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Geology, geochemistry, and geochronology of the Westcoast Crystalline Complex and related rocks, Vancouver… Isachsen, Clark 1984

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GEOLOGY, GEOCHEMISTRY, AND GEOCHRONOLOGY WESTCOAST CRYSTALLINE COMPLEX AND RELATED ROCKS, VANCOUVER ISLAND, BRITISH COLUMBIA by CLARK {ISACHSEN B.Sc. State U n i v e r s i t y of New York at Albany 1980 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE THE FACULTY OF GRADUATE STUDIES Department of G e o l o g i c a l Sciences We'accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September 1984 OF THE i n C l a r k Isachsen, 1984 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree a t the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t 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 study. 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 copying of 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 granted by the head o f my department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood t h a t copying or 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 allowed without my w r i t t e n p e r m i s s i o n . Department o f G?(5DL£>C<*itfH _ * S C*%JJC(Zr%> The U n i v e r s i t y of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date i i FRONTISPIECE View of Meares I s l a n d from Radar H i l l v i i i A b s t r a c t The Westcoast C r y s t a l l i n e Complex occurs as a b e l t of complexly mixed p l u t o n i c rocks along the west coast of Vancouver I s l a n d . I t i s composed mainly of heterogeneous a m p h i b o l i t i c country rock (Westcoast A m p h i b o l i t e ) , g r a n i t o i d s of t r o n d h j e m i t i c to gabbroic composition (Westcoast D i o r i t e ) , and v a r i a b l e mixtures of these two components (Westcoast Migmatite). V a r i o u s l i n e s of evidence suggest that these rocks were generated i n a magmatic arc s e t t i n g and i n t r u d e d the c r u s t in J u r a s s i c time. Major and t r a c e element chemistry of the Westcoast C r y s t a l l i n e Complex shows a s u b a l k a l i n e t h o l e i i t i c to c a l c - a l k a l i n e t rend compatible with p e t r o g e n e s i s i n a magmatic a r c . U-Pb i s o t o p i c dates of z i r c o n separates are c o n s i s t e n t l y concordant between 176 and 189 Ma and show no a p p r e c i a b l e evidence f o r an o l d e r c r u s t a l component. I n i t i a l Sr" r a t i o s o b tained f o r the Westcoast C r y s t a l l i n e Complex are between 0.70329 and 0.70360, compatible with a d e p l e t e d mantle source r e g i o n . The concave c o o l i n g curves d e r i v e d f o r Westcoast D i o r i t e s are not c o n s i s t e n t with iri s i t u c r u s t a l magmagenesis but, i n s t e a d , i n d i c a t e t h a t these rocks were in t r u d e d i n t o r e l a t i v e l y c o o l country rock. Based on age and chemistry, the Westcoast C r y s t a l l i n e Complex can be i n t e r p r e t e d as the deeper c r u s t a l e q u i v a l e n t of the more d i f f e r e n t i a t e d I s l a n d I n t r u s i o n s and Bonanza V o l c a n i c s . Taken together, these rocks provide a d i s r u p t e d i i i i a n d p e r h a p s i n c o m p l e t e c r o s s - s e c t i o n o f t h e J u r a s s i c m a g m a t i c a r c o f V a n c o u v e r I s l a n d . R e c o n n a i s a n c e o f t h e W a r k / C o l q u i t z C o m p l e x o f s o u t h e r n V a n c o u v e r I s l a n d s h o w s i t t o b e e s s e n t i a l l y i n d i s t i n g u i s h a b l e i n p e t r o g r a p h y , c h e m i s t r y a n d a g e f r o m t h e W e s t c o a s t C r y s t a l l i n e C o m p l e x , a n d a s i m i l a r h i s t o r y i s i n f e r r e d . A c a l c - a l k a l i n e c h e m i s t r y a n d l o w i n i t i a l S r r a t i o w e r e a l s o d e t e r m i n e d f o r a C a t f a c e I n t r u s i o n d a t e d a t 4 1 M a . T h i s i s a g a i n c o m p a t i b l e w i t h m a n t l e - d e r i v e d a r c m a g m a t i s m , b u t i t s p r o x i m i t y t o t h e c o e v a l t r e n c h i s e n i g m a t i c . V TABLE OF CONTENTS Page ABSTRACT i i i LIST OF FIGURES v i i LIST OF TABLES ' v i i i l ACKNOWLEDGEMENTS . . . J x INTRODUCTION 1 CHAPTER I : DESCRIPTION OF MAP UNITS 7 WESTCOAST CRYSTALLINE COMPLEX 7 WESTCOAST AMPHI BOLI TE 7 WESTCOAST MIGMATITE 10 WESTCOAST DIORITE 10 GABBRO-PERIDOTI TE 20 WESTCOAST METASEDIMENTS 22 DIABASE AND APLITE 25 FELSIC VOLCANICS 27 PACIFIC RIM COMPLEX 27 TERTIARY INTRUSIONS 28 CHAPTER II : GEOCHEMISTRY OF THE WESTCOAST CRYSTALLINE COMPLEX AND RELATED ROCKS 30 CHEMICAL ANALYSIS ; 30 CLASSIFICATION 30 TECTONIC SETTING DISCRIMINANTS 36 CONCLUSIONS 42 CHAPTER I I I : STRUCTURE 45 HETEROGENEOUS STRAIN 45 ORIENTATION OF STRUCTURAL FABRICS 49 DUCTILE-BRITTLE TRANSITION .55 BRITTLE FAULTS 57 CONCLUSIONS 59 CHAPTER IV : METAMORPHISM 61 AMPHI BOLI TES 61 CALCSILICATES 62 QUARTZO-FELDSPATHIC METASEDIMENTS 66 CHAPTER V : GEOCHRONOLGY OF THE WESTCOAST CRYSTALLINE COMPLEX AND RELATED ROCKS 69 WESTCOAST CRYSTALLINE COMPLEX 69 WARK COMPLEX 87 CATFACE INTRUSION 91 CHAPTER VI : COOLING HISTORY 97 WESTCOAST CRYSTALLINE COMPLEX 97 WARK COMPLEX 102 CATFACE INTRUSION 102 CHAPTER V I I : CONCLUSIONS 104 v i CHAPTER. VI11 :TECTONIC HISTORY OF VANCOUVER ISLAND 107 PALEOZOIC 107 TRIASSIC 108 JURASSIC 109 CRETACEOUS 113 TERTIARY 114 REFERENCES 119 APPENDIX 1 : SAMPLE DESCRIPTIONS AND LOCATIONS 128 APPENDIX 2 : MAJOR AND TRACE ELEMENT CHEMISTRY: WESTCOAST CRYSTALLINE COMPLEX, WARK COMPLEX AND TERTIARY INTRUSIONS 130 APPENDIX 3 : GEOCHRONOLOGY: ANALYTICAL METHODS AND RESULTS..137 GEOLOGICAL MAPS 4^ R—pos-k^ e-t v i i LIST OF FIGURES F i g u r e Pag^e F r o n t i s p i e c e i i 1 : G e n e r a l i z e d g e o l o g i c map of Vancouver I s l a n d 2 2 : G e o l o g i c a l map of Meares I s l a n d and adjacent jz^ £p ,c* JU-tSTi**^ s h o r e l i n e s . . . . .in pocke^-3 : L i n e a t i o n i n Westcoast Amphibolite 8 4 : P o s s i b l e cumulate t e x t u r e in Megacrystic Westcoast Amphibol i t e 8 5 : L i t - p a r - l i t i n j e c t i o n of banded g n e i s s , Westcoast Complex 11 6 : Agmatite, Westcoast Complex 11 7 : Complex mixture of Westcoast rock types 12 8 : P o s t - t e c t o n i c i n t r u s i o n , Westcoast Complex 14 9 : Screens of banded g n e i s s , Westcoast Complex 15 10 : Pluton - pegmatite c o n t a c t , Westcoast Complex 17 11 : F o l i a t e d and l i n e a t e d margin of the Indian I s l a n d i n t r u s i o n 18 12 : Schematic r e p r e s e n t a t i o n of major Westcoast rock types 21 13 : T i g h t l y f o l d e d and d i s r u p t e d a m phibolite and metapsammite, Westcoast Metasediments 2 4 14 : T i g h t l y f o l d e d i n t e r c a l a t i o n s of metapsammite i n a m p h i b o l i t e , Westcoast Metasediments 2 4 15 : Ambiguous c r o s s c u t t i n g r e l a t i o n s h i p between a p l i t e and diabase dikes i n e p i d i o r i t e , Westcoast Complex..26 16 : Hypabyssal dike and d i k e l e t of a n d e s i t e porphry i n t r u d i n g Catface I n t r u s i o n on Meares I s l a n d . . 2 9 17 : Major element v a r i a t i o n diagrams, Westcoast Complex and r e l a t e d rocks 31 18 : Trace element v a r i a t i o n diagrams, Westcoast Complex and r e l a t e d r o c k s . . . 3 2 19 : P l o t of a l k a l i s versus s i l i c a (wt. percent) 3 4 2 0 : AFM diagram 3 4 21 : P l o t of normative c o l o r index versus normative p l a g i o c l a s e composition 3 5 2 2 : Z r / T i 0 2 - S i 0 2 diagram. 3 7 2 3 : Nb/Y - Z r / T i O ? diagram 3 8 2 4 : T e c t o n i c s e t t i n g d i s c r i m i n a t i o n diagrams 4 0 2 5 : Normalized abundance p a t t e r n s of incompatible t r a c e elements, Westcoast Complex and Wark d i o r i t e 41 2 6 : Normalized abundance p a t t e r n s of incompatible t r a c e elements, u l t r a b a s i c rocks and T e r t i a r y i n t r u s i o n s . . 4 3 2 7 : G e n e r a l i z e d g e o l o g i c map showing s t r u c t u r a l data, ^ ^s. f . Z / ^ ^ j -Meares I s l a n d and adjacent s h o r e l i n e s tfi—pee-ke-t 2 8 : Sketch showing development of banded gneiss from i n c l u s i o n - r i c h g r a n i t o i d 4 7 2 9 : D e f l e c t i o n of f o l i a t i o n i n d i o r i t e i n t o m y l o n i t i c shear zone 4 8 3 0 : S t r u c t u r a l o r i e n t a t i o n s , Meares I s l a n d map area 5 0 31 : S i m i l a r - s t y l e f o l d s i n banded g n e i s s , Westcoast Complex 51 v i i i 32 : Boudinage of a m p h i b o l i t i c l a y e r s i n banded g n e i s s , Westcoast Complex 51 33 : Schematic diagram showing o r i e n t a t i o n of p r i n c i p l e s t r a i n axes 53 34 : F o l d i n g and boudinage of transposed l a y e r i n g i n c a l c s i l i c a t e , Westcoast Metasediments. .54 35 : Photomicrograph showing b r i t t l e f r a c t u r e of d u c t i l e s t r a i n f a b r i c along conjugate planes, Westcoast Amphibolite 56 36 : Photomicrograph showing c a t a c l a s i s of d u c t i l y deformed c a l c i t e r i b b o n s , Westcoast Metasediments... 56 37 : Sketch showing b r i t t l e deformation of a d u c t i l e shear zone, Westcoast Complex ....58 38 : H 2 0 - r i c h i s o b a r i c T-X diagram showing s t a b i l i t y f i e l d s f o r observed c a l c s i l i c a t e assemblages 64 39 : P -T diagram showing the ab-absent and an-absent i n v a r i a n t p o i n t s i n the system CKNASH 68 40 : Sample l o c a t i o n map f o r i s o t o p i c a n a l y s e s . . . . . 70 41 : Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s from the Indian I s l a n d i n t r u s i o n , Westcoast Complex.72 42 : Concordia diagram, Indian I s l a n d i n t r u s i o n 73 43 : Concordia diagram, T o f i n o I n l e t pluton 74 44 : Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s from the T o f i n o I n l e t p l u t o n , Westcoast Complex 75 45 : Concordia diagram, q u a r t z d i o r i t e g n e i s s , Westcoast Complex 76 46 : Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s from quartz d i o r i t e g n e i s s , Westcoast Complex 77 47 : Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s from the quartz d i o r i t e of Black R i v e r , Westcoast Complex 79 48 : Concordia diagram, q u a r t z d i o r i t e of Black River....80 49 : a) Sr e v o l u t i o n diagram, Indian I s l a n d i n t r u s i o n b) Sr e v o l u t i o n diagram, quartz d i o r i t e of Black R i v e r 83 50 : Sr e v o l u t i o n diagram, whole rock a n a l y s e s , Westcoast Complex 84 51 : Sr e v o l u t i o n diagram comparing data f o r the Westcoast Complex with I s l a n d I n t r u s i o n data 86 52 : Photomicrograph of spontaneous f i s s i o n t r a c k s i n a p a t i t e from the Indian I s l a n d i n t r u s i o n 88 53 Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s from the Wark Complex 89 54 : Concordia diagram, Wark d i o r i t e 90 55 : Concordia diagram, C a t f a c e I n t r u s i o n , R i t c h i e Bay...92 56 : Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s from the Catface I n t r u s i o n , R i t c h i e Bay .94 57 : Sr e v o l u t i o n diagram, C a t f a c e I n t r u s i o n , R i t c h i e Bay 95 58 : Photomicrograph of spontaneous f i s s i o n t r a c k s i n a p a t i t e from the C a t f a c e I n t r u s i o n , R i t c h i e Bay 96 59 : C o o l i n g curves 99 60 : Nonconformity between g e n t l y d i p p i n g s t r a t a of the . Carmanah Formation and the Westcoast Complex 118 v i i i i LIST OF TABLES page Table 1 : C a l c s i l i c a t e parageneses and p e r t i n e n t r e a c t i o n s . . 6 3 Table 2 : Summary of age determinations l i s t e d i n order of d e c r e a s i n g b l o c k i n g temperature 98 Table 3 : Comparative c a l c u l a t i o n s of u p l i f t r a t e f o r the Indian I s l a n d i n t r u s i o n 101 X Ac knowledgements R. L. Armstrong, t h e s i s s u p e r v i s o r , suggested the p r o j e c t and p r o v i d e d support, s u p e r v i s i o n and advice throughout the r e s e a r c h . Thanks are due to R. P a r r i s h , K. S c o t t , P. van der Heyden and S. Horsky f o r a b l e a s s i s t a n c e and guidance i n the l a b o r a t o r y . Ar analyses were pro v i d e d by J . Harakal and K analyses by K. S c o t t . A s s i s t a n c e from members of the s t a f f at the Department of Geology, U n i v e r s i t y of B r i t i s h Columbia, most notably E. Montgomery and B. Cranston, a l s o m e r i t s r e c o g n i t i o n . T h i s study 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 with R. P a r r i s h , J . V. Ross, H. J . Greenwood, J . E. M u l l e r and M. T. Brandon. F i n a l l y , the the w r i t e r would l i k e to thank S. Casanova for a s s i s t a n c e and moral support in the f i e l d and K. Chessman f o r p a t i e n c e and encouragement. N a t i o n a l Research C o u n c i l of Canada grant number A-8841 to R. L. Armstrong and a U n i v e r s i t y of B r i t i s h Columbia summer f e l l o w s h i p provided funding f o r t h i s p r o j e c t . 1 INTRODUCTION The Westcoast C r y s t a l l i n e Complex of Vancouver I s l a n d i s exposed i n t e r m i t t e n t l y i n a zone 10 to 15 km wide, extending from the San Juan F a u l t near Port Renfrew 280 km northwest to the Brooks P e n i n s u l a . S i m i l a r rocks, the Wark/Colquitz P l u t o n i c Complex, extend from the c i t y of V i c t o r i a northwestward approximately 45 km to the San Juan and Survey Mountain f a u l t s ( M u l l e r , 1977; F i g . 1). The f i r s t g e o l o g i c i n v e s t i g a t i o n of Vancouver I s l a n d ' s west coast was conducted f o r the G e o l o g i c a l Survey of Canada du r i n g the summer of 1902 by Mr. Arthur Webster and P r o f e s s o r Ernest Haycock. By means of a small s e a l i n g boat, they surveyed most of the major p r o t e c t e d i n l e t s along the west c o a s t . In a d d i t i o n to younger u n a l t e r e d sediments, they d e s c r i b e d a metamorphic and an i n t r u s i v e s e r i e s , acknowledging frequent d i f f i c u l t y i n d i s t i n g u i s h i n g between them (Haycock, 1903; Webster, 1903) In 1921, Dolmage i n v e s t i g a t e d the geology between Quatsino and Barkely Sounds and i d e n t i f i e d d i o r i t e and g r a n o d i o r i t e of Upper J u r a s s i c age b e l i e v e d to be r e l a t e d to the Coast Range B a t h o l i t h (Dolmage, 1921). J e l e t z k y (1950,1954a,b) d e s c r i b e d the b a t h o l i t h i c Coast I n t r u s i o n s and a t t r i b u t e d them to a major Middle J u r a s s i c orogeny. Eastwood (1965) introduced the name I s l a n d I n t r u s i o n s f o r these rocks to a v o i d c o n f u s i o n with p l u t o n i c rocks of the mainland. M u l l e r ( M u l l e r and Carson, 1969) noted a marked 2 F i g . 1 G e n e r a l i z e d g e o l o g i c map of Vancouver I s l a n d . Metamorphic complex east of 124° long, i s Wark/Colquitz; to the west i s the Westcoast C r y s t a l l i n e Complex. M o d i f i e d from M u l l e r (1977). 3 d i f f e r e n c e between t y p i c a l I s l a n d I n t r u s i o n s such as the Bedwell and Nanaimo B a t h o l i t h s and g r a n i t o i d s along the west coast and a p p l i e d the name Westcoast C r y s t a l l i n e Complex to the l a t t e r . A concordant U-Pb date of 264±7 Ma f o r a q u a r t z -p l a g i o c l a s e - b i o t i t e g n e i s s along with a 192±9 K-Ar date for a c r o s s - c u t t i n g a m p h i b o l i t i z e d dike on the south shore of G r i c e Bay near T o f i n o (Muller et al.,1974) supports M u l l e r ' s b e l i e f that the complex was d e r i v e d from Upper P a l e o z o i c v o l c a n i c rocks of the S i c k e r Group, migmatized in the J u r a s s i c , and thus i s a c r y s t a l l i n e 'basement' complex. The Wark/Colquitz Complex of southern Vancouver I s l a n d was f i r s t d e s c r i b e d and named by Clapp (1909,1912c,1913) and t e n t a t i v e l y c o r r e l a t e d with the Westcoast C r y s t a l l i n e Complex by M u l l e r (Muller and Carson, 1969). One z i r c o n age d e t e r m i n a t i o n f o r the C o l q u i t z Complex has y i e l d e d d i s c o r d a n t dates between 295 and 384 Ma ( M u l l e r , 1977). T h i s report i s based on d e t a i l e d mapping of Meares I s l a n d and adjacent s h o r e l i n e s near the f i s h i n g v i l l a g e of T o f i n o , B.C., as w e l l as the r e s u l t s of i s o t o p i c d a t i n g (U-Pb, K-Ar, Rb-Sr, and f i s s i o n t r a c k ) , major and t r a c e element a n a l y s i s (XRF), and p e t r o g r a p h i c i n v e s t i g a t i o n s of samples c o l l e c t e d from that a r e a . In a d d i t i o n , samples from the Westcoast C r y s t a l l i n e Complex near Pachena Po i n t and the Wark/Colquitz complex northwest of V i c t o r i a were s t u d i e d . T e c t o n i c S e t t i n g Vancouver I s l a n d , and the Queen C h a r l o t t e I s l a n d s to 4 the n o r t h make up a s i g n i f i c a n t p o r t i o n of W r a n g e l l i a , a l a r g e d i s p l a c e d t e r r a n e of the western Canadian C o r d i l l e r a (Jones et a l , 1977). T h i s t e c t o n o s t r a t i g r a p h i c package i s c h a r a c t e r i z e d by voluminous subaqueous to s u b a e r i a l t h o l e i i t i c b a s a l t s of Middle to Upper T r i a s s i c age capped disconformably by c a l c a r e o u s and c l a s t i c Upper T r i a s s i c sediments. T h i s d i s t i n c t i v e T r i a s s i c s t r a t i g r a p h y was d e p o s i t e d on the S i c k e r Group, a middle to l a t e P a l e o z o i c arc composed of v o l c a n i c , and subordinate carbonate, and c l a s t i c r o c k s . In the E a r l y J u r a s s i c magmatic a c t i v i t y resumed, with the emplacement of g r a n i t o i d b a t h o l i t h s and r e l a t e d v o l c a n i c rocks, termed the I s l a n d I n t r u s i o n s and Bonanza V o l c a n i c s , r e s p e c t i v e l y . W r a n g e l l i a i s now p a r t of the I n s u l a r b e l t , a composite t e c t o n i c b e l t which i n c l u d e s the Alexander Terrane, a heterogeneous assemblage of Precambrian, P a l e o z o i c , and Mesozoic rocks, i n t e r p r e t e d as another d i s p l a c e d c o n t i n e n t a l fragment by Jones et a l . (1972). The superjacent G r a v i n a -N u t z o t i n assemblage, an Upper J u r a s s i c to l a t e Lower Cretaceous b a s i n a l arc d e p o s i t of interbedded f l y s c h and v o l c a n i c rocks in southeastern A l a s k a , i n d i c a t e s amalgamation of the Alexander Terrane with W r a n g e l l i a by Late J u r a s s i c time (Berg et a l , 1972). Welding of t h i s composite te r r a n e to North America by the mid-Cretaceous to T e r t i a r y Coast P l u t o n i c Complex i n d i c a t e s a c c r e t i o n by mid-Cretaceous time (Monger et a l . , 1982). Van der Heyden (1982) has suggested that a c c r e t i o n of the composite 5 Wrangellia/Alexander t e r r a n e to S t i k i n i a as e a r l y as middle to upper J u r a s s i c may have been r e s p o n s i b l e f o r deformation in the c e n t r a l Coast P l u t o n i c Complex at that time. The P a c i f i c B e l t l i e s outboard of W r a n g e l l i a and i n c l u d e s the P a c i f i c Rim Complex and Leech R i v e r S c h i s t of Vancouver I s l a n d and the Chugach Terrane and Baranof Complex (Cowan, 1982) i n southern and southeastern A l a s k a . These u n i t s c o n s i s t mainly of Upper J u r a s s i c and Lower Cretaceous f l y s c h and melange, c o r r e l a t i v e with the F r a n c i s c a n Complex of C a l i f o r n i a . The P a c i f i c B e l t a l s o i n c l u d e s a c c r e t e d Cenozoic v o l c a n i c and sedimentary rocks. The a l l o c t h o n o u s nature of I n s u l a r B e l t t e r r a n e s i s i n d i c a t e d by paleomagnetic s t u d i e s . Data from s e v e r a l workers i n d i c a t e a l a r g e northward displacement r e l a t i v e to North America s i n c e the T r i a s s i c . T r i a s s i c b a s a l t s of the Karmutsen Formation on Vancouver I s l a n d have y i e l d e d a l a t e T r i a s s i c p a l e o l a t i t u d e of 18°N or S (Yole and I r v i n g , 1980). Recent determinations f o r the E a r l y J u r a s s i c Bonanza V o l c a n i c s i n d i c a t e a s i m i l a r p a l e o l a t i t u d e (21°N or S; I r v i n g , 1983) which would suggest that major northward t r a n s l a t i o n has occured s i n c e the E a r l y J u r a s s i c . T h i s view i s strengthened by s i m i l a r paleomagnetic data f o r I n s u l a r B e l t rocks i n southern Alaska ( H i l l h o u s e , 1977; Stone et a l . , 1982; Van der Voo et a l .,1980). As much as 13° of northward t r a n s l a t i o n s i n c e Lower Cretaceous has been s i m i l a r l y documented f o r S t i k i n i a subsequent to a c c r e t i o n to North America in about Middle J u r a s s i c time ( I r v i n g et a l . , 6 1980; Monger et a l . , 1982). 7 CHAPTER I DESCRIPTION OF MAP UNITS Westcoast C r y s t a l l i n e Complex Most of the f i e l d area shown in F i g u r e 2 ( i n pocket) i s u n d e r l a i n by the Westcoast C r y s t a l l i n e Complex, a heterogeneous assemblage of intermediate to b a s i c p l u t o n i c rocks, g n e i s s e s , m i g m a t i t i c rocks, a m p h i b o l i t e s , and metamorphosed c a l c a r e o u s sediments. De s p i t e pronounced t e x t u r a l and c o m p o s i t i o n a l v a r i a b i l i t y an attempt was made dur i n g mapping to d i f f e r e n t i a t e between dominantly mafic and dominantly f e l s i c components, h e r e a f t e r termed the Westcoast Amphibolite and Westcoast Migmatite, r e s p e c t i v e l y . A t h i r d l i t h o d e m i c u n i t , the Westcoast D i o r i t e s , i n c l u d e s small stocks of s l i g h t l y g n e i s s i c to massive d i o r i t e and quartz d i o r i t e as w e l l as the l a r g e r T o f i n o I n l e t p l u t o n . A metasedimentary u n i t (Westcoast Metasediments) and a gabbro-p e r i d o t i t e are a l s o d i s t i n g u i s h e d . Westcoast Amphibolite T h i s u n i t i s predominantly a m p h i b o l i t e which ranges from massive, medium to coarse g r a i n e d e p i d i o r i t e , a term a p p l i e d to a m p h i b o l i t i z e d d i o r i t e and gabbro with g r a n o b l a s t i c t e x t u r e (see F i g . 15), to f i n e g r a i n e d and w e l l l i n e a t e d amphibolite gneiss ( F i g . 3). In p l a c e s , the a m p h i b o l i t e has a d i s t i n c t i v e s p o t t ed t e x t u r e of rounded 1 to 3 cm i n diameter hornblende megacrysts i n a f i n e r g r a i n e d 8 F i g . 4 P o s s i b l e cumulate texture i n M e g a c r y s t i c Westcoast Amphibolite. 9 matrix of p l a g i o c l a s e and hornblende. O c c a s i o n a l l y , the megacrysts are a l i g n e d " l i k e beads on a s t r i n g " to y i e l d an even more s t r i k i n g t e x t u r e reminiscent of a cumulate ( F i g . 4). A l s o i n c l u d e d i n t h i s u n i t i s w e l l - f o l i a t e d grey d i o r i t e g n e i s s with metamorphic l a y e r i n g marked by hornblende- and p l a g i o c l a s e - r i c h laminae from 0.1 to 2 cm wide. I t f r e q u e n t l y e x h i b i t s a s c h l i e r e n t e x t u r e and i s g r a d a t i o n a l with the migmatite u n i t . T i g h t to i s o c l i n a l f o l d s are common in the d i o r i t e g n e i s ses which o f t e n appear "soupy" or " s w i r l y " . The amphibole i s blue-green to yellow-green p l e o c h r o i c hornblende which o f t e n d i s p l a y s s i e v e t e x t u r e and ragged t e r m i n a t i o n s . I t s modal abundance ranges from about 40 percent i n the grey g n e i s s e s to 90 percent i n the more mafic a m p h i b o l i t e g n e i s s e s . In some of the e p i d i o r i t e s with r e l i c t igneous t e x t u r e s , the hornblende i s c l e a r l y a replacement of hypersthene i n d i c a t i n g a probable gabbroic parentage. P l a g i o c l a s e , which c o n s t i t u t e s from 5 to 50 percent of these rocks, appears cloudy i n hand specimen and p a r t i a l l y to completely s a u s s u r i t i z e d when observed i n t h i n s e c t i o n . A l b i t e twins are f a i r l y common but are o f t e n obscured by a l t e r a t i o n . Where preser v e d , they i n d i c a t e a composition ranging from A n 4 0 to A n 7 0 . B i o t i t e i s a rare c o n s t i t u e n t of these rocks and i s l a r g e l y r e p l a c e d by c h l o r i t e . Quartz i s g e n e r a l l y absent except i n v e i n s and vugs. Epidote i s u s u a l l y present (up to 1 0 5 p e r c e n t ) , and sphene, a p a t i t e , and opaques are common a c c e s s o r i e s . Secondary p r e h n i t e i s f a i r l y common (up to 12 percent) and occurs i n v e i n s and c l o t t e d masses. A r e l a t i v e l y high n i c k e l c o n c e n t r a t i o n (56 to 211 ppm, see appendix 2) i s suggestive of o r i g i n as mafic igneous roc k s . Westcoast Migmatite The Westcoast C r y s t a l l i n e Complex i s in many p l a c e s m i g m a t i t i c . The migmatite i s of v a r i a b l e c h a r a c t e r but g e n e r a l l y c o n s i s t s of a m p h i b o l i t i c melasome i n t e r m i n g l e d with a t r o n d h j e m i t i c leucosome. T h i s u n i t i n c l u d e s a l l g r a d a t i o n s between banded g n e i s s , s c h l i e r e n g n e i s s , s c h o l l e n g n e i s s , stockwork, and agmatite (see F i g s . 5, 6, and 7). Quartz and p l a g i o c l a s e are the most abundant minerals of the leucosome and are present i n roughly equal amounts. A l t e r a t i o n i s evident i n extreme s a u s s u r i t i z a t i o n of p l a g i o c l a s e and c o n v e r s i o n of b i o t i t e to p r e h n i t e and c h l o r i t e . Amphibole i s scarce and K - f e l d s p a r never exceeds 3 to 4 p e r c e n t . Epidote i s a common v e i n f i l l i n g . Westcoast D i o r i t e Massive to f a i n t l y f o l i a t e d d i o r i t i c and t o n a l i t i c s t o c k s and plutons are common. The co m p o s i t i o n a l range of these i n t r u s i o n s i n c l u d e s h o r n b l e n d i t e , hornblende gabbro, hornblende d i o r i t e to qua r t z d i o r i t e and l e u c o t o n a l i t e or trondhjemite and rare g r a n o d i o r i t e . D i o r i t e and quartz 11 F i g . 6 Agmatite, Westcoast C r y s t a l l i n e Complex. 12 F i g . 7 Complex mix tu re of Westcoast rock t ypes i n c l u d i n g m e g a c r y s t i c a m p h i b o l i t e , p o r p h y r i t i c d i o r i t e , p e g m a t i t e , t r o n d h j e m i t i c a p l i t e , and d i a b a s e ; Westcoast C r y s t a l l i n e Complex. 13 d i o r i t e are by f a r the most voluminous. V a r i a t i o n i n s i z e and s t y l e of these i n t r u s i o n s i s great and ranges from small and obscure sub-map-scale bodies to the n e a r l y b a t h o l i t h i c p r o p o r t i o n s of the T o f i n o I n l e t p l u t o n . S y n t e c t o n i c plutons are v a r i a b l y f o l i a t e d , c o n t a i n angular to n e b u l i t i c a m p h i b o l i t i c x e n o l i t h s and have g r a d a t i o n a l c o n t a c t s with complexly f o l i a t e d and i n f o l d e d m i g m a t i t i c borders. P o s t - t e c t o n i c i n t r u s i o n s are g e n e r a l l y l e s s voluminous, more l e u c o c r a t i c , u n f o l i a t e d and s h a r p l y d i s c o r d a n t ( F i g . 8). Large r a f t s and near v e r t i c a l t a b u l a r bodies of d i o r i t e g neiss and migmatite up to 10 meters wide are o f t e n enveloped by some of the younger n o n - f o l i a t e d i n t r u s i v e s ( F i g . 9). In both types, x e n o l i t h mineralogy i s a p p a r e n t l y in e q u i l i b r i u m with the enveloping i n t r u s i o n and c o n s i s t s of e s s e n t i a l l y the same phases in d i f f e r e n t p r o p o r t i o n s . T h i s i s best i n t e r p r e t e d as a residuum-melt a s s o c i a t i o n . The h o r n b l e n d e - r i c h i n c l u s i o n s were l i k e l y d e r i v e d from pyroxene-bearing source rocks by e q u i l i b r a t i o n with the hydrous melt (White and C h a p p e l l , 1977). Sut h e r l a n d Brown (1966) d e s c r i b e d s i m i l a r " s y n t e c t o n i c b a t h o l i t h s " with a u r e o l e s of a m p h i b o l i t e and migmatite i n the Queen C h a r l o t t e I s l a n d s and c o r r e l a t e d the a m p h i b o l i t e s with Karmutsen b a s a l t s . The m i c r o s c o p i c t e x t u r e s e x h i b i t e d by the Westcoast D i o r i t e s are g e n e r a l l y hypidiomorphic to a l l o t r i o m o r p h i c g r a n u l a r . O c c a s i o n a l l y observed c o n t a c t s with l a t e stage 14 F i g . 8 P o s t - t e c t o n i c i n t r u s i o n showing sharp d i s c o r d a n c e with deformed g n e i s s e s , Westcoast C r y s t a l l i n e Complex. 15 F i g . 9 Screens of banded gneiss enveloped by l a t e - s t a g e i n t r u s i v e , Westcoast C r y s t a l l i n e Complex. 1 6 pegmatites of s i m i l a r m i n e r a l o g i c a l composition are sharp ( F i g . 10). Cloudy p l a g i o c l a s e (An„ 0 to A n 7 0 ) i s u b i q u i t o u s i n t h i s s u i t e of rocks and c o n s t i t u t e s from 10 to 50 percent of the rocks. P l a g i o c l a s e g r a i n s are u s u a l l y anhedral to subhedral and o f t e n e x h i b i t zoning with v a r y i n g degrees of a l t e r a t i o n . O r t h o c l a s e i s scarce or l a c k i n g i n most of these rocks. Hornblende i s by f a r the dominant mafic phase ranging from 90 percent abundance in the h o r n b l e n d i t e s to about 10 to 30 percent i n the quartz d i o r i t e s . I t i s o c c a s i o n a l l y seen as a replacement of c l i n o p y r o x e n e forming a u r a l i t e gabbro. B i o t i t e becomes the dominant mafic phase as the s u i t e becomes more l e u c o c r a t i c . I t i s o f t e n p a r t i a l l y a l t e r e d to c h l o r i t e . Modal quartz ranges from l e s s than 10 percent i n the d i o r i t e to 45 per cent i n trondhjemite. I t i s g e n e r a l l y f i n e r g r a i n e d than and i n t e r s t i t i a l to p l a g i o c l a s e . Minor phases i n c l u d e e p i d o t e , u s u a l l y i n v e i n s or as a l t e r a t i o n i n p l a g i o c l a s e , secondary p r e h n i t e , and a c c e s s o r y a p a t i t e , sphene, z i r c o n , and opaques. The two most notable i n t r u s i o n s s t u d i e d are the T o f i n o I n l e t p l u t o n , named by M u l l e r (Muller and Carson, 1969), i n the northeast corner o f the map area and a body on Indian I s l a n d termed h e r e a f t e r the Indian I s l a n d i n t r u s i o n . Both i n t r u s i o n s e x h i b i t f o l i a t e d and l i n e a t e d margins ( F i g l 11) concordant with adjacent gneisses and a m p h i b o l i t e s . O f f s h o o t s of the T o f i n o I n l e t p l u t o n are i n t i m a t e l y 17 F i g . 10 Sharp co n t a c t l a t e - s t a g e pegmatite C r y s t a l l i n e Complex. between a l t e r e d p l u t o n i c rock and with s i m i l a r mineralogy, Westcoast 18 19 i n t e r m i n g l e d with enveloping m i g m a t i t i c gneisses and are i n t e r p r e t e d as the r e s u l t of s y n t e c t o n i c emplacement. Both bodies are quartz d i o r i t i c c o n t a i n i n g hornblende and a l t e r e d b i o t i t e . Quartz i s mostly r e c r y s t a l l i z e d and i n t e r s t i t i a l to subhedral zoned g r a i n s of s a u s s u r i t i z e d p l a g i o c l a s e . A very s i m i l a r l i t h o l o g y occurs along the Westcoast T r a i l near the Black R i v e r (sample WCT-3) and w i l l be r e f e r r e d to h e r e i n as the quartz d i o r i t e of Black R i v e r . Two samples of d i o r i t e c o l l e c t e d from the Wark complex north of V i c t o r i a (samples V8. and V9) are l i k e w i s e very s i m i l a r i n p e t r o g r a p h i c c h a r a c t e r . Pervasive s a u s s u r i t i z a t i o n of p l a g i o c l a s e and t o t a l replacement of b i o t i t e by c h l o r i t e i n d i c a t e more e x t e n s i v e a l t e r a t i o n near V i c t o r i a , but i t i s d o u b t f u l that t h i s i s a c h a r a c t e r i s t i c of s p e c i a l s i g n i f i c a n c e . A s p i n e l - F e oxide a s s o c i a t i o n c o n s t i t u t e s about 8 percent of sample V9. The three major l i t h o d e m i c u n i t s thus d e s c r i b e d can be roughly c l a s s i f i e d i n terms of two dominant v a r i a b l e s : 1) the f r a c t i o n of l e u c o c r a t i c i n j e c t i o n , and 2) the degree of s y n p l u t o n i c deformation. The range of the f i r s t v a r i a b l e would be from a stockwork, i n which amphibolite i s predominant, to agmatite, i n which l e u c o c r a t i c v e i n s c o n s t i t u t e up to 60 percent of the outcrop, to p l u t o n i c rock c o n t a i n i n g up to 40 percent mafic i n c l u s i o n s . The second v a r i a b l e i s shown by the t r a n s i t i o n from undeformed agmatite c o n t a i n i n g angular blocks of country rock to s c h l i e r e n g n e i s s i n which the i n c l u s i o n s have been 20 f l a t t e n e d or elongated to banded gneiss with continuous mafic l a y e r s . F i g u r e 12 i s a g r a p h i c a l r e p r e s e n t a t i o n of t h i s two-dimensional c l a s s i f i c a t i o n scheme. A p o t e n t i a l t h i r d v a r i a b l e i s the degree of homogenization or mafic-f e l s i c c o n t r a s t . G a b b r o - P e r i d o t i t e S e v e r a l i s o l a t e d d i k e - l i k e masses of u r a l i t i z e d gabbroic rock outcrop near the c e n t r a l p a r t of Lemmens I n l e t . The t e x t u r e i s a l l o t r i o m o r p h i c g r a n u l a r , medium to coarse g r a i n e d , and u n f o l i a t e d . A u gite, f r e q u e n t l y showing d i a l l a g e , composes about 50 percent of these rocks. I t i s p o i k i l i t i c with p l a g i o c l a s e i n c l u s i o n s and o f t e n c o n t a i n s rods of i l m e n i t e along c r y s t a l l o g r a p h i c planes. Blue-green f i b r o u s u r a l i t e surrounds much of the pyroxene and c o n s t i t u t e s from 5 to 15 percent of the rock. E n s t a t i t e and o l i v i n e are l a r g e l y r e p l a c e d by s e r p e n t i n e which together make up from 25 to 35 percent of the rock. The remainder c o n s i s t s of about 10 percent s a u s s u r i t i z e d p l a g i o c l a s e ( A n 8 0 ) , 3 percent hornblende, 2 t o 5 percent chromite and magnetite, and minor c h l o r i t e . S i m i l a r c h e m i c a l l y to the g a b b r o - p e r i d o t i t e are a group of extremely sheared mafic rocks which crop out i n the northwest corner of Lemmens I n l e t . These comprise the g r e e n s c h i s t f a c i e s assemblage of a c t i n o l i t e - e p i d o t e -c h l o r i t e - p l a g i o c l a s e . The s i m i l a r chemistry and low grade of 2 1 Synplutonic Deformation F i g . 12 Schematic d i a g r a m d e p i c t i n g r e l a t i o n s h i p between f r a c t i o n _ of l e u c o c r a t i c i n j e c t i o n a n d degree o f s y n p l u t o n i c deformation in the W e s t c o a s t C r y s t a l l i n e Complex. 22 metamorphism of these mafic to u l t r a m a f i c rocks suggests they may be comagmatic and younger than the Westcoast am p h i b o l i t e s and g n e i s s e s . Sargent (1941) d e s c r i b e d s i m i l a r rocks i n t r u d i n g the J u r a s s i c Bedwell B a t h o l i t h near the Bedwell R i v e r . An i s o l a t e d occurrence of c h l o r i t e - a c t i n o l i t e s c h i s t was observed near the mouth of Windy Bay and may w e l l have been d e r i v e d from s i m i l a r r o c k s . Westcoast Metasediments Pendants and x e n o l i t h s of metasedimentary rocks are encountered w i t h i n the Westcoast C r y s t a l l i n e Complex and i n c l u d e metapsammitic and c a l c s i l i c a t e assemblages, both of which shed some l i g h t on metamorphic c o n d i t i o n s . Metapsammite Rocks of psammitic composition were found on the e a s t e r n s i d e of Indian I s l a n d , as x e n o l i t h s near the southern c o n t a c t of the T o f i n o I n l e t p l u t o n , and i n t e r c a l a t e d with c a l c a r e o u s sediments i n R i t c h i e Bay. On Indian I s l a n d , the metasediments are s t r o n g l y m y l o n i t i z e d and are conformable to the north and south with b r e c c i a t e d s i l i c e o u s v o l c a n i c s . M i n e r a l o g i c a l l y they are approximately 50 percent q u a r t z , 20 percent p l a g i o c l a s e ( A n 9 . 1 5 ) , 20 percent b i o t i t e , 5 percent garnet, 2 percent muscovite and the remaining 3 percent p y r i t e , epidote and a p a t i t e . B i o t i t e i s p a r t i a l l y r e t r o g r a d e d to p r e h n i t e . 23 Psammitic x e n o l i t h s i n the T o f i n o I n l e t pluton are s i m i l a r , c o n s i s t i n g of approximately 45 to 55 percent q u a r t z , 20 to 30 percent p l a g i o c l a s e , 5 to 15 percent b i o t i t e ( a l t e r i n g to p r e h n i t e ) , 2 percent muscovite, 3 percent p y r i t e and one percent s p i n e l . Minor phases i n c l u d e a p a t i t e , e p i d o t e , z i r c o n and c h l o r i t e . C l a s t i c i n t e r c a l a t i o n s w i t h i n banded c a l c s i l i c a t e on the north s i d e of R i t c h i e Bay appear to be more q u a r t z i t i c but c o n t a i n a s i m i l a r assemblage with up to 75 percent q u a r t z , 20 percent p l a g i o c l a s e ( A n 6 . 1 2 ) , 3 percent b i o t i t e , 2 percent garnet p o r p h y r o b l a s t s , and l e s s than one percent muscovite, c h l o r i t e and p r e h n i t e . The rock i s r a t h e r f i n e g r a i n e d on account of complete p o l y g o n i z a t i o n of quartz and f e l d s p a r . L a y e r i n g i s o f t e n accentuated by c o n c e n t r a t i o n s of f i n e g r a i n e d b i o t i t e . C a l c s i l i c a t e I s o c l i n a l l y f o l d e d banded c a l c s i l i c a t e and q u a r t z i t e crops out near the northern end of R i t c h i e Bay. The southern contact with an a m p h i b o l i t i c rock appears to be complexly f o l d e d . The amphibolite here c o n t a i n s abundant s t r e a k s of metapsammitic m a t e r i a l , t i g h t l y f o l d e d and d i s r u p t e d ( F i g s . 13 and 14), which may represent i n t e r c a l a t i o n s of sediments with b a s a l t i c e r u p t i v e s . The c a l c s i l i c a t e l a y e r s are q u i t e v a r i a b l e i n composition and c o n s i s t of v a r y i n g p r o p o r t i o n s of carbonate, w o l l a s t o n i t e , d i o p s i d e , s c a p o l i t e , q u a r t z , p l a g i o c l a s e 24 F i g . 14 T i g h t l y f o l d e d i n t e r c a l a t i o n s of metapsammite i n am p h i b o l i t e , Westcoast Metasediments. 25 ( A n 5 0 - 6 o ) / and z o i s i t e . The s t a b l e mineral assemblages from two r e p r e s e n t a t i v e samples (52-7B-1,-2) are d i s c u s s e d i n the s e c t i o n on metamorphism. Carbonate rocks are a l s o exposed i n the northern p a r t of Lemmens I n l e t . Here they occur as a massive 10 meter high c l i f f of f i n e g r a i n e d white marble and t i g h t l y f o l d e d and boudinaged l a y e r s of b l u i s h - g r e y d i o p s i d i t e . At the base of the c l i f f near the cont a c t with Westcoast c r y s t a l l i n e rocks i s a s t r o n g l y d i s r u p t e d zone of m y l o n i t i c and c a t a c l a s t i c c a l c s i l i c a t e . These rocks are m i n e r a l o g i c a l l y s i m i l a r to those exposed i n R i t c h i e Bay, with the ex c e p t i o n that w o l l a s t o n i t e i s r a r e , perhaps due to a retro g r a d e r e a c t i o n . S t a b l e assemblages from t h i s o u t crop are a l s o d i s c u s s e d i n the s e c t i o n on metamorphism (samples 50-4,a-j). Diabase and A p l i t e Diabase and a p l i t e d i k e s and d i k e l e t s are common i n the Westcoast C r y s t a l l i n e Complex. The a p l i t e d i k e s g e n e r a l l y c r o s s c u t the diabase, but t h e i r r e l a t i o n s h i p i s o f t e n ambiguous ( F i g . 15). Both appear to post-date deformation. S p i l i t i z a t i o n of the diabase i s u b i q u i t o u s . The t e x t u r e i s g e n e r a l l y very f i n e g r a i n e d , h y a l o p i l i t i c t o t r a c h y t i c , and v a r i o l i t i c . They are g e n e r a l l y composed of 40 to 90 percent a l b i t i c p l a g i o c l a s e , 2 to 30 percent c a l c i t e , 2 to 20 per cent c h l o r i t e and 0 to 10 percent q u a r t z , epidote, d i o p s i d e , and i l m e n i t e . A p l i t i c d i k e s i n the Westcoast C r y s t a l l i n e Complex are 26 F i g . 15 Ambiguous c r o s s c u t t i n g r e l a t i o n s h i p between a p l i t e and diabase d i k e l e t s i n e p i d i o r i t e , Westcoast C r y s t a l l i n e Complex. 27 t r o n d h j e m i t i c i n composition and are composed of subequal amounts of quartz and s a u s s u r i t i z e d p l a g i o c l a s e ( A n 5 0 . 6 0 ) t o t a l l i n g about 90 to 95 percent with the remainder c h l o r i t e , b i o t i t e , epidote and a p a t i t e . F e l s i c V o l c a n i c s A h i g h l y f r a c t u r e d f e l s i c v o l c a n i c u n i t crops out i n the southwest p o r t i o n of the map area near the Westcoast F a u l t . I t g e n e r a l l y c o n s i s t s of white to b l u i s h - g r e y to maroon t u f f a c e o u s a n d e s i t e and d a c i t e b r e c c i a . I t s r e l a t i o n s h i p with the Westcoast C r y s t a l l i n e Complex i s u n c l e a r . I t s proxim i t y to the Westcoast F a u l t suggests that i t may be a s l i v e r of e x o t i c m a t e r i a l p o s s i b l y c o r r e l a t i v e with the P a c i f i c Rim Complex. M u l l e r (1973) mapped these as Bonanza V o l c a n i c s . The rock c o n s i s t s mostly of a f i n e g r a i n e d hash of q u a r t z , a l b i t e , s e r i c i t e , and epidote with phenocrysts of f r a c t u r e d quartz and s a u s s u r i t e . F r a c t u r e s are f r e q u e n t l y f i l l e d with quartz and c a l c i t e . P a c i f i c Rim Complex The P a c i f i c Rim Complex was not s t u d i e d p e t r o g r a p h i c a l l y i n the present study. In the map area, i t i s composed of a complex mixture of greywacke, a r g i l l i t e , r ibbon c h e r t , and minor v o l c a n i c s . The greywacke i s a dark, f i n e to medium grain e d , massive u n i t c o n t a i n i n g p o o r l y s o r t e d c l a s t i c m a t e r i a l i n c l u d i n g b i t s of c h e r t , a r g i l l i t e , 28 and v o l c a n i c d e b r i s . The a r g i l l i t e i s grey to black i n c o l o r and i s g e n e r a l l y h i g h l y f r a c t u r e d which, in p l a c e s , develops a crude f r a c t u r e cleavage. Interbedded with the a r g i l l i t e s are o c c a s i o n a l 2 to 4 cm t h i c k ribbon c h e r t s . T e r t i a r y I n t r u s i o n s A l a r g e body of massive quartz d i o r i t e , mapped by M u l l e r as p a r t of the Catface I n t r u s i o n s (Muller and Carson, 1969), i n t r u d e s the Westcoast C r y s t a l l i n e Complex on the southern s i d e of R i t c h i e Bay. The rock i s hypidiomorphic g r a n u l a r i n t e x t u r e and c o n s i s t s of about 40 percent p l a g i o c l a s e showing obvious zoning ( A n 3 0 . 5 o ) and l i t t l e a l t e r a t i o n , 30 percent i n t e r s t i t i a l q u a r t z , 5 percent o r t h o c l a s e , 14 percent brown hornblende, 8 percent b i o t i t e a l t e r i n g to c h l o r i t e , 2 percent magnetite and one percent comprising s e r i c i t e , e p i d o t e , sphene, a p a t i t e and z i r c o n . Hypabyssal d i k e s of a n d e s i t e to d a c i t e porphyry with c h i l l e d margins are common i n the study area and intrude both the Westcoast C r y s t a l l i n e Complex and the Catface I n t r u s i o n s ( F i g . 16). Phenocryst phases i n c l u d e zoned p l a g i o c l a s e ( A n 2 3 . 5 o ) i q u a r t z , hornblende and b i o t i t e . A l t e r a t i o n products i n c l u d e u r a l i t e , c h l o r i t e , s e r i c i t e and e p i d o t e . 29 F i g . 16 Hypabyssal dike and d i k e l e t of a n d e s i t e porphry i n t r u d i n g C a t f a c e I n t r u s i o n on Meares I s l a n d . 30 CHAPTER II GEOCHEMISTRY OF THE WESTCOAST CRYSTALLINE COMPLEX AND RELATED ROCKS Chemical A n a l y s i s T h i r t e e n samples from the Westcoast C r y s t a l l i n e Complex as w e l l as three from the Wark Complex and two of T e r t i a r y i n t r u s i o n s were analyzed f o r major and t r a c e element composition by X-ray f l u o r e s c e n c e , using a whole-rock pressed-powder-pellet procedure developed by van der Heyden and others (van der Heyden, 1982). A l l samples were analyzed using a P h i l l i p s PW 1410 spectrometer. Operating c o n d i t i o n s and data r e d u c t i o n r o u t i n e s have been d e s c r i b e d by Berman (1979). A n a l y s i s f o r H 20 was done by l o s s on i g n i t i o n techniques a l s o d e s c r i b e d by Berman. R e s u l t s from these analyses are presented i n Appendix 2 and are d i s p l a y e d g r a p h i c a l l y i n F i g u r e s 17 and 18. C l a s s i f i c a t i o n V a r i o u s chemical c l a s s i f i c a t i o n schemes have been developed f o r v o l c a n i c rocks u s i n g major and t r a c e element abundances. S i m i l a r schemes f o r p l u t o n i c rocks, p a r t i c u l a r l y u s i ng t r a c e element c o n c e n t r a t i o n s , are l a c k i n g . T h i s may be p a r t i a l l y due to fewer a n a l y t i c a l data f o r g r a n i t o i d as compared to e x t r u s i v e rocks, the t r a d i t i o n a l use of modal c l a s s i f i c a t i o n schemes, and the view that not a l l g r a n i t o i d s represent t r u e l i q u i d compositions. In the f o l l o w i n g 31 O - - 0 . 0 m o o .Q._o a O -O Co u X UJ OL to Z UJ CaO -z o o UJ Q >< O o -0° ^ - -0- • -o o o o>. MgO TiO/o 0 •• 40 SO SO TQ 80 SiO, WT. PERCENT F i g . 17 Major element v a r i a t i o n diagrams f o r the Westcoast C r y s t a l l i n e Complex and r e l a t e d rocks; O = Westcoast C r y s t a l l i n e Complex, • = u l t r a b a s i t e s , A = Wark Complex, • » T e r t i a r y i n t r u s i o n s ; dashed l i n e s connect m a f i t e / f e l s i t e p a i r from a Westcoast Migmatite; dash-dot l i n e s connect m a f i t e / f e l s i t e p a i r from a Wark agmatite. 32 Nd - oo • » O 4 0 «r*^" Ba . or* • 'o o C e z" o < CC UI u z o CJ z UI a UI -I ui o I , ,nnj • 0 Ni o o- -A — — Y o • • 0 2 _ o- -Q O ° 0 a a. z" o z UI u z o C J u i a UI Zr o SP oo SiO, WT. PERCENT F i g . 18 Trace element v a r i a t i o n diagrams f o r the Westcoast C r y s t a l l i n e Complex and r e l a t e d rocks; symbols and connecting l i n e s as i n F i g . 17. 33 d i s c u s s i o n , the assumption i s made that a petrochemical s i m i l a r i t y e x i s t s between comagmatic i n t r u s i v e and e x t r u s i v e rocks and that they may be expected to share d i s t i n c t i v e chemical c h a r a c t e r i s t i c s . A standard major element c l a s s i f i c a t i o n scheme of I r v i n e and Baragar (1971) as w e l l as immobile element schemes of F l o y d and Winchester (1978) were employed in i n t e r p r e t a t i o n of the a n a l y s e s . The l a t t e r scheme i s c o n s i d e r e d more r e l i a b l e f o r a l t e r e d and metamorphosed rocks. The I r v i n e and Baragar c l a s s i f i c a t i o n s were f a c i l i t a t e d by the use of FORTRAN programs w r i t t e n by G. T. Nixon. FeO values were not measured but were approximated from measured t o t a l Fe ( r e p o r t e d as F e 2 0 3 ) using average FeO/Fe 20 3 r a t i o s proposed by Brooks (1976) f o r b a s a l t i c rocks. C0 2 was not determined f o r these samples and was presumably n e g l i g i b l e as modal carbonate was not p r e s e n t . If the major element analyses approximate magmatic compositions, the samples may be t e n t a t i v e l y c l a s s i f i e d as i n t r u s i v e e q u i v a l e n t s of a s u b a l k a l i n e s u i t e ranging from t h o l e i i t i c b a s a l t to c a l c - a l k a l i n e a n d e s i t e , d a c i t e and r h y o l i t e ( F i g u r e s 19, 20, and 21). The two samples p o r t r a y e d by s o l i d c i r c l e s i n F i g u r e s 19 and 20 are u l t r a b a s i c and may w e l l r e p r e s e n t cumulates (samples 12-2 and 15-12). T h e i r p o s i t i o n s on these diagrams t h e r e f o r e bear l i t t l e g enetic s i g n i f i c a n c e . They do not p l o t i n F i g u r e 21 as t h e i r normative p l a g i o c l a s e composition i s A n 1 0 0 . Two other 34 7 6^ O 5 CM I'* fW m Z 3H 2^ ALKALINE FIELD 40 o SUBALKALINE FIELD 50 SiO, 60 70 80 F i g . 19 P l o t of a l k a l i s versus s i l i c a (wt. p e r c e n t ) . Symbols as in F i g . 17. D i v i d i n g l i n e i s from I r v i n e and Baragar F i g . 20 AFM diagram (A = Na 20 + K 20, F = FeO + 0.S998 F e 2 0 3 , M = MgO, wt. p e r c e n t ) . Symbols as in F i g . 17. D i v i d i n g l i n e i s from I r v i n e and Baragar (1971). 35 80« NORMATIVE PLAGIOCLASE COMPOSITION F i g . 21 P l o t of normative c o l o r index versus normative p l a g i o c l a s e composition in percent c a t i o n e q u i v a l e n t s . Symbols as in F i g . 17. U l t r a b a s i c samples not shown (see t e x t ) . F i e l d boundaries are from I r v i n e and Baragar (1971), 36 analyses p l o t t i n g w i t h i n the a l k a l i n e f i e l d i n c l u d e a Westcoast Amphibolite (sample 35-19) and a Wark d i o r i t e (sample V9), both of which may a l s o represent cumulates. S a u s s u r i t i z e d p l a g i o c l a s e i n these rocks i n d i c a t e s hydrothermal s o l u t i o n s which c o u l d c o n c e i v a b l y have int r o d u c e d sodium to the rocks, thereby d r i v i n g them towards the a l k a l i n e f i e l d . On a F l o y d and Winchester (1978) p l o t of Z r / T i 0 2 versus S i 0 2 ( F i g . 22), the data c l e a r l y d e f i n e a s u b a l k a l i n e trend ranging from b a s a l t to r h y o l i t e . Again, the u l t r a b a s i c samples f a l l w i t h i n the a l k a l i n e f i e l d and may be cumulates from a more s i l i c a r i c h magma. On a p l o t of Nb/Y versus Z r / T i 0 2 ( F i g . 23), the data are s l i g h t l y more e q u i v o c a l but o v e r a l l s t i l l s u b a l k a l i n e . The d i s c r e p e n c i e s may be due to a s l i g h t l y higher c o n c e n t r a t i o n of Nb than i s found in average c a l c - a l k a l i n e rocks (Sun, 1980). Pearce and Cann (1973) proposed a Y/Nb r a t i o of 1.00 as the upper l i m i t f o r a l k a l i n e rocks which would c l a s s i f y a l l the data as t r a n s i t i o n a l to s u b a l k a l i n e . T e c t o n i c S e t t i n g D i s c r i m i n a n t s V a r i o u s diagrams employing immobile elements have been de v i s e d to determine the t e c t o n i c s e t t i n g of b a s a l t i c e r u p t i v e s . Three such diagrams developed by Pearce and Cann (1973) f o r b a s i c v o l c a n i c rocks to d i s t i n g u i s h between " w i t h i n - p l a t e " b a s a l t s , o c e a n - f l o o r b a s a l t s , low potassium t h o l e i i t e s , and c a l c - a l k a l i n e b a s a l t s are shown i n F i g u r e F i g . 22 Zr/TiOj - S i 0 2 diagram, Westcoast C r y s t a l l i n e Complex and r e l a t e d rocks; Symbols as in F i g . 17. F i e l d boundaries are from F l o y d and Winchester (1978). 28 1.0 4 0.1 o N 0.0 1 0 . 0 0 1 0 .01 P H O N O U T I S U B - A L K A L I N E B A S A L T y 0.10 A L K A L I B A S A L T B A S A N I T E N E P M E L I N I T E 1.00 10.0 Nb/Y F i g . 23 Nb/Y - 2 r / T i 0 2 diagram, Westcoast C r y s t a l l i n e Complex and r e l a t e d rocks; Symbols as i n F i g . 17. F i e l d boundaries are from F l o y d and Winchester (1978). 39 24. As d i f f e r e n t i a t i o n tends to i n c r e a s e the Z r / T i 0 2 r a t i o , only samples of b a s a l t i c to a n d e s i t i c composition have been p l o t t e d . The e f f e c t of d i f f e r e n t i a t i o n i s an i n c r e a s e d c a l c -a l k a l i n e c h a r a c t e r and i s i l l u s t r a t e d by the dashed l i n e s which connect a m a f i t e / f e l s i t e p a i r from the Wark Complex (samples V8-m and V 8 - f ) . Although the analyses p l o t t e d are from p l u t o n i c and metamorphic rocks showing v a r y i n g degrees of a l t e r a t i o n which may have a f f e c t e d the bulk chemistry, the o v e r a l l i d e n t i f i c a t i o n p o r t r a y e d by these diagrams i s , again, s u b a l k a l i n e t h o l e i i t i c to c a l c - a l k a l i n e , as would be found i n a magmatic a r c . F u r t h e r evidence f o r a magmatic arc s e t t i n g comes from a p l o t of normalized t r a c e element abundance p a t t e r n s . Sun (1980) showed that b a s a l t s erupted i n magmatic arc s e t t i n g s were e n r i c h e d i n Sr, K, Th, U, Ba, Rb, Cs, and Pb and d e p l e t e d i n T i , Zr, Nb, and Ta r e l a t i v e to mid-ocean r i d g e b a s a l t s and ocean i s l a n d b a s a l t s , r e s p e c t i v e l y . The p a t t e r n s d i s p l a y e d by the average compositions of three c a t e g o r i e s of the Westcoast C r y s t a l l i n e Complex, as w e l l as the average Wark d i o r i t e c l e a r l y show t h i s same gen e r a l p a t t e r n of enrichment and d e p l e t i o n ( F i g . 25). Comparison of these p a t t e r n s with the " t y p i c a l " p a t t e r n s of Sun ( i n s e t , F i g . 25) suggests that they are more c a l c - a l k a l i n e than t h o l e i i t i c . Again, the i n c r e a s e i n c a l c - a l k a l i n e c h a r a c t e r with d i f f e r e n t i a t i o n i s e v i d e n t . Extreme d e p l e t i o n of T i i n the average f e l s i t e i s an e x p r e s s i o n of the absence of magnetite i n these rocks. Ti/ 100 40 Ti/100 Zr Y*3 10000 E a a 200 F i g . 24 Tec t o n i c s e t t i n g d i s c r i m i n a t i o n diagrams showing d e l i m i t i n g f i e i d s f o r " w i t h i n - p l a t e b a s a l t s " (WPB), ocean-f l o o r b a s a l t s (OFB), low-potassium t h o l e i i t e s (LKT), and c a l c - a l k a l i n e b a s a l t s (CAB). Data p l o t t e d are from samples of b a s a l t i c to a n d e s i t i c composition. Dashed l i n e s connect m a f i t e / f e l s i t e p a i r from Wark agmatite. Symbols as in F i g . 17. F i e l d boundaries are from Pearce and Cann ( 1 9 7 3 ) . 41 100041 100 4 3 eo N E 10 '000, A OH M MUX ••MN average amphibolite average diorite > WCCx average felalte average diorite: Wark i i 11 l f f f f f f l > Rb Ba Nb K Ce Sr Nd Zr Ti Y F i g . 25 Normalized abundance p a t t e r n s of incompatible t r a c e elements, Westcoast C r y s t a l l i n e Complex and Wark d i o r i t e . Inset shows t y p i c a l p a t t e r n s for ocean i s l a n d , i s l a n d a r c , and ocean ridge b a s a l t s . Normalizing v a l u e s are from Sun (1980) . 42 D e p l e t i o n of V with d i f f e r e n t i a t i o n (see F i g . 18) can be s i m i l a r l y e x p l a i n e d . I n v e r s i o n of the Sr peak with d i f f e r e n t i a t i o n can be a t t r i b u t e d to d e c r e a s i n g r e c e p t i v i t y in p l a g i o c l a s e due to de c r e a s i n g a n o r t h i t e content. The normalized t r a c e element abundance p a t t e r n s f o r two T e r t i a r y i n t r u s i o n s (samples RBP and 14-5) a l s o d i s p l a y enrichment of Rb, Ba and K and d e p l e t i o n of Nb and T i c h a r a c t e r i s t i c of a magmatic arc a s s o c i a t i o n ( F i g . 26). The t e c t o n i c s e t t i n g of these i n t r u s i o n s , however, i s somewhat enigmatic i n that they are s i t u a t e d w e l l outboard of c o e v a l Eocene c a l c - a l k a l i n e arc v o l c a n i c s which extend from Wyoming to A l a s k a . The i m p l i c a t i o n s of t h i s are d i s c u s s e d l a t e r . The t r a c e element s i g n a t u r e s of the two u l t r a b a s i c samples are unique ( F i g . 26) and, i n p a r t , tend to show i n v e r s i o n of the enrichments and d e p l e t i o n s c h a r a c t e r i s t i c of the more evolved c a l c - a l k a l i n e rocks. T h i s i s p a r t i c u l a r l y notable f o r Rb, Ba, Nb, K, Nd, and Sr. T h i s o b s e r v a t i o n lends i t s e l f to i n t e r p r e t a t i o n of these rocks as r e s t i t e or cumulates, which were l a t e r i n j e c t e d i n t o the Westcoast C r y s t a l l i n e Complex. C o n c l u s i o n s P e t r o g r a p h i c and chemical data f o r the Westcoast C r y s t a l l i n e Complex, the Wark/Colquitz Complex, and the Catf a c e I n t r u s i o n s are c o n s i s t e n t with magma genesis i n a magmatic arc s e t t i n g . Enrichment of K, Rb, Ba, and Sr i n c a l c - a l k a l i n e magmas has been e x p l a i n e d by the m o b i l i t y of 43 10004 Tertiary Intrusions Catface qz diorite 1 +j t ? t t f t f f t Rb Ba Nb K Ce Sr Nd Zr Ti Y F i g . 26 Normalized abundance p a t t e r n s of incompatible t r a c e elements, u l t r a b a s i c rocks and T e r t i a r y i n t r u s i o n s . Normalizing values are from Sun ( i 9 6 0 ) . 44 these elements in aqueous f l u i d s which may have been i n t r o d u c e d to the mantle source r e g i o n by dehydration of subducted ocean c r u s t (Best, 1975; Saunders and Tarney, 1979). D e p l e t i o n of the incompatible elements Nb, Ce, Nd, Zr and Y i n c a l c - a l k a l i n e magmas may be f a c i l i t a t e d by in c r e a s e d 0 2 f u g a c i t y and a decreased mantle s o l i d u s which would together r e s u l t from h y d r a t i o n of the mantle (Pearce,1982). A higher 0 2 f u g a c i t y would tend to s t a b i l i z e r e s i d u a l oxides such as r u t i l e , sphene and z i r c o n and thus r e t a i n elements compatible with such minerals i n the residuum, while a depressed mantle s o l i d u s would r e s u l t i n a higher degree of p a r t i a l m e l t i n g , thereby d i l u t i n g whatever incompatible elements d i d enter the melt. I t i s a l s o c o n c e i v a b l e that the r e l a t i v e l y d e p l e t e d t r a c e elements of the magmas were the consequence of re m e l t i n g of p r e v i o u s l y d e p l e t e d mantle (Green,1973), but t h i s does not escape the need f o r a c o n t r i b u t i o n from the subducted s l a b . 4 5 CHAPTER III STRUCTURE Large s c a l e s t r u c t u r a l a n a l y s i s i n the area s t u d i e d i s complicated by the lack of a true s t r a t i g r a p h y c o n t a i n i n g d i s c r e t e marker h o r i z o n s , l i m i t e d and sporadic s h o r e l i n e exposures, and d i s r u p t i o n by p o s t - d e f o r m a t i o n a l i n t r u s i o n s . Observations on an outcrop s c a l e , however, were s u f f i c i e n t to i n d i c a t e the s t r u c t u r a l s t y l e of the region mapped ( F i g . 27, i n p o c k e t ) . Heterogeneous S t r a i n The most s t r i k i n g s t r u c t u r a l f e a t u r e i n the area s t u d i e d i s a pronounced v a r i a t i o n i n s t r a i n , e x e m p l i f i e d by both the a m p h i b o l i t i z e d country rock and the i n t r u s i v e s u i t e s . L o c a l l y superimposed on rocks showing a complex h i s t o r y of i n t r u s i o n , i s a heterogeneous, v a r i a b l y developed t e c t o n i c f a b r i c . In the g r a n i t o i d s , the f a b r i c , where developed, i s g e n e r a l l y d e f i n e d by a g n e i s s i c f o l i a t i o n ( S 0) which ranges from a very f a i n t alignment of mafic m i n e r a l s , f r e q u e n t l y only evident on weathered s u r f a c e s , to w e l l developed g r a n i t o i d g n e i s s . In the l a r g e r , s y n t e c t o n i c p l u t o n i c bodies, t h i s deformation i s l a r g e l y c o n f i n e d to t h e i r margins. In the a m p h i b o l i t i c country rock, a l l t r a n s i t i o n s e x i s t between u n s t r a i n e d a m p h i b o l i t i z e d metagabbro with i s o t r o p i c g r a n o b l a s t i c t e x t u r e s to s t r o n g l y f o l i a t e d and l i n e a t e d 46 a m p h i b o l i t e g n e i s s and s c h i s t . Mafic x e n o l i t h s i n c o r p o r a t e d i n the d i o r i t i c i n t r u s i v e s serve as e x c e l l e n t s t r a i n markers and, in p l a c e s , i n d i c a t e very steep s t r a i n g r a d i e n t s . F i g u r e 28 i s a sketch from a photograph showing a r a p i d t r a n s i t i o n i n which o r i g i n a l l y angular to subangular i n c l u s i o n s are i n c r e a s i n g l y f l a t t e n e d and elongated with i n c r e a s i n g shear s t r a i n . The r e s u l t of h i g h shear s t r a i n i s a banded g n e i s s i n which the mafic i n c l u s i o n s have been attenuated, elongated, and r o t a t e d i n t o the d i r e c t i o n of maximum e l o n g a t i o n towards p a r a l l e l i s m with the plane of simple shear (Ramsay and Graham, 1970; Simpson, 1983). At t h i s advanced stage of f o l i a t i o n development, the x e n o l i t h s are no longer r e c o g n i z a b l e as such and form more or l e s s continuous c o m p o s i t i o n a l bands w i t h i n the g n e i s s . R o t a t i o n of the s t r a i n e l l i p s o i d i s a l s o demonstrated in F i g u r e 29 where l o c a l very high shear s t r a i n has r e s u l t e d in a narrow m y l o n i t i c shear zone. The f o l i a t i o n i n the adjacent d i o r i t e i s c l e a r l y d e f l e c t e d i n t o t h i s zone and i n d i c a t e s d e x t r a l shear. The q u e s t i o n a r i s e s as to why the rocks show a v a r i a b l e response to a presumably r e g i o n a l shear s t r e s s . A p o s s i b l e e x p l a n a t i o n f o r heterogeneous s t r a i n put f o r t h by Ramsay and Graham (1970) i s " s t r a i n s o f t e n i n g " at regions of i n c i p i e n t deformation, perhaps as a r e s u l t of chemical m o d i f i c a t i o n and weakening of m a t e r i a l by i n t r o d u c t i o n of mobile c o n s t i t u e n t s . Another p o s s i b i l i t y which may apply more s p e c i f i c a l l y to these rocks i s suggested here; the i n j e c t i o n 47 F i g . 28 Sketch from a photograph showing development of banded gneiss from i n c l u s i o n - r i c h g r a n i t o i d by abrupt i n c r e a s e in shear s t r a i n , Westcoast C r y s t a l l i n e Complex. 48 F i g . 29 D e f l e c t i o n of f o l i a t i o n i n d i o r i t e i n t o m y l o n i t i c shear zone, Westcoast C r y s t a l l i n e Complex. 49 of g r a n i t o i d melts as small dikes and v e i n s i n t o somewhat c o o l e r country rock (see Chapt. VI) c o u l d c o n c i e v a b l y c r e a t e l o c a l temperature g r a d i e n t s r e s u l t i n g i n l o c a l d u c t i l i t y c o n t r a s t . A l s o , h e t e r o g e n e i t y i n the country rock i s l i k e l y to r e s u l t i n v a r i a b l e response to given boundary c o n d i t i o n s . For whatever reason, i t i s q u i t e apparent that the rocks behaved inhomogeneously during t h i s d e f o r m a t i o n a l event. O r i e n t a t i o n of S t r u c t u r a l F a b r i c s The S 0 f o l i a t i o n , although v a r i a b l y developed, shows a rat h e r c o n s i s t e n t o r i e n t a t i o n on a p l o t of pole s to S 0 ( F i g . 30a). The d e n s i t y d i s t r i b u t i o n i n d i c a t e s a g e n e r a l n o r t h w e s t e r l y s t r i k e and a predominantly steep n o r t h e a s t e r l y d i p with a maxima of about 317 69NE. The S 0 f o l i a t i o n i s f r e q u e n t l y i n v o l v e d i n t i g h t to i s o c l i n a l small s c a l e f o l d i n g ( F i g . 31) showing only minor and l o c a l i z e d development of an S, a x i a l planar f o l i a t i o n . D e spite an o f t e n i r r e g u l a r appearance i n outcrop, the axes to the minor f o l d s are c r u d e l y c o a x i a l with a g e n e r a l southeast p l u n g i n g , v a r i a b l y developed, s t r e t c h i n g l i n e a t i o n ( F i g . 30b). St e e p l y p l u n g i n g boudins of l e s s competent mafic m a t e r i a l w i t h i n the S 0 f o l i a t i o n are f a i r l y common i n the Westcoast g n e i s s e s ( F i g . 32). These f e a t u r e s lend support to the i n t e r p r e t a t i o n t h a t the primary l i n e a t i o n i s the r e s u l t of s t r e t c h i n g . A l l of these s t r u c t u r a l elements can be e x p l a i n e d i n 50 F i g . 30 S t r u c t u r a l o r i e n t a t i o n s , Meares I s l a n d map area-equal area p r o j e c t i o n s , a) P l o t of poles to S 0; contour i n t e r v a l s are 5%, 4%, 3%, 1.5%, 0%; 138 p o i n t s . b) P l o t of l i n e a t i o n s and minor f o l d axes; contour i n t e r v a l s are 9% 8%, 5%, 3%, 0%; 78 p o i n t s . F i g . 32 Boudinage of a m p h i b o l i t i c l a y e r s i n banded g n e i s s , Westcoast C r y s t a l l i n e Complex. 52 terms of s i m i l a r l y o r i e n t e d p r i n c i p a l s t r a i n axes. F i g u r e 33 i s an attempt to show these r e l a t i o n s h i p s d i a g r a m m a t i c a l l y . If these f e a t u r e s t r u l y represent a c o a x i a l s t r a i n h i s t o r y , with the lengths of boudins p e r p e n d i c u l a r to minor f o l d axes, they are best i n t e r p r e t e d as a r e s u l t of extension i n two p r i n c i p a l d i r e c t i o n s (X< > X 2 > 1). I d e a l l y , t h i s s t a t e of s t r a i n should produce the c l a s s i c c h o c o l a t e t a b l e t s t r u c t u r e , although f i e l d evidence f o r these s t r u c t u r e s i s l a c k i n g . T h i s f l a t t e n i n g s t r a i n can p o s s i b l y be e x p l a i n e d as a response to the emplacement of g r a n i t o i d p l u t o n s and the inherent volume problem they impose. A s i m i l a r p a r a l l e l i s m between l i n e a t i o n s and minor f o l d axes i s observed i n banded c a l c s i l i c a t e i n northern R i t c h i e Bay, but here they have a g e n e r a l l y steeper plunge to the ENE. The f o l i a t i o n i n these rocks (S,) , which i s transposed from an e a r l i e r f o l i a t i o n ( S 0 , bedding?), has a general n o r t h e a s t e r l y s t r i k e and d i p s 50 to 60° SE. The reason f o r t h i s d e v i a t i o n from o r i e n t a t i o n s i n the c r y s t a l l i n e g n e i s s e s i s p o o r l y understood but may r e s u l t from 1) r o t a t i o n due to f a u l t i n g , 2) p r e - J u r a s s i c deformation, or 3) emplacement of the nearby T e r t i a r y C atface I n t r u s i o n . The l i m i t e d exposure of t h i s u n i t makes r e s o l u t i o n of t h i s anomaly d i f f i c u l t . F i g u r e 34 i s a c l o s e - u p of pa r t of t h i s u n i t showing 1) the transposed S, f o l i a t i o n (note i s o c l i n a l f o l d c l o s u r e (F,) below p e n c i l ) , 2) more open s t y l e minor f o l d s ( F 2 , upper l e f t ) , 3) boudinage of more q u a r t z i t i c l a y e r s (middle l e f t ) , 53 F i g . 33 Schematic d iagram showing o r i e n t a t i o n of p r i n c i p l e s t r a i n axes for boud ins , l i n e a t i o n s , and minor f o l d s . 54 F i g . 34 C a l c s i l i c a t e showing transposed l a y e r i n g (S,) which has been f o l d e d and boudinaged; see t e x t f o r d e t a i l s . 55 and, l e s s c o n s p i c u o u s l y , 4) a small well-developed sheath f o l d ( d i r e c t l y to l e f t of p e n c i l ) with a doubly plunging f o l d a x i s and b i s e c t i n g s t r e t c h i n g l i n e a t i o n (not v i s i b l e i n photo) plunging 33 to 065 implying a s i m i l a r d i r e c t i o n of shear. D u c t i l e - B r i t t l e T r a n s i t i o n The d i s c u s s i o n up to t h i s p o i n t has d e a l t e x c l u s i v e l y with s t r u c t u r a l elements of a d u c t i l e nature. There i s , however, ample evidence on m i c r o s c o p i c and mesoscopic s c a l e s f o r a t r a n s i t i o n from d u c t i l e to b r i t t l e s t r a i n . F i g u r e 35 i s a photomicrograph of a w e l l l i n e a t e d amphibolite composed mainly of hornblende and p l a g i o c l a s e . The d u c t i l e s t r a i n f a b r i c i s d i s r u p t e d along two conjugate planes of b r i t t l e f r a c t u r e . Both the d u c t i l e and b r i t t l e deformation appear to be i n response to a s i n g l e d i r e c t i o n of maximum compressive s t r e s s . The d i r e c t i o n of p r i n c i p a l s h ortening i s g r a d u a l l y r o t a t e d by p r o g r e s s i v e simple shear so that with i n c r e a s i n g shear s t r a i n , the f o l i a t i o n , which develops at r i g h t angles to the maximum compressive s t r e s s (Ramsay and Graham, 1970), approaches p a r a l l e l i s m with the plane of shear. As the rock passes from a d u c t i l e to a b r i t t l e regime, the s t r a i n i s accomodated along conjugate b r i t t l e shears. Note the sense of movement on the f r a c t u r e s evidenced by the d e f l e c t i o n of the S 0 f o l i a t i o n . Another example of t h i s t r a n s i t i o n i s shown i n F i g u r e 36. In t h i s photomicrograph of a c a l c i t e marble from the 56 F i g . 3 5 P h o t o m i c r o g r a p h s h o w i n g b r i t t l e f r a c t u r e o f d u c t i l e s t r a i n f a b r i c a l o n g c o n j u g a t e p l a n e s , W e s t c o a s t A m p h i b o l i t e . F i g . 3 6 P h o t o m i c r o g r a p h d e f o r m e d c a l c i t e r i b b o n s , s h o w i n g c a t a c l a s i s o f W e s t c o a s t M e t a s e d i m e n t s . d u c t i l e l y 57 Westcoast Metasediments, the d u c t i l e deformation of the c a l c i t e i s obvious. The g r a i n s show m u l t i p l e twinning and are s t r o n g l y attenuated i n t o long ribbons. A n a l y s i s of the c a l c i t e twins using a method s i m i l a r to that d e s c r i b e d by Groshong (1972) i n d i c a t e s a high s t r a i n r a t e at f a i r l y high temperatures ( J . V. Ross, pers. comm.). The t r a n s i t i o n to a b r i t t l e regime i s c l e a r l y evidenced by the w e l l developed c a t a c l a s t i c f a b r i c . Mesoscopic f e a t u r e s i n d i c a t i n g a t r a n s i t i o n from d u c t i l e to b r i t t l e deformation are a l s o present i n the study area. The best i l l u s t r a t i v e example i s shown i n F i g u r e 37. Here, a 5 cm wide m y l o n i t i c shear i s deformed i n t o a Z - f o l d and subsequently d i s r u p t e d by b r i t t l e f r a c t u r e . B r i t t l e F a u l t s A major northwest-trending c r u s t a l f a u l t , named the Westcoast F a u l t by M u l l e r (Muller et a l . , 1974; M u l l e r , 1977), i s a r e g i o n a l l y mappable f e a t u r e along Vancouver I s l a n d ' s western margin and marks the boundary between the I n s u l a r and P a c i f i c B e l t s . In the study area, i t juxtaposes the Westcoast C r y s t a l l i n e Complex with the P a c i f i c Rim Complex, and i t s t r e n d , t h e r e f o r e , i s f a i r l y w e l l c o n s t r a i n e d by an abrupt change i n l i t h o l o g y and topography. I t s s t r u c t u r a l e x p r e s s i o n i s not, however, r e a d i l y apparent along most of i t s s t r i k e i n the area mapped. T h i s i s l i k e l y due to d i f f e r e n t i a l e r o s i o n of the more h i g h l y f r a c t u r e d rocks near the f a u l t r e s u l t i n g i n poor exposure of the f a u l t 58 F i g . 37 Sketch from a photograph showing d u c t i l e shear zone deformed i n t o a Z - f o l d and subsequently d i s r u p t e d by b r i t t l e f r a c t u r e , Westcoast C r y s t a l l i n e Complex. 59 zone. I t s general s t r u c t u r a l e x p r e s s i o n , where evident, appears to be of a b r i t t l e nature. Numerous s y n t h e t i c f a u l t s t r e n d i n g NNW and o r i e n t e d approximately 20 to 25° to the Westcoast F a u l t are i n t e r p r e t e d as R i e d e l shears r e l a t e d to t r a n s c u r r e n t d e x t r a l movement along t h i s f a u l t . Exposure of these f e a t u r e s i s l i k e w i s e poor, but they a l s o appear to be b r i t t l e shears. S l i c k e n s i d e s on one of these f a u l t s u r f a c e s i n d i c a t e d near h o r i z o n t a l d e x t r a l motion. C o n c l u s i o n s In summary, the most notable s t r u c t u r a l f e a t u r e d i s p l a y e d by rocks of the Westcoast C r y s t a l l i n e Complex i s a heterogeneous s t r a i n i n a d u c t i l e regime. L i m i t e d evidence suggests that there may have been two components to t h i s s t r a i n , p o s s i b l y a c t i n g i n unison; 1) a shear component, e x e m p l i f i e d by v a r i a b l y elongated and r o t a t e d x e n o l i t h s , perhaps r e l a t e d to r e g i o n a l t e c t o n i c s t r e s s e s a s s o c i a t e d with subduction, and 2) a component of f l a t t e n i n g , evidenced by minor f o l d s , boudins, and s t r e t c h i n g l i n e a t i o n s , as a response to g r a n i t o i d emplacement. With c o o l i n g and u p l i f t , the d u c t i l e deformation gave way to a b r i t t l e regime, r e s u l t i n g i n c a t a c l a s i z e d mylonites and b r i t t l e f r a c t u r e s . The development of major through-going f a u l t s , at l e a s t at the s t r u c t u r a l l e v e l now exposed, can be i n t e r p r e t e d as an exte n s i o n or c o n t i n u a t i o n of t h i s 60 b r i t t l e regime. 61 CHAPTER IV METAMORPHISM The determination of metamorphic c o n d i t i o n s i s an obvious goal i n s t u d i e s of metamorphic t e r r a i n s . Aluminous m e t a p e l i t e , a rock type u s e f u l f o r such d e t e r m i n a t i o n s , i s l a c k i n g i n the area s t u d i e d . P r e c u r s o r s to the Westcoast Metasediments p r e v i o u s l y d e s c r i b e d appear to have been l e s s r e v e a l i n g q u a r t z o - f e l d s p a t h i c c l a s t i c and c a l c a r e o u s sediments. E s t i m a t i o n of c o n d i t i o n s f o r these parageneses i s made d i f f i c u l t by widespread r e t r o g r a d e metamorphism and the l i k e l y s u p e r p o s i t i o n of r e g i o n a l and co n t a c t metamorphic c o n d i t i o n s ( i n t r a - and p o s t - J u r a s s i c ) . Amphibolites Metamorphism of mafic igneous rocks i n the Westcoast C r y s t a l l i n e Complex has r e s u l t e d i n the common amphibolite f a c i e s assemblage of intermediate to c a l c i c p l a g i o c l a s e and green hornblende. The absence of brown hornblende or metamorphic cli n o p y r o x e n e i s suggestive of middle amphibolite f a c i e s c o n d i t i o n s based on o b s e r v a t i o n s made by Engel and Engel (1962) i n the Adirondacks. The presence of a n a t e c t i c melts i n Westcoast rocks suggests t h a t , at l e a s t l o c a l l y , metamorphic c o n d i t i o n s were s u f f i c i e n t to i n i t i a t e p a r t i a l m e l t i n g . Whether t h i s i s a r e g i o n a l phenomena or merely the r e s u l t of l o c a l superposed contact metamorphism i s not d i s c e r n a b l e from the scope of t h i s study. A 62 temperature of T > 600°C but < 1000°C (approximate amphibole de h y d r a t i o n temperature; Yoder and T i l l e y , 1962; Lambert and W y l l i e , 1968; M i l l h o l l e n et a l . , 1974) f o r metamorphism of the Westcoast Amphibolites i s i n f e r r e d . C a l c s i l i c a t e s ^ C h a r a c t e r i s t i c metamorphic assemblages e x h i b i t e d by these rocks (determined from t h i n s e c t i o n s and v e r i f i e d in some cases by X-ray d i f f r a c t i o n ) are l i s t e d i n Table i along with p e r t i n e n t r e a c t i o n s . The c a l c s i l i c a t e assemblages a l l appear to be of a s i l i c a - s a t u r a t e d p a r a g e n e s i s. The main carbonate phase i s assumed to be c a l c i t e based on r a p i d r e a c t i o n with HC1 and the l i m i t e d s t a b i l i t y of other carbonates in the presence of q u a r t z . These r e p r e s e n t a t i v e assemblages c h a r a c t e r i z e two separate c a l c s i l i c a t e l o c a l i t i e s , northern Lemmens I n l e t (50-4) and northern R i t c h i e Bay (52-7B). The p r o x i m i t y of the l a t t e r to the T e r t i a r y C a t f a c e I n t r u s i o n makes i t of q u e s t i o n a b l e value i n the d e t e r m i n a t i o n of boundary c o n d i t i o n s f o r J u r a s s i c metamorphism. S t a b i l i t y f i e l d s f o r these two l o c a l i t i e s are i n d i c a t e d on an i s o b a r i c T-X C 0 2 diagram ( F i g . 38) drawn for 5 kb and compiled from c a l c u l a t i o n s of Rice (1983) and Slaughter et a l . (1975). • f The Lemmens I n l e t paragenesis i s l i m i t e d on t h i s diagram by the presence of g r o s s u l a r i t e and usual absence of w o l l a s t o n i t e in rocks c o n t a i n i n g both c a l c i t e and q u a r t z , 63 TABLE 1. C a l c s i l i c a t e parageneses and p e r t i n e n t r e a c t i o n s Sample no. Parageneses Reactions Lemmens Ir t l e t 50-4a,b,c cc - qtz - di t r cc + q t z di + vapor cc + qtz • = wo + 50-4f cc - qtz cc + q t r = = wo + CO 2 50-4h cc - qtz - d i cc + ojtz = wo + co? cc - qtz - wo t r + cc + q t z = d i + vapor qt z - d i - gr - sc zo a r + + cc + < 3 t 2 qtz = an + gr + wo vapor 50-4 j qtz - d i - gr - cc zo cc + qtz = 25 + vapor cc t r gr + + qtz cc + qtz = WO qtz = an d i + wo vapor R i t c h i e B< 52-7B-1 wo - sc - gr - d i t r zo + + cc + cc + qtz qtz d i + 25 + vapor vaDor cc qtz = WO + 52-7B-2 t r - an cc + ta + qtz = t r + vapor qtz - d i - d i t r + cc + qtz = a i + vapor wo - qtz cc + qtz = WO + c 6 2 an + < 3 r = zo qtz an WO - cjtz Notes: S t a b l e p o r t i o n s of the r e a c t i o n s are u n d e r l i n e d . an=anorthite, c c = c a l c i t e , d i = d i o p s i d e , g r = g r o s s u l a r i t e , qtz=quartz, t a = t a l c , t r = t r e m o l i t e , wo=wollastonite, z o = z o i s i t e . Sample l o c a t i o n s are l i s t e d i n Appendix 1 and shown in F i g u r e 2 ( i n p o c k e t ) . 64 F i g . 38 H 2 0 - r i c h p o r t i o n of 5 kb T-X Co 2 diagram showing s t a b i l i t y f i e l d s of observed c a l c s i l i c a t e assemblages; h o r i z o n t a l l i n e s : Lemmens I n l e t exposure, v e r t i c a l l i n e s : R i t c h i e Bay exposure; dotted l i n e shows the 2 kb p o s i t i o n of the r e a c t i o n gr+qtz=an+wo; s o l i d and dotted r e a c t i o n boundaries are from Rice (1983), dashed are from Slaughter et a l . ( 1 9 7 5 ) . 65 c o r r e s p o n d i n g to the e q u i l i b r i a cc + an/zo + q t z gr + vapor and cc + qtz = wo + C0 2, r e s p e c t i v e l y . I t i s l i m i t e d on the high temperature end by the presence of g r o s s u l a r i t e + quartz ( 710°C at 5 kb). T e x t u r a l evidence i n one t h i n s e c t i o n c o n t a i n i n g minor w o l l a s t o n i t e i n d i c a t e s a back r e a c t i o n to c a l c i t e + q u a r t z , but i t i s d o u b t f u l that a l l the c a l c i t e and quartz i n t h i s exposure are r e t r o g r a d e . T h i s may imply that the rocks were c l o s e to the r e a c t i o n boundary cc + qtz = wo + C0 2 p r i o r to r e t r o g r a d e metamorphism. The R i t c h i e Bay paragenesis appears to be w e l l i n t o the w o l l a s t o n i t e s t a b i l i t y f i e l d . The s t a b i l i t y of w o l l a s t o n i t e i s s t r o n g l y dependent on the p a r t i a l pressure of C0 2 which i s p r i m a r i l y c o n t r o l l e d by the pore f l u i d composition and the t o t a l vapor p r e s s u r e . I t s s t a b i l i t y at high pressures i s evidenced by i t s occurence i n g r a n u l i t e f a c i e s rocks i n the Adirondacks and i s f a c i l i t a t e d by a low C0 2/H 20 vapor phase. However, i t i s much more common as a product of shallow c o n t a c t metamorphism. I t s v a r i a t i o n i n s t a b i l i t y w i t h i n the study area tends to favor the l a t t e r c o n d i t i o n and i s most e a s i l y e x p l a i n e d i n t h i s case by p r o x i m i t y to the Catface I n t r u s i o n i n R i t c h i e Bay. Contact metamorphic e f f e c t s are a l s o e vident i n immediately adjacent a m p h i b o l i t e s which e x h i b i t a f i n e g r a i n e d h o r n f e l s i c t e x t u r e . Assuming that the R i t c h i e Bay parageneses were in e q u i l i b r i u m with the same pore f l u i d , the c o l l e c t i v e assemblage of gr-qtz-an-wo i s the most l i m i t i n g and p l a c e s the paragenesis on the r e a c t i o n boundary gr + q t z = an + wo. The temperature at which t h i s 66 r e a c t i o n takes p l a c e i s f a i r l y p r e s s u r e - s e n s i t i v e as i n d i c a t e d by the i t s p o s i t i o n at 2 kb (dotted l i n e ; R i c e , 1983). The e f f e c t of p l a g i o c l a s e s o l i d s o l u t i o n w i l l lower the temperature s l i g h t l y . An e s t i m a t i o n of temperature dur i n g J u r a s s i c metamorphism based on the observed c a l c s i l i c a t e assemblages i s complicated by p o s s i b l e T e r t i a r y r e h e a t i n g , but they do provide an upper temperature l i m i t of approximately 700°C . S t r a i n a n a l y s i s of c a l c i t e from the Lemmens I n l e t exposure suggests temperatures i n excess of 600°C dur i n g deformation ( J . V. Ross, pers. comm.). Q u a r t z o - f e l d s p a t h i c Metasediments E s t i m a t i o n of metamorphic c o n d i t i o n s from assemblages i n q u a r t z o - f e l d s p a t h i c metasediments i s a l s o e q u i v o c a l . The u b i q u i t o u s assemblage of q t z - p l - g t r b i - m s i s c o n s i s t e n t with the hornblende h o r n f e l s f a c i e s as w e l l as the medium grade amphibolite f a c i e s . The occurence of muscovite + quartz i n t h i s paragenesis tends to favor the higher pressure a l t e r n a t i v e (> 2 kb) as muscovite breaks down i n the presence of quartz above 600°C below 2 kb (Helgeson et a l . , 1978). Above 2 kb, muscovite + q u a r t z + p l a g i o c l a s e are s t a b l e up to about 650°C i n the presence of water, where they r e a c t to form s i l l i m a n i t e + l i q u i d . There was no evidence of p a r t i a l m e lting observed i n Westcoast Metasediments, but the Westcoast Migmatites, i f formed by i n  s i t u a n a t e x i s , i n d i c a t e that minimum melt temperatures were 67 a t t a i n e d , a t l e a s t l o c a l l y . For rocks of amphibolite composition with excess water, the temperature of i n i t i a l m e l t i n g i s only s l i g h t l y higher than that f o r muscovite g r a n i t e (=*625-700°C; W y l l i e , 1977). T h i s may imply that pressure c o n d i t i o n s were above the i n v a r i a n t point shown i n Fi g u r e 39. The e f f e c t of p l a g i o c l a s e composition on the p o s i t i o n of the i n v a r i a n t p o i n t as determined by Tracy (1978) i s a l s o shown. The p l a g i o c l a s e composition in the observed assemblages i s An,., 5. Varying X H 2p appears to have an even more profound e f f e c t on i t s p o s i t i o n , p a r t i c u l a r l y with respect to pr e s s u r e . K e r r i c k (1972) has i n d i c a t e d displacement of t h i s i n v a r i a n t point to about 7 .5 kb with X H2O = 0 • 5 . I t should be noted that equating the metamorphic c o n d i t i o n s f o r the metasediments and f o r the migmatites may be erroneous. I t i s q u i t e c o n c e i v a b l e that post-metamorphic f a u l t i n g c o u l d juxtapose rocks of d i f f e r e n t metamorphic grade. An e s t i m a t i o n of pressure as being g r e a t e r than 4 kb i s t h e r e f o r e made with c a u t i o n . C o n c l u s i o n s In summary, the c o n d i t i o n s during metamorphism of the Westcoast C r y s t a l l i n e Complex appear to be c o n s i s t e n t with the middle amphibolite f a c i e s , although t h i s determination i s not unequivocal for the reasons d i s c u s s e d . Nevertheless, T = 600-700°C, P > 4 kb, and f a i r l y low X c 0 2 are i n f e r r e d from the data. 68 F i g . 39 P H ? o ~ T diagram showing the Ab-absent and An-absent i n v a r i a n t p o i n t s in the system CKNASH; p o s s i b l e s t a b i l i t y f i e l d f o r q u a r t z o - f e l d s p a t h i c metasediments of the Westcoast C r y s t a l l i n e Complex i s i n d i c a t e d by the v e r t i c a l l y l i n e d area; m o d i f i e d from Tracy (1978). 69 CHAPTER V GEOCHRONOLOGY OF THE WESTCOAST CRYSTALLINE COMPLEX AND RELATED ROCKS One o b j e c t i v e of the present study was to determine i f the Westcoast C r y s t a l l i n e Complex i s c o e v a l with the Wark/Colquitz Complex and to determine i f they t r u l y represent the products of J u r a s s i c migmatization of S i c k e r and Vancouver Group rocks as proposed by M u l l e r et a l . (1974). For t h i s study, s e v e r a l samples were c o l l e c t e d from v a r i o u s l i t h o d e m i c u n i t s of the Westcoast C r y s t a l l i n e Complex and the Wark Complex and processed f o r z i r c o n and other m i n e r a l separates. A t o t a l of ten U-Pb z i r c o n dates f o r s i x d i f f e r e n t rock samples, e i g h t K-Ar mineral dates, three Rb-Sr b i o t i t e dates, one Rb-Sr whole rock isochron date, and three f i s s i o n t r a c k dates were determined. Sample l o c a t i o n s are i n d i c a t e d i n F i g u r e s 2 ( i n pocket) and 40 and are a l s o l i s t e d i n Appendix 1 with b r i e f sample d e s c r i p t i o n s . R e s u l t s of i s o t o p i c a n a l y ses and a n a l y t i c a l techniques are l i s t e d i n Appendix 3. Westcoast C r y s t a l l i n e Complex Z i r c o n separates from four l o c a l i t i e s w i t h i n the Westcoast C r y s t a l l i n e Complex, s p l i t i n t o v a r i o u s s i z e f r a c t i o n s , y i e l d e d a t o t a l of ten U-Pb a n a l y s e s . Nine of the ten a n a l y s e s p l o t very c l o s e to c o n c o r d i a and i n d i c a t e 1 C a t f a c e I n t r u s i o n (RHP) 2 Westcoast q u a r t z d i o r i t e g iu ' i ss 3 Westcoast A m p h i b o l i t e (27-22) 4 T o f i n p I n l e t p l u t o n (27-5) 5 I n d i a n I s l a n d i n t r u s i o n (26-2) 6 q u a r t z d i o r i t e of B l a c k R i v e r (WCT-3) 7 Wark d i o r i t e (V8) 8 Wark d i o r i t e (V9) F i g . 40 Sample l o c a t i o n map f o r i s o t o p i c a n a l y s e s . D e t a i l e d l o c a t i o n s a r e l i s t e d i n Appendix 1. Map legend as i n F i g . 1. 71 c r y s t a l l i z a t i o n ages for rocks of the Westcoast C r y s t a l l i n e Complex ranging from 176 ± 2 Ma to 189 ± 8 Ma. Within t h i s data age d i f f e r e n c e s are not c l e a r l y r e s o l v e d and there i s no c o n s i s t e n t evidence of any o l d e r z i r c o n component. The Indian I s l a n d i n t r u s i o n y i e l d e d three s i z e f r a c t i o n s of euhedral to subhedral, c l e a r to s l i g h t l y cloudy z i r c o n s . R e p r e s e n t a t i v e g r a i n s from the middle s i z e f r a c t i o n (75-150tt) are shown in F i g u r e 41. On a c o n c o r d i a diagram ( F i g . 42), the two c o a r s e r f r a c t i o n s are seen to be n e a r l y concordant at about 176 Ma. The f i n e s t f r a c t i o n appears to c o n t a i n a t r a c e of o l d e r Pb and t h i s i s suspected to be the r e s u l t of sample contamination i n the l a b o r a t o r y . I f the o l d e r Pb was g e o l o g i c a l l y a u t h e n t i c i t would be expected to be more evident i n the c o a r s e r f r a c t i o n s , whereas the f i n e s t f r a c t i o n i s the one most s u s c e p t i b l e to contamination. The T o f i n o I n l e t p l u t o n y i e l d e d two n e a r l y concordant dates of 196 i 3 Ma and 182 i 3 Ma ( F i g . 43). The average of these two r e s u l t s , 189 ± 8 Ma, i s r e p o r t e d here as the best estimate of the age of emplacement. Grains from the 45-75y f r a c t i o n are shown in F i g u r e 44. Three s i z e s p l i t s of z i r c o n from a sample of quartz d i o r i t e g n e i s s c o l l e c t e d from Meares I s l a n d (sample 11-6) y i e l d e d c l o s e l y grouped and e s s e n t i a l l y concordant dates averaging 183 i 3 Ma ( F i g . 45). Grains from the 45-75M f r a c t i o n are shown in F i g u r e 46. The concordance of these z i r c o n s i m p l i e s that the development of g n e i s s i c l a y e r i n g was contemporaneous with emplacement i n the Middle J u r a s s i c , F i g . 41 Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s (75-150M) from the Indian I s l a n d i n t r u s i o n , Westcoast C r y s t a l l i n e Complex. 73 Oil 4 2 . (Zoncor^a diagram, Indian I s l a n d i n t r u s i o n (26-2) Westcoast C r y s t a l l i n e Complex. u a x u n v^o ^; 74 F i g . 43 Concordia diagram, T o f i n o I n l e t p l u t o n (27-5) Westcoast C r y s t a l l i n e Complex. 75 F i g . 44 Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s (45-75M) from the T o f i n o I n l e t p l u t o n (27-5), Westcoast C r y s t a l l i n e Complex. 76 F i g . 45 C o n c o r d i a d i a g r a m , q u a r t z d i o r i t e g n e i s s ( 1 1 - 6 ) Westcoast C r y s t a l l i n e Complex. m o ; 77 F i g . 46 Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s (45-75M) from quartz d i o r i t e g n e i s s (11-6), Westcoast C r y s t a l l i n e Complex. 78 or that the z i r c o n s were t o t a l l y r e s e t by J u r a s s i c h e a t i n g and deformation of o l d e r rocks. A sample of the quartz d i o r i t e of Black River c o l l e c t e d along the Westcoast T r a i l south of Pachena Point (see F i g . 40) f u r n i s h e d two s i z e f r a c t i o n s of z i r c o n . The 45-75M f r a c t i o n p i c t u r e d i n F i g u r e 47 i s concordant at 187 ± 4 Ma ( F i g . 48). The 75-150M f r a c t i o n i s s l i g h t l y d i s c o r d a n t with a 2 0 6 P b / 2 3 8 U date of 166 i 2 Ma and a 2 0 7 P b / 2 3 5 U date of 171 ± 3 Ma. Only an average date of 178 ± 12 Ma can be. given without f u r t h e r a n a l y s e s . The g e o l o g i c i m p l i c a t i o n i s that the c o a r s e r f r a c t i o n experienced Pb l o s s through a l t e r a t i o n or a r e h e a t i n g event, while the f i n e r s p l i t d i d not, and t h i s i s d i f f i c u l t to e x p l a i n or accept. K-Ar dates on hornblende and b i o t i t e from Westcoast rocks i n d i c a t e the time at which the rocks c o o l e d past the b l o c k i n g temperatures f o r these m i n e r a l s (see Chapt. V I ) , and are t h e r e f o r e s l i g h t l y younger than U-Pb ages, which c l o s e l y approximate the time of c r y s t a l l i z a t i o n . Hornblende i s mostly p o i k i l i t i c with i n c l u s i o n s of p l a g i o c l a s e . B i o t i t e from Westcoast rocks u s u a l l y shows some a l t e r a t i o n to c h l o r i t e , and hornblende shows much l e s s . Both of these i m p u r i t i e s w i l l c o n t r i b u t e l i t t l e to the K or Ar observed. Hornblende from the Indian I s l a n d i n t r u s i o n gave a K-Ar date of 172 ± 6 Ma. T h i s i s i n good agreement with the U-Pb date f o r the same sample. B i o t i t e and hornblende from the quartz d i o r i t e of Black River y i e l d e d K-Ar dates of 162 ± 6 Ma and 144 ± 5 Ma, 79 F i g . 47 Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s ( 4 5 -7 5 M) from the quartz d i o r i t e of Black R i v e r (WCT - 3 ) , Westcoast C r y s t a l l i n e Complex. F i g . 48 C o n c o r d i a d iag ram, q u a r t z d i o r i t e of B lack R i v e r (WCT-3), Westcoast C r y s t a l l i n e Complex. 81 r e s p e c t i v e l y . T h i s i s anomolous i n that the c o n v e n t i o n a l b l o c k i n g temperature fo r b i o t i t e (^180-260°C) i s lower than that f o r hornblende (=*400-550°C) , and b i o t i t e should, t h e r e f o r e , normally y i e l d a younger date. The b i o t i t e K-Ar date i s confirmed by Rb-Sr (see below) which leaves the hornblende date p u z z l i n g l y low. Hornblende separate from Westcoast Amphibolite c o l l e c t e d near the mouth of Mosquito Harbor on Meares I s l a n d (sample 27-22) giv e s a K-Ar date of 151 ± 5 Ma. As p r e v i o u s l y mentioned, the metamorphism and s t r u c t u r a l f a b r i c e x h i b i t e d by the amphibolites i s b e l i e v e d to be g e n e t i c a l l y r e l a t e d to g r a n i t o i d emplacement. The K-Ar date r e f l e c t s the time at which the amphibolite c o o l e d through the 400-550°C isotherms and t r u l y r e p r e s e n t s a metamorphic age. Age determination of the p r e c u r s o r to the Westcoast Amphibolites, which were l i k e l y b a s a l t i c and/or gabbroic, i s impeded by 1) J u r a s s i c r e s e t t i n g ( i . e . A r - l o s s from hornblende), 2) the absence of z i r c o n s i n these rocks, and 3) t h e i r low Rb c o n t e n t s . Two Rb-Sr mineral isochrons were obtained from Westcoast D i o r i t e s (see Appendix 3 f o r a n a l y t i c a l data and r e g r e s s i o n ) . A f o u r - p o i n t isochron (whole rock, hornblende, p l a g i o c l a s e , and b i o t i t e ) determined f o r the Indian I s l a n d i n t r u s i o n i n d i c a t e s a date of 151 ± 15 Ma and an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.70356 ( F i g . 49a). The isochron slope i s h e a v i l y weighted by b i o t i t e because of i t s high Rb/Sr r a t i o , and consequently, the date r e p r e s e n t s the time s i n c e b i o t i t e 82 became c l o s e d with r e s p e c t to r a d i o g e n i c Sr l o s s ( c l o s u r e temperature about 300 ± 25°C). The Indian I s l a n d i n t r u s i o n thus passed t h r u t h i s temperature at 151 ± 15 Ma ago. The z i r c o n r e f e r e n c e isochron of 176 Ma i s shown i n F i g u r e 49a fo r comparison. By drawing t h i s i sochron through the whole rock p o i n t , an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.70354 i s i n d i c a t e d . The f o u r - p o i n t m i n e r a l isochron f o r the quartz d i o r i t e of Black River i s s i m i l a r l y dominated by the b i o t i t e and y i e l d s a date of 169 t 3 Ma with an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.70341 ( F i g . 49b). The 178 Ma z i r c o n r e f e r e n c e isochron i n d i c a t e s a s l i g h t l y lower i n i t i a l r a t i o of 0.70329. Rb-Sr analyses f o r v a r i e d l i t h o l o g i e s from the Westcoast C r y s t a l l i n e Complex (see Appendix 3) e x h i b i t a f a i r l y wide s c a t t e r on a Sr e v o l u t i o n diagram ( F i g . 50). The b e s t - f i t isochron (York I r e g r e s s i o n ) i n d i c a t e s a date of 143 Ma, and the wide s c a t t e r i s r e f l e c t e d i n the rather l a r g e e r r o r of 27 Ma. An. isochron of 177 Ma (average of z i r c o n dates) with an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.70342 (average of Indian I s l a n d and Black R i v e r i n t e r c e p t s ) i s shown f o r r e f e r e n c e . The degree of s c a t t e r e x h i b i t e d by the whole rock analyses exceeds experimental e r r o r , and may r e s u l t from 1) v a r y i n g degrees of a l t e r a t i o n , p o s s i b l y with seawater, or a s s i m i l a t i o n of marine sediments which c o u l d have modified the p r o t o l i t h i s o t o p i c compositions of Sr, 2) i n c l u s i o n of more than one c o g e n e t i c s u i t e ( i . e . a m p h i b o l i t e s 83 0.706 a) 0.705< 86Sr 0.704 Indian island intrusion • Whole Rock • Hornblende 0 Plagioclase • Biotite 0.7 14 b) 0.7 12* 0.7 10-0.708" r3L 6Sr 0.704-0.7033 0.703 0 0.5 8 7Rb/8 6Sr 1.0 Black River quartz diorite \* # ^ 0 0.1 0.2 0.3 0.4 2 3 8 7 — , / 86^ 'Rb/" S r F i g . 49 a) Sr e v o l u t i o n diagram, Indian I s l a n d i n t r u s i o n (26-2). b) Sr e v o l u t i o n diagram, quartz d i o r i t e of Black River (WCT-3). 84 Rb/"sr F i g . 50 Sr e v o l u t i o n diagram, whole rock a n a l y s e s , Westcoast C r y s t a l l i n e Complex. 8 5 vs. g r a n i t o i d s ) so that the p o i n t s represent rocks of d i f f e r e n t ages and i n i t i a l r a t i o s , or 3) open-system behavior so that rocks have v a r i a b l y gained or l o s t Rb, Sr or r a d i o g e n i c Sr. Comparison of these analyses with data f o r the I s l a n d I n t r u s i o n s at s c a t t e r e d s i t e s throughout Vancouver I s l a n d (Armstrong, i n prep.) i s shown in F i g u r e 51. The I s l a n d I n t r u s i o n data i n c l u d e s analyses of more f r a c t i o n a t e d g r a n o d i o r i t e and quartz monzonite, i n a d d i t i o n to d i o r i t i c i n t r u s i o n s and, hence, shows a g r e a t e r range of Rb/Sr r a t i o s . The b e s t - f i t isochron f o r the combined data i n d i c a t e s a date of 181 i 7 Ma with an i n t e r c e p t of 0.70360. T h i s date i s i n e x c e l l e n t agreement with U-Pb dates f o r the Westcoast C r y s t a l l i n e Complex, and the magnitude of i n i t i a l r a t i o v a r i a b i l i t y and the i n i t i a l r a t i o s are l i k e w i s e s i m i l a r . The s i m i l a r i t y i n age and i n i t i a l r a t i o thus demonstrated between I s l a n d I n t r u s i o n s and Westcoast g r a n i t o i d s i s completely compatible with a cogenetic r e l a t i o n s h i p . The i n t r u s i v e s t y l e and l e s s f r a c t i o n a t e d nature of p l u t o n s from the Westcoast C r y s t a l l i n e Complex suggests that they may represent deeper c r u s t a l e q u i v a l e n t s of the I s l a n d I n t r u s i o n s and, i n d i r e c t l y , the Bonanza V o l c a n i c s . The age d e t e r m i n a t i o n s thus obtained f o r the Westcoast C r y s t a l l i n e Complex are i n t e r n a l l y c o n s i s t e n t and c l e a r l y i n c o n f l i c t with the p r e v i o u s l y p u b l i s h e d G.S.C. date of 264 ± 7 (Muller et al.,1974). R e s o l u t i o n of 86 0.712 F i g . 51 Sr e v o l u t i o n diagram comparing data f o r the Westcoast C r y s t a l l i n e Complex ( t h i s study) with I s l a n d I n t r u s i o n data (Armstrong, i n p r e p . ) . 87 t h i s d i s c r e p e n c y by f u r t h e r i n v e s t i g a t i o n of the same sample l o c a t i o n i s paramount f o r an unequivocal i n t e r p r e t a t i o n to be made. Zir c o n and a p a t i t e from the Indian I s l a n d i n t r u s i o n were dated using the f i s s i o n t r a c k method (Naeser, 1976). Sample p r e p a r a t i o n and counting techniques at U.B.C. are d e s c r i b e d in d e t a i l by H a r r i s o n (1977). The e r r o r s quoted are 1a and were approximated using the standard e r r o r equation of Johnson et a l . (1979). They are l a r g e because of f a i r l y low t r a c k d e n s i t i e s (see F i g . 52). The z i r c o n y i e l d e d a date of 80 ± 20 Ma ( f o r a b l o c k i n g temperature i n t e r v a l of 150-200°C; H a r r i s o n et al.,1979),and a p a t i t e gave 31 ± 19 Ma ( f o r a b l o c k i n g temperature i n t e r v a l of 75-120°C; H a r r i s o n et a l . , 1979). The range of b l o c k i n g temperatures f o r the v a r i o u s i s o t o p i c systems a l l o w s a c o o l i n g h i s t o r y to be determined. T h i s w i l l be d i s c u s s e d i n the next c h a p t e r . Wark Complex For purposes of comparison, two samples of d i o r i t e were dated from the Wark Complex near V i c t o r i a . Sample V8 (see F i g . 40 f o r l o c a t i o n ) f u r n i s h e d two s i z e f r a c t i o n s of z i r c o n f o r U-Pb a n a l y s i s . The 45-75M s p l i t , shown i n F i g u r e 53, was n e a r l y concordant with a 2 0 6 P b / 2 3 8 U date of 165 ± 1.5 Ma and a 2 0 7 P b / 2 3 5 U date of 168 ± 2 Ma ( F i g . 54). The 75-150M s p l i t , a l s o n e a r l y concordant, gave a 2 0 6 P b / 2 3 8 U date of 168 ± 1.5 Ma and a 2 0 7 P b / 2 3 5 U date of 171 ± 2 Ma. The average 88 F i g . 52 Photomicrograph of spontaneous f i s s i o n t r a c k s i n a p a t i t e from the Indian I s l a n d i n t r u s i o n (26-2). 89 F i g . 5 3 Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s ( 4 5 -7 5 M ) from the Wark Complex ( V 8 ) . 90 F i g . 54 Concordia diagram, Wark d i o r i t e (V8). 91 date of 168 ± 3 Ma i s t h e r e f o r e p r e f e r r e d as the best d e s i g n a t i o n of age f o r the Wark Complex. The discordance r e p o r t e d by M u l l e r (1977) was not observed and i s t h e r e f o r e suspect. Hornblende separated from sample V8 y i e l d e d a K-Ar date of 133 ± 5 Ma. Another sample of Wark d i o r i t e (sample V9) c o l l e c t e d about 12 k i l o m e t e r s to the southeast produced a K-Ar hornblende date of 177 ± 7 Ma. These dates, although few i n number, s t r o n g l y suggest that g r a n i t o i d s of the Wark Complex and perhaps the c l o s e l y a s s o c i a t e d C o l q u i t z Complex were emplaced more or l e s s contemporaneously with the I s l a n d I n t r u s i o n s and the Westcoast C r y s t a l l i n e Complex in Middle J u r a s s i c time. C a t f a c e I n t r u s i o n s S e v e r a l e p i z o n a l p l u t o n s of T e r t i a r y age have been recog n i z e d along the west co a s t of Vancouver I s l a n d (Muller and Carson, 1968) and i n c l u d e the Sydney I n l e t p l u t o n , the T o f i n o p l u t o n , the Kennedy Lake pl u t o n s and the Catface I n t r u s i o n s . Previous K-Ar d e t e r m i n a t i o n s f o r these p l u t o n s have i n d i c a t e d a range in ages of 32 to 59 Ma (Wanless et al.,1974). F i g u r e 55 shows the r e s u l t s of U-Pb a n a l y s i s on two s i z e f r a c t i o n s of z i r c o n from the Catface I n t r u s i o n c o l l e c t e d from R i t c h i e Bay on Meares I s l a n d (sample RBP). Both the l a r g e and small f r a c t i o n s are concordant at 40 and 42 Ma, r e s p e c t i v e l y , i n d i c a t i n g a c r y s t a l l i z a t i o n age of 41 92 0.01S 0.01 206 2 3 8 Pb U 0.0084 Catface Intrusion : Ritchie Bay 20 60 40 •75 u 75-ISOjt o.os 207 Pb / 2 3 8 u 80 4 1 i 1 ( R B P ) . 5 5 C 0 n c ° r d i a diagram, Catface I n t r u s i o n , R i t c h i e Bay 93 ± 1 Ma. Z i r c o n s from the 45-75/u s p l i t are shown i n F i g u r e 56. K-Ar analyses on hornblende and b i o t i t e from the same l o c a l i t y y i e l d e d dates of 37 ± 1 Ma and 34 ± 1 Ma, r e s p e c t i v e l y , i n good agreement with the U-Pb data. A Rb-Sr mineral isochron g i v e s a somewhat o l d e r , but i n a c c u r a t e date of 48 ± 7 Ma ( F i g . 57). The d i f f e r e n c e observed i s probably a r e f l e c t i o n of a n a l y t i c a l e r r o r i n the b i o t i t e Rb/Sr r a t i o a n a l y s i s by X-ray f l u o r e s c e n c e . The 41 Ma r e f e r e n c e isochron shown i n F i g u r e 57 i n d i c a t e s an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.70372. The f i s s i o n t r a c k date f o r a p a t i t e from t h i s i n t r u s i o n i s 13 ± 12 Ma, the l a r g e e r r o r again due to low tr a c k d e n s i t y ( F i g . 58). The tr a c k d e n s i t y i n a s s o c i a t e d z i r c o n proved to be too low f o r a meaningful determination to be made. The c o o l i n g curve f o r t h i s i n t r u s i o n and i t s i m p l i c a t i o n s are presented in the next chapter. 94 F i g . 56 Photomicrograph of r e p r e s e n t a t i v e z i r c o n g r a i n s (45-75M) from the Catface I n t r u s i o n , R i t c h i e Bay (RBP). 95 n l i w i 5 7 S r . e * o l u t i o n diagram, Catface I n t r u s i o n , R i t c h i e Bay (RBP); symbols as i n P i g . 47. y 96 97 CHAPTER VI COOLING HISTORY The geochronometric r e s u l t s presented i n the l a s t chapter are summarized i n Table 2, l i s t e d from l e f t to r i g h t i n order of de c r e a s i n g b l o c k i n g temperature ( H a r r i s o n et a l . , 1979). On a p l o t of temperature versus time, the dates d e f i n e apparent c o o l i n g curves, the i m p l i c a t i o n s of which present v a r i o u s p o s s i b i l i t i e s r e garding emplacement and u p l i f t . Westcoast C r y s t a l l i n e Complex The data f o r the Indian I s l a n d i n t r u s i o n covers the widest range of b l o c k i n g temperatures and thus p r o v i d e s the best c o n s t r a i n e d c o o l i n g curve f o r the Westcoast C r y s t a l l i n e Complex. Three i n t e r p r e t a t i o n s of t h i s data are shown g r a p h i c a l l y i n F i g u r e 59. The simplest i n t e r p r e t a t i o n c o n s i s t s of p r o g r e s s i v e c o o l i n g of the i n t r u s i o n from the time of c r y s t a l l i z a t o n to the present i n d i c a t e d by the apparent c o o l i n g curve of Model 1. An a l t e r n a t e i n t e r p r e t a t i o n which i s u n r e s o l v a b l e from c o o l i n g curve data i n g e n e r a l takes i n t o account the subsequent thermal pulse a s s o c i a t e d with the C a t f a c e I n t r u s i o n s and i m p l i e s p a r t i a l a n n e a l i n g of z i r c o n f i s s i o n t r a c k s and complete r e s e t t i n g of the a p a t i t e (Models 2 and 3). In t h i s case, the c o o l i n g r a t e i s not i n d i c a t e d by the data. Models 2 and 3 represent p o s s i b l e slow and r a p i d c o o l i n g r a t e s , r e s p e c t i v e l y . Table 2 Summary of age determinations l i s t e d In order of decreasing blocking temperature (L to R) K-Ar •7 i Y-rnn Sample U-Pb zircon ( v o i r e ) hornblende ( 4 00-5 50° C) Rb-Sr b i o t i t e CV300°C) K-Ar b i o t i t e (180-260°C) F i s s i o n Track zircon a p a t i t e (150-200 C) (75-120 C) Indian Is. intrusi o n (26-2) quartz d i o r i t e of Black R. (WCT-3) Tofino Inlet pluton (27-5) quartz d i o r i t e gneiss (11-6) Westcoast Amphibolite (27-22) 176 ± 2 178 ± 12 189 ± 8 183 ± 3 Westcoast C r v s t a l l i n e Ikjj jnDiex 172 ± 6 151 ± 15 144 ± 51 151 ± 5 169 ± 3 162 ± 6 Wark Complex 80 ± 20 31 ± 19 d i o r i t e (V8) 168 ± 3 133 i 5 d i o r i t e (V9) 177 i 7 Catface Intrusions quartz d i o r i t e (RBP) 41 ± 1 37 ± I 48 ± 7 34 ± 1 13 ± 12 Notes: Blocking temperatures are from Harrison et a l . , 1979. Errors reported are for one standard deviation. 'Anomalously low, not used for cooling curve 99 F i g . 59 C o o l i n g curves; Catface I n t r u s i o n (4 ) , Wark Complex (±) , Indian I s l a n d i n t r u s i o n •(•) , and quartz d i o r i t e of Black R i v e r (•). 100 A l l of these p o s s i b l e c o o l i n g h i s t o r i e s i n v o l v e r a p i d i n i t i a l c o o l i n g between 176 Ma and 172 Ma. T h i s i s most e a s i l y e x p l a i n e d as the r e s u l t of l a t e r a l heat l o s s to c o o l e r country rock r a t h e r than r a p i d u p l i f t along a geothermal g r a d i e n t . T h i s supports the c o n c l u s i o n that magmagenesis took p l a c e at s u b c r u s t a l l e v e l s . - D etermination of average u p l i f t r a t e s f o r the three models i s d i f f i c u l t f o r the f o l l o w i n g reasons: 1) The curves only approximate true u p l i f t a f t e r the ambient temperature has been reached. T h i s i s d i f f i c u l t to p i n p o i n t but was probably i n the 300-500°C range. 2) I f the thermal h i s t o r y i n c l u d e s T e r t i a r y r e h e a t i n g the exact p o s i t i o n of the c o o l i n g curve i s u n r e s o l v a b l e and can only be i n f e r r e d . 3) E s t i m a t i o n of paleogeothermal g r a d i e n t i s c o m p l i c a t e d by the f a c t that Vancouver I s l a n d changed from a high heat flow arc s e t t i n g to a low heat flow f o r e a r c s e t t i n g between m i d - J u r a s s i c to mid-Cretaceous time. Thus, the geothermal g r a d i e n t probably v a r i e d with time as w e l l as depth. 4) The depth of emplacement of t h i s i n t r u s i o n , which would place c o n s t r a i n t s on the u p l i f t r a t e , i s not p r e c i s e l y determined. The c a t a z o n a l nature of the Westcoast C r y s t a l l i n e Complex i m p l i e s depths on the order of 15 to 20 km, c o n s i s t e n t with the p r e s s u r e estimate of P > 4 kb. Nonetheless, an attempt can be made to approximate upper and lower l i m i t s by v a r y i n g the unknown parameters w i t h i n a r e a l i s t i c range. Table 3 shows the r e s u l t s of c a l c u l a t i o n s f o r the three models assuming an ambient Table 3. Comparative c a l c u l a t i o n s of u p l i f t rate for the Indian I s l a n d i n t r u s i o n MODEL 1 No Reheat ing C o o l i n g r a t e °C/Ma 400 / 1 67 Ma = 2.4 Geothermal Gradient °C/km 10 20 30 40 50 U p l i f t r a t e km/Ma 0.24 0.12 0.08 0.06 0.05 U p l i f t km 40 20 1 3 10 8 MODEL 2 slow T e r t i a r y Reheating, c o o l i n g a f t e r emplacement C o o l i n g r a t e °C/Ma 400 / 1 1 4 Ma = 3.5 Geothermal Gradient °C/km 1 0 20 30 40 50 U p l i f t r a t e km/Ma 0.35 0.18 0.12 0.09 0.07 U p l i f t km 40 20 13 10 8 MODEL 3 f a s t T e r t i a r y Reheating, c o o l i n g a f t e r emplacement C o o l i n g r a t e °C/Ma 400 / 75 Ma * = 3.5 Geothermal Gradient °C/km 10 20 30 40 50 U p l i f t r a t e km/Ma 0.53 0.27 0.18 0.14 0.11 U p l i f t km 40 20 13 10 8 1 02 temperature of 400°C and geothermal g r a d i e n t s i n the range l0-50°/km c a l c u l a t e d at 10° i n t e r v a l s . The average c o o l i n g r a t e s used i n these c a l c u l a t i o n s are represented by dotted l i n e s i n Fi g u r e 59. The i d e n t i c a l amounts of u p l i f t i n d i c a t e d by the three models i s not f o r t u i t o u s but i n d i c a t e s that the m a j o r i t y of u p l i f t i n Models 2 and 3 was accomplished by 50 and 100 Ma ago, r e s p e c t i v e l y . The models i n d i c a t e r e l a t i v e l y modest u p l i f t r a t e s on the order of 0.1 to 0.5 km/Ma. The c o o l i n g curve f o r the qua r t z d i o r i t e of Black River ( F i g . 59) i s not c o n s t r a i n e d below 200°C but i n d i c a t e s s i m i l a r r a p i d i n i t i a l c o o l i n g and lends support to T e r t i a r y r e h e a t i n g of the Indian I s l a n d i n t r u s i o n . The anomalously young hornblende K-Ar date of 144 ± 5 Ma was omitted from c o n s i d e r a t i o n . Wark Complex The apparent c o o l i n g curve f o r the Wark Complex ( F i g . 59), only c o n s t r a i n e d by two p o i n t s , i s c o n s i d e r a b l y l e s s steep between 700°C and 500°C. T h i s may be accounted f o r by: 1) slower u p l i f t , 2) i n t r u s i o n i n t o h o t t e r country rock, or 3) p a r t i a l r e s e t t i n g of hornblende by a subsequent thermal event. Eocene i n t r u s i o n s i n the Leech River Complex may evidence such an event. C a t f a c e I n t r u s i o n The r a p i d c o o l i n g demonstrated by the data f o r the 1 03 Catf a c e I n t r u s i o n ( F i g . 59) i s i n accord with the e p i z o n a l c h a r a c t e r of these p l u t o n s . The f i s s i o n t r a c k dates f o r the Indian I s l a n d i n t r u s i o n i n d i c a t e that the Westcoast C r y s t a l l i n e Complex at the l e v e l p r e s e n t l y exposed was at a maximum temperature of 120°C when the Ca t f a c e I n t r u s i o n i n R i t c h i e Bay was emplaced. 0 1 04 CHAPTER VII CONCLUSIONS Previou s i n t e r p r e t a t i o n s of the Westcoast and Wark/Colquitz C r y s t a l l i n e Complexes a t t r i b u t e d them to E a r l y J u r a s s i c metamorphism and migmatization of P a l e o z o i c basement rocks (Muller and Carson, 1969; M u l l e r , 1973, 1977; Mu l l e r et a l . , 1974). A r e v i s e d i n t e r p r e t a t i o n i n l i g h t of the data presented in t h i s study seems necessary. The chemistry of the Westcoast C r y s t a l l i n e Complex i s compatible with p e t r o g e n e s i s i n a magmatic arc s e t t i n g . Both major and t r a c e element s i g n a t u r e s of the Westcoast Amphibolites and g r a n i t o i d s c l e a r l y d e f i n e a s u b a l k a l i n e t h o l e i i t i c to c a l c - a l k a l i n e t rend c h a r a c t e r i s t i c of magmas generated i n a magmatic a r c . The c h a r a c t e r i s t i c enrichment of K, Rb, Ba, and Sr and d e p l e t i o n of Nb, Ce, Nd, Zr, and Y i s e n t i r e l y compatible with magma genesis i n a hydrated mantle source region (Pearce, 1982). Furthermore, the presence of what appear to be cumulate gabbros i m p l i e s that the present l e v e l of e r o s i o n has exposed p o r t i o n s of magma chambers where c r y s t a l s e t t l i n g was o p e r a t i v e . The chemical evidence alone does not r u l e out a P a l e o z o i c p r o t o l i t h ; J u r a s s i c a n a t e x i s of S i c k e r v o l c a n i c rocks, which have a wide range i n s i l i c a content and l i k e l y have s i m i l a r c a l c - a l k a l i n e chemistry, c o u l d c o n c e i v a b l y produce the same chemical f e a t u r e s . T h i s i n t e r p r e t a t i o n i s not, however, supported by the i s o t o p i c data. The 1 05 c o n s i s t e n c y and general concordance of U~Pb dates i n the 176 to 189 Ma range obtained f o r the Westcoast C r y s t a l l i n e Complex in t h i s study argues f a i r l y s t r o n g l y a g a i n s t d e r i v a t i o n from P a l e o z o i c c r u s t a l rocks and c a s t s u n c e r t a i n t y on the p r e v i o u s l y p u b l i s h e d U-Pb date of 264 Ma (Muller et a l . , 1974). Sr i s o t o p i c data a l s o provide evidence f o r a s u b c r u s t a l magma source. The i n i t i a l 8 7 S r / 8 6 S r r a t i o f o r S i c k e r v o l c a n i c rocks i s about 0.7040 (Armstrong, i n prep.) which i s higher than those obtained f o r Westcoast rocks (0.70329 to 0.70360). By J u r a s s i c time, the S i c k e r v o l c a n i c rocks would be too r a d i o g e n i c to y i e l d the i n i t i a l r a t i o s observed fo r the Westcoast C r y s t a l l i n e Complex. Another l i n e of evidence f o r s u b c r u s t a l r a t h e r than i n  s i t u magmagenesis, although more tenuous, comes from the c o o l i n g h i s t o r y presented i n Chapter VI. The f a i r l y r a p i d c o o l i n g evidenced by the Indian I s l a n d i n t r u s i o n i m p l i e s that the adjacent c r u s t , at l e a s t at the l e v e l of emplacement, was c o n s i d e r a b l y c o o l e r than the magma. T h i s would be l e s s e a s i l y e x p l a i n e d i n terms of a c r u s t a l environment where a n a t e x i s was underway, and the c r u s t would presumably be q u i t e hot. A p r e f e r r e d i n t e r p r e t a t i o n , given our new data, i s that the Westcoast C r y s t a l l i n e Complex rep r e s e n t s the deep c r u s t a l p a r t of the J u r a s s i c magmatic arc invaded by predominantly mantle-derived magmas. Va r i o u s p r o t o l i t h s , i n c l u d i n g P a l e o z o i c and Mesozoic s u p r a c r u s t a l and e a r l y 1 06 s o l i d i f i e d magmas were converted i n t h i s environment to migmatites and metamorphic rocks. An ol d e r c r u s t a l component seems improbable as the primary magma source; i t may have made an undetectable but minor c o n t r i b u t i o n to the p l u t o n i c rocks s t u d i e d . The c o e v a l I s l a n d I n t r u s i o n s and Bonanza V o l c a n i c s are i n t e r p r e t e d as higher l e v e l comagmatic d i f f e r e n t i a t e s of the Westcoast D i o r i t e s . The more e a s t e r l y geographic p o s i t i o n of the more a c i d i c I s l a n d I n t r u s i o n s and Bonanza V o l c a n i c s i m p l i e s d i f f e r e n t i a l u p l i f t that was g r e a t e r to the west. A consequence of t h i s d i f f e r e n t i a l u p l i f t i s the present day exposure of a somewhat d i s r u p t e d and p o s s i b l y incomplete c r u s t a l c r o s s s e c t i o n through the J u r a s s i c magmatic arc of Vancouver I s l a n d . Reconnaisance of the Wark/Colquitz Complex of southern Vancouver I s l a n d shows i t to be e s s e n t i a l l y i n d i s t i n g u i s h a b l e i n age, petrography, and chemistry from the Westcoast C r y s t a l l i n e Complex, and a s i m i l a r h i s t o r y i s i n f e r r e d . The apparent o f f s e t a c r o s s the San Juan-Orcas f a u l t zone i s c o n s i s t e n t with Cenozoic l e f t - l a t e r a l displacement along that f a u l t proposed by M u l l e r (1977). 1 07 CHAPTER VIII TECTONIC HISTORY OF VANCOUVER ISLAND The t e c t o n i c e v o l u t i o n of Vancouver I s l a n d i s c h a r a c t e r i z e d by m u l t i p l e magmatic episodes of a v a r i e d nature spanning Middle P a l e o z o i c to T e r t i a r y time a s s o c i a t e d with l a r g e - s c a l e c r u s t a l movements i n c l u d i n g subduction, r i f t i n g , and t r a n s c u r r e n t f a u l t i n g . A b r i e f summary of the major l i t h o l o g i c u n i t s and how they r e l a t e to a s p e c t s of t h i s e v o l u t i o n i s presented here. In a d d i t i o n , c o r r e l a t i o n with other C o r d i l l e r a n t e c t o n o s t r a t i g r a p h i c u n i t s i s c o n s i d e r e d . The m a j o r i t y of the u n i t d e s c r i p t i o n s are borrowed from M u l l e r (1977, 1980b) and M u l l e r et a l . (1974, 1 981 ) P a l e o z o i c Rocks of the S i c k e r Group provide the only r e c o r d of a P a l e o z o i c h i s t o r y f o r Vancouver I s l a n d . M u l l e r (1980b) has s u b d i v i d e d t h i s group i n t o a b a s a l t i c to r h y o d a c i t i c b a s a l v o l c a n i c u n i t ( N i t i n a t and Myra Formations) o v e r l a i n by a g r e y w a c k e - a r g i l l i t e sequence ( S e d i m e n t - S i l l U nit) and capped by limestone ( B u t t l e Lake Formation). T h i s Group i s evidence of a Middle to Upper P a l e o z o i c v o l c a n i c arc t e r r a n e ; l a c k of evidence f o r an o l d e r c r y s t a l l i n e basement suggests that the arc may have been e n s i a l i c . P o s s i b l e c o r r e l a t i v e t e r r a n e s exposed elsewhere i n the C o r d i l l e r a occur in the Klamath-S i e r r a n arc i n C a l i f o r n i a (Schweickert, 1976; M u l l e r , 108 1977,1980b), the Roche Harbor t e r r a n e in the San Juan I s l a n d s ( M u l l e r , 1980b) and p o r t i o n s of the S t . E l i a s and Wrangell Mountains i n the Yukon and Alaska ( M u l l e r , 1980b). S i c k e r Group rocks are not exposed in the Queen C h a r l o t t e I s l a n d s but are presumed to be present beneath T r i a s s i c v o l c a n i c s of the Karmutsen Formation. Tr i a s s i c T r i a s s i c v o l c a n i c rocks of the Karmutsen Formation are exposed e x t e n s i v e l y on Vancouver I s l a n d and c o n s i s t mainly of b a s a l t i c p i l l o w l a v a s , p i l l o w b r e c c i a s , and s u b a e r i a l flows of t h o l e i i t i c composition. T h e i r t o t a l t h i c k n e s s on Vancouver I s l a n d ranges up to 6000 m, and they are o v e r l a i n by up to 1000 m of Upper T r i a s s i c carbonates and e l a s t i c s of the Quatsino and Parsons Bay Formations, r e s p e c t i v e l y . The age of the Karmutsen v o l c a n i c s i s w e l l c o n s t r a i n e d as l a t e L a d i n i a n - e a r l y Karnian, a span of 5 to 10 Ma (Muller et a l . , 1981). Voluminous outpouring of Karmutsen b a s a l t s and subsequent sedimentation has been a t t r i b u t e d to i n t e r - a r c r i f t i n g and b a s i n development ( M u l l e r , 1977). Rocks c o r r e l a t i v e with the Karmutsen Formation have been i d e n t i f i e d from Oregon to s o u t h - c e n t r a l Alaska and are the l i n k i n g element of the W r a n g e l l i a t e r r a n e . E x t e n s i v e paleomagnetic study of these rocks by v a r i o u s workers suggests that they represent d i s p l a c e d fragments of a once contiguous t e r r a n e ( H i l l h o u s e , 1977; Yole and I r v i n g , 1980; Panuska and Stone, 1981; H i l l h o u s e et a l . , 1982). 109 L o n g i t u d i n a l c o n t r o l on t e r r a n e displacements i s not provided by paleomagnetic data which le a v e s the source region f o r W r a n g e l l i a e n i g m a t i c . Some workers have argued f o r a s o u t h e r l y source along the western margin of the C o r d i l l e r a (e.g. Jones et a l . , 1972; Tipper and Richards, 1976; Schweickert, 1976; M u l l e r , 1977, 1980b) while others propose a Tethyan or Pangean b i r t h p l a c e (e.g. Nur and Ben-Avraham, 1977,1978; Danner, 1976; Stone et a l . , 1982). J u r a s s i c A t r a n s i t i o n from a dominantly e x t e n s i o n a l t e c t o n i c regime to one of compression and subduction between the l a t e s t T r i a s s i c and E a r l y J u r a s s i c i s suggested on Vancouver I s l a n d by renewed magmatic a c t i v i t y of changed, now c a l c -a l k a l i n e c h a r a c t e r . A v o l c a n i c / p l u t o n i c a r c , the Bonanza V o l c a n i c s , I s l a n d I n t r u s i o n s , and Westcoast C r y s t a l l i n e Complex, developed in the l a t e E a r l y J u r a s s i c along the western edge of the T r i a s s i c b a s a l t p l a t e a u and continued to be a c t i v e through most of the Middle J u r a s s i c . The youngest f o s s i l s a s s o c i a t e d with Bonanza V o l c a n i c s are P i i e n s b a c h i a n to T o a r c i a n ( J e l e t z k y , 1970) but range up to C a l l o v i a n i n c o r r e l a t i v e rocks on the Queen C h a r l o t t e I s l a n d s . The Bonanza V o l c a n i c s are c h a r a c t e r i z e d by l a v a flows ranging i n composition from b a s a l t i c a n d e s i t e to r h y o d a c i t e with a s s o c i a t e d t u f f s , b r e c c i a s , and i n t e r c a l a t e d c l a s t i c sediments. A t h i c k n e s s of up to 2500 m has been suggested (Muller et a l . , 1974). 1 10 The I s l a n d I n t r u s i o n s and Westcoast C r y s t a l l i n e Complex form s u c c e s s i v e l y deeper p o r t i o n s of the J u r a s s i c arc and are g e n e r a l l y g r a d a t i o n a l with one another i n chemistry and p e t r o l o g y . Data presented in t h i s study i n d i c a t e that p l u t o n i c emplacement i n deeper l e v e l s of the c r u s t began as e a r l y as T o a r c i a n and p o s s i b l y P l i e n s b a c h i a n with contemporaneous migmatization and deformation of the l o c a l country rocks. L o c a l development of f o l i a t i o n showing general p a r a l l e l i s m of s t r i k e with the arc a x i s and d i p p i n g to the northeast may i n d i c a t e a c r u s t a l response to n o r t h e a s t - d i r e c t e d subduction, although t h i s i s a d m i t t e d l y s p e c u l a t i v e . Arc a c t i v i t y appears to have abated i n Middle to Late J u r a s s i c time. T h i s i s roughly contemporaneous with a c c r e t i o n to S t i k i n i a / N o r t h America as proposed by Vance et a l . (1980) and van der Heyden (1982). The Upper J u r a s s i c to Lower Cretaceous Gambier and F i r e Lake Groups of the Coast P l u t o n i c Complex mark the i n i t i a t i o n of- arc magmatism i n that r e g i o n (Mu l l e r , 1977; van der Heyden, 1982). An e x p l a n a t i o n f o r the eastward m i g r a t i o n of arc a c t i v i t y i s d i f f i c u l t to give at t h i s time, but i t may have been a r e s u l t of c o l l i s i o n and t e c t o n i c s h o r t e n i n g , change of d i p of the subduction zone, landward t r e n c h jump, or subduction of p a r t of W r a n g e l l i a . The m a j o r i t y of tectonism a s s o c i a t e d with t h i s c o l l i s i o n occured east of Vancouver I s l a n d . The Jura-Cretaceous f l y s c h sequences of the P a c i f i c B e l t (Chugach, Baranof, P a c i f i c Rim, and Leech River Complexes) 111 probably represent c o e v a l subduction complexes, although t h i s n o t i o n has r e c e n t l y been questioned (Cowan and Brandon, 1981; F a i r c h i l d and Cowan, 1982; Brandon, 1984). E a r l y J u r a s s i c p a l e o l a t i t u d e s determined from paleomagnetic s t u d i e s of the Bonanza V o l c a n i c s on Vancouver I s l a n d are roughly e q u i v a l e n t to those obtained on the Karmutsen Formation ( I r v i n g , 1983; I r v i n g and Yole, 1984). If J u r a s s i c amalgamation of the Wrangellia/Alexander Terrane with S t i k i n i a i s accepted, i t appears to have occured f u r t h e r south. T h i s view i s c o n s i s t e n t with Cretaceous paleomagnetism for these two t e r r a n e s d i s c u s s e d i n the next sect i o n . A p e r i o d of u p l i f t and e r o s i o n commenced in the Late J u r a s s i c and continued i n t o the Cretaceous on Vancouver I s l a n d , which was now p a r t of the f o r e a r c r e g i o n . E a r l y to Middle J u r a s s i c arc a c t i v i t y has been recog n i z e d elsewhere i n the I n s u l a r B e l t . Burns and P e s s e l (1984) have r e c e n t l y d e s c r i b e d a 1000 km long d i s c o n t i n u o u s zone of J u r a s s i c a r c - t y p e p l u t o n i c rocks and a n d e s i t e s i n the Border Ranges of s o u t h - c e n t r a l A l a s k a . There, Mesozoic and Cenozoic u p l i f t has exposed "an almost complete c r o s s -s e c t i o n through the e n t i r e a r c " , complete with cumulate u l t r a m a f i c s , migmatites, intermediate p l u t o n s , and a n d e s i t e s . The resemblance to the J u r a s s i c Westcoast rocks on Vancouver I s l a n d i s s t r i k i n g . In the Queen C h a r l o t t e I s l a n d s , the Bonanza V o l c a n i c s have t h e i r e q u i v a l e n t s i n the Yakoun Formation. J u r a s s i c 1 1 2 p l u t o n s exposed along the west co a s t , d e s c r i b e d by Sutherland-Brown (1966) as predominantly s y n t e c t o n i c , represent the n o r t h e r l y c o n t i n u a t i o n of the Westcoast C r y s t a l l i n e Complex. In the San Juan I s l a n d s and northwestern and c e n t r a l Cascades, l o c a l i n t r u s i v e s u i t e s i n c l u d i n g gabbro, d i o r i t e , quartz d i o r i t e , trondhjemite, and a s s o c i a t e d amphibolite have been recognized w i t h i n a predominantly o p h i o l i t i c t e r r a n e . V a r i o u s age d e t e r m i n a t i o n s f o r these rocks, compiled by Vance et a l . ( l 9 8 0 ) , i n d i c a t e emplacement i n the Middle to Late J u r a s s i c f o l l o w e d c l o s e l y i n some areas by Late J u r a s s i c amphibolite f a c i e s metamorphism. Vance et a l . (1980) suggest that the o p h i o l i t e s i n t h i s r e g i o n represent the opening of a small Middle to Late J u r a s s i c oceanic b a s i n , s i m i l a r to that proposed by Davis et a l . (1978), which separated the W r a n g e l l i a n elements of Vancouver I s l a n d and e a s t e r n Oregon. If t h i s i n t e r p r e t a t i o n i s accepted, i t i m p l i e s t h at i n i t i a l a c c r e t i o n of W r a n g e l l i a was accomplished by the Middle J u r a s s i c r e v e r s e d b r i e f l y and then f i n a l i z e d by the end of the J u r a s s i c . Middle J u r a s s i c p e l a g i c c h e r t s i n the Bridge R i v e r Group, which l i e s between S t i k i n i a and W r a n g e l l i a i n the southern Coast Mountains of B r i t i s h Columbia, argue f o r the e x i s t e n c e of an ocean b a s i n at that time (Davis et a l . , 1978) and only post Middle J u r a s s i c s u t u r i n g of W r a n g e l l i a to S t i k i n i a . Major Cretaceous and T e r t i a r y f a u l t i n g , i n t r u s i o n , and metamorphism g r e a t l y complicate and obscure t h i s p i c t u r e . . 1 1 3 Cretaceous From Cretaceous time onward, Vancouver I s l a n d remained q u i e t l y i n a f o r e a r c s e t t i n g . During t h i s time, i t was mostly emergent. A minor mid-Cretaceous episode of i n t r u s i o n was c o n f i n e d to the easternmost p a r t of the i s l a n d at the southwestern edge of the Coast P l u t o n i c Complex (M u l l e r , 1977). C l a s t i c sediments shed from the J u r a s s i c v o l c a n i c / p l u t o n i c e d i f i c e are evident as a s h e l f sequence on the northern t i p of Vancouver I s l a n d . These sediments i n c l u d e the Lower Cretaceous Longarm Formation and the Middle to Upper Cretaceous Queen C h a r l o t t e Group. In the Late Cretaceous, a f o r e a r c basin developed on the east s i d e of Vancouver I s l a n d . C l a s t i c sediments shed i n t o t h i s shallow basin from adjacent highlands formed the Nanaimo Group. Paleomagnetic data obtained from a mid-Cretaceous magnetic o v e r p r i n t i n the Karmutsen Formation i n d i c a t e s that Vancouver I s l a n d was at that time s t i l l 20 degrees south of i t s present p o s i t i o n r e l a t i v e to c e n t r a l North America ( I r v i n g and Yole, 1984). S i m i l a r l y , S t i k i n i a was as much as 13 degrees south i n the E a r l y Cretaceous ( I r v i n g et a l . , 1980). These ap p a r e n t l y d i f f e r e n t p a l e o l a t i t u d e s are c o n s i s t e n t with d e x t r a l movement on the Yalakom f a u l t , a c t i v e u n t i l the Late Cretaceous (Tipper and R i c h a r d s , 1976), and E a r l y T e r t i a r y o f f s e t along the S t r a i g h t Creek f a u l t (Davis et a l . , 1978), but the known displacement on these f a u l t s i s l e s s than the i m p l i e d movements. 1 1 4 Paleomagnetic data f o r the Chugach Terrane i n south-c e n t r a l Alaska i n d i c a t e s that i t was only about 10 degrees from the equator i n the l a t e s t Cretaceous (Stone et a l . , 1982). T h i s i m p l i e s e q u i v a l e n t or gr e a t e r o f f s e t on f a u l t s west of Vancouver I s l a n d . Assuming that Vancouver I s l a n d reached i t s present l a t i t u d e i n the Middle T e r t i a r y , an average l a t i t u d i n a l v e l o c i t y of 3 cm/yr i s i n d i c a t e d between mid-Cretaceous and m i d - T e r t i a r y time. A l a t i t u d i n a l v e l o c i t y of 6 cm/yr was proposed by Stone et a l . (1982) f o r south-c e n t r a l Alaskan t e r r a n e s although a value c l o s e r to 8 cm/yr seems more a p p r o p r i a t e f o r t h e i r post-Cretaceous Chugach data. T h i s i n d i c a t e s d e x t r a l motion on f a u l t s west of the I n s u l a r B e l t at a r a t e of at l e a s t 3 to 5 cm/yr. Gromme and H i l l h o u s e (1981) have c a l l e d f o r 25° of l a t i t u d i n a l displacement f o r 41 Ma pl u t o n s i n t r u d i n g the Chugach Terrane based on paleomagnetic data, while r e c o n s t r u c t i o n s of Cowan (1982) f o r the same time p e r i o d r e q u i r e only about 10° based p u r e l y on g e o l o g i c grounds. T e r t i a r y T e r t i a r y , rocks on Vancouver I s l a n d are represented by the Metchosin v o l c a n i c s , . s e v e r a l i s o l a t e d T e r t i a r y i n t r u s i o n s , the A l e r t Bay v o l c a n i c s , and sediments of the Carmanah Formation. The Lower to Middle Eocene Metchosin V o l c a n i c s are s i t u a t e d south of the Leech River f a u l t on the southern t i p of Vancouver I s l a n d and c o n s i s t of t h o l e i i t i c p i l l o w l a v a s , t u f f , b r e c c i a , and s u b a e r i a l flows, u n d e r l a i n 1 15 by gabbroic rocks of the Sooke I n t r u s i o n s . The t r a c e element chemistry of the v o l c a n i c s i s s i m i l a r to that of the Crescent Formation exposed in the Olympic Mountains and i s most c o n s i s t e n t with formation i n an I c e l a n d i c - t y p e i s l a n d -r i d g e s e t t i n g ( M u l l e r , 1980a). The Metchosin and Crescent oceanic t e r r a n e was a c c r e t e d to North America subsequent to metamorphism (-40 Ma) and a c c r e t i o n (post-40 Ma) of the Leech River Complex to the n o r t h ( F a i r c h i l d and Cowan, 1982; Cowan, 1982). Emplacement of the Leech River Complex along the San Juan f a u l t predated d e p o s i t i o n of the Late Eocene-Early Oligocene Carmanah Formation which in p l a c e s o v e r l i e s the f a u l t ( M u l l e r , 1977). The emplacement of the Metchosin V o l c a n i c s along the Leech River f a u l t i s s i m i l a r l y c o n s t r a i n e d by Upper Oligocene s t r a t a o v e r l y i n g that s t r u c t u r a l boundary ( F a i r c h i l d and Cowan, 1982). S e v e r a l quartz d i o r i t i c s tocks and p l u t o n s and a s s o c i a t e d d i k e s and s i l l s of mostly Eocene age are s c a t t e r e d a c r o s s Vancouver I s l a n d . The c a l c - a l k a l i n e chemistry and low i n i t i a l Sr r a t i o determined i n t h i s study are compatible with mantle d e r i v e d arc magmatism. However, i n t r u s i o n of these bodies occured i n the f o r e a r c region of the Eocene magmatic arc represented by v o l c a n i c rocks of the Kamloops Group, but a f t e r the c u l m i n a t i o n of arc magmatism. Ewing (1981) demonstrated a waning in magmatic arc a c t i v i t y n orth of 49° l a t i t u d e at about 42 Ma and a t t r i b u t e d i t to the i n c e p t i o n of a transform regime at the c o n t i n e n t a l 1 16 margin. T h i s i s c l o s e to the age of the Catface I n t r u s i o n s and a cause and e f f e c t r e l a t i o n s h i p may have been o p e r a t i v e although the d e t a i l e d and undoubtedly complicated mechanism r e q u i r e d to produce the necessary heat flow and the r e s u l t a n t chemistry are e l u s i v e at t h i s stage. S i m i l a r T e r t i a r y near-trench magmatism has been recognized i n southern Alaska (Burk, 1965; Moore, 1969; Moore, 1973) and has been a t t r i b u t e d to r i d g e subduction (Marshak and K a r i g , 1977) and a n a t e x i s w i t h i n an over-deepened a c c r e t i o n a r y prism (Hudson et a l . , 1979). Marshak and K a r i g p o s t u l a t e d that m i g r a t i o n of a r i d g e -t r e n c h - t r e n c h t r i p l e j u n c t i o n i n v o l v i n g the A l e u t i a n t r e n c h and the K u l a - F a r a l l o n r i d g e was r e s p o n s i b l e f o r i n c r e a s e d heat flow and p a r t i a l m e l t i n g w i t h i n the a c c r e t i o n a r y prism. Ewing's (1981) r e c o n s t r u c t i o n s f o r Eocene p l a t e c o n f i g u r a t i o n s o f f the west coast of Vancouver I s l a n d a l s o invoke a r i d g e - t r e n c h - t r e n c h t r i p l e j u n c t i o n i n v o l v i n g the P a c i f i c - F a r a l l o n r i d g e . Hence, the p l a t e geometries may have been c o n s i s t e n t with t h i s i n t e r p r e t a t i o n . I t f a i l s to e x p l a i n , however, the low i n i t i a l Sr r a t i o observed i n the Catface I n t r u s i o n s . P a r t i a l m e l t i n g of the a c c r e t i o n a r y prism would l i k e l y y i e l d a more r a d i o g e n i c and Rb-enriched magma than that observed. The h y p o t h e s i s of Hudson et a l . (1979), although c o n s i s t e n t with the i n i t i a l Sr r a t i o s they r e p o r t e d , s i m i l a r l y doesn't seem to apply here. App a r e n t l y , a mechanism i s r e q u i r e d which would e x p l a i n s u b c r u s t a l magmagenesis i n a f o r e a r c s e t t i n g . The answer may come from 1 1 7 a more thorough understanding of l i t h o s p h e r i c responses and heat flow r e l a t e d to the e v o l u t i o n of t r a n s c u r r e n t p l a t e boundaries. The Late Eocene to Oligocene Carmanah Formation occurs as a f r i n g e of c l a s t i c sediments along the west coast of Vancouver I s l a n d . I t l i e s unconformably on Mesozoic rocks i n c l u d i n g the Leech River s c h i s t , Bonanza V o l c a n i c s , and the Westcoast C r y s t a l l i n e Complex near Pachena P o i n t ( F i g . 60) and has been i n t e r p r e t e d as a b a t h y a l fan complex (Cameron, 1975). Subsequent f a u l t i n g and u p l i f t have t i l t e d and d i s r u p t e d t h i s u n i t . The youngest magmatic episode on Vancouver I s l a n d i s represented by the Late Miocene to E a r l y P l i o c e n e A l e r t Bay V o l c a n i c s exposed f o r the most p a r t between A l i c e Lake and Port M c N e i l l . The v o l c a n i c c e n t e r s appear to be c o l i n e a r with a f a u l t zone which extends from Brooks Pen i n s u l a to Port M c N e i l l and are l i k e l y r e l a t e d to recent p l a t e i n t e r a c t i o n s along the P a c i f i c margin d e s c r i b e d by Riddihough (1977). 118 F i g . 60 Nonconformity between g e n t l y d i p p i n g s t r a t a of the Carmanah Formation and d i o r i t e of the Westcoast C r y s t a l l i n e Complex near Pachena P o i n t . Rock s t i c k i n g up i n foreground i s a stack of Westcoast D i o r i t e surrounded by the younger sediments i n background and underfoot. 119 REFERENCES Berg, H. C , Jones, D. L., and R i c h t e r , D. H., 1972 G r a v i n a - N u t z o t i n B e l t - - T e c t o n i c s i g n i f i c a n c e of an upper Mesozoic sedimentary and v o l c a n i c sequence i n southern and south- eastern A l a s k a : U.S. G e o l o g i c a l Survey P r o f e s s i o n a l Paper 800-D, pp. D1-D24. Berman, R. 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APPENDIX J_ SAMPLE DESCRIPTIONS AND LOCATIONS 129 Sample Descriptions and Locations Latitude Westcoast Crystalline Complex 27-22 megacrystic amphibolite 49° i r 46" 125° 47' 45" 35-19 amphibolite 49° n» 08" 125° 53» 23" 34-3b,d amphibolite, trondhjemite 49° 08' 29" 125° 48' 00" 30-1 trondhjemite 49° 11' 56" 125° 51' 23" 14-5 trondhjemite 49° 12' 25" 125° 53' 15" 35-17 d i o r i t e 49° 11» 15" 125° 53' 26" 26-2 ( l i s ) quartz d i o r i t e 49° 06' 44" 125° 45' 50" 27-5 (TIP) quartz d i o r i t e 49° 13' 22" 125° 47' 25" WCT-3 quartz d i o r i t e 48° 43' 55" 125° 06' 18" 11-6 quartz d i o r i t e gneiss 49° 12' 02" 125° 52' 56" 12-2 gabbro-peridotite 49° 11' 16" 125° 52' 46" 15-12 sheared u l t r a b a s i t e 49° 12' 51" 125° 53' 21" 50-4a-j massive c a l c s i l i c a t e 49° 12' 42" 125° 51' 37" 52-7B-1.2 banded c a l c s i l i c a t e Wark Complex 49° 14' 10" 125° 53' 30" V8(m,f) amphibolite, d i o r i t e agmatite 48° 33' 54" 123° 33' 54" V9 diorite-hornblende gabbro 48° 28' 00" 123° 29' 24" T e r t i a r y Intrusions RBP quartz d i o r i t e 49° 13' 30" 125° 55' 24" 37-14 dacit e porphyry dike 49° 08' 04" 125° 50' 14" APPENDIX 2 MAJOR AND TRACE ELEMENT CHEMISTRY WESTCOAST CRYSTALLINE COMPLEX WARK COMPLEX AND TERTIARY INTRUSIONS 131 MAJOR ELEMENT COMPOSITION WESTCOAST CRYSTALLINE COMPLEX Westcoc ist Amph: b o l i t e Westcoc ist Cx-fe >lsite Sample-* 27-22 35-19 34-3b 34-3d 30-1 1 4-5 S i 0 2 44.60 42.25 50.85 74.06 76.50 77.60 A 1 2 0 3 15.42 15.19 1 2.62 12.92 12.85 12.50 F e 2 0 3 1 1 .89 1 4.54 9.59 2.79 1 .27 1 .86 MgO 1 1 .87 10.80 1 1 .65 1 .27 0.30 0.39 CaO 12.49 1 1 .84 9.36 2.09 1 .08 1.81 Na 20 1 .59 1 .76 3.05 4.34 4.37 3.97 K 20 0.34 0.21 0.87 1 .36 2.93 1 .22 T i 0 2 0.76 0.89 0.70 0.23 0.06 0.03 MnO 0.15 0.14 0.17 0.13 0.11 0.09 P 2 0 5 0.05 0.06 0.21 0.06 0.02 0.03 H 20" 0.06 0.15 0.06 0.11 0.06 0. 10 H 2 0 + 0.77 2.16 0.88 0.64 0.45 0.39 NOTES: 1) Values are in weight percent, normalized to 100% 2) T o t a l Fe reported as F e 2 0 3 3) C0 2 was not determined 132 MAJOR ELEMENT COMPOSITION WESTCOAST CRYSTALLINE COMPLEX V Jestcoast . D i o r i t e ;s D i o r i t e Gnei ss Westc U l t r a t :oast j a s i t e s Sample-* 35-17 l i s TIP WCT-3 1 1-6 1 2-2 15-12 S i 0 2 45.66 57.50 70.40 55.05 62.02 37.38 38.81 A1 20 3 1 6.66 1 5.75 14.55 1 5.55 14.24 1 1 .99 1 4.84 F e 2 0 3 6.11 8.32 3.19 9. 44 7.81 12.68 13.05 MgO 9.82 4.09 1.27 5.29 3.65 23.56 19.27 CaO 17.41 6.31 3.53 6.62 4.25 8.37 9.07 Na 20 2.08 3.69 4.44 2.99 5.02 0.56 1 . 06 K 20 0.15 1 .41 1 .05 1 .60 0.81 0.11 0.20 T i 0 2 0.23 0.89 0.28 0.91 0.79 0.17 0.20 MnO 0.17 0.18 0. 18 0.18 0 . 1 8 0.14 0.16 P 2 0 5 0.05 0.26 0.10 0.22 0.22 0.03 0.04 H 20~ 0. 12 0.10 0.16 0.24 0.11 0.32 0.24 H 20 + 1 .55 1 .50 0.85 1 .92 0.91 4.70 3.05 NOTES: 1) Values are i n weight percent, normalized to 100% 2) T o t a l Fe r e p o r t e d as F e 2 0 3 3) C0 2 was not determined MAJOR ELEMENT COMPOSITION WARK COMPLEX AND TERTIARY INTRUSIONS Wc irk Comp! .ex Tert I n t r i . i a r y i s i o n s Sample-* V8-m V8-f V9 RBP 37-14 S i 0 2 50.57 65. 35 45. 17 61 .84 64.90 A 1 2 0 3 15.29 13.89 16 . 91 15.63 14.46 F e 2 0 3 12.51 7.71 1 3.02 5.68 5.71 MgO 6.95 2.64 7.84 3.28 2.37 CaO 7.90 4.54 9.75 5.87 3.49 Na 20 3.14 3.58 2.88 4.20 4.49 K 20 1.16 0.92 0.52 0.98 1 .56 T i 0 2 0.86 0.44 0.96 0.65 0.40 MnO 0.09 0.18 0.17 0.18 0.18 P 2 O 5 0.25 0.18 0.22 0.16 0.10 H 20' 0.07 0.09 0. 19 0.07 0.17 H 20 + 1 .20 0.49 2.37 1 .47 2.18 NOTES: 1) Values are i n weight percent, normalized to 100% 2) T o t a l Fe repo r t e d as F e 2 0 3 3) C0 2 was not determined 134 TRACE ELEMENT COMPOSITION WESTCOAST CRYSTALLINE COMPLEX Westcoc ist Amph: b o l i t e Westcoa ist Cx-fe >lsite Sample-*- 27-22 35-19 34-3b 34-3d 30-1 1 4-5 Ba 86 72 294 976 1 486 669 Ce 8 7 17 58 1 7 23 Nb 2 3 8 6 7 9 Nd 6 3 9 27 6 1 1 Ni 56 21 1 101 6 5 1 1 Rb 5 6 35 47 46 25 Sr 353 200 372 451 249 1 94 V 324 307 272 25 1 2 12 Y 13 22 1 5 6 1 2 10 Zr 31 38 72 213 66 65 NOTE: A l l values i n ppm 135 TRACE ELEMENT COMPOSITION WESTCOAST CRYSTALLINE COMPLEX V Jestcoast . D i o r i t e D i o r i t e Gneiss Westc U l t r a ! oast ) a s i t e s Sample-* 35-17 l i s TIP WCT-3 1 1-6 1 2-2 15-12 Ba 75 563 615 547 444 1 3 57 Ce 5 34 36 26 44 3 6 Nb 2 7 8 8 8 3 4 Nd 6 1 7 1 6 1 5 21 4 2 Ni 26 9 3 1 7 29 281 196 Rb 5 39 1 6 40 18 5 6 Sr 665 429 396 341 372 46 217 V 1 30 1 74 34 236 1 23 1 43 1 10 Y 3 25 10 24 52 5 5 Zr 22 1 05 1 20 1 1 2 1 79 22 29 NOTE: A l l values i n ppm TRACE ELEMENT COMPOSITION WARK COMPLEX AND TERTIARY INTRUSIONS Wc irk Comp] .ex Tert I n t r i . i a r y i s i o n s Sample-* V8-m V8-f V9 RBP 37-1 4 Ba 534 433 253 319 838 Ce 18 30 9 30 39 Nb 4 4 .3 8 8 Nd 1 1 1 3 3 1 5 18 Ni 54 33 26 18 1 5 Rb 38 34 1 2 25 41 Sr 41 1 502 485 393 277 V 267 87 383 1 04 56 Y 25 1 1 1 1 1 3 1 6 Zr 66 1 18 29 1 25 143 NOTE: A l l values i n ppm APPENDIX 3 GEOCHRONOLOGY ANALYTICAL METHODS AND RESULTS U-Pb isotopic data Zircon Isotopic abundance,' Pb 206 = 100 Pb Ratios and ages (Ma) 2.3 Measured ( M ) (mg) (ppm) (ppm) 2 0 8 207 204 ^Pb/^Pb A D i a n K ™ 2 0 6 P b / 2 3 8 U 2 0 7Pb / 2 3 5 U 2 0 7 p b / 2 0 6 p b Sample 26-2, Indian Island i n t r u s i o n i 45-75 19. .3 255s 1 7. 72 11. . 5 7 6.236 0.0356 716 4.0 0.02968(189) 0 .2339(213) 0.05718(498) 75-150 28. .7 244.5 6. 94 12. .65 5 . 4 2 9 0.0287 1331 2.2 0.02778(177) 0 .1917(178) 0.05007(198) >150 20. ,2 216.2 6. 22 12. .85 5.547 0.0475 1 0 8 8 1 . 7 0.02800(178) 0 .1871(174) 0.04848(121) Sample 27-5, Tofino Inlet pluton 45 - 7 5 2 2 . .0 1 3 1 . 8 3 . 8 5 1 0 . . 61 6 . 2 9 9 0.0856 104 10.5 0.02852(181) 0 .1980(183) 0.05036(212) 7 5 - 1 5 0 1 6 . ,0 1 1 4 . 9 3 . 6 5 1 1 . . 0 2 6.590 0.1054 1 0 6 3. 1 0.03075(195) 0 .2137(197) 0.05041(214) Sample 11-6, Westcoast d i o r i t e gneiss <45 7. .4 6 9 4 . 9 2 0 . 2 9 1 2 . .07 5.887 0.0541 5 7 5 5.1 0.02848 (181) 0 .2000(185) 0.05094(238) 4 5 - 7 5 3 2 , .4 473.4 1 3 . 7 8 1 1 . . 5 9 5 . 9 7 6 0.0645 8 1 4 2.3 0.02843(181) 0 .1971(183) 0.05029(208) 7 5 - 1 5 0 1 6 . . 0 524.4 1 5 . 1 0 10 . . 0 4 5 . 3 7 1 0.0214 8 2 9 2 .9 0.02888(184) 0 .2013(186) 0.05058(222) Sample WCT-3, quartz d i o r i t e of Black River 45-75 2 6 . .4 321.3 1 0 . 07 1 3 . . 1 0 6 . 9 4 8 0.1344 4 9 4 3.5 0.02958(188) 0 .2027(187) 0.04973(182) 75-150 20, .5 . 287.7 7 . 6 1 1 2 . . 3 4 5 . 3 9 1 0.0197 354 6.0 Sample V 8 , Wark d i o r i t e 0.02601(166) 0 .1829(171) 0.05104(239) 45 - 7 5 1 8 . . 8 7 3 3 . 4 2 0 . 5 6 1 8 . . 8 5 5 . 5 7 7 0.0354 1 2 2 0 2.2 0.02587(165) 0 .1803(168) 0.05057(221) 7 5 - 1 5 0 12 , .4 4 8 1.0 1 3 . 5 3 17 . . 8 2 5.404 0.0239 402 5.4 Sample RBP, Catface Intrusion 0.02639(168) 0 .1839(171) 0.05053(219) 4 5 - 7 5 2 1 . 0 2 8 6 . 9 2 . 0 4 17 . . 6 4 6 . 5 7 7 0.1231 230 4.5 0.00649(41.7) 0 .0426(42.4) 0.0476(79.2) 7 5 - 1 5 0 2 2 , . 0 2 4 7 . 7 1. 6 2 14. . 7 1 5.033 0.0272 423 5.4 0.00633(40.7) 0 .0404(40.3) 0.0463(13.9) Notes: A l l s p l i t s were nonmagnetic; zircons were generally pale pink and clear although some were cloudy. 'Corrected for blank with composition ^Pb/^Pb: 37.00; 2 0 7Pb/ 2 0 4Pb: 15.57; ^ P b / ^ P b : 17.75. 2 I s o t o p i c composition of common Pb i s based upon 180 Ma Pb derived from the growth curve of Stacey and Kramers (1975) except for Sample RBP which i s based upon 40 Ma Pb. 3A 2 3 e = 0.155125 x 10"7yr.; X 2 3 5 = 0.98485 x 10'7yr.; 2 3 8 U/ 2 3 5 U = 137.88. A n a l y t i c a l techniques; Zircon separate was prepared using standard grinding and heavy mineral separation procedures. Dissolution and i s o l a t i o n of U and Pb were done using the procedure of Krogh (1973). Samples were analyzed using single Re filament and s i l i c a gel techniques on a V.G. IS0MASS 54R mass spectrometer. Data a q u i s i t i o n and reduction were accomplished on a HEWLETT PACKARD HP 85 computer. CO 139 K-Ar a n a l y t i c a l data Ar Sample Description %K (*10 cc/gm) %ZAr Date and la.error (Ma) Sample 26-2, Indian Island i n t r u s i o n IIs-Hb hornblende 1.10 7.714 94.1 172 ± 6 Sample WCT-3, quartz d i o r i t e of Black River WCT-Hb hornblende 0.869 5.076 82.3 144 ± 5 WCT-Bi b i o t i t e 2.30 15.194 84.6 162 ± 6 Sample 27-22, Westcoast Amphibolite 27-22-Hb hornblende 0.284 1.742 83.2 151 ± 5 Sample V8, Wark d i o r i t e V8-Hb hornblende 0.721 3.857 90.3 133 i 5 Sample V9, Wark d i o r i t e V9-Hb hornblende 0.138 0.998 48.5 177 ± 7 Sample RBP, Catface Intrusion RBP-Hb hornblende 0.321 0.467 48.7 37.1 ± 1.3 RBP-Bi b i o t i t e 4.77 6.380 55.8 34.1 ± 1.3 An a l y t i c a l techniques: K i s determined i n duplicate by atomic absorption using a Techtron AA4 spectrophotometer and Ar by isotope d i l u t i o n using an AEI MS - 10 mass spectrometer and high purity Ar spike. Errors reported are for one standard deviation. The constants used are: \ k 0 K t - 0.581 x 1 0 - 1 0 y _ 1 , Xk0Ko - 4.962 x I0-10 y - l p k0K/K = 0.01167 atom percent, K analyses were done by K.L. Scott, Ar analyses were done by J.E. Harakal Rb-Sr a n a l y t i c a l data Sample Description Sr (ppm) Rb (ppm) 8 7Rb/ 8 6Sr 8 7Sr/ 8 6Sr Sample 26-2, Indian Island i n t r u s i o n IIs-WR hb-bi quartz d i o r i t e 452 38.5 0.2465 0.70416 ± 6 IIs - P l plagioclase 608 28.7 0.1362 0.70374 ± 4 • IIs-Hb hornblende 66.6 6.1 0.2661 0.70418 ± 8 II s - B i b i o t i t e 123 42.8 1.0065 0.70570 ± 17 Sample WCT-3, quartz d i o r i t e of Black River WCT-WR hb-bi quartz d i o r i t e 351 37.7 0.3113 0.70408 + 4 WCT-P1 plagioclase 539 35.1 0.1886 0.70395 ± 5 • WCT-Hb hornblende 86.7 10.8 0.3590 0.70427 ± 8 WCT-Bi b i o t i t e 58.6 86.0 4.2505 0.71362 ± 6 Whole rock analyses, Westcoast C r y s t a l l i n e Com 30-1-WR trondhj emite 251 43.4 0.5008 0.70463 ± 5 TIP-WR quartz d i o r i t e 404 13.6 0.0975 0.70383 ± 7 IIs-WR quartz d i o r i t e 452 38.5 0.2465 0.70416 ± 6 WCT-WR quartz d i o r i t e 351 37.7 0.3113 0.70408 ± 4 • 35-17-WR d i o r i t e 698 2.4 0.0098 0.70365 ± 11 11-6-WR d i o r i t e gneiss 378 15.2 0.1166 0.70406 ± 3 27-22-WR amphibolite 361 4.3 0.0344 0.70344 ± 6 Sample RBP , Catface Intrusion RBP-WR quartz d i o r i t e 414 23.5 0.1637 0.70382 ± 5 RBP-P1 plagioclase 488 13.7 0.0810 0.70368 ± 4 RBP-Hb hornblende 79.8 5.9 0.2158 0.70387 ± 9 RBP-Bi b i o t i t e 35.3 144 11.823 0.71181 ± 30 Date (Ma) I n i t i a l ^Sr/^Sr Zr' I n i t i a l ^Sr/^Sr 151 ± 15 0.70356 ± 8 169 ± 3 K8.4 ± 7 0.70341 ± 5 143 ± 27 0.70360 ± 9 0.70368 ± 3 0.70354 0.70329 0.70372 Notes: A l l errors are reported at the 1 a confidence i n t e r v a l . The slopes and intercepts were calculated using a York I regression (York, 1967). Rb A 8 7 = 1.42 x I0" 1 1/yr. 1Zr i n i t i a l ^Sr/^Sr was determined by drawing the U-Pb zircon reference isochron through the whole rock point. 141 A n a l y t i c a l Techniques: Rb and Sr concentrations were determined by r e p l i c a t e analysis of pressed powder p e l l e t s using X-ray fluorescence. U. S. Geological Survey rock standards were used for c a l i b r a t i o n ; mass absorption c o e f f i c i e n t s were obtained from Mo Ka Compton scattering measurements. Rb/Sr r a t i o s have a p r e c i s i o n of 2% (1 a) and concentrations a p r e c i s i o n of 5% (1 a ) . Sr i s o t o p i c composition was measured on unspiked samples prepared using standard ion exchange techniques. The mass spectrometer, a V.G. ISOMASS 54R, has data a q u i s i t i o n d i g i t i z e d and automated using a HEWLETT PACKARD HP 85 computer. Experimental data have been normalized to a 8 6 S r / 8 8 S r r a t i o of 0.1194 and adjusted so that the NBS standard SrC0 3 (SRM987) gives a 8 7 S r / 8 6 S r r a t i o of 0.71020±2 and the Eimer and Amend Sr a r a t i o of 0.70800±2. The p r e c i s i o n of a si n g l e 8 7 S r / 8 6 S r r a t i o i s 0.00010 (1 a). Rb-Sr dates are based on a Rb decay constant of 1.42 x 1 0 _ 1 1 y - 1 . The regressions are calculated according to the technique of York (1967). F i s s i o n Track A n a l y t i c a l Data P S P i * 1 2 3 Sample Mineral ( 1 0 6 t / c m 2 ) Tracks,s ( 1 0 6 t / c m 2 ) Tracks,! ( 1 0 l V c « 2 ) n » < f l ' * > l i s z i r c o n 8.58 Indian Island i n t r u s i o n 1623 1.33 251 2.08 80 + 20 15/15 l i s a patite 0.64 475 0.26 190 2.08 31 ± 19 45/45 RBP apatite 0.20 Catface Intrusion, R i t c h i e Bay 148 0.19 141 2.08 13 ± 12 30/30 ' V spontaneous track density; Pj=induced track density; 4>=thermal neutron dose S = standard error number of grains or f i e l d s counted: s, spontaneous; i , induced decay constants: X 2 3 8 ( f i s s i o n ) = 7 . 0 0 x 10" 1 7/yr; A 2 3 8(alpha)=l.55125 x 10~ 1 0/yr; other constants: U 2 3 8/U 2 3 5=137.88; X 2 3 5=580 x 10~ 2Vm 2 143 F i s s i o n Track Dating  A n a l y t i c a l Techniques Techniques were adapted from Naeser (1976) and Gleadow and L o v e r i n g (1975). A p a t i t e and z i r c o n were dated by the p o p u l a t i o n and e x t e r n a l d e t e c t o r methods, r e s p e c t i v e l y . For a p a t i t e , two s p l i t s were made, one being annealed at 480°-520°C f o r at l e a s t two hours to remove spontaneous, n a t u r a l l y o c c u r i n g t r a c k s . T h i s annealed s p l i t was i r r a d i a t e d and subsequently mounted, p o l i s h e d , and etched together with the other s p l i t which r e t a i n e d the n a t u r a l t r a c k s . A p a t i t e e t c h i n g c o n d i t i o n s were 7% n i t r i c a c i d at 22°-24°C f o r about 30 seconds. Tracks were counted at 800x. Z i r c o n s were mounted and etched a c c o r d i n g to the procedure of Gleadow et a l . (1976) using FEP t e f l o n and a e u t e c t i c KOH-NaOH etch at 200°-210°C u n t i l n a t u r a l t r a c k s were f u l l y exposed (about 48 hours). Muscovite d e t e c t o r s were used to record the induced track d e n s i t y and were etched i n 48% HF f o r 12 minutes at 22°-24°C. Tracks i n z i r c o n were counted at 2000x in o i l . A geometry f a c t o r of 0.5 was assumed for the e x t e r n a l d e t e c t o r method. Dates were c a l c u l a t e d a c c o r d i n g to the f i s s i o n track age equation, date=ln[1 + ( p s / p i ) * ( X 2 3 8 alpha/X 2 3 8 f i s s i o n ) ( U 2 3 5 /U 2 3 8 )X 2 3 s ] x ( 1 / X 2 3 8 a l p h a ) where X 2 3 8 a l p h a and X 2 3 8 f i s s i o n are U 2 3 8 decay constants f o r alpha and f i s s i o n , r e s p e c t i v e l y , * i s the thermal neutron 144 dose, u 2 3 5 / U 2 3 " i s the atomic r a t i o of uranium i s o t o p e s , and X 2 3 S i s the c r o s s - s e c t i o n f o r neutron f i s s i o n r e a c t i o n of U 2 3 S . Thermal neutron i r r a d i a t i o n was performed i n Denver, Colorado at the USGS TRIGA re s e a r c h r e a c t o r under the s u p e r v i s i o n of C.W.Naeser and D.Rusling. Neutron dose was determined by b a c k - c a l c u l a t i o n using the counted t r a c k d e n s i t y r a t i o , p s / p i , f o r a p a t i t e and z i r c o n standards from the t u f f of F i s h Canyon assuming t h e i r age as 27.9 Ma (Steven et a l . , 1967; new c o n s t a n t s ) . Reported e r r o r s f o r dates are c a l c u l a t e d a c c o r d i n g to formulae given by Johnson et a l . (1979). 

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