . 2 4 0 . 2 7 0 , , 2 9 0 . 2 9 0 . 3 0 0 . 2 9 0 . 2 9 0 . 3 2 N o r m a t i v e M i n e r a l s Oz 0 . 0 7 Or 2 . 9 3 2 . 8 0 3 , . 2 0 2 , , 7 5 3 . , 0 3 3 .01 3 . 0 2 3 . 4 7 3 . 76 3 . 2 3 3 .31 3 .41 3 . . 6 0 3 . 6 1 Ab 2 3 . 6 9 2 9 , , 9 8 3 3 . , 1 4 2 3 . .81 3 0 . 92 30 . 7 6 29 . 3 8 30 . 8 5 3 4 . , 0 2 31 . 6 7 3 3 , . 1 6 3 0 . . 7 9 34 . 2 5 3 5 . 6 9 An 2 9 . 3 5 2 5 , , 9 8 2 3 . ,57 2 9 , . 4 4 2 8 . , 2 6 25 . 2 4 26 . 0 7 26 . 2 8 2 2 . 41 24 . 1 8 24 . 9 3 25 . 2 4 2 3 . , 3 3 2 4 . 6 0 Di 1 0 . 1 8 1 3 , . 3 8 14, . 5 5 10. , 8 8 1 0 . 82 13 . 0 0 12 . 3 9 11 .77 1 5 . ,13 13 . 8 8 14 .01 12, , 5 8 15 .01 1 3 . 3 3 En 1 8 . 0 8 3 , , 5 0 3 . 39 12. , 8 6 0 . 28 9 .61 12. . 7 6 6 . 1 3 2 . 96 6 . 3 7 7, . 5 0 1 , .81 4 . 5 5 Fs 9 . 4 8 1 . 9 0 2 , ,07 7. , 3 2 0 . 16 6 . 3 0 8 . 5 4 3 . 3 9 1 . 6 6 3 . 7 5 4 . 3 6 1 .10 2 . 9 0 Fo - 10. , 0 0 8 . .14 3 . 89 1 2 . 3 0 3 , . 4 2 0 . , 9 9 7 .41 8 . 59 6 . , 4 2 1 0 , . 9 0 6 . 02 8 , .75 5 . 4 4 Fa - 5 . ,97 5 . , 5 0 2 . 44 7 . 66 2 .47 0 . , 7 3 4 . 5 2 5 . 32 4 .17 7 , .21 3 , , 8 6 5 , . 8 8 3 . 8 1 Mt 2 . 7 1 2 . 87 2 . 86 2 , 97 2 . 91 2, . 8 5 2 , , 8 3 2 . 7 2 2 . 70 2 . ,81 2 , . 8 5 2 . , 7 8 2 . 77 2 . 5 4 11 2 . 8 4 2 . 92 2 . 89 2 . . 9 6 3 . 0 4 , 2 , . 7 5 2 . ,73 2, . 8 3 2 . 78 2 . . 8 4 2 . 84 2 . 77 2 . 83 2 . 8 1 Ap 0 . 6 8 0 . 70 0 . 69 0 . 68 0 . 6 3 0 , . 5 8 0 . 56 0 , . 6 3 0 . 68 0 . 68 0 . 68 0 . 68 0 . 68 0 . 7 3 D . I . 2 6 . 6 8 3 2 . 78 3 6 . 34 2 6 . 56 3 3 . 9 5 3 3 , , 7 6 3 2 . 40 34 . 2 2 3 7 . 78 3 4 . , 9 0 3 6 . 56 3 4 . ,20 3 7 . 84 3 9 . 3 0 An % 5 5 . 3 4 4 6 . 43 4 1 . 56 5 5 , 29 4 7 . 75 4 5 , . 0 8 4 7 . 01 4 6 , . 0 0 3 9 . 72 4 3 . 29 4 2 . 92 4 5 . , 0 5 4 0 . 51 4 0 . 8 1 * F e 2 0 3 and FeO c o n t e n t s i n a n a l y s e s r e c a l c u l a t e d t o 100 p e r c e n t v o l a t i l e f r e e d e t e r m i n e d u s i n g r a t i o s g i v e n i n p r o c e e d i n g t e x t . T A B L E X V I I I . C H E M I C A L A N A L Y S E S AND CIPW NORMATIVE M I N E R A L S OF THE HELM C R E E K L A V A , THE C I N D E R CONE 215 N u m b e r : 1 2 3 4 5 S a m p l e : 5 8 0 1 6 7 4 4 5 3 7 4 4 0 4 7 4 4 1 1 7 4 4 5 1 S i 0 2 4 8 . 9 2 4 9 . 8 0 5 3 . 5 3 5 3 . 9 1 5 4 . 0 2 T i 0 2 1 . 8 8 1 . 3 1 1 . 1 1 , , 0 . 9 9 0 . 9 9 A 1 2 ° 3 1 8 . 3 3 1 7 . 3 1 1 6 . 8 4 1 7 . 6 3 1 6 . 3 8 F e 2 0 3 1 2 . 2 0 1 0 . 7 5 7 . 8 5 • 7 . 6 1 7 . 2 1 MnO 0 . 2 5 0 . 1 5 0 . 1 2 0 . 1 3 0 . 1 2 MgO 5 . 8 7 5 . 8 9 3 . 9 7 4 . 9 2 5 . 5 7 CaO 8 . 9 2 9 . 3 4 7 . 8 6 7 . 0 2 7 . 3 4 N a 2 0 3 . 4 2 3 . 7 8 5 . 6 5 5 . 6 8 5 . 6 5 K j O 0 . 6 4 0 . 9 2 1 . 4 8 0 . 9 9 1 . 4 2 P 2 ° 5 0 . 1 8 0 . 3 0 0 . 6 9 0 . 4 0 0 . 5 2 H 2 0 + 0 . 3 3 0 . 3 3 0 . 3 5 0 . 1 9 0 . 0 8 H 2 0 " 0 . 1 3 - - - 0 . 0 3 T o t a l 1 0 1 . 0 7 9 9 . 7 0 9 9 . 4 5 9 9 . 4 7 9 9 . 3 1 T r a c e e l e m e n t s ( P P M ) Nb 6 1 3 Y - - 21 1 6 Zr _ _ 1 0 0 - 1 1 4 Rb - 7 10 - 12 S r 9 1 8 1 8 1 0 - 1 5 5 7 B a - 331 7 0 6 - 6 0 6 Cu - 4 6 61 - 4 9 Zn - 8 2 1 0 0 - 8 0 N i _ 5 5 1 0 0 - 9 6 C r _ 77 1 1 2 1 3 9 2 0 V - 2 0 4 1 4 9 1 6 3 4 9 K/Rb _ 1 0 3 2 1241 - 9 7 4 K / S r - 8 . 3 6 . 8 - 7 . 6 K / B a - 2 3 . 1 1 7 . 4 - 1 9 . 5 C a / S r - 7 2 . 7 3 1 . 0 - 3 3 . 7 R b / S r - 0 . 0 0 8 1 0 . 0 0 5 5 - 0 . 0 0 7 8 C r / V - 0 . 4 0 . 8 0 . 9 0 . 4 R e c a l c u l a t e d V o l a t i l e F r e e S i 0 2 4 9 . 1 2 5 0 . 5 7 5 4 . 3 7 5 4 . 6 5 5 4 . 7 9 T i 0 2 1 . 8 9 1 . 3 3 1 . 1 3 1 . 0 0 1 . 0 0 A 1 2 ° 3 1 8 . 4 0 1 7 . 3 9 1 7 . 1 1 1 7 . 8 7 1 6 . 5 9 F e 2 ° 3 * 2 . 1 0 1 . 8 7 1 . 3 7 1 . 3 2 1 . 2 5 F e O * 9 . 1 3 8 . 1 4 5 . 9 5 5 . 7 5 5 . 4 5 MnO 0 . 2 5 0 . 1 5 0 . 1 2 0 . 1 3 0 . 1 2 MgO 5 . 8 9 5 . 9 8 4 . 0 3 4 . 9 9 5 . 6 5 CaO 8 . 9 6 9 . 4 8 7 . 9 8 7 . 1 2 7 . 4 4 N a 2 0 3 . 4 3 3 . 8 4 5 . 7 4 5 . 7 6 5 . 7 6 K j O 0 . 6 4 0 . 9 3 1 . 5 0 1 . 0 0 1 . 4 4 P 2 ° 5 0 . 1 8 0 . 3 0 0 . 7 0 0 . 4 1 0 . 5 3 N o r m a t i v e i M i n e r a l s O r 3 . 8 0 5 . 5 2 8 . 8 8 5 . 9 3 8 . 5 1 Ab 2 9 . 0 5 3 2 . 4 8 4 2 . 5 6 4 6 . 6 1 4 3 . 0 7 A n 3 2 . 9 1 2 7 . 4 8 1 6 . 4 7 1 9 . 9 6 1 5 . 3 0 Ne - - 3 . 2 5 1 . 1 4 2 . 9 3 D i 8 . 5 1 1 4 . 4 3 1 5 . 2 5 1 0 . 3 6 1 4 . 7 5 En 3 . 8 8 0 . 0 6 - - -Fs 3 . 2 8 0 . 0 5 - - -Fo 5 . 9 7 7 . 5 9 4 . 1 4 6 . 5 5 6 . 5 9 Fa 5 . 5 6 6 . 4 5 3 . 7 0 4 . 6 9 3 . 9 0 M t 3 . 0 4 2 . 7 1 1 . 9 8 1 . 9 2 1 . 8 2 11 3 . 5 8 2 . 5 3 2 . 1 4 1 . 9 1 1 . 9 1 A p 0 . 4 2 0 . 7 1 1 . 6 2 0 . 9 4 1 . 2 2 D . I . 3 2 . 8 5 3 8 . 0 0 5 4 . 6 9 5 3 . 6 8 5 4 . 5 1 A n 51 5 3 . 1 1 4 5 . 8 3 2 7 . 9 1 2 9 . 9 8 2 6 . 2 1 * F e 2 0 3 a n d FeO c o n t e n t s i n a n a l y s e s r e c a l c u l a t e d t o 1 0 0 p e r c e n t v o l a t i l e f r e e d e t e r m i n e d u s i n g r a t i o s g i v e n i n p r e c e e d i n g t e x t . TABLE X I X . CHEMICAL ANALYSES AND CIPW NORMATIVE MINERALS OF THE DESOLATION VALLEY (THE CINDER C O N E ) , SPHINX MORAINE AND ENOSTUCK MEADOWS B A S A L T I C - A N D E S I T E S U n i t : D e s o l a t i o n V a l l e y S p h i n x M o r a i n e E n o s t u c k Meadows N u m b e r : 1 2 3 4 5 6 7 8 9 S a m p l e : 5 8 0 1 3 7 4 2 0 8 7 4 4 5 0 7 4 5 5 7 7 4 4 1 0 74591 7 4 5 9 8 5 8 0 1 2 7 4 6 8 8 S i 0 2 5 4 . 6 2 5 5 . 3 8 5 6 . 0 3 5 6 . 0 9 5 6 . 4 8 5 4 . 5 8 5 4 . 9 6 5 5 . 9 7 5 6 . 4 1 T i 0 2 1 . 1 2 0 . 9 7 0 . 8 4 0 . 9 8 1 . 0 8 0 . 8 7 0 . 8 8 1 . 0 2 0 . 9 5 A 1 2 ° 3 1 9 . 2 7 1 7 . 4 3 1 6 . 8 5 1 7 . 5 2 1 7 . 5 4 1 7 . 3 1 1 7 . 2 9 1 6 . 5 7 1 6 . 4 2 F e 2 ° 3 9 . 0 9 7 . 5 9 6 . 9 5 7 . 5 8 7 . 7 5 6 . 7 6 6 . 7 1 7 . 3 6 6 . 5 6 MnO 0 . 0 9 0 . 1 4 0 . 1 2 0 . 1 2 0 . 1 3 0 . 1 0 0 . 1 0 0 . 1 4 0 . 2 5 MgO 4 . 9 3 5 . 3 9 5 . 4 6 5 . 1 1 4 . 8 9 6 . 7 3 5 . 8 0 5 . 8 2 5 . 7 0 CaO 6 . 9 6 6 . 9 6 6 . 7 8 7 . 1 1 7 . 0 3 8 . 2 5 8 . 0 5 8 . 0 6 7 . 6 9 N a 2 0 3 . 8 8 4 . 5 0 4 . 7 7 4 . 4 8 4 . 1 8 3 . 9 7 3 . 9 8 3 . 2 5 3 . 9 9 K^O 0 . 6 4 1 . 0 2 1 . 1 4 0 . 9 9 1 . 0 8 1 . 1 1 1 . 0 9 1 . 2 4 1 . 6 0 P 2 ° 5 0 . 1 4 0 . 4 1 0 . 4 0 0 . 4 1 0 . 4 1 0 . 3 1 0 . 3 1 0 . 1 6 0 . 4 2 H 2 0 + 0 . 1 0 0 . 0 9 0 . 2 8 0 . 0 8 0 . 1 2 0 . 1 7 0 . 4 6 0 . 7 7 0 . 3 0 H 2 0 " 0 . 1 8 - 0 . 2 2 - - - - 0 . 1 4 0 . 2 1 T o t a l 1 0 1 . 0 2 9 9 . 8 8 9 9 . 8 4 1 0 0 . 4 7 1 0 0 . 6 9 1 0 0 . 1 6 9 9 . 6 2 1 0 0 . 5 0 1 0 0 . 5 0 T r a c e e l e m e n t s ( P P M ) Nb 10 12 9 3 5 _ 5 Y _ 23 20 21 27 22 - - 21 Z r 141 1 2 5 119 8 8 124 - - 2 0 9 Rb _ 10 12 9 10 13 13 - 24 S r _ 752 9 6 4 7 7 5 8 0 4 1 3 0 0 1 3 0 9 - 1 3 4 5 Ba _ 3 4 8 4 6 2 3 0 8 3 0 5 - 4 6 2 - 7 4 5 Cu _ 51 39 41 38 - 56 - 67 Zn _ 84 81 79 8 3 - 55 - 74 Ni - 9 5 87 72 77 89 111 - 8 0 C r _ 107 1 0 5 117 142 135 104 - 9 2 V - 163 120 159 142 74 141 - 144 K/Rb 847 8 0 9 9 3 4 897 6 8 8 7 0 6 5 8 8 K / S r - 1 1 . 3 9 . 8 1 0 . 6 1 1 . 2 7 . 1 6 . 8 - 9 . 9 K / B a - 2 4 . 3 2 0 . 5 2 6 . 7 2 9 . 3 - 1 9 . 4 - 1 7 . 8 C a / S r - 6 6 . 1 5 0 . 3 6 5 . 6 6 2 . 5 4 5 . 4 4 4 . 0 - 4 0 . 9 R b / S r _ 0 . 0 1 3 3 0 . 0 1 2 1 0 . 0 1 1 4 0 . 0 1 2 4 0 . 0 1 0 3 0 . 0 0 9 7 - 0 . 0 1 7 7 C r / V - 0 . 7 0 . 9 0 . 7 1 . 0 1 . 8 0 . 7 - 0 . 6 R e c a l c u l a t e d V o l a t i l e F r e e S i 0 2 5 4 . 63 5 5 , , 8 5 5 6 . 67 5 6 . 16 5 6 . 4 5 5 4 . 89 5 5 , , 74 5 6 . 5 5 5 6 . , 6 6 T i 0 2 1 . 12 0 . . 9 8 0 . 8 5 0 . 9 8 1 . 0 8 0 . 87 0 . 89 1 . 0 3 0 , , 9 5 A 1 2 ° 3 1 9 . 27 17. . 5 8 17 . 04 17 . . 5 4 17 . 53 17 . .41 17 . 53 1 6 . 7 4 1 6 , . 4 9 F e 2 ° 3 * 1 . . 5 6 1 .31 2 . 37 2 . 56 2 . .61 1 , , 1 6 1 . 17 1 . 2 7 2 , . 2 2 F e O * 6 . . 7 8 5 . .71 4 . . 1 9 4 . . 5 2 4 . . 6 2 5 . . 0 7 5 . 0 7 5 . 5 4 3 , . 9 3 MnO 0 . . 0 9 0 . . 1 4 0 , . 1 2 0 . 12 0 . . 1 3 0 , 10 0 , . 1 0 0 . 1 4 0 , . 2 5 MgO 4 . . 9 3 5 . 4 4 5 . . 5 2 5 . .12 4 . 8 9 6 , .77 5 . 8 8 5 . 8 8 5 . 7 3 CaO 6 . . 9 6 7 . 0 2 6 . . 8 6 7 . . 1 2 7 . 0 3 8 . . 3 0 8 . 1 6 8 . 1 4 7 . 7 2 N a 2 0 3 . . 8 8 4 . 5 4 4 . . 8 2 4 , , 4 9 4 . 1 8 3 , . 9 9 4 . 0 4 3 . 2 8 4 .01 K 2 0 0 . . 6 4 1 . 0 3 1 . 1 5 0 . . 9 9 1 . 0 8 1 . 1 2 1 . 1 0 1 . 2 5 1 .61 P 2 ° 5 0 . 1 4 0 .41 0 . . 4 0 0 .41 0 . .41 0 .31 0 .31 0 . 1 6 0 . 4 2 N o r m a t i v e M i n e r a l s Oz 3 . 4 9 1 . .51 2 , , 4 8 4 , , 0 6 6 . 0 1 2 . , 0 6 6 . 0 1 4 . 2 3 Or 3 . 7 8 6 . , 0 8 6 , .81 5 . . 8 6 6 . 3 8 5 , . 6 0 6 , , 4 7 7 . 4 0 9 , . 5 0 Ab 3 2 . 8 3 3 8 , , 4 0 4 0 , . 8 2 3 7 , , 9 5 3 5 . 3 5 3 3 . . 7 8 34 . 1 5 2 7 . 7 8 3 3 , .91 An 3 3 . 2 8 2 4 . , 5 6 21 . 4 4 2 4 , . 8 0 2 5 . 8 9 2 6 . . 7 8 2 6 , . 4 9 2 7 . 2 4 22 . 2 7 D i 0 . 2 8 6 , . 1 6 7 . 9 6 6 , . 2 9 5 . 0 6 1 0 , . 3 5 9 , . 6 7 9 . 8 1 1 0 , . 6 3 En 1 2 . 2 0 11 . 6 6 10 . 8 9 10 . 5 4 1 0 . 4 3 1 3 , . 2 3 11 . 4 9 1 1 . 5 2 10 . 3 5 Fs 9 . 4 2 6 , . 9 3 3 .61 3 . . 9 7 4 . 0 9 5 . 5 6 5 , 5 5 6 . 0 5 3 . 1 0 Fo - - 0 . 0 8 -Fa - - 0 . 0 4 -Mt 2 . 2 6 1 . 9 0 3 . 4 4 3 , .71 3 . 7 9 1 , . 6 9 1 . 6 9 1 . 8 5 3 . 2 2 n 2 . 1 3 1 . 8 6 1 .61 1 . 8 6 2 . 0 5 1 . 6 6 1 . 6 9 1 . 9 6 1 .81 Ap 0 . 3 2 0 , . 9 6 0 , . 9 4 0 . . 9 5 0 . 9 5 0 , . 7 2 0 , . 7 3 0 . 3 7 0 , , 9 8 D . I . 4 0 . 1 1 4 5 . 9 9 50 .11 4 7 , . 8 7 4 7 . 7 4 4 0 . . 3 8 4 2 , . 6 8 4 1 . 1 9 4 7 , . 6 4 An % 5 0 . 3 4 3 9 , .01 34 . 4 4 3 9 . . 5 2 4 2 . 2 8 4 3 . 76 4 3 . . 6 9 4 9 . 5 1 3 9 . , 6 4 * ^^2^3 a n d FeO c o n t e n t s i n a n a l y s e s r e c a l c u l a t e d t o 100 p e r c e n t v o l a t i l e f r e e d e t e r m i n e d u s i n g r a t i o s g i v e n i n p r e c e e d i n g t e x t . T A B L E X X . C H E M I C A L A N A L Y S E S AND CIPW NORMATIVE M I N E R A L S UF THE T A B L E LAVAS U n i t : T a b l e M e a d o w A n d e s i t e s T h e T a b l e A n d e s i t e s N u n m b e r : 1 2 3 4 5 6 7 8 9 10 11 S a m p l e : 7 4 6 4 6 7 4 6 4 8 7 4 6 5 1 7 4 6 5 3 7 4 6 3 6 7 4 6 3 7 7 4 6 3 8 7 4 6 4 1 7 4 6 4 2 7 4 6 6 7 58011 S i 0 2 57 . 9 2 5 9 , . 2 4 59 . 9 3 5 9 . . 6 8 6 0 , . 2 4 5 6 , . 6 3 5 8 . 0 7 5 8 . 1 8 5 8 . . 1 9 57 . 6 8 5 7 . 4 8 T i 0 2 0 . . 6 6 0 . 6 9 0 . . 6 9 0 , , 6 9 0 , . 6 4 0 . 6 3 0 . 6 5 0 . 6 3 0 , . 6 7 0 . 6 5 0 . 9 0 A 1 2 0 3 1 8 . 4 6 1 8 , . 4 8 1 8 , . 3 2 1 8 , . 7 5 1 8 , . 5 0 18 . 2 2 1 8 . 1 5 18 . 7 2 1 8 , . 5 6 18 . 7 5 1 8 . 9 7 F e 2 ° 3 6 . 5 0 5 , . 6 0 5 , .71 5 , . 6 0 5 , . 7 3 6 , .21 6 . 3 0 5 . . 8 1 ' 6 , , 2 2 6 , . 0 4 6 . 6 2 MnO 0 .11 0 , , 1 0 0 , , 1 0 0 , . 1 0 0 , . 1 0 0 . 0 8 0 . 1 1 0 , . 1 0 0 , ,11 0 , .11 0 . 1 6 MgO 3 . . 8 1 3 , , 1 0 3 . . 1 9 3 , , 1 5 3 . , 4 0 3 , . 8 6 3 . 5 9 3 . . 6 7 4 . 6 0 3 , . 6 7 3 . 1 8 CaO 6 , . 6 9 5 , . 7 4 5 , , 8 5 5 , . 7 9 5 , . 9 8 6 , . 8 0 6 . 7 0 6 , . 5 6 6 , . 7 4 6 . . 6 3 6 . 6 2 N a 2 0 4 . 2 0 4 , . 7 8 3 . . 8 3 4 , . 5 6 3 . , 6 3 4 , . 9 6 4 . 6 5 4 . 9 0 3 . , 7 5 4 . 5 6 4 . 1 2 K j O 1 . 2 2 1 , . 5 5 1 . . 3 8 1 . 3 9 1 . . 2 0 1 , . 1 7 1 . 2 6 1 . 3 3 1 . , 2 9 1 . . 2 4 1 . 0 4 P 2 ° 5 0 . , 3 4 0 . 3 0 0 . 3 0 0 . 3 0 0 . 2 4 0 . , 3 5 0 . 3 9 0 , , 3 7 0 . 4 0 0 . , 3 7 0 . 2 8 H 2 0 + 0 . , 5 6 0 . 31 0 . 4 5 0 . 3 5 0 . 19 0 , . 2 7 0 . 3 2 0 , , 1 9 0 . , 1 2 0 . , 2 7 0 . 7 6 H 2 0 " 0 . , 3 0 0 . 3 3 0 . 0 6 0 . . 0 9 0 . 3 2 0 . , 2 7 0 . 3 0 0 . .31 0 . 0 5 0 . , 5 4 0 . 1 2 T o t a l 1 0 0 . . 8 0 1 0 0 . 2 2 9 9 . 81 1 0 0 . 4 5 1 0 0 . 17 9 9 . 4 5 1 0 0 . 4 9 1 0 0 . .77 1 0 0 . 70 1 0 0 . 51 1 0 0 . 2 5 T r a c e E l e m e n t s ( P P M ) Nb - - - - 2 4 - 7 _ 5 Y - - - - 16 12 _ 18 _ 16 Z r - - - - 9 9 7 5 - 8 5 - 9 2 Rb 11 1 8 - 16 13 1 3 13 1 3 11 11 S r 1 1 8 6 9 7 5 - 1 0 0 6 9 2 6 1 2 6 0 1311 1 2 8 7 1 3 9 7 1 3 0 8 B a 4 4 4 7 1 0 - - 4 1 7 4 7 0 411 4 8 2 4 6 0 4 9 2 Cu 34 27 3 0 - 32 4 3 5 8 50 12 3 8 Zn 73 62 6 5 - 1 0 2 67 71 69 72 77 N i 33 2 6 2 6 - 39 37 29 3 3 30 32 C r 3 6 21 2 3 - 2 3 3 9 31 2 9 29 2 6 V 1 2 8 1 0 5 104 - 117 91 121 121 1 2 3 1 2 3 K/Rb 8 9 6 7 1 5 - 7 4 5 791 771 8 2 4 8 5 6 9 3 9 9 5 3 K / S r 8 . 5 1 3 . 2 - 1 1 . 5 1 0 . 8 7 . 7 8 . 0 8 . 6 7 . 7 7 . 9 K / B a 2 2 . 8 1 8 . 1 - - 2 3 . 9 2 0 . 6 2 5 . 5 2 2 . 9 2 3 . 3 2 0 . 9 C a / S r 4 0 . 3 4 2 . 1 - 41 .1 4 6 . 2 3 8 . 6 3 6 . 5 3 6 . 4 3 4 . 5 3 6 . 2 R b / S r 0 . 0 0 9 5 0 . 0 1 8 5 - 1 0 . 0 1 5 4 0 . 0 1 3 6 0 . 0 1 0 0 0 . 0 0 9 7 0 . 0 1 0 0 0 . 0 0 8 2 0 . 0 0 8 3 C r / V 0 . 3 0 . 2 0 . 2 - 0 . 2 0 . 4 0 . 3 0 . 2 0 . 2 0 . 2 R e c a l c u l a t e d V o l a t i l e F r e e S i 0 2 5 8 . 2 1 59 .71 6 0 . 5 8 59 . 9 0 6 0 . 6 8 5 7 . 4 9 5 8 . 3 9 5 8 . 2 5 58 . 1 2 5 8 . 0 9 5 8 . 1 0 T i 0 2 0 . . 6 6 0 . 7 0 0 . 7 0 0 . . 6 9 0 . 6 4 0 . 6 4 0 . 6 5 0 . 6 3 0 . 6 7 0 , . 6 5 0 . 9 1 A 1 2 ° 3 1 8 . 5 8 1 8 . 6 3 1 8 . 5 2 18 . 8 2 1 8 . 6 3 1 8 . 5 0 18 . 2 5 1 8 . 7 4 18 . 5 4 18 . 8 8 1 9 . 1 8 F e 2 ° 3 * 2 . 2 0 1 . 9 0 1 . 9 5 1 . 9 0 1 . 9 5 2 . 1 3 2 . 1 4 1 . 9 6 2 . 1 0 2 , . 0 5 2 . 2 6 F e O * 3 . . 8 9 3 . 3 7 3 . 4 4 3 , . 3 5 3 . 4 4 3 . 7 6 3 , . 7 8 3 . . 4 7 3 . 7 0 3 , . 6 3 3 . 9 9 MnO 0 .11 0 . 1 0 0 . 1 0 0 . 1 0 0 . 1 0 0 . 0 8 0 . .11 0 , . 1 0 0 .11 0 . .11 0 . 1 6 MgO 3 . . 8 3 3 . . 1 2 3 . 2 2 . 3 . . 1 6 3 . 4 2 3 . 9 2 3 , . 6 1 3 , . 6 7 4 . 5 9 3 , . 7 0 3 . 2 1 CaO 6 . 7 2 5 . 7 9 5 . 9 1 5 . 8 1 6 . 0 2 6 . 9 0 6 . . 7 4 6 , . 5 7 6 . 7 3 6 . . 6 8 6 . 6 9 N a 2 0 4 . 2 2 4 . 8 2 3 . 8 7 4 . 5 8 3 . 6 6 5 . 0 4 4 . 6 8 4 . 9 1 3 . . 7 5 4 . 5 9 4 . 1 6 K j O 1 . 2 3 1 . 5 6 1 . 4 0 1 . 4 0 1 . 2 1 1 . 1 9 1 . . 2 7 1 . 3 3 1 . 2 9 1 . 2 5 1 . 0 5 P 2 ° 5 0 . . 3 4 0 . . 3 0 0 . 3 0 0 , , 3 0 0 . 2 4 0 . 3 6 0 . , 3 9 0 , . 3 7 0 . 4 0 0 . . 3 7 0 . 2 8 N o r m a t i v e M i n e r a l s Qz 8 , . 4 4 8 . . 3 1 1 4 . 4 0 1 0 , .01 1 5 . 7 1 3 . 7 3 6 . 9 3 5 , . 4 2 9 , . 5 4 6 . 72 9 . 7 6 O r 7 . 2 4 9 . . 2 3 8 . 2 4 8 , . 2 4 7 . 1 4 7 . 0 2 7 . , 4 9 7 , , 8 7 7 , .61 7 . . 3 8 6 . 2 1 Ab 3 5 , .71 4 0 . 7 7 3 2 . 7 6 3 8 , . 7 2 3 0 . 9 4 4 2 . 6 1 3 9 . 56 4 1 , .51 3 1 , . 6 9 3 8 . . 8 6 3 5 . 2 4 A n 2 8 . 1 4 2 4 . . 5 9 2 7 . 3 6 2 6 , . 6 9 2 8 . 3 0 2 4 . 3 6 2 5 . . 0 7 2 5 , . 1 9 2 9 , . 9 7 2 7 . . 2 2 3 0 . 5 2 D i 2 .41 1 . 7 3 - 0 . . 1 4 - 6 . 1 0 4 . . 6 8 4 , .01 0 , . 6 6 2 . 8 0 0 . 6 7 En 8 . . 7 4 7 . .21 8 . 0 3 7 , . 8 3 8 . 5 3 7 . 7 1 7 . 4 6 7 . . 7 9 1 1 , .21 8 . , 2 7 7 . 7 9 Fs 4 , . 0 7 3 , . 3 8 3 . 7 5 3 . .61 3 . 8 3 3 . 3 5 3 . 5 6 3 . ,31 4 , . 0 8 3 . 67 4 . 1 5 M t 3 . . 2 0 2 . . 7 6 2 . 8 2 2 . . 7 5 2 . 8 2 3 . 0 8 3 . 10 2 . , 8 5 3 . , 0 4 2 . 9 8 3 . 2 7 11 1 . 2 6 1 . 3 2 1 . 3 2 1 . . 3 2 1 . 2 2 1 . 2 1 1 . . 2 4 1 . , 2 0 1 . . 2 7 1 . . 2 4 1 . 7 3 Ap 0 . , 7 9 0 . . 7 0 0 . 7 0 0 , , 7 0 0 . 5 6 0 . 8 2 0 . ,91 0 . 8 6 0 . 9 3 0 . . 8 6 0 . 6 6 Co 0 . 6 1 0 . 9 4 - -D . I . 5 1 , . 4 0 5 8 , .31 5 5 . 4 0 5 6 . 9 8 5 3 . 7 9 5 3 . 3 6 5 3 . 9 8 5 4 . , 7 9 4 8 , , 8 4 5 2 . 9 6 51 .21 An * 4 4 , . 0 7 3 7 , . 6 2 4 5 . 5 1 4 0 . 8 0 4 7 . 7 8 3 6 . 3 8 3 8 . 79 3 7 . 76 4 8 . . 6 0 41 . 2 0 4 6 . 4 2 * F e 2 ° 3 a n d F e 0 c o n t e " t s i n a n a l y s e s r e c a l c u l a t e d t o 1 0 0 p e r c e n t v o l a t i l e f r e e d e t e r m i n e d u s i n g r a t i o s g i v e n i n p r e c e e d i n g t e x t . 218 T A B L E X X I . C H E M I C A L A N A L Y S E S A N D C I P W N O R M A T I V E M I N E R A L S O F T H E B L A C K T U S K L A V A S U n i t : P l a t y A n d e s i t e M i c r o w a v e B l u f f M o r a i n e • A n d e s i t e s W e s t B l u f f A n d e s i t e s P l u g Dome Summi t L a v a N u m b e r : 1 2 4 5 e 7 E 1 S i 10 11 12 1 3 14 1 5 S a m p l e : 7 4 6 1 0 7 4 6 1 1 7 4 4 1 9 7 4 4 2 1 7 4 6 2 5 7 4 6 2 8 7 4 6 3 1 7 4 6 1 3 7 4 6 1 8 7 4 6 2 2 7 4 6 2 4 7 4 6 3 4 7 4 6 3 5 7 4 6 0 5 5 8 0 0 9 S i 0 2 5 8 . 9 2 • 5 9 . 9 9 6 4 . 2 9 6 2 . 5 2 6 0 . 1 5 5 9 . 01 6 0 . 14 5 9 . 1 0 5 9 . 5 4 6 2 . 2 0 5 8 . 2 5 5 9 . 7 0 6 0 . 8 9 6 0 . 5 9 5 9 . 6 5 T i 0 2 0 . 8 0 1 . 0 6 0 . 5 0 0 . 4 9 • 0 . 8 0 0 . 7 8 0 . 6 2 0 . 7 8 0 . 77 0 . 5 8 0 . 8 0 0 . 6 2 0 . 6 2 0 . 6 5 0 . 7 6 A 1 2 ° 3 1 8 . 3 0 1 8 . 6 2 1 8 . 3 2 1 7 . 7 8 1 8 . 0 7 1 8 . 12 1 8 . 3 3 1 8 . 2 6 1 8 . 2 0 1 9 . 0 6 1 7 . 3 4 1 8 . 5 0 1 8 . 9 3 1 8 . 7 5 1 9 . 1 4 F e 2 0 3 6 . 41 5 . 5 0 4 . 3 8 4 . 2 7 6 . 61 6 . 2 6 5 . 5 5 6 . 2 8 6 . 2 8 5 . 0 7 6 . 2 6 5 . 4 1 5 . 31 5 . 6 2 5 . 9 2 MnO 0 . 11 0 . 1 1 0 . 0 8 0 . 0 8 0 . 11 0 . 11 0 . 1 0 0 . 11 0 . 11 ' 0 . 1 0 0 . 1 1 0 . 0 9 0 . 0 9 0 . 10 0 . 1 4 MgO 2 . 6 3 2 . 2 7 1 . 6 8 2 . 0 1 3 . 01 3 . 01 2. 54 2 . 7 5 2 . 9 8 2 . 3 3 2 . 9 0 2 . 8 7 2 . 5 6 2 . 9 5 3 . 0 5 CaO 6 . 0 5 5 . 9 4 5 . 0 5 5 . 1 0 5 . 9 7 6 . 0 3 5 . 9 0 6 . 12 5 . 9 9 5 . 3 6 6 . 0 5 5 . 9 3 5 . 8 3 6 . 0 6 6 . 1 2 N a 2 0 4 . 3 2 4 . 9 9 2 . 7 3 4 . 5 7 3 . 27 3 . 9 0 4 . . 0 8 4 . . 8 5 3 . 12 4 . 0 0 4 . 7 4 4 . 8 0 3 . 6 3 4 . 0 4 3 . 7 7 K ? 0 1 . 16 1 . 5 1 1 . 5 8 1 . 5 3 1 . 1 8 1 . 2 5 1 . . 2 1 1 . . 2 6 1 . 2 6 0 . 9 0 1 . 2 0 1 . 2 5 1 . . 3 8 1 . 21 1 . 2 4 P 2 ° 5 0 . 3 7 0 . 3 6 0 . 1 9 0 . 1 8 0 . 3 0 0 . 3 2 0 . . 2 6 0 . . 3 6 0 . 3 4 0 . 2 2 0 . 3 3 0 . 2 6 0 . 2 3 0 . 2 5 0 . 0 9 H 2 0 + 0 . 3 4 0 . 2 0 0 . 41 0 . 3 2 0 . 1 5 0 . 2 7 0 . . 3 0 0 . 2 3 0 . 3 3 0 . 1 6 0 . 3 6 0 . 3 4 0 . 3 2 0 . 0 3 0 . 5 6 H 2 0 " 0 . 8 9 0 . 2 7 0 . 5 0 0 . 9 6 0 . 4 9 0 . 52 0 . . 2 4 0 . 3 6 - 0 . 4 0 0 . 3 3 0 . 1 9 0 . 0 3 T o t a l 1 0 0 . 3 0 1 0 0 . 8 2 9 9 . 71 9 9 . 8 1 9 9 . 6 2 9 9 . 5 5 9 9 . 5 5 1 0 0 . . 3 4 9 9 . 2 8 9 9 . 9 8 9 8 . 7 4 1 0 0 . 1 0 9 9 . 9 8 1 0 0 . 2 5 1 0 0 . 4 7 T r a c e e l e m e n t s ( P P M ) Nb 4 5 1 Y _ _ 2 2 - 1 9 16 _ i Z r - - 1 5 6 - 1 1 7 9 6 -Rb 1 11 _ 2 6 2 0 11 11 14 12 8 13 1 3 12 1 3 -S r 8 6 7 - 9 1 0 8 9 8 8 7 5 8 ' 18 9 3 4 831 8 1 8 8 5 6 911 9 3 0 9 4 2 -B a - 3 6 5 3 9 9 4 2 9 4 8 3 4 0 3 5 3 3 2 5 8 5 5 5 4 1 9 4 1 7 3 6 3 -Cu - 21 2 2 3 6 32 4 3 3 7 2 8 3 5 3 7 i 16 2 6 -Zn _ 4 9 5 0 7 3 7 9 6 3 1 30 6 4 7 9 6 6 7 0 7 5 -Ni - 10 13 2 9 2 9 31 3 0 1 8 29 3 9 3 7 1 59 -C r 27 8 12 3 2 3 0 2 9 2 8 12 31 2 2 31 3 7 -V 1 0 6 - 8 5 8 5 1 2 0 1 2 9 111 1 2 0 1 7 4 8 7 1 1 5 1 1 4 1 2 4 -K / R b 8 9 1 _ 5 0 6 6 3 5 891 9 1 8 7 4 4 8 7 9 9 5 8 7 6 6 8 3 0 9 5 5 7 8 4 K / S r 11 .1 - 14 . 4 1 4 . 1 11 . 2 1 2 . . 2 10 . 8 12 . 6 9 . 1 1 1 . 6 11 . 4 12 . 3 10 . 7 -K / B a - 3 5 . 9 31 . 8 3 3 . 8 21 . 5 2 4 . 9 1 9 . 6 2 9 . 0 1 7 . 9 2 4 . 8 27 . 5 27 . 7 -C a / S r 4 9 . 9 - 3 9 . 7 4 0 . 6 4 8 . 8 5 0 . 8 4 5 . 1 5 2 . 6 4 6 . 8 5 0 . 5 4 6 . 5 4 4 . 8 4 6 . 0 -R b / S r 0 . 0 1 2 5 - 0 . 0 2 8 5 0 . 0 2 2 3 0 . 0 1 2 6 0 . 0 1 3 3 0 . 0 1 4 5 0 . 0 1 4 3 0 . 0 0 9 5 0 . 0 1 5 2 0 . 0 1 3 7 0 . 0 1 2 9 0 . 0 1 3 6 -C r / V 0 . 3 - 0 .1 0 . 1 0 . 3 0 . 2 0 . 3 0 . 2 0 . 1 0 . 4 0 . 2 0 . 3 0 . 3 -R e c a l c u l a t e d V o l a t i ' l e F r e e S i 0 2 5 9 . 7 3 6 0 . 0 0 6 5 . 2 6 6 3 . 6 4 6 0 . 7 4 59 . 9 9 61 . 1 4 69 . 4 3 6 0 . 5 5 6 2 . 5 2 5 9 . 7 0 6 0 . 2 6 61 . 4 3 6 0 . 6 8 5 9 . 9 6 T i 0 2 0 . 8 1 1 . 0 6 0 . 5 1 0 . 5 0 0 . 8 1 0 . 7 9 0 . 6 3 0 . 7 8 0 . 7 8 0 . 5 8 0 . 8 2 0 . 6 3 0 . 6 3 0 . 6 5 0 . 7 6 A 1 2 ° 3 1 8 . 5 5 1 8 . 6 2 1 8 . 6 0 1 8 . 1 0 18 . 2 5 1 8 . 4 2 18 . 6 4 1 8 . 3 6 18 . 5 4 1 9 . 1 6 1 7 . 7 7 1 8 . 6 7 1 9 . 1 0 18 . 7 8 1 9 . 2 4 F e 2 ° 3 * 2 . 1 9 1 . 8 6 1 . 5 0 1 . 4 7 2 . 2 5 2 . 1 5 1 . 9 0 2 . 1 3 2 . 1 6 1 . 7 2 2 . 1 6 1 . 8 4 1 . 8 1 1 . 9 0 2 . 0 1 F e O * 3 . 8 7 3 . 2 8 2 . 6 5 2 . 5 9 3 . 9 8 3 . 7 9 3 . 3 6 3 . 7 6 3 . 8 1 3 . 0 4 3 . 8 3 3 . 2 6 ' 3 . 1 9 3 . 3 6 3 . 5 5 MnO 0 . 1 1 0 . 1 1 0 . 0 8 0 . 0 8 0 . 1 1 0 . 1 1 0 . 1 0 0 . 1 1 0 .11 0 . 1 0 0 . 1 1 0 . 0 9 0 . 0 9 0 . 1 0 0 . 1 4 MgO 2 . 6 7 2 . 2 7 1 .71 2 . 0 5 3 . 0 4 3 . 0 6 2 . 5 8 2 . 7 7 3 . 0 4 2 . 3 4 2 . 9 7 2 . 9 0 2 . 5 8 2 . 9 5 3 . 0 7 CaO 6 . 1 3 5 . 9 4 5 . 1 3 5 . 1 9 6 . 0 3 6 . 1 3 6 . 0 0 6 . 1 5 6 . 1 0 5 . 3 9 6 . 2 0 5 . 9 9 5 . 8 8 6 . 0 7 6 . 1 5 N a 2 0 4 . 3 8 4 . 9 9 2 . 7 7 4 . 6 5 3 . 3 0 3 . 9 6 4 . 1 5 4 . 8 8 3 . 1 8 4 . 0 2 4 . 8 6 4 . 8 5 3 . 6 6 4 . 0 5 3 . 7 9 K j O 1 . 1 8 1 . 5 1 1 . 6 0 1 . 5 6 1 . 1 9 1 . 2 7 1 . 2 3 1 . 2 7 1 . 2 8 0 . 9 0 1 . 2 3 1 . 2 6 1 . 3 9 1 . 2 1 1 . 2 5 P 2 ° 5 0 . 3 8 0 . 3 6 0 . 1 9 0 . 1 8 0 . 3 0 0 . 3 3 0 . 2 6 0 . 3 6 0 . 3 5 0 . 2 2 0 . 3 4 0 . 2 6 0 . 2 3 0 . 2 5 0 . 0 9 N o r m a t i v e M i n e r a l s O z 11 . 9 0 9 , 4 4 2 8 . 6 8 1 5 . . 8 0 1 8 . 4 1 1 3 . . 4 2 1 4 . 6 4 8 . 8 7 1 8 . 7 5 1 9 . 7 2 9 . 3 0 9 . 7 5 1 7 . 4 1 1 4 . 1 2 1 3 . . 8 9 O r 6 . . 9 5 8 . 9 2 9 . 4 8 9 . 2 0 7 . . 0 4 7 . 5 1 7 . 2 7 7 . 4 9 7 . . 5 8 5 . 3 5 7 . . 2 7 7 . 4 6" 8 . 2 3 7 . . 1 6 7 . . 3 7 A b 3 7 . . 0 6 4 2 . 2 3 2 3 . 4 5 3 9 . . 3 6 2 7 . 9 4 3 3 . 5 4 3 5 . 1 0 4 1 . 2 6 2 6 . . 8 9 3 4 . . 0 2 4 1 . 1 1 4 1 . 0 0 3 0 . 9 9 3 4 . 2 4 3 2 . 0 6 A n 2 7 . 4 9 2 3 . 9 5 2 4 . 1 7 2 3 . . 9 0 2 7 . 9 3 2 8 . 2 8 2 8 . 0 3 2 4 . 4 7 2 8 . 0 1 2 5 . 2 8 2 3 . 0 6 2 5 . 4 8 2 7 . 6 6 2 8 . 4 7 2 9 . 9 3 D i 0 . . 4 0 2 . 5 7 0 . . 5 3 4 . 4 6 2 . . 0 2 E n 6 . 5 2 4 . 8 7 4 . 2 5 4 . 9 3 7 . 5 7 7 . 6 2 6 . 4 3 5 . 9 7 7 . 5 6 5 . 8 3 6 . 0 1 6 . . 5 6 6 . 4 3 7 . 3 6 7 . 6 4 F s 4 . 1 0 2 . 5 2 2 . 9 4 2 . . 7 8 4 . 3 2 4 . 0 9 3 . . 7 5 3 . 5 2 4 . 1 3 3 . 3 8 3 . 3 2 3 . 2 7 3 . 5 1 3 . 7 0 3 . 8 6 M t 3 . 1 8 2 . 6 9 2 . 1 8 2 . 1 3 3 . 2 7 3 . 11 2 . 7 6 3 . 0 9 3 . . 1 3 2 . 4 9 3 . . 1 4 2 . 6 7 2 . 6 2 2 . 7 5 2 . 9 1 11 1 . 5 4 2 . 0 1 0 . . 9 6 0 . . 9 5 1 . 5 3 1 . 51 1 . 2 0 1 . 4 9 1 . 4 9 1 . 1 1 1 . 5 6 1 . 1 9 1 . 1 9 1 . 2 4 1 . 4 5 A p 0 . 8 7 0 . 8 3 0 . . 4 5 0 . . 4 2 0 . 7 0 0 . 7 5 0 . . 6 1 0 . 8 4 0 . . 8 0 0 . 51 0 . . 7 8 0 . 6 1 0 . . 5 4 0 . . 5 8 0 . 21 C o 3 . 4 4 1 . 2 9 0 . 1 6 0 . . 2 1 1 . 6 6 2 . 3 0 1 . 4 3 0 . 3 8 0 . . 6 9 D . I . 5 5 . 9 1 6 0 . 5 9 61 . . 6 1 6 4 . 3 6 5 3 . 3 9 5 4 . 4 8 5 7 . 0 1 5 7 . 6 2 5 3 . . 2 2 5 9 . 0 9 5 7 . 6 8 5 8 . 2 0 5 6 . 6 2 5 5 . 5 2 5 3 . 3 2 A n % 4 2 . 5 9 3 6 . 1 9 5 0 . . 7 6 3 7 . . 7 8 4 9 . 9 9 4 5 . 7 5 4 4 . 4 0 3 7 . 2 2 51 . 0 2 4 2 . 6 3 3 5 . 9 3 3 8 . . 3 3 4 7 . 1 7 4 5 . 4 1 4 8 . . 2 8 * £^2^3 a n c l c o n t e n t s i n a n a l y s e s r e c a l c u l a t e d t o 1 0 0 p e r c e n t v o l a t i l e f r e e d e t e r m i n e d u s i n g r a t i o s g i v e n i n p r e c e e d i n g t e x t . T A B L E X X I I . C H E M I C A L A N A L Y S E S AND CIPW NORMATIVE M I N E R A L S OF MOUNT P R I C E LAVAS 219 u " i t : T a b l e B a y S u m m i t L a v a W e l d e d - a s h P r i c e B a y C u l l i t o n C r e e k N u m b e r : 1 2 3 4 5 6 7 8 9 S a m p l e : 7 4 5 2 3 7 4 5 2 2 7 4 5 1 9 7 4 5 4 3 7 4 5 6 5 7 4 5 4 2 7 4 5 3 5 7 4 5 7 9 7 4 5 8 1 S i 0 2 5 8 . 3 7 6 2 . 4 5 6 3 . 1 1 6 1 . 6 9 6 2 . 1 5 6 3 . 6 9 6 2 . 5 3 5 9 . 8 2 6 0 . 9 3 T i 0 2 0 . 7 2 0 . 5 8 0 . 5 4 0 . 4 7 0 . 5 5 0 . 5 2 0 . 5 5 0 . 6 1 0 . 7 1 A 1 2 ° 3 1 8 . 5 5 1 8 . 9 6 1 7 . 7 7 1 8 . 4 9 1 8 . 4 1 1 7 . 3 1 1 7 . 6 8 1 7 . 8 4 1 8 . 1 4 F e 2 0 3 fi.02 5 . 1 0 4 . 9 1 4 . 3 3 4 . 8 6 4 . 5 4 4 . 7 0 5 . 5 6 5 . 4 8 MnO 0 . 0 9 0 . 1 0 0 . 0 9 0 . 0 8 0 . 0 9 0 . 0 8 0 . 1 0 0 . 1 0 0 . 0 9 MgO 3 . 5 7 2 . 3 3 2 . 9 3 2 . 3 9 2 . 3 5 2 . 2 3 2 . 8 1 2 . 6 3 2 . 8 9 CaO 6 . 6 7 5 . 3 6 5 . 1 6 4 . 8 8 5 . 2 3 4 . 5 7 5 . 1 9 5 . 6 8 5 . 8 8 N a 2 0 4 . 5 4 4 . 2 5 3 . 5 1 4 . 5 3 3 . 1 3 4 . 5 3 4 . 5 2 4 . 5 9 4 . 2 9 1^0 1 . 1 2 1 . 1 4 1 . 3 6 1 . 3 5 1 . 2 5 1 . 6 5 1 . 2 2 1 . 4 7 1 . 4 8 P 2 ° 5 0 . 3 1 0 . 2 2 0 . 2 5 0 . 2 6 0 . 2 5 0 . 2 0 0 . 3 5 0 . 3 5 0 . 3 4 H 2 0 + 0 . 7 2 0 . 1 6 0 . 2 6 0 . 2 7 0 . 4 5 0 . 1 2 0 . 0 3 0 . 3 2 0 . 0 1 H 2 0 " 0 . 1 5 - 0 . 3 2 1 . 0 3 0 . 6 6 0 . 0 5 0 . 0 2 0 . 3 3 0 . 0 7 T o t a l 1 0 0 . 8 3 1 0 0 . 6 5 1 0 0 . 2 1 9 9 . 7 7 9 9 . 3 8 9 9 . 4 9 9 9 . 7 0 9 9 . 3 0 1 0 0 . 3 1 T r a c e e l e m e n t s ( P P M ) Nb Y - - - - - - - - -Z r - - - _ _ Rb 12 - 12 1 5 15 1 3 14 1 5 _ S r 1 3 0 4 - 8 5 6 8 4 6 8 7 4 751 8 6 5 1 0 2 2 _ Ba 4 9 9 - 4 1 6 481 5 4 0 6 6 8 5 4 5 7 0 5 _ Cu 50 - 1 8 18 14 16 2 8 19 _ Zn 77 - 56 52 53 4 4 54 57 N i 4 9 - 2 3 21 31 2 3 24 25 _ C r 4 0 - 22 21 2 0 3 2 2 3 29 _ V 1 4 0 - 8 4 1 0 5 9 7 7 5 79 122 K/Rb 781 _ 9 5 7 747 7 1 5 1 0 7 8 7 0 8 8 3 0 K / S r 7 . 1 - 1 3 . 2 1 3 . 2 1 1 . 9 1 8 . 2 11 . 7 11 . 9 _ K / B a 1 8 . 6 - 2 7 . 1 2 3 . 3 1 9 . 2 2 0 . 5 1 8 . 6 1 7 . 3 _ C a / S r 3 6 . 6 - 4 3 . 1 4 1 . 2 4 2 . 8 4 3 . 5 4 2 . 9 3 9 . 7 R b / S r 0 . 0 0 9 1 - 0 . 0 1 3 8 0 . 0 1 7 7 0 . 0 1 6 6 0 . 0 1 6 9 0 . 0 1 6 5 0 . 0 1 4 4 _ C r / V 0 . 3 0 . 3 0 . 2 0 . 2 0 . 4 0 . 3 0 . 2 -R e c a l c u l a t e d V o l a t i l e F r e e S i 0 2 5 8 . 6 3 6 2 . 3 6 6 3 . 5 5 6 2 . 8 3 6 3 . 4 5 6 4 . 3 2 6 2 . 9 5 6 0 . 8 7 61 . 0 1 T i 0 2 0 . 7 2 0 . 5 8 0 . 5 4 0 . 4 8 0 . 5 6 0 . 5 3 0 . 5 5 0 . 6 2 0 . 7 1 A 1 2 0 3 1 8 . 6 3 1 8 . 9 3 1 7 . 8 9 1 8 . 8 3 1 8 . 8 0 1 7 . 4 8 1 7 . 8 0 1 7 . 6 8 1 8 . 4 7 F e 2 ° 3 * 2 . 0 4 1 . 7 2 1 . 6 7 1 . 4 9 1 . 6 7 1 . 5 5 1 . 6 0 1 . 9 1 1 . 8 5 F e O * 3 . 6 1 3 . 0 4 2 . 9 5 2 . 6 3 2 . 9 6 2 . 7 3 2 . 8 2 3 . 3 7 3 . 2 7 MnO 0 . 0 9 0 . 1 0 0 . 0 9 0 . 0 8 0 . 0 9 0 . 0 8 0 . 1 0 0 . 1 0 0 . 1 0 MgO 3 . 5 9 2 . 3 3 2 . 9 5 2 . 4 3 2 . 4 0 2 . 2 5 2 . 8 3 2 . 6 8 2 . 8 9 CaO 6 . 7 0 5 . 3 5 5 . 2 0 4 . 9 7 5 . 3 4 4 . 6 2 5 . 2 2 5 . 7 8 5 . 8 9 N a 2 0 4 . 5 6 4 . 2 4 3 . 5 3 4 . 6 1 3 . 2 0 4 . 5 7 4 . 5 5 4 . 6 7 4 . 3 0 K j O 1 . 1 2 1 . 1 4 1 . 3 7 1 . 3 7 1 . 2 8 1 . 6 7 1 . 2 3 1 . 5 0 1 . 4 8 P 2 ° 5 0 . 3 1 0 . 2 2 0 . 2 5 0 . 2 6 0 . 2 6 0 . 2 0 0 . 3 5 0 . 3 6 0 . 3 4 N o r m a t i v e M i n e r a l s Oz 8 . 0 9 1 7 . 4 6 2 1 . 4 2 1 5 . 8 5 2 4 . 1 8 1 7 . 2 8 1 5 . 9 4 11 . 3 6 1 3 . 0 4 O r 6 . 6 5 6 . 7 3 8 . 0 9 8 . 1 3 7 . 5 4 9 . 8 5 7 . 2 6 8 . 8 4 8 . 7 6 Ab 3 8 . 5 8 3 5 . 9 1 2 9 . 9 1 3 9 . 0 4 2 7 . 0 4 3 8 . 7 1 3 8 . 5 0 3 9 . 5 2 3 6 . 3 5 A n 2 7 . 0 5 2 5 . 1 2 2 4 . 1 3 2 2 . 9 3 2 4 . 8 2 2 1 . 5 8 2 3 . 6 2 2 4 . 1 5 2 5 . 9 0 Di 3 . 3 5 - - - - - - 1 . 7 8 0 . 8 7 En 7 . 8 1 5 . 7 9 7 . 3 5 6 . 0 6 5 . 9 8 5 . 6 1 7 . 0 5 6 . 1 1 6 . 9 2 Fs 3 . 4 2 3 . 3 9 3 . 3 1 2 . 9 6 3 . 2 9 3 . 0 2 3 . 1 3 3 . 4 7 3 . 3 3 Mt 2 . 9 6 2 . 4 9 2 . 4 2 2 . 1 6 2 . 4 3 2 . 2 4 2 . 3 1 2 . 7 7 2 . 6 8 n 1 . 3 7 1 . 1 0 1 . 0 3 0 . 9 1 1 . 0 7 1 . 0 0 1 . 0 5 1 . 1 8 1 . 3 5 Ap . 0 . 7 2 0 . 5 1 0 . 5 8 0 . 6 1 0 . 5 9 0 . 4 7 0 . 8 2 0 . 8 3 0 . 7 9 Co - 1 . 5 1 1 . 7 5 1 . 3 5 3 . 0 6 0 . 2 4 0 . 3 3 - -D . I . 5 3 . 3 3 6 0 . 0 9 5 9 . 4 3 6 3 . 0 2 5 8 . 7 6 6 5 . 8 4 6 1 . 6 9 5 9 . 7 2 5 8 . 1 4 An % 4 1 . 2 1 41 . 1 6 4 4 . 6 6 3 7 . 0 0 4 7 . 8 6 3 5 . 7 9 3 8 . 0 2 3 7 . 9 3 4 1 . 6 1 * F e 2 0 3 a n d FeO c o n t e n t i n a n a l y s e s r e c a l c u l a t e d t o 1 0 0 p e r c e n t v o l a t i l e f r e e d e t e r m i n e d u s i n g r a t i o s g i v e n i n p r e c e e d i n g t e x t . T A B L E X X I I . CONTINUED U n i t : B a r r i e r A n d e s i t e N u m b e r : 10 11 12 1 3 14 1 5 16 17 1 8 S a m p l e : 7 4 5 3 1 7 4 5 3 3 7 4 5 3 4 7 4 5 4 7 7 4 5 8 4 5 8 0 0 6 2 7 0 2 0 2 7 0 1 9 2 7 0 1 8 S i 0 2 5 9 . 4 4 5 9 . 0 3 5 9 . 7 5 6 0 . 1 2 6 0 . 4 9 6 1 . 0 7 6 0 . 9 8 6 0 . 7 5 6 1 . 7 3 T i 0 2 0 . 7 9 0 . 7 5 0 . 6 8 0 . 9 0 0 . 7 3 0 . 5 6 0 . 7 1 0 . 7 4 0 . 6 1 A 1 2 ° 3 1 7 . 3 8 1 8 . 2 4 1 8 . 1 2 1 8 . 0 9 1 7 . 6 6 1 8 . 4 6 1 7 . 6 1 1 7 . 5 5 1 7 . 6 7 F e 2 ° 3 5 . 6 7 5 . 9 4 5 . 5 5 4 . 9 9 5 . 5 7 5 . 9 3 5 . 9 1 5 . 9 2 5 . 5 4 MnO 0 . 0 9 0 . 1 0 0 . 0 9 0 . 0 9 0 . 1 0 0 . 1 4 0 . 1 1 0 . 1 1 0 . 1 0 MgO 3 . 1 5 3 . 3 9 3 . 5 1 3 . 1 5 2 . 7 6 3 . 2 2 2 . 7 3 2 . 8 4 2 . 6 9 CaO 5 . 9 5 6 . 0 6 6 . 0 4 6 . 4 6 5 . 8 0 5 . 7 8 5 . 8 6 5 . 7 2 5 . 8 1 N a 2 0 4 . 2 8 4 . 1 4 3 . 7 7 4 . 5 5 4 . 7 4 4 . 2 3 4 . 5 7 4 . 5 7 4 . 4 4 K^O 1 . 5 6 1 . 2 6 1 . 3 5 1 . 2 6 1 . 4 7 0 . 9 9 1 . 2 4 1 . 4 6 1 . 1 6 P 2 ° 5 0 . 3 7 0 . 2 4 0 . 3 0 0 . 2 8 0 . 3 5 0 . 1 6 0 . 2 9 0 . 3 4 0 . 2 5 H 2 0 + 0 . 1 0 0 . 3 4 0 . 1 3 0 . 2 3 0 . 1 5 0 . 0 7 - - -H 2 0 " 0 . 4 5 0 . 1 8 0 . 0 7 0 . 1 1 0 . 1 4 - - - -T o t a l 9 9 . 2 3 9 9 . 6 7 9 9 . 3 6 1 0 0 . 2 3 9 9 . 9 6 1 0 0 . 6 1 1 0 0 . 0 0 9 9 . 9 9 1 0 0 . 0 0 T r a c e e l e m e n t s ( P P M ) Nb 8 _ 8 - - - -Y _ _ 17 - 1 8 - - - -Z r _ 9 4 - 1 2 8 - - - -Rb _ • 1 4 1 5 1 8 - 12 17 11 S r 9 3 9 1 0 5 6 9 6 4 1011 - 8 9 8 1 0 5 2 9 0 6 B a _ 4 5 9 4 7 6 521 5 2 2 - 4 4 7 5 1 9 4 2 2 Cu - 16 21 2 6 9 - - - -Zn _ 5 6 6 0 5 6 61 - - - -N i _ 2 5 29 3 3 27 - 2 5 27 27 C r 2 5 2 6 2 8 3 2 2 6 - - - -V 1 1 3 1 0 9 1 1 0 1 1 2 1 0 3 - - - ~ K/Rb 6 7 5 8 1 2 7 1 6 6 9 7 - 8 4 4 6 9 7 8 9 0 K / S r _ 1 1 . 1 1 0 . 6 1 0 . 9 1 2 . 1 - 1 1 . 5 1 1 . 5 1 0 . 6 K / B a 2 2 . 8 2 3 . 5 2 0 . 1 2 3 . 4 - 2 3 . 0 2 3 . 4 2 2 . 8 C a / S r _ 4 6 . 1 4 0 . 9 4 7 . 9 4 1 . 0 - 4 6 . 6 3 8 . 9 4 5 . 8 R b / S r - 0 . 0 1 6 5 0 . 0 1 3 1 0 . 0 1 5 1 0 . 0 1 7 3 - 0 . 0 1 3 6 0 . 0 1 6 5 0 . 0 1 1 9 C r / V 0 . 2 0 . 2 0 . 3 0 . 3 0 . 3 - ~ ~ ~ R e c a l c u l a t e d V o l a t i l e F r e e S i 0 2 6 0 . 4 7 5 9 . 7 7 6 0 . 4 8 6 0 . 3 9 6 0 . 9 2 6 0 . 9 8 61 . 2 2 6 0 . 9 9 6 1 . 9 5 T i 0 2 0 . 8 0 0 . 7 6 0 . 6 9 0 . 9 0 0 . 7 4 0 . 5 6 0 . 7 1 0 . 7 4 0 . 6 2 A 1 2 ° 3 1 7 . 6 8 1 8 . 4 7 1 8 . 3 4 1 8 . 1 7 1 7 . 7 8 1 8 . 4 3 1 7 . 6 8 1 7 . 6 2 1 7 . 7 3 F e 2 0 3 * 1 . 9 5 2 . 0 3 1 . 9 0 1 . 6 9 1 . 8 9 2 . 0 0 2 . 0 0 2 . 0 1 1 . 8 8 F e O * 3 . 4 4 3 . 5 9 3 . 3 5 2 . 9 9 3 . 3 4 3 . 5 3 3 . 5 4 3 . 5 4 3 . 3 2 MnO 0 . 0 9 0 . 1 0 0 . 0 9 0 . 0 9 0 . 1 0 0 . 1 4 0 . 1 1 0 . 1 1 0 . 1 1 MgO 3 . 2 0 3 . 4 3 3 . 5 5 3 . 1 6 2 . 7 8 3 . 2 2 2 . 7 4 2 . 8 5 2 . 7 0 CaO 6 . 0 5 6 . 1 4 6 . 1 1 6 . 4 9 5 . 8 4 5 . 7 7 5 . 8 8 5 . 7 4 5 . 8 3 N a 2 0 4 . 3 5 4 . 1 9 3 . 8 2 4 . 5 7 4 . 7 7 4 . 2 2 4 . 5 9 4 . 5 9 4 . 4 6 K 2 0 1 . 5 9 1 . 2 8 1 . 3 7 1 . 2 7 1 . 4 8 0 . 9 9 1 . 2 4 1 . 4 7 1 . 1 6 P 2 ° 5 0 . 3 8 0 . 2 4 0 . 3 0 0 . 2 8 0 . 3 5 0 . 1 6 0 . 2 9 0 . 3 4 0 . 2 5 N o r m a t i v e M i n e r a l s Qz 1 1 . 5 4 1 1 . 4 0 1 3 . 8 7 1 0 . 9 6 11 . 0 6 1 4 . 0 2 1 2 . 8 5 1 2 . 0 3 1 4 . 5 9 O r 9 . 3 8 7 . 5 4 8 . 0 8 7 . 4 8 8 . 7 5 5 . 8 4 7 . 3 6 8 . 6 6 6 . 8 7 Ab 3 6 . 3 4 3 5 . 4 7 3 2 . 2 9 3 8 . 6 7 4 0 . 3 9 3 5 . 7 4 3 8 . 8 2 3 8 . 8 2 3 7 . 7 1 A n 2 4 . 0 1 2 7 . 8 1 2 8 . 3 5 2 5 . 3 3 2 2 . 7 3 2 7 . 5 9 2 3 . 9 7 2 3 . 1 6 2 4 . 9 6 Di 2 . 8 7 0 . 8 4 - 4 . 0 3 3 . 2 0 - 2 . 6 9 2 . 5 2 1 . 8 9 En 7 . 0 2 8 . 2 7 8 . 8 5 6 . 4 5 5 . 9 0 8 . 0 1 5 . 9 9 6 . 3 1 6 . 1 3 Fs 3 . 1 2 3 . 7 2 3 . 6 2 2 . 2 6 3 . 0 3 4 . 1 7 3 . 3 9 3 . 4 0 3 . 3 8 M t 2 . 8 2 2 . 9 4 2 . 7 5 2 . 4 5 2 . 7 4 2 . 9 0 2 . 9 0 2 . 9 1 2 . 7 2 11 1 . 5 3 1 . 4 4 1 . 3 1 1 . 7 2 1 . 4 0 1 . 0 6 1 . 3 5 1 . 4 0 1 . 1 7 A p 0 . 8 7 0 . 5 6 0 . 7 0 0 . 6 5 0 . 8 2 0 . 3 7 0 . 6 7 0 . 7 9 0 . 5 8 Co - - 0 . 2 0 - - 0 . 3 0 - - ~ D . I . 5 7 . 7 5 5 4 . 5 1 5 4 . 2 3 5 7 . 1 1 6 0 . 1 9 5 5 . 6 0 5 9 . 0 3 5 9 . 5 2 5 9 . 1 6 A n % 3 9 . 4 5 4 3 . 9 5 4 6 . 7 5 3 9 . 5 8 3 6 . 0 1 4 3 . 5 7 3 8 . 1 7 3 7 . 3 6 3 9 . 8 3 * F e 2 0 3 a n d FeO c o n t e n t s i n a n a l y s e s r e c a l c u l a t e d t o 1 0 0 p e r c e n t v o l a t i l e f r e e d e t e r m i n e d u s i n g r a t i o s g i v e n i n p r e c e e d i n g t e x t . 2 2 1 APPENDIX I I . MAJOR ELEMENT PARTIAL MELTING CALCULATIONS FOR GARIBALDI LAKE ANDESITES Chapter I I I p r e s e n t s arguments t h a t t r a n s f o r m e d o c e a n i c c r u s t ( e c l o g i t e ) , s p i n e l - or g a r n e t - p e r i d o t i t e , or r h y o d a c i t e - e n r i c h e d p e r i d o t i t e c o u l d be p o t e n t i a l s o u r c e m a t e r i a l s f o r the G a r i b a l d i Lake a n d e s i t e s , i n a d d i t i o n t o h i g h - l e v e l f r a c t i o n a t i o n o f b a s a l t i c magma. I f t h e s e a n d e s i t e s r e p r e s e n t u n m o d i f i e d p a r t i a l m e l t s o f one of the p o s s i b l e p a r e n t a l m i n e r a l assemblages, then the p a r t i a l m e l t s and r e f r a c t o r y r e s i d u e must have a b u l k c o m p o s i t i o n e q u i v a l e n t t o t h a t o f the s o u r c e r o c k . To e v a l u a t e t h e s e m e l t i n g hypotheses q u a n t i t a t i v e l y , the p r o p o r t i o n s o f magma and r e f r a c t o r y m i n e r a l s w i t h which a magma c o u l d have been s a t u r a t e d a t o r n e a r i t s l i q u i d u s have been determined u s i n g the l e a s t - s q u a r e s m i x i n g program XALFRAC ( J . N i c h o l l s , p e r s o n a l communication). These c a l c u l a t i o n s w i l l n o t d e f i n e p r e c i s e l y the c o m p o s i t i o n of the s o u r c e m a t e r i a l " , but r a t h e r t e s t t h e - q u a n t i t a t i v e r e a s o n a b l e n e s s o f v a r i o u s m e l t i n g h y p o t h e s e s . The approach t a k e n i s s i m i l a r t o t h a t d e s c r i b e d by G i l l (1974). The parameter f o r goodness o f f i t i n the c a l c u l a t i o n s i s e x p r e s s e d as a sum o f squares o f r e s i d u a l s 2 (Er ) , where the r e s i d u a l s a r e t h e d e v i a t i o n between o b s e r v e d and c a l c u l a t e d major element o x i d e v a l u e s ( S i O ^ , A ^ O ^ , H O 2 , FeO, MgO, CaO, N a 2 0 , and ^ 0 ) . What v a l u e t h i s term s h o u l d r e a c h b e f o r e a model i s r e j e c t e d i s a m a t t e r o f p e r s o n a l judgement. The subducted o c e a n i c l i t h o s p h e r e i s presumed t o po s s e s s a b u l k c o m p o s i t i o n comparable, to a f i r s t a p p r o x i m a t i o n , t o the t h o l e i i t i c b a s a l t ( f r e s h o r a l t e r e d ) o f the a d j a c e n t Juan de Fuca P l a t e ( T a b l e X X I I I ) . The c o m p o s i t i o n s o f g a r n e t and c l i n o p y r o x e n e used as 222 r e f r a c t o r y phases ( T a b l e XXIV) r e p r e s e n t l i q u i d u s o r n e a r - l i q u i d u s phases o b t a i n e d i n h i g h - p r e s s u r e e x p e r i m e n t a l s t u d i e s on o c e a n i c t h o l e i i t e or a n d e s i t e c o m p o s i t i o n s ( G i l l , 1974; T.H. Green and Ringwood, 1968). R e s u l t s of c a l c u l a t i o n s based on a model i n v o l v i n g p a r t i a l m e l t i n g of a n ' e c l o g i t l c m i n e r a l assemblage o f the assumed c o m p o s i t i o n s a r e p r e s e n t e d i n T a b l e XXV. Al t h o u g h s p i n e l - l h e r z o l i t e n o d u l e s have been r e p o r t e d i n a l k a l i n e l a v a s o f the W e l l s Grey - Quesnel r e g i o n o f e a s t e r n B r i t i s h Columbia ( L i t t l e j o h n and Greenwood, 1974; F i e s i n g e r , 1975), s i m i l a r m a n t l e - d e r i v e d i n c l u s i o n s , which might p r o v i d e i n f o r m a t i o n about the zone o f p a r t i a l m e l t i n g , a r e absent i n t h e l a v a s o f the G a r i b a l d i Lake a r e a . C o n s e q u e n t l y , h y p o t h e t i c a l mantle c o m p o s i t i o n s ( i n c l u d i n g t h e assemblages OLIV + OPX + CPX + AMPB + SP, OLIV + OPX + CPX + AMPB + PL, OLIV + OPX + CPX + AMPB + GARN, and OLIV + OPX + CPX + SP computed by Leeman (1974); T a b l e X X I I I ) have been used t o model p o s s i b l e s o u r c e p e r i d o t i t e c o m p o s i t i o n s . M i n e r a l c o m p o s i t i o n s used t o r e p r e s e n t r e f r a c t o r y phases a r e the same as those used t o c a l c u l a t e the l h e r z o l i t e c o m p o s i t i o n s ( T a b l e XXIV). A b e t t e r m a t h e m a t i c a l f i t between observed and p r e d i c t e d major element c o n c e n t r a t i o n s o f the a n d e s i t e s i s o b t a i n e d ( T a b l e XXVI) u s i n g ? t h e s e l h e r z o l i t e models (Er v a l u e s of 0.007-0.092 compared t o 0.200-1.600 f o r the e c l o g i t e m o d e l s ) . The p e r c e n t a g e o f m e l t p r e d i c t e d i n the c a l c u l a t i o n s , however, n e c e s s a r i l y r e p r e s e n t s t h e minimum amount o f l i q u i d t h a t can be g e n e r a t e d . Whereas t h e e c l o g i t e - t y p e f r a c t i o n a t i o n p o r t r a y s o n l y a h i g h - p r e s s u r e p r o c e s s , i t i s apparent t h a t the r e f r a c t o r y m i n e r a l s i n the p e r i d o t i t e models c o u l d remain as l i q u i d u s o r n e a r -l i q u i d u s phases over a c o n s i d e r a b l e p o r t i o n o f a magma's a s c e n t . The 223 amount o f l i q u i d produced a l s o appears p r o p o r t i o n a l t o the modal abundance o f amphibole i n the mantle p e r i d o t i t e ; the goodness o f f i t of the models d e c r e a s e s r a p i d l y , however, as the amphibole c o n t e n t o f the l h e r z o l i t e s r i s e s above 2 p e r c e n t . Ringwood (1974) proposed t h a t hydrous r h y o d a c i t e l i q u i d g e n e r a t e d by m e l t i n g w i t h i n subducted o c e a n i c c r u s t r i s e s i n t o the o v e r l y i n g m antle and i n s t i g a t e s p a r t i a l m e l t i n g . The c o m p o s i t i o n o f such a contaminated s o u r c e p e r i d o t i t e i s i n d e t e r m i n a t e , but may be approximated by adding v a r y i n g amounts of r h y o d a c i t e l i q u i d t o the p r e v i o u s l y c a l c u l a t e d l h e r z o l i t e models (minus t h e i r amphibole component). The a d d i t i o n o f t h i s s i l i c i c l i q u i d (5, 10, 15 and 20 p e r c e n t ; T a b l e X X I I I ) to a d e p l e t e d mantle assemblage does n o t a p p r e c i a b l y a l t e r the r e s u l t s o b t a i n e d i n T a b l e XXVI, but i n c r e a s e s the amount o f m e l t produced i n d i r e c t p r o p o r t i o n to the melt f r a c t i o n 2 added. The m a t h e m a t i c a l f i t i s o p t i m i z e d (Er = 0.016-0.028) when 10 p e r c e n t r h y o d a c i t e l i q u i d i s i n t r o d u c e d ( T a b l e X X V I I). 224 T A B L E X X I I I . P A R E N T A L C O M P O S I T I O N S USED IN M A J O R - E L E M E N T M E L T I N G MODELS U n d e p l e t e d M a n t l e P e r i d o t i t e T A M P T P G P T R h y o d a c i t e - e n r i c h e d P e r i d o t i t e L E S P L E P P L E G P J a c q u e L a k e N o d u l e  J L 3 9 J L 1 0 s i o 2 4 2 . 8 2 4 4 . 3 1 4 4 . 8 5 4 4 . 0 0 4 5 - 2 3 4 6 . 8 8 4 6 . 1 6 4 4 . 6 6 4 7 . 0 2 T i 0 2 0 . 41 0 . 3 4 0 . 2 0 0 . 2 2 0 . 3 8 0 . 17 0 . 1 9 0 . 1 4 0 . 18 A 1 2 0 3 7- 13 4 . 5 5 7 . 0 8 6 . 0 5 7 . 91 7- 7 9 6 . 8 4 2 . 2 9 3 . 7 9 FeO 9 . . 3 * 1 2 . 0 6 1 0 . 91 1 1 . 5 7 8 . 9 4 1 0 . 3 4 1 0 . 9 6 8 . 3 5 7 . 9 2 MgO 3 4 . . 6 3 3 4 . 7 0 3 1 . . 8 7 3 3 - 9 3 3 2 . 10 2 9 . 4 5 31 . 4 2 4 0 . 3 0 3 6 . 7 0 C a O 4 . . 4 9 3 . 1 9 4 . , 2 1 3 . 4 3 4 . 4 8 4 . 17 3 . 4 5 3 . 2 6 3 . 4 7 N a 2 0 0 . . 5 0 0 . 6 7 0 . 7 8 0 . 5 4 0 . 7 5 1 . , 0 0 0 . 7 8 0 . 3 1 0 . 7 4 K 2 0 0 . . 0 4 0 . 1 0 0 , , 0 5 0 . 0 4 0 . . 2 0 0 . , 2 0 0 . 1 9 0 . 0 1 0 . 18 M o d e ( W t . P e t . ) 0 1 i v i n e 5 0 . . 0 0 5 8 . 1 0 5 3 . , 9 0 5 3 . 9 0 , 4 5 , . 6 9 4 9 , , 5 0 4 9 . 5 0 71 . 0 0 6 3 . 9 0 C I i n o p y r o x e n e 2 2 . . 2 0 1 5 - 9 0 1 4 . . 7 0 1 4 . 7 0 2 0 , . 2 8 1 3 . • 5 0 1 3 - 5 0 1 6 . 0 0 1 4 . . 4 0 O r t h o p y r o x e n e 21 , . 2 0 2 1 . 0 0 19 . 6 0 1 9 - 6 0 1 9 , . 3 7 1 8 , . 0 0 1 8 . 0 0 1 2 . 0 0 1 0 , , 8 0 S p i n e l 5 . 1 0 - - 4 . 6 6 - 1 . 0 0 0 , . 9 0 G a r n e t - 9 . 8 0 9 - 0 0 -P 1 a g i o c 1 a s e - 9 . 8 0 - 9 . 0 0 - -A m p h i b o l e 1 . 5 0 5 . 0 0 2 . 0 0 2 . 0 0 - -R h y o d a c i t e 1 i q u i d • 10 . 0 0 10 . 0 0 1 0 . 0 0 1 0 . . 0 0 R h y o d a c i t e L i q u i d s J u a n d e F u c a R i d g e RDY2 R D Y 3 SVR1 J D F 2 J D F D S i O , A , 2 0 3 F e O MgO C a O K 2 0 6 5 . 3 0 6 5 - 0 0 5 1 . 18 4 8 . 0 5 4 8 . 17 4 9 . 7 2 0 . 7 0 0 . 5 6 1 . 2 7 1 . 0 6 1 . 16 1 . 13 1 6 . . 0 0 1 7 - 0 0 1 6 . 12 1 7 - 0 6 1 8 . 9 8 1 8 . 41 4 , 7 0 3 . 4 0 9 - 21 9 . 2 6 8 , . 1 3 7 . . 8 9 1 , 7 0 1 . 7 0 7- 7 5 1 0 . 4 0 8 . . 4 9 8 . . 2 4 5 . . 0 0 5 . 2 0 1 1 . 7 2 1 1 . 5 4 1 2 . . 4 0 1 2 , . 0 3 3 . . 6 0 4 . 5 0 2 , , 5 1 2 . 5 0 2 , , 0 2 1 . • 9 6 2 . , 1 0 1 . 7 0 0 , . 0 7 0 . 1 4 0 , . 5 8 0 , . 5 6 C o m m e n t s : S P P T - s p i n e 1 - 1 h e r z o l i t e m o d e l f r o m C a r t e r ( 1 9 7 0 ) w i t h M3HB a m p h i b o l e a d d e d ; M2 p r e f i x e d m i n e r a l s u s e d i n c a l c u l a t i o n s . A M P T - a m p h I b o 1 e - b e a r i n g l h e r z o l i t e c a l c u l a t e d u s i n g M 3 . p r e f i x e d m i n e r a l s . P G P T -a m p h I b o l e - b e a r i n g p i a g i o c 1 a s e - I h e r z o l i t e c a l c u l a t e d u s i n g M3 p r e f i x e d m i n e r a l s a n d M3PG p l a g i o c l a s e . G T P T - - a m p h i b o l e - b e a r i n g g a m e t - I h e r z o I i t e c a l c u l a t e d u s i n g M3 p r e f i x e d m i n e r a l s a n d M3GT g a r n e t . L E S P -r h y o d a c i t e - e n r i c h e d s p i n e 1 - 1 h e r z o l i t e c a l c u l a t e d u s i n g M2 p r e f i x e d m i n e r a l s a n d 10 p e r c e n t R D Y 2 l i q u i d . L E P P - r h y o d a c i t e - e n r i c h e d p i a g i o c 1 a s e - 1 h e r z o 1 i t e c a l c u l a t e d u s i n g M3 p r e f i x e d m i n e r a l s a n d RDY2 l i q u i d . L E G P -r h y o d a c i t e c o n t a m i n a t e d g a m e t - 1 h e r z o 1 i t e c a l c u l a t e d u s i n g R D Y 2 l i q u i d a n d M3 p r e f i x e d m i n e r a l s . J L 3 9 ~ J a c q u e L a k e N o d u l e c a l c u l a t e d f r o m m o d e ( L i t t l e j o h n a n d G r e e n w o o d , 1 9 7 * 0 a n d J 9 p r e f i x e d m i n e r a l s . J L 1 0 -s a m e a s J L 3 9 b u t c o n t a m i n a t e d w i t h 10 p e r c e n t R D Y 3 l i q u i d . RDY2 - r h y o d a c i t e l i q u i d d e r i v e d f r o m e x p e r i m e n t a l d a t a o f G r e e n a n d R i n g w o o d ( 1 9 6 8 ) . R D Y 3 - r h y o d a c i t e l i q u i d d e r i v e d f r o m e x p e r i m e n t a l d a t a o f G r e e n ( 1 9 7 2 ) . J D F C , J D F D , J D F 2 a n d SVR1 - J u a n d e F u c a R i d g e b a s a l t s f r o m B a r r ( 1 9 7 2 ) . S P P T , A M P T , P G P T a n d G T P T - a f t e r L e e m a n ( 1 9 7 * 0 . TABLE X X I V . MINERAL COMPOSITIONS USED IN MAJOR-ELEMENT MODELS C 1 i n o p y r o x e n e Ga m e t K y a n i t e M 2 C P M3CP J 9 C P E C 2 P E C 3 P M3GT E C 2 G E C 3 G K Y N T s i o 2 5 0 . 5 5 5 1 . 1 1 5 3 . 1 6 50 . 2 3 5 1 . 0 0 4 0 . 9 0 4 0 . 0 0 4 0 . 6 0 3 6 . 7 0 T i 0 2 0 . 7 0 0.50 0 .64 0 . 0 9 0 . 6 5 0 . 1 9 1 . 2 0 1 . 3 0 -A 1 2 0 3 9 . 0 8 1 2 . 5 6 6 .84 1 5 - 0 9 1 2 . 4 0 2 2 . 7 9 2 2 . 8 0 2 2 . 3 0 6 2 . 7 3 F e O * 4 . 3 8 4 . 9 8 2 . 6 9 6 . 2 0 7 . 8 0 6 . 9 6 1 3 - 8 0 1 6 . 4 0 0 . 6 3 MgO 1 4 . 3 8 1 3 - 1 2 14 . 6 0 1 0 . 5 4 1 0 . 6 0 21 . 4 2 1 3 . 3 0 1 0 . 6 0 -C a O 1 9 . 1 3 1 4 . 7 5 1 9 . 6 0 1 4 . 7 8 1 5 . 9 0 7 . 7 4 8 . 9 0 1 0 . 0 0 -N a 2 0 1.34 2 . 9 9 1 . 5 4 3 . 0 0 2 . 3 0 - - - -K 2 0 - 0 . 0 1 0 . 0 7 0 . 0 7 - - -O r t h o p y r o x e n e 0 1 i v i n e S p i n e l P I a g i o c l a s e M 2 0 P M 3 0 P J 9 0 P M 3 B M 2 B J9B M 2 S P J9SP M 3 P G s i o 2 54.84 5 2 . 2 3 5 5 - 6 0 3 9 - 8 8 3 9 . 2 8 4 1 . 5 2 0 . 2 2 - 48 . 8 6 T i 0 2 0 . 1 6 0 . 2 0 0 . 2 6 - 0 . 0 2 - 2 . 5 1 0 . 1 8 -A 1 2 0 3 5-57 8.74 4 . 6 8 - 1 . 1 4 - 64 . 1 8 6 3 . 5 7 3 2 . 9 5 F e O * 7 . 2 2 8 . 7 6 6 . 4 2 1 5 - 2 8 12.65 9 . 9 1 8 . 4 7 1 1 . 2 9 0 . 0 8 MgO 3 1 . 4 3 2 9 - 0 7 31 • 3 0 4 4 . 5 9 4 6 . 9 6 4 7 . 9 0 2 4 . 6 3 2 0 . 1 5 -C a O 0 . 5 7 0 . 8 2 0 . 8 2 0 . 2 5 - 0 . 0 3 - - 1 5 . 5 5 N a 2 0 0 . 2 0 0 . 1 8 0 . 3 2 - - 0 . 6 1 - - 2.50 K 2 0 - 0 . 01 - - - - - 0 . 0 6 A m p h i b o l e M i c a R u t i l e M 3 H B X 1 A M X I P h RUT s i o 2 4 0 • 6 9 4 2 . 4 3 3 8 . 5 5 -T i 0 2 4 .46 1 . 8 8 3 - 5 5 1 0 0 . 0 0 * T o t a l F e a s F e O A 1 2 0 3 14 . 0 0 1 5 - 3 3 1 3 - 9 5 -F e O * 11 . 2 9 1 0 . 6 7 1 3 - 9 5 -MgO 13 . 0 4 1 4 . 3 7 1 9 . 1 9 -C a O 1 0 . 3 5 1 1 . 0 9 - -N a 2 0 3 . 0 7 2 . 6 8 0 . 7 2 -K 2 0 2 . 0 7 0 . 2 9 9 . 0 2 N o t e s : M i n e r a l s h a v i n g M3 p r e f i x a r e t a k e n f r o m L e e m a n ( 1 9 7 4 ; c o m p o s i t i o n s a t F o 8 y ) • t h o s e h a v i n g M2 p r e f i x a r e f r o m W r i g h t ( 1 9 7 1 , T a b l e 9 ; m i n o r a m o u n t s o f T i 0 2 a r e a d d e d t o p y r o x e n e a f t e