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UBC Theses and Dissertations

Geology and thermal history of an area near Okanagan Lake, Southern British Columbia Medford, Gary Allen 1976

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GEOLOGY AND THERMAL HISTORY OF AN AREA NEAR OKANAGAN LAKE, SOUTHERN BRITISH COLUMBIA. by Gary A l l a n Medford B.Sc.(Hon.) M c G i l l U n i v e r s i t y 1968 M.Sc. M c G i l l U n i v e r s i t y 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e Department o f G e o l o g i c a l S c i e n c e s We a c c e p t thi.s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d . THE UNIVERSITY OF BRITISH COLUMBIA J a n u a r y , 1976 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I ag ree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d that c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depa rtment The U n i v e r s i t y o f B r i t i s h Co lumbia V a n c o u v e r 8, Canada Date i ABSTRACT F i v e phases o f d e f o r m a t i o n a r e r e c o g n i z e d i n Shuswap (Monashee Group) g n e i s s i n an a r e a e a s t o f Okanagan Lake, s o u t h e r n B r i t i s h Columbia.. The f i r s t i s d e l i n e a t e d by n o r t h t r e n d i n g m e s o s c o p i c s t r u c t u r e s . The second c o m p r i s e s a s o u t h c l o s i n g m e g a s c o p i c s y n f o r m w i t h a h o r i z o n t a l ESE a x i a l d i r e c t i o n . T h i s s t r u c t u r e has i n t u r n been c o a x i a l l y r e f o l d e d i n t o a more open phase 3 s y n f o r m . The second d e f o r m a t i o n was a s s o c i a t e d w i t h e x t e n s i v e i n t r o d u c t i o n o f s y n k i n e m a t i c q u a r t z m o n z o n i t e and g r a n o d i o r i t e t h a t c o m p r i s e s much o f t h e a r e a , and c u l m i n a t e d w i t h a m p h i b o l i t e grade metamorphism. Phase 3 d e f o r m a t i o n was f o l l o w e d by e x t e n s i v e l o c a l r e c r y s t a l 1 i z a t i o n and meta-somatism which d e s t r o y e d e a r l i e r f a b r i c elements o f t h e g n e i s s e s . Phases 1 t o 3 a r e p r e - m i d - C a r b o n i f e r o u s based on poor f o s s i l e v i d e n c e whereas phases 4 and 5 a r e T e r t i a r y . Phase 4 c o m p r i s e s open f l e x u r a l s l i p f o l d i n g about NE t r e n d i n g axes and Phase 5 c o n s i s t s o f broad warps about h o r i z o n t a l ESE axes. These d e f o r m a t i o n a l e v e n t s were a s s o c i a t e d w i t h h i g h l e v e l t hermal and h y d r o t h e r m a l a c t i v i t y which a p p e a r s t o be most i n t e n s e i n a r e a s o f h i g h grade Shuswap g n e i s s , where i t has r e s e t K-Ar d a t e s t o about 50 m i l l i o n y e a r s (paper no. 1 ) . T h e r m a l l y s e n s i t i v e f i s s i o n t r a c k a p a t i t e d a t e s i n d i c a t e t h a t t h e h i g h thermal g r a d i e n t s can be t r a c e d i n t o t h e p l u t o n i c r o c k s west o f t h e Okanagan V a l l e y i n which K-Ar d a t e s have been much l e s s a f f e c t e d and range between 130 and 200 m i l l i o n y e a r s . Thus perhaps o n l y the o l d e s t d a t e s r e p r e s e n t minimum emplacement d a t e s . The s t a t i s t i c a l methods used i n a c q u i r i n g t h e a p a t i t e d a t e s a r e d i s c u s s e d and d e v e l o p e d beyond t h a t a v a i l a b l e i n t h e l i t e r a t u r e (paper no. 2 ) . i i TABLE OF CONTENTS Page ABSTRACT i TABLE OF CONTENTS i i LIST OF FIGURES: GEOLOGY OF THE OKANAGAN MOUNTAIN AREA v LIST OF TABLES: GEOLOGY OF THE OKANAGAN MOUNTAIN AREA v i i LIST OF PLATES: GEOLOGY OF THE OKANAGAN MOUNTAIN AREA v i i LIST OF TABLES: PAPER NO. 1 v i i LIST OF PLATES: PAPER NO. 1 v i i LIST OF TABLES: PAPER NO. 2 v i i ACKNOWLEDGEMENTS v i i i A. GEOLOGY OF THE OKANAGAN MOUNTAIN AREA 1 1 - INTRODUCTION TO THE FIELD STUDY AND RELATED PAPERS 1. Gen e r a l I n t r o d u c t i o n 1 G e n e r a l Geology 8 2 - STRUCTURAL SUCCESSION 14 U n i t 1: Laminated A m p h i b o l i t e 16 U n i t 2: Hornblende ( b i o t i t e ) G r a n i t o i d G n e i s s 18 U n i t 3: Augen G n e i s s 19 U n i t 4: U n d i f f e r e n t i a t e d G r a n i t o i d P a r a g n e i s s 21 U n i t A: L e u c o - q u a r t z M o n z o n i t e 21 U n i t B: F o l i a t e d G r a n o d i o r i t e 23 U n i t C: D i o r i t e 25 U n i t D: U n f o l i a t e d G r a n o d i o r i t e 25 U n i t E: Q u a r t z M o n z o n i t e Dikes 28 U n i t F: P r o t o c l a s t i c Q u a r t z M o n z o n i t e 28 B r e c c i a s and U l t r a m y l o n i t e s 30 i i i Page 3 - STRUCTURE 31 Phase 1 ( e a r l i e s t ) 34 Phase 2 37 Phase 3 43 Subset 3a 45 Subset 3b 54 Phase 4 54 Phase 5 62 Summary 67 4 - METAMORPHISM AND THE EMPLACEMENT OF IGNEOUS ROCKS 63 E a r l y metamorphism 68 Phase 3 metamorphism 70 L a t e o r p o s t phase 3 metamorphism T e r t i a r y metamorphism 72 5 - DISCUSSION 77 C o r r e l a t i o n w i t h nearby a r e a s 77 Ti m i n g o f d e f o r m a t i o n and metamorphism 80 O r i g i n o f sediments 82 Re g i o n a l i n t e r p r e t a t i o n 83 B. PAPER NO. 1: K-Ar AND FISSION TRACK GEOCHRONOMETRY OF AN EOCENE THERMAL EVENT IN THE KETTLE RIVER (WEST HALF) MAP AREA, SOUTHERN BRITISH COLUMBIA 86 A b s t r a c t 87 I n t r o d u c t i o n 88 A n a l y t i c a l t e c h n i q u e s 88 D i s t r i b u t i o n o f K-Ar d a t e s 89 F i s s i o n t r a c k a n a l y s i s 92 C o r r e l a t i o n o f the T e r t i a r y event w i t h g e o p h y s i c a l e v i d e n c e 96 Reg i o n a l a s p e c t s 98 Summary 99 i v Page C, PAPER NO, 2; ON THE COMPUTATION OF STATISTICAL ERROR IN FISSION TRACK ANALYSIS 101 I n t r o d u c t i o n 101 F i s s i o n t r a c k age e q u a t i o n 102 T e c h n i q u e s o f f i s s i o n t r a c k a n a l y s i s 103 D. REFERENCES 116 E. APPENDIX 1: F i s s i o n - t r a c k and K-Ar a n a l y t i c a l t e c h n i q u e s APPENDIX 2: C o o l i n g - r a t e c a l c u l a t i o n s 122 126 V LIST OF FIGURES; GEOLOGY OF OKANAGAN MOUNTAIN Page 1-1 G e n e r a l i z e d g e o l o g y o f s o u t h e a s t e r n B.C. 2 1-2 S t r u c t u r a l elements o f the Canadian C o r d i l l e r a 3 1-3 L o c a t i o n map o f a r e a s o f s t u d y 9 1- 4 General g e o l o g y o f the Okanagan V a l l e y 11 2- 1 U n i t 1. Photo o f hand specimen 17 2-2 U n i t 2. Photo o f hand specimen 17 2-3 U n i t 3. Photo o f hand specimen 20 2-4 U n i t 3. Photo o f hand specimen 20 2-5 U n i t A. Photo o f hand specimen 22 2-6 U n i t A. Photo o f hand specimen 22 2-7 U n i t B. Photo o f o u t c r o p 24 2-8 U n i t B. Photo o f o u t c r o p 24 2-9 U n i t C. Photo o f hand specimen 26 2-10 U n i t D. Photo o f o u t c r o p 27 2-11 U n i t D. Photo o f hand specimen 27 2- 12 U n i t E. Photo o f hand specimen 29 3- 1 S t r u c t u r a l domains, s o u t h p a r t o f map a r e a 35 3-2 Photo o f phase 1 f o l d deformed by phase 2 f o l d . Phase 3 c l e a v a g e 36 3-3 Photo o f phase 1 f o l d i n u n i t 2 36 3-4 Photo o f hand specimen o f a phase 2 f o l d 38 3-5 T h i n s e c t i o n o f nose o f F i g . 3-4 39 3-6 T h i n s e c t i o n o f nose o f F i g . 3-5 39 3-7 S t a c k e d r o o t l e s s phase 2 i s o c l i n e s i n o u t c r o p o f u n i t 2 40 3-8 Phase 2 f o l d s i n l a m i n a t e d a m p h i b o l i t e . Hand specimen 40 3-9 Phase 2 f o l d s d e v e l o p e d i n u n i t 3 42 3-10 S t e r e o p l o t o f phase 1 and 2 s t r u c t u r e s 44 3-11 I s o m e t r i c view o f phase 2 and 3 m e g a s t r u c t u r e s 46 3-12 S t e r e o p l o t o f phase 3 s t r u c t u r e s 47 3-13 S y n o p s i s o f phase 2 and 3 a x i a l d i r e c t i o n s 48 v i Page 3 -14 P h o t o o f p h a s e 3a f o l d s i n m y l o n i t i z e d u n i t 3, O u t c r o p 50 3 -15 P h o t o . D e t a i l o f f o l d i n F i g , 3-14 50 3 -16 P h o t o o f p h a s e 3a f o l d i n a u g e n g n e i s s . U n i t 3 51 3 ; -17 D e t a i l o f p h a s e 3a f o l d i l l u s t r a t i n g c l e a v a g e F 3 a 51 3--18 P h o t o o f p h a s e 2 f o l d d e f o r m e d by p h a s e 3 f o l d i n o u t c r o p o f u n i t 1 52 3--19 P h o t o o f f o l d m u l l i o n i n a m p h i b o l i t e ( u n i t 1) o u t c r o p 52 3--20 P h o t o o f p h a s e 3a f o l d i n u n i t B 53 3--21 P h o t o o f p h a s e 3 f o l d i n u n i t B 53 3--22 P h o t o o f s u b s e t 3b f o l d i n l a m i n a t e d a m p h i b o l i t e 54 3--23 P h o t o o f p h a s e 3 s l i d e i n l a m i n a t e d a m p h i b o l i t e 55 3--24 S t e r e o p l o t o f p o l e s t o F2 and c o m p o s i t i o n a l l a y e r i n g 56 3--25 S c h e m a t i c i l l u s t r a t i o n o f p h a s e 3 s t r u c t u r e s i n u n i t 1 57 3--26 P h o t o o f p h a s e 4 f o l d 58 3-•27 S t e r e o p l o t o f p h a s e 4 d i s t o r t i o n o f p h a s e 2 e l e m e n t s 60 3-•28 S t e r e o p l o t o f p h a s e 4 s t r u c t u r a l e l e m e n t s 61 3-•29 S t e r e o p l o t o f m e a s u r e d f r a c t u r e s 63 3-•30 P h o t o o f a p h a s e 5 f o l d 64 3-•31 P h o t o o f p r o t o c l a s t i c d i k e ( u n i t F) c u t t i n g p h a s e 4 f o l d 64 3-•32 S t e r e o p l o t o f p h a s e 5 d i k e s 65 3-•33 C r o s s - s e c t i o n i l l u s t r a t i n g p h a s e 5 s t r u c t u r e s 66 4-•1 P h o t o m i c r o g r a p h o f a m p h i b o l i t e g r a d e a s s e m b l a g e i n u n i t 1 69 4- •2 P h o t o m i c r o g r a p h o f h o r n b l e n d e a l i g n m e n t p a r a l l e l t o l_2 69 4-•3 P h o t o m i c r o g r a p h o f d i o p s i d e a l i g n m e n t p a r a l l e l t o L2 71 4- •4 P h o t o o f q u a r t z m o n z o n i t e s i l l , u n i t A, i n t r u d i n g g n e i s s 71 4- •5 P h o t o m i c r o g r a p h o f s t r a i n e d q u a r t z c a u s e d by p h a s e 2 73 4- •6 P h o t o m i c r o g r a p h o f F3 s t r a i n p a t t e r n i n q u a r t z i n n o s e o f f o l d o f u n i t 3 73 4-•7 P h o t o m i c r o g r a p h o f F3 s t r a i n p a t t e r n i n q u a r t z i n g r a n o d i o r i t e , u n i t A 74 4- •8 P h o t o m i c r o g r a p h o f c h l o r i t e v e i n i n g and s e r i c i t e a l t e r -a t i o n c a u s e d by T e r t i a r y h y d r o t h e r m a l a c t i v i t y 74 4- 9 P h o t o m i c r o g r a p h o f s e n s i t i z a t i o n o f f e l d s p a r c a u s e d by T e r t i a r y h y d r o t h e r m a l a c t i v i t y 75 5- 1 C o r r e l a t i o n o f s t r u c t u r e i n t h e s o u t h O k a nagan. C r o s s -s e c t i o n 79 VI 1 L I S T OF TABLES: GEOLOGY OF OKANAGAN MOUNTAIN 3-1 N o m e n c l a t u r e o f s t r u c t u r a l e l e m e n t s L I S T OF PLATES: GEOLOGY OF OKANAGAN MOUNTAIN ( i n pockot) Map Gb. 1 G e o l o g y o f t h e Okanagan M o u n t a i n a r e a 2 V e r t i c a l c r o s s - s e c t i o n o f t h e s o u t h p o r t i o n o f t h e Okana g a n M o u n t a i n a r e a L I S T OF TABLES: PAPER NO. 1 1 P o t a s s i u m - a r g o n a n a l y t i c a l d a t a 2 F i s s i o n t r a c k a n a l y t i c a l d a t a L I S T OF PLATES: PAPER NO. 1 (4n p o c k e t ) flu? C*b > S" 1 G e o c h r o n o l o g y o f t h e K e t t l e R i v e r ( w e s t h a l f ) map s h e e t L I S T OF FIGURES: PAPER NO. 2 1 T r a c k d e n s i t y r a t i o s + 3 s t a n d a r d d e v i a t i o n s 2 O b s e r v e d v e r s u s e x p e c t e d t r a c k f r e q u e n c i e s L I S T OF TABLES: PAPER NO. 2 1 Computed e r r o r s u s i n g P o i s s o n model and e q u a t i o n 11 2 C o m p a r i s o n o f e r r o r s d e t e r m i n e d i n t h i s p a p e r w i t h t h o s e s t a t e d i n t h e l i t e r a t u r e 3 S u g g e s t e d e r r o r v a l u e s f o r d a t a p r e s e n t e d i n t h e 1 i t e r a t u r e vi i i ACKNOWLEDGEMENTS The author would l i k e to thank Dr. J .V . Ross for suggesting the f i e l d problem and supplying funds through National Research Council of Canada Grant A-2134. Thanks are also extended to Mrs. V. Bobik and Mr. J . HarakaT for assistance with the K-Ar analyses, and to Dr. P. Christopher for introducing the author to the f i s s i o n track technique. The author would l i k e to express appreciat ion to a l l those who gave of time and in terest during the preparation of the t h e s i s , espec ia l l y the technical s ta f f of the department. The author acknowledges aid provided through two National Research Council Scholarships, 1970-72, and a Univers i ty of B r i t i s h Columbia Graduate Scholar-sh ip , 1972-73. Further support was obtained through Teaching Assistantships in the Department of Geological Sciences, 1970-1974. 1 INTRODUCTION Genera l I n t r o d u c t i o n T h i s s t u d y was u n d e r t a k e n t o examine t h e s t r u c t u r e o f t h e Okanagan Mountain r e g i o n , and t h e n a t u r e , e x t e n t and s i g n i f i c a n c e o f a T e r t i a r y thermal e v e n t i n the K e t t l e R i v e r map a r e a (west h a l f ) , which i n c l u d e s t h e Okanagan Mountain r e g i o n . Okanagan Mountain i s l o c a t e d on the e a s t s i d e o f t h e Okanagan V a l l e y o f s o u t h c e n t r a l B r i t i s h Columbia i n an a r e a o f h i g h - g r a d e metamorphic r o c k s known as t h e Shuswap Metamorphic Complex ( F i g . 1-1). The Shuswap Metamorphic T e r r a i n , as o r i g i n a l l y d e s c r i b e d by G.M. Dawson (1877) , i s an a r e a o f h i g h g r a d e s c h i s t s and g n e i s s e s i n t h e v i c i n i t y o f Shuswap Lake, some 80 m i l e s n o r t h o f t h e Okanagan Mountain map a r e a . T h i s s u r v e y was f o l l o w e d by t h e p u b l i c a t i o n o f h i s "Shuswap She e t " i n 1898 (Dawson, 1898). S i n c e t h a t t i m e the Shuswap has been expanded t o i n c l u d e a b e l t o f h i g h - g r a d e ( g e n e r a l l y s i l l i m a n i t e g r a d e ) metamorphic r o c k s ( R e e s o r , 1970) o c c u p y i n g t h e s o u t h e r n p a r t o f t h e c o r e zone o f t h e E a s t e r n C o r d i l l e r a n F o l d B e l t ( F i g . 1-2) e x t e n d i n g i n t o n o r t h - c e n t r a l Washington. T h i s c o r e zone, known as t h e Omineca G e a n t i c l i n e (Wheeler, 1970) c o n s i s t s o f n o r t h e r l y t r e n d i n g groups o f metamorphic r o c k s bounded on t h e west by the p r e d o m i n a n t l y v o l c a n i c Inter-Montane Zone and on the e a s t by t h e c o m p l e x l y f o l d e d Kootenay A r c ( R o s s , 1970). D a l y (1912) i n h i s g e o l o g i c a l s u r v e y o f t h e 49th p a r a l l e l o f the North American C o r d i l l e r a , mapped m e t a s e d i m e n t a r y , v o l c a n i c , and p l u t o n i c r o c k s but he d i d n o t a s s o c i a t e them w i t h t h e Shuswap complex. In 1911, D a l y re-examined the a r e a p r e v i o u s l y mapped by Dawson and was t h e f i r s t t o e x t end t h e term Shuswap T e r r a i n t o c o v e r metamorphic and g r a n i t i c r o c k s f a r t h e r t o the s o u t h . He ( D a l y , 1915, 1917) c o n s i d e r e d the a r e a t o be a p r o d u c t o f l o a d metamorphism 2 Okanagan Lake GENERALIZED GEOLOGY SOUTHEASTERN B.C. Granitic Rock M - Tertiary Volcanics Mesozoic - Upper Paleozoic Rock Shuswap Metamorphic Complex Lower Paleozoic- Windermere Strata Belt Purcell Strata GNEISS DOMES 1. Frenchman's Cap 2. Thor-Odin 3. Valhalla FIGURE l-l 3 STRUCTURAL ELEMENTS CANADIAN CORDILLERA FIGURE 1-2 4 induced by deep b u r i a l , and resu l t ing in sch is tos i t y and bedding "which nearly always l i e r igorously p a r a l l e l " (Daly, 1915, p. 45). G i l l u l y (1934) disagreed with th is concept a f ter completing a petrofabr ic analysis of two of Daly 's samples and suggested d i f f e ren t i a l movement or shear pa ra l l e l with the layer ing to be of prime importance. Many con f l i c t s pertaining to the ages of rocks involved and the timing of metamorphic events a r ise because much of the te r ra in has been mapped only on a reconnaissance basis ( L i t t l e , 1961) and i t s structure and strat igraphy is poorly known. Correlat ion across the regional s t r i ke i s d i f f i c u l t because of the high grade of metamorphism which has ob l i te ra ted f o s s i l s and the o r ig ina l character of the rock. Ages determined by isotop ic invest igat ions are not abundant and are predominantly by the K-Ar method. These have given information on more recent metamorphic and plutonic events, but not the age of the rock involved in the metamorphism. Dawson (1898) believed the rocks of his invest igat ion to be Archean because of the i r s i m i l a r i t y to Grenv i l le Series rocks of eastern Canada, and Daly (1915) lent credence to th is suggestion by proposing that Precambrian Bel t sedimentary rocks rest unconformably on Shuswap rocks in Albert Canyon, east of Revelstoke, B r i t i s h Columbia. This unconformity was la te r disproved by Gunning (1928) although no serious opposit ion to the Shuswap as a Precambrian metamorphic te r ra in was voiced unt i l Cairnes (1939) had completed his examination of the complex. In opposit ion to Dawson and Daly, Cairnes bel ieved the rocks to be of several ages and the metamorphism related to the in t rus ion of numerous Mesozoic plutons. His c r i t e r i on for del ineat ing Shuswap rock was based on the presence of structures conformable to bedding which excluded less metamorphosed formations character ized by more upright s t ructures. The di f ferences were ascribed to var ia t ion in the depth of bur ial 5 during Mesozoic deformation and metamorphism. In the Vernon map area, Jones (1959) mapped the Shuswap as unconformably over la in by Cache Creek rocks of possible Carboniferous and Permian age. He redefined the strat igraphy and divided the Shuswap into the Mt. Ida, Monashee, and Chapperon Groups whose s t ra t ig raph ic re la t ions to one another have not been yet es tab l ished. Jones recognized younger and older periods of deform-ation and bel ieved these a l l to be pre-Permian and possibly Precambrian. Preto (1964) examined three of the areas mentioned by Jones where Cache Creek rocks might unconformably over l i e the Shuswap but was unable to f ind evidence for unconformities except at Salmon River . L i t t l e (1961), in his examination and compilation of the Kett le River (west ha l f ) map sheet, revised Cairnes' work with emphasis on strat igraphy and structure but did not obtain su f f i c i en t s t ructura l data wi thin the area to del ineate fo lds . He mapped the in t rus ives and he separated Nelson "Plutonics from the s l i g h t l y younger Va lha l la Plutonics (both of Mesozoic age) based on petro logic s i m i l a r i t y to these rocks in the i r type area some 60 miles to the east. The layered gneiss of the map area was referred to as the Monashee Group by Jones (1959) who designated the oldest rocks in his map area by th is name. At the eastern margin of the complex Hyndman (1968) noted T r i ass i c rocks of the Slocan group to have undergone the same phases of deformation as the underlying Shuswap rocks thus ind icat ing at least a p re -Tr iass ic time of sedimentation but pos t -Tr iass ic deformation. Ross (1968) suggested that Hudsonian basement rocks are involved in the core of the Frenchman's Cap gneiss dome on the eastern margin of the complex (F ig . 1-1), forming wedges in the cores of refolded recumbent i s o c l i n e s , and has 6 iden t i f i ed three phases of deformation in rocks known to be of la te Protero-zoic and ear ly Paleozoic age. He suggested that two phases of deformation and metamorphism predate the T r i ass i c (Ross, 1970). Reesor (1970) postulated the mantles of the ser ies of domes along the eastern margin of the complex to have been twice deformed in pos t -M iss iss -ipp ian , possib ly as la te as pos t -Tr iass ic t ime. The core gneiss was considered to be metasomatically derived'from sediments as old as la te Precambrian (Windermere Group). These domes have received much attent ion and include Frenchman's Cap gneiss dome (Ross, 1968; Fy les , 1970; McMi l lan, 1970, 1973; Blenkinsop, 1972) the Va lha l la gneiss dome (Reesor, 1965) and the Thor-Odin gneiss dome (Reesor, 1970; Reesor and Moore, 1971). Timing of some e a r l i e s t phases of deformation in the southwestern part of the complex has recent ly been shown to be at least pre-mid-Carboniferous (Ross and Barnes, 1972) and possibly related to the Caribooan orogeny.(White, 1959). The evidence i s based on the presence of non-metamorphosed sedimentary rocks in the v i c i n i t y of Keremeos, B r i t i s h Columbia which contain poorly preserved f o s s i l s of la te Miss iss ipp ian to ear ly Pennsylvanian age. This sequence over l ies rocks with structures s im i la r to those of the Vaseaux Formation, the most westerly exposed part of the Shuswap in the Okanagan Val ley of southern B r i t i s h Columbia (Ross and C h r i s t i e , 1969; C h r i s t i e , 1973). Farther north near Shuswap Lake, however, Fyson (1970) f inds metamorphism and four phases of deformation in rocks bel ieved to be of Paleozoic age. Isotopic work accomplished to date is not extensive. The K-Ar method has been most frequently employed and has general ly f a i l e d to give information on the emplacement ages of many rocks because of the complicated metamorphic h is tory of the t e r r a i n , and the occurrence of a widespread thermal event in 7 the ear ly Ter t ia ry . A few dates, ch ie f l y on over ly ing Ter t iary vo lcan ics , have been reported by Mathews (1964), White et a l . (1968) and Church (1970). Fa i r -bairn and Hurley (1964) attempted some Rb-Sr work but were unsuccessful because of low Rb/Sr r a t i o s . Ryan (1973) has achieved somewhat more success in the Okanagan Val ley immediately north of the internat ional border but the oldest whole-rock age obtained, 170 m i l l i on years , is c l ea r l y not that of even the la tes t pre-Carboniferous deformation (Ross and Barnes, 1972). Three occurrences of gneiss on the east margin of complex in the Revel stoke v i c i n i t y have been interpreted as c r y s t a l l i n e basement (Ross, 1968; Campbell, 1968; Giovane l la , 1968; Campbell and Campbell, 1970); these have given Rb-Sr isochrons of about 800 m i l l i on years (Blenkinsop, 1972) and a 207 206 Pb /Pb age of 722 m i l l i on years (Campbell, 1973). These ages are older than associated s t r a t i f i e d rocks and as such indicate basement. Data from the lead age suggests the system has suffered a loss of lead and hence the gneiss could be older than the quoted 722 m i l l i on years. C lea r l y , both s t ra t ig raph ic and iso top ic invest igat ions do not give a consistent and un i f ied p icture of the timing of metamorphism and deformations nor re la t i ve ages of rocks comprising the Shuswap Complex. These events may be pos t -Tr iass ic in some areas and pre-Cenozoic in others (Reesor and Moore, 1971). The only uni fy ing aspect is that a l l areas examined in deta i l d isplay at least one intense deformation resu l t ing in recumbent fo ld ing with var iab le axia l t rends, and involve amphibolite or more s p e c i f i c a l l y s i l l i m a n i t e grade metamorphism. The thesis area occupies part of the southwestern margin of the Shuswap Complex where a number of deta i led projects have been carr ied out at the Univers i ty 8 of B r i t i s h Columbia (Ross and C h r i s t i e , 1969; O k u l i t c h , 1969; Ross and Barnes, 1972; C h r i s i t e , 1973; Ryan, 1973). By c o n c e n t r a t i n g on a r e l a t i v e l y small area of the complex i t i s p o s s i b l e to avoid some o f the c o r r e l a t i o n problems a l l u d e d to i n the previous paragraphs. To t h i s end d e t a i l e d s t r u c t u r a l , l i t h o l o g i c , and i s o t o p i c i n v e s t i g a t i o n s have been c a r r i e d out. S p e c i f i c d e t a i l s o f the work are f u r n i s h e d i n the f o l l o w i n g s e c t i o n . General Geology Okanagan Mountain i s l o c a t e d near the Okanagan V a l l e y o f s o u t h - c e n t r a l B r i t i s h Columbia centered at l a t i t u d e 49°43' N and l o n g i t u d e 119°36' W. The map area, comprising about one hundred square m i l e s , i s l o c a t e d between the major Okanagan c i t i e s o f Kelowna i n the north and Pent i c t o n i n the south ( F i g . 1-3). Good access i s provided to the boundaries o f the area from both c i t i e s . A d i r t road winds through the area v i a Chute Lake, and i s commonly i n d i s r e p a i r from Chute Lake northwards. Another d i r t t r a c k v i a Wildhorse Canyon i s f r e q u e n t l y impassible a t the south end, and the road to the top of Okanagan Mountain i s usable only by f o u r wheel d r i v e v e h i c l e near the top. Access to the southern p o r t i o n o f the area i s p a r t l y f a c i l i t a t e d by the C.P.R. K e t t l e R i v e r V a l l e y l i n e which switchbacks up the mountain, as well as a few l o g g i n g and Naramata Water Board roads. The area i n which T e r t i a r y heating was i n v e s t i g a t e d i s a l s o o u t l i n e d i n F i g . 1-3. The borders correspond to those of the K e t t l e R i v e r V a l l e y (west h a l f ) f o u r m i l e g e o l o g i c a l map of L i t t l e (1961). F i e l d work was c a r r i e d out over a t o t a l period of about s i x months during the summers of 1971, 1972 and 1973 using a i r photographs and 4"=1 m i l e base maps enlarged from 1:50,000 n a t i o n a l topographic maps. The area north o f Okanagan mountain was not covered as e x t e n s i v e l y as the southern s e c t i o n because o f poor 9 10 exposure, d i f f i c u l t access and a g e n e r a l l y homogenous l i t h o l o g y . The map area forms part of the Okanagan highland created by the d i s s e c t i o n of an e a r l y T e r t -i a r y e r o s i o n s u r f a c e (Holland, 1964). Okanagan Mountain, the h i g h e s t peak i n the area, i s about 5200 f e e t i n e l e v a t i o n or approximately 4100 f e e t above the mean l e v e l of Okanagan Lake (1123'). Topography i s moderately steep w i t h i n about one or two miles o f the l a k e but f l a t t e n s above 3000 f e e t . A n o r t h e r l y trending f r a c t u r e p a t t e r n , accentuated by g l a c i a t i o n , has r e s u l t e d i n an extrem-e l y c l e f t y t e r r a i n which makes t r a v e r s i n g i n an east-west d i r e c t i o n very tedious i n many parts of the area. Evidence of g l a c i a t i o n i s present a t a l l e l e v a t i o n s i n the form of p o l i s h e d bedrock exposures and g l a c i a l d e p o s i t s rimming Okanagan Lake. The g l a c i a l h i s t o r y of the area has been d i s c u s s e d by Nasmith (1962). The area under c o n s i d e r a t i o n i s part of the Monashee Group as mapped by L i t t l e (1961). The p o r t i o n centred about Okanagan Mountain i s shown as Nelson P l u t o n i c s but i s best considered p a r t of the Monashee Group as i n t h i s study no pluton has been found. The l o c a l geology i s i l l u s t r a t e d i n F i g . 1-4. Farther south Oku!itch (1969), Ross and C h r i s t i e (1969), C h r i s t i e (1973), and Ryan (1973) have d e l i n e a t e d polyphase deformation i n the Kobau, Vaseaux, and A n a r c h i s t Formations (see Bostock, 1940, 1941a) r e s p e c t i v e l y . L i t t l e (1961) mapped the l a t t e r two formations as p a r t of the Monashee Group and these have s i n c e been found to e x h i b i t c o n s i s t e n t s t r u c t u r e as well as l i t h o l o g i c s i m i l a r i t y . The s t r u c t u r e i n the v i c i n i t y of Okanagan Mountain i s a l s o com-parable but l i t h o l o g i c c o r r e l a t i o n i s dangerous as the area i s l o c a t e d some twenty miles north. Within the l i m i t s of d e t a i l e d study to date i t appears that the Monashee group has undergone an e a r l y phase of deformation which the Kobau and a d j o i n i n g (Late P a l e o z o i c ? see Neugebauer, 1965) Old Tom and Shoe-maker Formations d i d not experience. In a l l , f i v e phases of deformation are recognized. The f i r s t three are c h a r a c t e r i s t i c a l l y i s o c l i n a l recumbent s t r u c t u r e s 11 LEGEND Sedimentary and Volcanic Lower Tertiary. 5 | Upper Triassic. Triassic. Anarchist Gp. Pennsylvanian and Permian. Cache Cr. Gp., Blind Cr. Fm. (3A). Kobau Gp. Lower Palaeozoic and/or Pre-Cambrian. Shuswap Complex, Monashee Gp. Intrusive Jurassic and/or Cretaceous. Okanagan Batholith Complex including Osoyoos(a), Similkmeen (b), Colville(c), Oliver(d), Krugerfe), Fairview(f) intrusive bodies. ————— Faults MILES 10 20 i 49c 120° GENERAL GEOLOGY of the OKANAGAN V A L L E Y FIGURE 1-4 12 of probable p re - la te -Miss iss ipp ian age (Ross and Barnes, 1972). The l as t two are open structures of Eocene age and associated with widespread block f a u l t i n g , vo lcan ic , and hydrothermal a c t i v i t y . Rocks of the map area grade eastward into a fo l i a ted granod ior i t i c te r ra in of homogenous l i t ho logy . On the west and north they appear to be fau l t bounded by Mesozoic in t rus ives of the Okanagan Composite Bathol i th (Daly, 1912; Peto, 1971, 1973a,b) with the exception of a th in s l i v e r of gneiss on the west shore of Okanagan Lake. To the south they merge with the 1 i t ho log i ca l l y s im i la r Vaseaux Formation. Early Ter t iary sediments and volcanics are present .•t in the region but are not found on the Okanagan Mountain map area. The Okanagan Mountain area has been interpreted as a gneiss dome ar i s ing from the upwelling of a granite magma within i t s core with subsequent in jec t ion into f l a t ly ing gneiss layers of the complex (Brock, 1934). Several other mountains (e.g. L i t t l e White Mountain) in the area were believed to be formed by a s im i la r mechanism. The f i r s t portion of th is thesis was thus undertaken in the ant ic ipa t ion that the dome might be explained with a set of structures consistent with those found in nearby areas which have been studied in d e t a i l . The second part of the study was i n i t i a t e d because of a 50 m i l l i on year K-Ar date obtained from Okanagan Mountain on a hornblende sample extracted from the gneiss. If the rocks concerned were deformed and metamorphosed in pre-mid-Carboniferous time (Ross and Barnes, 1972) or Jurass ic time (Monger et a l . , 1972) then the apparent age is low and has been reset in the early Ter t ia ry , coeval with extensive volcanism in the area (Mathews, 1964; Church, 1970). Because many te r t i a r y erosion surfaces are found in surrounding areas there is the suggestion that , in the absence of large displacement block fau l t ing and u p l i f t of some areas re l a t i ve to others, s i gn i f i can t heating must have 13 o c c u r r e d a t v e r y h i g h l e v e l s i n t h e c r u s t . F i s s i o n t r a c k d a t i n g and more e x t e n s i v e K - A r work was u n d e r t a k e n t o g a i n more knowledge of t h e s p a t i a l d i s t r i b u t i o n and i n t e n s i t y o f t h e e v e n t w i t h i n t h e K e t t l e R i v e r (wes t h a l f ) map s h e e t ( L i t t l e , 1961) and t o see i f t h e use o f d a t i n g methods w i t h d i f f e r e n t s u s c e p t i b i l i t i e s t o t h e r m a l r e s e t t i n g c o u l d g i v e a b e t t e r p i c t u r e o f t h e r e c e n t t h e r m a l h i s t o r y o f t h e a r e a . The r e s u l t s o f t h i s work a r e s u b m i t t e d as p a p e r #1. In t h e c o u r s e o f a n a l y s i s o f t h e f i s s i o n t r a c k d a t a i i was found t h a t i n some i n s t a n c e s e r r o r s e x i s t i n t h e s t a t i s t i c a l p r o c e s s i n g o f f i s s i o n t r a c k c o u n t i n g d a t a and i n o t h e r s improvemen ts c o u l d be made. A l t e r n a t i v e a p p r o a c h e s w h i c h d i f f e r f rom t h o s e c u r r e n t l y i n t h e l i t e r a t u r e have been d e v e l o p e d and t h e s e a r e p r e s e n t e d as p a p e r #2. 14 STRUCTURAL SUCCESSION The Monashee Group was not subdivided by L i t t l e (1961) in his reconnaiss-ance survey. It includes the Vaseaux Formation located about th i r ty miles south of Okanagan Mountain (Fig, 1-3), This formation has since been subdivided by Ross and Christ ie (1969), They have outlined a structural succession exceeding 4000 feet in present thickness. Five map units were defined of which two, an amphibolite and a schist , form l i tho log ica l ly d ist inct markers. Ryan (1973) has observed similar l i thology and structure farther south in the area bordering Osoyoos Lake. Three paragneiss units are recognized in the Okanagan Mountain map area, of which only one is dist inct ive but of l imited distr ibut ion (see Plate 1,2). This is an amphibolite, unit 1, which appears to be similar to those observed by Ross and Chr is t ie , and Ryan. No schist is found anywhere in the area. The other two units are d i f f i cu l t to distinguish and are characterized by the presence of abundant hornblende and minor biot i te in one (unit 2), and characteris-t i c large orthoclase augen in the other (unit 3). These units are confined to the southern part of the map area. The north section bordering the lake is an undifferentiated paragneiss sequence (unit 4) composed of gneiss, mylonite, and minor massive amphibolite. No major continuous units are observed here and the contact with the central granodiorite (unit B) is gradational. It is impossible to correlate any parts of unit 4 with those in the south but i t is l i ke ly that i t represents the remains of a paragneiss sequence into which abundant later synkinematic (phase 2) grano-dior i te (unit B) was intruded. The topography, for the most part, is sub-paral lel to the dip of the fo l ia t ion of the unit so very l i t t l e vert ical section 15 is exposed to view. Minor amounts of units A and C (see below are also found in this area. Intrusive igneous units are lettered A to E. Unit A is spat ia l ly confined to the area of paragneiss and is believed to be an early second-phase synkinematic intrusive of leuco-granite to leuco-quartz monzonite. The central portion of the area is composed largely of granitoid rock which probably represents a late second-phase intrusive (unit B). This unit contains scattered screens and s t r i n g e r s of amphibolite. It is impossible, however, to define any units within this re lat ively homogeneous mass of granitoid gneiss. Units C to F are cross-cutting igneous rocks whose emplacement postdated phase 2 deformation. Unit C is a dior i te s i l l complex which predates at least the fourth phase of deformation whereas unit D is a coarsely crysta l l ine grano-dior i te which postdates phase three and is a result of recrystal l izat ion and metasomatic alteration of unit B. Units E and F are leuco-quartz monzonite dikes probably of Tertiary age and believed associated with the Coryell intrusives to the east. Mylonitization has produced a mappable phase within unit 3 but is not extensive enough to delineate elsewhere in the area. Brecciation is common throughout the area and is associated with local high angle fau l ts . The absolute ages of the map units cannot be defined so they are described by numbering outwards from the core of the ear l iest recognized large scale structure (phase 2). The units are i l lustrated in Plate 2. The present thickness of units is obtained from cross sections in Plate 2. Unit 1 is 200 or 300 feet thick. Unit 2 does not exceed 600 feet and where in contact with unit 1 thins to only half that amount. Because of antipathetic variation in the thickness of units 1 and 2, the aggregate thickness of both is 16 e s s e n t i a l l y c o n s t a n t and abou t 600 - 700 f e e t . U n i t 3 i s by f a r t h e t h i c k e s t . The c o n t a c t o f t h i s u n i t w i t h one s t r u c t u r a l l y l o w e r i s no t f o u n d i n t h e a r e a and i t i s t h e r e b y known t o have a minimum t h i c k n e s s o f a b o u t 3000 f e e t . The r e l a t i o n s h i p o f t h e p r e s e n t t h i c k n e s s t o t h e o r i g i n a l t h i c k n e s s i s w e l l d i s g u i s e d by t h e r i v a l p r o c e s s e s o f t e c t o n i c t h i n n i n g d u r i n g d e f o r m a t i o n and d i l a t i o n because o f s y n k i n e m a t i c i g n e o u s a c t i v i t y . The u n i t s a r e d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n s t o g i v e a g e n e r a l i m p r e s s i o n o f t h e i r a p p e a r a n c e . D e t a i l e d i n f o r m a t i o n p e r t a i n i n g to m i c r o s c o p i c and m e s o s c o p i c s t r u c t u r e s i s g i v e n i n s e c t i o n 3 . In g e n e r a l , t h e r o c k s a r e composed o f v a r i o u s c o m b i n a t i o n s o f h o r n b l e n d e , b i o t i t e , f e l d s p a r , and q u a r t z . No o t h e r d i s t i n c t i v e m i n e r a l o g y can be used t o s e p a r a t e f o r m a t i o n s and t h i s l e a d s t o p r o b l e m s where g r a d a t i o n a l changes o c c u r . T e x t u r a l c o n s i d e r a t i o n can be i n v o k e d i n some c a s e s . U n i t 1 - L a m i n a t e d A m p h i b o l i t e T h i s u n i t i s o f l i m i t e d a r e a ! e x t e n t and f o rms a band s t r i k i n g s o u t h o f e a s t a c r o s s t h e s o u t h e r n p o r t i o n o f t h e map a r e a . I t i s e a s y t o r e c o g n i z e i n t h e f i e l d and w e a t h e r s d a r k g r e y t o r u s t y brown bu t i s o t h e r w i s e a deep g r e e n . More i m p o r t a n t l y , i t p o s s e s s e s a c h a r a c t e r i s t i c s t r i p e d a p p e a r a n c e c a u s e d by a l t e r n a t i n g l a y e r s o f h o r n b l e n d e and p l a g i o c l a s e ( F i g . 2 - 1 , 2 - 1 3 ) . B a n d i n g i s on t h e s c a l e o f f r om 1/8 t o 3 i n c h e s . The g r a i n s i z e o f t h e c o n s t i t u e n t h o r n b l e n d e i s v a r i a b l e and r a n g e s f rom 1/16 t o 1/4 i n c h . T h i s m i n e r a l commonly o u t l i n e s a p e n e t r a t i v e l i n e a t i o n bu t o f t e n i s p r e s e n t as a m o s a i c o f i n t e r l o c k i n g g r a i n s . M e s o s c o p i c f o l d h i n g e s a r e common and t h e b a n d i n g , where v e r y t h i n , u s u a l l y d e f i n e s numerous s m a l l r o o t l e s s i s o c l i n a l f o l d s . L o c a l l y t h e banded m a t e r i a l g r a d e s t o c o a r s e - g r a i n e d m a s s i v e a m p h i b o l i t e . 17 2-1 Unit 1. Laminated amphibolite. Light c o l -oured rock at l e f t i s part of a Tert iary (?) dike. Scale bar = 1 cm. 2-2 Unit 2. Hornblende-biotite granito id gneiss i l l u s t r a t i n g folded compositional layering common in th i s un i t . Scale bar = 1 cm. 18 In th in sect ion the dark bands are composed of more than 60 percent dark green hornblende with the remainder p lagioc lase (An4g). Dark brown b io t i t e may const i tu te 10-15 percent of the dark minerals and sometimes i s segregated into bands with almost complete absence of hornblende. The l i gh te r bands are composed ch ie f l y of p lagioc lase (An4g, occasional ly An5g). Many contain laths of anhedral p o i k i l o b l a s t i c diopside which is sometimes p a r t i a l l y replaced by hornblende and p lag ioc lase. Occasional ly the p lagioc lase is a l tered to c l i n o z o i s i t e and the diopside shows sporadic development of c lusters of epidote. Sphehe is pa r t i c -u la r l y abundant in some l i gh t coloured pods within the amphiboli te. These pods, which sometimes core small i s o c l i n a l f o l d s , a lso contain abundant d iopside. Mesoscopic structures are well preserved in the un i t , espec ia l l y those associated with deformation phases 2 and 3, such as the fo ld mull ion i l l u s t r a t e d in F i g . 3-19. Folds are sometimes out l ined by b i o t i t e r i ch layers wi thin other-wise massive amphibolite and wh'ere these structures are weathered out. a micaceous sheen i s imparted to the outcrop. Unit 2 - Hornblende (b io t i te ) Grani to id Gneiss, Granul i te Unit 2 is defined by the presence of 40 to 60 percent mafics with hornblende predominating over b i o t i t e as well as the common presence of macroscopic root less i soc l ines of between a few inches and a few feet in s i z e . The leucocrat ic con-st i tuents normally include modal orthoclase as well as p lagioclase and quartz. The percentages, however, are var iab le . The uni t weathers dark brown but i t often possesses a pink hue where orthoclase i s abundant. For the most part the unit is coarsely c r y s t a l l i n e and does not readi ly preserve structures such as f o l i a t i o n and cleavage caused by post phase 2 deformation. A photograph of a sample from th is Textural term having no metamorphic fac ies connotation. 19 unit is given in F i g , 2-2 as well as F i g . 3-7. No other d i s t i n c t i v e minerals were observed. The uni t i s approximately granodior i te in bulk composition but i t s provenance cannot be ascerta ined. The abundant folded mafic layer ing strongly suggests a metasedimentary ancestry but the p o s s i b i l i t y of metamorphic segregation layer ing cannot be discounted. A var ia t ion of th is uni t includes rock containing th in b i o t i t e -r i ch layers separated by quartz-p lagioclase r i ch bands. Such outcrops contain small fo lds which tend to weather out in r e l i e f as opposed to the aforemen-tioned c r y s t a l l i n e material which weathers uniformly (see F i g . 3-4). Unit 3 - Augen Gneiss The sa l ien t di f ference between th is unit and uni t 2 as far as f i e l d recognit ion i s concerned i s the abundance of orthoclase augen (F ig . 2-3). Although about 50 percent of th is material appears dark grey to b lack, the mafic mineral content is r e l a t i v e l y low. Hornblende i s not common. The darker regions are general ly composed of strained and rec r ys ta l l i zed quartz and feldspar rimmed by a th in network of b i o t i t e and minor ch l o r i t e . The matrix fe ldspar is almost en t i re l y orthoclase (with plagioclase An25-35) as are the porphyroclasts. The l i gh te r bands are those in which quartz and feldspar predominate and the grain s ize is sometimes larger in these bands. The observed f o l i a t i o n is associated with t ransposi t ion developed ch ie f l y during the second phase of deformation. Highly appressed remnant fo ld hinges are sometimes found along with a penetrative l i nea t ion associated with hornblende growth, alignment of fe ldspar porphyroclasts, and quartz rodding. This uni t is extremely mylonit ized in one area and forms a mappable band (Plate 1 ) . A photo from th is band is presented in F i g . 2-4. 20 2-3 Unit 3. Augen gneiss. Plane of photo is perpendicular to Lg. Despite the abundant dark material the mafic mineral content is low ( = 10%). Scale bar = 1 cm. 2-4 Unit 3. More intensely mylonit ized var ie ty of the augen gneiss. 21 Unit 4 - Undif ferent iated Granitoid Paragneiss Unit 4 is a combination of grani to id gneiss , amphibolite lenses and bands, mafic screens, mylonite and u l t ramyloni te* . No continuous l i tho logy was found and hence no attempt was made to examine the area in as much deta i l as in the south. Unit A - Leuco-quartz monzonite Unit A i s a ser ies of leuco-quartz monzonite s i l l s emplaced during the l a t t e r stages of the second phase of deformation. As such they are strongly fo l ia ted and penetrat ively l ineated by the development of quartz rodding and the alignment of elongate orthoclase megacrysts. The rock i s composed of 35 to 50 percent quartz, the remainder being feldspar of which s i g n i f i c a n t l y more than half is or thoclase. The plagioclase is about An35. Two textural va r ie t ies are present: The porphyr i t ic var ie ty (F ig . 2-5) contains potassium feldspar mega-crysts which appear to be replacements of o r ig ina l p lagioc lase phenocrysts. Where these c las ts are in contact with p lag ioc lase , myrmekitic intergrowths are present. The other var ie ty (F ig . 2-6) is s im i la r but has smaller fe ldspar gra ins. It may simply be a more highly myloni t ic form of the same rock. The var ie ty with the large feldspar c las ts is dominantly pink whereas the other has a cream to bleached white colour. Unit A occurs as d iscrete s i l l s (Plates 1,2) but is more pervasive than indicated on the map. Close examination of some of the areas of unit 2 and 3 reveals that th is material is disseminated throughout the country rock. Because of th is un i t ' s rather p a l l i d and subdued colour , i t i s frequently d i f f i c u l t to perceive. *Nomenclature according to Higgins, 1971. 22 2-5 Unit A. Porphyr i t ic var ie ty of leucoquartz monzonite. Plagioclase phenocrysts have been almost en t i re ly replaced by or thoclase. Stained for pot-assium. Light grey i s or thoc lase, white is p lag io-c lase . Scale bar = 1 cm. 2-6 Non-porphyrit ic var ie ty of the above. Lower slab has been stained for potassium. Light grey is or thoclase, white is p lag ioc lase. Scale bar = 1 cm. 23 In the s i l l s of un i t A, most of the fe ldspar reveals i nc ip ien t s e n s i -t i z a t i o n and randomly d is t r ibu ted secondary f ractures f i l l e d with epidote, . which imparts a d i s t i n c t greenish cast to the rock. This a l t e ra t i on i s believed to have occurred much l a te r than emplacement and to be associated with a Ter t ia ry thermal event. Unit B - Fo l ia ted Granodior i te This grey to cream-white un i t covers the centra l part of the map area (Plate 1) . I t consists predominantly of f o l i a t e d hornblende granodior i te and grades to hornblende quartz monzonite in areas where large subhedral porphyro-b lasts comprise much of the rock. The porphyroblasts, along with elongate hornblende, give r i se to a penetrat ive l i nea t ion associated with the second phase of deformation and l i e r igorous ly wi th in a f o l i a t i o n plane otherwise defined by l e n t i c l e s of fe ldspar and quartz. The texture of th is un i t i s i l l u s t r a t e d in F i g . 2-7 , 2-8. Mineralogy of th i s un i t i s simple and consis ts mainly of o r thoc lase, p lagioc lase (An25-35), and quartz. Hornblende i s the p r inc ipa l mafic c o n s t i -tuent with b i o t i t e absent or in low abundance. Sphene, apa t i t e , and z i rcon are common accessories and diopside a f ter hornblende i s noted occas iona l l y . In the area bordering uni ts to the south b i o t i t e appears to increase in quanti ty but does not produce a d i s t i n c t l y mappable phase. In add i t i on , the abundance of large orthoclase c rys ta l s appears greater wi th in th i s region but more deta i led mapping would be necessary to substant iate t h i s observat ion. Dark grey to black screens up to a foot th ick are found occas iona l ly wi th in 24 2-7 Unit B. Plane of photograph i s para l le l to F 2 . The outcrop is f i l l e d with large crys ta ls of orthoclase which trend 110°. 2-8 Non-porphyrit ic var ie ty of unit B. Plane of photograph is para l le l to F?. Hornblende mineral l ineat ions trend 110°. Pod'of amphibolite is t run-cated by movement on one of many northerly trending f rac tures . 25 th is un i t and cons is t pr imar i ly of hornblende and b i o t i t e , They are conformable with the f o l i a t i o n of the granodior i te but are much more deformed i n t e r n a l l y , The f o l i a t i o n developed wi th in the screens p a r a l l e l s that of the surrounding granodior i te , Unit C - D io r i te Unit C i s a s i l l complex cons is t ing of d i o r i t e which has been intruded pa ra l l e l to the f o l i a t i o n developed in the second phase of deformation. The rock i s equigranular, of g ra ins ize about 1/16", and cons is ts of an unstrained mosaic of p lag ioc lase (An^ ) and hornblende with about one percent i n t e r s t i t i a l oxides. These s i l l s have been affected by the fourth phase of deformation but have apparently developed no observable associated penetrat ive s t ruc tures. The un i t , which has a cha rac te r i s t i c s a l t and pepper texture (F ig . 2 -9) , often does not occur in large enough bodies to map. A few of these s i l l s are indicated on Plate 1. Unit D - Unfol iated Granodior i te Unit D i s of l imi ted d i s t r i b u t i o n and appears to be a resu l t of s t a t i c r e c r y s t a l l i z a t i o n or metasomatism of un i t B, and contacts with un i t B are gradational over several fee t . I t possesses no penetrat ive f ab r i c but commonly includes d isor iented s t r ingers of amphibolite (F ig . 2-10). F i g . 2-11 portrays the texture and gra ins ize of the mate r ia l . The sample contains 10 to 15 percent hornblende, 10 percent quartz and about equal amounts of p lag ioc lase (An^ ) and or thoclase. Sphene and apat i te are accessor ies. The average gra in s i z e exceeds 1/8". 26 2-9 Unit C. D io r i t e . Equigranular unstrained mosaic of hornblende and p lag ioc lase. Scale bar = 1 cm. 27 2-10 Unit D. At top of Okanagan Mountain. D is -oriented st r ingers of amphibolite suggest attainment of a l i qu id phase. 2-11 Unit D. Recrys ta l l i zed or remelted port ion of unit B i l l u s t r a t i n g the g ran i t i c texture which has succeeded strongly fo l i a ted ( F 2 ) granodior i te . 28 This material forms a few small- patches in the map area, one of which i s at the centre of Okanagan Mountain. Brock's (1934) in terpretat ion of a pine-tree structure for the mountain was influenced strongly by the presence of th is mater ia l . Unit E - Quartz Monzonite Dikes Unit E consists of quartz monzonite dikes which range in colour from pink to white and are believed related to the Coryel l plutonics ( L i t t l e , 1961). The sample shown here (F ig . 2-12) consists of phenocrysts of p lagioclase (An 24) up to 1" in length commonly almost completely replaced by or thoclase. These two minerals const i tute about 80 percent of the rock and quartz the remainder. Diopside and sphene are present in trace quant i t ies . Apparently the surrounding rocks became hot during emplacement as there are no c h i l l e d contacts and where the laminated amphibolite has been .intruded, local d i s to r t i on and contort ion of the layer ing has occurred (F ig . 2-13). Unit F - Pro toc las t ic Quartz Monzonite This unit is found as a ser ies of shallow south dipping d ikes , dark grey in appearance and frequently very d i f f i c u l t to d is t ingu ish from the host rock at a glance. The rock possesses a very strong f o l i a t i o n and l inea t ion formed by mineral alignment and is composed of about 40 percent p lagioclase (An3g), 25 percent or thoclase, and 25 percent quartz. Oxides account for about 5 percent of the volume and secondary mica rimming grains the remaining 5 percent. An example of th is material i s i l l u s t r a t ed in F ig . 3-3V. 29 2-12 Unit E. Tert iary (?) dike intruding lamin-ated amphibolite (see below). Stained for potassium. Dark grey is or thoc lase, white is p lag ioc lase. Scale = 1 cm. 2-13 Unit 1. Laminated amphibolite shouldered aside and deformed by invading Ter t iary (?) d ikes. 30 Breccias and Ultramylonites Microbreccias derived from the various grani to id rocks in the area are common. They are observed where fau l ts are present and are otherwise l o c a l l y associated with NNE trending f ractures which are found throughout the area. Just northeast of Squally Point they are espec ia l l y abundant and coincident with extensive development of f rac tu r ing . Mylonites and ul tramylonites are present and have been noted espec ia l l y in the northwest part of the map area. Hand specimens of th is material are dark grey to black, possess a subconchoidal f racture and may well be of pseudotachyl i t ic o r i g i n , perhaps a resu l t of shearing in the waning stages of phase two deformation or associated with f l e x u r a l - s l i p fo ld ing in the Ter i ta ry . 31 STRUCTURE Five phases of deformation are recognized in the Okanagan Mountain map area. Most information i s derived from the layered rocks south of Okanagan Mountain as st ructura l features are more p len t i fu l within these units than in the homogenous grani to id ones. This sect ion out l ines the evidence for the interpreted sequence of s t ructura l events and gives a descr ipt ion of the geometries and interference re la t ionships of the successive phases of fo ld ing . The ea r l i es t recognizable deformation (phase 1) resulted in i soc l i na l fo ld ing about northerly trending axes, This was fo l lowed, perhaps c lose ly in time, by i s o c l i n a l fo ld ing about a west-northwesterly axis (phase 2) and extensive late-kinematic in t rus ion of grani to id rock. Amphibolite grade metamorphism was attained during phase 2 deformation. Phase 3a folds developed approximately co -ax ia l l y with those of phase 2 about shallow south dipping axia l planes. Folding was considerably more open in s t y l e . Subset 3b fo lds formed la te r about north dipping ax ia l planes. Phase 4 resulted in concentric fo lds about north-northeasterly axes with near ve r t i ca l ax ia l planes sometimes delineated by c lose ly spaced f rac tures . F i n a l l y , and perhaps coeval ly with phase 4 deformation, warping about a west-northwesterly axis (phase 5) in te r -fered with phase 4 fo lds to impart a crude domical shape to the area. The sequence of deformation is deduced, in part , from interference re lat ionships of mesoscopic fo lds . Evidence of a l l periods of fo ld ing i s not present at any one locat ion and so fo lds associated with a par t i cu la r s t ruc-tural event must of necessity be related by s ty le and or ien ta t ion . In th is way fo lds and associated fab r i c elements have been organized into groups which are considered representative of a spec i f i c period or phase of deform-at ion . The phases of deformation may or may not have overlapped in time. 32 The e a r l i e s t recognisable period of deformation (phase 1) i s by fa r the most poorly represented and resolved in the preceding scheme. The or ien ta t ion of associated mesoscopic s t ruc tures , however, i s consistent with those of the e a r l i e s t phase of deformation out l ined by Ross and Ch r i s t i e (1969), Ch r i s t i e (1973), and Ryan (1973). The sequence of development of phases 2, 3 and 4 , on the other hand, i s read i l y deduced. Warping associated with phase 5 deforma-t ion cannot d e f i n i t e l y be resolved as post-phase 4 from information obtained wi th in the map area, although evidence presented la te r in th is sect ion strongly supports such an in te rp re ta t ion . In add i t i on , the or ien ta t ion and s t y le are s im i l a r to fo lds mapped by C h r i s t i e (1973) in an adjoining area where the sequence can be demonstrated. A resume'of the s t ruc tura l elements, t he i r o r ien ta t ion and nomenclature i s presented in Table 3-1. The presence of o r ig ina l bedding, F Q , i s quest ion-able as no r e l i c t sedimentary structures are found. Best examples of possib le sedimentary layer ing are found in un i t 2 (F ig . 3-5,6) and consis t of th in b i o t i t e - r i c h bands a l ternat ing with quar tz-p lag ioc lase layers . The lamina-t ions wi th in un i t 1 may also represent primary (igneous ?) layer ing but the p o s s i b i l i t y of metamorphic segregation cannot be ruled out. Other planer structures F-j-F4, are defined in Table 3-1 and elaborated upon in the sect ions that fo l low. Mesoscopic l inea t ions L-j-L4 include three types: those formed by mineral growth (mostly hornblende and or thoc lase) , fo ld hinges of minor f o l d s , and crenulat ions or r e l i c t micro-hinges. Megascopic fo lds associated with phases 2 and 3 can only be del ineated in the layered uni ts in the southern part of the map area. There i s thus a l imi ted region in which data have been acquired for domain ana l ys i s . S p e c i f i c -a l l y , domains homogenous with respect to the e f fec ts of phases 2 and 3 can be 33 TABLE 3-1 DEFORMATION AND AXIAL TREND STYLE MEASURED STRUCTURES FQ - o r ig ina l bedding (?) D] N-S Recumbent, i soc l i na l L-| - crenulat ions, mineral growth, folds (?) fo ld hinges D2 E-SE Recumbent, i soc l i na l folds L 2 - mineral growth, mainly hornblende and orthoclase - quartz rodding, fo ld hinges. F 2 - ax ia l planes minor f o l ds , cleavage. Associated f o l i a -t ion in igneous un i ts . D3 E-SE Recumbent, more open fo ld ing than D2 L3 - fo ld hinges F3A- ax ia l planes minor f o l ds , cleavage. F3g- ax ia l plane minor fo ld conjugate (?) to F3A D4 N-NE Upright f l e x u r a l - s l i p open warps s t a t i s t i -c a l l y def ined, and t igh ter mesoscopic fo lds . L4 - fo ld hinges, s t a t i s t i c a l l y defined fo ld axes. F4 - ax ia l planes. D5 E-SE F lexu ra l - s i i p open warps, s t a t i s t i c a l l y def ined. L5 - s t a t i s t i c a l l y defined mega-scopic fo ld ax i s . F5 - estimated from megascopic fo ld . 34 out l ined by use of the ax ia l traces of major phase 2 and 3 fo lds (F ig . 3-1) as domain boundaries. Because of access d i f f i c u l t i e s and poor outcrop only two of these areas (1 and 2) permitted acqu is i t ion of a reasonable number of s t ructura l measurements. In add i t i on , phase 4 fo ld ing i s extremely well developed within the region, and, because of the presence of fo lds such as i l l u s t ra ted in F ig . 3-26, early s t ructura l elements have been reor iented. Phase 5 has also contributed to the i r reor ientat ion but to a lesser extent. Thus, e f fec t i ve domain analysis is precluded and reor ientat ion of ea r l i e r structures by successive phases of fo ld ing cannot be defined r igorous ly . Further information pertaining to the above i s presented in the fol lowing sections in which the f i ve phases of deformation are described in chronolog-i ca l order. The associated emplacement of igneous rock into the succession and concommitant metamorphism are discussed in a la te r sect ion. Phase 1 (ea r l i es t ) Phase 1 is poorly defined and is delineated by the presence of north trending penetrative mineral l ineat ions and crenulat ions found on f ine-grained and micaceous compositional layer ing. These elements are not abundant and are found only within units 1 and 2. L ineat ions, L ] , are plotted with st ructura l data associated with phase 2 deformation in F ig . 3-10 and trend approximately 010°. Because measurements of L] at the noses of phase 2 minor fo lds were not obtained, and because of l imi ted data, no systematic reor ien-tat ion of L-| by phase 2 deformation can be deduced. S im i la r l y the ef fect of phase 3 cannot be assessed because L-j measurements have not been observed in domains 2 and 4. Figure 3-2 i l l u s t r a t e s a refolded set of phase 1 fo lds in the core of STRUCTURAL DOMAINS -south part of map area-36 3-2 Phase 1 fo lds in the core of a phase 2 i soc l i ne developed in unit 2. F3 cleavage is f a i n t l y v i s i b l e . Facing east. 3-3 View along hinge of phase 1 fo ld developed in c r ys ta l l i ne part of uni t 2. Facing south. Axis trends 188°, ax ia l plane dips west because of phase 4 fo ld ing . 37 a recumbent phase 2 i soc l i ne in uni t 2. The s ty le of these fo lds (F ig . 3-3) i s s im i la r to those described below which developed in phase 2 deformation. The axis of the fo ld i l l u s t r a t e d in F i g . 3-3, however, is oriented almost at r ight angles to the ax ia l d i rect ions of nearby phase 2 fo lds . Ross and Chr i s t i e (1969) and Chr i s t i e (1973) have mapped a megascopic phase 1 fo ld which closes to the west and brings about repe t i t ion in the succession near Vaseaux Lake (F ig . 1-3), and Ryan (1973) has out l ined a few large, poorly defined phase 1 s t ructures. The absence of d i s t i n c t i v e l i t h -ology and l imi ted area of outcrop of layered rock precludes such success in the Okanagan Mountain map area. Nevertheless, the evidence obtained does support the presence of pre-phase 2 deformation s im i la r in or ientat ion to that out l ined in nearby map areas. Destruction of most of the early structure of the area by subsequent deformation has thwarted any attempt to out l ine the geometry of phase 1 st ructures. Phase 2 Phase 2 fo lds are recumbent i soc l ines found in a range of s i z e s . The largest is a south-closing synform (Plate 2) at least four miles in amplitude, whereas the smallest fo lds have amplitudes measuring but a few inches. Meso-scopic fo lds are best represented in uni t 2 where compositional layer ing ( F Q / F - , ) i s well developed (see sect ion 2) . Many fo lds weather out and are s imi la r to that i l l u s t r a t ed in F ig . 3-4 (these are c lass Ic according to the geometrical c l a s s i f i c a t i o n of Ramsay, 1967). Here the fo ld is defined by th in lamellae of brown b io t i t e in a quartz-p lagioc lase r i ch rock (F ig . 3-5, F ig . 3-6). Axial plane f o l i a t i o n (F 2 ) is v i s i b l e in th in sect ion but is not read i ly observed in hand samples. F i g . 3-7 i l l u s t r a t e s a stacked set of 39 3-5 Thin sect ion of nose of fo ld i l l u s t r a t ed in F ig . 3-4. Cut perpendicular to L.£. Dark bands are b i o t i t e r i c h . F 2 i s well d isplayed. Cross polar-ized l i g h t . F ie ld of view measures 4 cm across. 3-5 Close-up of hinge of fo ld in F ig . 3-5. Bio-t i t e has not general ly grown para l le l to F 2 but rather is bent around the nose of the fo ld and thus may represent layer ing F O / F T . Cross polar ized l i g h t . F ie ld of view measures 5 mm across. 40 3-7 Stacked rootless isoclines developed in the crystal l ine part of unit 2. Facing east. 1 ) . Folds commonly contain cores of l ight coloured material composed mainly of plagioclase and diopside. 41 r o o t l e s s i s o c l i n e s i n the more c r y s t a l l i n e p a r t of u n i t 2. Phase 2 f o l d s d e v e l o p e d i n u n i t 1 a r e s i m i l a r i n appearance, w i t h more a n g u l a r hinges i n most c a s e s . They a r e sometimes c o r e d by d i s c o n t i n u o u s pods and l e n s e s o f i l i g h t e r c o l o u r e d p l a g i o c l a s e - d i o p s i d e a g g r e g a t e s ( F i g . 3-8). In g e n e r a l the f o l d s have subrounded t o s u b a n g u l a r h i n g e s , a re near-i s o c l i n a l and a r e f r e q u e n t l y r o o t l e s s , making i t i m p o s s i b l e t o deduce sense o f movement. F 2 c l e a v a g e i s not w e l l d e v e l o p e d and i s p r o b a b l y o f t e n i n d i s -t i n g u i s h a b l e from c o m p o s i t i o n a l l a y e r i n g (Fg/F-|) because o f extreme t r a n s -p o s i t i o n . Where s e n s i b l y d e v e l o p e d , i s d e f i n e d by p o r p h y r o b l a s t i c or p o r p h y r o c l a s t i c o r t h o c l a s e , and mica. I t i s thus e x t r e m e l y p e n e t r a t i v e and o u t l i n e s an e a s t - s o u t h e a s t e r l y p l u n g i n g l i n e a t i o n , L 2 , d e f i n e d by e l o n g a t e m i n e r a l growth, q u a r t z r o d d i n g , and i n t e r s e c t i o n o f F 2 w i t h F Q / F - | . M e s o s c o p i c f o l d c l o s u r e s a r e seldom found i n u n i t 3. Where ob s e r v e d they a r e d e f i n e d by t h i n t i g h t l y f o l d e d d i s c o n t i n u o u s s t r i n g e r s of f e . l d s -p a t h i c m i n e r a l s ( F i g . 3-9). The pre p o n d e r a n t m y l o n i t e w i t h i n t h i s u n i t i s one r e a s o n f o r t h e l a c k of minor f o l d s and i s a r e s u l t o f i n t e n s e phase 2 d e f o r m a t i o n . T h i s d e f o r m a t i o n has r e s u l t e d i n a s t r o n g p e n e t r a t i v e l i n e a t i o n , L 2 , d e f i n e d by e l o n g a t e f e l d s p a r p o r p h y r o c l a s t s , o c c a s i o n a l hornblende m i n e r a l l i n e a t i o n , and s t r e a k i n g o f m i n e r a l s . W i t h i n u n i t B which c o v e r s t he c e n t r a l p a r t of the map area no phase 2 c l o s u r e s a r e found as t h i s m a t e r i a l was i n t r o d u c e d as a l a t e k i n e m a t i c i n t r u s i o n . N e v e r t h e l e s s , a s t r o n g f o l i a t i o n ( F 2 ) and a p e n e t r a t i v e l i n e a t i o n ( L 2 ) formed by the growth o f t a b u l a r and e l o n g a t e m i n e r a l s i s found i n t h i s a r e a . The r e l a t i o n s h i p between t h i s f a b r i c and phase 2 d e f o r m a t i o n i s deduced by the p a r a l l e l i s m o f the f o l i a t i o n p l a n e t o the a x i a l plane o f phase 2 f o l d s i n a d j o i n i n g l a y e r e d u n i t s . S i m i l a r l y t he l i n e a t i o n i n u n i t B, which i s 42 3-9 R o o t l e s s i s o c l i n e s d e v e l o p e d i n u n i t 3. Phase 3 f r a c t u r e c l e a v a g e i s v i s i b l e . Photo measures about 15 cm a c r o s s . F a c i n g e a s t . » 43 usual ly a resu l t of alignment of hornblende and orthoclase megacrysts, i s para l le l to the ax ia l d i rec t ions of minor f o l d s . Although phase 3 fo lds are essen t ia l l y coaxial with phase 2 f o l d s , the plane F 2 in uni t B is observed to be folded into phase 3 folds and so no ambiguity ex is ts in assigning th is d i s t i nc t i ve f o l i a t i o n and l inea t ion to phase 2 of deformation. The present dip of F 2 for the megascopic synform in the southern part of the area is about 20° south. In F ig . 3-10 poles to ax ia l planes of phase 2 fo lds in domain 1 (F ig . 3-1) are plotted (open c i r c l e s ) and poles of measurements of F 2 from unit .B north of the phase 5 megafold (Plate 1) are plotted as dots. These poles form two d i s t i n c t groups which are a resu l t of phase 5 fo ld ing . Furthermore they show a spread caused by phase 4 fo ld ing about the indicated axis obtained from st ructura l analysis in F i g . 3-28. This spread i s more pronounced in the data from the south dipping layered rock, and in the f i e l d th is observation i s corre lated with higher pe r iod ic i t y in the development of phase 4 f o l d s . L 2 from the north and south sides of the phase 5 megafold which passes through the top of Okanagan Mountain are intermixed on the stereoplot and, indeed, systematic reor ientat ion of l_2 by Lg would be ant ic ipated to be minimal compared with measurement error as both l ineat ions are essen t ia l l y coax ia l . On the s tereoplot , those L 2 which plunge steeply are measurements that have been taken from the f lanks of phase 4 fo lds . In sp i te of reor ientat ion of L 2 by la te r f o l d i n g , the ax ia l d i rec t ion of phase 2 structures can be seen to be approximately 105°. The ax ia l plane of the megascopic phase two synform now dips south at about 20°. Phase 3 Phase 3 fo lds are wel1-developed in the layered rocks of the southern Phases 182 a L i from domain I + L 2 north of phase 5 mega-fold n H n II II F 2 * L 2 south of phase 5 mega-fold r- II II II II II o p 2 FIGURE 3-10 45 port ion of the map area but are not as read i l y perceived in the homogeneous widespread granodior i te (uni t B) . For convenience of descr ip t ion they can be div ided into subsets 3a and 3b, of which subset 3a i s more common. Subset 3a. The megascopic expression of th is fo ld set i s the Robinson Creek synform, defined by well-developed congruent minor fo lds on both limbs (F i g . 3-11). The ax ia l surface dips 20-30° to the south-southwest (F ig . 3-11, Plates 1,2) and has an ax ia l d i r e c t i o n , L 3 , s im i l a r in d i rec t ion to L 2 (about 105°). As with phase 2, l a te r deformations have caused reor ienta t ion of data. This reor ienta t ion i s best displayed by examination of the d i s t r i b u t i o n of poles to ax ia l planes of observed phase 3 minor f o l d s . The measurements were taken from the lower limb of the Robinson Creek synform, which are homogeneous with respect to phase 2 and 3 megafolds (domain 1) . The data i s p lot ted in F i g . 3-12 which a lso i l l u s t r a t e s the theore t ica l d i s t o r t i on caused by phase 4 and phase 5 f o l d i ng . The scat ter of poles appears to be accounted for by these two phases of deformation. The pos i t ion of measured phase 3 fo ld hinges (F ig . 3-12), when compared with s im i l a r phase 2 data (F ig . 3-10), d ic ta tes that the fo ld ing associated with phases 2 and 3 of deformation must be considered coaxial wi th in the bounds of measurement e r ro r , and scat ter caused by l a te r deformation (Fig,. 3-13). Minor fo lds associated with th i s deformation are considerably more open than phase 2 fo lds and are equal ly wel l represented in a l l the paragneiss un i t s . As a consequence of t he i r open form, the sense of movement can almost always be deduced whereas th i s i s never so with phase 1 and 2 mesoscopic f o l d s . In uni ts 2 and 3 the fo ld hinges are general ly rounded to subrounded (F ig . 3-14 FIGURE 3-12 FIGURE 3- 13 49 to 3-17). Ax ia l -p lane cleavage, F 3 a , i s general ly v i s i b l e only as d iscon t in -uous f ractures (F ig . 3-2, 3-15, 3-17), Measured l inear s t ruc tures, L.3a, are from the hinges of th i rd phase minor f o lds . These fo lds are not characterized by mineral growth para l le l to L.3a (e .g . hornblende mineral l ineat ion) as in phase 2 deformation. Unit B also exhib i ts fo lds about F 3 a , but the lack of compositional layer ing makes the i r presence more d i f f i c u l t to perceive (F ig . 3-20). In some instances they are out l ined by a l k a l i fe ldspar porphyroblasts (F ig . 3-21). The s ty le of phase 3 fo lds in unit 1 d i f f e r s markedly. They are general ly extremely open, large arcuate fo lds (F ig . 3-18) terminated by s l i d e s * (F ig . 3-23) and frequently weather out as fo ld mull ion (F ig . 3-19). The s l ides are believed associated with formation of subset 3b fo lds and the i r formation i s discussed in the fo l lowing sec t ion . Most of the southern part of the map area i s part of the lower limb of the Robinson Creek synform (F ig . 3-11) and contains minor fo lds congruent with the megascopic f o l d . Towards the core the sense of movement is l os t but the fo lds are not appreciably t ighter here than elsewhere. On the upper limb the fo lds are s im i la r in appearance to those on the lower limb but with the opposite sense of movement. Poles to ax ia l plane surfaces of phase 2 structures (F 2 ) and compositional layer ing (FQ) were taken from domains 1 and 2 and plotted in F ig . 3-24 in order to examine the reor ientat ion of these structures by phase 3 fo ld ing . These poles do not define a great c i r c l e about L 3 and, as i s the case with phase 3 ax ia l surfaces (F ig . 3-12), better r e f l ec t the ef fects of phases 4 and 5 fo ld ing . * a fau l t formed in close connection with f o l d i ng , conformable with the fo ld limb or ax ia l surface; Fleuty (1964). 50 3- 14 Phase 3a folds deform myloni t ic laminat ion/ compositional layer ing of unit 3. Facing west. Sense of movement indicates lower limb of the Robinson Creek synform which closes to the south. 3-15 Detai l of F ig . 3-14 i l l u s t r a t e s the l imi ted development of F3 a in th is un i t . Folded layers are compositional l ayer ing , myloni t ic laminat ion, or both. 51 3-]6 Phase 3a fo ld developed in augen gneiss, unit 3. Light coloured bands occasional ly preserve phase 2 fo lds . Facing east. 3-17 Detail of phase 3a fo ld in unit 3. Axial plane cleavage is f a i n t l y v i s i b l e . 52 3-18 Phase 2 fo ld in amphibolite taken around a broad open phase 3a fo ld in uni t 1. Photo facing east. 3-19 Development of fo ld mull ion in amphibolite of uni t 1 by phase 3 fo ld ing . Hinges trend about 110°. 53 3-20 Fain t ly v i s i b l e phase 3a fo ld in unit B formed by fo ld ing of e a r l i e r f o l i a t i o n , F 2 . Hammer l i e s along ax ia l plane. 3-21 Phase 3 fo ld in unit B accentuated by growth of orthoclase porphyroblasts. 54 Subset 3b. Subset 3b folds are confined to unit 1 and only two or three have been noted. They have the opposite sense of movement to 3a fo lds and are developed about a north-dipping ax ia l plane; possibly bearing a conjugate re la t ionsh ip to (F ig . 3-22, 3-25). The 3b axia l planes appear to represent inc ip ien t s l ides which are common in unit 1 (F ig . 3-23), truncate phase 3a f o l d s , and are thus post subset 3a in time. The development of these s l i des i s schematical ly depicted in F ig . 3-25 which also i l l u s t r a t e s how structures s imi la r to those in F ig . 3-23 may have formed by movement on 3b surfaces. Phase 3b structures have not been observed in the other units and i t would appear that they are res t r i c ted to the amphibolite (unit 1) which is composit ional ly the most d i s t i n c t rock type. The occurrence of structures related to th is subset are too l imi ted to assess the i r e f fect on ea r l i e r s t ructura l elements. There i s no evidence of an associated megascopic expression of th is event. Phase 4 Phase 4 f lexura l s l i p fo ld ing trends approximately 035° with a 70° east dipping ax ia l plane. Large open warps are found in the northern part of the map area with up to 150° between the l imbs. In the south the fo lds are better developed on a mesoscopic scale and measure 110° or less between the limbs (F ig . 3-25). The sense of movement of these fo lds indicates that the area occupies the west f lank of an ant ic l inor ium which apparently closes east of the map area. Such mesoscopic fo ld development has not been observed in the northern and central part of the map area. Here broad open f lexures have been delineated by careful measurement of the or ientat ion of F ? which, 55 3-22 Subset 3b fo ld in laminated amphibolite. North dipping ax ia l plane intruded by a Ter t iary d ike. Facing east. 3-23 Sl ide in laminated amphibolite terminates arcuate phase 3 f o l d . Facing east. L. 4 A + • . E l . (Fig.3-12) L3 'from Domain I Domain 2 * + Poles to compositional layering Poles to F2 F I G U R E 3 - 2 4 58 3-26 Phase 4 f o l d . Axial plane dips steeply east. Photo taken facing north. 59 in th is area, has not been extensively reoriented by phase 3 fo ld ing . An example is the fo ld ing defined south of Squally Point (Plate 1) in F i g . 3-27. Folds may be t igh ter in the south because of the presence of layer ing and f a c i l i t y of fo ld formation by react iva t ion of th is layer ing . A cross-sect ion i s presented in p late 2. The d i s t r i bu t i on of poles to ax ia l planes, F^, defines an average axia l plane of 036/70E which contains the associated south plunging measured fo ld axes, L4 (F ig . 3-28). Those L4 plunging north (because of phase 5 fo ld ing) do not exact ly l i e in th is plane. This could be caused by measurement error but as a l l poles f a l l oh one side of the ax ia l plane (F ig . 3-28) the cause could be small c i r c l e rotat ion of L4 about L^. Possib le d ispers ion by concentric fo ld ing of L4 by Lg i s designated by the dashed small c i r c l e on the s tereoplot . Predominant f rac tur ing in the area trends about 015° and is ve r t i ca l (F ig . 3-29). The maxima of poles to f ractures do not correspond to the observed posi t ions of poles to phase 4 ax ia l planes, although a cer ta in small component of the measured fractures may be representat ive of F4. Most of the f rac tur ing i s probably related to f a i l u r e towards the end of phase 4 fo ld ing . Some movement is indicated by o f fset structures (F ig . 2-8) and occasional ca tac las i te in zones pa ra l l e l to these f rac tu res , but the amount of o f fse t cannot be deduced. The majori ty of f racture planes have developed obl iquely to the average pos i t ion of F4 ( i . e . about 30° away from F4). A s im i la r s i tua t ion was also suspected by Chr i s t i e (1973) in the Vaseaux Lake area but was not proved because the exact trend of phase 4 fo ld ing could not be del ineated. Thus C h r i s t i e ' s conclusion that the f ractures represent a set of shear f ractures re lated to f a i l u r e at the f i na l stage of phase 4 Phase 4 + Lz mineral lineations • Poles to F2 FIGURE 3-2 7 61 Phase 4 + l_4 Fold axes o Poles to F 4 • l_2 on flanks of phase 4 meso-folds F I G U R E 3 - 2 8 62 fo ld ing seems tenable. Phase 5 . The most obvious expression of phase 5 deformation i s the large gentle antiformal st ructure with ' i t s almost hor izontal axis passing through Okanagan Mountain, and trending about 105°. This f o l d , by inter ference with phase 4 open f o l d s , has imparted a crude domical s t ructure to the area which i s evident from Plate 1. The fo ld ing i s very open and gentle and not always read i l y del ineated mesoscopically in the f i e l d , but can be out l ined e a s i l y on a larger scale (Plate 1). F ig . 3-30 i l l u s t r a t e s a phase 5 f lexure defined by a f o l i a t i o n and fur ther enhanced by part ing pa ra l l e l to th is f o l i a t i o n . Phase 5 fo ld ing appears re la ted to a ser ies of p ro toc l as t i c * quartz monzonite dikes cons is tent ly emplaced with a shallow southerly dip of between 10 and 40 degrees. A few of these dikes are mapped on P la te 1. They range in thickness from 2 to 60 feet and possess a streak l i nea t ion which pa ra l l e l s L5 (F ig . 3-32). F ig . 3-31 i l l u s t r a t e s one of these dikes cut t ing a phase 4 f o l d . I t i s l i k e l y these dikes occupy one set of f ractures of a conjugate set whose intermediate s t ress axis pa ra l l e l s L5 (F i g . 3-33). Perhaps f o r t -u i t o u s l y , , the or ientat ion of L5 i s perpendicular to the average plane of f rac tur ing in the Ter t iary (F ig . 3-29). Otherwise, the or ien ta t ion may have been dictated by complex competition between the necessi ty of bending L4 fo ld axes and the j o s t l i n g of d iscre te b locks, defined by phase 4 f r ac tu r i ng , against one another in order to release the stresses during phase 5 deformation. * Nomenclature according to Higgins, 1971. 63 L , ' \ Principal concentration ^s cf poles to F4 Poles to Fractures (Tertiary) 186 points Contoured at 1 3 5 , 8 . 0 , 2.7 Percent FIGURE 3 -29 64 3-30 Phase 5 warp defined by parting parallel to faint fo l ia t ion ( F 2 ) . Taken in area transitional between unit 3 and unit B. 3-31 Grey protoclastic quartz monzonite (unit F) cross-cuts phase 4 fo ld . Photo taken facing approx-imately NE. kes associated with phase 5 deformation • poles * lineations FIGURE 3-32 P H A S E 5 S T R U C T U R E S mm. N o r t h , ° P E N P H max Okanogan Lake en phase 4 flexures, trending 0 3 5 ° Corrugated with phase 4 folds, trending 055° . SOUth Okanagan Mountain Naramata max. Open folding Failure by frocturing -with emplacement of dikes. CD C JO rn OJ i w Ol SCALE: I inch = 2miles mm. 67 Summary The fo l lowing st ructura l events have occurred in the area: 1) Phase 1 fo ld ing occurred along axes now possessing a north to north-easter ly trend. They may have been o r i g i n a l l y i s o c l i n a l or l a te r closed by phase 2 fo ld ing . 2) Phase 2 fo ld ing ob l i tera ted much of the evidence of the e a r l i e r deformation as recumbent i s o c l i n a l fo lds formed about an axis trending about 105°. The ax ia l plane of the megascopic fo ld now dips 15 - 20° south but i t s o r ig ina l or ienta t ion i s unknown. 3) Phase 3 deformation resulted in much more open fo ld ing about an axis essen t ia l l y pa ra l le l to phase 2. The megascopic expression i s the Robinson Greek synform developed about a 25 - .30° south dipping ax ia l plane. 4) Phase 4 deformation caused open fo ld ing in the nor th, and t igh ter f lexura l s l i p fo lds to form in the south. The di f ference in degree might have been caused by f a c i l i t y in react iva t ing ex is t ing s l i p surfaces formed by e a r l i e r deformations in layered rock to the south. Fractur ing on steeply dipping northeasterly trending planes followed th is fo ld ing and resul ted • in the f i na l r e l i e f of s t ress . j 5) Phase 5 deformation, essen t i a l l y coaxial with phases 2 and 3, produced broad warps, the largest one being that through the centre of Okanagan Mountain. Interference with phase 4 fo ld ing imparted a crude domical st ructure which culminates at the top of Okanagan Mountain. If phase 5 fo ld ing was rea l l y a resu l t of l a te ra l r e s t r i c t i o n that i s commonly found in thrust systems, then phase 4 and 5 fo ld ing may have been contemporaneous, and associated with a s ing le episode of deformation ( i . e . Laramide orogeny). 68 METAMORPHISM AND THE EMPLACEMENT OF IGNEOUS ROCKS The st ructura l h istory described in the previous sect ion was punctuated by the emplacement of large quant i t ies of granodior i te as well as some quartz monzonite and grani te. Most of th is material was introduced during the second phase of deformation, associated with amphibolite grade metamorphism of the meta-sediments. Af ter phase 3 deformation loca l r e c r y s t a l l i z a t i o n destroyed e a r l i e r structures in the rock, espec ia l l y those within the g ran i t i c un i t s . F i n a l l y , in the Ter t ia ry , c h l o r i t i z a t i o n and ep idot iza t ion (p r inc ipa l l y wi thin f ractures) occurred throughout the area. The res t r i c ted range of bulk composition of a l l units in the map area precludes a f r u i t f u l invest igat ion of the metamorphic geology. Information of metamorphic grade is found only in l imi ted areas of unit 1, and is assoc-iated with phase 2 of deformation. In the other units there i s ample evidence of mineral growth related to th is deformation but l i t t l e more information can be obtained read i l y . Early metamorphism. Information on early metamorphism is derived from assemblages that formed during the second deformation. They are representat ive of amphibolite grade metamorphism and are found within the pods frequently located in the cores of phase 2 fo lds in laminated amphibolite (unit 1 ) . They comprise: hornblende + plagioclase (An4o) + diopside + b io t i t e + ep ido te /c l i nozo is i t e + sphene, as might be expected i n , but not confined t o , metamorphosed basic rocks (F ig . 4-1). No other d i s t i n c t i v e assemblages or index minerals have been found in the 69 4-1 Unit 1. Assemblage including hornblende (hb), diopside (d i ) , epidote (ep), b i o t i t e (b t ) , p lagioc lase (p i ) . Cross polar ized l i g h t . F ie ld of view i s 1 .2 mm across. 4-2 Unit 1. Hornblende (hb) al igned para l le l to L-2. Grid cleavage i s rare ly v i s i b l e as sect ion i s cut pa ra l le l to L 2 . Plane-polar ized l i g h t . F ie ld of view i s 4.5 mm across. 70 examination of about 100 thin sections of a l l un i t s . Although information about the metamorphic grade attained i s minimal, extensive growth of minerals took place during phase 2 deformation. In the amphibolites (unit 1) hornblende has grown r igorously pa ra l le l with L 2 as i l l u s t r a t e d in F ig . 4-2. Within the l i gh t coloured pods in the cores of phase 2 fo lds diopsidic-pyroxene also pa ra l l e l s L 2 (F ig . 4-3) . In the other un i t s , the sa l ien t feature of phase two deformation and metamorphism is the pervasive growth of hornblende and orthoclase para l le l to L 2 . Phase two deformation was c lose ly associated with late-stage conformable plutonism. Intrusion of leuco-quartz monzonite (unit A, F i g . 4-4) was followed by granodior i te (unit B) which forms part of a large ba tho l i th ic (orthogneiss) complex extending east of the area. The f o l i a t i o n in these s i l l s i s nowhere deformed into phase 2 fo lds thus ind icat ing in t rus ion in the l a t t e r stages of the event. Information of metamorphism associated with phase 1 has not been found in the area. Phase 3 Metamorphic assemblages that may have formed during phase 3 metamorphism have not been observed. The degree of mineral growth is minimal in comparison with that during phase 2. Axia l d i rec t ions of phase 3 fo lds are not character-ized by mineral l ineat ions ( i . e . hornblende,orthoclase) and those phase 2 mineral assemblages found in unit 1 have not responded percept ib ly to the event. Extensive development of phase three mesoscopic fo lds d ic tates that at 71 4-3 Unit 1. Thin sect ion cut pa ra l le l to L 2 . Pyroxene porphyroblast i s part of a ser ies of c rys ta ls para l le l to L 2 . Cross-polar ized l i g h t . F ie ld of view i s 5 mm across. 4-4 Unit A. Leuco-quartz monzonite s i l l in jected para l le l to F 2 . 72 least some mineralogic reorganizat ion or growth must have occurred. Evidence for th is has only been observed microscopica l ly in the form of pronounced s t ra in patterns in quartz pa ra l le l to F3 (F ig . 4 -5 , 6, 7 ) . Occasional growth of b i o t i t e para l le l to F3 indicates that lower or middle greenschist metamorphism may have prevai led during phase 3 deformation. Late or post phase 3 metamorphism. The next event resu l t ing in local mineralogical reconst i tu t ion of the gneiss post-dated phase 3 f o l d i ng , or may represent a continuation of the phase three metamorphic event fo l lowing the cessation of deformation. A few small areas (unit D, Plate 1; F ig . 2-10, 11) contain non- fo l ia ted , non-l ineated granodior i te which grades within a few feet into rock bearing the fabr ic of phase 2 and 3 deformation. One of these areas occupies the top of Okanagan Mountain (F ig . 2-10) and was in part responsible for Brock's (1934) in terpretat ion that the domical shape of Okanagan Mountain was caused by upwelling of g ran i t i c material into the core of the s t ructure. Ter t iary metamorphism. Ter t ia ry metamorphism was essen t ia l l y a hydrothermal low pressure event associated with hot spring development and extensive a l te ra t ion in the area mapped by Chr i s t i e (1973). In the Okanagan Mountain area the event i s characterized by the presence of epidote and ch lo r i t e veins wi thin f ractures (F ig . 4-8) often imparting a greenish s ta in or caste to the rock. S e r i c i t i -za t ion , kao l in iza t ion and other a l te ra t ion of minerals i s l o c a l l y common (F ig . 4-9) . 73 4-5 Extreme deformation in unit 3 caused by s t ra in ing of quartz (qt) during phase 2 (Fo). Deformation caused by phase 3 i s also present (F3). Section cut perpendicular to L 2 . Feldspar (f) has remained r i g i d . Cross polar ized l i g h t . F ie ld of view is 4.5 mm across. 4-6 St ra in pattern in quartz out l ines F3 in nose of phase 3 fo ld in uni t 3. Cross polar ized l i g h t . F ie ld of view i s 3 mm across. 74 4 - 7 Faint development of F 3 in granodior i te , uni t A, out l ined by strained quartz. Cross polar ized l i gh t F ie ld of view is 3 mm across. 4-8 Chlor i te veins and strong s e r i c i t i z a t i o n , e tc . of other minerals a t test to hydrothermal a c t i v i t y in the ear ly Ter t ia ry . Epidote veins are also common. This sect ion i s one of those most intensely a l te red . Plane polar ized l i g h t . F ie ld of view is 4 . 5 mm across 75 4-9 Sen" c i t i zat ion of feldspars records Ter t iary hydrothermal a c t i v i t y in uni t A. Plane polar ized l i g h t . F ie ld of view is 4.5 mm across. 76 Ter t ia ry volcanism and associated high leve l plutonism was coeval or shor t ly fol lowed th is a l te ra t ion and involved the emplacement of pink to white quartz monzonite or grani te dikes (uni t E). These dikes occupy f rac tu res , ax ia l planes of ear ly fo lds (F ig . 3-22) and are general ly l oca l i zed along planes of weakness. The host rocks appear to have been hot during in jec t ion as indicated by the d i s to r t i on of laminated amphibolite (F ig . 2-13). With one except ion, the dikes were found to be unfo l ia ted ind ica t ing emplacement a f te r phase 4 deformation. The s ing le exception was a north trending v e r t i c a l dike with a strong ve r t i ca l f o l i a t i o n . Phase 5 deformation was associated with the emplacement of p ro toc las t i c quartz monzonite dikes (unit F) which d i f f e r s i g n i f i c a n t l y in co lour , tex ture , and or ienta t ion (see Sections 2,3) from those of un i t E. Contrary to the view expressed by Chr i s t i e (1973) that phase 5 f rac tur ing was important in l o c a l i z i n g hydrothermal a l t e r a t i o n , i t i s evident that a l te ra t i on had ceased and the country rock had cooled p r io r to the in jec t ion of p ro toc las t i c quartz monzonite dikes during phase 5 deformation. These dikes are remarkably f resh with no replacement of t he i r wel l zoned p lag ioc lase phenocrysts as i s found in other Ter t iary dikes (see F ig . 2-12). I t may thus be concluded that the stresses associated with phase 5 deformation out lasted any a l te ra t ion or hydrothermal ef fects associated with the Ter t iary thermal event. 77 DISCUSSION Five phases of deformation in order of occurrence have been deduced within the Okanagan Mountain map area. In add i t ion , macroscopic fo lds associated with phases 2, 3 and 5 have been out l ined. Three metamorphic events have occurred in the area. The most intense was that associated with phase 2 of deformation. Extensive r e c r y s t a l l i z a t i o n and growth of new minerals accompanied th is metamorphism and resulted in a strong penetrative fabr ic throughout the map area. Amphibolite fac ies metamorphic grade was at ta ined, and was associated with synkinematic emplacement of gran-i t o i d rock. Evidence of Phase 1 deformation therefore has not been found in the grani to id rock and has been large ly ob l i te ra ted by the second deformation in the other rocks. The second metamorphic event followed phase 3 deformation and resulted in local rec rys ta l1 iza t ion of the gneisses, thereby destroying the i r f a b r i c . This metamorphism may have been a continuation of that accompany-ing phase 3 deformation, or a la te r event. The f i na l metamorphism was associated with phase 4 of deformation. It involved shallow level heating and hydrothermal a c t i v i t y and resul ted in extensive c h l o r i t i z a t i o n and ep idot iza t ion of ex is t ing fractures in the area. Corre la t ion with nearby areas. Three adjoining areas have been mapped in deta i l some th i r t y miles south of the present locat ion (Ross and C h r i s t i e , 1969; Oku l i tch , 1970; C h r i s t i e , 1973; Ryan, 1973) as part of deta i led s t ructura l study of the west margin of the Shuswap metamorphic t e r ra i n . From th is a reasonably consistent st ructural h is tory has emerged. Correlat ion of l i tho logy on a uni t to unit basis has not been possible 78 between the areas but compositional s i m i l a r i t i e s suggest equivalent l i t ho logy . The Okanagan Mountain map area bears a strong st ructura l resemblance to the areas east of the Okanagan Val ley both in s ty le and or ientat ion of fo ld se ts , and in the number of d i s t i nc t phases of deformation (5) recognized. The lower grade Kobau Group west of the Okanagan Val ley as mapped by Okul i tch (1969) does not show evidence of the f i r s t deformation. A map and cross-sect ion are presented in F ig . 5-1. The s t r i k ing di f ference between the present study area and those to the south is the preponderance of second phase synkinematic grani to id rock. The re la t i ve st ructura l posi t ion of the map area cannot d e f i n i t e l y be establ ished although i t might be speculated that i t represents a deeper level which was more thoroughly permeated and digested during phase 2 deformation. Post phase 3 intrusions are not represented in the map area although they may in fact be present at a shallow depth (F ig . 5-1). The south-southwesterly vergence of phase 2 fo ld ing agrees wi th. that found to the south. Further information can be establ ished regarding phase 3 fo ld ing . The r e l a t i v e l y t ight phase 3 megafold mapped by Chr i s t i e (1973) contrasts strongly with the cor re la t i ve more open phase 3 fo ld defined by Ryan (1973). Ch r i s t i e believed the fo ld to have been closed by in jec t ion of a phase 3 pluton (F ig . 5-1). This in terpretat ion was supported by Ryan (1973) who thereby accounted for the openness of his fo ld ing because of i t s s t ructura l posi t ion above the pluton. The s ty le of the fo ld ing was thus believed to be strongly contro l led by local events. The s im i la r s ty le of the phase 3 megafold in th is study to that mapped by Chr i s t i e (1973) suggests that development is much more regional in extent and not necessar i ly contro l led by emplacement of sheets of phase 3 igneous rock. 80 Timing of deformation and metamorphism. The absolute timing of the ear ly deformational events out l ined in th is study remains uncertain. Isotopic work does not. provide sa t i s fac to ry information because of the complex thermal h is tory of the t e r ra i n . In conjunction with st ructura l s tud ies , however, some constraints can be def ined. I t is apparent that the f i ve phases of deformation recognized in map areas east of the Okanagan Val ley include structures consistent in both s ty le and or ien ta t ion . West of the Val ley and north of Keremeos, Ross and Barnes (1973) have suggested that deformations believed equivalent to phases 2 and 3 are pre-mid-Carboniferous, based on poor f o s s i l evidence in undeformed rock found above an unconformity. This evidence would necessar i ly place phases 2 and 3 as pre-mid-Carboniferous but l i t t l e more can be said of the absolute timing of the f i r s t 3 phases. Cer ta in ly , iso top ic work has not given any pre-mid-Carboniferous dates. Ryan's (1973) oldest Rb-Sr date i s appreciably younger, only 170 m i l l i o n years. Chr i s t i e (1973), pr ior to the work by Ross and Barnes (1973), placed phase 3 deformation at pre-144 m i l l i on years as th is was the age obtained by White et a l . (1968) on the Ol iver quartz monzonite which intrudes the Shuswap gneisses. This age is probably low, because of an intense thermal event about 50 m i l l i on years ago which has completely or p a r t i a l l y reset K-Ar dates throughout the area. The 170 m i l l i o n year Rb-Sr age i s probably also affected somewhat but the extent is unclear. The present author favours the in terpretat ion that the oldest ages obtained by the K-Ar method in the surrounding Okanagan and Similkameen Plutonic Complex (about 200 m i l l i on years) should at present be considered minimum ages for the termination of phase 3 fo ld ing . Uranium-lead dating may prove to be the only method of gett ing older dates in these rocks as high strontium concentrations l i m i t the usefulness of Rb-Sr (Ryan, personal communication, 1973). Post phase 3 81 plutonism may thus at least straddle the Permo-Triassic boundary and poss ib ly extend back into the Permian or Carboniferous. In e f f e c t , older dates that might coincide with metamorphism associated with the pre-mid-Carbonif-erous deformation suggested by Ross and Barnes (1973) may have been ob l i te ra ted by a rather complex thermal h is tory which fol lowed deformation. Conceivably, the magmattsm fo l lowing the Caribooan orogeny could have been continuous over t h i s period and may have in fac t continued r igh t in to the Jurass ic and Cretaceous (Columbian orogeny) re f lec ted i n cur rent ly ava i lab le K-Ar dates. Phase 4 structures are s im i l a r in s t y le and or ienta t ion to those in the map areas to the south, Ch r i s t i e (1973) has deduced phase 4 deformation to have occurred in the ear ly Ter t iary as rhomb porphyry dikes and s i l l s s i m i l a r in appearance to volcanic rocks of the Marron Formation (K-Ar date 51.6 m i l l i o n years ; Church, 1970) are deformed by phase 4 s t ruc tures . Emplacement of other l i g h t to dark coloured igneous rocks (pe t ro log ica l l y equivalent to the younger parts of the Marron vo lcan ics) was strongly cont ro l led by the north trending f ractures in his area. These fractures are s im i l a r in or ien ta t ion to those found in the Okanagan Mountain map area and the i r formation was probably coeva l , although emplacement of dikes wi th in the f ractures was ra re ly observed. The above evidence suggests that the north trending f ractures are l a te phase 4. Data from the Okanagan Mountain i area show ? in add i t i on , that there is a d i f ference in or ienta t ion between ax ia l planes to phase 4 fo lds and the average pos i t ion of the north trending f rac ture set (see F i g . 3-28). Phase 4 deformation was apparently associated with an intense hydrothermal event (Ross, 1974). Most of the f ractures in the region are c h l o r i t i z e d and ep ido t i zed , and Ch r i s t i e (1973) has noted intense hydrothermal a l te ra t i on in part of h is map area. He suggested that f rac tu r ing associated with phase 5 deformation 82 may have provided channels for the movement of f l u i d s . He has found a l te ra t i on in the gneisses only as high as the base of the over ly ing Ter t ia ry volcanics and so hydrothermal a c t i v i t y must have terminated before extrusion of the vo lcan ics . The emplacement of p ro toc las t i c dikes associated with phase f i v e fo ld ing in the Okanagan Mountain map area, however, suggests that the host rock was at l eas t cool enough at that time to support f r ac tu r i ng . Furthermore, the freshness of the dike rock suggests hydrothermal a l te ra t ion had ceased p r i o r to emplacement. In summary the evidence ava i lab le thus fa r from deta i led studies at the southwest margin of the Shuswap Complex would relegate phases 1 to 3 to pre-mid-Carboniferous time based on s t ructura l considerat ions. This t im ing , however, i s in disagreement with information which i s ava i lab le in the Vernon Map area (Jones, 1959), approximately 40 miles north of Okanagan Mountain, in which evidence for two penetrat ive phases of deformation i s present in rocks of probable T r i a s s i c age (Oku l i t ch , personal communication, 1974). This d i spa r i t y i s at present inadequately resolved and i s predicted upon the a v a i l a b i l i t y of only poor f o s s i l evidence. At any ra te , these ear ly events were fol lowed by plutonism which extends possib ly into the Cretaceous but evidence for th i s warrants fur ther examination. F i n a l l y high leve l heating of the crust associated with hydrothermal a c t i v i t y occurred between 50 and 45 m i l l i o n years ago and was c lose ly re la ted to phases 4 and 5 of deformation. Origin of sediments. Ch r i s i t e (1973) favoured the in terpre ta t ion that the metamorphosed rock had a greywacke a f f i n i t y , and included shales and basic volcanics as part of the succession. Ryan (1973), using i so top ic evidence, supported the suggestion that the amphibolites were derived from volcanics as opposed to l ime- r i ch pe l i t es and moreover showed that the amphibolites had a f f i n i t i e s with andesites found on the 83 cont inental s ide of subduction zones. He also found an iso la ted st ructure in the amphibolite which may have been o r i g i n a l l y vo lcan ic . Regional In terpretat ion. The high grade gneisses found east of the Okanagan Ga l ley d isp lay a s im i l a r s t ruc tura l h i s to ry . To the west, the Kobau Group mapped by Okul i tch lacks e v i -dence of the e a r l i e s t deformation but exh ib i ts consistent l a te r s t ruc tures. Kobau rocks may thus project s t r uc tu ra l l y above those to the east and Okul i tch (1969) has suggested that the Kobau sediments were derived from r i s i n g nappe structures developed during phase 1 fo ld ing of the rocks to the east . Whatever the reason for the absence of the ear ly deformation, i t seems c lear that th is deformation is not present in any lower grade rocks to the west of the Monashee Group, inc luding the Old Tom and Shoemaker Formations (Ross and Barnes, 1972; Ryan, 1973, p.168). Moreover, fa r ther to the west at Hedley, the T r i a s s i c Hedley Formation (Bostock, 1940) does not contain i s o c l i n a l recumbent fo lds which might be equivalent to phase 3 fo lds (Ryan, 1973). Thus phases 1, 2 and 3 of fo ld ing may only be present in older (pre-mid-Carboniferous) rocks. In terms of current p la te - tec ton ic models the deformational h is tory of the C o r d i l l e r a i s s t i l l somewhat confused and con t rove rs ia l . Phases 4 and 5 of deformation, however, are much more amenable to d iscussion and are considered f i r s t . Both phases have occurred about 50 m i l l i o n years ago (Ross, 1974) and as such are c o r r e c t a b l e with the extensive Laramide phase of Cord i l l e ran tectogenesis. Coney (1972) has a t t r ibu ted th is orogeny to major reorganizat ion of p late motions resu l t ing from the separation of Europe and North America which commenced about 80 m i l l i o n years ago and resul ted in southwesterly ro ta t ion of the North America plate away from Europe. Subsequent to about 40 m i l l i o n y e a r s ago p l a t e mo t i on was r e d u c e d and 34 deformation ceased because of slowing of spreading in the North A t l a n t i c . The associated subduction model for the west coast i s more e laborate ly discussed by Atwater (1970) and involves in te rac t ion of the Fara l lon p late with western North America, rapid subduction, and associated andes i t i c volcanism. The re lated ef fects include f o l d i ng , as encountered in the western part of the cont inent , and thin-skinned deformation in the Rocky Mountains. The area studied would occupy the inner arc in the context of Miyashiro's(l972) paired metamorphic b e l t , and the Coast Mountain u p l i f t (Culbert , 1971) would correspond to events in the arc -trench gap. The only d i ssa t i s f y i ng aspect in the app l ica t ion of th is model i s the rather great width (200 mi les) of the metamorphic pa i r in B.C. as opposed to those discussed by Miyashiro(60 m i l es ) . Coney (1972) explains th is discrepancy by emphasizing the importance of the ro ta t ion of North America in causing deform-a t i on , in contrast with simple subduction where stresses are transmitted over a r e l a t i v e l y short d is tance. The time gap between pre-mid-Carboniferous and Ter t ia ry deformation (Ross and Barnes, 1972) i s large wi th in the areas studied in d e t a i l . Elsewhere, however, evidence for the Columbian orogeny of Jurass ic and Cretaceous age i s found (Monger et a l . , 1972). Folding which may be associated with th is deformation has been noted by Ryan (1973) in T r i a s s i c rocks of the Hedley Formation some 30 mi les west of the Okanagan Va l ley . Monger et a l . (1972), in f a c t , favour the hypothesis that the main metamorphism in the Omineca Bel t is T r i a s s i c in the north and Jurass ic in the south. Spec i f i c de ta i l s of the pos i t ion of subduction zones are few. The P i n c h i -Tes l in lineament (Coney, 1972; Paterson, 1973) i s at leas t one p o s s i b i l i t y fo r a suture in Permo-Traissic time but probably only one of a ser ies of p a r a l l e l , west-migrating subduction zones which existed throughout the Phanerozoic. Unfor-85 tunately evidence for the re la t i ve motions of in terac t ing oceanic plates ear ly on in the development of the Cord i l l e ra connot now be found wi th in the present day P a c i f i c ocean, f o r , as pointed out by Coney (1972), the P a c i f i c oceanic plate was not in contact with the North American plate during the time of in te res t . 86 K-Ar and f i s s i o n track geochronometry of an Eocene thermal event in the Kett le River (west hal f ) map area, southern B r i t i sh Columbia. PAPER NO. 1 G.A. Medford Department of Geological Sciences The Universi ty of B r i t i s h Columbia Vancouver, B r i t i sh Columbia 87 ABSTRACT The Okanagan and Similkameen plutonic complexes west of the Okanagan Val ley of south-central B r i t i s h Columbia y i e l d K-Ar dates that range from 185 to 133 m i l l i on years. East of the Okanagan Val ley Shuswap gneisses into which the plutonics intrude, and which may be as old as pre-mid-Carboniferous in age (Ross and Barnes, 1972), y i e l d K-Ar dates between 59.9 and 47.4 m i l l i on years. This abrupt change, which approximately coincides with the Okanagan Va l ley , is a consequence of an intense thermal event in the ear ly Tert iary which has reset K-Ar dates in the gneisses at shallow depths. Comparison of K-Ar, sphene and apat i te f i s s i o n track dates demon-strates that the heating affected the plutons west of the Okanagan Val ley and that cool ing of the Shuswap gneisses occurred at a rate in excess of 25°C. per m i l l i on years. The scat ter observed in the older K-Ar dates of the plutonic complexes could be caused by post-emplacement heating with var iable par t ia l argon loss rather than by separate magmatic events. Thus only the oldest K-Ar dates obtained from the plutons may be s ign i f i can t as minimum ages for emplacement. 88 Introduction This invest igat ion was begun when an early Eocene date (49.9 m i l l i on years) was obtained for a hornblende sample (1-150) extracted from ortho-gneiss of the Shuswap metamorphic complex east of Okanagan Lake. This date was unexpected because Jurass ic plutons are known to intrude the gneiss and because many nearby patches of Eocene vo lcan ics , ranging in age from about 50 to 45 m i l l i on years (Mathews, 1964; Church, 1970), s i t upon fanglomerates rest ing on an early Ter t iary erosion surface. In the absence of block fau l t ing with large displacement and deep erosion of up l i f ted b locks, the hornblende date must have been reset at a high level in the crust perhaps by a nearby unobserved body of Coryel l granite ( L i t t l e , 1961). In th is paper, the nature, extent, and s ign i f i cance of the high level Tert iary thermal event are out l ined in the area contained within the Kett le River (west ha l f ) map sheet. This event has lowered K-Ar dates, espec ia l l y those obtained east of the Okanagan Val ley where local intense hydrothermal a l te ra t ion has been observed (Ross, 1974). An apat i te f i s s i o n track study reveals that th is thermal event affected areas where K-Ar dates have not been great ly a l te red. A few sphene dates were also obtained and comparison of these dates with those from the apat i tes indicates very rapid cool ing of the rock when the thermal event ended. Ana ly t ica l Techniques The K-Ar analy t ica l procedure used i s described by White et a l . (1967). In addit ion samples were baked to about 130°C for approximately 18 hours to el iminate or reduce the ef fects of atmospheric argon contamination. F iss ion 89 track dates were calcu lated using the equation in Naeser (1967). Deta i ls of the ana ly t i ca l methods are given in Appendix 1. D is t r i bu t ion of K-Ar Dates. A few K-Ar dates west of the Okanagan Va l ley have previously been reported fo r the Okanagan and Similkameen Complexes (see Peto, 1973a,b) which intrude gneisses of the Shuswap metamorphic complex. White et al .(1968) found the O l iver quartz monzonite (Plate 1) to have a muscovite K-Ar date of 144 m i l l i o n years . Farther west near Hedley, Roddick et a l . (1972) obtained dates between 140 and 180 m i l l i o n years fo r various uni ts of the Okanagan and Similkameen Complex. K-Ar dates on hornblende and b i o t i t e obtained in th is study on gneisses east of the Okanagan Val ley a l l f a l l i n the in terva l of 51 to 47 m i l l i o n years (Plate 1, Table 1) with one exception (sample 3-10) at 59.9 m i l l i o n years . In add i t i on , concurrent Rb-Sr work by Ryan (1973) in the gneisses j us t north of the Internat ional Boundary east of Osoyoos ind icates reset t ing of minerals and some sch is ts to dates of between 30 and 50 m i l l i o n years . On the other hand, dates from the plutonic rocks on the west s ide of the Okanagan Val ley range between 185 and 133 m i l l i o n years (Plate 1, Table 1 ) . A sharp break in K-Ar dates which approximately fo l lows the Okanagan Va l ley i s thus coincident with the contact between the area containing p lutonic bodies and the Shuswap gneisses. The Okanagan Val ley has long been considered f au l t cont ro l led (see L i t t l e , 1961). Na tu ra l l y , the young dates on the east s ide might r e f l e c t u p l i f t and erosion which have exposed an area s u f f i c i e n t l y deeply buried and hot enough to reset K-Ar dates during the lower Eocene. Ava i lab le evidence does not support t h i s in te rp re ta t ion . Widespread patches of vo lcanic rocks ranging 90 TABLE 1 POTASSIUM-ARGON ANALYTICAL DATA No. LOCATION ROCK TYPE,UNIT,MINERAL Ar40r PERCENT Ar40r TO"5 Ar40r K+S A740T ccSTP/g ((40 APPARENT AGE M .y. 1- 50* 49°36.7'N 119°34.6'W 1-160* 49"39.2'N 119°33.8'W 1-148* 49"43.9'N 119°31.3 W 1-150* 49°44.5'N 119°31.0'W 1-178* 49°42.6'N 119o36.0'M 3- 3 49"25.2'N 119047.2'W 3- 5 49"51.6'N 119°17.2'W 3- 7 49"03.5'N 119°38.5'W 3- 8 49"47.5'N 119°08.0'W 3- 10 49"16.5'N 119°31.2'W 3- 13 49"36.0'N 119°34.5'W 3- 18 49"36.6'N 119047.8'W 3- 20 49"42.5'N 119°48.8'W 3- 21 49U42.8'N 119016.5'W 3- 22 -49v15.0'N ~119°10.5'W South shallow dipping dike Highly sheared Whole rock Coarse-grained granodio-rite dike Whole rock Recrystallized granodio-rite gneiss. Monashee Group - Hornblende Granodiorite gneiss. Hornblende lineated. Monashee Group Hornblende Granodiorite gneiss. Hornblende lineated. Monashee Group Hornblende Similkameen quartz diorite. Similkameen Complex. Biotite Augen gneiss. Hornblende lineated. Hornblende Kruger Syenite. Similkameen Complex. Hornblende Paragneiss. Monashee Group. Hornblende Vaseaux Formation para-gneiss. Monashee Group. Hornblende Granodiorite gneiss. Monashee Group. Hornblende Similkameen quartz diorite. Okanagan Complex. Biotite - 25% Hornblende Valhalla Granodiorite. Okanagan Complex. Biotite Granodiorite gneiss. Monashee Group. Biotite Valhalla Granodiorite. Biotite 3.18i0.003 0.937 0.6038 0.002805 47.4*1.5 1.19+0.003 0.792 0.2339 0.002904 49.0+1.6 1.24+0.003 0.469 0.2411 0.002873 48.5+1.5 1.1U0.004 0.744 0.2220 0.002955 49.9±1.7 1.05±0.004 0.713 0.2145 0.003033 51.1*1.6 3.82±0.034 0.912 2.9456 0.0T13924 185 +6.6 1.33±0.003 0.735 0.2788 0.003097 52.2+1.5 0.58±0.006 0.775 0.4098 0.010476 171 +6.4 1.49±0.012 0.831 0.3197 0.003170 53.4±1.9 1.40±0.009 0.730 0.3374- 0.003560 59.9+2.0 1.34+0.000 0.839 0.2816 0.003104 52.3±1.4 2.50+0.081 0.712 1.7103 0.010107 165 +9.7 6;44+0.023 0.950 3.5024 0.008035 133 ±4.1 4.85+0.019 0.596 1.0089 0.003073 51.8±1.6 6.78±0.026 0.919 1.4086 0.003070 51.8±1.6 Argon analyses by J.Harakal (*) and G.Medford using MS-10 mass spectrometer. 91 in age from 50. to 45 m i l l i o n years are found unconformably over ly ing the gneisses and plutonic rocks throughout the area. I t i s poss ib le , of course, that the area east of the Okanagan Va l ley was up l i f t ed at a greater rate p r io r to the Ter t ia ry and d isp lays the e f fec ts of up l i f t ed isotherms. This area might then have been qu ick ly eroded jus t before the extrusion of the Eocene vo lcan ics . In the absence of an abnormally steep geothermal gradient , the u p l i f t must have been both rapid and have involved r e l a t i ve la rge-sca le d i s -placement. Although th is p o s s i b i l i t y cannot be en t i re l y precluded, evidence obtained from deta i led mapping near Ol iver in the south argues against i t . Ross and Chr i s t i e (1969), Ch r i s t i e (1973) and Ryan (1973) do not note large displacement fau l t i ng in the i r map areas and Ross and Ch r i s t i e (1969) have mapped continuous l i tho logy and st ructure across the Okanagan Va l ley . I t i s considered more l i k e l y that the in tens i t y of the shallow thermal event diminishes sharply to the west. Although White et a l . (1968) obtain a 144 m i l l i o n year K-Ar date on muscovite fo r the Ol iver quartz monzonite, they a lso report several b i o t i t e ages of between 118 and 82 m i l l i o n years from nearby parts of the same pluton. S i m i l a r l y , at the Brenda Mine in the north-west corner of the area (Plate 1 ) , White et a l . (1968) obtain a b i o t i t e K-Ar date of 148 m i l l i o n years whereas hornblende y i e l ds 168 m i l l i o n years . This discrepancy, however, may only r e f l e c t dating of secondary b i o t i t e associated with m inera l i za t ion . In any case, there i s some evidence that Ter t ia ry heating may have occurred west of the Okanagan Va l l ey , but i t was apparently i n s u f f i c -ient to reset the K-Ar dates as has happened to the east . In order to see i f evidence of such heating could be detected in the west several apat i te f i s s i o n track dates were determined. 92 F iss ion Track Analys is and Cooling-Rate Determination Apat i te i s the most thermally sens i t i ve mineral used in f i s s i o n track dating at present. Track- loss curves have been experimental ly determined as a funct ion of temperature and time (Naeser and F a u l , 1969). Track retent ion (for a period of 10^ to 10^ years) i s essen t i a l l y complete in apat i te at temperatures below about 50°C and in sphene below 275°C. On the other hand, a conservat ively low estimate of the closure temperature of the hornblende K-Ar clock i s about 150°C (see Damon, 1968, p.23, 24). Because of i t s s e n s i t i v i t y to rese t t i ng , apat i te has been used to date recent u p l i f t . S p e c i f i c a l l y , the age obtained represents the time at which the rock passed through the c r i t i c a l isotherm for track re ten t ion , which in many instances may represent only 3 or 4 ki lometers of b u r i a l . For high leve l in t rus ion of magma into cool rock (rapid coo l i ng ) , however, f i s s i o n track dates concordant with other i so top ic methods have been obtained (Chr istopher, 1973). S im i l a r l y a high leve l regional thermal event which i s s u f f i c i e n t l y intense to reset K-Ar b i o t i t e and hornblende dates might be expected to resu l t i n concordance between the K-Ar and apat i te ages, as cool ing upon cessat ion of the heat input would be expected to be r e l a t i v e l y rap id . In the fo l lowing paragraphs, a reasonable minimum c^obling-rate for the Shuswap gneisses i s ca lcu la ted . Deta i ls of the ca lcu la t ion are presented in Appendix 2. Apat i te dates obtained in th is study are presented in Table 2 and P la te 1. On the east s ide of the Val ley at each locat ion the apat i te and sphene dates are concordant with the K-Ar dates, wi th in the stated l i m i t s of e r ro r . The mean hornblende K-Ar date i s 51.2 m i l l i o n years (excluding sampleQ^ lOj ih ich i s unusually h igh) , the mean sphene date i s 53.6 m i l l i o n years and the mean apat i te date i s 48.4, not very much l e s s . On the west side of the Okanagan Va l l ey , however, the mean apat i te date i s 54.3 whereas the K-Ar dates vary but TABLE 2 FISSION TRACK ANALYTICAL DATA No, LOCATION MINERAL ROCK TYPE.UNIT TRACK DENSITY RATIO APPARENT AGE 1-145 49 41 . 2 ' N A p a t i t e 119°34.4'W G r a n o d i o r i t e g n e i s s , Monashee Group. 0 . 591±0.108 41 .2 + 7.7 1-148 49 "43 .9 'N A p a t i t e 119 31 .3'W R e c r y s t a l 1 i zed g r a n o d i o r i t e g n e i s s , Monashee Group. 0.686+0.034 47.8+3.1 1-150 4 9 U 4 4 . 5 ' N A p a t i t e 1 1 9° 31 .O'W G r a n o d i o r i t e g n e i s s . Monashee Group. 0.609±0.038 42.413.1 1-154 4 9 U 3 9 . 7 ' N A p a t i t e 119°34.3'W G r a n o d i o r i t e g n e i s s , Monashee Group. 0.889+0.094 61 .9 + 7.2 1-173 4 9 U 3 9 . 9 ' N A p a t i t e 119°38.0'W G r a n o d i o r i t e g n e i s s . Monashee Group. 0.770*0.087 53.6+6.4 2- 83 49^37.6 'N A p a t i t e 119°34.0'W G r a n o d i o r i t e gne iss Monashee Group. 0.629+0.028 43.8+2.7 3- 0 4 9 U 1 1 . 9 ' N A p a t i t e 119°35.3'W 01i ver i n t rus i ve . Simi lkameen Complex, 0.646+0.062 45.0+4.6 3- 1 49°01.8 'N A p a t i t e 119°41 .1 'W G r a n o d i o r i t i c i n t r u -s i v e . Simi lkameen Complex. 0.774+0.081 53.9+6.0 3- 5 4 9 51 .6 'N A p a t i t e • 119°17.2'W Augen g n e i s s . Monashee Group. 0.666+0.060 46.4+4.6 3- 6 49 27 .9 'N A p a t i t e 119°41 . 2 ' W Jura g r a n o d i o r i t e . Okanagan Complex. 0.978+0.085 6 8 . 0 i 6 . 5 3 . 8 4 9 U 4 7 . ' 5 ' N A p a t i t e 119°08 . 0 ' W P a r a g n e i s s . Monashee Group. 0.761+0.055 53.0+4.4 3- 10 49°16.5 'N A p a t i t e 119°31.2 ' W Vaseaux Format ion paragnei s s . Monashee Group. 0.688+0.079 47 . 9±5 . 8 3- 1 2 49 u 16.1 'N A p a t i t e 119°22.3'W Orthognei s s . Monashee Group, 0.907±0.145 63.1±10 . 4 3- 13 49 26 .0 'N A p a t i t e 11 9°34.5'W G r a n o d i o r i t e gne iss Monashee Group. 0.649+0.048 45.2+3.9 3- 14 49°28.6 'N A p a t i t e C o r y e l l i n t r u s i v e . 119 u 38.1 'W G r a n i t e . 0.822+0.064 57.2+5.0 3- 16 49°32.7 'N A p a t i t e Paragne iss 1 1 9 38.1 'W Monashee Group. 1 .01610.066 70.6+5.4 3- 17 49°31.O'N A p a t i t e Augen gne iss 119 U 32.7 'W Monashee Grouf 0.51OiO.045 35.6+3.5 FISSION TRACK ANALYTICAL DATA TRACK DENSITY APPARENT No. LOCATION MINERAL ROCK TYPE,UNIT RATIO AGE 3 - 18 49°36. 119°47. 6'N ,8'W Apa t i te Similkameen quar tz d i o r i t e . Okanagan Complex. 0. .698+0. 046 48, .6+3.8 3 - 20 49°42. 119°48. 5 ' N ,8'W Apat i te V a l h a l l a g r a n o d i o r i t e Okanagan Complex. 0. .544+0. 026 37, ,9+2.4 3 - 21 49°42. 119°16. 8' N 5'W Apat i te G r a n o d i o r i t e g n e i s s . Monashee Group. 0. 677±0. 049 47 . ,2+3.9 3 - 22 -49°15. -119°10. 0 ' N 5'W Apat i te V a l h a l l a g r a n o d i o r i t e 0. ,689+0. 059 48. ,0+4.6 1 -178 49°42. 119°36. 6'N O'W S p h e n e: G r a n o d i o r i t e g n e i s s . Monashee group. 0. ,816+0. 026 56, ,8±7.8 3 - 5 49°51 . 119°17. , 6 ' N ,2'W Sphene Augen gnei ss . Monashee Group. 0, ,662t0 . 1 66 46, .1+11.5 3 - 13 49°26. 11 9°34.. .O'N .5'W Sphene G r a n o d i o r i t e g n e i s s . Monashee Group. 0, .745+0. 085 51 .9 + 11 .4 3 - 20 49°42, 119°48 . 5 ' N ;8'W Sphene V a l h a l l a g r a n o d i o r i t e Okanagan Complex. 0 . 569 i0 . 1 20 39 .7+8.4 3 - 22 ~49°1 5 -119°10 .O'N .5'W Sphene V a l h a l l a g r a n o d i o r i t e 0 .855+0. 175 59 .5+12.2 Thermal neutron f l u e n c e : ( 1 1 . 4 ± 0 . 5 ) x 1 0 1 5 n e u t r o n s / c m 2 Quoted e r r o r : one standard d e v i a t i o n Model age i s c a l c u l a t e d accord ing to Naeser ( 1 967-) Etch c o n d i t i o n s : A p a t i t e ; 75 percent HN0 ? , 20 seconds. Room tempera ture . Sphene; 1 H F , 2 H N 0 v 3 H C 1 , 6 H ? 0 , 12 minu tes . Room tempera tu re . 95 are always considerably higher. I t would seem, then, that the increased thermal gradients were common to both sides of the Va l l ey , but that heating was less intense in the west so that the K-Ar dates are only s l i g h t l y reduced. The d i f ference between the mean hornblende K-Ar date and the mean apat i te date fo r samples east of the Okanagan Val ley i s 2.8 m i l l i o n years . During th i s time the area must have cooled from at leas t 150°C. to a temperature below 75°C. (see Appendix 2) . Hence a cool ing rate in excess of 25°C. per m i l l i o n years i s impl ied. S i m i l a r l y , the d i f ference in mean dates fo r sphene and apat i te i s 5.2 m i l l i o n years during which time the area cooled over a range of 200°C, or at a rate of about 40°C. per m i l l i o n years . This estimate i s independent of errors in decay constants and f l ux c a l i b r a t i o n . The cool ing rates calcu lated above may be compared with those ca lcu lated for the u p l i f t - c o o l i n g model of the Alps (Clark and Jager, 1969). In the high-grade core of the Alps rocks have cooled from a maximum of 400°C. to 650°C. to surface temperatures in about 30 m i l l i o n years , and th is i s compatible with a denudation rate of 0.4 to 1.0 mm per year . Despite the continuous, rapid u p l i f t and erosion displayed by the Alps the cool ing rate there i s only 13°C. to 22°C. per m i l l i o n years , somewhat less than that estimated for the Shuswap rocks east of the Okanagan Va l ley . Examination of ind iv idual dates allows a number of in teres t ing observa-t ions to be made. Sample 3-20 y i e l ds the lowest K-Ar date (133 m i l l i o n years) obtained in th is study west of the Okanagan Va l ley . The apat i te date, 37.9 m i l l i o n years , i s also r e l a t i v e l y low here and i s well determined ( + 2 . 4 ) . A sphene date at th is loca t ion i s s im i l a r to the apat i te date (39.7 m i l l i o n years ) . The oldest K-Ar date obtained (185 m i l l i o n yea rs ) , on the other hand, i s found at locat ion 3-3 which i s about 1000 feet below a known Ter t ia ry erosion sur face. In add i t i on , nearby apat i te dates are r e l a t i v e l y high (samples 3-6 and 3-16). This general region may thus have escaped most of 96 the ef fects of Ter t iary heating. If t h i s i s so then the 185 m i l l i o n year K-Ar date, which i s among the oldest obtained in surrounding areas (Hibbard, 1971; Roddick et a l . , 1972; Rinehart and Fox, 1972) may r e a l l y be a minimum age for the p lutonic complex and the others are scattered due to pa r t i a l rese t t i ng . I t i s in teres t ing to note that Peto (1970, 1973a,b) postulated a genetic s i m i l a r i t y between in t rus ive uni ts of the Okanagan Complex based or> chemical arguments but retracted the hypothesis because of the data of Roddick et al .(1972) which suggested a time di f ference of 20-30 m i l l i o n years between in t rus ive events. In view of the above discussion Peto 's o r ig ina l conclusion may be v a l i d . The scat ter of K-Ar dates in areas af fected by Ter t iary heating probably r e f l ec t s pa r t i a l argon l o s s , although the p o s s i b i l i t y of argon gain in a region peripheral to an area being degassed by an intense thermal event cannot be overlooked. Sample 3-20 may r e f l e c t th is phenomenon as the sphene date i s qui te low here (39.7 m.y.) whereas the b i o t i t e date i s s i g n i f i c a n t l y higher (133 m.y.) . The anomalously high hornblende date at locat ion 3-10 (59.9 m.y.) might be explained s i m i l a r l y . To date, however, Ar gain has only been observed i n minerals r e c r y s t a l l i z e d wi th in basement te r ra ins where an Ar overpressure has developed (see fo r example, Bocquet et a l . , 1974). Notwithstanding, the reso lu t ion of emplacement dates might best be l e f t to other techniques. A pa i r of well-determined low apat i te dates (samples 3-17, 35.6 + 3 .5 ; and 3-20, 37.9 + 2.4) provide evidence that heating continued l o c a l l y long a f te r the main event about 50 m i l l i o n years ago. Corre la t ion of the Ter t ia ry Event with Geophysical Evidence The crust i s known to be r e l a t i v e l y th in between the Rocky Mountain Trench and the Hope Fau l t , not more than 30 kilometers as opposed to 45-55 kilometers elsewhere in the Co rd i l l e ra (Berry et a l . , 1971). In add i t i on , 97 a highly conductive layer below th is zone i s now interpreted to be caused by hydration and possible par t ia l mel t ing, leaving a r i g i d crust of only 10-15 ki lometres. At the present time a thermal anomaly in the northwestern United States out l ined by Blackwell (1969) appears to extend into Canada (Jessop and Judge, 1971) and may be associated with continued or intermit tent heating fol lowing the main thermal event in the ear ly Eocene. Later heating is suggested by a few re l a t i ve l y low (35 m i l l i on year) dates obtained on apat i te . C i rcu la t ing water is a convenient method of thermal energy transport . Ross (1974) has noted extensive a l te ra t ion in the southeast part of the map area and has associated th is with a high level hydrothermal event co in -cident with extensive plutonism (e.g. Coryel l p lu ton ics ; L i t t l e , 1957, 1961) and volcanic ac t i v i t y (Kamloops Group or Princeton Group vo lcan ics ; see Mathews, 1964) during the in terval 50 to 45 m i l l i on years ago. Much.of the area dated at 50 m i l l i on years did not show evidence of such extreme a l t e r a -t i o n , but common c h l o r i t i z a t i o n and ep idot izat ion of f ractures may represent co r re la t i ve phenomena. Otherwise, heat may have been introduced by shallow in t rus ion of large convecting bodies of Coryel l g ran i t i c rock. A few small stocks are found within the area (Plate 1) as well as numerous dikes and these conceivably tap a large bathol i th s im i la r to that unroofed in the Kett le River (east hal f ) map area ( L i t t l e , 1957). The rather abrupt decrease in the in tens i ty of the event at the Okanagan Val ley (which i s commonly considered fau l t con t ro l led , a lbe i t of probably minor displacement), may re f l ec t s t ructural control on the movement of the magma. The change from andesi t ic volcanism to extrusion of a l k a l i - b a s a l t s in the Miocene throughout B r i t i s h Columbia (Souther, 1970) is not re f lec ted in 98 isotop ic ages obtained thus fa r . This is reasonable considering the source of basalts is deep and they are quickly t ransferred to the surface with contact ef fects only next to feeder d ikes. In contrast , the disturbance discussed thus far i s a shallow crusta l phenomenon of regional extent. Regional Aspects. It has become apparent' that heating associated with ear ly Tert iary igneous events is widespread and found in many parts of B r i t i s h Columbia and the northwest United States. Armstrong (1974), Fox et a ] . , (1973), and M i l l e r (1974) noted comparable regional reset t ing in K-Ar dates in plutonic rocks of Idaho and Washington, and B i rn ie (personal communication, 1974) has obtained several Ter t iary K-Ar dates from Shuswap gneiss near Revelstoke, as well as discordant hornblende (146 m i l l i on years) and b i o t i t e (51 m i l l i on years) dates from an in t rus ive into the gneisses. Examination of the dates in south central B r i t i sh Columbia compiled by Uanless (1969) reveals a large number of ear ly Ter t iary dates throughout the south-central Cord i l l e ra as well as frequent discordance in the older measurements. It seems apparent that the resolut ion of d i s t i n c t plutonic episodes may be rendered quite d i f f i c u l t i f Tert iary heating is widespread and intense. Erroneous time re la t ionsh ips may be proposed, for example, i f dikes intruding other rocks are dated without ensuring that both dike and host rock have not been reset . The del ineat ion of general areas strongly affected by the Tert iary thermal event must await rather extensive dating of the plutonic and gneiss complexes of the province. The thermally sens i t i ve apat i te f i s s i o n track technique can prove useful in detecting areas where modif icat ion of K-Ar dates is l i k e l y to have occurred. Obviously an old apat i te date which is 99 concordant with a K-Ar date on a oogenetic mineral supports the absence of l a te r pa r t i a l rese t t ing . The spat ia l cor re la t ion of the 50 m i l l i on year K-Ar dates ( i . e . , areas of most intense heating) with high grade gneisses of the Shuswap metamorphic complex, o r i g i n a l l y metamorphosed in T r i a s s i c / J u r a s s i c (Monger et a l . , 1972) or possibly pre-mid-Carboniferous time (Ross and Barnes, 1972), is remarkable within the Kett le River (west hal f ) map sheet. Whether or not th is corre la t ion is consistent over a larger region is a question that requires much more isotop ic work, although the dates (mentioned above) obtained by B i r n i e , and those compiled by Wanless (1969) are in agreement with this; p o s s i b i l i t y . Armstrong (personal communication, 1974) has noted a s im i l a r coincidence in Idaho. The apparent spat ia l overlap of the Eocene thermal event and the high grade gneisses of the Shuswap Complex is at present unexplained. Summary. Evidence has been presented to suggest that a high level thermal event which was less intense west of the Okanagan Val ley has reset K-Ar dates in the older Shuswap gneisses to about 50 m i l l i on years and has probably a l tered some of the K-Ar dates of the T r iass ic and Jurass ic plutonics of the Okanagan and Similkameen complexes. Apat i te f i s s i o n track dating has traced the event into areas where K-Ar radiometric dates have been much less af fected. In the Kett le River (west ha l f ) map area only the oldest K-Ar dates obtained may thus have any real s i gn i f i cance , and, assuming no excess argon is present, represent a minimum age of in t rus ion . Obviously d i f f i c u l t y w i l l always be encountered in deducing d i s t i n c t magmatic events in areas where l a t e r 100 heating has occurred. Apat i te f i s s i o n track dates are useful as detectors of thermal events that are too weak to t o t a l l y reset K-Ar dates yet responsible for s i gn i f i can t degrees of Ar l oss . 101 On the computation of s t a t i s t i c a l error in f i s s i on track ana lys is . PAPER NO. 2 G.A. Medford Department of Geological Sciences The Univers i ty of B r i t i sh Columbia Vancouver, B r i t i sh Columbia. 102 Introduction This paper is concerned with two aspects of def ining error l im i t s in f i s s i on track ana lys is . F i r s t l y i t i l l u s t r a t e s that in some instances operator variance can exceed that predicted by Poisson s t a t i s t i c s and that th is should be taken into account when deriving error l im i t s for f i s s i o n track dates. Secondly i t deals with combination of errors in the var iables Ps, P i , and 0 (defined below) and i l l u s t r a t e s d i f f i c u l t i e s that can ar ise i f care is not taken. The discussion that fo l lows was prompted by the nature of data co l lec ted by the author in the course of f i s s i o n track ana lys is . These data, as well as that from other pub-l ishea papers, are used to demonstrate the methods employed. The s t a t i s t i c s for the d i f fe rent procedures of f i s s i o n track analysis (see below, cases 1, 2 and 3) are discussed separately. F iss ion track age equation. The standard equation used to determine date based on f i s s i o n tracks is (Maeser, 1967) (1) A - 6.49 x 10 9 In (1 + 9.45 x 10-18 £|. 0) where A = age (mi l l ion years) Ps = spontaneous track density t racks/un i t area Pi = induced track density from t racks/un i t area neutron i r rad ia t i on 0 = thermal neutron fluence neutrons/cm The var iables Ps , Pi and 0 are a l l determined by counting f i s s i o n t racks. The quantity 0 usual ly is obtained by counting tracks on a glass standard of known uranium content for which a conversion factor re la t ing track density to neutron fluence i s ava i lab le . 103 Precis ion of the calculated value, A, is related to the errors assigned to the var iables Ps, Pi and 0 : namely o s , aj and O 0 . Because counting data are involved, Poisson s t a t i s t i c s (Freund, 1962) have been invoked to estimate these parameters. Techniques of f i s s i on track ana lys is . Three d i f fe rent techniques are employed in determining a f i s s i o n track age and these are discussed separately in the fo l lowing sect ions. Par t i cu la r attent ion is given to determining the error in P s / P i . Combination of th is error with that in 0 is considered la te r . Case I One determines Ps by counting the number of tracks per uni t area in the polished surfaces of a number of gra ins. The value of Pi is obtained s im i l a r l y in grains that have been annealed to remove spontaneous t racks , and then i r rad ia ted, Assuming x s and x-j track counts are obtained on each sample, one can obtain s u i t -able error l im i t s using Poisson s t a t i s t i c s . The appropriate Poisson parameters (Bennett and Franklin,1967) are n s _ z j x s _ A x , = i X i 1 n i where n i and n g are the number of observations (or grids counted). The variances 2 2 o s and a are 2 _ , » s s n p (3) s 2 _ A i a i n. l 104 For n>50 the central l im i t theorem applies (Bennett and Frankl in, 1967, p.119) and the l im i t i ng form of the Poisson d is t r i bu t ion is the normal d i s t r i bu t i on with 2 2 -variances a and a .. and means Pi and Ps where (4) Ps = A S Pi = A . i It is necessary to derive the resul tant error in the quotient Ps /P i and th is can be readi ly accomplished by Taylor ser ies expansion (Greenwalt and Schultz,1962). Def ining: - Pi (5) z = z = the track density ra t io Ps and the percentage error in z is (7) a % = -^zr X 100 z From equation (6) i t i s evident one can also wri te 2 2 2 (8) c< z% ~ a j% + a % This formula, in f ac t , has been used by most f i s s i o n track workers (CW. Naeser, wr i t ten communication, 1973). In order to invest igate the p o s s i b i l i t y that the estimate of a using Poisson s t a t i s t i c s might at times be too low, each sample of a sui te being dated by the author was counted on three d i f fe rent occasions over a period of about two 105 months. The track r a t i o s , obtained on each t r i a l are p lot ted in F i g . 1 as well as er ror bars representing three standard deviat ions (3o z ) - Given one estimate of z i t i s un l i ke l y that any subsequent determination would f a l l outside — the p robab i l i t y of such an event being about .003%. F ig . 1, however, indicates that several samples do exh ib i t an unduly large var ia t ion in i . More than ha l f are wi thin the l i m i t s of var ia t ion expected assuming the Poisson estimates of o . and 0s are v a l i d , and a few are marginal. The anomalous samples could be a resu l t of the fo l l ow ing : 1) The count-data are not Poisson d is t r ibu ted (e .g . inhomogenous uranium content) 2) Interpretat ional va r ia t ion is larger than that predicted by Poisson s t a t i s t i c s (e .g . f a i n t l y etched tracks might be included in the count at some times and not others) i . e . operator variance i s higher in some instances. In order to check for the f i r s t p o s s i b i l i t y a l l data were obtained using a 6 x 6 gr idocular i n which the number of t racks appearing in each subdiv is ion was recorded. Those subdiv is ions containing grain imperfections were omitted from the count. I t was thus possible to construct a frequency diagram for each sample to check the d i s t r i bu t i on of the data. Two samples which showed anomalous behaviour were selected and the data are i l l u s t r a t e d in F i g . 2. The observed data do not depart s i g n i f i c a n t l y from that expected under the assumptions of Poisson d i s t r i bu t i on at the Chi-square 0.01 percent level of s i gn i f i cance . Hence i t must be concluded that an addi t ional component of variance ex is ts in the data ( i . e . operator var iance) . I t i s thus necessary to t a i l o r the method of obtaining to the qua l i t y of the data co l lec ted . In those cases where Poisson assumptions ho ld , the best 106 T R A C K D E N S I T Y R A T I O S No 0.5 0.6 0.7 0.8 0.9 1.0 l.l 1.2 I - 145 ^ • i « i I - 148 ' ' t . . ' • i i i I- 150 ' ' t • - —• \ 1-154 ' i ' - ( »' t 1- 173 ' • ' ' " i ' * ' i '  2 - 83 ', ' " ' '. "' 3- 0 ' i' , » , 3- I ' ' t - ' • " J '  3" 5 ' • = ^ 3- 6 i 3" 8 J '.' , 3- 10 '» ' . - 1 ( 3- 12 ' 1 3- 13 ' • f „ . . ' , — . ( 3- 14 *t • • t  i , 1 3- 16 I — « I 0 3- 17 , ' T~"*t- „ -t 3- 18 ' 1 ' 1 * . " ' i • » 3- 20 t> « t t 3- 21 i f » t ' • -i 3- 22 , ' « > ' —' 3- 24 ! ' » t ' 3- 25 ' >f • ^ ( 3- ST ' t ' ~ ' FIGURE I 107 OBSERVED VERSUS EXPECTED TRACK FREQUENCIES Sample 3 - 0 200h lOOh J l m-0 1 2 3 4 0.86 412 ' Z 3 4 S 6 1.56 510 C 1 2 i 4 S 0.92 368 0 1 2 J i 5 1.24 453 0 1 2 3 4 0.92 472 0 1 2 3 4 5 6 1.41 379 trial trial 2 trial 3 4001 Sample 1-145 200 0 1 2 3 0.33 499 1 0 1 2 3 0.56 457 LJD 0 1 2 0.23 525 0 1 2 3 0.56 51 5 0 1 2 0.38 523 !E UJJL 0 1 2 3 0.49 506 trial I trial 2 trial 3 m = estimated mean track density n = number of fields counted S- spontaneous tracks I= induced tracks Observed freq : U Expected f req : F I G U R E 2 108 estimate of A . (or ^ ) i s (Bennett and Frankl in 1954, p.608) E N p. ( q N _ j i j for determinations on j occasions M E • n . . n . . = number of f i e l ds counted on J J sample i on occasion j Pi = average track density per f i e l d of sample i on occasion j from which su i tab le error l im i ts can be der ived: (10) / ~ 1 1 E . n. . J i J Otherwise, i t is probably better to consider each track r a t i o , z . (determined on a par t i cu la r occasion), as an independent estimate from a d i s t r i bu t ion with larger variance. That i s ( ID a 2= = J J - l z = the overal l mean for deter-z j ruinations on j occasions In cases where two values of z . are c lose together and one remote, i t might be best to count a fourth or f i f t h sample before deciding which of the above pro-cedures to use. Any ou t l i e r can then be discarded. Table 1 i l l u s t r a t e s the raw data obtained and resul ts obtained by using both of the above models. It is most conservative to use the method which generates the largest percentage er ror . Note that in some cases estimated percentage errors d i f f e r by a factor of 2. The preceding discussion i l l u s t r a t e s the usefulness of counting the same sample several times as opposed to counting very many tracks (high n) in a sample on one occasion. In th is study, about f i f teen grains were counted at each time and involved between 100 and 400 t racks , depending on the uranium content of the sample. Anomalous v a r i a b i l i t y in the data was then readi ly detected. EQUATION 11 POISSON MODEL SAMPLE Z l Z2 Z3 Z °Z 0% i . Ps Xs Pi Xi Ps Z= — Pi °Z 0% ] - 145 .5914 .4041 .7763 0 .591 .108 18.2 .3115 481 .5338 789 0 .584 .034 5.8 1- 148 .5696 .7542 .7355 0 .686 .034 5.0 .5711 884 .8027 1233 0 .712 .031 4.4 150 .5916 .6043 .6582 0 .618 .020 3.3 .2827 406 .4642 738 0 .609 .038 6.2 154 .7070 1.0176 .9431 0 .889 .094 10.5 .4310 653 .4769 754 0 .904 .048 5.3 173 .6139 .9134 .7814 0 770 .087 11.3 .3629 560 .4611 712 0 .787 .045 5.7 2- 83 .6622 .6598 .6209 0 648 .013 2.0 .6303 762 1.0020 1530 0 .629 .028 4.4 3- 0 .6533 .7491 .5368 0 646 .062 9.5 .9022 1134 1.4266 1710 0 634 .024 3.8 3- 1 .9171 .7677 .6379 0 774 .081 10.4 .3177 338 .4345 663 0 731 .049 6.7 3- 5 .5666 .6559 .7742 0 666 .060 9.0 .1328 234 .2062 413 0 644 .053 8.2 3- 6 1 .0642 1 .0610 .8081 0 978 .085 8.7 .5798 759 .6230 985 0 931 .045 4.8 3- 8 .7061 .7061 .8704 0 761 .055 7.2 .6735 982 .8788 1254 0 766 .033 4.3 3- 10 .8470 .5947 .6258 0 688 .079 11.5 ' 3628 747 .5143 1076 0 705 .034 4.8 3- 12 1 .0560 .6175 1 .0488 0 907 .145 16.0 2088 208 .2321 344 0 900 .079 8.8 3- 13 .5599 .7240 .6640 0 649 .048 7.4 4128 637 .6281 998 0 657 .033 5.1 3- 14 .8904 ..7888 .8519 0 844 .027 3.5 2373 267 .2888 428. 0 822 .064 7.8 3- 16 .9089 1.1358 1 .0042 1. 016 .066 6.5 3555 625 .3622 648 0 982 .055 5.6 3- 17 .5782 .4247 .5257 0. 510 .045 8.9 3138 381 .6178 868 0. 508 .031 6.1 3- 18 6863 .7690 .6363 0. 697 .039 5.6 4555 302 .6525 892 0. 698 .046 6.7 3- 20 5537 .5071 .5749 0. 545 ' .020 3.7 4371 660 .8041 1371 0. 544 .026 4.7 3- 21 6493 .6075 .7739 0. 677 .049 7.2 2676 393 .4000 628 0. 699 .043 6.4 3- 22 6496 .8073 6216 0. 693 .058 8.3 2301 202 .3342 403 0. 689 .059 8.6 3- 24 7937 .7718 7244' 0. 763 .021 2.7 ;466 462 .4479 499 0. 774 .050 6.5 3- 25 7754 .6913 6581 0. 708 .035 J,]9 378 .3425 509 0. 730 .050 6.8 Symbols defined in text Xs,Xi=number of tracks counted 110 Case II Another method of obtaining Ps and Pi involves counting the spontaneous track density on several grains and then pressing a detector plate against the polished grain surfaces. Induced tracks are then counted in the detector plate af ter i r r a d i a t i o n . The s t a t i s t i c a l procedure in th is case is ident ica l to that involv ing equations (9) and (10). In the l i t e r a t u r e , however, one can f ind ages based on each grain count which are then averaged to obtain the sample age. The standard error is obtained using an equation analogous to equation (11). This is incorrect for two reasons. F i r s t l y , the fluence e r ro r , a^, i s common to each age estimate and cannot be reduced by j -1 (see equation 11) as for independent estimates. Thus the track r a t i o s , z , must be treated separately. In general , one can determine (or the track ra t io of any given sample) as prec ise ly as possible or desired but th is error must la te r be combined with the fluence error o ^ . Otherwise the error a ^ , common to each grain est imate, is reduced without j u s t i -f i c a t i o n . Secondly, the use of equation (10) i s not warranted unless the va r i a -b i l i t y in z from grain to grain is higher than that predicted by the Poisson d i s t r i b u t i o n , as discussed in the preceding sect ion. To i l l u s t r a t e the arguments set out above one can consider the data on apat i te obtained by Stuckless and Naeser (1972) (Table 2) . Each grain age has an error of between 8.7 and 16 m i l l i on years. These f igures presumably include the fluence e r ro r , the exact value of which i s not stated in the paper. The standard error of the mean age (times 2) is calculated to be + 0.8 m i l l i on years which i s very small considering the errors on the ind iv idual estimates based on Poisson s t a t i s t i c s . Assuming fo r the moment that there i s no fluence error and i n TABLE 2 APATITE AP-202 mi 11 ion years 49.9 ± 8.8 48.9 ± 1 6 5 Q J 50.5 + 8.7 51.0 + 12 Data from Stuckless and Naeser 1972 0.8 = two standard errors of the mean From th is study suggested age is 50.1 ± 5.9 5.9 = one standard error of the mean 112 the age errors are proportional to the track r a t i o s , z , the ind iv idual ages could be combined as (12) Mean Age = E i n d i v 1 d u a 1 a 9 e s J n with error (12) 2 _ " ,1.2 2 o~ • - variance in each Age * 0 mean . , V " j J estimate on a grain n = 4 in th is case The variance estimate i s eas i l y obtained from the Taylor series expansion (Greenwalt and Shultz 1962) with equation (12) as the source funct ion. The standard error obtained from the apat i te data is + 5.9, a more r e a l i s t i c f i gu re . It is in terest ing to note that using an equation s im i la r to (11) gives a very small standard er ror . Nevertheless, the grain ra t ios z are each dependent on the parameters Ps and Pi which can only be known, at the best of t imes, as prec ise ly as spec i f ied by the appropriate Poisson d i s t r i bu t i ons . The indiv idual grain ra t io estimates perhaps l i e close together because grains to be used can be preselected and v a r i a b i l i t y caused by poorly po l ished, s l i g h t l y a l tered (e tc . ) grains i s el iminated using the detector plate technique. In add i t ion , the same area is examined when determining Ps and Pi and hence v a r i a b i l i t y because of inhomogeneity in uranium is removed. This is not always the case, however. The data presented by Nagpal and Nagpaul (1973) i l l u s t r a t e further the d iscrep-ancies which can occur depending on the method of ana lys is . The resu l ts from four samples dated in that paper are reproduced in Table 3. In addit ion the s ta t -i s t i c s calculated by the present author are included for comparison. Nagpal and Nagpaul (1973) state that the quoted overal l error for each sample is 'the standard deviat ion from the mean value" although the numbers seem to be most 113 TABLE 3 Data from Nagpal & Nagpaul (1973) Sample Table 1, #7 756 + 30 750 + 22 747 + 6* 744 + 28 Table 1 , #8 803 + 40 746 + 74 768 + 25 755 + 67 Table 2, #1 538+16 518 + 25 519 + 15 501 + 20 Table 2, #3 457 + 9 300+15 515 + 10 428 + 77 384 + 12 485 + 7 Original data: mean + "standard deviat ion of the mean". j - i 31 19 68 Standard deviat ion 18 11 31 Equation 13 Standard error of the mean. See Equation 11. 16 36 12 Calculat ions in th is paper. tactual mean appears to be 750, not 747. 114 s imi la r to the square root of the sample variance as calculated by the author. The standard error of the mean (Eg. 11) is also calculated as well as the error according to Eq. 13 which is based purely on counting s t a t i s t i c s . If the suggestion made in th is paper i s fo l lowed, the largest of these two errors would be quoted. These numbers are marked with as ter isks in Table 3 and, in some cases, can be seen to d i f f e r great ly from each other and from those given by Nagpal and Nagpaul. Case III Here spontaneous track densi t ies are determined on polished surfaces of several gra ins. The grains are then i r rad ia ted and re-etched to reveal induced t racks. The induced track density i s obtained by subtract ing the spontaneous track density from the to ta l track density a f ter i r r ad ia t i on (see Bigazzi and Ferrara 1971). In th is case the track density ra t io i s (14) z = Ps Ps = spontaneous track density P t - P s Pt = to ta l track density The propagated error is given by Taylor ser ies expansion (Greenwalt and Shultz 1962). Here 2 2 I 6iV 2 (15) a\= p - 1 °s + from which i t can be shown <16> oz%J__h , .s + a t P - Ps I t s 2 2 °t Ps (P t-Ps)J In teres t ing ly , as P t approaches Ps ( this can happen i f an unsuitable neutron 1 1 5 fluence i s used for any given age) the error increases quite d r a s t i c a l l y because of the subtract ion in the denominator of equation ( 1 4 ) which resu l ts in a small number with a large error . For th is reason i t is probably best to avoid using th is method in favour of detector p la tes . If used i t i s necessary to choose a su i tab le reactor fluence which resul ts in as low a ra t io as possible (see equation ( 1 ) : 0 < x P i for any given age). This presupposes a rough knowledge of the age of the sample which is not always ava i lab le . The parameters Ps and Pi along with the i r standard errors can be obtained using equations ( 9 ) and ( 1 0 ) . Combination of track ra t io error and fluence error . As pointed out e a r l i e r (see case II) i t i s necessary f i r s t to deal with the track ra t io error and then to combine in the fluence error . This can be done with the fol lowing formula: 2 2 , 2 ( 1 5 ) a A g e % = a 2% + a Q% which i s a simple extension of equations ( 5 ) and ( 6 ) . S t r i c t l y speaking the error obtained from the r igh t hand side of equation ( 1 5 ) should be appl ied within the argument of equation ( 1 ) . S ince, however, the funct ion In varies almost l i nea r l y wi thin the range of arguments used the percentage error can be applied d i r ec t l y to the age with only a f rac t ion of a percent e r ror . An in terest ing s ide l i gh t to the combination of the ra t io and fluence error involves discordancy of ages. Obviously i f one wishes to look at two ages the same reactor run i t i s unnecessary to combine in the fluence error which i s common to both. These samples can be compared in terms of t he i r track density ra t i os . 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Potassium-argon ages of igneous rocks from the area near Hedley, southern B r i t i s h Columbia. Can. J . Earth S c i . 9, pp. 1632-1639. Ross, J . V . , 1968. Structural re la t ions at the eastern margin of the Shuswap Complex, near Revel stoke, southern B r i t i s h Columbia. Can. Jour . Earth S c i . , 5, pp. 813-849. Ross, J . V . , 1970. Structural evolut ion of the Kootenay A rc , southeastern B r i t i s h Columbia. Geol . Assoc. Can, Spec. Paper, 6, pp. 53-65. Ross, J . V . , 1974. A Ter t iary thermal event in south-central B r i t i s h Columbia, Can. J . Earth S c i . 11, pp. 1116-1122. Ross, J . V . and C h r i s t i e , J . C , 1969. Polyphase deformation wi th in the Shuswap Terrane of the southern Okanagan Va l l ey , B r i t i s h Columbia. Geol . Soc. Am., abst r . Part 3 , p. 57. Ross, J . V . and Barnes, W . C , 1972. Evidence for "Caribooan Orogeny" in the Southern Okanagan region of B r i t i s h Columbia. Can. J . Earth S c i . 9, pp. 1693-1702. 121 Ryan, B .D. , 1973. Geology and R b r - S r Geochronology of the Anarchist Mountain area, southcentral B r i t i s h Columbia. Unpublished Ph.D. t h e s i s , the Univers i ty of B r i t i s h Columbia, Vancouver, B.C. Souther, J . G . , 1970. Volcanism and i t s re la t i on to recent crusta l movements in the Canadian C o r d i l l e r a , Can, J . Earth S c i . 7, pp. 553-568. Stuck less , J . S . and Naeser, C.W., 1972. Rb-Sr and f i s s i on - t r ack age determin-at ions in the Precambrian plutonic basement around the Superst i t ion vo lcanic f i e l d , Ar izona. U.S. Geol . Surv. Prof . Pap. 800-B, pp. 191-194. Turner, F . J . , 1968. Metamorphic petrology, mineralogical and f i e l d aspects. McGraw-Hill Inc. Wanless, R .K . , 1969. Isotopic age map of Canada, Geol . Surv. Can. , Map 1256A. Wheeler, J . O . , 1970. Summary and d iscuss ion . Geol , Assoc. Can. Spec. Paper, 6, pp. 155-166. White, W.H., 1959. Cord i l le ran tectonics in B r i t i s h Columbia. Amer. Assoc. P e t r o l . Geol . B u l l . , 43, pp. 60-100. White, W.H., Er ickson, G . P . , Northcote, K . E . , Dirom, G . E . , and Harakal , J . E . , 1967. Isotopic dating of the Guichon ba tho l i t h , B r i t i s h Columbia: Can. J . Earth S c i . 4,pp.677-690. White, W.H., Harakal , J . E . , and Car ter , N .C. , 1968. Potassium-argon ages of some ore deposits in B r i t i s h Columbia. Canadian Min. and Met. B u l l , pp. 1326-1334. Winkler, H .G.F . , 1967. Petrogenesis of metamorphic rocks. Springer-Verlag New York Inc . , New York. 122 APPENDIX 1 F iss ion track and K-Ar ana ly t i ca l techniques. The ca lcu la t i on required to obtain a f i s s i o n track age i s given in Naeser (1967), namely: Age (y r . ) = 6.49 x 10 9ln(l + 9 ' 4 5 * R 0 \ v 1(T where R = the ra t i o of natural track density (ps) to induced t rack densi ty (fxO 0 = to ta l neutron f l ux (thermal) x 1 0 ^ Samples of apat i te which had been annealed of spontaneous tracks by heating at 600°C fo r 24 hours were sent to the Nuclear Science and Technology F a c i l i t y State Univers i ty of New York at Buffalo Rotary Road Buf fa lo , New York 14214 15 with a request for a to ta l thermal neutron exposure of approximately 1 x 10 2 neutrons per cm . Sphene samples which had been pre-pol ished and etched to reveal natural tracks were a lso inc luded. Each sample was wrapped in an aluminum f o i l packet and, together, a l l samples occupied a volume of approx-2 imately 8 cm . In order to determine the actual neutron exposure, 3 standards cons is t ing of apat i te of a known age were inc luded. Two of these apat i te samples were obtained from the K i t l ey Lake Member of the Marron Formation (Church, 1970) with a K-Ar date of 51.6 + 1.8 m.y. A th i rd apat i te was obtained from Dr. C.W. Naeser of the U.S. Geological Survey. This sample was taken from a tu f f on which concordant dates (27.2 +0 .7 m.y.) on the minerals p lag ioc lase , horn-blende, b i o t i t e , and sanidine had been obtained. Measurement of the ra t i o of natural to induced track dens i t ies for these samples allowed est imation of the neutron exposure by rewr i t ing the age equation as fo l lows : 123 By subs t i tu t ing the appropriate age (A) fo r a given sample and measuring the track densi ty ra t i o (R), 0 could be measured. The resu l ts were as fo l lows : R 3% 0 x 1 0 1 4 C.W. Naeser 1s apat i te .398 +7.56 11.40+.84 K i t l ey Lake apat i te (1) .763 + 6.71 11.07 + .74 K i t l ey Lake apat i te (2) .708 +6 .99 11.92 + .83 Mean 0 = 11.4 + 0.5 x 1 0 1 4 neutrons/cm 2 Samples were counted using a magnif icat ion of 1200 with a 6 x 6 square gr idocu lar . S t a t i s t i c a l methods of treatment of the data are presented in Paper #2. Approximately 30 mineral grains of apat i te containing natural t racks were examined. Track densi ty wi th in each gr id subf ie ld was recorded. Subf ie lds containing imperfections were omitted from the count. The track densi ty r a t i o was subsequently ca lcu lated (tracks per f i e l d ) . In the case of the sphene samples, polished grains were pre-etched to reveal natural t racks . These grains were then re-etched a f te r i r r ad ia t i on and the induced track densi ty obtained by subt ract ion. In th i s case, the induced track density had to be doubled to account fo r tracks that would have been contributed from the part of the mineral removed by po l i sh ing . The K-Ar analyses were car r ied out using the mass spectrometry system of the Univers i ty of B r i t i s h Columbia located in the Department of Geophysics and Astronomy, and operated j o i n t l y by the above department and the Department of Geological Sciences. The system i s iden t i ca l to that described by White et a l . (1967) except that the sample and ent i re fus ion system were baked at 130°C fo r 18 hours to el iminate or reduce atmospheric argon contamination (Roddick and Far ra r , 1971). A schematic diagram of the argon ext ract ion and ana ly t i ca l system i s presented on page 124. TO PUMPS ELECTROMETER M g AIR PIPET Ar 30 ((0)) ION PUMP A WW SPIKE PIPET 1 MASS SPECTRO-METER 7 / PTT^^ r I o - J o o o A rr / nr / t z j LEAK VALVE I TO PUMP SC I rM E T A L VALVES c TRAP — f p . ivy-Vp c TRAP m ^-s\ SAMPLE IN g Mo CRUCIBLE ° / Ti IN Si0 2 TUBE r-7—r~r-/ J / /1 i T I TO PUMPS! a UBC K/Ar LABORATORY ARGON EXTRACTION a ANALYT1CAI S Y S T E M 125 Potassium analyses were obtained by flame photometry as described in White et a l . (1967). The number of rep l i ca te analyses was selected such that the standard er ror of the mean was less than about 3%. This was necessary as impur i t ies in some samples resul ted in var iab le sampling errors (Ingamells, et a l . , 1972)(see footnote of Table I, p. 90). 126 APPENDIX 2 Cool ing-rate ca lcuat ions . In order to obtain a minimum estimate of the coo l ing- ra te of the Shuswap gneisses, i t was necessary to se lec t a c losure temperature for the argon clock which i s conservat ively low, and a track retent ion temperature l i m i t fo r f i s s i o i t rack dates which i s conservat ively high. Thus, the temperature range over which cool ing occurs was made as small as poss ib le . In the f i r s t coo l ing-ra te est imate, the hornblende blocking temperature was selected as 150°C. This estimate was taken from "the f rac t iona l argon l oss " versus "duration of elevated temperature" curve (case I) computed by Damon (1968). This estimate i s qui te a reasonable one as i t can be observed that heating hornblende fo r 100 m.y. at 155°C would resu l t in less than approximately 2% argon l o s s . On the other hand, the temperature of complete track retent ion in apat i te was selected as 75°C, or 25°C higher than the estimate presented by Naeser and Faul (1969). Thus a minimum cool ing in te rva l of 75°C was obtained. The time over which cool ing occurred has been estimated by comparing 2jthe mean hornblende K-Ar date (51.2 + 4.0 m.y . ) , excluding sample 3-10 ( 5 9 J ^ m.y.) which i s unusually h igh, possib ly because of Ar gain with the mean apat i te date (48.4 +7.5 m.y . ) , inc lud ing sample 3-17 (35.6) which i s unusually low probably because of l a t e r heat ing. Inclusion and exclusion of these numbers makes a small d i f ference in the means (e .g . 51.2 becomes 52.5, and 48.4 becomes 49.3 m.y.) and does not appreciably a l t e r the coo l ing- ra te est im-ate. As i t i s phys ica l l y reasonable that the mean K-Ar date should be greater than mean apat i te date , one sided confidence l i m i t s fo r d i f ference between the means ( i . e . 2.8 m.y.) resu l t s i n a range of poss ib le values of 0 to 7.3 m.y. at 127 the 5% leve l of s ign i f i cance . Using these extremit ies the coo l ing- ra te var ies between an i n f i n i t e value and 10°C/m.y. The ca lcu lated d i f fe rence , 2.8 m.y., however, i s nevertheless a reasonable estimate and resu l t s in a minimum cool ing rate of approximately 25°C/m.y. A s i m i l a r argument using the sphene mean date (53.6 +_ 5.9 m.y.) and the apat i te mean date (48.4 + 7.5 m.y.) resu l t s in d i f fe rence of 5.2 m.y. wi th one-sided 95% confidence l i m i t s of 0 and 11.8 m.y. Using the 11.8 m.y. extremity, and a temperature in terva l of 200°C (somewhat smaller than that given by the data of Naeser and Fau l , 1969), the cool ing rate must be at l eas t 17°C/m.y.. The observed mean d i f fe rence , on the other hand, resu l t s i n an estimate of approximately 40°C per m.y. Thus, regardless of how one wishes to manipulate the data, rapid cool ing of the Shuswap gneiss i s an inescapable conclusion and i s consistent with a model invo lv ing shallow leve l heating of the crust fol lowed by quick conduc-t i ve cool ing upon cessat ion of the heat input. V E R T I C A L C R O S S - S E C T I O N S O K A N A G A N M O U N T A I N A R E A Unit I : Laminated amphibolite, minor mossive amphi-bolite, granulile. Unit 2 = Hornblende (biotite)granitoid gneiss.granulite Unit 3 : Augen gneiss Unit A Leuco-quartz monzonite ,early synkinematic phase 2. Unit 8 • Foliated Granodiorite, mainly hornblende granodiorite (Bi) gradational into biotite-rich hornblende granodiorite (Bz), late syn-kinematic phase 2. MILES • 2 3 4 5~ FEET THOUSANDS NORTH OKANAGAN L A K E SOUTH ELEVATION FEET - SOOO - 4 0 0 0 - 3000 2000 A' 3000 12000 IOOO Hsooo H2000 IOOO OKANAGAN L A K E isooo H4000 i 3 0 0 0 2000 IOOO PLATE 2 G E O C H R O N O L O G Y K E T T L E R IVER ( W E S T H A L F ) L_ 12 10 PRINCETON GROUP VOLCANiCS CORYELL INTRUSIVES VALHALLA GRANODIORITE NELSON PLUTONICS EMPRESS GRANITE McNULTY CREEK QT/.MONZONITE JURA GRANODIORITE SIMILKAMEEN OTZ.DIORITE SUMMERLAND DIORITE KIRTON DIORITE NICOLA GROUP 0 : 0LD TOM .SHOEMAKER, BAR SLOW F M . b KOBAU ^iROUP c MONASHEE GROUP, ANARCHIST GROUP NOTE Geology generalized from mop by Little (1961). Intrusive contacts west of Okanogan Lake are from Peto(l973a,b). K Potassium-Argon date b-biotite h hornblende m muscovite wwhole rock A Apatite dote S Sphene date Underlined dates from White et al (1968) '!> 5« K h 5 2 . 2 ± l 5 A 4 6 . 4 M 6 S 46. I-l I 5 3 - 2 i « K b 5 l . 8 * 1 . 6 A 47.2*39 S H U S W A P GNEISS 2 ; / / 3 •Kh53 .4 i | . 9 A 5 3 0 - 4 . 4 12 l/l \ II \ 10 \ / \ r u 10 BEAVERDELL 10 3-10 Kh59.9-2.0 <J A 4 7 9 * 5 8 Km "44*6 Kb 82,99,118 \ 3 - 2 2 » Kb54.8*1.6 A 4 8 0 - 4 6 S 59.5*12.2 Id '3 -o»A45.0*4.6 < lb Khl7l*6.4 b OLIVER O O < < < < < > o r> w z <J - J < > < > SHUSWAP GNEISS i c I// / \ / / I / ' 2 / 50° 0 0 ' 4 9 3 3 0 ' / i c \ SIMILKAMEEN COMPLEX 3-i«A 53 9*6 .0 5? Ax) 12 i c s 4 9 ° 0 0 I 2 0 ° C 0 ' l 9 ° 0 O ' 

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