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Geology of the Pinchi Lake area, central British Columbia Paterson, Ian Arthur 1973

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THE GEOLOGY OF THE PINCHI LAKE AREA, CENTRAL BRITISH COLUMBIA by IAN ARTHUR PATERSON B.Sc. (Hons.) , Aberdeen University, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of Geological Sciences We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June 1973 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of GEOLOGICAL SCIENCES The University of British Columbia Vancouver 8, Canada ABSTRACT THE GEOLOGY OF THE PINCHI LAKE AREA The area mapped, 75 square miles, l i e s a s t ride the Pinchi Fault near Fort St. James i n central B r i t i s h Columbia. Northeast of the f a u l t system i s the lower Mesozoic Takla Group composed of greywackes, conglomerates and minor limestones. Southwest of the f a u l t system i s the Pennsylvanian-Permian Cache Creek Group, made up of limestones, cherts, a r g i l l i t e s and greenstones. Between these regions, a complex northwesterly-trending f a u l t system involves a series of elongate fault-bounded blocks of contrasting l i t h o l o g y and/or metamorphic grade. Rock types making up i n d i v i d u a l blocks include: Ca) lawsonite-glaucophane metasediments and meta-volcanics, Cb) pumpellyite-aragonite greenstones, Cc) serpentinized harzburgites and dunites and (d) a sequence of amphibolitized gabbro-diabase basalt. Boulders of lawsonite-glaucophane .^eclogite are also found i n the area. Within the glaucophanitic rocks a metamorphic f o l i a t i o n CS^) p a r a l l e l s the bedding and contains a mineral l i n e a t i o n i i (L^). This f o l i a t i o n i s deformed by F^ folds and mullions which are accompanied by a prominent crenulate l i n e a t i o n . These structures are deformed by kink f o l d s . S i g n i f i c a n t mineral assemblages within the glauco-phanitic rocks include: (i) quartz + lawsonite + sphene + phengite + glaucophane ± carbonaceous material (metacherts) ( i i ) aragonite + dolomite (limestones) ( i i i ) j a d e i t i c pyroxene + lawsonite + quartz + white mica + c h l o r i t e (metagreywacke) (iv) quartz + white mica + lawsonite ± glaucophane + p y r i t e + carbonaceous material (carbonaceous schists) (v) acmitic pyroxene + lawsonite + sphene + c h l o r i t e (metavolcanics) (vi) glaucophane + lawsonite + sphene + c h l o r i t e (metavolcanics). Comparison of these assemblages with experimentally determined phase e q u i l i b r i a favours the hypothesis that the glaucophanitic rocks formed at high l i t h o s t a t i c pressures and r e l a t i v e l y low temperatures. Because there i s no evidence for a metamorphic zonation with respect to the Pinchi Fault, and because metagreywackes and metavolcanics are commonly unsheared, tectonic overpressures are not considered to have contributed appreciably to the t o t a l pressure. In the late stages of metamorphism the f l u i d phase i n carbonaceous schists became progressively more reducing and may have contained appreciable methane. This f l u i d reacted with acmitic metavolcanics (v) to give glaucophane bearing assemblages t v i ) . Thus high pressure mineral assemblages with high 3+ 2+ Fe /Fe (.i.e. v) existed within the metavolcanics p r i o r to reaction with the reducing f l u i d . A three stage tectonic model i s proposed. F i r s t l y , during the Late Permian, a narrow wedge of Upper Paleozoic sediment was metamorphosed i n the blueschist f a c i e s along an easterly dipping subduction zone approximately on the s i t e of the Pinchi Fault. This event i s considered to be contemporaneous with the formation of structures. At a higher c r u s t a l l e v e l , an o p h i o l i t e sequence of which unit (d) i s a remnant was obducted over the Cache Creek Group sediments which had accumulated above the subduction zone. During the second stage, a change i n the stress system converted the subduction zone into a r i g h t - l a t e r a l s t r i k e -s l i p or o b l i q u e - s l i p f a u l t . This event marked the beginning of the deformation. S t r i k e - s l i p movement resulted i n formation of zones of low pressure where there were deflections or o f f s e t s i n the o r i g i n a l f a u l t . At Pi n c h i , such zones were at once f i l l e d from below by d i a p i r s of low average density consisting of subducted blueschist, serpentinized ultramafites and minor e c l o g i t e which rose i v i n the crust to a l e v e l governed by isostasy. I t i s con-sidered that the Middle or Upper T r i a s s i c K-Ar dates obtained from phengitic micas (211, 214, 216 and 218 ± 7 m yrs) record either the time of cooling of the u p l i f t e d blueschists or the end of the F 2 deformation. The t h i r d stage, during the Upper T r i a s s i c , involved erosion of the o p h i o l i t e s exposed on a topographic high to the west of the Pinchi Fault and f l y s c h deposition i n the adjacent trough to the east. The deformation was contemporaneous with carbon-a t i z a t i o n of f a u l t zones and possibly occurred during the Eocene. v TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES xiv LIST OF FIGURES xvi LIST OF PLATES xviv LIST OF MAPS xx ACKNOWLEDGEMENTS xxi I. INTRODUCTION 1 General Introduction 1 Location and A c c e s s i b i l i t y 1 Physiography and Gl a c i a t i o n 3 Previous Work 4 Work Done i n thi s Study 6 I I . GENERAL GEOLOGY 7 Regional Setting 7 Cache Creek Group 11 Introduction 11 Greenstones of Pinchi Mountain 13 Lithology and petrology 14 Internal structure and contacts 14 Age 15 Origin 16 v i Page Glaucophane-lawsonite bearing rocks around Pinchi Lake 16 D i s t r i b u t i o n . . . i 16 Rock types and proportions 17 Petrology 18 Internal structure 21 External contacts 2 2 Age 22 Origin 23 Basic rocks south of Pinchi Lake . . . . . . 24 D i s t r i b u t i o n and petrology 24 Internal structure and contacts 25 Age and o r i g i n 2 6 Massive limestones and cherts of the Mount Pope b e l t 27 D i s t r i b u t i o n 27 Lithology and petrology 27 Fauna and age 29 Internal structure and contacts 29 Origin 30 Takla Group 32 Introduction 32 Lithology 34 Conglomerate 36 Internal structure and contacts 39 v i i Page Fauna 40 Origin 41 D i o r i t e 43 Cretaceous (?) Conglomerate 44 I I I . ULTRAMAFITES AND SILICA-CARBONATE ROCKS . . . . 46 Ultramafites 46 Introduction 46 Rock types 46 Internal structure . . . 48 Contact re l a t i o n s h i p s 51 Petrology . . . . . 51 Discussion on the o r i g i n of ultramafites 53 Origin of Pinchi ultramafites 57 Silica-carbonate Rocks 62 Di s t r i b u t i o n 62 Rock types, i n t e r n a l and external structure 62 Petrology 64 Age and o r i g i n 65 IV. METAMORPHISM 68 PART I - METAMORPHISM IN GREENSTONE AND BLUESCHIST FAULT BLOCKS 68 Introduction 68 Paragenetic Sequence of Minerals 71 v i i i Page Mineral Assemblages 74 Metamorphic Reactions 7 8 Metabasic rocks 78 Metasediments 8 3 Relevant Phase E q u i l i b r i a 84 Aragonite s t a b i l i t y 8 4 Jadeite and acmite-jadeite s t a b i l i t y 87 Phengite s t a b i l i t y 90 S t a b i l i t y of sodic amphiboles 91 Lawsonite s t a b i l i t y 91 Pumpellyite and prehnite s t a b i l i t y 9 2 Eclo g i t e s t a b i l i t y 93 Oxygen isotopes 94 Pressure-temperature Conditions of Metamorphism 96 F l u i d Phase Composition 96 F l u i d Phase Composition at 10 kb and 327°C . . . 104 Evolution of F l u i d Phase 108 Pressure-Temperature Trajectories of Pinchi Rocks 11° PART II - METAMORPHISM IN THE REMAINING FAULT BLOCKS 112 Metabasic Rocks South of Pinchi Lake 112 Mount Pope Belt 114 Takla Group 114 ix Page Ultramafites 116 V. STRUCTURAL GEOLOGY 118 Introduction . . ' 118 Structural Elements i n the Lawsonite-Glaucophane Bearing Rocks 123 F^ Deformation 125 ^2 Deformation 127 F^ Deformation 12 8 Microscopic Analyses 130 Timing of Metamorphism and Deformation 134 Structural Geology of Remaining Fault Blocks . . 139 Greenstones of Pinchi Mountain 139 Mount Pope b e l t 139 Ultramafites 140 Basic rocks south of Pinchi Lake 141 Takla Group 142 Important Faults i n the Pinchi Area . . . . . . 142 Pinchi Fault (No.l) 143 Fault system no. 2 145 Fa u l t system no. 3 147 Fault system no. 4 148 Fault system no. 5 148 Faults i n the v i c i n i t y of Pinchi Mine . . . . 149 x Page VI. TECTONIC IMPLICATIONS 150 Introduction 15 0 Constraints to Tectonic Model (Factual and Inferred) 15 2 Conclusions of t h i s study 152 Inferred c r u s t a l structure i n central B r i t i s h Columbia 153 The Pinchi Fault as a "Suture Zone" 157 Evidence for s t r i k e - s l i p movement on the Pinchi Fault 160 P o s s i b i l i t y that the Pinchi Geanticline was overlain by oceanic crust 161 Absence of metamorphic zonation within the Cache Creek Group . 161 Tectonic Models 162 Subduction model 162 Subduction/obduction model . . . . 168 Mesozoic and Tertiary events 172 REFERENCES CITED 174 APPENDIX I - FOSSIL LOCALITIES IN THE PINCHI AREA 188 APPENDIX II - MINERALOGY 191 A n a l y t i c a l Methods 191 Minerals . . . . . . 193 Sodic amphiboles 193 Brown and green amphiboles 198 Pyroxenes . . . . . 200 x i Page Ch l o r i t e 207 Aragonite 209 White mica 212 Celadonite 215 A l b i t i c plagioclase 215 Pumpellyite 216 Stilpnomelane 216 Lawsonite 216 Prehnite . 217 Garnet . 217 Deerite 217 Opaques 218 Carbonaceous material 220 APPENDIX III - BULK CHEMICAL ANALYSES 221 Metabasic rocks 2 21 Metagreywackes . 228 APPENDIX IV - PETROLOGY 231 Greenstones of Pinchi Mountain . . . . . . . . 231 Lawsonite-Glaucophane Bearing Rocks 2 37 Metabasic rocks 237 Dolomitic carbonates 2 41 Metagreywackes 245 Metacherts 247 Quartz-carbonate rocks and schists 249 x i i Page APPENDIX V - ECLOGITE BOULDERS 250 APPENDIX VI - POTASSIUM-ARGON RADIOMETRIC DATES . . . 253 APPENDIX VII - CALCULATION OF EQUILIBRIUM CONSTANT FOR REACTION: PLAGIOCLASE = JADEITIC PYROXENE + QUARTZ 256 APPENDIX VIII - SPECIMEN..NUMBERING SYSTEM 258 PLATES (following page 261) MAPS (at end of thesis) \»\ evwelo|>e, -C; ie I 6a s ide . x i i i LIST OF TABLES Table Page 1. TABLE OF FORMATIONS IN PINCHI AREA 9 2. MODAL ANALYSES OF GREYWACKES FROM THE TAKLA GROUP 33 3. PEBBLE CONTENT OF CONGLOMERATES WITHIN TAKLA GROUP 38 4. GENERALIZED PARAGENETIC SEQUENCE OF METABASALTS 7 5 5a. POSSIBLE REACTIONS IN METABASIC ROCKS . . . . 76 5b. MINERAL COMPOSITIONS USED IN REACTIONS . . . . 77 6. OPTICAL PROPERTIES OF PYROXENES AND AMPHIBOLES 194 7. CELL DIMENSIONS FOR GLAUCOPHANE AND JADEITIC PYROXENE 195 8. ELECTRON MICROPROBE ANALYSES OF GLAUCOPHANES 196 9. ELECTRON MICROPROBE ANALYSES OF RELICT AUG ITE 199 10. ELECTRON MICROPROBE ANALYSES OF METAMORPHIC PYROXENES 201-202 11. ELECTRON MICROPROBE ANALYSES OF CHLORITES 208 12. CARBONATE MINERALOGY AND SAMPLE DISTRIBUTION 211 13. PARTIAL ELECTRON MICROPROBE ANALYSES OF PHENGITES AND CELADONITE 213 14. STANDARD DEVIATIONS FOR SELECTED MINERAL ANALYSES 214 15. ACCURACY OF BULK CHEMICAL ANALYSES 222 16a. CHEMICAL ANALYSES OF METABASALTS FROM THE PINCHI AREA 223 xiv Table Page 16b. ADJUSTED CHEMICAL ANALYSES OF METABASALTS . 224 17. C.I.P.W. NORMS FOR ANALYZED META-BASALTIC ROCKS 224 . 18. SUMMARY OF SIGNIFICANT CHEMICAL CHARACTER-ISTICS OF METABASALTS 227 19. CHEMICAL ANALYSES OF METAGREYWACKES 22 9 20. EQUILIBRIUM MINERAL ASSEMBLAGES IN THE GREENSTONES OF PINCHI MOUNTAIN 2 32 21. TEXTURAL DOMAINS WITHIN THE PINCHI MOUNTAIN GREENSTONES . . 233 22. EQUILIBRIUM MINERAL ASSEMBLAGES IN THE LAWSONITE-GLAUCOPHANE BEARING METAVOLCANICS . . . . . . . . . . . . . . . . 238 23. TEXTURAL DOMAINS IN LAWSONITE-GLAUCOPHANE BEARING METABASIC ROCKS . . 239 24. EQUILIBRIUM MINERAL ASSEMBLAGES IN METAGREYWACKES 244 25. EQUILIBRIUM MINERAL ASSEMBLAGES IN. CHERTS AND CHERTY GRAPHITE SCHISTS 244 26. SAMPLE LOCATIONS AND ANALYTICAL DATA FOR POTASSIUM-ARGON ANALYSES 254 xv LIST OF FIGURES Figure Page 1. Location and regional geology map 2 2. Major tectonic elements of B r i t i s h Columbia 8 3. Geological subdivisions at Pinchi Lake . . . 12 4. Takla Group: sections, f o s s i l and specimen l o c a l i t i e s 12 5. St r u c t u r a l features i n Harzburgites . . . . 47 6. Paragenetic sequence of metamorphic minerals i n metabasalts and eclogites . . . . 72 7. Paragenetic sequence of metamorphic minerals i n metasediments 7 3 8. Phase e q u i l i b r i a relevant to formation of mineral assemblages 85-86 9. Pressure-temperature conditions of metamorphism of greenstones, blueschists and eclogites 95 10. Phase e q u i l i b r i a relevant to f l u i d phase chemistry 97-99 11. Schematic i l l u s t r a t i o n of f l u i d phase chemistry 105-106 12. T-f.Q diagram i l l u s t r a t i n g evolution of f l u i d phase composition 106 13. Pressure-temperature t r a j e c t o r i e s for greenstone, blueschist and eclogite I l l 14. Phase e q u i l i b r i a i n the system MgO-Si02-H 20 I l l 15. Stereographic projections i l l u s t r a t i n g s l r L2 a n c ^ ^3 orientations xv i Figure Page 16. St r u c t u r a l elements i n s c h i s t s 120 17. F2 folds 121 18. Recumbent i s o c l i n a l F2 folds C22a, 22b) and F2 folded by F3 (22c) 122 19. Relation of metamorphic r e c r y s t a l l i z a t i o n to deformation . 131 20. Depth-time trajectory for Pinchi rocks . . . 135 21. Crustal model for central B r i t i s h Columbia 154 22. Generalized diagram i l l u s t r a t i n g location of major geologic units i n central B r i t i s h Columbia i n r e l a t i o n to the problematical b e l t of Paleozoic rocks between the Takla Group and the Omineca Geanticline 158 23. Subduction model 163 24. Subduction - obduction model 166-167 25. Possible mechanism for intrusion of blue-schists and serpentinized ultramafites along s t r i k e - s l i p f a u l t s 169 26. Element v a r i a t i o n in. zoned glaucophane . . . 197 27. Compositional v a r i a t i o n of pyroxenes from Pinchi and C a l i f o r n i a 203-205 28. Map of aragonite occurrences i n Pinchi area 210 29. Map of prehnite, pumpellyite and selected glaucophane-lawsonite occurrences 210 30. Textures i n opaque minerals 219 31. A-F-M diagram i l l u s t r a t i n g basalt compositions 225 32. A l k a l i - s i l i c a v a r i a t i o n diagram for metabasalts . . . . . . 225 xv i i Figure Page 33. Textures i n Pinchi Mountain greenstones. . 236 34. Textures i n lawsonite-glaucophane bearing metabasic rocks 240 35. Textures i n carbonates 242 36. Textures i n metasediments 246 37. Ecl o g i t e l o c a l i t i e s and source areas as in f e r r e d from g l a c i a l transport directions 251 x v i i i Plate 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. LIST OF PLATES (following p. 2 61) View of the Pinchi Lake area looking northwest from the summit of Mount Pope Laminated s i l t s t o n e s and sandstones i n the Takla Group Northwesterly plunging mullions i n quartz-mica carbonate schists Angular limestone cobble conglomerates i n Takla Group Northerly dipping compositional layering i n silica-carbonate rocks Primary (?) two phase f l u i d i n c l u s i o n i n metacherts Late pyroxenite layers p a r a l l e l to dunite r i c h layers i n harzburgite Late discordant pyroxenite layer cross-cutting i r r e g u l a r dunite Smooth weathering dunite layer p a r a l l e l to f o l i a t i o n i n harzburgite outlined by varying o l i v i n e : pyroxene r a t i o Folded early pyroxenite layers Folded dunite layers i n f o l i a t e d harzburgite xviv LIST OF MAPS (Following p.263 at end of thesis) Map I. Geology of Pinchi Lake area I I . Cross sections of Pinchi Lake area I I I . Aeromagnetic map of Pinchi area IV. Geology i n the v i c i n i t y of Pinchi Mine V. Geology of an area on the northwest shore of Pinchi Lake VI. Important f a u l t s i n the Pinchi area VII. Specimen location map xx ACKNOWLEDGEMENTS Cominco Ltd. permitted access to the i r claims which l i e within the area mapped. The kindness, cooperation and advice of employees i n the course of this study i s grate-f u l l y acknowledged, e s p e c i a l l y G. Warning, geologist at Pinchi Lake Mercury Mine. The author i s deeply indebted to Drs. H.J. Greenwood and K.C. McTaggart of the University of B r i t i s h Columbia who introduced the writer to the project, provided early supervision and made constructive c r i t i c i s m s of the manu-s c r i p t . Helpful suggestions for improvements to various parts of the manuscript were also made by Drs. W.C. Barnes, W.R. Danner and P.B. Read1"' Call at U.B.C). F o s s i l i d e n t i f i -cations were made by Drs. W.R. Danner CU.B.C), B.E.B. Cameron and H. Frebold (G.S.C.). K-Ar radiometric dates were obtained by J.E. Harakal .and V. Bobik i n the laborator-ies of the departments of Geological Sciences and Geophysics at the University of B r i t i s h Columbia. Assistance and tra i n i n g i n the use of the electron microprobe at the University of Washington were provided by Dr. B.W.. Evans and L. L e i t z . The author also wishes to thank members of the Vancouver branch of the Geological Survey of Canada who gave generously of t h e i r time i n h e l p f u l discussions and were xxi instrumental i n obtaining f i f t e e n chemical analyses from the G.S.C. laboratory i n Ottawa. Costs of f i e l d work were defrayed by a grant from the Geological Survey of Canada and l o g i s t i c a l support was provided by Cominco Ltd. From 1968-71, the author was supported by a scholarship from the National Research Council of Canada. F i n a l l y , the author's sincere thanks are extended to the f a c u l t y , technical s t a f f and graduate students at /the University of B r i t i s h Columbia for the i r i n t e r e s t and constructive suggestions i n the course of t h i s study. Special thanks are due to my wife Barbara for assistance i n the f i e l d and i n edi t i n g the manuscript. x x i i ADDENDA p. 65, Line 23. Recent (1972) radiometric dates from the Sustut Group show i t to be of Upper Cretaceous to Eocene age (G.H. Eisbacher, oral commun.) p. 180 The following reference was omitted: Fyfe, W.S., Turner, F.J., and Verhoogen, J., 1958, Metamorphic reactions and metamorphic facies, Geol. Soc. Am., Mem 73. x x i i i I INTRODUCTION General Introduction The purpose of t h i s thesis i s to describe the general geology i n the v i c i n i t y of Pinchi Lake, central B r i t i s h Columbia. This area i s 325 square km i n extent and l i e s a s t r ide the Pinchi Fault zone, one of the major tectonic lineaments i n the Canadian C o r d i l l e r a . P a r t i c u l a r attention i s paid to the structure, metamorphism and petrogenesis of rocks of the lawsonite-glaucophane f a c i e s . Location and A c c e s s i b i l i t y The map area i s situated i n central B r i t i s h Columbia, 600 km north of Vancouver, and can be reached v i a Prince George and Fort St. James (Fig. 1). A road 49 km long connects Fort St. James with Pinchi Lake Mercury Mine on the northern shores of Pinchi Lake. The region between Pinchi and Stuart Lakes i s served by the B r i t i s h Columbia Railway and a gravel road leading to Tachie Indian Reserve (Map I ) . L O C A T I O N a G E N E R A L G E O L O G Y LEGEND Cre taceou s ft T e r t i a r y r o c k s Me so zo i c i n t r u s i v e s Lower M e s o z o i c (Takla ft Haze i t on G r o u p s ) U l t r a m a f i t e s (Trembleur I n t r u s i v e s ) Ca rbon i fe rous ft Pe rmian P r o t e r o z o i c 6 C a m b r i a n 4 0 mi les 64 5 km Com p i l e d b y W. H W f t i t * ( 1 9 6 6 ) 3 Physiography and G l a c i a t i o n There i s a close r e l a t i o n s h i p between the geology and the topography within the Pinchi Lake area. Mount Pope (1472 m) i s the highest point on a long limestone ridge between Pinchi and Stuart Lakes, forming a prominent feature i n the landscape (Plate 1). Other landmarks to the northeast of t h i s ridge are Pinchi Mountain (127 0 m) and Murray Ridge (14 00 m) both of which are composed of u l t r a -mafic rocks. Between these landmarks and the limestone ridge the area i s of subdued r e l i e f , broken only occasion-a l l y by b l u f f s of limestone or carbonatized serpentinite. The area i s forested with spruce, s i l v e r b i r c h and lodgepole pine. Alpine f i r and juniper are found above 1000 m. Armstrong (1949) and Tipper (1971) give accounts of the Pleistocene g l a c i a l history of the area. In b r i e f , the following events took place which strongly influenced the development of the landforms: (a) the development of the C o r d i l l e r a n icecap led to the deposition of an extensive cover of g l a c i a l t i l l ; (b) a f t e r the i c e retreated, i t advanced again and formed a drumlinised t i l l p l a i n . The d i r e c t i o n of i c e flow was towards the east-northeast; 4 (c) during recession of the i c e , drainage channels were blocked and g l a c i a l lakes formed. One such lake, of which Pinchi Lake i s a remnant, covered much of the ground below 840 m. S i l t , sand and clay deposited i n the lake and the e a r l i e r d r i f t deposits cover most of the area and are responsible for the s c a r c i t y of outcrops. Previous Work On the grounds of l i t h o l o g i c s i m i l a r i t y , A.R.C. Selwyn (1877) correlated rocks i n the Fort St. James area with those i n the Lower Cache Creek Group which he had previously defined (1872) i n the Cache Creek area. In t h i s area he had found brachiopods i n d i c a t i v e of a Late Paleozoic age. In the course of a journey across central B r i t i s h Columbia i n 187 6, G.M. Dawson c o l l e c t e d f u s u l i n i d bearing limestones from the Mount Pope area and v e r i f i e d Selwyn's c o r r e l a t i o n . J.G. Gray (1938) mapped the area for the Geological Survey of Canada and worked out the general stratigraphy i n the area. He also discovered cinnabar i n 1937 on a limestone b l u f f on the north shore of Pinchi Lake. During the Second World War, a mercury shortage stimulated prospecting and a development i n the region. Pinchi Mine, on the s i t e of Gray's discovery, came into 5 production i n 1940. Stevenson (1940) published a report which included a section on the geology of the mine and v i c i n i t y . He recognized that the Cache Creek Group sediments had been dynamically metamorphosed and recorded the presence of glaucophane. The cinnabar mineralization took place a f t e r the glaucophanitic metamorphism and was associated with considerable metasomatic replacement and complex f a u l t i n g . In 1942, A.C. Freeze completed a Ph.D. thesis on the geology of the Pinchi Lake area. He divided the s t r a t i f i e d rocks into l i t h o l o g i c a l l y and s t r u c t u r a l l y d i s t i n c t series separated by unconformities: a pre-Upper T r i a s s i c series (the Cache Creek Group), an Upper T r i a s s i c series (the Takla Group) and a Cretaceous-Tertiary (?) s e r i e s . He also suggested that the formation of glaucophane was due to soda metasomatism at r e l a t i v e l y shallow depths and moderate temperatures. J.E. Armstrong of the Geological Survey of Canada published a number of reports on the Pinchi Lake mercury b e l t (1942, 1944) and produced a memoir on the Fort St. James map-area (1949). A major contribution was the recognition of the Pinchi F a u l t which juxtaposed l a t e Paleozoic Cache Creek Group rocks to the west with Mesozoic Takla Group rocks to the east. An important observation, also made by Freeze (1942) was that the diagnostic Upper T r i a s s i c pelecypod Monotis sub circular is occurred i n close 6 a s s o c i a t i o n w i t h a c o n g l o m e r a t e c o n t a i n i n g d e t r i t a l c h r o m i t e . A r m s t r o n g a l s o n o t e d t h e a s s o c i a t i o n b e t w e e n t h e f a u l t a n d m e r c u r y d e p o s i t s . A r e v i e w o f t h e l i t e r a t u r e p e r t a i n i n g t o t h e g e n e s i s o f g l a u c o p h a n e s c h i s t s w i l l be g i v e n i n C h a p t e r TV. Work Done i n T h i s S t u d y G e o l o g i c a l m a p p i n g o f a 40 km by 8 km a r e a a s t r i d e t h e P i n c h i F a u l t was c a r r i e d o u t d u r i n g 1968 (1 m o n t h ) , 1969 (3 months) , a n d 1971 (.3 weeks) . Two l o c a l i t i e s i n t h e C a c h e C r e e k G r o u p w e r e mapped i n d e t a i l i n a n a t t e m p t t o i n t e r p r e t s t r a t i g r a p h i c a n d s t r u c t u r a l r e l a t i o n s a d j a c e n t t o t h e f a u l t z o n e . D r i l l e d c r o s s s e c t i o n s w e r e l o g g e d a n d s a m p l e d . R e c o n n a i s s a n c e t r i p s w e r e made t o t h e n o r t h - w e s t arm o f S t u a r t L a k e , T s a y t a L a k e a n d t h e V i t a l M o u n t a i n s i n o r d e r t o c ompare C a c h e C r e e k G r o u p r o c k s e x p o s e d e l s e w h e r e w i t h t h o s e a t P i n c h i L a k e . L a b o r a t o r y w o r k h a s i n v o l v e d t h e e x a m i n a t i o n o f 23 0 t h i n s e c t i o n s a nd s u p p l e m e n t a l e x a m i n a t i o n s o f X - r a y d i f f r a c t i o n t r a c e s w h e r e n e c e s s a r y . B u l k c h e m i c a l a n a l y s e s o f 15 r o c k s w e r e c a r r i e d o u t b y t h e G e o l o g i c a l S u r v e y o f C a n a d a a n d m i n e r a l a n a l y s e s w e r e o b t a i n e d w i t h a n A.R.L. m i c r o p r o b e b e l o n g i n g t o t h e U n i v e r s i t y o f W a s h i n g t o n . D e t a i l s o f a n a l y t i c a l t e c h n i q u e s a r e g i v e n i n t h e a p p e n d i c e s . II GENERAL GEOLOGY Regional Setting In central B r i t i s h Columbia, the 450 km-long Pinchi Fault demarcates the Pinchi Geanticline, l a r g e l y consisting of the Late Paleozoic Cache Creek Group, from the Quesnel Trough", . composed of the Lower Mesozoic Takla Group. To the east of the Quesnel Trough i s the Omineca Geanticline, a mountainous region carved from rocks of Proterozoic and Cambrian age (Figs. 1 & 2). The Late Paleozoic Cache Creek Group to the west of the Pinchi Fault consists of interbedded chert, cherty p h y l l i t e , a r g i l l i t e , greenstone, minor greywacke and massive limestone, the l a s t mentioned forming a conspicuous unit s t r i k i n g p a r a l l e l to the Pinchi Fault. During Permo-T r i a s s i c times, these rocks were: (a) deformed about northerly or northeasterly trending f o l d axes; (b) intruded by alpine type peri&dotites and (c) metamorphosed under lower greenschist or l o c a l l y lawsonite-glaucophane facies conditions. These rocks were then intruded by d i o r i t e s , granodiorites and granites during the Jurassic Period. 8 F I G . 2 9 TABLE 1 FORMATIONS PRESENT IN THE PINCHI AREA ERA PERIOD OR EPOCH GROUP OR FORMATION LIXHOLOGi MESOZOIC CRETACEOUS (?) USI.IKA FORMATION chert conglomerate UNCONFORMITY MESOZOIC JURASSIC UPPER TRIASSIC OMINECA INTRUSIONS hornblende diorite TAKLA GROUP plagloclase arkose, slltstones limestones, conglomerates and minor tuff. • \ FAULT CONTACT PALEOZOIC OR KESCZCIC PRE-UPPER TRIASSIC POST-LOWER PERMIAN TREMBLEUR INTRUSIONS serpentlnlte, harzburglte, dunlte and pyroxenlte. FAULT CONTACT PALEOZOIC LOWER PERMIAN PENN SYLVANIAH CACHE CREEK CROUP MOUNT POPE BELT limestone, chert, minor volcanic breccia MISSISSIPPI A;; (?) ro PERMIAN CACHE CREEK CROUP (?) GREENSTONES OF PINCHI MOUNTAIN pumpellylte bearing basalts, minor aragonltlc limestone and graphite schist BASIC ROCKS SOUTH .OF PINCHI LAKE basalt, diabase, gabbro GLAUCGPHANITIC ROCKS OF PINCHI LAKE metabasalts, metacherts, schists, greywacke, dolomitic limestone. 10 The Takla Group l i e s to the east of the f a u l t and everywhere appears to be i n f a u l t contact with the Cache Creek Group. A dominantly sedimentary Upper T r i a s s i c sequence of greywackes, s i l t s t o n e s , limestones and occasional conglomerates i s o v e r l a i n by Jurassic andesites, breccias, t u f f s and intercalated sediments. In contrast to the Cache Creek Group, Takla Group rocks are unfoliated and have undergone a low grade b u r i a l rather than dynamic metamorphism. Also, f a u l t i n g rather than fol d i n g i s the c h a r a c t e r i s t i c type of deformation i n these younger rocks. Extensive emplacement of d i o r i t e s and granodiorites occurred during the Mesozoic. Cretaceous or Paleocene continental sediments were deposited unconformably on Takla Group rocks, and o u t l i e r s are preserved at various l o c a l i t i e s along the Pinchi Fault zone. Such rocks, described by Roots (1954) i n the Aiken Lake area to the north, have been involved i n post-Paleocene f a u l t movements. These disturbances were followed by eruption of Oligocene and Miocene flood basalts, remnants of which are preserved mainly i n the southern part of the Fort St. James area. The foregoing sequence of events i s summarized i n the table of formations (Table 1). The area mapped (Map I) includes most of the above mentioned rock units and l i e s a s t r ide the Pinchi Fault 11 system. For descriptive purposes, rock units are considered under f i v e headings: Cache Creek Group, Ultramafites, Takla Group, D i o r i t e and Cretaceous (?) conglomerate. Cache Creek Group  Introduction In 187 5, A.R.C. Selwyn noted the l i t h o l o g i c a l s i m i l a r -i t y of limestones i n the v i c i n i t y of Mount Pope with those of Late Paleozoic age which he had found previously on the banks of the Thompson River i n the Cache Creek area. The following year, G.M. Dawson (18 76) v i s i t e d Mount Pope and c o l l e c t e d f u s u l i n i d s which confirmed the c o r r e l a t i o n . Armstrong (1949) concluded that The Cache Creek Group may.be defined as a very thick assemblage 20,000 f t or more of interbedded sedimentary and volcanic rocks, mainly of Permian age, but also probably i n part of Pennsylvanian age. The whole of the Permian may be represented. Foraminiferal limestones and ribbon cherts are c h a r a c t e r i s t i c of the group. He recognized four main l i t h o l o g i c a l d i v i s i o n s i n the Cache Creek Group, namely s l a t e , ribbon chert, limestone and greenstone d i v i s i o n s . On account of s t r u c t u r a l complexity, stratigraphic r e l a t i o n s are i n doubt but he suggested that the greenstone d i v i s i o n predominates i n the upper part of the group. 12 F i g . 3 G E O L O G I C A L S U B D I V I S I O N S AT PINCHI L A K E . Em m «3J Greenstones of F l n c h l Mountain Lawsonite-glaucophane r o c k s of P i n c h i Lake B a s i c r o c k s south of P i n c h i Lake Limestones and c h e r t s of Kount Pope b e l t U l t r a r a a f i t e l\ , 6 \ I T a k l a Group mm 2 miles 3-2 km D i o r i t e |o>g ;»i Cretaceous (?) L= ^ conglomerate F a u l t (approx. or assumed) Unconformity (?) Fig. 4 T A K L A GROUP : STRATIGRAPHIC S E C T I O N S , F O S S I L a S P E C I M E N L O C A L I T I E S . 13 Within the area covered by t h i s thesis, four sub-d i v i s i o n s (Fig. 3) have been made i n rocks thought to belong to the Cache Creek Group, namely: (a) greenstones of Pinchi Mountain, (b) glaucophane bearing rocks around Pinchi Lake, (c) basic rocks south of Pinchi Lake and (d) massive limestones and cherts of the Mount Pope b e l t . Each of these units i s bounded by f a u l t s which juxtapose blocks of contrasting metamorphic grade. In t h i s chapter only the l i t h o l o g i c a l and microscopic features r e l a t e d to the primary rock type w i l l be discussed. Description of metamorphism and structure i s reserved for Chapters IV and V respectively. Greenstones of Pinchi Mountain The main occurrence of greenstone i s on the southern slopes of Pinchi Mountain between the lawsonite-glaucophane bearing rocks and the Pinchi Mountain ultramafite to the north. Exposure i s poor and much information has been obtained from trenches and d r i l l holes. Rocks correlated with these are found at the west end of Murray Ridge and 11 km northeast of Fort St. James (Map I ) . 14 1. Lithology and petrology This u n i t consists dominantly of brown weathering, highly fractured basalts or, l o c a l l y , f i n e grained diabasic i n t r u s i v e s . Some members are por p h y r i t i c , containing euhedral plagioclase laths Cl cm) or augite phenocrysts (8 mm) set i n a green aphanitic matrix; elongate 4 mm c h l o r i t e blebs are also common. Within t h i s dominantly volcanic unit, occasional outcrops and d r i l l hole data show the presence of two sedimentary units separated by 28 m of greenstone. The upper unit consists of 15 m of graphitic chert i n 1 cm beds, grading into a grey limestone containing s t y o l i t e s . The lower unit contains green laminated t u f f , limestone and graphitic s c h i s t . In t h i n section, the greenstones consist of r e l i c t augite or a l b i t i s e d plagioclase phenocrysts set i n a fin e grained matrix of metamorphic minerals (see Appendix IV). C h l o r i t e - f i l l e d blebs are common and may o r i g i n a l l y have been amygdules, or o l i v i n e or hornblende phenocrysts. R e l i c t ilmenite i s occasionally present i n diabasic rocks. 2. Internal structure and contacts The metabasalts on the northern slopes of Pinchi Mountain s t r i k e at 100° and dip to the north at 45°, approxi-mately p a r a l l e l to the f a u l t which separates, the unit from 15 silica-carbonate rocks and ultramafites to the north. To the south, the unit i s juxtaposed against rocks of the lawsonite-glaucophane facies by a nearly v e r t i c a l f a u l t which i s l o c a l l y carbonatized. A similar s i t u a t i o n e x i s t s on the Darbar claim group, where a northeasterly dipping f a u l t separates porphyritic augite basalts from lawsonite-glaucophane bearing rocks. The existence and attitude of a l l the f a u l t s mentioned above have been confirmed by diamond d r i l l holes. Thicknesses are i n doubt owing to poor exposure and pervasive shearing. I t i s suggested that the unit i s not less than 450 m thick, of which approximately 30 m i s composed of intercalated sediments. 3. Age Freeze (1942) and Armstrong (1949) included these rocks i n the Takla Group. This p o s s i b i l i t y cannot d e f i n i t e l y be excluded but evidence that the unit may belong to the Cache Creek Group includes the following: (a) the presence of aragonite, acmitic pyroxene and possible lawsonite (Chapter IV) suggests that metamorphism was contemporaneous with the T r i a s s i c glaucophanitic metamorphism of the Cache Creek Group, and therefore that the unit i s of pre-Takla Group age; 16 (b) graphitic cherts interbedded with the basalts suggest an a f f i n i t y with the Cache Creek Group. It i s concluded that t h i s unit may be representative of the "upper greenstone d i v i s i o n " of the Cache Creek Group (Armstrong, 1949) and presumably of l a t e Permian age. 4. Origin These rocks have been c a l l e d basalts or diabases because of t h e i r basic composition and r e l i c t mineralogy. Chemical evidence (Appendix III) indicates that the basalts are of the a l k a l i - o l i v i n e type. The presence of i n t e r -calated limestones, graphitic cherts and laminated t u f f s suggests that the basalts may have been extruded i n a submarine environment. Glaucophane-lawsonite bearing rocks around Pinchi Lake 1. D i s t r i b u t i o n Glaucophane-lawsonite-bearing rocks form a narrow fault-bounded b e l t , 2.5 km wide, extending the length of the map area. The best exposures are i n the immediate v i c i n i t y of Pinchi Lake Mercury Mine and along the north-west shore of Pinchi Lake; both of these areas were mapped i n d e t a i l (Maps IV and V). Glaucophanitic rocks are also found on the Darbar claim group, (Map I ) , 11 km northeast 17 of F o r t S t . James, and i t i s presumed t h a t such r o c k s u n d e r l i e much of the low unexposed ground between Murray Ridge and Mount Pope. An exposure, of importance because of i t s p r o x i m i t y to the southwest margin of the f a u l t b l o c k , l i e s 3.6 km south o f P i n c h i Mine on a s m a l l i s l a n d near the southwest shore of P i n c h i Lake. The most n o r t h w e s t e r l y outcrops are 1.6 km south of the west end of Tezzeron Lake. Beyond t h i s , l i t t l e i s known about the geology because of poor exposure and d i f f i c u l t a c c e s s . 2. Rock types and p r o p o r t i o n s The glaucophane-lawsonite b e a r i n g b e l t c o n s i s t s o f metabasic r o c k s , d o l o m i t i c l i m e s t o n e s , c h e r t s , g r a p h i t i c c h e r t s , greywackes and s c h i s t s . E s t i m a t e s of l i t h o l o g i c a l p r o p o r t i o n s a re d i f f i c u l t because of poor exposure, but i t i s f a i r l y c e r t a i n t h a t much of the low ground i s u n d e r l a i n by g r a p h i t i c c h e r t s which grade i n t o greywacke and s c h i s t s . Because o f r e s i s t a n c e to e r o s i o n , l i m e s t o n e s , b a s i c r o c k s and c h e r t s tend t o form r i d g e s w i t h outcrops g i v i n g a f a l s e i mpression of t h e i r abundance. L i t h o l o g i c a l p r o p o r t i o n s are e stimated as f o l l o w s : l imestone, 10%; metabasic r o c k s , 5%; metagreywackes, 15%; metacherts, 25%; g r a p h i t i c c h e r t s and s c h i s t s , 45%. 18 3. Petrology (a) Metabasic rocks Metabasic rocks are associated with the limestone lenses along the northern shores of Pinchi Lake. Metabasic rocks were also intersected i n diamond d r i l l holes 2.4 km northwest of Pinchi Mine. For descriptive purposes, these rocks are subdivided into f o l i a t e d and massive rocks but rock types with intermediate c h a r a c t e r i s t i c s are very common. Fol i a t e d rocks are blue, f i n e to medium grained and have a well defined f o l i a t i o n outlined by p a r a l l e l a c i c u l a r c r y s t a l s of glaucophane. They may also show a compositional layering consisting of 1 cm alt e r n a t i n g blue and green layers p a r a l l e l to the f o l i a t i o n . Commonly crenulations can be seen on f o l i a t e d surfaces which cross-cut the glaucophane mineral l i n e a t i o n . Massive metabasic rocks are recognized i n outcrop by brownish weathering, complex f r a c t u r i n g and lack of measurable structures. Fractures are commonly f i l l e d with quartz, carbonate or glaucophane, the l a t t e r growing perpendicular to the walls. Fresh rock between veins i s green, f i n e grained, and occasionally microporphyritic or amygdaloidal. One d i s t i n c t i v e layer contains 1 cm blastoporphyritic augites set i n a l i g h t green aphanitic matrix. 19 In t h i n section, f o l i a t e d rocks are thoroughly metamorphically reconstituted. Massive rocks, however, display a v a r i e t y of r e l i c t textures similar to those observed i n the metabasic rocks on Pinchi Mountain. R e l i c t amygdules contain aragonite or c h l o r i t e and r e l i c t p lagio-clase laths contain lawsonite, c h l o r i t e and white mica. Blastoporphyritic augites are common and trachytic or py r o c l a s t i c textures occur occasionally. Two large metabasic boulders, containing a d i f f e r e n t mineralogy from anything previously mentioned were found i n the area. A glaucophane-eclogite boulder i s located 9 km east of Pinchi Mercury Mine on the Tezzeron Lake logging road ( l o c a l i t y E on Map I ) . I t measures 12 by 4 by 3 m, i s rounded and i s embedded i n g l a c i a l t i l l . I t appears to have been transported eastwards from the v i c i n i t y of Pinchi Mine during g l a c i a t i o n . The boulder contains f o l i a t e d and lineated blocks up to 30 cm i n diameter consisting of green pyroxene, and garnet a l t e r i n g to stilpnomelane. Glaucophane and lawsonite occupy the i n t e r s t i c e s between the blocks and also permeate them. Late cross-cutting fractures are f i l l e d with c h l o r i t e and a brown amphibole. A second boulder i s located 3.2 km north-west of the mine (location Br on Map I ) . I t l i e s i n a b e l t of low ground between two silica-carbonate b l u f f s a s t r i d e a major f a u l t zone and i s thought to be l i t t l e transported f l o a t . This boulder contains green brecciated 20 fragments up to 2 cm i n diameter set i n a dark blue glau-cophanitic matrix. (b) Metasedimentary rocks Dolomitic limestones grading into c a l c i t i c dolomites form a discontinuous series of steep b l u f f s (180 m high) along the shore of Pinchi Lake northwest of the mine. They are grey weathering massive r e c r y s t a l l i z e d rocks consisting of varying proportions of dolomite, metamorphic aragonite, c a l c i t e and carbonaceous material. The dolomite content varies from 20% to 90% and occurs as evenly d i s t r i b u t e d rhombs or mottled anastamosing patches. Locally the lime-stones possess a well developed f o l i a t i o n , characterized by alternating streaky laminations of grey aragonite, carbonaceous material and white aragonite with occasional dolomitic laminae (Fig. 35). Petrographic data on the limestones i s given i n Appendix IV. Massive metagreywackes are of l i m i t e d extent and form a unit 30 m i n thickness 1.6 km northwest of Pinchi Mine. Fresh surfaces are grey or. b l u i s h green and 1 mm white c l a s t s or 2 mm carbonaceous fragments can usually be seen in.hand specimen. Thin sections show that metagrey-wackes possess.a r e c r y s t a l l i z e d f a b r i c , but r e l i c t angular poorly sorted, quartz, grains, and. pseudomorphs a f t e r plagioclase can s t i l l be detected. A steep dipping fracture cleavage i s sporadically developed and l a t e quartz or carbonate veins (less than 1 mm i n width) are common. Metacherts are f a i r l y r e s i s t a n t to erosion and are best exposed immediately north of the large limestone b l u f f west of the mine. They weather grey and the fresh surface has a pale b l u i s h sheen due to the presence of ac i c u l a r nematoblastic glaucophane. Thin schistose laminae containing white mica and glaucophane occur between cherty layers (2 to 5 cm thick) and show a well developed l i n e a t i o n defined by crenulations. Poorly exposed p y r i t i c carbonaceous schists and quartz + white mica + lawsonite ± glaucophane schists are interbedded with the cherts and limestones. Being incompe-tent rocks, they tend to be contorted and on exposure to weathering decompose to a micaceous or black carbonaceous mud. A t y p i c a l specimen of carbonaceous s c h i s t contains white beds of cherty quartzite generally less than 3 cm i n thickness with i n t e r l a y e r s of micaceous carbonaceous s c h i s t . Quartz + mica + lawsonite ± glaucophane schists are green with white 2 mm lawsonite and 6 mm blue ac i c u l a r glaucophane porphyroblasts. 4. Internal structure Limestones form i s o l a t e d lenses, s t r i k i n g north-westerly and dipping at approximately 6 0° northeast. Generally the lenses show no trace of bedding, but a 22 f o l i a t i o n may be present l o c a l l y . The rapid v a r i a t i o n i n thickness and the lensing out of the limestone may either be the r e s u l t of folding (Chapter V) or a primary depositional phenomenon. Some contacts may be faulted but at Pinchi Mine massive limestone grades into well bedded (15 cm max) cherty quartzite, mica s c h i s t and limestone with no suggestion of a f a u l t contact. Metavolcanic rocks have sharp contacts and appear i n many cases to have an inter-tonguing r e l a t i o n s h i p with limestones. This r e l a t i o n s h i p could also be attributed to f o l d i n g . D r i l l holes 1.6 km northwest of Pinchi Mine indicate that carbonaceous cherts and schists have gradational contacts with greywackes. 5. External contacts A l l glaucophane-lawsonite bearing rocks are faulted against adjacent rocks. In some places contacts have been d r i l l e d , giving intersections of f a u l t breccias and carbonatized rocks. 6. Age The association of rock types i s similar to that found i n the Cache Creek Group. However, no f o s s i l s have been found i n thi s subdivision and the p o s s i b i l i t y remains that these rocks are older than the Pennsylvanian-Permian Cache Creek Group. Since i t i s to be i n f e r r e d (Chapter IV) that these rocks have been buried to far greater depths than the Cache Creek rocks to the southwest, they may be of Mississippian age or older. L i t h o l o g i c a l l y , these rocks are also similar to those of the Mississippian Slide Mountain Group (Sutherland-Brown 1963) . 7. Origin Prior to metamorphism and deformation, the sediments consisted of black shale, chert, carbonaceous dolomitic limestone and greywacke i n order of decreasing abundance. Porphyritic basalt flows and t u f f s were cl o s e l y associated with the dolomitic limestones. On account of the complexity of the structure, the largely unknown stratigraphic r e l a t i o n s and c o n f l i c t i n g opinions on the o r i g i n of dolomitic carbonates, cherts and p y r i t i c black shales, the nature of the depositional environment i s highly speculative. According to PettiJohn (1957, p. 421) early formation of dolomite suggests a near shore f a c i e s , a s i t u a t i o n which receives some confirmation with the presence of greywackes at Pinchi. Bedded cherts have been assigned a deep water o r i g i n by Rich (1951), and Trumpy (1960). However, Danner (1967) suggests a shallow water o r i g i n for at least some cherts and th e i r depth sig n i f i c a n c e i s therefore s t i l l uncertain. The presence 24 of p y r i t i c black shales suggests accumulation under stagnant reducing conditions but i t i s also possible that the reduc-ing environment formed i n the sediment during diagenesis. I t appears, therefore, that a l l that can be said about the depositional environment i s that i t may have been adjacent to a land mass. Basic rocks south of Pinchi Lake 1. D i s t r i b u t i o n and petrology A b e l t of basic rocks l i e s to the southwest of Pinchi Lake, between a s l i v e r of Takla Group sediments and an elongate serpentinite body (Map I ) . I t i s at l e a s t 16 km long and 1500 m i n maximum width. The basic rocks are crudely layered p a r a l l e l to the elongation of the body. The southern half of the b e l t consists of brown weathering massive gabbro, with equi-granular texture, containing 60% mafic constituents and 40% feldspar. Quartz veins and epidotization along fractures occur l o c a l l y . Towards the northeast, f i n e r grained rocks with diabasic texture predominate and contain sporadic anastamosing patches of medium grained gabbro. These rocks are highly sheared and c h l o r i t i z e d along fractures. F i n a l l y , the most northerly rocks i n the b e l t are basalts (?) containing 2 mm c a l c i t e blebs. 25 The gabbro consists primarily of equigranular sub-hedral plagioclase (2 mm average) and i n t e r s t i t i a l augite; ilmenomagnetite and orthopyroxene are also present. Plagioclase i s a l b i t i c and i s mottled with s e r i c i t e or granular epidote making i d e n t i f i c a t i o n d i f f i c u l t . Inter-s t i t i a l pyroxenes commonly are replaced by a green pleochroic amphibole (Z"c = 22° max) and a l l that remains of anhedral ilmenomagnetite grains i s a r e t i c u l a t e sphene skeleton. Diabasic rocks are altered i n a similar fashion, the assemblage commonly consisting of saussuritized plagioclase + a c t i n o l i t e + c h l o r i t e + sphene. Average grain s i z e i s approximately 0.4 mm. Basalts are inequigranular, containing augite pheno-crysts and a l b i t i c s e r i c i t i s e d plagioclase (1 mm max) i n a sub-ophitic fine-grained matrix of augite, a l b i t e , c h l o r i t e , and sphene. 2. Internal structure and contacts Takla Group sediments appear to be conformable with the northern contact of the basic rocks. They also young northwards and are overturned. As pebbley limestones near the base of the sequence contain.skeletal ilmenomagnetite and c h l o r i t i s e d amphibolitic pebbles l i t h o l o g i c a l l y similar to the basic rocks (Table 3) , i t may be that the northern contact i s an unconformity which has been overturned and dips to the southwest at 6 0°. 3. Age and o r i g i n The age and o r i g i n of these basic rocks i s problematic-a l . A summary of facts and reasonable inferences i s as follows: Ca) gabbro occurs lowest i n the sequence followed by diabase and basic volcanics (?) at the top. Relationships between the rock types are apparently gradational; (b) the gabbro and diabase belong to the lower green-s c h i s t f a c i e s ; (c) a serpentinite b e l t bounds the gabbro to the south; (d) there may be an unconformity between Upper T r i a s s i c sediments and the underlying basic sequence, as the l a t t e r appears to provide detrit u s to the Takla Group (Table 3 ); and Ce) i f an unconformity i s present, the sequence has been overturned. The basic rocks apparently l i e below Upper T r i a s s i c (?) sediments.and are presumably of Cache Creek Group or T r i a s s i c age. I t i s suggested that the greenschist f a c i e s metamor-phism was contemporaneous with the lower greenschist fac i e s metamorphism i n the Cache Creek Group (see Chapter IV, Part 2). The apparent succession serpentinite-gabbro-diabase-basalt i s of great i n t e r e s t . Such basic or o p h i o l i t i c complexes are being increasingly recognized elsewhere as oceanic crust CBailey et al., 1970 and Page, 1972). This p o s s i b i l i t y w i l l be further discussed i n Chapter VI. Massive limestones and cherts of the Mount Pope b e l t 1. D i s t r i b u t i o n Between Pinchi and Stuart Lakes the topography i s controlled by a continuous ridge of limestone which stretches the length of the map-area. This ridge i s almost e n t i r e l y composed of massive c r y s t a l l i n e limestone with two minor in t e r c a l a t i o n s of volcanic breccia. Flanking t h i s limestone to the northeast and southwest are b e l t s of chert, l o c a l l y containing interbeds of t u f f and s i l t s t o n e . 2. Lithology and petrology The limestone ranges from l i g h t grey to dark grey i n colour and weathers l i g h t grey. In contrast to the lime-stones north of Pinchi Lake, i t i s only l o c a l l y carbonaceous, and dolomite i s a minor constitutent which occurs i n l a t e veins. Breccia ted iron stained zones occur adjacent to fractures. C r i n o i d a l discs and columnals are the most abundant faunal elements, occurring as fragments up to 1 cm i n length. Fusulinids, corals and a l g a l structures are also occasionally 28 seen i n hand specimen. The proportion of b i o c l a s t i c a l l o -chemical constituents ranges between 40% and 7 0% i n the four specimens studied i n thin section. The framework grains are poorly sorted and set i n a m i c r i t i c matrix. Most limestones could be c l a s s i f i e d as f i n e , medium or coarse b i o m i c r i t i c calcarenites (FolK, 1968) l o c a l l y grading into f i n e or medium c a l c i r u d i t e s . Flanking the limestone b e l t are grey to black bedded cherts with thin argillaceous partings. Individual beds are generally from 2 to 5 cm thick but occasionally may reach 12 cm. Siliceous s i l t s t o n e s are commonly i n t e r -bedded with cherts. In t h i n section, cherts are crypto-c r y s t a l l i n e with m i c r o c r y s t a l l i n e quartz replacing r a d i o l a r i a t e s t s . The grey to black colour i s caused by d i f f u s e opaque stringers which have a black amorphous appearance i n r e f l e c t e d l i g h t and are probably carbonaceous. Cross cutting d i l a t a t i o n a l quartz veins occasionally give the chert a brecciated appearance. Volcanic breccias occur at two l o c a l i t i e s on the southwestern flanks of the limestone. Fragments include green and purple flow rocks and minor s i l t s t o n e set i n a sparry c a l c i t e cement. In thin section, plagioclase laths i n volcanic fragments are highly s e r i c i t i s e d and set i n a green or dark brown c h l o r i t i c matrix. A schistose t u f f , i ntercalated with chert on the north side of the limestone, contains blastoporphyritic augite and quartz i n a c h l o r i t i c matrix. 29 3. Fauna and age Faunas from the Mount Pope b e l t have been c o l l e c t e d by several workers. A l l previous l o c a l i t i e s , together with several found i n the course of t h i s work, are tabulated i n Appendix I. G.M. Dawson c o l l e c t e d f u s u l i n i d s from the Mount Pope area, which were l a t e r dated as Early Permian (Freeze, 1942) . Thompson (1953) c o l l e c t e d from a number s of l o c a l i t i e s and concluded that i n the v i c i n i t y of Fort St. James the limestone b e l t ranged i n age from Pennsylvanian to Early Permian. Forty miles north, between Trembleur and Kloch Lakes, Armstrong (1949) c o l l e c t e d f u s u l i n i d s of Leonardian and Guadalupian age. No such f u s u l i n i d s were found i n the Mount Pope area. A l g a l structures, bryozoa, echinoderm fragments, c r i n o i d a l debris and occasional corals have also been found i n the area but few are diagnostic of age. Radiolaria tests were found by the author i n cherts north of Mount Pope. 4. Internal structure, and contacts Most bedded cherts flanking the limestone b e l t dip southwest. The oldest f u s u l i n i d s of Desmoinesian age (Thompson, 1965) are found adjacent to cherts at two l o c a l i t i e s : on the northwest shore of Pinchi Lake and 30 two m i l e s n o r t h w e s t o f F o r t S t . James a t t h e base o f Mount Pope (Map I ) . Thompson a l s o s t a t e s t h a t f u s u l i n i d s become p r o g r e s s i v e l y younger s t r a t i g r a p h i c a l l y towards t h e summit of Mount Pope and northwestwards a l o n g t h e shore o f S t u a r t Lake. From t h e s e o b s e r v a t i o n s t h e f o l l o w i n g c o u l d be i n f e r r e d : (a) l i m e s t o n e s o v e r l i e the c h e r t s ; (b) t h e p a r t of the l i m e s t o n e b e l t s o u t h o f Mount Pope has been l o c a l l y o v e r t u r n e d and (c) the l i m e s t o n e b e l t c o n t a i n s a s y n c l i n e w i t h a x i a l p l a n e d i p p i n g southwest. U n d o u b t e d l y , the s t r u c t u r e i s n o t as s i m p l e as t h i s , b u t f u r t h e r u n r a v e l l i n g must a w a i t d e t a i l e d f a u n a l s t u d i e s . The n o r t h e r n boundary o f t h e b e l t a ppears t o be the s i t e o f a major f a u l t c l o s e l y a s s o c i a t e d w i t h s l i v e r s o f u l t r a m a f i c r o c k f o r much o f i t s l e n g t h . 5. O r i g i n The l i m e s t o n e b e l t i s remarkably c o n t i n u o u s . I n c e n t r a l B r i t i s h C o l u m b i a , l i m e s t o n e s o f s i m i l a r age and l i t h o l o g y form a p e r s i s t e n t b e l t , 200 km i n l e n g t h , s t r i k i n g s u b - p a r a l l e l t o the P i n c h i F a u l t . S i m i l a r l i m e s t o n e s o c c u r on the A t l i n H o r s t ( A i k e n , 1959 and Monger, 19 69) and i n the Cache C r e e k a r e a (Selwyn, 1872 and Danner, 1964, p. 109). The c o n t i n u i t y o f t h e b e l t s u g g e s t s t h a t i t may have formed a b a r r i e r r e e f d u r i n g t h e L a t e P a l e o z o i c . Whether i t formed a d j a c e n t t o a l a n d mass i s n o t known.. 31 The o r i g i n and depositional environment of bedded cherts are controversial. In the past, many geologists f e l t that they originated from inorganic p r e c i p i t a t e s r e s u l t -ing from submarine volcanism and s i l i c a saturation of sea water Ce.g. Bailey, Irwin and Jones, 1964). However, t h i s i s not considered a v a l i d argument where there i s no evidence for volcanic a c t i v i t y (Krauskopf, 1967, p. 169). I t i s now commonly accepted that most bedded cherts formed as a r e s u l t of accumulation of s i l i c e o u s organic skeletons (Bramlette, 1946) either at great depths (Trumpy, 1960 and Dietz, 1966) or i n shallow water (Danner, 1967). In the l a t t e r case, Danner noted the close association of organically derived bedded cherts and f u s u l i n i d or a l g a l limestones i n the Cache Creek area, and was l e d to postulate a shallow water o r i g i n for the cherts. The cherts occur p r e f e r e n t i a l l y i n the lower part of the Cache Creek succession and grade upwards into limestones (Danner, 1964, p. 109). With respect to the o r i g i n of cherts i n the Mount Pope b e l t three factors are of s i g n i f i c a n c e : (a) the cherts contain r a d i o l a r i a and are not a s s o c i -ated with s i g n i f i c a n t amounts of volcanic material; (b) they appear to underlie shallow water limestones; (c) the chert-limestone contact i s sharp. There are three possible explanations for these r e l a t i o n -ships. F i r s t l y , the cherts could have been deposited i n 32 deep water and subsequently raised to a near surface environment where limestones were deposited. Secondly, cherts and limestones may have originated i n a shallow basin with rate of subsidence roughly equal to rate of deposition. Lastly, shallow water limestones may have been thrust over deep water cherts. Takla Group Introduction The term Takla Group was f i r s t used by Armstrong (1949) to describe rocks of sedimentary and volcanic o r i g i n deposited during the Upper T r i a s s i c and Jurassic. Armstrong recognized two subdivisions: (a) Upper T r i a s s i c Monotis • •—bearing sedimentary rocks, (b) Undivided Takla Group consisting of basic volcanics, p y r o c l a s t i c s and interbedded sediments. The rocks exposed at Pinchi belong to the f i r s t subdivision and occur mainly on the northeast side of the Pinchi Fault zone. A b e l t of sediments allocated to the Takla Group i s also found on the southwest side of the f a u l t , s t r i k i n g subparallel to the southern shore of Pinchi Lake (Fig. 3). 33 TABLE 2 MODAL ANALYSES OF GREYWACKES FROM THE TAKLA GROUP Specimen Number 4 7 1 Feldspar 43.0* 26.7 44.9 Matrix 40.6 50.5 30.1 Hornblende - - 12.2 Clinopyroxene - 3.9 -Rock fragments 10.0 14.4 10.1 Quartz 6.4 3.6 2.6 Notes: 1. 1000 point counts made on each th i n section. 2. Rock fragments include hornblende plagioclase porphyry, and trachytic basic volcanics. 3. Sample locations are given i n F i g . 4. * 4. Minor K-Feldspar. 34 Lithology The rock types present within the area are, i n decreasing order of abundance, arkosic greywacke, laminated s i l t s t o n e , limestone, conglomerate, basic intrusives and basic volcanics. Arkosic greywackes are dark grey weathering rocks, generally thick to massive bedded (1-3 m) and possessing shale or laminated s i l t s t o n e interbeds (8-30 cm). On fresh surfaces, they are dark grey or green, and c l a s t s of f e l d s p a r , b i o t i t e and hornblende can be seen. Most beds are graded, with p a r t i c l e size averaging 1 mm at the base grading into s i l t s ize at the top. Black l i t h i c c l a s t s of s i l t s t o n e occur sporadically. Thin sections (Table 2) show that the dominant constituent i s plagioclase, but augite, brown hornblende, volcanic l i t h i c fragments, and c a l c i t e can a t t a i n appreciable amounts. Accessory constitutents include quartz , K-feldspar, pyrite and hematite. Grains are poorly sorted, angular and embedded i n an i n c i p i e n t l y metamorphosed micr o c r y s t a l l i n e matrix consisting of c h l o r i t i c or l i m o n i t i c material, plagioclase and c a l c i t e . Porphyritic volcanic fragments, with phenocrysts of plagioclase and/or hornblende i n a microgranular t r a c h y t i c or f e l t e d matrix, r a r e l y constitute more than 2% of the rock. Micaceous quartzite c l a s t s and tremolite-epidote aggregates were also i d e n t i f i e d . Interbedded s i l t s t o n e s 35 possess similar mineralogy and textures. S i l t s t o n e s , calcareous s i l t s t o n e s and s i l t y shales are dark grey weathering, thin-bedded (2-10 cm) rocks with occasional buff weathering f i n e grained sandstone interbeds (Plate 2). Beds may show fin e s i l t y intralaminae and graded bedding, f i n e crossbedding, convolutions or intraformational s i l t s t o n e c l a s t s . Plagioclase i s again the dominant constituent, with quartz also abundant i n a few samples. Clinopyroxene and hornblende have decomposed to c h l o r i t e which constitutes the matrix along with carbonate. Limestones l i e i n the b e l t of sediments southwest of Pinchi Lake, between Murray Ridge and Pinchi Lake and intermittently along the north shore of the lake. At the f i r s t l o c a l i t y the limestones are grey weathering, dark grey to black rocks which emit a bituminous odour on f r a c t u r i n g . Well bedded b i o m i c r i t i c calcarenites grade into, or are interbedded with, dark grey c a l c i l u t i t e s , calcareous s i l t s t o n e s and pebbley calc a r e n i t e s . Limestones at the second l o c a l i t y are massive, bituminous and contain c o l o n i a l corals and c r i n o i d a l debris. At the t h i r d l o c a l i t y , on the north shore of Pinchi Lake, the limestones are i n d i s t i n c t l y bedded, weather grey and are buff coloured on a fresh surface. They are dominantly poorly sorted b i o m i c r i t i c c a l c a r e n i t e s containing abundant c r i n o i d a l d i s c s , 36 cor a l and s h e l l fragments. Pebbles (1 cm max) are commonly found i n th i n layers or are d i s t r i b u t e d sporadically through the limestones. Limestones associated with con-glomerates at t h i s l o c a l i t y are discussed l a t e r . Basic rocks were found at only two l o c a l i t i e s (Map I ) . A b a s a l t i c l a p i l l i t u f f i s found 2 00 m east of the conglomerate on the north shore of Pinchi Lake and an unusual biotite-quartz gabbro i s located on the south shore of Pinchi Lake 1.6 km west of the east end of the lake. Conglomerate Conglomerate has been treated separately because of i t s importance regarding the depositional environment and provenance of the Takla Group sediments. The most int e r e s t i n g l o c a l i t y (Fig. 4, No. 2) l i e s on a promontary on the north shore of the lake 6.4 km from the east end. The limestone beds associated with the conglomerate are near v e r t i c a l and the way-up could not be determined. A stratigraphic section i s as follows. 37 150 f t . 47 m. M w 0 O O o o o 0 0 O o M l i m e s t o n e c o b b l e c o n g l o m e r a t e p o l y m i c t p e b b l e c o n g l o m e r a t e w e l l - b e d d e d (30 cm) l i m e s t o n e Monotis suboivoulavis F i n e g r a i n e d , w e l l b e d d e d l i m e s t o n e s c o n t a i n i n g Monotis g r a d e i n t o i n t r a f o r m a t i o n a l c o n g l o m e r a t e s c o n s i s t i n g o f a n g u l a r p o o r l y s o r t e d l i m e s t o n e c o b b l e s ( P l a t e 4) . T h e c o b b l e s a l s o c o n t a i n Monotis a n d a r e u p t o 18 cm i n d i a m e t e r . A d j a c e n t t o t h i s c o b b l e c o n g l o m e r a t e i s a v e r y i n t e r e s t i n g 5 m t h i c k b e d o f p o l y m i c t p e b b l e c o n g l o m e r a t e . T h e p e b b l e s (4 cm m a x ) a r e g r e y t o b l a c k , s u b - r o u n d e d , p o o r l y s o r t e d a n d a r e e m b e d d e d i n a b i o m i c r i t i c c a l c a r e n i t e m a t r i x . T h e c o m p o s i t i o n o f t h e p e b b l e s i s g i v e n i n T a b l e 3 ( l o c . 2). S u b - a n g u l a r i n t r a f o r m a t i o n a l l i m e s t o n e c o b b l e s (10 cm max) a r e a l s o p r e s e n t i n t h i s l a y e r . T h e p r e s e n c e o f d e t r i t a l c h r o m i t e a t t h i s l o c a l i t y was f i r s t n o t e d b y F r e e z e (1942). I n t h e c o u r s e o f t h i s 38 TABLE 3 PEBBLE CONTENT OF CONGLOMERATES WITHIN TAKLA GROUP LOCATION (See Fig. k ) PETROLOGY OF FEBBLES ABUNDANCE (% of total i-ebbles ln specimen) SOUiiCE K0CK N . shore of Pinchi T y p p A i anphlbolltlsed Lake. Loc. 2. medium grained; ab-actln -sph-ep-qtz-rellct opx. gabbro. Type B i 50* metadiabase, fine grained; ab-actln- metabasalt. chl i relict diabasic texture. Type C i 2% basalt. ab laths ln micro-granular matrix; trachytic texture Type D i 5% bedded chert. cryptocrystalllne chert Type E i 156 harzburgite, chromite grains dunite. East of Pinchi Type A i 75% bedded chert.. Mine, Loc. 5 cryptocrystalllne chert Type B i 5% basalt. trachytic basalt. intrusives ? coarse grained plagioclase 20% and quartz grains are also present. S.- shore of Pinchi Type A i 50% metabasalt. Lake. Loc. 6 ab-sph-chl ; porphyrltlc or equlgranular texture. Type B i »5% bedded chert. cryptocrystalllne chert Type C i assorted grains i 5% gabbro, skeletal llmenomas-netlte, eerpentlnlte ? . cllnopyroxene, serpentine. ab=albltei actlr.=actlnoll te i sph=sphenei qtz=quartzs opx=orthopyroxenei chl=chlorlte. 39 study the i d e n t i f i c a t i o n of chromite was v e r i f i e d by p a r t i a l electron probe analyses and r e f l e c t i n g microscopy. Two types of chromite were noted, opaque chromite and dark brown translucent p i c o t i t e . Both v a r i e t i e s were observed i n nearby ultramafites. The chromite grains are monominer-a l i c and are embedded i n the limestone matrix. Within the basic amphibolitic pebbles, the opaque grains were found to consist of s k e l e t a l ilmenomagnetite altered to sphene and hematite, suggesting that these basic pebbles could not have been the source for the chromite. Conglomerates are also found elsewhere i n the Takla Group (Table 3). Two beds of medium pebble conglomerate (8-18 m thick) are interbedded with s i l t s t o n e s at the east end of Pinchi Lake. They contain rounded, moderately sorted grey and black chert pebbles (1-2 cm max) i n a s i l i c e o u s matrix. Pebbley (5 mm) sandstones were found 4.8 km east of Pinchi Mine (Fig. 4, l o c . 5) and i n the b e l t of Takla Group sediments south of Pinchi Lake (loc. 6). Internal structure and contacts The lack of d i s t i n c t i v e marker horizons, poor exposure and paucity of f o s s i l s have obscured stratigraphic r e l a t i o n s of the Takla Group within the area. The stratigraphy observed i n areas of favourable exposure i s given i n F i g . 4 together with approximate thicknesses. Correlation between sections i s problematical but i t appears that there i s at 40 least 900 m of Takla Group sediments i n the area. Northeast of the Pinchi Fault zone, the rocks of the Takla Group seem to be highly faulted rather than folded. A marked change i n s t r i k e i n the northeastern part of the map area suggests that a major f a u l t passes from Murray Ridge to Tezzeron Lake. South of Pinchi Lake, the northern margin of the Takla Group i s faulted against glaucophane bearing metacherts. Because sediments at the base of t h i s b e l t contain gabbro and basalt d e t r i t u s , i t may be that the southern contact i s an unconformity rather than a f a u l t ( s e e also p. 25 ) . Fauna Armstrong (1949) made f o s s i l c o l l e c t i o n s at two l o c a l i -t i e s i n the Takla Group. In addition to these, three new l o c a l i t i e s (Fig. 4) were discovered i n the course of t h i s study. Descriptions of the fauna are as follows: 1. Limestones on the north shore of Pinchi Lake 6.5 km from the east end contain Monotis sub-oivoulavis of Upper T r i a s s i c age (Armstrong, 1949) . 2. Sandstones 6.4 km east of Pinchi Mine, 60 m south of the road contain Eevinea, Astarte and Tvigonia i n d i c a t i n g a Jurassic age (Armstrong, 1949). 3. A b i o c l a s t i c limestone i n Takla Group sediments south of Pinchi Lake contains c r i n o i d columnals, bivalve fragments, o o l i t e s or pseudoolites, echinoid spines and echinoid s h e l l s (Cameron, G.S.C, written commun.).. Cameron also suggested that the limestone formed i n a high energy environment and was of la t e Paleozoic or early Mesozoic age. 4. An ammonite fragment was found i n a shale bed at the west end of the island at the east end of Pinchi Lake. H. Frebold considered i t to be Jurassic (oral commun.). 5. An outcrop of limestone 30 m north of a carbona-t i z e d ultramafite between Murray Ridge and Pinchi Lake contains c o l o n i a l corals thought to be of Upper T r i a s s i c age (Danner, o r a l commun.) 6. Carbonaceous wood fragments were found i n sandstone at two l o c a l i t i e s . Origin The oldest rocks appear to be Monotis bearing lime-stones and associated conglomerates, s i l t s t o n e s and grey-wackes. The shel l y fauna, the presence of intraformational limestone breccias and pebble conglomerates suggest shallow water deposition.. According to Souther and Armstrong (1966) , conglomerates occur at i n t e r v a l s along the eastern margin 42 of the Pinchi Fault zone and mark the time of emergence of the Pinchi Geanticline at the beginning of the Upper T r i a s s i c . The stratigraphic r e l a t i o n s h i p of the thick arkosic greywacke sequence to the above mentioned limestones i s unknown/ but i t i s suggested that the former was deposited on the l a t t e r during rapid subsidence of a basin l y i n g northeast of the f a u l t zone. Graded bedding and the poorly sorted angular nature of the c l a s t s imply deposition by t u r b i d i t y currents. The composition of the arkosic greywacke and of the pebbles i n the conglomerates (Table 3) indicate that the source for the Takla Group sediments included the following rocks: (a) amphibolitis^ed gabbro, diabase, amphibolite, and c h l o r i t e s c h i s t ; (b) porphyritic basic volcanics with phenocrysts of plagioclase, hornblende and occasionally clinopyroxene or b i o t i t e ; (c) chert; (d) limestone; (e) K-feldspar bearing intrusives or volcanics; (f) chromite bearing ultramafites. Porphyritic basic volcanics and hornblendized basic i n t r u s -ives seem to have provided the bulk of the d e t r i t u s . Because 43 basic i n t r u s i v e pebbles contain a l b i t e , amphibole and epidote (a lower greenschist f a c i e s mineralogy) i t seems that metamorphic rocks must have been exposed at the surface p r i o r to deposition of the Upper T r i a s s i c . The presence of ultramafites at the surface i s implied by the presence of d e t r i t a l chromite within Takla Group sediments. It could be argued that the chromite originated i n a layered basic i n t r u s i o n . However, chromite bearing gabbroic rocks have not yet been found i n the Cache Creek Group. The most reasonable hypothesis for the provenance of the Takla Group sediments i s that the d e t r i t u s originated from the limestones, cherts, gabbros and greenstones belong-ing to the Cache Creek Group, southwest of the f a u l t zone. It i s s i g n i f i c a n t that many of the basic pebbles are very similar to the gabbros and basalts exposed south of Pinchi Lake. The presence of carbonaceous fragments suggests that the area being eroded was nearby and covered with vegetation. D i o r i t e A body of hornblende d i o r i t e apparently intrudes Takla Group greywackes east of Pinchi Lake Mercury Mine, ju s t north of the road. I t i s grey-green or brownish weathering and i s well f o l i a t e d ; l o c a l l y i t i s por p h y r i t i c or contains greenstone xenoliths. Hand specimens show 44 aligned phenocrysts of plagioclase (5 mm) and a c i c u l a r hornblende (5 mm) set i n a grey aphanitic matrix. In thin section, 75% of the rock consists of euhedral plagioclase laths, well zoned with cores i n the range A n80-70 a n < ^ e x t r e m e l y altered to s e r i c i t e , epidote (?) and carbonate. Patchy low r e l i e f suggests p a r t i a l a l b i t i z a t i o n . Subhedral brown pleochroic hornblende, and minor c l i n o -pyroxene and b i o t i t e phenocrysts are also present, the f i r s t being highly altered to c a l c i t e , c h l o r i t e and sphene. The matrix consists of a microgranular mixture of plagio-clase, quartz, c h l o r i t e and sphene. Contacts, where observed, are faulted and a r g i l l i t e s showing no signs of thermal metamorphism can be seen within a few metres of the d i o r i t e . The northeast contact has not been examined. The age of the i n t r u s i v e i s uncertain. The i n c i p i e n t metamorphism or a l t e r a t i o n suggests an Early Mesozoic age. Cretaceous (?) Conglomerate Cretaceous (?) conglomerate i s exposed only at the western end of Murray Ridge where i t forms two elongate knolls 1.6 km southwest of the east end of Pinchi Lake. The pebbles and cobbles are poorly sorted, sub-rounded and up to 12 cm i n diameter. They consist of green chert (.85%) , black chert (.2%), red chert (2%) and grey limestone 45 (3%) i n a red weathering sandstone matrix (8%). The unit dips to the south at 50° and appears to be faulted against greenstones to the north. The age i s uncertain, but because of i t s poorly indurated nature and l i t h o l o g i c a l resemblance to conglomer-ates described by Armstrong (.194 9) , i t i s thought to be of Cretaceous or Paleocene age. A reddish l i m o n i t i c matrix suggests subaerial deposi-t i o n i n a continental environment. Presumably the chert and limestone pebbles were derived from the Cache Creek Group. I l l ULTRAMAFITES AND SILICA-CARBONATE ROCKS Ultramafites Introduction Three ultramafic bodies, on Murray Ridge, Pinchi Mountain and the northern slopes of Mount Pope, form conspic-uous northwest s t r i k i n g ridges (Map I). . Each of these ultramafites i s l e n t i c u l a r i n ou t l i n e and appears to be bounded by f a u l t s . Swampy areas, which are bel t s of magnetic highs, l i e along the sides of the ultramafites and also along some f a u l t zones. Diamond d r i l l i n g of a magnetic high east of Pinchi Mercury Mine and near the Pinchi Fault zone intersected serpentinite. This suggests that such anomalies elsewhere i n the map area are also the location of buried serpentinites. Aeromagnetic data (Map III) has been used i n extrapolating serpentinite contacts beneath d r i f t cover. Rock Types The dominant rock type, constituting 95% of the exposures, i s harzburgite. In outcrop, i t i s generally 47 Fig. 5 STRUCTURAL FEATURES IN H ARZBURG JTES L a t e f r ac tu re s with red __. xE-arly concordant pyroxenite a l t e r e d m a r g i n s ^^7_A--—)---^'sS. 8 c r n w ide d u n i t e •'\)( "^N> s t r ingers . • ( a ) ' Concordant of M u r r a y dunite R i d g o . and py roxen i t e layers in h a r z b u r g i t e ot wes te rn edge Dunite H a r z b u r g i t e (b)> Dunite - pyroxeni te ve in in h a r b u r g i t e Murray R i d g e (c)r F o l d e d p y r o x e n i t e l a ye r cut by late p y r o x e n i t e . 48 massive or blocky, weathers reddish brown to buff and possesses a rough mottled surface. Locally i t may be inten-sely serpentinized or fractured and d i f f i c u l t to recognize. However, the platey b a s t i t e serpentine which replaces 1 cm orthopyroxene grains generally p e r s i s t s . Dunites, comprising over 4% of the ultramafites, have a c h a r a c t e r i s t i c buff weathered surface which i s smooth i n comparison to that of the harzburgite, and are highly fractured. Pyroxenite layers are minor but are conspicuous by t h e i r d i f f e r e n t i a l weathering c h a r a c t e r i s t i c s (Plates 7 & 8). Internal structure Much of the harzburgite i s massive with no obvious f o l i a t i o n . However, i n the v i c i n i t y of dunite or pyroxenite layers, a f o l i a t i o n outlined by d i f f e r i n g olivine-orthopyro-xene r a t i o s can occasionally be discerned (Plate 9). Contacts of the dunite and pyroxenite layers with the harzburgite are gradational over 1 cm. Dunite bodies are of two v a r i e t i e s : s i l l or vein-l i k e dunites up to 1.5 m i n thickness, and i r r e g u l a r dunite bodies up to 100 m i n diameter. The former are commonly concordant with the layering i n the harzburgite and generally contain chromite stringers (Fig. 5a, Plate 9). Occasionally, rootless folds with.limbs of variable thickness are outlined by the s i l l - l i k e dunites (Plate 11). Dunite veins, up to 49 5 cms i n width, were occasionally observed giving way to pyroxenite layers (Fig. 5b). The t r a n s i t i o n from dunite to pyroxenite i s sharp and occurs where the dunite vein narrows down to 2 cm i n thickness. Irregular dunite bodies are r e l a t i v e l y rare, the only well defined example being on the southern slopes of Pinchi Mountain. Contacts with harzburgite are sharp and very i r r e g u l a r . Two d i s t i n c t types of pyroxenite layer were observed: early concordant layers and l a t e discordant l a y e r s . However, many layers could not be categorized. Early concordant pyroxenite layers occur i n swarms p a r a l l e l to the harzburgite f o l i a t i o n (Plate 10). Individual layers range from 2 to 6 cm i n thickness.' They do not have great l a t e r a l continuity, and commonly wedge out, disappear into shear zones or are cut o f f by shears. Occasionally, the pyroxenite layers are folded (Plate 10) and i n one case (Fig. 5c) a cross-cutting pyroxenite layer has an a x i a l planar orientation with respect to e a r l i e r l ayers. The orientation of these layers i s f a i r l y regular i n some regions and e r r a t i c i n others. Strikes are generally westerly to northwesterly with steep northerly dips. Late discordant pyroxenite layers are commonly up to 10 cm i n width. Within harzburgite, they occasionally occur i n groups (Plate 7) p a r a l l e l to dunite r i c h layers, but usually they occur as single pyroxenite layers which cross-cut e a r l i e r layers and i r r e g u l a r dunite bodies (Plate 8 ) . A d i s t i n c t i v e feature at a few l o c a l i t i e s i s a black-weathering, serpentine-rich zone, 5 cm wide, i n the harzburgite or dunite adjacent to the contacts of the pyroxenite layers. Folding of l a t e pyroxenite layers i s related to movement along l a t e fractures. Joints are well developed and are highly variable i n frequency and or i e n t a t i o n . One sub-horizontal and two v e r t i c a l j o i n t sets can be detected i n most outcrops. Commonly one p a r t i c u l a r j o i n t set becomes dominant and clo s e l y spaced (1 cm), i n which case the term cleavage might be more appropriate, e s p e c i a l l y where there are indications of movement along the planes of d i s c o n t i n u i t y . I t appears that the spacing of the j o i n t or cleavage planes depends on the l i t h o l o g y and the amount of l o c a l deformation. Generally, dunites are f r i a b l e because of the presence of a pervasive fracture system which subdivides the rock into a series of rhombs. Weathered pyroxenites possess a conspicuous columnar structure (Plates 7 and 8) caused by d i f f e r e n t i a l weathering along the serpentihized cleavage planes which transect the pyroxenite layers and o f f s e t the contacts. In summary, f i e l d observations indicate the following sequence of events: 51 (a) formation of early pyroxenite layers and dunite bodies, (b) f o l d i n g , Cc) formation of l a t e pyroxenite layers, Cd) pervasive serpentinization, and Ce) formation of j o i n t s and cleavages and minor serpentinization. Contact relationships As i s the case with most alpine ultramafites, contacts are poorly exposed. Where contacts are v i s i b l e , i n outcrop or i n d r i l l holes, highly sheared serpentinite i s juxtaposed against s i l i c e o u s sediments or volcanics showing no signs of thermal metamorphism. In general, contacts are thought to be the s i t e s of major f a u l t zones (Chapter V) which have been l o c a l l y carbonatized. Petrology Harzburgites have an average o l i v i n e : pyroxene r a t i o of 7:3 and occasionally contain up to 3% clinopyroxene. P i c o t i t e constitutes 2% of the rock, and serpentine a minimum of 20%. O l i v i n e grains (4 mm) are anhedral and veined by serpentine, giving r i s e to a mesh texture i n highly serpentinized rocks. Disseminated grains or stringers of magnetite are commonly associated with the serpentine. Kink 52 bands or deformation lamellae (Loney et a l . , 1971) are present i n o l i v i n e i n most thi n sections. Enstatite (En^-En^) p a r t l y replaced by platey b a s t i t e ( l i z a r d i t e according to Page, 1967) forms 1 cm grains which occasionally appear to have an intergranular r e l a t i o n s h i p to o l i v i n e or to have o l i v i n e i n c l u s i o n s . Such textures suggest magmatic c r y s t a l l i z a t i o n of o l i v i n e followed by orthopyroxene. Most en s t a t i t e grains contain f a i n t exsolution lamellae p a r a l l e l to (010) and blebs of augite aligned along cleavage planes. L o c a l l y , the lamellae are deformed by fractures. There i s a marked tendency for both augite blebs and exsolution lamellae to survive replacement by b a s t i t e . Contrary to the observations of Raleigh (1963), there i s no i n d i c a t i o n of o f f s e t t i n g of exsolution lamellae because of expansion during replacement by b a s t i t e . Clinopyroxene has an intergranular r e l a t i o n s h i p to both orthopyroxene and o l i v i n e . P i c o t i t e grains are usually intergranular and equant but may be i r r e g u l a r . Commonly they are cross-cut by stringers of serpentine. Dunites contain, on average, 7 0% serpentine and brucite, 30% o l i v i n e and 1% p i c o t i t e or chromite. Textural features of o l i v i n e and spinel are similar to those i n the harzburgites. Three types of serpentine were noted: (a) grey banded mesh serpentine, (b) cross-cutting colourless ribbon serpentine and (c) brownish serpentine grading into i d d i n g s i t e . The brownish serpentine appears to be related to the pervasive fracture cleavage and the ribbon serpentine i s often found near fractures associated with chromite grains. The presence of brucite i n three dunite samples was confirmed by X-ray d i f f r a c t i o n . The early pyroxenite layers contain on average 88% enst a t i t e , 10% o l i v i n e and 2% p i c o t i t e . Enstatite contains the c h a r a c t e r i s t i c (010) exsolution lamellae, occasionally contains o l i v i n e inclusions and may possess kink bands. Coarse blebs of clinopyroxene are aligned along the cleavage planes. Olivine occurs as i r r e g u l a r intergranular grains which have rounded boundaries with the e n s t a t i t e . P i c o t i t e i s intergranular and associated with o l i v i n e . The minerals i n the pyroxenite layers are remarkably un-altered, with serpentine limited to the margins of the layers and to c a t a c l a s t i c zones formed by cross-cutting cleavages. Only one specimen was obtained from the l a t e d i s -cordant pyroxenite layers. I t consists of 90% ortho-pyroxene, 8% clinopyroxene and 2% p i c o t i t e . Discussion on the o r i g i n of ultramafites The problem of the o r i g i n of alpine ultramafites, that i s harzburgite-dunite bodies possessing no apparent association with stratiform cumulate type ultramafites, i s longstanding and no attempt w i l l be made to review the various hypotheses. This has been adequately done by 54 Turner and Verhooge.n (1960, p. 307) . Recent sampling of ultramafites i n oceanic areas (Bonatti, 1971; Aumento, 1971) and a renewed i n t e r e s t i n t e r r e s t r i a l ultramafites has shed much l i g h t on the i r o r i g i n and on c r u s t a l structure i n oceanic areas. Geolog-i s t s working i n Cyprus (Moores and Vine, 1971) and C a l i f o r n i a (Bailey et a l . , 197 0) think that ultramafites are derived from the upper mantle and that r e l a t i v e l y un-disturbed sections of oceanic crust can be found on the continents. The suggestion has been made (Monger, Souther and Gabrielse, 1972; Danner, o r a l communication) that much of c e n t r a l B r i t i s h Columbia i s underlain by oceanic crust. This i s because the Cache Creek Group possesses no apparent continental basement and contains an o p h i o l i t i c association of rock types. Ten years ago, similar ideas were t e n t a t i v e l y suggested for the Franciscan Group, and now many geologists (Ernst, 1965; Page, 1972) accept the view that the Franciscan Group and part of the Great Valley sequence were deposited on oceanic crust. This hypothesis must be tested seriously for B r i t i s h Columbia. Concerning the o r i g i n of the ultramafites i n the area, two theories w i l l be discussed as representative of current hypotheses. McTaggart (1971) suggested that "Ultramafic bodies originated as cumulates i n basic magma chambers high, i n the crust. . . . These were subsequently folded, or dismembered by f a u l t i n g , and because of their high density, subsided during tectonism, to form cold f a u l t enclosed intrusions." The main advantage of t h i s hypothesis i s that i t explains why bodies of high density should be found at high l e v e l s i n the earth's crust amidst sediments of r e l a t i v e l y low density. I t does, however, possess serious d e f i c i e n c i e s . F i r s t , i n the cases where data on f a b r i c and mineral compositions are a v a i l a b l e , (Loney et al., 1971) i t has been demonstrated that the alpine ultramafites are metamorphic tectonites which have r e c r y s t a l l i z e d at approximately 1200°C after primary magmatic c r y s t a l l i z a t i o n . High temperature r e c r y s t a l l i z a t i o n could not have taken place during tectonism of an ultramafite situated within the low greenschist f a c i e s Cache Creek Group rocks. Secondly, McTaggart suggests that ultramafites of B r i t i s h Columbia are complementary to Late Paleozoic or T r i a s s i c volcanics. However, according to Stueber and Murthy (1966), t y p i c a l 87/86 alpine ultramafites possess unique Rb/Sr and Sr values which are not related g e n e t i c a l l y to s p a t i a l l y associated gabbros and volcanics. Because data on f a b r i c and Rb/Sr values are not available for ultramafites i n B r i t i s h Columbia, McTaggart's hypothesis cannot yet be disproved. A second, more a t t r a c t i v e theory i s that proposed by the C a l i f o r n i a n school (Coleman and Keith., 1971; Coleman, 1971).. Their model. involves c r y s t a l l i z a t i o n of a primary ultramafic magma followed by p l a s t i c deformation and r e c r y s t a l l i z a t i o n during which a mineralogical f o l i a t i o n , 56 o l i v i n e microfabric and c h a r a c t e r i s t i c i n t e r l o c k i n g textures were formed. Fracturing and serpentinization of the cooled p e r i d o t i t e occurred on tectonic emplacement i n the Franciscan melange. This model i s , of course, influenced by current ideas on plate tectonics. I t i s envisaged that the primary p e r i d o t i t e forms at an active oceanic ridge, acquires a tectonic f a b r i c i n the upper mantle and undergoes cooling, f r a c t u r i n g , emplacement and serpentinization at a subduction or obduction zone at the plate margin. The main weakness of t h i s theory i s the problem, e f f e c t i v e l y dealt with by McTaggart, of emplacement of dense peridotites' high; i n the crust. Presumably t h i s i s overcome by density lowering during p a r t i a l serpentinization, tectonic emplace-ment and/or " r a f t i n g up" of ultramafites by r e l a t i v e l y l i g h t sediments on i s o s t a t i c u p l i f t (Burch, 1968). Recent intensive work on the Burro Mountain p e r i d o t i t e i n C a l i f o r n i a (Burch, 1968; Page, 1967; Loney et al.} 1971; Coleman and Keith, 1971) has contributed to the formation of the above mentioned model. This ultramafite c l o s e l y resembles those i n the Pinchi area, both p e t r o l o g i c a l l y and s t r u c t u r a l l y . Fabric data and detailed s t r u c t u r a l information are not available for the Pinchi ultramafites, but i t i s f e l t that the s i m i l a r i t y to Burro Mountain i s s u f f i c i e n t l y great to warrant some extrapolation of conclusions. 57 Origin of Pinchi ultramafites The o r i g i n of the primary mineralogy and textures i n ultramafites i s conjectural. Loney et al. (1971) suggest that harzburgite and dunite c r y s t a l l i z e d penecontem-poraneously from gen e t i c a l l y related ultramafic magmas rather than having d i f f e r e n t i a t e d from a basic magma or having formed as a.residual product of p a r t i a l melting of the primitive mantle. They substantiate t h i s by pointing out the lack of c r y p t i c or rhythmic layering i n alpine ultrama-f i t e s and the dearth of cumulate textures compared with stratiform layered complexes such as the Bushveld and S t i l l -water. As p o s i t i v e evidence for a magmatic o r i g i n , they state that the differences i n the chemistry of o l i v i n e and chromite i n d i f f e r e n t dunite bodies suggest that the harzburgite has been intruded by dunite magmas of contrasting composition. Ringwood (1962) suggested that alpine u l t r a -mafites are the residual products of p a r t i a l melting of the primitive mantle. This p o s s i b i l i t y i s given some support by the Rb/Sr work of Stueber and Murthy (1966) who suggested that alpine ultramafites are re s i d u a l and were probably depleted of l i t h o p h i l e elements at some early stage i n the i r hi s t o r y . On the basis of the work done at Pinchi so f a r , the above mentioned hypotheses cannot be substantiated or refuted. However, textures which are suggestive of magmatic 58 c r y s t a l l i z a t i o n (p. 52 ) do e x i s t and may be a r e l i c t feature of a primary magmatic o r i g i n . The o r i g i n of the concordant harzburgite-dunite-pyroxenite layering i s also problematical. Four hypotheses have been suggested: (a) c r y s t a l s e t t l i n g from a magma (Raleigh, 1965), (b) magmatic intrusion of dunite and pyroxenite dykes and s i l l s (Loney et al., 1971), (c) metasomatic replacement along fractures (Bowen, 1949) and (d) metamorphic d i f f e r e n t i a t i o n caused by shearing stress and d i f f e r i n g physical properties of o l i v i n e and pyroxene (Burch, 1968). The f i r s t hypothesis receives support from a study by Raleigh (1965) car r i e d out on the Cypress Island p e r i d o t i t e , considered to be a t y p i c a l alpine type ultramafite. I t contains a dunite-chromitite-harzburgite layering and accumulative textures which Raleigh considered to have formed by c r y s t a l s e t t l i n g from a magma. Later r e c r y s t a l l i z a t i o n and penetrative deformation produced a preferred orientation of o l i v i n e . However, Burch (1968) decided against such an o r i g i n for the pyroxenite-dunite-harzburgite layering i n the Burro Mountain p e r i d o t i t e because of the sharpness of contacts between layers and absence of cry p t i c layering, rhythmic layering, graded bedding and well defined cumulate 59 textures. This also appears to be the case at Pinchi but, because l i t t l e i s known about the primary magmatic conditions and the extent of modification by r e c r y s t a l l i z a t i o n and deformation, c r y s t a l s e t t l i n g must be considered a possible mechanism for layer formation. Magmatic intrusion (b) i s a plausible hypothesis for the o r i g i n of the ir r e g u l a r dunites. However, i t does not explain the common p a r a l l e l i s m of dunite and pyroxenite 'layers, the s i m i l a r i t y i n thickness of pyroxenite layers and the absence of cross-cutting relationships between dunite layers and early pyroxenites. Bowen (1949) suggested that SiC ^ - d e f i c i e n t f l u i d s moving along fractures (c) could r e s u l t i n replacement of orthopyroxene by o l i v i n e , forming dunite layers. Presumably, i t would also be possible for SiC ^ - r i c h f l u i d s to cause replacement of o l i v i n e by orthopyroxene. This hypothesis possesses similar d e f i c i e n c i e s to that of magmatic in t r u s i o n . In addition, i t i s d i f f i c u l t to explain why metasomatic f l u i d s should be of d i f f e r e n t compositions i n adjacent layers. Burch (1968) suggested that pyroxenite and dunite layers are formed by metamorphic d i f f e r e n t i a t i o n along shears because of the d i f f e r i n g physical properties of o l i v i n e and orthopyroxene. He argued that dunite would tend to concentrate i n areas of shear, as o l i v i n e y i e l d s r e a d i l y by p l a s t i c flow, and pyroxene would concentrate i n areas of l e a s t stress or p o t e n t i a l tension fractures. 60 As dunite and pyroxenite layers are concordant, o l i v i n e segregation i n a shear plane may have l e f t a pyroxene-r i c h residue, thus giving r i s e to the observed p a r a l l e l i s m of dunite and pyroxenite layers. This hypothesis i s also considered acceptable because i t explains the absence of early pyroxenite layers i n dunites. The pyroxenite and dunite layers within the Pinchi ultramafites have been folded. Similar structures have been observed at Burro Mountain (Loney et al.3 1971) where o l i v i n e f a b r i c data indicate: the presence of a pervasive planar f a b r i c which cross-cuts harzburgite-dunite contacts and i s a x i a l planar to minor, f o l d s . Analyses of co-e x i s t i n g o l i v i n e and chrome spinel suggest that t h i s f a b r i c r e c r y s t a l l i z e d at an approximate temperature of 12 00°C. This conclusion i s supported by experimental work on o l i v i n e deformation at 1200°C which produced a f a b r i c similar to that present i n dunites from Burro Mountain (Ave L'Allement and Carter, 1969). On the basis of s i m i l a r i t i e s i n i n t e r n a l structure between the Burro Mountain and Pinchi ultramafites i t i s suggested that the l a t t e r has also undergone a deep-seated p l a s t i c deformation at 1200°C. The o r i g i n of the late discordant pyroxenite layers can possibly be explained by fol d i n g of pre-existing layers and formation of a new generation of shear planes cross-cutting the early layers. However, t h i s hypothesis cannot explain the occurrence of l a t e pyroxenite veins 61 cutting i r r e g u l a r dunite bodies. I t i s therefore concluded that the o r i g i n of the late discordant pyroxenites has to be accounted for by magmatic in t r u s i o n of an "orthopyroxenite magma," or, more l i k e l y , by metasomatic a c t i v i t y . The formation of kink bands i n o l i v i n e i s related to the g l i d e system {Okl} [10 0]. Experimental evidence (Ave L'Allemant, 1968) indicates that t h i s g l i d e system operates predominantly i n the temperature range 90 0° to 1200°C and suggests that kink band formation took place a f t e r r e c r y s t a l l i z a t i o n at 1200°C and p r i o r to se r p e n t i n i -zation . The main period of serpentinization i s believed to have been contemporaneous with f r a c t u r i n g and emplacement during the F ^ and F^ deformations (Chapter V) . The pressure-temperature conditions of serpentinization are discussed i n Chapter IV (p. 116 ) . Two hypotheses are considered for the emplacement of ultramafites. The f i r s t involves overthrusting of oceanic crust during Permo-Triassic tectonism, i n which case, the Pinchi ultramafites may be downfaulted remnants of an o p h i o l i t i c cover to the Cache Creek Group. The second hypothesis suggests s o l i d emplacement of a fault-bounded block of low average density (2.7-2.8 gms/cc)containing cherts limestones, s c h i s t and serpentinized p e r i d o t i t e , along zones of low pressure i n the Pinchi Fault system. These hypotheses are further discussed i n Chapter VI. 62 The l a t e s t deformational event within the u l t r a -mafites was the formation of a complex fracture cleavage associated with minor serpentinization. This may have occurred during the l a t t e r stages of the F^ deformation or during the F^ deformation (Chapter V). Silica-carbonate Rocks  D i s t r i b u t i o n Because of t h e i r resistance to erosion, carbonatized serpentinites are conspicuous i n the f i e l d . Generally they form a colinear series of rusty weathering south facing b l u f f s up to 120 m wide occurring sporadically along f a u l t zones at the margins of ultramafites. The best examples are on the southern slopes of Pinchi Mountain and at the southeast end of Pinchi Lake. Carbonatized ultramafites are also found i n the v i c i n i t y of Pinchi Mercury Mine and the Darbar claim group, 7 miles northeast of Fort St. James. Rock types, i n t e r n a l and external structure Ferroan magnesite, the predominant constituent i n the silica-carbonate rocks, i s orange-brown and has a rough weathered surface because of the presence of anastamosing quartz v e i n l e t s . R e l i c t chromite grains or pale green mariposite (fuchsite) are commonly seen i n hand specimen. 6 3 Layers of sugary white magnesite, up to 1 m i n thickness are occasionally found. These layers contain inclusions of ferroan magnesite. Compositional layering within the ferroan magnesite zones i s p a r a l l e l to the contacts and i s defined by s i l i c i f i e d breccia zones and white magnesite layers (Plate 5). The contacts of r e l i c t lenses of serpentinized harzburgite are concordant with the layering and are highly sheared and veined by magnesite. Dips are to the north or northeast at approximately 60°. A magnesite zone on the south side of Pinchi Mountain l i e s s t r u c t u r a l l y beneath ultramafic and glaucophanitic rocks and o v e r l i e s the Pinchi Mountain greenstones. However, on the assumed path of the Pinchi Fault, at the southeast end of Pinchi Lake, silica-carbonate rocks appear to pass under Upper T r i a s s i c rocks. On the Darbar claim group the f o l i a t i o n i n carbonatized serpentinites p a r a l l e l s a f a u l t zone which dips under greenstones at 60° to the northeast. A l l the evidence i s compatible with the hypothesis that the layering originated during contemporan-eous carbonatization and movement along major f a u l t s . Late brecciation affected a l l the above mentioned rocks. D i l a t a t i o n a l fractures or voids were f i l l e d i n turn by dolomite, chalcedonic quartz or agate and c r y s t a l l i n e quartz. Dolomite veins cut by quartz veins are common, the former possessing s t r i a t e d c r y s t a l s with long axes 64 perpendicular to the sides of fractures. These late veins are cross-cut by a near v e r t i c a l set of north-south s tr iking frac tures. Petrology The silica-carbonate rocks consist of ferroan magnesite ± quartz ± serpentine. Magnesite and quartz are usually m i c r o c r y s t a l l i n e but may be l o c a l l y coarse grained. R e l i c t subhedral chromite possessing reddish translucent margins i s common i n a l l specimens. Annabergite ("nickel bloom") i s found on fracture surfaces near Pinchi Mine. The presence of r e l i c t chromite and lenses of serpen-t i n i z e d harzburgite within the silica-carbonate rocks demon-strates that they were ultramafites p r i o r to a l t e r a t i o n by along f a u l t zones. The most common equilibrium mineral assemblage i s magnesite + quartz, but an t i g o r i t e + magnesite i s also present i n one specimen. Talc i s not present. Con-sidering these assemblages, the reaction: serpentine + CC^ = magnesite + quartz + H^ O i s applicable. This has been studied experimentally and th e o r e t i c a l l y by Greenwood (1967) and Johannes (1969). Their data i n d i c a t e : (a) the assemblage a n t i g o r i t e + magnesite must have formed at low p a r t i a l pressure of CC^ ( x Co less than 0.03 at 1 kb); (b) the predominant assemblage magnesite + quartz i s stable at X _ greater than 0.03 at 1 kb and 300°C and (c) the temperature of the reaction was less than 350°C at 4 kb and less than 310°C at 1 kb. An t i g o r i t e i s considered a "high temperature" serpentine mineral (Wenner, 1971; Coleman, 1971). Therefore, the occurrence of a n t i g o r i t e and the quartz-magnesite assemblage suggest that the temperature of formation of the s i l i c a -carbonate rocks was i n the 2 00° to 300°C range. Age and o r i g i n Carbonatization i s r e s t r i c t e d to the major f a u l t s i n the area. Because of the presence of s i l i c i f i e d magnesite breccias i t i s considered l i k e l y that the layering within the silica-carbonate rocks was controlled by movement along f a u l t s during a l t e r a t i o n . Therefore, the problem of the timing of the a l t e r a t i o n i s associated with the timing of active f a u l t i n g . A number of authors consider that active f a u l t i n g associated with the Laramide orogeny took place before the Late Eocene. In the McConnell Creek map area and on the S p a t s i z i Plateau, Sustut Group rocks of Upper Cretaceous to Paleocene age have been a c t i v e l y involved i n northeasterly directed thrust f a u l t i n g (Lord, 1949; Eisbacher, 1969). North of Fort St. James, conglomerate beds of Cretaceous 66 or Paleocene age are found intermittently along the Pinchi Fault zone (Armstrong, 1949) and such a conglomerate i s found at the west end of Murray Ridge apparently i n f a u l t contact with older rocks. Roots (1953) staterd that rocks of the Takla Group are carbonatized adjacent to a mercury mineralized f a u l t zone the continuation of which i s believed to displace rocks of Cretaceous or Paleocene age i n the Aiken Lake map area. For these reasons, evidence favours a period of active f a u l t i n g associated with carbonatization i n the Fort St. James area during the Eocene. The formation of silica-carbonate rocks i s commonly considered as an early stage of the hydrothermal a c t i v i t y which la t e r r e s u l t s i n the deposition of cinnabar (Bailey, 1963; Henderson, 1968). Cinnabar mineralization i s generally considered epithermal, and cinnabar i s at present being deposited by hot springs at various locations i n the western United States (White, 1968, p. 1675). Henderson (1968) states that cinnabar deposits i n the silica-carbonate rocks of C a l i f o r n i a occur at depths of less than 8 00 m and commonly the sequence of events i s : (a) silica-carbonate a l t e r a t i o n of serpentinite, (b) fr a c t u r i n g of silica-carbonate rock, and Cc) cinnabar vein mineralization of the fractures. In the Pinchi area, cinnabar mineralization occurs as fracture f i l l i n g s i n silica-carbonate rocks at two l o c a l i t i e s and i t i s suggested that the history of mineralization i s similar to that of the C a l i f o r n i a n occurrences and that the mineralization took place i n a near surface environment after the formation of the silica-carbonate rocks. J.E. Armstrong (1966) and H.W. Tipper (oral communication) also consider the mercury mineralization i n B r i t i s h Columbia to be of Tertiary age. In summary, evidence favours the following sequence of events: (a) The silica-carbonate rocks were formed during a re a c t i v a t i o n of the Pinchi Fault i n the Eocene. Carbon d i o x i d e — r i c h f l u i d s , possibly acting as lubricants i n the f a u l t plane, reacted with adjacent ultramafites giving r i s e to the magnesite + quartz assemblage. The type of f a u l t i n g and sense of movement i s unknown, but northeasterly directed thrusting was widespread 240 km to the north during the Eocene (Eisbacher, 197 0). The common occurrence of northeasterly dipping f a u l t planes suggests that s i m i l a r l y oriented stresses may have given r i s e to underthrusting i n the Pinchi area, but t h i s i s speculative. (b) Fracturing of the silica-carbonate rocks occurred during the Eocene, Oligocene or Miocene contem-poraneous with hot spring a c t i v i t y and mercury mineralization. IV METAMORPHISM I. METAMORPHISM IN GREENSTONE AND BLUESCHIST FAULT BLOCKS  Introduction Four possible hypotheses currently e x i s t which attempt to explain the occurrence of the "high pressure" blueschist f a c i e s mineralogy. The hypotheses are l i s t e d below together with t h e i r p r i n c i p a l exponents. (a) The necessary pressures are attained by tectonic overpressures i n addition to l i t h o s t a t i c pressures (Blake, Irwin and Coleman, 1967, 1969). (b) High pressures are attained by an " i n t e r n a l l y created gas overpressure" (Brothers, 1970). (c) Blueschist fac i e s minerals are formed metastably at lower pressures than those indicated i n experi-mental studies because of reducing conditions i n the pore f l u i d accompanying metamorphism (Gresens, 1969) . (d) Blueschist fac i e s mineral assemblages are formed at high l i t h o s t a t i c pressures and r e l a t i v e l y low temperatures (Fyfe, Turner and Verhoogen, 1958, p. 226; Ernst, 1965, 1971b). 69 A b r i e f summary of each of these hypotheses follows with emphasis on aspects of the metamorphism at Pinchi which bear d i r e c t l y on the problem of discriminating between the suggested hypotheses. The hypothesis of Blake, Irwin and Coleman (1967, 1969} that tectonic overpressures i n a f a u l t zone can explain blueschist mineralogy has been shown to be u n l i k e l y by Brace et al., (1970) who showed experimentally that Franciscan greywacke cannot support even one kilobar of tectonic overpressure and by Ernst (1971b) who concludes that none of the geologic, s t r u c t u r a l , petrographic or experimental work i s consistent with the existence of s u f f i c i e n t tectonic overpressure. At Pinchi, the hypothesis of tectonic overpressures finds no support i n structure or tectonic f a b r i c . No gradation e x i s t s i n metamorphic rec o n s t i t u t i o n towards the Pinchi Fault and metabasalts and j a d e i t i z e d metagreywackes commonly contain r e l i c t igneous or c l a s t i c textures (Appendix IV). Their unsheared nature excludes the p o s s i b i l i t y of a s i g n i f i c a n t contribution from tectonic overpressure. There may well be a tectonic-genetic r e l a t i o n s h i p between the glaucophanitic rocks and the Pinchi Fault but there i s no evidence that movement on the f a u l t produced tectonic overpressure i n the adjacent rocks. Brothers (197 0) proposed an hypothesis involving the c r y s t a l l i z a t i o n of lawsonite-aragonite rocks i n an 70 environment with an " i n t e r n a l l y created gas overpressure." This overpressure was thought to be contained by an impermeable tectonic seal i n the form of an overlying ultramafite. Under such conditions, P^, . , = P f l u i d s o l i d s CG reenwood, 1961) but l o c a l l y , ^ f ] _ u j ^ — ^ s o l i d s > , , 4 . 4 . • • This hypothesis i s unsupported by the 1 1 t n o s ta t i c Pinchi rocks because i t seems u n l i k e l y that Pf]_uj_£ could exceed P, ... +..•-• throughout a regionally metamorphosed l i t n o s t a r i c t e r r a i n and because there i s no evidence for an "impermeable tectonic sea l " which could have encapsulated the blueschists. Gresens (1968) emphasized the global association of ultramafites with blueschists and suggested that the presence of ultramafites undergoing serpentinization and oxidation of iron resulted i n the formation of a reducing pore f l u i d i n the adjacent sediments. This pore f l u i d induced the metastable growth of the c h a r a c t e r i s t i c blueschist f a c i e s minerals at lower pressures than those indicated by experimentally produced P/T s t a b i l i t y diagrams. In addition to the arguments advanced by Ernst (1971b) against t h i s hypothesis, f i e l d evidence at Pinchi demonstrates that ultramafites are commonly associated with greenstones belonging to the prehnite-pumpellyite facies rather than the lawsonite-glaucophane bearing rocks. Gresens 1 hypothesis i s therefore not d i r e c t l y applicable 71 to the Pinchi blueschists. However, there i s evidence at Pinchi that indicates some of the blueschist f a c i e s minerals c r y s t a l l i z e d under reducing conditions and c e r t a i n aspects of the hypothesis are therefore worthy of considera-t i o n . These are discussed l a t e r (p. 96 )• The fourth hypothesis for blueschist formation involves high l i t h o s t a t i c pressures and r e l a t i v e l y low temperatures (Fyfe, Turner and Verhoogan, 1958; Ernst, 1965, 1970, e t c . ) . The hypothesis i s based larg e l y on the favourable comparison of the observed mineral paragenesis with experimentally determined phase e q u i l i b r i a . In the succeeding sections, i t i s demonstrated (a) that the Pinchi blueschists formed at high pressures, and (b) that methane may have been an important constitu-ent of the f l u i d phase i n metamorphic assemblages which contain carbonaceous material. Paragenetic Sequence of Minerals Possible mineral paragenetic sequences for metabasic rocks and metacherts based on textural and f i e l d evidence are given i n Figs. 6 & 7. As extensive work has been done recently on progressive metamorphism of Franciscan grey-' wackes, a paragenetic sequence i s taken from Ernst (1971) and the stage of evolution of the Pinchi greywackes i s indicated for comparison. 72 FIG. 6 KINEF1AL PARAGEMESES METABASALTS Minerals Greenstones Blueschists Lato minerals a lb l te Bodlo pyroxene acmlte-JadeIte ch lor i te quartz ca l c i te aragonite pumpellylte white mica celadonlte Ephene glaucophane lawsonite stllpnomelane prehnite brown amphlbole deerite pyr i te magnetite hematite ? ? Increase ln P : ^ Decrease ln P ^ ECLOGITE Early minerals Late minerals omphaclte garnet r u t l l e glaucophane lawsonite sphene brown amphlbole + ch lor i te ? Decrease ln P (?) > — — ; major constituent : minor constituent : accessory const ituent: 73 FIG. 7 MINERAL PARACENBSES KETACHERTS Minerals Early minerals Blueschist facies Late minerals glaucophane lawsonlte quartz white mica pyrite hematite alblte carbonaceous material sphene acmltlc pyroxene ?  METAGREYWACKES Minerale Diablo Range (Ernst, 1 9 7 1 ) Increasing grade Finchl area clastic biotite pumpellylte lawsonite alblte Jadeltic pyroxene glaucophane white mica chlorite stilpnomelane calcite aragonlte quartz rock fragments sphene : major constituent s minor constituent : accessory constituent 74 Mineral Assemblages Details of the petrography, mineral assemblages and specimen locations of the f a u l t bounded blocks containing the greenstones of Pinchi Mountain and the glaucophanitic rocks are given i n Appendix IV. Equilibrium phase assemblages can be summarized as follows: (a) Greenstones of Pinchi Mountain (i) ab + NaPx + c h l + sph ± wh m ± arag ± pump ± celad Cii) qtz + NaPx + c h l + sph ± wh m ± arag ( i i i ) arag + dol (only i n inter c a l a t e d limestones) (iv) ab + c h l + pump + sph ± prehn ± cc ± celad ± wh m (only found at west end of Murray Ridge) (b) Glaucophane-lawsonite bearing assemblages Metabasic rocks (v) acm-jd + lws + sph + c h l ± wh m ± arag ± glph (massive) (vi) glph + lws + sph ± chl ± wh m (foliated) Limestone (vii) arag + carbonaceous material ± dol ± qtz Metasediments ( v i i i ) arag + qtz ± wh m (ix) qtz + lws + glph + wh m + c h l (xl qtz + wh m ± lws + glph ± carb mat ± sph ± py •^•Abbreviations glph = glaucophane; NaPx = sodic pyroxene; acm-jd = acmite-jadeite; lws = lawsonite; qtz = quartz; ab = a l b i t e ; c h l = c h l o r i t e ; sph = sphene; wh m = white mica (phengite); arag= aragonite; pump = pumpellyite; celad.= celadonite; prehn = prehnite; cc = c a l c i t e ; dol = dolomite; s t i l p = stilpnomelane; py = py r i t e ; carb mat = carbonaceous material. 75 TABLE 4 GENERALIZED PARAGENETIC SEQUENCE OF METABASALTS BASALT -> GREENSTONE -> PINCHI MT. GREENSTONE augite plagioclase ilmenite o l i v i n e / / retrograde reaction I \ c h l o r i t e a l b i t e sphene zeolites prehnite c a l c i t e ECLOGITE omphacite garnet r u t i l e \ FOLIATED GLAUCOPHANITIC METABASALTS glaucophane lawsonite sphene c h l o r i t e stilpnomelane c h l o r i t e a l b i t e sphene sodic pyroxene pumpellyite aragonite prehnite celadonite white mica quartz ACMITE-JADEITE METABASALT acmite-j adeite lawsonite c h l o r i t e sphene aragonite phengite glaucophane stilpnomelane deerite TABLE 5a POSSIBLE REACTIONS IN METABASIC ROCKS (A) Metastnble reactions (1) plagioclase - x alblte + (1-x) anorthlte (2) 2 llmenlte + | 0 2 » hematite + 2 r u t l l e (3) anorthlte • r u t l l e •» sphene + AljSlO^ (Al-Sl component l n phenglte o r chlorite) (4) 7 anorthlte + 5 diopside + 10H20 = 6 prehnlte + chllnochlore + 3 quartz (5) 6Ca(Kg,Fe)Sl 20 6 + 4H20 + 6 C 0 2 = (Mg.Fe) 6 S1^0 1 0(OH)g + 6 C a C 0 3 + 8S102 augite chl o r i t e c a l c i t e (B) Significant prograde reactions (I.e. greenstones — — laKSonlte-glaucophane metabasltes) * (6) c a l c i t e = aragonite **(?) laumontlte + prehnlte + chlorite = pumpellylte + quartz + H20 (8) amesite(chlorlte) + 8 prehnlte + 2H20 = 4 pumpellylte + 2 quartz * (9) laumontlte = lawsonite + 2 quartz + 2H20 * (10) heulandlte = lawsonite + 5 quartz + 4H20 ••(11) Ca(Mg,Fe)Si 20 6 + Na + • (Al.Fe) 3* = Na(Al,Fe)Si 20 6 + C a 2 + + (Kg,Fe) 2 + augite EOln. Jadelte-acmlte soln. comp. ln pyroxene (12) NaAlSljOg + 0.33?ejOk + 0.1702 = NaFeSl 2 0 6 + 0.5 A1 20 3 + S10 2 a l b l t e hematite acmlte comp. Al-Sl component l n p h e n g l t e o r l n pyroxene c h l o r i t e (13) NaAlSljOg + ( K e . F e ) 6 S l 4 0 1 0 + 0.1202 = N a A l ^ ^ F e ^ + (Kg,Fe) $ > 5A1 _^51 ^ O ^ f O H j g al b l t e ehlorlte - 1 ecmlte-Jadelte chlorlte-2 + 1.12 S i 0 2 * (14) a l b l t e = Jadelte + quartz **(15) 2 alblte + C a 2 + + 2H20 = lawsonite + 2 Na* + 4 S10 2 H2 *°2 (16a) 2 acmlte + 3 hematite + 4 quartz + CH^ = r l e b e c k l t e + m a g n e t i t e + C + H20 * • H 2 ° ° 2 H 2 . 2 5 0 2 (16bj 2 acmlte +1 . 5 hematite + 4 quartz + CH^ + . 250 2 = rlebecklte + C + H 20 H2o . 7 5 0 2 f l u i d f l u i d (17) 2 alb l t e + 0.5 c h l o r i t e ( s e r p e n t i n e ) = glaucophane + water (C) Possible reactions relating Pinchi assemblages, with higher temperature ( i . e . greenschist facies) assemblages. * (18) lawsonite = anorthlte + 2H20 * (19) 4 lawsonite = 2 zo l s l t e + kyanlte + quartz + 7H20 E l l l l m a n l t e * (20) 12 prehnlte + 6 cllnochlore + 2 quartz = 12 c l l n o z o l s i t e + 5 serpentine + 10H20 * (21) 20 pumpellylte = 16 c l l n o z o l s i t e + 5 ameslte + 16 grossular + 14 quartz + 2H20 (22) 24CaAl 2Sl 20 ? ( 0 H) 2.H 20 + SMg^Sl^O „)(OH) 8 = 12Ce 2Al S I ( O H ) + 6 l 1 E ^ A l 2 S l 3 0 1 0 ( O K ) 0 lawsonite chlorite c l l n o z o l s i t e c h l o r i t e , • + 14S102 + 38H 0 (Turner, 1968. p. 155) * (23) f e r r o g l e u c o p h n n e • i 0 ? = 2 e l b l t e + 2 q u a r t z • m a g n e t i t e • H20 * e x p e r i m e n t a l d e t e r m i n a t i o n - see ? l j . ; . C f o r p o s i t i o n c f e q u i l i b r i u m c u r v e ond a u t h o r ** u n b a l a n c e d o r i o n i c r e a c t i o n m l n c r n l c o n - . p o n ltlonR o ro g i v e n l n 'pi l . lc 5b 77 TABLE 5b MINERAL COMPOSITIONS pumpellyite: Ca 4MgAl 50 COHl3 CSi 20 7) 2 (Si0 4) 2 • 2H20; prehnite: C a ^ l ^ i - j O . ^ (OHl 2; grossular : Ca^A^ (SL^O^l i c l i n o z o i s i t e : Ca^-L^Si-jO.^ '> amesite: Mg^Al^Si-jO^g (OH) g; cli n o c h l o r e : M g 5 A l 2 S i 3 O 1 0 COH)g; serpentine: Mg^-SiO.^ (OH) g; laumonite: C a A l 2 S i 4 0 1 2 * 4H20; heulandite: C aAl 2Si 70 1 8» 6H20; glaucophane: N a 2 M g 3 A l 2 S i g 0 2 2 (OH) 2; +2 +3 rie b e c k i t e : Na 2Fe^ F e 2 S i g 0 2 2 (OH) 2; + 2 ferroglaucophane: NaFe^ A l 2 S i g 0 2 2 (OH) 2j 78 (c) Eclogite (xi) garnet + omphacite (xii) glph + lws + sph + s t i l p (retrograde). Metamorphic Reactions Mineral chemistry i s given i n Appendix II and bulk rock chemistry i n Appendix I I I . Metabasic rocks Prior to metamorphism, the mineralogy of the metabasic rocks was augite + plagioclase + ilmenite ± o l i v i n e . These minerals are metastable under low grade metamorphic conditions i n the presence of water. Introduction of H^ O may have had a c a t a l y t i c e f f e c t on th e i r breakdown. Reactions i l l u s t r a t -ing breakdown of primary minerals are i l l u s t r a t e d i n Table 5. C h l o r i t e , a l b i t e , c a l c i t e and sphene were probably the most important breakdown products, with prehnite and the z e o l i t e s heulandite and laumontite as possible add i t i o n a l phases. Progressive depth zonation involving such minerals has been demonstrated by Coombs (1961) and J o l l y and Smith (1972). I t i s suggested that the Pinchi greenstones may also have passed through t h i s stage during b u r i a l . The greenstones of Pinchi Mountain are characterized by the absence of c a l c i t e , z e o l i t e s and prehnite and the presence of aragonite, pumpellyite and sodic pyroxene. Glaucophane and lawsonite are present as minor constituents i n two samples. Aragonite presumably r e c r y s t a l l i z e d from c a l c i t e present i n amygdules, veins or interbedded lime-stones. Pumpellyite formation i s problematical. Coombs (1961) considers that reaction 7 (Table 5) i s applicable and Hinrichsen and Schurmann (1972) experimentally i n v e s t i -gated breakdown of pumpellyite i n reaction 8. The absence of-the z e o l i t e s laiimontite and heulandite can be explained by reactions 9 and 10. Sodic pyroxene may have formed as the r e s u l t of two processes. F i r s t l y , r e l i c t augite may have taken part i n a cation exchange reaction with the f l u i d phase (reaction 11). E p i t a x i a l sodic pyroxenes are common and analyses (p. 2 06) show them to be poorer i n Ca and Mg and ric h e r i n Na, A l and Fe with respect to r e l i c t augites. Secondly, Kerrick (1971) proposed reaction 12 as being of importance i n the formation of the acmitic component i n j a d e i t i c pyroxene i n metagreywackes. In the Pinchi rocks, iron oxides would have been available on the break-down of ilmenite at low temperatures. On depletion of iron oxides, a l b i t e may coexist with sodic pyroxene, a compata-b i l i t y often observed. Hematite and a l b i t e were not found together i n any specimen. The re s i d u a l A^O^ and Si02 most l i k e l y take - part i n reactions forming c h l o r i t e or pumpellyite., The assemblage sodic pyroxene + c h l o r i t e + sphene + quartz ± aragonite, present i n a few rocks, 80 presumably r e f l e c t s reaction 12 having gone to completion because of suitable compositional requirements, /_ or u2 k i n e t i c s . There i s no evidence to suggest that the absence of alb'ite i s because of higher pressure conditions. The lawsonite-glaucophane bearing rocks probably developed by r e c o n s t i t u t i o n of the assemblages present i n the Pinchi Mountain Greenstones. These blueschists are distinguished by the absence of pumpellyite, prehnite, a l b i t e and celadonite and the presence of widespread glaucophane, lawsonite and jadeite-acmite. Stilpnomelane, phengite and deerite may also be present. Pyroxene, glaucophane and c h l o r i t e compositions are given i n Appendix I I . Two main assemblages are commonly recognized within the matabasic rocks and are characterized by the presence of acmite-jadeite or glaucophane. Textural relationships indicate that the glaucophanitic assemblages formed l a t e r . Therefore, i t appears that greenstones reacted to form acmite-jadeite assemblages which i n turn reacted to give glaucophanitic assemblages (Table 4). The main minerals within acmite-jadeite metabasalts are lawsonite, c h l o r i t e , acmite-jadeite and sphene. Lawsonite formation (Table 5, reaction 15) commonly occurs within a l b i t e pseudomorphs. I t appears that on the break-down of a l b i t e , Ca entered the pseudomorphs and lawsonite nucleated. The calcium originated from the breakdown of r e l i c t augite (Reaction 11), pumpellyite or prehnite. 81 The presence of white mica and c h l o r i t e i n pseudomorphs associated with lawsonite, suggests inward d i f f u s i o n of potassium, magnesium and iron i n addition to calcium. The jadeite-acmite component i n the pyroxene progressively increased as a r e s u l t of reactions 11 and 12 (described previously). Reaction 13 may have been of importance but there i s l i t t l e evidence to support i t from the available c h l o r i t e analyses. The absence of celadonite as a separate phase can be accounted for by increase i n extent of s o l i d solution i n phengite (see p. 90 ). Glaucophanitic rocks appear to have formed from acmite-jadeite assemblages. Evidence for t h i s i s seen i n the common occurrence of glaucophane veins which cross-cut acmite-jadeite bearing assemblages. Many rocks show t r a n s i t i o n a l c h a r a c t e r i s t i c s and contain glaucophane and acmite-jadeite. In such rocks, glaucophane commonly rims or cross-cuts acmite-jadeite porphyroblasts. Inspection of bulk compositions (Table 16a) reveals only minor differences between acmite-jadeite and glaucophane rocks and i t i s suggested that glaucophanitic assemblages were formed from acmite-jadeite assemblages as a r e s u l t of a reaction similar to no. 16 (Table 5). Glaucophane may also have been produced by reaction of a l b i t e and c h l o r i t e . This reaction (17) can be combined with no. 16 to give c r o s s i t e . 82 I t i s generally considered that the prehnite-pumpellyite and blueschist f a c i e s pass into the green-s c h i s t f a c i e s with increase i n temperature. Associated with t h i s t r a n s i t i o n i s the breakdown of such minerals as prehnite, pumpellyite and lawsonite and the growth of epidote and t r e m o l i t e - a c t i n o l i t e (Table 5 ). The absence of these l a s t mentioned minerals at Pinchi puts a somewhat i l l -defined upper l i m i t on the temperature of metamorphic r e c r y s t a l l i z a t i o n . The occurrence of eclogite boulders i n the Pinchi area might suggest, as i l l u s t r a t e d i n Table 4, that they are the product of the next stage i n the metamorphism of the metabasic rocks. Presumably, the c r i t i c a l reaction must be of the type: lws + acm-jd + glph + sph + c h l = omphacite + garnet + r u t i l e + H 2 0 Being a dehydration reaction, i t would be favoured by increase i n temperature as univariant curves for such reactions have steep slopes on the petrogenetic g r i d . However, according to Morgan (197 0) eclogites may form d i r e c t l y from dry basalts without the involvement of ^ 0 . Within the e c l o g i t e , garnet and omphacite are replaced by glaucophane, lawsonite, sphene and stilpnomelane. I t i s suggested that t h i s occurred during retrogressive metamorphism associated with decrease i n temperature and pressure, reversal of the above 83 reaction and re-entry into the glaucophane-lawsonite f a c i e s . Metasediments Assemblages i n metasedimentary rocks are given i n Tables 24 and 25. L i t t l e can be said about prograde metamorphic reactions as the mineralogy of lower grade rocks i s not known with c e r t a i n t y . Within metacherts, glaucophane c r y s t a l s may be zoned with Fe-rich cores (crossite) and margins (Fig. 26). Mg and A l show an antipathetic v a r i a t i o n . In the sample studied (No. 151) glaucophane co-exists with quartz, lawsonite, phengite, carbonaceous material and magnetite. P a r t i a l microprobe analyses of phengites (Table 13) indicate that they are compositionally homogeneous and presumably have equilibr a t e d with the F e - r i c h rims of glaucophane c r y s t a l s . This zoning could be a r e s u l t of the changing composition of the f l u i d phase during metamorphism but the presence of carbonaceous material, lawsonite and magnetite defines a narrow range of f l u i d composition (see p. 104)• A second al t e r n a t i v e i s that the zoning i n the glaucophane r e f l e c t s the changing composition of the co-existing phengite with increase and decrease i n pressure. According to Velde (1965) there i s evidence that increase i n pressure favours s o l i d solution between muscovite and various celadonite end-members. The e f f e c t of pressure on the 84 e x t e n t o f p h e n g i t e s o l i d s o l u t i o n w i t h Mg-Al c e l a d o n i t e ( F i g . 8c) i s p a r t i c u l a r l y n o t a b l e b u t i s a l s o a p p r e c i a b l e +3 w i t h Mg-Fe c e l a d o n i t e s . I t i s suggested t h a t an F e - r i c h p h e n g i t e may c o e x i s t w i t h glaucophane a t h i g h p r e s s u r e s . Decrease i n p r e s s u r e would r e s u l t i n i n s t a b i l i t y o f the F e - p h e n g i t e and r e a c t i o n w i t h glaucophane t o g i v e p h e n g i t e + c r o s s i t e . T h i s s u g g e s t s t h a t the c r o s s i t e c o r e s , zoned t o glaucophane, c r y s t a l l i z e d d u r i n g an e p i s o d e of i n c r e a s i n g p r e s s u r e and t h a t the narrow c r o s s i t e r i m s formed d u r i n g a r e t r o g r e s s i v e phase of metamorphism a s s o c i a t e d w i t h d e c r e a s e i n p r e s s u r e . An a l t e r n a t i v e e x p l a n a t i o n o f the z o n i n g c o u l d be sought i n R a l e i g h f r a c t i o n a t i o n d u r i n g growth a t c o n s t a n t P and T ( H o l l i s t e r , 1966). However, t h i s does n o t e x p l a i n the F e - r i c h r i m s i n t h e glaucophane c r y s t a l s . R e l e v a n t Phase E q u i l i b r i a A r a g o n i t e s t a b i l i t y F o r t h e c a l c i t e - a r a g o n i t e t r a n s i t i o n , t he e x p e r i m e n t a l r e s u l t s o f B o e t t c h e r and W y l l i e (1968) and C r a w f o r d and F y f e (1964) a r e shown i n F i g u r e 8a. R e v e r s a l s were o b t a i n e d by C r a w f o r d and F y f e between 4.2 and 4.5 kb a t 100°C and by B o e t t c h e r and W y l l i e between 7.8 and 8.6 kb a t 400°C. The common o c c u r r e n c e i n n a t u r e o f a r a g o n i t e c o - e x i s t i n g T° C Figs. 8 a to 8 f ! Experimentally determined phene e q u i l i b r i a relevant to the formation of minerals In Pinchi rocke. Where H-0 Is Involved ln the reaction Pfiuid « Ptotal* 86 F I G. 8 (cont . ) 1 - 8 -6 -4 -2 0 Fig. 8g P-X diagram illustrating effect of solid solution ln lowering the pressure of formation of jadeltio pyroxene from albite. The experimental results of Newton and Smith ln the system NaAlSioOg-KaFeSi at 600°C are Indicated by a solid line. Calculated equilibrium curves for different temperatures and values of K = are Indicated by dashed lines (see also Appendix VII). Values of for each of the analysed pyroxenes (Fig. 27b) are also shown. 87 w i t h d e f i n i t i v e h i g h p r e s s u r e m i n e r a l s such as j a d e i t i c pyroxene s u b s t a n t i a t e s the experimental r e s u l t s . However, a number of authors have gi v e n evidence f o r , or i m p l i e d the "metastable" c r y s t a l l i z a t i o n of a r a g o n i t e a t p r e s s u r e s s i g n i f i c a n t l y lower than t h a t g i v e n by accepted t r a n s i t i o n s . Newton et al. , (1969) demonstrated the growth of a r a g o n i t e from s t r a i n e d c a l c i t e s e v e r a l k i l o b a r s below the s t a b i l i t y f i e l d g i v e n by p r e v i o u s a u t h o r s . 2+ + B i s c h o f f and F y f e (1968) showed t h a t a h i g h Mg /H r a t i o i n aqueous s o l u t i o n s i n h i b i t e d the growth of c a l c i t e n u c l e i w i t h i n the c a l c i t e s t a b i l i t y f i e l d . T h i s c o u l d r e s u l t i n metastable p r e c i p i t a t i o n of a r a g o n i t e i f the formation of a r a g o n i t e n u c l e i i was not i n h i b i t e d . Vance (1968) noted the occurrence of a r a g o n i t e i n p r e h n i t e - p u m p e l l y i t e . b e a r i n g metabasic r o c k s and suggested on g e o l o g i c grounds t h a t the p r e s s u r e d u r i n g metamorphism d i d not exceed 3 kb. T h i s i s 1.3 kb l e s s than the accepted t r a n s i t i o n p r e s s u r e a t temperatures of 100°C. A t P i n c h i , a r a g o n i t e occurs i n s e v e r a l d i f f e r e n t a s s o c i a t i o n s . F i r s t l y , i t i s the common CaCO^ polymorph w i t h i n d o l o m i t i c l i m e s t o n e s , lawsonite-glaucophane b e a r i n g metabasic r o c k s and j a d e i t i c metagreywackes. Secondly, a r a g o n i t e c o - e x i s t s w i t h the greenstone assemblage a l b i t e + s o d i c pyroxene + sphene + c h l o r i t e ± p u m p e l l y i t e . The presence of s o d i c pyroxene and the absence of p r e h n i t e d i f f e r e n t i a t e s the P i n c h i assemblages from those d e s c r i b e d 88 by Vance (1968) i n the San Juan Islands. Thirdly, an occurrence of aragonite was noted within Upper T r i a s s i c (?) limestones south of Pinchi Lake (see F i g . 28 for l o c a t i o n ) . This was the only occurrence discovered outside of units 1 and 2 (Fig. 3) . Greenstones at the west end of Murray Ridge contain c a l c i t e coexisting with prehnite and pumpellyite. There are no indications that the c a l c i t e has formed by inversion from aragonite and i t i s inferred that c a l c i t e was the stable polymorph during metamorphism. I t i s perhaps s i g -n i f i c a n t that t h i s i s the only region where prehnite occurs i n greenstones. Despite the p o s s i b i l i t y of metastable c r y s t a l l i z a t i o n , i t i s concluded from t h i s evidence at Pinchi Lake that the aragonite-calcite t r a n s i t i o n i s a v a l i d geobarometer. This means that the dolomitic limestones, metagreywackes, lawsonite-glaucophane metavolcanics and most of the greenstones c r y s t a l l i z e d at pressures above the t r a n s i t i o n . The prehnite-c a l c i t e bearing greenstones formed at lower pressures. The presence of aragonite i n the Upper T r i a s s i c (?) limestones i s problematical. There i s no evidence for deep b u r i a l and the preservation of d e t r i t a l aragonite seems u n l i k e l y . 89 Jadeite and acmite-jadeite s t a b i l i t y The equilibrium curve for the reaction a l b i t e = jadeite + quartz according to Newton and Smith (1967) i s given i n F i g . 8b. This curve was used rather than that of Birch and LeComte (1960) as i t takes into account the entropy change of disordering of a l b i t e . Also shown on F i g . 8b are values of the equilibrium constant for the reaction plagioclase = j a d e i t i c pyroxene + quartz. In t h i s case, K = X ? ^ / x j ^ ^ assuming unit a c t i v i t i e s for pure phases and mole f r a c t i o n (X) = a c t i v i t y . If the plagioclase i s pure a l b i t e , the values of K correspond to mole f r a c t i o n of jadeite i n clinopyroxene. For d e t a i l s of c a l c u l a t i o n see Appendix VII. Newton and Smith also investigated the e f f e c t of Fe i n the system using synthetic glasses with or without mineral seeds. They concluded that the presence of 10 mole per cent acmite has at the most a few hundred bars e f f e c t on the s t a b i l i t y of jadeite with quartz (Fig. 8g). Using 40 mole per cent acmite, the pressure of the a l b i t e breakdown curve i s lowered by 1.6 kb at 600°C. The calculated pressures for the t r a n s i t i o n for d i f f e r e n t values of X t j ^ are compatible with Newton and Smith's experimental r e s u l t s at 6 00°C. Microprobe analyses of pyroxenes from the Pinchi area are given i n Table 10 and i l l u s t r a t e d i n F i g . 27. Two groups of pyroxenes were noted; acmite-jadeites from 90 metavolcanics and j a d e i t i c pyroxenes from metagreywackes. Both are considered to have formed under similar P-T conditions and the compositional difference i s assigned to the contrast i n bulk composition. The minimum pressure of c r y s t a l l i z a t i o n of the j a d e i t i c pyroxene (coexisting with quartz) can be obtained from the calculated s t a b i l i t y curves for d i f f e r e n t values of the equilibrium constant (Fig. 8g) . For the pyroxene J ( ^ 7 8 A c i i D ^ i i ( N o- 7^) , K = 0.78 and the minimum pressure of c r y s t a l l i z a t i o n i s 9.5 kb at 300°C and 7.6 kb at 200°C. These values l i e approximately 8 00 and 60 0 bars below the s t a b i l i t y of pure jadeite with quartz. The minimum pressures for formation of the acmite-jadeite pyroxenes are below those for j a d e i t i c pyroxenes and can be obtained from F i g . 8g-Phengite s t a b i l i t y Velde (1965) c a r r i e d out a study of the s t a b i l i t y of phengitic micas. His r e s u l t s show that the extent of s o l i d solution i s dependent on pressure, temperature and composition of the celadonite end-member.. Increase i n s o l i d solution i s favoured by high pressures, low temperatures and involve-ment of the Mg-Al celadonite end-member. Other end-members show a less pronounced increase i n s o l i d solution with high pressures. Figure 8c indicates the equilibrium boundary for the reaction M u s c 7 n C e l ^ n going to muscovite, b i o t i t e , 91 K-feldspar, quartz and f l u i d as given by Velde. The e q u i l i b -rium r e f e r s to the KAlMgSi^O^Q(OH)^ celadonite end-member. The approximate p o s i t i o n of the Musc^gCel^g boundary shown on the figure was obtained from graphical extrapolation of Velde's r e s u l t s . S t a b i l i t y of sodic amphiboles Experimental work on the s t a b i l i t y of glaucophane and riebeckite (Ernst, 1961, 1962) demonstrates that these sodic amphiboles have broad s t a b i l i t y f i e l d s and that they do not impose r e s t r i c t i o n s on the P-T conditions of c r y s t a l l i z a t i o n of blueschist f a c i e s minerals. More recently, Hoffman (197 2) has determined the s t a b i l i t y of natural and synthetic ferroglaucophane (Fig. 8d). The breakdown curve has a steep slope and i s l i t t l e affected by change of oxygen buffer. Thus ferroglaucophane, when i d e n t i f i e d , could r e s t r i c t metamorphic temperatures to less than 360°C. The T-log fQ s t a b i l i t y f i e l d s of sodic amphiboles w i l l be discussed i n the section on f l u i d phase chemistry. Lawsonite s t a b i l i t y The breakdown curve for lawsonite was investigated by Newton and Kennedy (1963) and i s shown i n F i g . 8e. This gives a maximum temperature for the s t a b i l i t y of lawsonite at a given pressure. In nature, coupled reactions with 92 minerals such as c h l o r i t e w i l l doubtless s h i f t the curve to lower temperatures. Phase r e l a t i o n s for the reaction: laumontite = lawsonite + quartz + H 20 have been determined separately by Liou (.1971) , Thompson (1970) and Nitsch (1968). The univariant curves for the reaction are f a i r l y compatable and suggest that lawsonite i s unstable at pressures less than 2.5 kb at 200°C and 3 kb at 300°C i n the presence of quartz and water. The equilibrium curve for the reaction: heulandite = lawsonite + quartz + f l u i d according to Nitsch (1968) i s also given on F i g . 8e. Pumpellyite and prehnite s t a b i l i t y Hinrichsen and Schurmann (1972) defined the curve for the breakdown of Fe-free pumpellyite shown i n Figure 8f. During t h e i r experiments, they noticed that, with the i n c l u s i o n of iron i n the system, reaction rates were increased and equilibrium curves shifted towards lower temperatures. The pleochroism scheme and extinction angle of Pinchi pumpellyites shows them to be iron bearing,and therefore at 5 kb temperatures i n the greenstones must have been less than 315°C. Reaction 20 (Fig. 8f) defines the breakdown of prehnite i n the presence of chlinochlore and quartz to give c l i n o z o i s i t e , serpentine and water. This experimental determination was not well defined i n that there was some doubt about the compositions of the 93 c h l o r i t e s involved, but i t does y i e l d a maximum temperature of 285°C at 5 kb for the metamorphism of the prehnite bearing greenstones at the west end of Murray Ridge. Eclogite s t a b i l i t y Extrapolations of the garnet gr a n u l i t e - e c l o g i t e t r a n s i t i o n to temperatures below 800°C have been made by Green and Ringwood (1967) and Ito and Kennedy (1971) and are shown on F i g . 9b. Neither of these extrapolations have a sound t h e o r e t i c a l basis and the large difference i n pressure between the two may be e n t i r e l y caused by d i f f e r i n g bulk compositions. Green and Ringwood studied mineral assemblages i n basalts at pressures from 1 to 30 kb and at temperatures above 1000°C. The lin e a r extrapolation of th e i r experimental data on the disappearance of plagio-clase inte r s e c t s the 5 00°C isotherm at pressures of 5 to 8 kb and the temperature axis at around 200°C at atmospheric pressure. This implies the s t a b i l i t y of eclogite i n a wide range of geological conditions. The r e c r y s t a l l i z a t i o n of a t h o l e i i t i c basalt to garnet granulite and to eclogite was experimentally investigated by Ito and Kennedy (1971) at temperatures between 8 00° and 12 00°C. The extrapolation of t h e i r eclogite/plagioclase e c l o g i t e t r a n s i t i o n curve to lower temperatures l i e s 400 bars below the a l b i t e / jadeite + quartz boundary as determined by Newton and Smith 94 (1966) . At 500°C, t h i s y i e l d s a pressure of 14 kb for the disappearance of plagioclase, 6 to 9 kb higher than the equivalent reaction i n Green and Ringwood's experiments. There i s some evidence that the extrapolation of Green and Ringwood l i e s at too low a pressure. An analysed TOX omphacitic pyroxene from a Pinchi eclogite has Xtj^ = 0.26. On F i g . 8b, t h i s gives a minimum pressure of formation of 9.1 kb at 5 00°C which i s 1.1 kb above the extrapolation of Green and Ringwood. Because of t h i s f a c t and compatability with the commonly accepted high pressure o r i g i n of eclogites the data of Ito and Kennedy are preferred. Oxygen isotopes Taylor and Coleman (1968) obtained temperatures of formation of glaucophanitic rocks from Cazadero using 18 16 0 /0 f r a c t i o n a t i o n between coexisting minerals. They conclude that in s i t u Type II and III blueschists (Coleman and Lee, 1963) form at temperatures of 200° to 325°C, where-as higher grade blueschists from New Caledonia and Type IV tectonic blocks from C a l i f o r n i a form at 400° to 550°C. Mineral assemblages at Pinchi are similar to those of Cazadero Type II and III blueschists and therefore possibly formed at similar temperatures. The glaucophane bearing e c l o g i t e boulders at P i n c h i contain a similar mineralogy to Type IV blueschists and possibly formed between 400° and 500°C. 95 , , , , 1 1 1 ( 1 ! 1 1 1 1 1 1 1 1 FIG. 9a P-T CONDITIONS OF METAMORPHISM OF GREENSTONES . TEMPERATURE, °C r- T 1 1 « • 1 i 1 1 FIG. 9b P-T CONDITIONS OF METAMORPHISM OF BLUESCHIST AND T°C 96 Pressure-temperature Gonditions of Metamorphism Constraints on the P-T conditions of metamorphism are given by phase equilibrium studies and oxygen isotope geothermometry. These are consistent with a minimum geothermal gradient of 6°C/km. Inferred metamorphic conditions for the various fault-bounded blocks are i l l u s t r a t e d i n Figs. 9a and 9b. For the Pinchi Greenstones (Fig. 9a), the high temperature l i m i t i s somewhat a r b i t r a r y and i s based on the s t a b i l i t y of Fe-free pumpellyite given by Hinrichsen and Schurmann (1972). Fe-pumpellyite would l i m i t the f i e l d even more but the s h i f t i n the equilibrium boundary towards lower temperatures i s not known. Estimated P-T conditions are as follows: Murray Ridge greenstones 3-6 kb Pinchi Mountain Greenstones 4.5-9 kb Blueschists 8-12 kb.. Eclogite > 12-15 kb (Ito & Kennedy, 1971) > 9 kb (from X?* at 5 00°C) > 3-6 kb (Green & Ringwood, 1967) F l u i d Phase Composition The presence of lawsonite, quartz and sphene i n an assemblage severely l i m i t s the C0 2 content of the coexisting f l u i d . Experimental studies by Nitsch (1972) demonstrate 100-225°C 100-250°C 225-325°C 400-550°C 97 F i g . 10a P-T diagram i l l u s t r a t i n g pressure of C0 2 (.i.e. P E ) i n equilibrium with the reaction: c a l c i t e + quartz CO 2 + r u t i l e + sphene + C0 2 (after Ernst, 1971). Estimated P-T conditions for formation of Pinchi blueschists are also shown. F i g . 10b T-X^Q diagram i l l u s t r a t i n g phase e q u i l i b r i a at 4 kb and 7 kb i n the system Ca0-Al 20 3-Si0 2-H 20-C0 2 (after Nitsch, 1972) . py = pyr o p h y l l i t e ; zo = z o i s i t e ; lws = lawsonite; cc = c a l c i -te; qtz = quartz. F i g . 10c diagram i l l u s t r a t i n g v a r i a t i o n i n p a r t i a l pressure of gaseous species at 3 27°C and P g a s = 10 kb with change i n f^ . The estimated oxygen fugacity within lawsonite-quartz-magnetite-carbonaceous cherts i s indicated by the shaded area. Selected values of nco2^ nC0 2 + nH20^ a r e a-"-so shown. Assuming i d e a l behaviour of gaseous phases, lawsonite + quartz i s stable at values of n_-- / (n^^ +n„ ,_) less than *-02 "-2, .05 (Fig. 10b) i n the s t a b i l i t y f i e l d of f a y a l i t e . Dis-placement of s t a b i l i t y f i e l d of lawsonite + quartz to values of £Q above the quartz-fayalite-magnetite buffer i s attributed to non-ideal behaviour of gaseous species. F i g . lOd T-log fg diagram i l l u s t r a t i n g experimental and calcu-lated phase e q u i l i b r i a involving riebeckite and ferroglau-cophane (after Ernst, 1962; Hoffman, 1972). hem = hematite; qtz = quartz; mt = magnetite; acm = acmite; fay = f a y a l i t e ; f e g l = ferroglaucophane; alb = a l b i t e ; arfved = arfvedsonite s o l i d solution. (b) System CoO-AlgOj-SiC^-Hp-COg 4 0 0 r Lws + Qtz T C 3 0 0 r 200\ \ experimentally determined y' schematic metastable extension after Nitsch (1972) •01 •02 X •03 •04 C0„ 99 100 that lawsonite and quartz can coexist with a f l u i d only i f n ^ <-/(n„ <+ n„ -») does not exceed 0.03 ± 0.02 at 4 kb cc^*' - ' " 2 (Fig. 10b). Calculations by Thompson (1971) support t h i s conclusion. Schuiling and Vink (1967) determined the equilibrium for the reaction: c a l c i t e + quartz + r u t i l e = sphene + CC^. The univariant curve demonstrated that sphene formation can only take place at extremely low P C Q (e.g. less than 35 bars at 300°C with P t o t a l = p c o ) 2 2 otherwise, the assemblage c a l c i t e + quartz + r u t i l e would occur. Using t h i s curve and ad d i t i o n a l thermochemical data, Ernst (1972) made a semiquantitative estimate of the maximum p a r t i a l pressure of C0 2 i n equilibrium with sphene in .conditions where P C Q < PfT_ u ;j_£ (Fig. 10a) . Values for X C 0 were calculated independently and suggested that high grade Franciscan rocks equilibrated with a f l u i d phase i n which X C Q was less than 0.01. This compares favourably 2 with Nitsch's experimental r e s u l t s for the s t a b i l i t y of lawsonite + quartz. From these data, i t i s concluded that for the Pinchi rocks at 7 kb, the assemblage lawsonite + quartz formed at n '/(n__ + n„ „) < 0.02 ± 0.02. Sphene L.U2 ^~^2 2 — bearing assemblages c r y s t a l l i z e d with n C Q ^ / ( n C Q ^ + nH 0^  — 0.01 or with P C 0 2 a t a m a x i m u m value of 1500 bars. Phengitic mica coexists with lawsonite i n the Pinchi metacherts. Velde (1965) and Ernst (1963) suggest that the presence of phengite i s i n d i c a t i v e of metamorphism under s i g n i f i c a n t water pressures although Pj^ o n e e c ^ n o * - e < 3 u a l P l o a d * 101 Carbonaceous material i s abundant i n s c h i s t s , meta-cherts and massive limestones. X-ray d i f f r a c t i o n (Appendix II) indicated the presence of nearly amorphous g r a p h i t i c material i n one sample (graphite-d^, according to Landis' c l a s s i f i c a t i o n , 1971) and amorphous material i n 4 others. Calculations by French (1966) i n the system C-H-0 are s t r i c t l y applicable only to c r y s t a l l i n e graphite and not to amorphous carbonaceous material. They also depend on the assumption of i d e a l behavior of the gaseous species involved. It i s considered, however, that p a r t i a l e q u i l i b r a t i o n has taken place between the carbonaceous material and the f l u i d phase and that the c a l c u l a t i o n s have some a p p l i c a b i l i t y . F i g . 10c i s adapted from French (1966, F i g . 1) and gives the f l u i d phase composition i n the system C-H-0 at 327°C and 10 kb, conditions approximating those of blueschist f a c i e s metamorphism. In rocks containing carbonaceous material, lawsonite and quartz, the experimental work of Nitsch requires that nQQ^/ ^ n H 2 0 + n C 0 2 ^ c o u l c ^ n o t have exceeded 0.04 at 7 kb and possibly even less at 10 kb. From t h i s and the r a t i o s of the p a r t i a l pressures of CO2 and i l l u s t r a t e d i n F i g . 10c i t can be i n f e r r e d that -33 the fn must have been less than 10 bars. However, the u2 presence of magnetite + quartz i n graphitic metacherts -33 implies an fQ greater than 10 bars. I t i s suggested that the excessively low values of fn i n f e r r e d from the u2 102 s t a b i l i t y of lawsonite and graphite i s brought about by the assumption of i d e a l i t y and that a reasonable estimate -33 -32 for the f_ i s between 10 and 10 bars. If t h i s i s °2 the case, methane must have been a dominant constituent i n the f l u i d phase i n carbonaceous rocks. Ernst (1962) constructed a log -T diagram showing the s t a b i l i t y f i e l d s of riebeckite and acmite at 2 kb (Fig. lOd). The curve which defines the reaction acmite + hematite + quartz + f l u i d = r i e b e c k i t e + oxygen approximately p a r a l l e l s the abscissa at temperatures of 500°C. The po s i t i o n of t h i s reaction curve at 10 kb can be approximately determined by r e c a l c u l a t i n g the position of the buffer curves and by graphical extrapolation of the experimental data on acmite/riebeckite s t a b i l i t y to high pressures. At 500°C, the increase i n pressure from 2 to 10 kb r a i s e s the -17 -15 equilibrium fQ from 10 to 10 bars. The extrapolation 2 of the reaction curve to lower temperatures, applicable to blueschist f a c i e s metamorphism, i s c r i t i c a l but neither experimental nor thermochemical data are a v a i l a b l e . I t i s suggested that the curve takes on a p o s i t i v e slope with decrease i n temperature. This i s compatible with the steepening of the s o l i d phase oxygen buffer curves with decrease i n temperature. At 300°C and 3 kb, ferroglaucophane i s stable at an fQ of le s s than 10~ 3 2 bars (Hoffman, 1972). An extra-103 polation similar to that c a r r i e d out i n the case of r i e -beckite enables an approximation of the s t a b i l i t y f i e l d at 10 kb (Fig. lOd). Presumably, sodic amphiboles of i n t e r -mediate composition have breakdown curves on a log -T 2 diagram between those of ferroglaucophane and r i e b e c k i t e . Oxidation r a t i o s (Chinner, 1960), obtained from bulk analyses (Table 16a) should give a crude measure of the oxygen pressure at the p r e v a i l i n g pressure and temperature (Miyashiro, 1964). Within the unfoliated metavolcanics, oxidation r a t i o s are commonly i n the range 40 to 55 . Rocks with a high modal per cent of acmite-jadeite y i e l d the highest r a t i o s and presumably e q u i l i b r a t e d with an oxi d i s i n g f l u i d phase. F o l i a t e d glaucophanitic meta-volcanics with similar bulk composition have lower oxidation r a t i o s (e.g. no. 55 , Table 16a) and probably formed at lower oxygen pressures. Opaque oxides can give clues as to f l u i d phase composition. Within carbonaceous cherts, magnetite coexists with glaucophane, This l i m i t s the /_ to values less than u 2 -27 10 bars, at 10 kb and 325°C. If the coexisting carbon-aceous material behaved as graphite, the would be -31 reduced to less than 10 bars at 10 kb and 325°C. Hematite i s also found i n cherts but i s believed to have formed during retrograde metamorphism as i t commonly rims magnetite. Opaque oxides are rare i n metabasic rocks and where observed are composite, consisting of p y r i t e 104 rimmed by magnetite and hematite i n turn (Fig. 30). I t i s uncertain whether p y r i t e , magnetite or hematite was stable during blueschist metamorphism. Two phase f l u i d inclusions are present i n metachert samples and provide evidence for. the presence of a f l u i d phase during metamorphism. In general, they can be divided into two groups; 20y inclusions containing 3y gas bubbles (Plate 6) and aggregates of small ly inclusions along p a r t l y healed fractures. Roedder (1968) considers that large inclusions showing no rela t i o n s h i p to r e l i c t fracture zones are more l i k e l y to be primary. Therefore i t i s possible that the f i r s t type represent" primary syn-metamorphic inclusions and the second type represent secondary in c l u s i o n s . F l u i d Phase Composition at 10 kb and 327°C The f l u i d composition at 327°C and 10 kb, within contrasting rock types, i s i l l u s t r a t e d i n F i g . 11. Water, methane and carbon dioxide are considered to be the major components with P„ _ + P_,„ + p__. = P.... . , = p, ,. The * CH4 C 0 2 f l u i d load calculations of French (1966) suggest that CO and H 2 are of minor importance at the temperature and pressure of blueschist formation. Within acmite-jadeite metavolcanics, the presence of lawsonite, quartz and sphene requires that the n C Q ^ / (n„ A + ) be less than 0.04 at 10 kb. The s t a b i l i t y ti2>-' of acmite-jadeite suggests that the coexisting f l u i d was 105 F i g . 11 d iagram i l l u s t r a t i n g f l u i d phase c o m p o s i t i o n i n P i n c h i b l u e s c h i s t s a t 10 kb and 3 2 7 ° C . (a)„ The l i n e AC Con the g r a p h i t e buffer) , r e p r e s e n t s p o s s i b l e f l u i d c o m p o s i t i o n s i n e q u i l i b r i u m w i t h carbonaceous m a t e r i a l , -32 9 l a w s o n i t e , q u a r t z and m a g n e t i t e between f = 1 0 * CA) 2 -33 and 10 bars CC) as o b t a i n e d from F i g . 10c. Cb) D i a g r a m -mat i c phase e q u i l i b r i u m b o u n d a r i e s i n the system C a - A l - S i -C - O - H . The i n f e r r e d s t a b i l i t y o f l a w s o n i t e + q u a r t z i s i n d i c a t e d by s h a d i n g . F i g . 12 T - l o g f. d iagram i l l u s t r a t i n g f l u i d c o m p o s i t i o n s 2 i n P i n c h i r o c k s . E x p l a n a t i o n i s g i v e n i n t e x t . 106 FIG. II FLUID PHASE COMPOSITION AT 10 KB a 3 2 7 ° C HX> f 0 > I O l o b a r s ( ? ) massive metabasalt acm-jd + lws«sph+chl g laucophane veins trans it ion fo l iated metabasalt glph+lws+sph*chl carbonaceous cherts, schists - 4 0 Fig. 12 Temp, range of blueschist metamorphism < •> metamorphic conditions in metacher t s 8 carbonaceous schists. 2 0 0 4 0 0 T C 6 0 0 107 r e l a t i v e l y o x i d i z i n g , possibly with greater than °2 -25 10 bars. Methane i s incompatible with j\_ greater than °2 -30 10 bars and therefore i t appears that H^ O was the main component i n the f l u i d phase. In the ternary system B^O - CC>2 - CH^ the f l u i d composition i n equilibrium with graphite at varying /_ u2 l i e s on the graphite buffer curve AD. I t was infe r r e d e a r l i e r , that rocks containing coexisting lawsonite, quartz, carbonaceous material and magnetite may have c r y s t a l l i z e d —33 -32 i n the range 10 to 10 bars ( i . e . just above the u2 quartz-fayalite-magnetite buffer curve). I f t h i s i s the case, f l u i d compositions l i e on the curve AC. For J*0 = 10~ 3 2 bars ( i . e . point C), P C Q = 500 bars, P R Q = 2 2 2 3200 bars and P-,„ = 6300 bars. Decrease i n value of n_,_ / (n T T n + n_,_ ) (and f_ ) r e s u l t s i n increase of methane CO2 CO2 O2 water r a t i o (Fig. 10c) and the f l u i d composition migrates towards A. On the extension of t h i s curve (CD),lawsonite i s considered to be unstable i n the presence of graphite and quartz. Diagrammatic phase equilibrium boundaries i n the system Ca-Al-Si-C-O-H and the inferred s t a b i l i t y l i m i t of the assemblage lawsonite + quartz are also shown i n F i g . 11. The carbonaceous metasediments are far more abundant than the metavolcanics. and i t i s to be expected that with progressive metamorphism and deformation a methane-water mixture with a r b i t r a r y composition B permeated active 108 shear zones i n the matavolcanics. The oxidized acmite-jadeite assemblage reacted with the f l u i d to give a reduced glaucophanitic assemblage which coexists with methane-water. A reaction involving the acmite end-member (Table 5, No. 16a) consumes methane, lib e r a t e s water and p r e c i p i t a t e s graphite. As graphite was not i d e n t i f i e d i n glaucophane veins or f o l i a t e d metavolcanics, i t appears that methane and water f u g a c i t i e s were externally c o n t r o l l e d and the f l u i d phase composition i n the metasediments migrated from B towards C. Evolution of F l u i d Phase The changing composition of the f l u i d phase with progressive metamorphism can also be i l l u s t r a t e d with r e f e r -ence to a T - / Q diagram (Fig. 12) . Curves 1, 2 and 3 are respectively the hematite-magnetite, the q u a r t z - f a y a l i t e -magnetite and the magnetite-iron oxygen buffer curves calculated at 10 kb. The i n equilibrium with pure water at 1 bar and 10 kb (curves 4 and 5) were calculated a f t e r Miyashiro (1964). Curve 6 represents the gas-graphite buffer i n the system C-0 calculated at 10 kb employing an equation given by French and Eugster (1965). This curve i s very close to the graphite buffer as calculated i n the system C-H-0 employing an H/O r a t i o of 2/1 (French, 1966). Curve 7 represents the hypothetical extrapolation 109 at 10 kb of the reaction acmite + quartz + hematite + f l u i d = riebeckite + oxygen described i n the previous section. During diagenesis of the carbonaceous sediments, the i n t e r s t i t i a l l i q u i d was probably a saline brine. I t i s supposed that the i n the l i q u i d was i n i t i a l l y s i m i l a r to that of pure water, i t can be inferred from -30 curves 4 and 5 that /_ was approximately 10 bars ( i . e . °2 point B). With diagenesis and metamorphism, oxygen reacted with carbonaceous material to give CC^ and the changed along the a r b i t r a r y trajectory BX. Presumably, at temperatures i n the 200° to 300°C range (shaded area) the carbonaceous material behaved as graphite with the r e s u l t that the fugacities of the various components can be sp e c i f i e d uniquely i n terms of / ' at f i x e d temperature and U2 t o t a l pressure (French, 1966) . Basalts within the sedimentary rocks may have under-gone hydration i n the z e o l i t e or prehnite-pumpellyite facies during progressive b u r i a l and/or deformation. Pre-sumably, the water originated i n the adjacent metasediments p r i o r to lowering of / and e q u i l i b r a t i o n with graphite. °2 After hydration, i t appears that massive basalt units behaved e s s e n t i a l l y as closed systems and the fn changed u2 along the curve BY. The p o s i t i o n of th i s curve i s d i r e c t l y dependent on the extrapolation of the acmite/riebeckite reaction (Fig. 12, No. 7). Oxidation of iron to form acmite must have lowered the fn i n the hydrous pore f l u i d to U2 110 values below that of pure water (curves 4 and 5) but the presence of acmite-jadeite bearing rocks which do not contain glaucophane or c r o s s i t e suggests that the f_ i n the f l u i d °2 remained within the acmite s t a b i l i t y f i e l d and did not equ i l i b r a t e with graphite as temperature increased. At Y, acmitic rocks were i n disequilibrium with the reservoir of methane r i c h f l u i d and glaucophane forming reactions probably took place along shears and fractures. Pressure-Temperature Trajectories of Pinchi Rocks Three assumptions have been made in drawing the pressure-temperature t r a j e c t o r i e s shown i n F i g . 13. They are: (a) an average c r u s t a l density of 3.1 grams/cc, (b) a minimum geothermal gradient of 6°C/km and (c) a maximum geothermal gradient of 12°C/km. The f i r s t assumption i s based on a tectonic model (Chapter V) involving a major proportion of basic or ultramafic rocks. The second i s hypothetical, but geothermal gradients as low as 8.56°C/km have been measured (Clark, 1957). . The l a s t assumption i s based on a study of aragonite/calcite k i n e t i c s by Brown et at., (1962), who inferred from t h i s that for survival of metamorphic aragonite the geothermal gradient could not have greatly exceeded 10°C/km. This estimate was dependent on the calcite/aragonite t r a n s i t i o n as determined by Clark (1957) and Jamieson (1953). Later work by I l l , , - , - T - 1 - i - r 1 r— P-T Irojectories for eclogite, blueschist and greenstone. Fig. 13 400 600 800 T 0 C 112 Boettcher and Wyllie (1967) lowered the t r a n s i t i o n pressure, and geothermal gradients of 12°C/km can be accommodated. I t i s therefore concluded that the aragonite bearing rocks at Pinchi crossed the aragonite/calcite t r a n s i t i o n at tempera-tures less than 300°C on their return to the surface. U p l i f t was accompanied by retrograde metamorphism. Eclogites were veined and p a r t l y replaced by glaucophane, lawsonite and stilpnomelane i n the i r passage through the blueschist f a c i e s . Late a l b i t e veins i n the blueschists must have formed below the a l b i t e / j a d e i t e + quartz t r a n s i t i o n at pressure-temperature conditions similar to the metamorphism of the greenstones. II METAMORPHISM IN THE REMAINING FAULT BLOCKS Metabasic Rocks South of Pinchi Lake (Map,I, Unit 4) The following metamorphic mineral assemblages were noted during examination of six thin sections. Minerals are thought to be i n equilibrium with one another except for white mica which only occurs within plagioclase. (al metabasalts: a l b i t e + sphene + celadonite i white mica ( c a l c i t e + c h l o r i t e veins) (bX metagabbro: a l b i t e + a c t i n o l i t e + sphene ± white mica ± serpentine ± epidote (?) 113 R e l i c t minerals, namely orthopyroxene, clinopyroxene, ilmenomagnetite and c a l c i c plagioclase are abundant i n the gabbros and may constitute up to 9 0% of the rock. Metadiabase and metabasalt tend to be more highly r e c o n s t i t u -ted with few r e l i c t s . S i g n i f i c a n t l y , a c t i n o l i t e i s absent from the metavolcanic unit which also i s apparently the highest i n the sequence isee p. 24 ) . The presence of a pale green amphibole and possible epidote suggests that the lower gabbro unit was metamorphosed in the greenschist f a c i e s . No meaningful pressure estimates can be made, but the temperature may have been i n the 200° to 400°C range based on the facies diagrams of Turner (1968, p. 366} and Liou (1971) . The a l b i t e / c e l a d o n i t e - c a l c i t e association i n the overlying basalts suggests that they were metamorphosed i n conditions equivalent to the prehnite-pumpellyite f a c i e s . I t was suggested (p. 27 ) that the basic rocks may represent part of an o p h i o l i t e sequence. I t i s therefore s i g n i f i c a n t that the metamorphic mineral assemblages i n these rocks are similar to those recently investigated from oceanic environments (Aumento et al. 3 1970). Such rocks, characterized by a greenschist fa c i e s mineralogy, and general absence of a tectonite f a b r i c are t y p i c a l of ocean f l o o r metamorphism (Miyashiro, 1972). ' 114 Mount Pope Belt (Map I, Units 7, 8 and 9) Sensitive indicators of metamorphic grade are rare within the Mount Pope b e l t , as limestone and chert are the dominant l i t h o l o g i e s . A s c h i s t interlayered with cherts on the northern margin of the limestone contains the assemblage: a l b i t e + c h l o r i t e + c a l c i t e + quartz + white mica + unidentified opaques and amorphous material. Out-side of the map area on the northwest arm of Stuart Lake, the author noted a regionally metamorphosed basic assemblage containing a c t i n o l i t e + c h l o r i t e + c l i n o z o i s i t e + a l b i t e + sphene. Similar assemblages within the Cache Creek Group have been reported by Armstrong (1949). I t i s concluded that the Mount Pope b e l t was metamorphosed under lower greenschist facies conditions. Takla Group (Map. I, Units 10, 11 and 12) The greywackes and s i l t s t o n e s of the Takla Group are only i n c i p i e n t l y metamorphosed. D e t r i t a l plagioclase, the main constituent of the sediments, commonly displays a turbid a l t e r a t i o n and may be l o c a l l y a l b i t i s e d . R e l i c t mafic c l a s t s (e.g. hornblende) are generally altered to c h l o r i t e + sphene + c a l c i t e . Matrix minerals include microgranular c h l o r i t e , s e r i c i t e and the occasional grain: of celadonite. At one l o c a l i t y , a dark grey, hard s i l t s t o n e contains analcite dodecahedra coexisting with c h l o r i t e , quartz and p a r t l y a l b i t i s e d plagioclase. However, t h i s l o c a l i t y i s near the 115 Pinchi Fault and the analcite may have been formed as a r e s u l t of hydrothermal a c t i v i t y . Only one small outcrop of basalt was encountered within the area and contained the following domain assem-blages : matrix: a l b i t e + c h l o r i t e + sphene + c a l c i t e amygdules: Prehnite + quartz veins: Epidote + c a l c i t e A puzzling feature i s the occurrence of minor aragonite (.Table 12, No. 23 0) i n a b i o c l a s t i c limestone thought to be of Upper T r i a s s i c age south of Pinchi Lake. Aragonite was not found i n adjacent limestone beds or i n Upper T r i a s s i c limestones to the northeast of the f a u l t . Aragonite at th i s l o c a l i t y replaces rectangular c l a s t s , which were possibly o r i g i n a l l y c r i n o i d columnals. The presence of prehnite and epidote i n b a s a l t i c rocks suggests that the Takla Group was metamorphosed i n t r a n s i t i o n a l conditions between the prehnite-pumpellyite and greenschist f a c i e s . Lord (,1949) estimated a thickness of 23,000 f t C7 km) for Takla Group sediments and volcanics i n the McConnell Creek area so that l i t h o s t a t i c pressures i n the Pinchi area may well have approached 3 kb. The occurrence of aragonite i n Takla Group (?) rocks southwest of the f a u l t may indicate that higher pressures were attained l o c a l l y ( i . e . 5.5 kb at 200°C). 116 Ultramafites (Map I, Units 14a and 14b) Harzburgites and dunites at Pinchi have been thoroughly serpentinized. D.B. Wenner (.Written communication! examined three harzburgites from the Pinchi area and i d e n t i f i e d the serpentine mineralogy by X-ray d i f f r a c t i o n . The r e s u l t s were: Spec. 85 (from Pinchi Mt.) : l i z a r d i t e + c h r y s o t i l e Spec. 88 (from Pinchi Mt.) : ch r y s o t i l e Spec. 236 (from Murray Ridge) : l i z a r d i t e + c h r y s o t i l e Brucite was i d e n t i f i e d by X-ray d i f f r a c t i o n i n three dunite samples (Nos. 87, 88, 240) and one harzburgite (No. 239). S t a b i l i t y of serpentine-bearing mineral assemblages i s i l l u s t r a t e d on F i g . 14. At 10 kb and 585°C serpentine breaks down to f o r s t e r i t e , t a l c and water (Kitahara et al.3 1966). Brucite coexisting with serpentine lowers the s t a b i l i t y l i m i t . t o 460°C at 10 kb (Johannes, 196.8). The occurrence of serpentine and brucite at Pinchi therefore indicates that serpentinization took place below 460°C at 10 kb and 390°C at 3 kb. Oxygen isotope work (Wenner and Taylor, 1969) ca r r i e d out on serpentinites from B r i t i s h Columbia and western North America indicates that serpentine minerals equilibrated with meteoric water at low temperature. Fractionation of oxygen isotopes between coexisting serpentine and magnetite suggest that continental l i z a r d i t e - c h r y s o t i l e serpentinites 117 equilibrated at 85° to 115°C and oceanic l i z a r d i t e - c h r y s o t i l e serpentinites at 130° to 185°C (.Wenner and Taylor, 1971). An t i g o r i t e e q uilibrates at temperatures above 200°C. From th i s i t may be inferred that a major episode of serpentiniz-ation occurred i n the Pinchi ultramafites at temperatures less than 185°C to give the c h r y s o t i l e - l i z a r d i t e association. For the s u r v i v a l of metamorphic aragonite, the f a u l t blocks containing greenstones' and blueschists must have been rap i d l y u p l i f t e d (p. 110 ) along the P-T trajectory i n F i g . 13. The p o s s i b i l i t y exists that ultramafic bodies, which are clo s e l y associated with these rocks, underwent a similar rapid u p l i f t and a possible P-T trajectory i n the system MgO-Si0 2 -H 20 i s i l l u s t r a t e d i n F i g . 14. From the oxygen isotope geothermometry i t appears that a n t i g o r i t e i s the stable serpentine mineral at blueschist facies temperatures. The absence of a n t i g o r i t e suggests that either a n t i g o r i t e never formed or that i t was completely replaced by the lower temperature l i z a r d i t e - c h r y s o t i l e mineralogy.during u p l i f t and cooling. In the writers opinion much of t h i s serpentiniz-ation occurred during a period of s t r i k e - s l i p f a u l t i n g associated with u p l i f t and the F^ deformation (Chapters V and VI). V STRUCTURAL GEOLOGY Introduction The emphasis i n t h i s chapter i s placed on the i n t e r n a l structure of the f a u l t block containing the lawsonite-glaucophane bearing rocks (Fig. 3, Unit 2). I t was hoped that a s t r u c t u r a l study of these rocks, which l i e adjacent to the Pinchi Fault, would help to elucidate the history of movements on the f a u l t . To t h i s purpose, s t r u c t u r a l mapping was c a r r i e d out i n the v i c i n i t y of the mine (Map IV) and along the northwest shore of Pinchi Lake (Map V). Structural data c o l l e c t e d i n the v i c i n i t y of the mine were plotted on stereographic projections (Fig. 15). I t should be emphasized that s t r u c t u r a l interpretation i s seriously constrained by (a) the poor exposure, (b) the undoubtedly complex deformational history and (c) the lack of a s t r a t i g r a p h i c sequence. Structural geology of the remaining f a u l t blocks has been adequately dealt with i n Chapters II and III and w i l l only be summarized i n t h i s section. 119 FIG. 15 EQUAL ABE A STEREOGRAPH IC PROJECTIONS OF STRUCTURAL DATA OBTAINED IN THE VICINITY OF PINCHI KINE ( HAP IV ) Poles to foliation (S±) Llneatlons (L2) and fold axes (Fj). Fold exes (F-) a i 1 1 6 points contoured using Kalsbeck counting net (Kalsbeck, I 9 6 3 ) . Contour Intervals at 1,4,and 6% of points per 1% area of net. b 1 1 5 3 points contoured using Kalsbeck counting net (Kalsbeck, I 9 6 3 ) . Contour intervals at 2 , 4 , 6 and 8% of points per 1% area of net. c t 5 9 points contoured using the Mellis method (Turner * Weiss, I 9 6 3 , p. 6 2 ) contour Intervals at 1 , 2 end k% of points per 1% area of net. 120 S . F O L I A T I O N F i g . 16a Structural elements i n f o l i a t e d glaucophane r i c h metavolcanic (drawn from hand specimen!. F i g . 16b f _ f o l d i n metachert. F , KINK F O L D S J O I N T S F i g . 16c F~ kink folds i n metachert. 121 F i g . 17a F2 f o l d i n quartz-carbonate-mica s c h i s t . Note lens-ing out of micaceous layers. F i g . 17b ^2 folds i n metachert. Note v a r i a t i o n i n plunge and curving a x i a l plane of F2 f o l d s . Folds are cross-cut by q u a r t z - f i l l e d j o i n t . 122 2 cms F i g s . 18a and b: Recumbent I s o c l i n a l F2 folds i * i metachert. These F2 folds appear to have been refolded by l a t e t ? l F2 f o l d s . F i g . 18c F2 f o l d i n metachert folded by F 3 . 123 Structural Elements i n the Lawsonite-Glaucophane Bearing  Rocks (al F o l i a t i o n s tS-surfaces) : The most obvious st r u c t u r a l feature of the metasediments and the f o l i a t e d metavolcanics i s the f o l i a t i o n tSj) which appears to be concordant with the l i t h o l o g i c layering or bedding (S^). In l i t h o l o g i e s r i c h i n white mica and glaucophane, t h i s f o l i a t i o n might be termed a s c h i s t o s i t y . Within limestones, i s characterized by alternating layers of dolomite and streaky carbonaceous aragonite (Fig. 35). No trace of i s seen i n greywackes, massive metavolcanics and much of the limestone. The S-^ f o l i a t i o n i s cross-cut by an s t r a i n - s l i p cleavage best developed i n f o l i a t e d metavolcanics and glaucophane-mica s c h i s t s . generally cannot be recognized i n metacherts, metagreywackes and massive metavolcanics. Growth of mimetic minerals i s not associated with S^-Metacherts possess a fracture cleavage (S^) with similar o r i e n t a t i o n to the j o i n t s described below. Where well developed, the cleavage i s widely spaced (1 cm) and i s a x i a l planar to kink f o l d s . On account of d i f f i c u l t y of recognition and confusion with complex fractures related to f a u l t s , only a few measurements of were made i n the f i e l d . 124 (b) Lineations : The e a r l i e s t structure (L-^) i s a mineral l i n e a t i o n , best defined i n f o l i a t e d metavolcanics and metacherts by nematoblastic glaucophane (Figs. 16a, 16b) . " Very few measurements were made of i n the f i e l d because of the s c a r c i t y of rocks i n which the glaucophane was s u f f i c i e n t l y coarse grained to discern the l i n e a t i o n . Within the f o l i a t e d metavolcanics, i s deformed by an i 2 crenulate l i n e a t i o n (Fig. 16b). In the metacherts, occurs as a prominent crenulation on surfaces, defined by spindle-shaped aggregates of quartz and crenulated white micas. The l i n e a t i o n commonly p a r a l l e l s the mineral l i n e a t i o n within the metacherts but i n samples possessing t i g h t recumbent F^ folds the orientation i s v a riable (Fig. 18b) . Coaxial and 'L^ may r e s u l t from ro t a t i o n of glaucophane grains during the F^ deformation. At some l o c a l i t i e s , metacherts and quartz-carbonate schists form i r r e g u l a r mullions (Wilson, 1953) or elongate boudins with attitudes similar to (Plate 3). (c) J o i n t s : Most exposures of metachert and metavolcanic rock exhibit a prominent series of j o i n t s oriented approxi-mately perpendicular to (Fig. 16b) . Frequently, they are f i l l e d by vuggy, undeformed quartz veins. 125 (d) Folds : Three d i s t i n c t periods of folding have been recognized and are termed F^, F^ and F^, from oldest to youngest respectively. Convincing f o l d closures associated with F^ are seldom seen. The presence of mesoscopic t i g h t i s o c l i n e s with a x i a l planes p a r a l l e l to i s inferred from the lensing out of compositional layering (Fig. 17) and apparent r e p e t i t i o n of l i t h o l o g i c layering (on mesoscopic and macroscopic s c a l e s ) . Within metacherts, mesoscopic F^ folds (Figs. 17 and 18) are very conspicuous and, inasmuch as they do not show appreciable thickening of the hinge zones, are of concentric type. S l i g h t thickening of f o l d hinges does occur i n incompetent l i t h o l o g i e s such as interbedded limestones and s c h i s t s . F^ f o l d axes are p a r a l l e l to and i n areas where F^ i s of mild i n t e n s i t y they trend at 300° and plunge at 55° (Fig. 15b). F^ folds generally have a complex geometry with curving a x i a l planes and may grade into f o l d mullions. Kink folds (Fig. 16c) are c h a r a c t e r i s t i c of F^ and deform F^ folds and associated l i n e a r structures. F^ Deformation Both mesoscopic and macroscopic f o l d closures associ-ated with F^ are very d i f f i c u l t to demonstrate. The only possible example of a mesoscopic closure i s located on a 12 6 small i s l a n d 4 km south of Pinchi Mine. Southerly dipping, bedded cherts form a t i g h t southwesterly plunging i s o c l i n a l synform containing a schistose greywacke with a x i a l plane cleavage deformed by crenulations. The possible existence of i s o c l i n a l folds suggests that transposition of bedding may have caused the apparent p a r a l l e l i s m of the primary l i t h o l o g i c layering with 5 ^ . Bedding transposition can be i n f e r r e d i n incompetent units such as graphitic cherts, where q u a r t z i t i c layers are highly sheared and wedge out a f t e r a few inches. However, c e r t a i n l i t h o l o g i e s such as massive metavolcanics and metagreywackes generally show no signs of the pervasive shearing which must accompany transposition, and p a r a l l e l s the l i t h o l o g i c layering (Sg). I t i s considered l i k e l y that transposition has occurred only i n the hinge zones of competent units thus explaining the r a r i t y of convincing F^ f o l d closures. In summary, i t appears that the F^ deformation i n -volved i s o c l i n a l f o l d i n g , transposition of the bedding p a r a l l e l to S^ and synkinematic r e c r y s t a l l i z a t i o n of a blueschist facies mineralogy on attaining the necessary depth of b u r i a l (Fig. 19). The l i n e a t i o n possibly p a r a l l e l s F, f o l d axes. 127 F^ Deformation Mesoscopic F^ structures are widespread within the area. and mullions p a r a l l e l the axes.of minor folds which deform and have an average trend and plunge of 300° and 5.5° (Fig. 15b) . Minor folds are generally of the symmetric concentric type; both t i g h t and open folds are common. A x i a l planes of minor folds are d i f f i c u l t to measure because of the prevalence of disharmonic f o l d i n g and mullioning. Because of widespread disruption by f a u l t i n g and flexures associated with F^, macroscopic F^ folds are d i f f i c u l t to recognize. In the mine area within domains of low F^ i n t e n s i t y , the average s t r i k e and dip of the enveloping surface i s 120° and 57° northeast (Fig. 15a and Map IV). V a r i a t i o n i n s t r i k e i s common i n the v i c i n i t y of large limestone lenses which impose a l o c a l control on attitudes. However, on the northwest shore of Pinchi Lake, dip to the southwest along the lakeshore but dip to the northeast farther inland (Map V). Southerly dipping occur on the small island. 4 km south of Pinchi Mine. These attitudes suggest the existence.of a macroscopic antiformal structure with a x i a l trace approximately centred on the long dimension of the glaucophanitic f a u l t block and trending at 100°. There i s no obvious g i r d l e i n the p l o t of poles to 5 1 (Fig. 15a) which would demonstrate 128 the existence of thi s structure. This i s because the struc-t u r a l data were obtained from the northern limb of the postulated antiform. A second possible antiform was i d e n t i f i e d 130 m north of the limestone lens 1.5 km west of Pinchi Mine. There i s a marked difference i n plunge of between the northwest Pinchi area (Map V) and the mine area (Map IV); i n the former the average plunge i s 10°W and i n the l a t t e r i t i s 55°W. I t i s considered that the steeper plunges are found immediately adjacent to the main Pinchi Fault where the in t e n s i t y of the F^ deformation was stronger. In summary, the F^ deformation i s characterized by a prominent l i n e a t i o n and concentric, t i g h t to open f o l d s . Mullions are common and there i s some suggestion that the in t e n s i t y of deformation i s related to the proximity of the Pinchi Fault. Major structures are int e r p r e t a t i v e but a probable east-west trending antiform i s centered on Pinchi Lake. Blueschist facies minerals have been deformed by F^ and metamorphic r e c r y s t a l l i z a t i o n i s l i m i t e d to a l b i t e , quartz and hematite (see p. 133)• F^ Deformation Structures related to F^ are cl o s e l y associated with f a u l t s and are best demonstrated i n the Pinchi Mine area (Map IV) where a series of macroscopic F^ cross folds deform e a r l i e r structures. Mesoscopic kink folds appear to be 129 associated with these cross-folds and trend 100° to 125° and plunge 10° to 60° southeast. The a x i a l plane (S^) i s generally coplanar with the j o i n t s perpendicular to F^ s t r i k i n g from 60° to 90° and dipping southeast at 45°. The v a r i a t i o n i n trend of F^ f o l d axes depends on the i n i t i a l o rientation of with respect to F^> Therefore, varying orientations of theoretically, should dispose F^ f o l d axes i n a great c i r c l e whichidefines S^. The spread of F^ f o l d axes i n F i g . 15c bears t h i s out. F^ folds deform F^ folds and lineations.as i l l u s -trated i n F i g . 18c. T h e o r e t i c a l l y , for concentric fol d i n g of ^2 situated on a plane of known orientation (e.g. S^), the locus of L 2 should l i e on a p a r t i a l small c i r c l e on the l o c a l "b" kinematic axis (Turner and Weiss, p. 498). L i t t l e can be said as to whether the spread i n i n F i g . 15b defines a great c i r c l e or a small c i r c l e . On the northern slopes of Pinchi Mine h i l l , s t r i k e s east-west and, rather unexpectedly, dips are southerly. This gives the impression that on the mine h i l l , the c o n t r o l l i n g structure i s an easterly trending F^ synform which plunges at 30° (Map IV). The occurrence of similar l i t h o l o g i c sequences on both north and south limbs reinforces t h i s i n t e r p r e t a t i o n . However, the a x i a l zone of t h i s synform i s highly faulted and carbonatized and r e l a t i o n s between the limbs are obscure. Similar easterly trending 130 folds were mapped at various locations south of the f a u l t separating the glaucophanitic rocks from the greenstones. I t appears that the only way to make sense out of the structure i n the mine area i s to divide the area into domains which display some degree of homogeneity and draw fa u l t s along the domain boundaries. This i s subjective to some degree but i t presents a complex pattern of warping, f a u l t i n g , block r o t a t i o n and carbonatization along f a u l t zones which i s possibly close to the truth. Consideration of the trend and plunge of folds and of the sense of warping i n the limestone beds suggest that r i g h t - l a t e r a l movement with a d i p - s l i p component may have taken place along f a u l t number 2 (Map VI) during F^. Microscopic Analyses The r e l a t i o n s h i p between growth of metamorphic minerals and deformation i s i l l u s t r a t e d i n F i g . 19. Within the metacherts, thin layers of l e p i d o b l a s t i c white mica, a c i c u l a r glaucophane and tabular lawsonite define and L^. In f o l i a t e d metavolcanics the preferred orientation of glaucophane defines and L^. Some l i t h o l o g i e s such as metagreywackes and massive metavolcanics do not have a preferred orientation of t h e i r metamorphic mineralogy and are unfoliated. Both l i t h o l o g i e s do, however, grade into schistose types. I t i s assumed that 131 F ig . 19 Relat ionship of metamorpic recrystal l izat ion to deformation T Y P E OF RECRYSTALL I ZAT ION Mineral Synk inemat ic (F,) Transitional 1 stage 1 Synkinematic (Fz) acmitic pyroxene 1 1 jadeitic pyroxene I 1 lawsonite 1 l phengite I 1 sphene I I chlorite i i glaucophane aragonite I i ca l c i te — i " 1 stilpnomelane 1 1 brown amphibole 1 I albite + magnetite ?-- 1 I hematite 1 v. Increase in P and T Decrease in P a n d T possibly accompanied by strike slip faulting Uncerta inty 132 unfoliated rocks possessed s u f f i c i e n t strength to withstand the F^ penetrative deformation which gave r i s e to the f o l i a t e d zones. Some features o f f e r f a i r l y d i s t i n c t i v e evidence that at least some minerals c h a r a c t e r i s t i c of the blueschist fa c i e s continued to r e c r y s t a l l i z e a f t e r the F^ deformation. For instance, aragonite veins cross-cut i n limestones tFig. 35) and the mineral commonly i s found c l o s e l y associated with undeformed radiating clusters of a brown amphibole growing along fractures. A network of glaucophane or glaucophane + quartz veins t y p i f i e s massive volcanics. Glaucophane i s aligned either p a r a l l e l or perpendicular to the walls. The problem arises as to whether these veins formed during or after the F^ deformation. In hand specimen, some of them appear to be shears f i l l e d with glaucophane growing along small thrust f a u l t s which o f f s e t e a r l i e r glaucophane veins. Other veins are sigmoidal, resembling tension gashes, and i n one specimen a glaucophane vein was seen cross-cutting a weak f o l i a t i o n (S^) . I t was previously suggested i n Chapter IV (p. 110) that minute fractures acted as channels for f l u i d s which reacted with jadeite-acmite i n the metavolcanics to form glaucophane. A reasonable explanation for the o r i g i n of the fractures i s that some are shears related to F^ and that others may be post-F^ tension fractures which formed during u p l i f t and/or the early stages of the F„ deformation. Presumably, 133 th i s r e c r y s t a l l i z a t i o n came to a h a l t with decrease i n pressure and temperature. Blueschist facies minerals are deformed by F^ crenulations. Glaucophane and lawsonite are commonly fractured, and white mica forms contorted swathes i n meta-cherts (Fig. 36d). . Quartz shows sutured boundaries and undulose extinction i n some samples, but i n others, s t r a i n -free polygonal aggregates suggest that r e c r y s t a l l i z a t i o n accompanied F^. Restricted r e c r y s t a l l i z a t i o n of retrograde minerals occurred during F^. A l b i t e veins form i n metacherts and metavolcanics and also heal fractures i n glaucophane c r y s t a l s (Fig. 36c). Magnetite grains, believed to have formed during the glaucophanitic metamorphism, are commonly rimmed by retrograde hematite. Brecciation of the limestones resulted i n p a r t i a l inversion of aragonite to c a l c i t e and veining by sparry c a l c i t e . F^ deformation was accompanied by carbonatization along active f a u l t zones. Vuggy quartz veins which commonly f i l l j o i n t s i n metacherts may be associated with the l a t t e r stages of F ^ . In summary, three stages of metamorphic r e c r y s t a l l i z -ation are proposed (Fig. 19). The f i r s t i s a synkinematic stage associated with blueschist facies metamorphism and the formation of and the mineral l i n e a t i o n L^. The second i s an i l l - d e f i n e d t r a n s i t i o n a l stage between F , and 134 ^2 accompanied by r e c r y s t a l l i z a t i o n of minor glaucophane, aragonite, stilpnomelane and brown amphibole. A t h i r d stage involved deformation of blueschist f a c i e s minerals and r e c r y s t a l l i z a t i o n of quartz, and minor a l b i t e , white mica and hematite. This stage must have occurred at much lower pressure-temperature conditions than the f i r s t stage. Timing of Metamorphism and Deformation Because of the absence of f o s s i l s , the age of deposition of sediments i n the blueschist bearing f a u l t block i s not known with certainty, but the l i t h o l o g i e s present are similar to those i n the Cache Creek or Sli d e Mountain Groups, suggesting a Mississippian to Permian age. Armstrong (1949) gave evidence for a Permo-Triassic deformation of the Cache Creek Group. I t i s i n f e r r e d that t h i s deformation was contemporaneous with the F^ deformation i n the blueschists, but the former occurred at a higher s true tura1 1eve1. A number of authors (e.g. Ernst, 1965) have argued that blueschist f a c i e s metamorphism takes place during rapid b u r i a l of sediments, creating conditions of low temperature and high pressure within the crust. For preservation of the c h a r a c t e r i s t i c mineralogy, i t i s e s s e n t i a l that the metasediments have undergone a rapid u p l i f t (Brown et al., 1962). Metamorphic ages i n a ra p i d l y u p l i f t e d t e r r a i n should c l o s e l y approximate the 135 FIG. 20 D E P T H - T I M E T R A J E C T O R Y FOR GLAUCOPHANITIC ROCKS _ _ j 1 PERMIAN - ' TRIASSIC « ? >' JUR. i ! i ! Upper [Lower 8 Middle J Upper J Lower Note : a) The T r i a s s i c - J u r a s s i c boundary i s a r b i t r a r i l y placed at 200 m.yrs. with the uncertainty indicated. Data are from Tozer, 196k (190-200 m.yrs.); Bochkarev & Pogorelov, 196? (204 m.yrs.); Armstrong, 1970 (210 m.yrs.). b) The Middle-Upper T r i a s s i c (230 m. yrs.) i s taken from Borsi and Ferrara ( 1 9 6 7 ) . . c) The Permo-Triassic boundary (255-260 m.yrs.) i s obtained from Bochkarev & Pogorelov (1967). 136 actual age of metamorphism (Suppe,. 1972) . In the case of the Pinchi rocks, i t has been argued that r e c r y s t a l l i z a t i o n of blueschist facies minerals was largely synkinematic but there i s also evidence for a phase of post E ' ^ r e c r y s t -a l l i z a t i o n which could only have occurred during the u p l i f t . Three K-Ar dates were obtained from phengitic micas present i n schistose layers i n cherts. D e t a i l s of specimen location and a n a l y t i c a l techniques are given i n Appendix V. The dates obtained were 211, 214 and 216 * 7 m yrs. A fourth date of 218 m yrs was obtained from a micaceous ec l o g i t e . These absolute ages correspond to Middle or Upper T r i a s s i c geologic ages depending on which time scale i s used. I f the T r i a s s i c - J u r a s s i c boundary i s taken at 190-2 00 m yrs (Tozer, 1964) i t i s conceivable that the dates could be Middle T r i a s s i c . Armstrong (1971) proposed a boundary between the periods at 210 m yrs and Borsi and Ferrara (1967) established a date of 230 m yrs for the Middle-Upper T r i a s s i c boundary. This being the case, the ages obtained at Pinchi are Upper T r i a s s i c . If i t i s assumed that the micas did not absorb excess argon, two factors would have been responsible for these T r i a s s i c dates. They could r e f l e c t (a) the age of u p l i f t and cooling to a temperature below the c r i t i c a l isotherm for argon retention and (b) the age of the 137 crenulation (L^) which deforms the micas and i s associated with the F^ deformation. Typical F^ structures such as minor f o l d s , crenulations and mullions are not present i n the Upper T r i a s s i c Takla Group even adjacent to the f a u l t zone. This f a c t , together with the age dates, suggests that the F^ deformation was i n progress during deposition of the Upper T r i a s s i c but a f f e c t i n g rocks at a lower s t r u c t u r a l l e v e l . U p l i f t and cooling of the f a u l t block to below 2 00°C must have occurred either before or during the F^ deformation. The foregoing facts and inferences suggest that F1 deformation, metamorphism, u p l i f t and F^ deformation a l l took place between the beginning of the late Permian and the end of the T r i a s s i c ( i . e . between 245 and 200 m yrs ago) . Possible depth time t r a j e c t o r i e s for the blueschists are i l l u s t r a t e d i n F i g . 20. If a subduction model i s invoked (see Chapter VI), the rate of b u r i a l of sediments i s approximately dependent on the rate of underthrusting and the dip of the subduction zone. For a subduction zone dipping at 45°, the times taken to descend 30 km at under-thrusting rates of 10 cm per year (AC) and 1 cm per year (BCl are 0.35 m yrs and 3.5 m yrs r e s p e c t i v e l y . The present day underthrusting rates of 10 cm per year i n the Japan Trench (Oxburgh, 1971) suggest that the former appears 138 the more r e a l i s t i c t i m e . A t C, d u r i n g b l u e s c h i s t f a c i e s metamorphism, i t i s assumed t h a t t h e r e was a r e l a x a t i o n o r change i n o r i e n t a t i o n o f the s t r e s s w h i c h caused the extreme d e p t h of b u r i a l , and t h e b l o c k was a l l o w e d t o i s o s t a t i c a l l y r e - e q u i l i b r a t e . Maximum u p l i f t r a t e s o f t h e o r d e r o f 1 cm per y e a r have been used t o r e c o n s t r u c t the CD p a r t o f t h e t r a j e c t o r y . Presumably, t h i s u p l i f t s t a g e was accompanied by minor b l u e s c h i s t f a c i e s r e c r y s t a l l i z a t i o n u n t i l c o o l i n g r e s u l t e d i n a " f r e e z i n g " o f the metamorphic m i n e r a l o g y . A t D, assuming a geothermal g r a d i e n t of 12°C/km, the g l a u c o p h a n i t i c r o c k s p a s s e d t h r o u g h t h e 200°C i s o t h e r m w h i c h r e p r e s e n t s the a p p r o x i m a t e t e m p e r a t u r e f o r argon r e t e n t i o n i n m u s c o v i t e s (Suppe, 1972). T h e o r e t i c a l l y t h i s s h o u l d r e p r e s e n t the maximum age d a t e o b t a i n a b l e f rom the b l u e s c h i s t s . From D t o E the b l o c k c o n t i n u e d t o r i s e a t a d e c r e a s i n g r a t e because o f approach t o i s o s t a t i c e q u i l i b r i u m . The F ^  d e f o r m a t i o n must have o c c u r r e d d u r i n g t h i s p a r t o f the t r a j e c t o r y , p o s s i b l y c a u s i n g argon l o s s i n the micas and r e s e t t i n g t h e age d a t e s . P o i n t E i s somewhat a r b i t r a r i l y chosen a t 4.6 km from t h e s u r f a c e . T h i s d e p t h r e p r e s e n t s the amount of r o c k eroded i n 230 m i l l i o n y e a r s , assuming an;.4-_\ e r o s i o n r a t e o f 1 m p e r 50,000 y e a r s . F^ d e f o r m a t i o n was accompanied by c a r b o n a t i z a t i o n a l o n g f a u l t zones. I t was p r e v i o u s l y i n f e r r e d (p. 67 ) t h a t the c a r b o n a t i z a t i o n o c c u r r e d d u r i n g the Eocene. 139 Structural Geology of Remaining Fault Blocks The i n t e r n a l structure of the remaining f a u l t blocks has been adequately dealt with i n the appropriate sections i n Chapter I I . The following summary f a c i l i t a t e s comparison of structures i n d i f f e r e n t f a u l t blocks. Greenstones of Pinchi Mountain (Map I, Units 5 and 6) These rocks are well f o l i a t e d and interbedded graphite schists possess an S^. D r i l l hole intersections of an intercalated limestone unit indicate that these rocks dip north or northeast at 45°. This dip i s conformable with the layering i n the silica-carbonate rocks which form the northeastern boundary of the greenstone unit . Mount Pope b e l t (Map I, Units 7, 8 and 9) The Mount Pope b e l t i s of Pennsylvanian to Middle Permian age and was deformed during the Permo-Triassic. The basic rocks belong to the lower greenschist f a c i e s . A r g i l l i t e s possess a slaty cleavage which commonly cross-cuts the bedding i n the hinge zones of early f o l d s . To the west of the map-area, a p a r t i a l cross section of the Cache Creek Group i s revealed i n the north arm of Stuart Lake. The northwesterly s t r i k i n g cleavage i s fanned and dips grade from v e r t i c a l i n the west to 25° southwest towards the east. Deformation i s of Permo-Triassic age 140 (Armstrong, 1949) and i s considered to be the low pressure, high l e v e l expression of the F^ deformation i n the blue-schists . Paleontological evidence (Appendix I) demonstrates the presence of a syncline which appears to run the length of the map area. Judging from the southwest dip of chert beds on either side of the limestone b e l t , i t would appear that the syncline i s asymmetric with a southwesterly dipping a x i a l plane. At inte r v a l s along the s t r i k e of the b e l t , the limestone appears to cut out; t h i s i s thought to be due to l o c a l culminations i n the plunge of the syncline. Ultramafites (Map I, Units 14a and 14b) Structural relationships outlined i n Chapter I I I suggest that the following sequence of events has taken place within the ultramafites. Correlation of deformational phases with those recognized i n the blueschists i s tentative. (a) Magmatic c r y s t a l l i z a t i o n of harzburgite and intrusion of dunite pods, (b) Formation of early pyroxenite and dunite layers, tc) Folding and high temperature metamorphic r e c r y s t a l l i z a t i o n within the mantle (?), (d) Formation of late pyroxenite layers, (e) Emplacement, pervasive f r a c t u r i n g , introduction of connate (?) water and serpentinization (This 141 event was possibly contemporaneous with F^ and/or F^ deformations), Cf). Formation of l a t e fracture cleavage, f o l i a t i o n of the serpentinite along f a u l t zones and minor serpentinization (.contemporaneous with F^ or F^ deformations). Basic rocks south of Pinchi Lake (Map I, Unit 4) This b e l t of rocks i s bounded to the north by Takla Group sediments of Upper T r i a s i c (?) age and to the south by an elongate serpentinite body thought to be associated with a major f a u l t (Map VI, No. 3). I t i s uncertain whether the northern contact of unit 4 i s a f a u l t or an unconformity. An unconformity i s more probable because of the presence of a conglomerate containing pebbles of a metabasic rock near the base of a Takla Group (?) sedimentary sequence (unit 10). If t h i s i s an unconformity, the following sequence of events must have taken place: (a) f r a c t u r i n g , introduction of water and amphiboliz-ation of metabasic rocks p r i o r to or during Permo-Triassic deformation, ( h \ deposition of Upper T r i a s s i c (?) sediments of the Takla Group, and (cl overturning of the sequence during Mesozoic or T e r t i a r y deformation. 142 Takla Group (Map I, Units 10, 11 and 12) Rocks belonging to t h i s group are unfoliated and have been subject to low grade b u r i a l metamorphism rather than the dynamic metamorphism which characterizes the Cache Creek Group. Structural elements include a fracture cleavage i n s i l t s t o n e s and sparse kink bands. Folds are rare and appear to be gentle warps i n the v i c i n i t y of f a u l t s . The intensive f a u l t i n g i s the r e s u l t of Mesozoic or T e r t i a r y deformation. Important Faults i n the Pinchi Area The f a u l t pattern at Pinchi i s highly complex and has divided the area into a series of elongate tectonic s l i c e s of contrasting metamorphic grade. A l l f a u l t s have been numbered for reference purposes (Map VI) with splay f a u l t s indicated by an alphabetical subscript (e.g. 2a, 2b). In areas of poor exposure, f a u l t recognition i s extremely d i f f i c u l t . Some of the c r i t e r i a used at Pinchi are as follows: (a) abrupt change i n metamorphic grade, s t r u c t u r a l complexity or l i t h o l o g y , (b) presence of carbonatized zones i n ultramafites or metasediments, (c) presence of serpentinites and associated l i n e a r magnetic highs, 143 (d) occurrence of l i n e a r topographic lows containing sporadic sag ponds, and (e) f a u l t breccias encountered i n d r i l l core or, r a r e l y , i n outcrop. Once a f a u l t has been established on these c r i t e r i a , i t i s even more d i f f i c u l t to demonstrate the age of f a u l t -ing and type of movement. This d i f f i c u l t y i s larg e l y because of the s c a r c i t y of rocks younger than Lower J u r a s s i c . However, information on the age and type of f a u l t a c t i v i t y can be obtained from (a) proven relationships i n adjacent map-areas, (b) minor structures such as drag f o l d s , l i n e a t i o n s and slickensides, and (c) metamorphic grade of rocks which have been juxtaposed. An addi t i o n a l factor which must be considered, es p e c i a l l y i n areas of major f a u l t s , i s the p o s s i b i l i t y of r e a c t i v a t i o n of an old f a u l t under a d i f f e r e n t stress system. Pinchi Fault (No. 1) Within the map area, the Pinchi Fault juxtaposes rocks of the Takla and Cache.Creek Groups. To the southwest of the f a u l t zone, the Cache Creek Group i s c l o s e l y associated with ultramafic bodies and has been metamorphosed under lower greenschist or blueschist f a c i e s conditions. 144 Evidence has been given for two periods of deformation, F^ and F^, p r i o r to the deposition of the Upper T r i a s s i c . Most information on the f a u l t zone was obtained just east of Pinchi Mine where i t appears to be about 2 000 f t wide. Percussion d r i l l cores across the f a u l t indicated the presence of s l i v e r s of serpentinite, brecciated volcanics, carbonatized ultramafites and contorted carbon-aceous s c h i s t s . Ground magnetics indicate an excellent c o r r e l a t i o n between serpentinite s l i v e r s and li n e a r magnetic highs. The s l i g h t o f f s e t of the magnetic highs with respect to the d r i l l hole information suggests that the serpentin-i t e s dip steeply to the northeast. The only other evidence on the attitude of the f a u l t zone i s the occurrence of a northeasterly dipping fracture cleavage within the ultramafites along the northeast margin of Murray Ridge. These planar structures (^^?) probably p a r a l l e l the f a u l t zone i n the Pinchi area. However, 30 miles farther north, symmetric aeromagnetic p r o f i l e s across the f a u l t zone suggest that i t i s v e r t i c a l , and indicate that the dip of the f a u l t zone may be variable along i t s length. The Pinchi Fault i s one of the major tectonic lineaments i n central B r i t i s h Columbia. I t has been traced from near Quesnel 500 km north-northwest to the McConnell Creek map-area where i t appears to dissipate i n a number of splay f a u l t s . Lord (19491 and Eisbacher (1969) demonstrated 145 that i n the McConnell Creek area Sustut Group rocks of late Cretaceous to Eocene age have been involved i n north-easterly directed thrust f a u l t i n g . Eisbacher (oral communication) believes that the presence of the extension of the Pinchi Fault at depth in a Paleozoic-early Mesozoic basement may have controlled the positioning of thrust f a u l t s i n the cover rocks. This l i n e of reasoning suggests active movement of the f a u l t during the Eocene p r i o r to the mercury mineralization (p. 67 ). Along the northern part of the f a u l t zone, rocks of the Cache Greek Group are faulted against the Hogem batho l i t h which, according to Armstrong (1949), i s of Jurassic age. A K-Ar radiometric date of 170 m i l l i o n years was obtained for the Hogem (Koo, 1968). S i g n i f i c a n t l y , metasediments of the Cache Creek Group adjacent to the f a u l t zone are highly sheared and show no sign of contact thermal metamorphism. These data also indicate that active f a u l t i n g must have taken place after the emplacement of the Hogem bath o l i t h ( i . e . post Middle J u r a s s i c ) . The history of movement on the Pinchi Fault i s complex and c l o s e l y linked to the deformations as discussed i n Chapter VI. Fault system no. 2 The trace of t h i s f a u l t system i s sub-parallel to the Pinchi Fault and numerous splays i n t e r s e c t i t . The most persistent f a u l t separates the Pinchi Mountain green-146 stones to the north from the lawsonite-glaucophane bearing rocks. D r i l l intersections of f a u l t 2d indicate that i t dips to the northeast at 60° on the Darbar claim group and immediately east of Pinchi Mine. However, 3 km west of Pinchi Mine, the f a u l t i s apparently v e r t i c a l . Splay f a u l t s juxtapose ultramafites, greenstones and blueschists. Carbonatization along these f a u l t zones i s l o c a l l y extensive and ultramafic s l i v e r s have been p a r t l y converted to the silica-carbonate rocks (p. 63 ; Plate 5). The f o l i a t i o n i n the silica-carbonate outcrop generally dips to the north or northeast at 60° to 70° and i s p a r a l l e l to the f a u l t plane as v e r i f i e d by d r i l l hole intersections and ground magnetic p r o f i l e s . A s l i v e r of chert pebble conglomerate i s located i n the footwall of f a u l t no.2d on the Darbar claim group. The only conglomerate with which i t could possibly be correlated i s found at the west end of Murray Ridge and i s believed to be of Cretaceous or Paleocene age (p. 44 ). At the Darbar l o c a l i t y , f a u l t 2d dips to the northeast at 60° and separates greenstones from a chaotic mixture of glauco-phanitic metabasic rocks, carbonatized serpentinites, sand-stones and conglomerates constituting the footwall. The involvement of a Cretaceous or Paleocene conglomerate i n the f a u l t i n g suggests that t h i s f a u l t was active during Eocene tectonic a c t i v i t y . Presumably, southwesterly directed thrusting of the greenstones caused the formation of a melange zone i n the footwall. 147 A s i m i l a r s i t u a t i o n e x i s t s j u s t t o t h e e a s t o f P i n c h i M i n e where t h e so c a l l e d " b r e c c i a " zone i s f o u n d a l o n g t h e n o r t h e a s t e r l y d i p p i n g g r e e n s t o n e c o n t a c t . T h i s " b r e c c i a " m a i n l y c o n s i s t s o f a n g u l a r c h e r t f r a g m e n t s i n a m a g n e s i t i c m a t r i x , b u t t h e p r e s e n c e o f r a r e , r o u n d e d c h e r t p e b b l e s s u g g e s t s t h a t t h e " b r e c c i a " may o r i g i n a l l y have been a c o n g l o m e r a t e . I t i s p o s s i b l e , however, t h a t t h e r o u n d e d c h e r t p e b b l e s o r i g i n a t e d a s a n g u l a r f r a g m e n t s b u t were r e s o r b e d d u r i n g t h e c a r b o n a t i z a t i o n . F a u l t s y s t e m no. 3 T h i s f a u l t s y s t e m l i e s between t h e Mount Pope l i m e -s t o n e b e l t a n d t h e s o u t h w e s t s h o r e o f P i n c h i L a k e . A l o n g i t s l e n g t h , l a w s o n i t e - g l a u c o p h a n e b e a r i n g r o c k s t o t h e n o r t h e a s t a r e f a u l t e d a g a i n s t u l t r a m a f i t e s , c h e r t s and Upper T r i a s s i c (?) l i m e s t o n e s and s i l t s t o n e s . The r e l a t i o n s h i p among t h e l a s t m e n t i o n e d s e d i m e n t s , g a b b r o s and u l t r a m a f i t e s i s n o t c l e a r l y u n d e r s t o o d and t h e c o n t a c t s may o r may n o t be t h e s i t e s o f f a u l t s a s s o c i a t e d w i t h t h i s s y s t e m . J u d g i n g f r o m t h e a e r o m a g n e t i c p r o f i l e s a c r o s s t h e s e r p e n t i n i t e s l i v e r w h i c h p a r a l l e l s t h e f a u l t z one, i t w o u l d a p p e a r t h a t t h e d i p o f t h e f a u l t v a r i e s a l o n g t h e s t r i k e . N o r t h o f Mount Pope, t h e p r o f i l e GH (Map. I I ) . s u g g e s t s t h a t t h e s e r p e n t i n i t e has a m o d e r a t e s o u t h w e s t d i p , s i m i l a r t o t h e a t t i t u d e o f t h e b e d d i n g i n c h e r t s a d j a c e n t t o t h e f a u l t 148 zone. Towards the north-northwest however, cherts near the f a u l t zone dip to the southwest at 80° and the aeromagnetic anomaly decreases i n i n t e n s i t y , suggesting that the serpen-t i n i t e s l i v e r i s no longer present. Fault system no. 4 The lineaments 4a and 4c trend east-northeast and may well be major fracture zones rather than f a u l t s as there i s no sign of o f f s e t of l i t h o l o g i c units. They form low lyi n g swampy areas, and folds within the limestone b e l t tend to plunge away from the fracture zones. Faults 4b and 4d are i n f e r r e d , f i r s t l y because they occupy topographic lows and secondly because of the apparent r i g h t - l a t e r a l o f f s e t of the lin e a r magnetic high which follows the serpentinite b e l t (Map III) southwest of Pinchi Lake. Whether t h i s "offset" i s the r e s u l t of normal or s t r i k e - s l i p movement i s uncertain. I t i s int e r e s t i n g to note that the Pinchi Fault appears to have undergone l e f t - l a t e r a l o f f s e t by f a u l t l a . Fault system no. 5 The only evidence for t h i s f a u l t i s an abrupt change in s t r i k e of the Takla sediments on either side of i t . The f a u l t i s p a r a l l e l to the Pinchi Fault and appears to i n t e r -sect i t north of Murray Ridge. 149 Faults i n the v i c i n i t y of Pinchi Mine (Nos. 2c and 6) The f a u l t pattern around the mine i s exceedingly complex and only the well documented f a u l t s w i l l be l i s t e d here. Fault 2c trends southeast and dips to the northeast at 80°. I t may well be an early f a u l t as i t i s associated with ultramafic s l i v e r s . Faults 6b (the "south fault") and 6a dip to the southwest at 50° to 60° and are post-mineraliz-ation. According to Armstrong (1949) and the mine geologists, f a u l t 6a i s a thrust f a u l t with l e f t l a t e r a l o b l i q u e - s l i p displacement. These f a u l t s are crosscut by late northerly trending normal f a u l t s with near v e r t i c a l dip. VI TECTONIC IMPLICATIONS Introduction The general conclusion arrived at i n recent studies i s that the formation of orogenic belts containing blue-schists i s related to the i n t e r a c t i o n of l i t h o s p h e r i c plates. Ernst (JL970) suggested that blueschist facies metamorphism r e s u l t s from, the down-warping of the earth's crust i n a subduction zone and deep b u r i a l of sediments. Coleman (.1971). agrees that t h i s may be a v a l i d mechanism but also suggests that blueschist formation takes place as the r e s u l t of tectonic overpressures developed i n shallow dipping zones underneath obducted oceanic crust. In contrast to subduction zones, obduction zones are characterized by a complete lack of associated volcanic a c t i v i t y . An important feature of orogenic belts containing blueschists i s t h e i r association with o p h i o l i t e s , and their presence i s commonly taken as evidence for involvement of oceanic crust during plate i n t e r a c t i o n (Dewey and Bi r d , 1970; Coleman, 1971). Commonly c i t e d examples occur i n New Caledonia. ( L i l l i e and Brothers, 1970) and western 151 C a l i f o r n i a CBailey et al.s 1970; Page, 1972) where ophio-l i t i c sequences are believed to have been thrust over sediments undergoing blueschist facies metamorphism presum-ably during active subduction or obduction. According to Ernst (1971), former subduction zones can be recognized by a c h a r a c t e r i s t i c metamorphic zonation with respect to a major f a u l t . He gives examples from Japan, western C a l i f o r n i a and the Alps where the commonly developed metamorphic sequence towards the f a u l t i s : (a) z e o l i t i z e d rocks, (b) pumpellyite bearing rocks, (c) greenschists and/or blueschists and (d) a l b i t e amphibo-l i t e s . The preservation of t h i s zonation i s at t r i b u t e d to a change i n orientation of the stress f i e l d which caused the formation of the subduction, and i s o s t a t i c u p l i f t of the buried sediments. An i n t e r e s t i n g feature of the areas referred to i s the occurrence of p o s t - u p l i f t s t r i k e - s l i p movement on the major f a u l t zones. These f a u l t zones separate regions with contrasting sedimentary records, s t r u c t u r a l s t y l e and grade of metamor-phism, and are believed to be zones of plate i n t e r a c t i o n . The term "suture zone" i s used to describe the contact between two such juxtaposed regions. Examples commonly given are the Median Tectonic Line i n Japan, the Coast Range Thrust i n C a l i f o r n i a and the Insubric Line i n the Swiss and I t a l i a n Alps. These suture zones are thought to represent fundamental zones of weakness i n the earth's 152 crust which may have been the locus of underthrusting during subduction giving r i s e to Benioff zones. Thereafter, the suture zone may have been the s i t e of post-subduction up-l i f t or s t r i k e - s l i p movement. Constraints to Tectonic Model (Factual and Inferred) Conclusions of th i s study (a) The inferred pressure-temperature conditions for metamorphism within the various fault-bounded blocks are as follows: Murray Ridge greenstones 3 - 6 kb 100-225°C Pinchi Mountain greenstones 4.5-9 kb 100-250°C Blueschists 8-12 kb 225-325°C Eclogite 12-15 kb 400-550°C (b) The blueschist facies metamorphism was contemporan-eous with the F^ deformation. This was c l o s e l y followed by the F^ deformation which seems to have been associated with u p l i f t and cooling. The K-Ar dates of 211, 214, 216 and 218 ± 7 m yrs are con-sidered to record cooling below 200°C and/or the close of the F^ deformation (p. 136). tc) Greywackes and conglomerates within the Upper T r i a s s i c Takla Group contain d e t r i t u s which originated i n a landmass consisting of basalt, amphibolitized gabbro, ultramafite and chert tp. 42 ) . 153 (d) South of Pinchi Lake, serpentinite i s overlain by amphibolitized gabbro, diabase, basalt and Upper T r i a s s i c C?l conglomerate. These basic and metabasic rocks may represent an o p h i o l i t i c sequence Cp- 2 6 ) . Ce) The deformation was contemporaneous with the carbonatization of f a u l t zones and possibly occurred during the Eocene (p. 6 7 ). Inferred c r u s t a l structure i n central B r i t i s h Columbia Largely because there are no i n l i e r s of older rocks i n the Cache Creek Group and because i t contains o p h i o l i t i c sequences, Monger et al. 3 (1972) and Dercourt (1972) have implied that the Group i s underlain by oceanic crust. By analogy with other regions (Bailey et al.3 1970; Page, 1972), such an oceanic crust would be expected to consist of basalts grading downwards into diabase, gabbro, harz-burgite and dunite. In central B r i t i s h Columbia seismic and gravity data suggest that the M-discontinuity l i e s between 30 and 35 km below the surface (Berry, et al. 3 1971). Evidence suggests that the oceanic M-discontinuity represents a gabbrd/harz-burgite t r a n s i t i o n (Coleman, 1971; Aumento et al.3 1970). Therefore i t would appear that the Cache Creek Group within the Pinchi Geanticline must be at le a s t 25 km thick, allow-ing for 5 km of basalt and gabbro. However, Armstrong FIG. 21 CRUSTAL MODEL FOR CENTRAL BRITISH COLUMBIA PINCHI GEANTICLINE QUESNEL TROUGH OMINECA GEANTICLINE ROCKY MOUNTAIN THRUST BELT Km 0 10 20 30-Toklo Fault Pinchi Fault Wolverine Fault Rocky Mf. Trench Topley Intrusions Cache Creek Group Hogem Bafholifh Paleozoic Lower Mesozoic volcanics.clastics Proterozoic ond Lower Paleozoic Proterozoic 8 Paleozoic — Conrad Discontinuity /y// highly conductive (hydroted ?) lower crust (Dragert, 1970; Caner, 1970) •• M approximate depth to Mohorovicic discontinuity (Berry et al., 1971; D.A.G. Forsyth, M.Sc. thesis in prep., U.B.C, 1973) For section location, see Fig. I, A-B • — M 20 km 155 (1949) estimates the maximum thickness of the Cache Creek Group as being 6 km. This i s considered an overestimate, as wherever de t a i l e d work has been done, rep e t i t i o n s of strata have been recognized (Douglas et al.3 1970, p. 416). I t follows that i f the interpretations of seismic and gravity data are v a l i d , a surprising amount of tectonic thickening of the Cache Creek Group must have taken place to give a depth of over 25:'km to oceanic crust. Geomagnetic depth sounding and magnetotelluric studies i n c e n t r a l B r i t i s h Columbia (Dragert, 1970) suggest the presence of an important d i s c o n t i n u i t y i n the conductivity of the earth's crust at a depth of 10 to 15 km. The t r a n s i t i o n from a poorly conductive surface layer to a highly conductive lower layer i s interpreted as being due to p a r t i a l melting or hydration of the lower crust (Caner, 1970). P a r t i a l melting at depths of 10 to 15 km commences at approximately 7 00°C i n a "wet" granite and would imply steep, u n r e a l i s t i c , geothermal gradients of the order of 40°to 70°C per km. Moreover, i f p a r t i a l melting d i d e x i s t , i t would be expected to give r i s e to Quaternary v o l c a n i c i t y at the surface. Study of nodules within Quaternary basalts from B r i t i s h Columbia (LittleJohn, 1972), suggests that they had a f a i r l y deep-seated source, possibly around 35 to 70 km, and could not therefore be related to p a r t i a l melting between 10 and 30 km. Therefore, hydration seems 156 to be the best mechanism for explaining the increase i n conductivity of the lower crust. The presence of a 10 to 15 km discontinuity beneath the Cache Creek Group therefore suggests that the somewhat u n r e a l i s t i c thickness of over 25 km infer r e d from seismic and gravity r e s u l t s may be i n error. To compromise the geological and geophysical evidence, a c r u s t a l model i s proposed (Fig. 21). i n which the Pinchi Geanticline, con-s i s t i n g of t e c t o n i c a l l y thickened Cache Creek Group with associated o p h i o l i t e s , i s 10 to 15 km thick and the lower crust i s a mixture of serpentinized p e r i d o t i t e , amphibolite, s l i v e r s of Cache Creek Group and possibly minor e c l o g i t e . Increasing temperatures may r e s u l t i n dehydration reactions which give r i s e to i n t e r s t i t i a l water vapour, thus creating the conductivity anomaly. The discontinuity at 3 0 to 35 km proposed by gravity and seismic models may represent the t r a n s i t i o n to anhydrous p e r i d o t i t e . A possible alternative model would show the Cache Creek Group and associated o p h i o l i t e s as being underlain by lower Paleozoic rocks and/or basement gneisses down to the M-discontinuity. Such a basement could have been thrust under the Cache Creek Group during Mesozoic tectonism. This model i s thought u n l i k e l y as lower Paleozoic or gneissic rocks which could be interpreted as basement have not been recognized i n the intermontane region. Also, serpentinite and p e r i d o t i t e have a much lower r e s i s t i v i t y than g r a n i t o i d 157 rocks (Caner, 1970) and would therefore be more l i k e l y to y i e l d the conductivity anomaly below 15 km. The Pinchi Fault as a "Suture Zone" The p o s s i b i l i t y e x i s t s that the Pinchi Fault l i e s within a Permo-Triassic "suture zone" which welded together two tectonic b e l t s of contrasting age and/or primary depositional environment. For much of the length of the f a u l t , Cache Creek Group rocks are i n contact with lower Mesozoic greywackes, volcanics and intrusives (Fig. 22). To the east of the Mesozoic i s a b e l t of rocks formerly mapped as late Paleozoic by Armstrong (1949) and Roots (1954). These rocks were re-examined by Monger (197 3) and his preliminary s t r a t i g r a p h i c data are shown i n F i g . 22. A number of important observations emerge from these studies. F i r s t l y , the b e l t contains rocks ranging i n age from Proterozoic to Middle Pennsylvanian. Younger rocks may be present but diagnostic f o s s i l s have not been found. Secondly, i n the Lay Range, Pennsylvanian rocks are intruded by the P o l a r i s ultramafite. Thirdly, the Pennsylvanian rocks, of similar age to the lower part of the Cache Creek Group west of the Pinchi Fault, consist of volcanic sandstone, agglomerate, carbonate pods, red r a d i o l a r i a n chert and a r g i l l i t e . I t therefore appears that rocks of similar age, association and metamorphic grade are found on both sides of the Pinchi Fault and there i s no obvious j u s t i f i c a t i o n for c a l l i n g 158 F i g . 22 Generalized diagram i l l u s t r a t i n g l o c a t i o n of major geologic units i n c e n t r a l B r i t i s h Columbia i n r e l a t i o n to the problematical b e l t of Paleozoic rocks between the Takla Group and the Omineca Geanticline Cstippled area). The stratigraphy i n t h i s b e l t was reported on by Monger (1973). Area "A" i n the Lay Range contains a highly faulted section consisting of (a) Early Pennsylvanian (?) volcanic sandstone and agglomerates, and (b) Middle Pennsylvanian carbonate, breccia and sandstone. Area "B", near Germansen Landing, contains a section extending from l a t e s t Proterozoic to possible M i s s i s s i p p i a n . 15 9 the f a u l t a "suture zone." However, i n the eastern b e l t a much longer depositional history i s recorded extending from the Proterozoic to the Upper Paleozoic. There are also differences i n l i t h o l o g i e s between the two b e l t s , the most important of which are the paucity of bedded chert and the occurrence of an appreciable thickness of volcanic sandstone and agglomerate within the b e l t to the east of the f a u l t . Whether these two factors present s u f f i c i e n t j u s t i f i c a t i o n to c a l l the Pinchi Fault a suture zone i s open to speculation. Perhaps the term may equally well be applied to the major f a u l t which separates the Omineca Geanticline from the Paleozoic rocks to the east ( i . e . the Wolverine F a u l t ) . Paleontological evidence suggests that the Pinchi Fault separates two l a t e Paleozoic faunal belts (Monger and Ross, 1970) but there i s uncertainty as to whether the d i f f e r e n t faunas are the r e s u l t of l o c a l v a r i a t i o n i n depositional environment or the "juxtaposition of o r i g i n a l l y i s o l a t e d biogeographic provinces by major c r u s t a l movements." On account of A s i a t i c faunal a f f i n i t i e s , Wilson (1968) suggested that the western part of the Canadian C o r d i l l e r a had d r i f t e d across from A s i a . The c o l l i s i o n zone as represented by Wilson l i e s to the west of the Omineca Geanticline and presumably follows the Pinchi or Wolverine f a u l t zones. 160 Evidence for s t r i k e - s l i p movement on the Pinchi Fault The length (450 km) and straightness of the Pinchi Fault suggest that at some time i n i t s history, i t may have been a s t r i k e - s l i p f a u l t . In the McConnell Creek map-area, Upper T r i a s s i c Takla Group rocks are found astride the continuation of the f a u l t , suggesting that i f there was s i g n i f i c a n t s t r i k e - s l i p displacement i t must have taken place p r i o r to the Late T r i a s s i c . As the f a u l t cuts across folded Cache Creek Group limestones, s t r i k e - s l i p movement probably post-dates Permo-Triassic (F^) deformation and may have been contemporaneous with the T r i a s s i c F^ deformation i n the blueschists. I t i s inter e s t i n g to speculate whether o b l i q u e - s l i p movement on the f a u l t could have given r i s e to the F^ structures at Pinchi. These structures are characterized by moderately plunging folds (55°) which trend sub-parallel to the Pinchi Fault and are associated with mullion structures i n incompetent rocks and a crenulate l i n e a t i o n ( I ^ ) p a r a l l e l to the f o l d axes. If d i p - s l i p movement on the f a u l t resulted i n elevation of the Pinchi Geanticline during the Middle and/or Upper T r i a s s i c , i t i s conceivable that the F 2 structures i n the blueschists may well have formed as the r e s u l t of r i g h t - l a t e r a l o b l i q u e - s l i p movement on the Pinchi Fault. Steeply plunging folds adjacent to major f a u l t zones have also been described i n the South 161 Island of New Zealand. L i l l i e 01964) has suggested that they formed during an episode of s t r i k e - s l i p f a u l t i n g . P o s s i b i l i t y that the Pinchi Geanticline was overlain  by oceanic crust There i s some evidence which suggests that the Cache Creek Group i n central B r i t i s h Columbia may have been over-l a i n by oceanic crust. The Upper T r i a s s i c rocks at Pinchi contain abundant detritu s which indicates erosion of a landmass consisting of amphibolitized gabbro, basic volcanics and cherts. The presence of conglomerate and carbonaceous wood fragments i n the Upper Tr i a s s i c . suggests that the landmass was close-by, most l i k e l y on the s i t e of what i s now the Pinchi Geanticline. South of Pinchi Lake, a serpen-tinite-amphibolitized gabbro-diabase-basalt sequence i s overlain by Upper T r i a s s i c (?) pebble conglomerates, s i l t -stones and limestones. I t i s suggested that these basic rocks might represent a downfaulted remnant of the cover of the Pinchi Geanticline during the Upper T r i a s s i c and that the basalt/pebble conglomerate contact i s a disconformity. Absence of metamorphic zonation within the Cache  Creek Group Ernst (1971) has related progressive metamorphic zonations i n Japan, C a l i f o r n i a and the Alps to descent of li t h o s p h e r i c plates down subduction zones. Reconnaissance 162 work by Armstrong (1949) and the writer indicates that the bulk of the Cache Creek Group west of the f a u l t zone has been metamorphosed i n the lower greenschist f a c i e s , and apart from the elongate fault-bounded wedge of blue-s c h i s t adjacent to the f a u l t there i s apparently no change i n metamorphic grade. There i s therefore no evidence within the Cache Creek Group of a metamorphic zonation which could be attributed to plate descent. Tectonic Models Subduction model If hypotheses involving tectonic overpressures are discarded (p. 69), the most plausible mechanism for the generation of blueschists i s by subduction. This model involves descent of an oceanic plate under the continental crust and formation of an eastward dipping Benioff zone. Downwarping of the earth's crust i s accompanied by deep b u r i a l , deformation and metamorphism of the sediments which accumulated i n an offshore oceanic trench. Consumption of oceanic crust i s believed to be related to c a l c - a l k a l i n e plutonism and volcanism above a descending plate (Dickenson, 1968). Therefore the absence of Permo-Triassic volcanics or plutonics to the east of the Pinchi Fault could be looked upon as a major drawback to the subduction model. However, 163 F i g . 23 Subduction model 164 major s t r i k e - s l i p movements on the f a u l t may have taken place and the igneous rocks associated with the subduction have to be sought elsewhere. One p o s s i b i l i t y i s the Asitka Group which i s situated i n the McConnell Creek map-area 240 km north of Fort St. James. I t i s considered to be of Permo-Triassic age (Monger, 1973) and consists of 2000 m of r h y o l i t e and minor andesite. Intrusives are present i n the Omineca Geanticline and could represent the source for overlying volcanics which have since been eroded, i n which case the T e r t i a r y K-Ar dates (Douglas et a l . , 1970) would r e f l e c t a regional u p l i f t or a reheating event. The Omineca Geanticline may therefore represent the high temperature-high pressure zone i n a Permo-Triassic paired metamorphic b e l t which formed above an active east-dipping subduction zone. On relaxation of the stresses which caused subduction, i s o s t a t i c readjustment of the deeply buried sediment p i l e gave r i s e to a regional u p l i f t r e s u l t i n g i n the formation of the Pinchi Geanticline. The formation of the Quesnel Trough i to the east of the Pinchi Geanticline may well have been caused by downwarping of the crust above the exhumed subduction zone. Such a mechanism i s believed to be responsible,for the formation of the Po Basin and the Great Valley of C a l i f o r n i a (Ernst, 1971). The Pinchi Fault may o r i g i n a l l y have been one of the major f a u l t s i n the 165 subduction zone. During the u p l i f t , the sense of movement may well have been reversed and deeply buried sediments such as the blueschist-bearing fault-block exploited the f a u l t zone as an easy upward path i n establishing i s o s t a t i c equilibrium. There are three main drawbacks to t h i s model. F i r s t l y , i t i s d i f f i c u l t to explain the occurrence of o p h i o l i t e s exposed at the surface of the u p l i f t e d Pinchi Geanticline. Presumably, the o p h i o l i t e s were thrust over the Cache Creek Group either from the west or the east. Secondly, i t i s to be expected that a regional u p l i f t of the metasedimentary p i l e should give r i s e to a progressive metamorphic zonation with respect to the Pinchi Fault as seems to have happened i n Japan, the Alps and C a l i f o r n i a (Ernst, 1971). However, the bulk of the Cache Creek Group i s i n the lower greenschist fa c i e s and the blueschists form an i s o l a t e d fault-bounded wedge adjacent to the f a u l t . The t h i r d drawback i s the apparent absence of c a l c - a l k a l i n e plutonism and volcanism contemporaneous with subduction. A possible explanation could be that the subduction zone had a very shallow dip and was s i m i l a r to an obduction zone. Coleman (1971) states that obduction zones are characterized by a complete lack of volcanic a c t i v i t y . However, a shallow dipping obduction zone would make i t d i f f i c u l t to explain the high l i t h o s t a t i c pressures necessary to create the P i n c h i blueschists without resorting to tectonic overpressures. 166 F i g . 24 Subduction-obduction model i l l u s t r a t i n g evolution of main tectonic features i n central B r i t i s h Columbia (a) During the Pennsylvanian, rocks of the S l i d e Mountain Group were obducted over the Proterozoic and Lower Cambrian rocks con-s t i t u t i n g the Omineca Geanticline (after Monger, et al.3 1972). (b) The formation of an east-wards dipping subduction zone gave r i s e to blue-s c h i s t f a c i e s metamorphism and deformation of Cache Creek Group sediments. (c) An i s l a n d arc impinged on the subduction zone and because i t contained a large thickness of buoyant low density material, was not subducted. Obduction of an o p h i o l i t e slab, either from the east or west appears to have accompanied t h i s event. The crust below the Pinchi Geanticline may well have been thickened by underthrusting during or a f t e r t h i s period to give the observed depth of 30 km to the M-discontinuity. (d) A change i n o r i e n t a t i o n of the stress regime, resulted i n r i g h t - l a t e r a l o b l i q u e - s l i p movement along the Pinchi Fault zone and u p l i f t of the Pinchi Geanticline. Blueschist and serpentinized ultramafite d i a p i r s were emplaced along the f a u l t . Detritus from o p h i o l i t i c rocks exposed on the Pinchi Geanticline was shed into the Quesnel 'T.r.ough. 167 Pinchi Foult Cache Creek Group sediments Pinchi Geanticline I Quesnel Trough elastics limestone v basalt c gabbro II//}/ peridotite 20 km (d) LATE Rocky Mt. Trench ( c ) EARLY TRIASSIC melting (?) Island arc trench obducted slab (Slide Mt (a) PENNSYLVANIAN 168 Subduction/obduction model Because of the drawbacks of the subduction model, a second model i s suggested which involves a combination of subduction and obduction (Fig. 24). At a number of l o c a l i t i e s i n the C o r d i l l e r a , late Paleozoic volcanics, cherts and minor volcanics o v e r l i e older miogeoclinal rocks and i t has been suggested (Monger et a l . , 1972; Dercourt, 1972) that the late Paleozoic rocks are allochthonous, having been thrust easterly over the miogeoclinal e l a s t i c s con-s t i t u t i n g the Omineca Geanticline during the Caribooan Orogeny. This s i t u a t i o n i s i l l u s t r a t e d i n the eastern half of F i g . 24a. During the Pennsylvanian, an i s l a n d arc with associated limestones was situated at an unknown distance from the North American craton (Fig. 24a). The limestones continued to grow i n thickness during the Pennsylvanian and Permian and may have formed an extensive b a r r i e r reef type deposit adjacent to the i s l a n d arc. At the close of the Permian an east-west p r i n c i p a l stress, possibly the r e s u l t of ocean f l o o r spreading, caused the formation of a subduction zone, and the sea f l o o r between the arc and the continent was consumed. There i s no c e r t a i n evidence that v o l c a n i c i t y accompanied the subduction. Eventually the island arc approached the subduction zone and because i t contained a large thickness of buoyant low density rocks, did not descend the subduction zone. The s c a r c i t y of arc 169 (a) development of en echelon (b) s t r i k e - s l i p movement fractures r e s u l t s i n "rhombo-chasm" formation (Carey, 1958) (c) d i a p i r i c i n t r usion of blueschist and serpen-t i n i z e d ultramafite along zones of low l i t h o s t a t i c pressure F i g . 25 Possible mechanism for in t r u s i o n of blueschists and serpentinized ultramafites along s t r i k e - s l i p f a u l t s . 170 type volcanic rocks associated with the limestones i n the Pinchi Geanticline i s a major problem. I t i s possible that the volcanic basement C?) of the islan d arc was subducted. The c o l l i s i o n of the island arc with the subduction zone was possibly followed by overthrusting or obduction of oceanic crust over the accumulation of Cache Creek Group sediments above the subduction zone. A change i n orientation of the p r i n c i p a l stresses brought about o b l i q u e - s l i p movement along the main f a u l t i n the subduction zone ( i . e . the Pinchi Fault ?) . I t i s suggested that a s t r i k e - s l i p component of movement on the f a u l t created zones of low pressure sporadically along the f a u l t zone , which were at once f i l l e d from below by i n t r u s i o n of low density f a u l t bounded d i a p i r s attempting to regain i s o s t a t i c equilibrium and o r i g i n a t i n g i n the depths of the former subduction zone. These low pressure zones can possibly be compared with Carey's (1956) rhombochasms (Fig. 25). Using t h i s mechanism, composite blocks of low average density containing blueschist, serpentinized ultramafite and eclogite could have worked t h e i r way up to a higher s t r u c t u r a l l e v e l and are therefore representative of the lower crust or mantle at that time. As the blueschists rose i n the crust, they cooled to temperatures below those necessary for argon retention (c. 200°C) and were deformed by F^. The Upper l?l T r i a s s i c K-Ar dates are considered to mark the close of the F~ deformation. 171 During the Upper T r i a s s i c , u p l i f t and erosion of the Pinchi Geanticline brought about deposition of conglomerates and f l y s c h i n the adjacent Quesnel Trough. The composition of the sediments i s compatible with a land-mass which consisted of basic rocks, cherts and u l t r a -mafites . This model i s preferred for a number of reasons. F i r s t l y , the presence of an obducted o p h i o l i t e cover to the Cache Creek Group i s compatible with the presence of basic detrit u s i n the Upper T r i a s s i c sediments. I t i s considered possible that the o p h i o l i t e sequence south of Pinchi Lake, which appears to be overlain conformably by Upper T r i a s s i c with no intervening Cache Creek Group sediments, i s a remnant of t h i s cover. Secondly, the presence of a major zone of c r u s t a l weakness associated with a subduction zone accounts for the development and subsequent r e a c t i v a t i o n of the Pinchi Fault. The absence of a metamorphic zonation i n the Cache Creek Group with respect to the Pinchi Fault i s problematical. Perhaps such zonations are c h a r a c t e r i s t i c of regions where large amounts of material have been subducted and exhumed. I t may be that a r e l a t i v e l y small volume of rock was sub-ducted and metamorphosed i n the blueschist facies along the Pinchi Fault and a s i g n i f i c a n t i s o s t a t i c anomaly which could produce a regional u p l i f t and exhumation of a metamorphic zonation was not formed. 172 Mesozoic and Tertiary events Mesozoic movement on the Pinchi Fault i s poorly recorded. Movement may well have occurred during the l a t e Mesozoic, as the f a u l t forms the western margin of the 170 m i l l i o n year old Hogem batholith. However, t h i s feature may also have formed as the r e s u l t of T e r t i a r y a c t i v i t y . The occurrence of Cretaceous or Paleocene conglomerate involved i n f a u l t i n g i n the Pinchi area i s convincing evidence for a period of T e r t i a r y Fault a c t i v i t y . I t was infe r r e d Cp. 67 ) that silica-carbonate rocks formed during a r e a c t i v a t i o n of the Pinchi Fault during the Eocene. Because of the association of carbonatization and intense F^ deformation i n the mine area, i t i s believed that formation of silica-carbonate rocks was contemporaneous with f a u l t i n g and F^ deformation. Consideration of the trend and plunge of F^ folds and of the sense of warping i n the limestone units suggests that r i g h t - l a t e r a l movement with a d i p - s l i p component may have taken place along f a u l t NO. 2 (Map VI) during F^. Northeasterly directed thrusting was widespread 240 km to the north during the Eocene (Eisbacher, 1970). The common occurrence of northeasterly dipping f a u l t planes suggests that s i m i l a r l y oriented stresses may have given r i s e to underthrusting i n the Pinchi area, but t h i s i s speculative. 173 Mercury mineralization occurred a f t e r the carbonatiz-ation, possibly associated with hot spring a c t i v i t y . Faulting continued aft e r mineralization as demonstrated by the occurrence of post-mineralization thrust f a u l t s i n the mine area. REFERENCES CITED A g r e l l , S.D., Brown, M.G., and McKie, D., 1965, Deerite, howieite, and zussmanite, three new minerals from the Franciscan of the Layton v i l l e D i s t r i c t , Mendocino County, C a l i f o r n i a C a b s . ) : Am. Mineral, v o l . 50, p. 278. Aitken, J.D., 1959, A t l i n Map-area, B r i t i s h Columbia: Geol. Surv. Canada, Mem. 3 07. Armstrong, J.E., 1942, The Pinchi Lake Mercury Belt: Geol. Surv. Canada, Paper 42-11. , 1944, Northern Part of the Pinchi Lake Mercury Belt: Geol. Surv. Canada, Paper 44-5. , 1949, Fort St. James Map-area, Cassiar and Coast D i s t r i c t s , B r i t i s h Columbia: Geol. Surv. Canada, Mem. 252. , 1966, Tectonics and-mercury deposits i n B r i t i s h Columbia: i n Tectonic History and Mineral Deposits of the Western C o r d i l l e r a , Can. Inst. Mining Metal, Spec. v o l . 8, p. 341. Armstrong, R.L., and Besancon, J . , 1970, A T r i a s s i c time scale dilemma: K-Ar dating of Upper T r i a s s i c mafic igneous rocks, eastern U.S.A. and Canada and post-Upper T r i a s s i c plutons, western Idaho, U.S.A.: Eclogae geol. Helv., v o l . 63/1, p. 15. Aumento, F., Loncarevic, B.D., and-Ross, D.I., 1970, Hudson Geotraverse: geology of the Mid-Atlantic Ridge at 45°N: P h i l . Trans, Roy. Soc. Lond., A., v o l . 268, p. 623. ; Ave L'Allemant, H.G., 1967, Structural and petrofabric analysis of an "Alpine type" p e r i d o t i t e . The l h e r z o l i t e of the French Pyrenees: Leid. geol. Meded., v o l . 42. Ave L'Allemant, H.G. and Carter, N.L., 1969, Syntectonic r e c r y s t a l l i z a t i o n of o l i v i n e C a b s . ) : Trans. Am. Geophys. Un., v o l . 50, p. 324. 175 Ave L'Allemant, H.G., Schiffmann, P.M., and Carter, N.L., 1968, Ductile Flow of p e r i d o t i t e and quart z i t e : Trans. Am. Geophys. Un., v o l . 49, p. 313. Bailey, E.H., Blake, M.C., and Jones, D.L., 1970, On-land Mesozoic crust i n C a l i f o r n i a Coast Ranges: U.S. Geol. Surv. Prof. Paper 7 00-C, p. C70. Bailey, E.H., Irwin, W.P., and Jones, D.L., 1964, Franciscan and related rocks and the i r significance i n the geology of western C a l i f o r n i a : C a l i f o r n i a Div. Mines and Geol. B u l l . , no. 183, 171 p. Banno, S., 1959, Aegerine-augites from c r y s t a l l i n e schists i n Shikoku: Geol. Soc. Japan Jour., v o l . 65, p. 652. _ , 19 64, Petrologic studies on Sanbagawa c r y s t a l l i n e s c hists i n the Bessi-Ino d i s t r i c t , c e n t r al Sikoku, Japan: Tokyo Univ. Fac. S c i . Jour., Sec. 11, v o l . 15, p. 203. Berry, M.J., Jacoby, W.R., N i b l e t t , E.R., and Stacey, R.A., 1971, A review of geophysical studies i n the Canadian C o r d i l l e r a : Can. Jour. Earth S c i . , v o l . 8, p. 788. Birch, F., and LeComte, P., 1960, Temperature-pressure plane for a l b i t e composition: Am. Jour. S c i . , v o l . 258, p. 209. Bischoff, J.L., and Fyfe, W.S., 1968, C a t a l y s i s , i n h i b i t i o n , and the calcite-aragonite problem. (I): Am Jour. S c i . , v o l . 266, p. 65. Blake, M.C., Irwin, W.P., and Coleman, R.G., 1967, Upside down metamorphic zonation, blueschist f a c i e s , along a regional thrust i n C a l i f o r n i a and Oregon: U.S. Geol. Survey Prof. Paper 575-C, p. C l . , 1969, Blueschist-facies metamorphism related to regional thrust f a u l t i n g : Tectonophys., v o l . 8, p. 237. Bloxam, T.W., 1956, Jadeite bearing metagreywackes i n C a l i f o r n i a : Am. Min., v o l . 41, p. 488. Bochkarev, V.S., and Pogorelov, B.S., 1967, New data on the chronologic boundary of the Permian and T r i a s s i c systems: Akad. Nauk. S.S.R. Doklady, v o l . 173, p. 153. 176 Boettcher, A.L., and Wyllie, P.J., 1968, The c a l c i t e -aragonite t r a n s i t i o n measured i n the system CaO-H^O-C0 2: Jour. Geol., v o l . 76, p. 314. Bonatti, E., Honnerez, J . , and Ferrara, G., 1971, P e r i d o t i t e gabbro-basalt complex from the equatorial Mid-Atlanti Ridge: P h i l . Trans. Roy. Soc. Lond., A., v o l . 268, p. 385. Borsi, S., and Ferrara, G., 1967, Determinazione delteta d e l l e rocce i n t r u s i v e d i Predazzo con i metodi del Rb/Sr e K/Ar: Miner. Petrogr. Acta, v o l . 13, p. 45. Bowen, N.L., and T u t t l e , F., 1949, The system MgO-Si0 2-H 20: Geol. Soc. Am. B u l l . , v o l . 60, p. 439. Brace, W.F., Ernst, W.G., and Kallberg, R.W., 1970, An experimental study of tectonic overpressure i n Franciscan rocks: Geol. Soc. Am. B u l l . , v o l . 81, p. 1325. Bramlette, M.N., 1946, The Monterey formation of C a l i f o r n i a and the o r i g i n of i t s s i l i c e o u s rocks: U.S. Geol. Surv. Prof. Paper 212. Brothers, R.N., 197 0, Lawsonite-albite s c h i s t s from northern most New Caledonia: Contr. Mineral and P e t r o l . , v o l . 25, p. 185. Brown, W.H., Fyfe, W.S., and Turner, F.J., 1962, Argonite i n C a l i f o r n i a glaucophane schists and the k i n e t i c s of the aragonite-calcite transformation: Jour. P e t r o l . , v o l . 3, p. 566. Burch, S.H., 1968, Tectonic emplacement of the Burro Mountain ultramafic body, Santa Lucia Range, C a l i f o r -n i a: Geol. Soc. Am. B u l l . , v o l . 79, p. 527. Caner, B., 1970, E l e c t r i c a l conductivity structure of the lower crust and upper mantle i n western Canada: Unpubl. Ph»D. thesis, Univ. of B r i t i s h Columbia. Carey, S.W., 1958, A tectonic approach to continental d r i f t : iri Continental D r i f t : A Symposium, University of Tasmania, p. 117. Chinner, G.A., 1960, P e l i t i c gneisses with varying ferrous/ f e r r i c r a t i o from Glen Clova, Angus, Scotland: Jour. P e t r o l . , v o l . 1, p. 178. 177 Clark, S.P., 1957a, A note on calcite-aragonite equilibrium: Am. Min., v o l . 42, p. 564. , 1957b, Heat flow at Grass V a l l e y , C a l i f o r n i a : Am. Geophys. Union Trans., v o l . 38, p. 239. Coleman, R.G., 1971, Plate tectonic emplacement of upper mantle pe r i d o t i t e s along continental edges: Jour. Geophys. Res. • v o l . 76, p. 1212. Coleman, R.G., and Clark, J.R., 1968, Pyroxenes i n the Blueschist facies of C a l i f o r n i a : Am. Jour. S c i . , v o l . 266, p. 43. Coleman, R.G., and Keith, T.E., 1971, A chemical study of serpentinization--Burro Mountain, C a l i f o r n i a : Jour. P e t r o l . , v o l . 12, p. 311. Coleman, R.G., and Lee, D.E., 1963, Glaucophane-bearing metamorphic rock of the Cazadero area, C a l i f o r n i a : Jour. P e t r o l . , v o l . 4, p. 260. Coleman, R.G., Lee, D.E., Beatty, L.B., and Brannock, W.W., 1965, Eclogites and e c l o g i t e s : t h e i r differences and s i m i l a r i t i e s : Geol. Soc. Am. B u l l . , v o l . 76, p. 483. Coleman, R.G., and Papike, J . J . , 1968, A l k a l i amphiboles from the blueschists of Cazadero, C a l i f o r n i a : Jour. P e t r o l . , v o l . 9, p. 105. Coombs, D.S., 1961, Some recent work on the lower grades of metamorphism: Aust. Jour. S c i . , v o l . 24, p. 203. Crawford, W.A., and Fyfe, W.S., 1964, Calcite-aragonite equilibrium at 100°C: Science, v o l . 144, p. 1569. Danner, W.R., 1964, i n Geological History of Western Canada: Alberta Soc. P e t r o l . Geol., Calgary, p. 109. , 1967, Organic, shallow-water o r i g i n of bedded chert i n the eugeosynclinal environment: Geol. Soc. Am. Abs., New Orleans, p. 42. Dawson, G.M., 1878, Report on explorations in B r i t i s h Columbia, c h i e f l y i n the basins of the Blackwater, Salmon, and Nechacco Rivers, and on Francois Lake: Geol. Surv. Canada, Rept. of Progr. 1876-1877, pt. I l l , p. 17. 178 Deer, W.A., Howie, R.A., and Zussman, J . , 1963, Rock Form-ing Minerals, vols. 1-5: Longmans, Green and Co. Ltd., London Dercourt, J . , 1972, The Canadian C o r d i l l e r a , the Hellenides, and the sea-floor spreading theory: Can. Jour. Earth S c i . , v o l . 9, p. 709. Dewey, J.F., and Bird, J.M., 197 0, Mountain belts and the new global tectonics: Jour. Geophys. Res., v o l . 75, p. 2625. Dickenson, W.R., 1968, Evolution of c a l c - a l k a l i n e rocks i n the geosynclinal system of C a l i f o r n i a and Oregon: Proc. of the Andesite Conference, Oregon, Dept. Geol. Mineral. Ind. B u l l . , v o l . 65, p. 143. Dietz, R.S., and Holden, J . C , 1966, Deep-sea deposits i n but not on the continents: Am. Assoc. Petroleum Geologists B u l l . , v o l . 50, p. 351. Douglas, R.J.W., Gabrielse, H., Wheeler, J.O., Stott, D.F., and Belyea, H.R., 1970, Geology of Western Canada: i n Geology and Economic Mineral Deposits of Canada, Canada Geol. Surv. Econ. Geol. Rept. I, p. 366. Dragert, H., 1970, A geomagnetic, depth-sounding p r o f i l e across c e n t r a l B r i t i s h Columbia: Unpubl. M. Sc. thesis, Univ. of B r i t i s h Columbia. Eisbacher, G.H.,1970, Tectonic framework of Sustut and Si f t o n Basins, B r i t i s h Columbia: i n Report of A c t i v i t i e s , A p r i l to October, 1969, Geol. Surv. Canada, Paper 70-1, pt. A, p. 36. , 1971, Tectonic framework of Sustut and S i f t o n Basins, B r i t i s h Columbia: i i i Report of A c t i v i t i e s , A p r i l to October, 1970, Geol. Surv. Canada, Paper 71-1, pt. A, p. 20. Engel, A.E.J., Engel, C.G., and Havens, R.G., 1965, Chemical c h a r a c t e r i s t i c s of oceanic basalts and the upper mantle: Geol. Soc. Am. B u l l . , v o l . 76, p. 719. Ernst, W.G., 1961, S t a b i l i t y r e l a t i o n s of glaucophane: Am. Jour. S c i . , v o l . 259, p. 735. , 1962, Synthesis, s t a b i l i t y r e l a t i o n s , and occurrence of riebeckite and riebeckite-arfvedsonite s o l i d solutions: Jour. Geol., v o l . 70, p. 689. 17 9 Ernst, W.G., 1963a, Polymorphism i n a l k a l i amphiboles: Am. Mineral., v o l . 48, p. 241. , 1963b, Significance of phengitic micas from low-grade s c h i s t s : Am. Mineral., v o l . 48, p. 1357. , 1965 , Mineral parageneses i n Franciscan metamorphic rocks, Panoche Pass, C a l i f o r n i a : Geol. Soc. Am. B u l l . , v o l . 76, p. 879. , 1970r Tectonic contact between the Franciscan melange and the Great Valley sequence—crustal expression of a Late Mesozoic Benioff Zone: Jour. Geophys. Res., v o l . 75, p. 88 6. , 1971a, Metamorphic zonations on presumably sub-ducted li t h o s p h e r i c plates from Japan, C a l i f o r n i a and the Alps: Contr. Mineral, and P e t r o l , , v o l . 34, p. 43. , 1971b, Do mineral parageneses r e f l e c t unusually high-pressure conditions of Franciscan metamorphism?: Am. Jour. S c i . , v o l . 270, p. 81. Ernst, W.G., Seki, Y., Onuki, H., and G i l b e r t , M.C., 197 0, Comparative study of low-grade metamorphism i n the C a l i f o r n i a Coast Ranges and the outer metamorphic b e l t of Japan: Geol. Soc. Am., Mem. 124. Essene, E.J., and Fyfe, W.S., 1967, Omphacite i n C a l i f o r n i a n rocks: Contr. Mineral, and P e t r o l . , v o l . 15, p. 1. Evans, H.T. J r . , Appleman, D.E., and Handwerker, D.S., 1963, The least squares refinement of c r y s t a l unit c e l l s with powder d i f f r a c t i o n data by an automatic computer indexing method (abs.): Amer. C r y s t a l l o g r . Assoc. Ann. Meet., March 28, 1963, Cambridge, Mass., Progr. Abstr., 42. Folk, R.L., 1968, Petrology of Sedimentary Rocks, University of Texas: Hemphill's, Austin. Freeze, A.C, 1942, Geology of Pinchi Lake, B r i t i s h Columbia: unpubl. Ph.D. thesis., Princeton University. French, B.M., 1964, Graphitization of organic material i n a progressively metamorphosed Precambrian iron formation: Science, v o l . 146, p. 917. 180 French, B.M., 1964, Graphitization of organic material i n a progressively metamorphosed Precambrian iron formation: Science, v o l . 146, p. 917. , 1966, Some geological implications of equilibrium between graphite and a C-H-0 gas phase at high temperatures and pressures: Rev. Geophys, v o l . 4, p. 223. French, B.M., and Eugster, H.P., 1965, Experimental control of oxygen fugacities by graphite-gas equilibriums: Jour. Geophys. Res., v o l . 70, p. 1529. Friedman, G.M., 1959, I d e n t i f i c a t i o n of carbonate minerals by staining methods: Jour. Sed. P e t r o l . , v o l . 29, p. 87 . Gray, J.G., 1938, Fort Fraser map-area, B r i t i s h Columbia: Geol, Surv. Canada, Paper 38-14. Green, D.H., and Ringwood, A.E., 1967, An experimental i n -vest i g a t i o n of the gabbro to ec l o g i t e transformation and i t s p e t r o l o g i c a l applications: Geochim. et Cos-mochim. Acta, v o l . 31, p. 767. Greenwood, H.J., 1961, The system NaAlSi20g-H20-argon: t o t a l pressure and water pressure i n metamorphism: Jour. Geophys. Res., v o l . 66, 3923. 1967, Mineral e q u i l i b r i a i n the system MgO-Si02~ H 2 O - C O 2: i n Researches i n Geochemistry, Abelson, P.H., ed.: John Wiley and Sons, New York, London, Sydney, p. 542. Gresens, R.L., 1969, Blueschist a l t e r a t i o n during sepentiniz-ation: Contr. Mineral and P e t r o l . , v o l . 24, p. 93. Henderson, F.B. I l l , 1969, Hydrothermal a l t e r a t i o n and ore deposits i n serpentinite-type mercury deposits: Econ. Geol., v o l . 64, p. 489. Hey, M.H., 1954, A new review of the c h l o r i t e s : Mineralog. Mag., v o l . 30, p. 277. Hinrichsen, T., and Schurmann, K., 1972, Mineral reactions i n b u r i a l metamorphism: Neues Jahrb. Mineral., Monatch., no. 1, p. 35. Hoffman, C , 1972, Natural and synthetic ferroglaucophane: Contr. Mineral, and P e t r o l . , vol.. 34, p. 135. H o l l i s t e r , L.S., 1966, Garnet zoning: an interpretation based on Rayleigh f r a c t i o n a t i o n model: Science, v o l . 154, p. 1647. 181 Irvine, T.N. and Baragar, W.R.A., 1971, A guide to the chemical c l a s s i f i c a t i o n of the common volcanic rocks: Canad.. Jour. Earth S c i . , v o l . 8, p. 523. Ito, K, and Kennedy, G.C., 1971, An experimental study of the basalt-garnet gr a n u l i t e - e c l o g i t e t r a n s i t i o n : i n The Structure and. Physical Properties of the Earth's Crust: Geophys. Monograph 14, John G. Heacock, editor, Amer. Geophys. Union Publication, Washington, D .C . , p. 303. Iwasaki, M., 1963, Metamorphic rocks of the Kotu-Bizan area, eastern Shikoku: Univ. Tokyo Jour. Fac. S c i . , Sec. I I , v o l . 15, p. 1. Jamieson, J . C , 1953, Phase e q u i l i b r i a i n the system calcite-aragonite: Jour. Chem. Phys., v o l . 21, p. 1385. Johannes, W., 1968, Experimental investigation of the reaction: f o r s t e r i t e + H2O = serpentine + brucite: Contr. Mineral, and P e t r o l . , v o l . 19, p. 309. 1969, An experimental investigation of the system MgO - S i 0 2-H 2 0 - C 0 2: Am. Jour. S c i . , v o l . 267, p. 1083. J o l l y , W.T., and Smith, R.E., 1972, Degradation and metamor-phic d i f f e r e n t i a t i o n i n the Keweenawan t h o l e i i t i c lavas of northern Michigan, U.S.A.: Jour. P e t r o l . , v o l . 13, p. 273. Kerrick, D.M. and Cotton, W.R., 1971, S t a b i l i t y r e l a t i o n s of jadeite pyroxene i n Franciscan metagreywackes near San Jose, C a l i f o r n i a : Am. Jour. S c i . , v o l . 271, p. 350. Kitahara, S., Takenouchi, S., and Kennedy, G.C, 1966 , Phase r e l a t i o n s i n the system MgO - S i 0 2 _ H 2 0 at high temperatures and pressures: Am. Jour. S c i . , v o l . 264, p. 223. Koo, J.H., 1968, Geology and mineralization i n the Lorraine Property Area, Omineca Mining D i v i s i o n , B r i t i s h Columbia: Unpubl. M.Sc. thesis, Univ. of B r i t i s h Columbia. Krauskopf, K.B., 1967, Introduction to Geochemistry: McGraw-H i l l , New York. Landis, C.A., 1971, Graphitization of dispersed carbonaceous material i n metamorphic rocks: Contr. Mineral, and P e t r o l . , v o l . 30, p. 34. 182 L i l l i e , A.R., and Brothers, R.N., 1970, The geology of New Caledonia: New Zealand Jour. Geol. Geophys., v o l . 13, p. 145. L i l l i e , A.R., and Gunn, B.M., 1964, Steeply plunging folds i n the Sealy Range, Southern Alps: New Zealand Journ. Geol. Geophys., v o l . 7, p. 403. Liou, J.G., 1971, P-T s t a b i l i t i e s of laumontite, wairakite lawsonite, and related minerals i n the system CaAl 2Si20g-Si0 2-H 20: Jour. P e t r o l . , v o l . 12, p. 379. L i t t l e j o h n , A.L., 1972, A comparative study of l h e r z o l i t e nodules i n b a s a l t i c rocks from B r i t i s h Columbia: Unpubl. M. Sc. thesis, University of B r i t i s h Columbia. Logan, B.W. , Rezak., R., and Ginsburg, R.N., 1964, C l a s s i f i c a t i o n and environmental si g n i f i c a n c e of a l g a l stromatolites: Jour. Geol., v o l . 72, p. 68. Loney, R.A., Himmelberg, G.R., and Coleman, R.G., 1971, Structure and petrology of the alpine-type p e r i d o t i t e at Burro Mountain, C a l i f o r n i a , U.S.A.: Jour. P e t r o l . , v o l . 12, p. 245. Lord, C.S., 1949, McConnell Creek map-area, Cassiar D i s t r i c t , B r i t i s h Columbia: Geol. Surv. Canada, Mem. 251. MacDonald, G.A., 1968, Composition and o r i g i n of Hawaiian lavas: Geol. Soc. Am., Mem. 116, p. 477. McConnell, J.D.C., and McKie, D., 1960, The k i n e t i c s of the ordering process i n t r i c l i n i c NaAlSi30g: Mineralog. Mag., v o l . 32, p. 436. McTaggart, K.C., 1971, On the o r i g i n of ultramafic rocks: Geol. Soc. Am. B u l l . , v o l . 82, p. 23. Miyashiro, A., 1964, Oxidation and reduction i n the earth's crust with special reference to the r o l e of graphite: Geochim. et Cosmochim. Acta, v o l . 28, p. 717. , 1972, Metamorphism and related magmatism i n plate tectonics: Am. Jour. S c i . , v o l . 272, p. 629, Miyashiro, A., and Seki, Y., 1958, Mineral assemblages and subfacies of the glaucophane-schist f a c i e s : Jap. Jour. Geol. and Geogr., v o l . 29, p. 199. 183 Monger, J.W.H., 1969, Stratigraphy and structure of Upper Paleozoic rocks, northeast Dease Lake map-area, B r i t i s h Columbia; Geol. Surv. Canada, Paper 68-48. , 1973, Upper Paleozoic rocks of the western Canadian C o r d i l l e r a : i n Report of A c t i v i t i e s , A p r i l to October, 1972, Geol. Surv. Canada, Paper 73-1, pt. A, p. 27. Monger, J.W.H. and Ross, CA., 1971, D i s t r i b u t i o n of Fusulin-aceans i n the western Canadian C o r d i l l e r a : Can. Jour. Earth S c i . , v o l . 8, p. 259. Monger, J.W.H., Souther, J.G., and Gabrielse, H., 197 2, Evolution of the Canadian C o r d i l l e r a : a plate tectonic model: Am. Jour. S c i . , v o l . 272, p. 577. Moores, E.M., and Vine, F.J., 1971, The Troodos Massif, Cyprus and other o p h i o l i t e s as oceanic crust: evaluation and implications: P h i l . Trans. Roy. Soc. Lond., A, v o l . 268, p. 443. Morgan, B.A., 1970, Petrology and mineralogy of eclogite and garnet amphibolite from Puerto Cabello, Venezuela: Jour. P e t r o l . , v o l . 11, p. 101. Newton, R.C, Goldsmith, J.R., and Smith, J.V., 1969, Aragonite c r y s t a l l i z a t i o n from strained c a l c i t e at reduced pressures and i t s bearing on aragonite i n low-grade metamorphism: Contr. Mineral, and Pe t r o l . , v o l . 22, p. 335. Newton, R.C. and Kennedy, G.C., 1963, Some equilibrium reactions i n the j o i n CaAl2Si20g-H 20: Jour. Geophys. Res., v o l . 68, p. 2 967. Newton, R.C, and Smith, J.V., 1967, Investigations concern-ing the breakdown of a l b i t e at depth i n the earth: Jour. Geol., v o l . 75, p. 2 68. Nitsch, K.-H., 19 68, Die S t a b i l i t a t von Lawsonit: Naturwiss., v o l . 55, p. 388. , 1972, Das P-T-X c o S t a b i l i t a t s f e l d von Lawsonit: Contr. Mineral, and. 2 P e t r o l . , v o l . 34, p. 116. Oxburgh, E.R., and Turcotte, D.L., 1971, Origin of paired metamorphic belts and c r u s t a l d i l a t i o n i n island arc regions: Jour. Geophys. Res., v o l . 76, p. 1315. 184 Page, B., 1972, Oceanic crust and mantle fragment i n sub-duction complex near San Luis Obispo, C a l i f o r n i a : Geol. Soc. Am. B u l l . , v o l . 83, p. 957. Page, N.J., 1967, Serpentinization at Burro Mt., C a l i f o r n i a : Contr. mineral P e t r o l . , v o l . 14, p. 321. Pettijohn, F.J., 1957, Sedimentary Rocks: Harper and Row, New York. Plas, L. van der, 1959, Petrology of the northern Adula region, Switzerland (with p a r t i c u l a r reference to glaucophane bearing rocks): Leidse Geol. Meded., v o l . 24, p. 415. Raleigh, C.B., 1963, Fabrics of naturally and experimentally deformed o l i v i n e : Ph.D. thesis, Univ. of C a l i f o r n i a , Los Angeles, C a l i f o r n i a . , 1965, Structure and petrology of an alpine peridote Cypress Island, Washington, U.S.A.: B e i t r . Miner. Petrog., v o l . 11, p. 719. Rich, J.L., 1951, Three c r i t i c a l environments of deposition, and c r i t e r i a for recognition of rocks i n each of them: B u l l . Geol. Soc. Am., v o l . 62, p. 1. Ringwood, A.E., 1962, A model for the upper mantle: Jour. Geophys. Res., v o l . 67, p. 857. Robie, R.A.., and Waldbaum, D.R., 1968, Thermodynamic properties of minerals and re l a t e d substances at 298.15°K and one atmosphere (1.013b) pressure and at higher temperatures. U.S. Geol. Surv. B u l l . 1259. Roedder, E., 1967, F l u i d inclusions as samples of ore f l u i d s : i n Geochemistry of Hydrothermal Ore Deposits, H.L. Barnes, ed., Holt, Rinehart and Winston, Inc., New York. Roots, E.F., 1954, Geology and mineral deposits of the Aiken Lake map-area: Geol. Surv. Canada, Mem. 274. Rucklidge, J . C , Gasparrini, E. , Smith, J.V., and Knowles, C.R., 1971, X-ray emission microanalysis of rock forming minerals: Can. Jour. Earth S c i . , v o l . 8, p. 1171. 185 Scarfe, CM., and Wyllie, P.J., 1967, Serpentine dehydration curves and t h e i r bearing on serpentinite deformation in orogenesis: Nature, v o l . 215, p. 945. Schuiling, R.D., and Vink, B.W., 1967, S t a b i l i t y r e l a t i o n s of some titanium minerals (sphene, perovskite, r u t i l e , anatasel: Geochim. Cosmochim. Acta, v o l . 31, p. 2399. Selwyn, A.R.C, 1872, Journal and report of preliminary explorations i n B r i t i s h Columbia: Geol. Surv. Canada, Rept. of Progr. 1871-72, p. 16. , 1877, Report on exploration i n B r i t i s h Columbia i n 1875: Geol. Surv. Canada, Rept. of Progr. 187 5-76, p. 29. Shido, F., 1959, Notes on rock-forming minerals (9). Horn-blende bearing eclogite from Gongenyama of Higasi-Akaishi i n the Bessi D i s t r i c t , Sikoku: Geol. Soc. Japan Jour., v o l . 65, p. 701. Souther, J.G., 1970, Volcanism and i t s rel a t i o n s h i p to recent c r u s t a l movements i n the Canadian C o r d i l l e r a : Can. Jour. Earth S c i . , v o l . 7, p. 553. Souther, J.G., and Armstrong, J.E., 1966, North central b e l t of the c o r d i l l e r a of B r i t i s h Columbia: i n Tectonic history and mineral deposits of the western C o r d i l l e r a , Can. Inst. Min. Metal., S p e c , v o l . 8, p. 171. Stevenson, J.S. , 1940, Mercury deposits of B r i t i s h Columbia: Dept. of Mines, B.C., B u l l , No. 5, p. 18. Stueber, A.M. and Murthy, V.R., 1966, Strontium isotope and a l k a l i element abundances i n ultramafic rocks: Geochim. Cosmochim. Acta, v o l . 30, p. 1243. Suppe, J . , and Armstrong, R.L., 1972, Potassium-argon dating of Franciscan metamorphic rocks: Am. Jour. S c i . , v o l . 272, p. 217. Sutherland-Brown, A., 1963, Geology of the Cariboo River area, B r i t i s h Columbia: B.C. Dept. of Mines, B u l l . 47. 18 16 Taylor, H.P., J r . , and Coleman, R.G., 1968, 0 /0 r a t i o s of coexisting minerals i n glaucophane bearing metamor-phic rocks; Geol. Soc. Am. B u l l . , v o l . 79, p. 1727. 186 Thompson, A.B., 1970, Laumontite e q u i l i b r i a and the z e o l i t e f a c i e s : Am. Jour. S c i . , v o l . 269, p. 267. , 1971, Pco9 •"•n low-grade metamorphism; z e o l i t e , carbonate, clay mineral, prehnite r e l a t i o n s i n the system CaO-Al203-Si02-C02-H20: Contr. Mineral, P e t r o l . , v o l . 33, p. 145. Thompson, M.L., 1965,Pennsylvanian and Early Permian fus u l i n i d s from Fort St. James area, B r i t i s h Columbia, Canada: Jour. Paleontol. v o l . 39, p. 224. Tipper, H.W., 1971, G l a c i a l geomorphology and Pleistocene history of central B r i t i s h Columbia: Geol. Surv. Canada, B u l l . 196. Tozer, E.T., 1964, The T r i a s s i c Period in The Phanerozoic Time Scale: Geol. Soc. Lond. Quart. Jour., v o l . 1205, p. 207. Trumpy, R., 1960, Paleotectonic evolution of the central and western Alps: Geol. Soc. Am. B u l l . , v o l . 71, p. 843. Turner, F.J., 1968, Metamorphic Petrology: McGraw-Hill Co., New York. Turner, F.J., and Verhoogan, J . , 1960, Igneous and Metamorphic Petrology: McGraw-Hill, New York. Turner, F.J., and Weiss, L.E., 1963, Structural Analyses of Metamorphic Tectonites: McGraw-Hill, New York. Vance, J.A., 1968, Metamorphic aragonites i n the prehnite-pumpellyite f a c i e s , Northwest Washington: Am. Jour. S c i . , v o l . 266, p. 299. Velde, B., 1965, Phengitic micas: synthesis, s t a b i l i t y and natural occurrence: Am. Jour. S c i . , v o l . 263, p. 886. 18 Wenner, D.B., and Taylor, H.P. J r . , 1969, 6D and 60 studies of serpentinization of ultramafic rocks C a b s . ) : Geol. Soc. Am. Annual Meeting, A t l a n t i c C i t y , N.J. , 1971, Temperatures of serpentinization of u l t r a -mafic rocks based on O-^/O.lS f r a c t i o n a t i o n between coexisting serpentine and magnetite: Contr. Mineral. P e t r o l . , v o l . 32, p. 165. 187 White, D.E., 1967, Mercury and base-metal deposits with associated thermal and mineral waters: i n Geo-chemistry of Hydrothermal Ore Deposits, H.L. Barnes ed., Holt, Rinehart and Winston, Inc., New York, p. 575. White, W.H., Erickson, G.P., Northcote, K.E., Dirom, G.E., and Harakal, J.E., 1967, Isotopic dating of the Guichon Batholith: Canad. Jour. Earth S c i . , v o l . 4 p. 677. Wilson, G., 1953, Mullion and rodding structures i n the Moine series of Scotland: Geol. Assoc. P r o c , v o l . 64, p. 118. Wilson, J.T., 1968, S t a t i c of mobile earth: the current s c i e n t i f i c revolution: Proc. Am. P h i l . Soc., v o l . 112, p. 309. Winchell, A.N., 1936, A t h i r d study of c h l o r i t e : Am. Mineral., v o l . 21, p. 642. Yoder, H.S. and T i l l e y , C.E., 1962, Origin of basalt magma an experimental study of natural and synthetic rock systems: Jour. P e t r o l . , v o l . 3, p. 342. Zwart, H., 1969, Metamorphic facies series i n the European orogenic belts and t h e i r bearing on the causes of orogeny: i n Age Relations i n High Grade Metamorphic Terrains, Geol. Assoc. Canada, Spec. Paper no. 5, p. 7. APPENDIX I FOSSIL LOCALITIES IN THE PINCHI AREA The positions of the following f o s s i l l o c a l i t i e s are shown i n Map I. In the l i s t which follows, l o c a l i t i e s assigned by other authors are given i n brackets immediately a f t e r the l o c a l i t y number used i n t h i s study. A. Collected by I. Paterson; i d e n t i f i e d by W.R. Danner Fauna from f o s s i l l o c a l i t i e s F - l to F-6 are given on p. 40 . B. Collected by J.E. Armstrong and i d e n t i f i e d by various workers F -7 (.51 : southeast end of a ridge l y i n g between Pinchi v i l l a g e , Stuart Lake and Pinchi Lake; Tvitioites; age; probably Upper Pennsylvanian. F -8 (.81 : south side of Mount Pope; Lonsdaleia-l i k e c o r a l and c r i n o i d discs and stems. Age; Carboniferous (?1. C. Collected and i d e n t i f i e d by M.L. Thompson. F-9 (BC-12); Eosohubevte I la ; Millerella; Favamillevella . F-10 (BC-141; Fusulinella jamesensis: Fusulinella; Pseudostaffella sandersoni; Millevella; ParamiIlereI la. 189 F- 11 (BC- 17} : Pseudostaffella sandevsoni; F u s u l i n e l l a jamesensis C?); P a v a m i l l e r e l l a ; M i l l e v e l l a F- 12 (BC- 11) : Eosehubevtella; F u s u l i n e l l a . F- 13 CBC- 13) : Fusulina ? ocoasa ? F- 14 C B C - 15) : Quasifusulina popensis n. sp.; Sohubertella popensis n. sp. F- 15 C B C - 18) : Quasifusulina ?; Sohubertella. F- 16 (BC-•52) : Sohubertella ?; F u s u l i n e l l a . F- 17 tBC-•24) : P r o f u s u l i n e l l a ?; Pseudostaffella; F- 18 tBC- 23) : Fusulina ? ocoasa n. sp.; Pseudostaffella; Schubevtella. F- 19 CBC- 49) : Fusulina p i t v a t i n. sp.; F u s i l i n a ; F u s u l i n e l l a ; Sohubertella. F- 20 (BC-•48} : Fusulina p i t r a t i n. sp.; Fusulina; F u s u l i n e l l a ; Sohubertella. F-•21 C B C - •21} : Quasifusulina amerioana n. sp.; T r i t i o i t e s pinohiensis n. sp.; Sohubertella kingi? Oketaella; T r i t i o i t e s stuartensis n. sp.? F-•22 C B C - •53) : Pseudoschwagerina arta; T r i t i o i t e s stuartensis n. sp. F- 23 CBC- 19). : Fusulina p i t r a t i ? ; F u s u l i n e l l a ; Eosohuber-t e l l a ; Pseudostaf f e l l a . 190 D. Collected by I.A. Paterson, i d e n t i f i e d by W.R. Danner. F-24: F u s u l i n e l l a jamesensis; A k i y o s h i e l l a sp.? Middle Pennsylvanian. F-25: Sahwagerina sp. Early Permian. F-26: Tetvataxis sp. Pennsylvanian and Permian range. A l g a l structures, bryozoa and echinoderm c l a s t s are also present. APPENDIX II MINERALOGY A n a l y t i c a l Methods Approximately 200 thi n sections and 10 polished sections of greenstones, glaucophanitic rocks and carbonates were examined. Whole rock or single mineral d i f f r a c t i o n traces were used to confirm o p t i c a l i d e n t i f i c a t i o n of minerals. X-ray standards were prepared by repeatedly centrifuging the 200-250 mesh f r a c t i o n i n diodomethane or bromoform. Measurements of r e f r a c t i v e indices were not made because of uncertainty of c o r r e l a t i o n of r e f r a c t i v e index with chemical composition. C e l l parameters of three glaucophanes and one j a d e i t i c pyroxene are given along with the d i f f r a c t i o n method i n Table 7. To i l l u s t r a t e textures i n carbonate rocks, slabs from 7 dolomitic limestones were etched with HCl and stained using a l i z a r i n e red solution and F i e g l ' s solution (Friedman, 19591. Because of fine grain s i z e , f a b r i c heterogeniety and d i f f i c u l t y of recognition of phases, accurate modal analyses are d i f f i c u l t to obtain. However, modal analyses using a Zeiss micrometer eyepiece were obtained for four rocks and 192 estimates made for many more (.Tables 20, 22, 24 and 25). Mineral compositions were determined using an A.R.L. electron microprobe at the University of Washington. Operat-ing conditions were as follows: 15 KV accelerating p o t e n t i a l , 1-2 u. spot s i z e , specimen current of 0.09 y-amps for major elements and 0.18 y-amps for minor elements. The time taken for a constant amount of current to pass was monitored for a l l points. A check was also made on the specimen current p r i o r to analyses of each mineral i n order to monitor sample conductivity. Within each polished thin section, three to f i v e grains of each mineral were analysed. Approx-imately 5 spots were probed on each grain. Compositional heterogeniety gave r i s e to a spread of 1-2 wt.% for major elements within i n d i v i d u a l grains and from grain to grain. Any grain or spot with an analysis d i f f e r i n g s i g n i f i c a n t l y from the norm was omitted i n the f i n a l averaging. In order to obtain a crude estimate of the a n a l y t i c a l p r e c i s i o n , a normal d i s t r i b u t i o n of grain composition within one s l i d e was assumed and the standard deviations for each element i n a pyroxene (Px-74) and a glaucophane (Gl-103) were calculated (Table 14). Chlo r i t e compositions were obtained from a working curve prepared for each element using c h l o r i t e standards. Glaucophanes and pyroxenes were analyzed using natural and synthetic pyroxene standards for Mg, Fe, A l , T i , S i , Na and Ca, a garnet standard for Mn and a muscovite standard 193 f o r K. A l l s t a n d a r d s were o b t a i n e d from t h e U n i v e r s i t y o f Washington c o l l e c t i o n . A b s o r p t i o n , f l u o r e s c e n c e and atomic number c o r r e c t i o n s were c a r r i e d o u t i n a d d i t i o n t o those f o r deadtime, d r i f t and background u s i n g computer programmes (UWPROBE, EMX2A) i n t h e p o s s e s s i o n of t h e U n i v e r s i t y o f Washington. A c o n s i d e r a b l e i n c r e a s e i n • t o t a l s was noted a f t e r c o r r e c t i o n on a c c o u n t o f h i g h a b s o r p -t i o n by Fe o f r a d i a t i o n from elements of low ato m i c number. 3+ 2+ As no d i s t i n c t i o n can be made between Fe and Fe u s i n g the m i c r o p r o b e , r e s u l t s a r e r e p o r t e d as FeO or Fe20^ depending o'n w h i c h o x i d e predominates i n p u b l i s h e d wet c h e m i c a l a n a l y s e s o f s i m i l a r m i n e r a l s . T h e r e f o r e , i r o n i s g i v e n as FeO i n c h l o r i t e and glaucophane. I n the case o f the pyroxenes FeO and Fe20.j v a l u e s have been c a l c u l a t e d . M i n e r a l s S o d i c amphiboles S o d i c amphiboles a r e w i d e l y d i s t r i b u t e d w i t h i n the l a w s o n i t e - g l a u c o p h a n e b e a r i n g r o c k s , where they o c c u r i n m e t a b a s i c r o c k s , m e t a c h e r t s and i n t e r l a y e r e d s c h i s t s . A few s m a l l g r a i n s o f glaucophane were a l s o o b s e r v e d i n the gr e e n s t o n e s o f P i n c h i M o u n t a i n a p p r o x i m a t e l y 50 f e e t n o r t h of a f a u l t e d c o n t a c t w i t h t h e g l a u c o p h a n i t i c b l o c k . 194 TABLE 6 OPTICAL PROPERTIES OF PYROXENES AND AMPHIBOLES Spec, no. Mineral PleochrolBm a e Optic plane 2V y A c A 0*. c 26 glaucophane colourless lavendej: blue 010 - 10° 31 « N • - 9 ° 33 M • II - 17° 1*9* II - * • • - 4° 205* n . m • - - 20° 103* H • m • - 2 0 ° 9 ° 210 H • m • -38°*4°# 5° 134 H m • - 1 5 ° 8 ° 155 green amphlbole pale green green blue-green - 5 0 ° 2 0 ° 36 1 201 J brown amphlbole f aim pale brown dark brown 0 ° - 8 ° »° 27 r e l i c t euglte - - - + 5 5 ° # 46° 211 r e l i c t augite - - - +60°# 205*. acmltic pyroxene - • - - - -0°-2° 49* acmltic pyroxene pale green fawn fawn - - - 0° 103* omphacltlc pyroxene pale green pale green green +75° • 39° Jadeltlo pyroxene - - - - - 1*0° • analysed mineral # 2V determined using universal stage A l l other 2V measurements were estimated from curvature and separation of lsogyres. 195 TABLE 7 CELL DIMENSIONS FOR GLAUCOPHANE.AND JADEITIC PYROXENE C e l l Dimen-sions Gl-206 Gl-208. Gl-207 Px-209 0 o o 0 a 9.636±6 A 9 .571±9 A 9 .602±3 A . 9.484±3 A o 17 0 o o b 'IT". 909+6 A .79±3 A 17 .860±9 A 8.633±3 A 0 o o o c 5.30 ±1 A 5 .314±5 A 5 .316±4 A 5.244±2 A B 103°20'±8 • 103 °33'±6' 103 °24 1±3' 107 o27 ,±2 , V 890±2 i 3 880±1 A 3 887±1 A 3 409.6±0.2 A 3 (a) Diffractometer patterns were obtained from a P h i l l i p ' s X-ray diffractometer. For the j a d e i t i c pyroxene (Px-209), a K-Br i n t e r n a l standard was employed and the 8 or 9 best - peaks i n 3 o s c i l l a t i o n s were measured to within 0.01° 2 6 and averaged. The chart speed was 5 x 240 mm/hr and the scan rate ,i-°/min. A similar method was used for the glaucophanes except that a s i l i c o n i n t e r n a l standard was employed to measure the 6 best peaks. Peaks were indexed by comparison with published data for similar minerals: j a d e i t i c pyroxene-Coleman and Clark, 1968; glaucophane-Coleman and Papike, 19 68. (b) C e l l parameters were calculated to three decimal places using a programme which gave a least squares refinement of the unit c e l l (author: Evans et al. , 1963; supplier: E.P. Meagher, U.B.C). (c) Given errors are standard errors and refer to the f i n a l decimal place i n the c e l l dimensions. (d) Specimens have not been analyzed. Locations are given i n Map VII. 196 TABLE 8 ELECTRON MICROPROBE ANALYSES OF GLAUCOPHANES Gl-103 Gl-49 Gl-205 s ± o 2 56.5 56.2 56.7 A l 2 ° 3 8.9 7.4 6.3 T i 0 2 0.00 0.13 0.09 t 1 FeO 15.1 17.2 21.5 MnO 0.03 0.19 0.03 MgO 9.2 8.4 5.9 CaO 0.9 1.0 0.3 Na 20 6.9 6.8 7.3 K 20 0. 02 0.07 0.04 H20* 2.1 2.1 2.1 Total 99.7 99.5 100.3 assumed H 20 content on basis of average analyses of two glaucophanes reported by Rucklidge et a l . , (1971)'. t t o t a l Fe calculated as FeO. 103: assemblage i s glaucophane + lawsonite + sphene + stilpnomelane; occurs as a l t e r a t i o n of garnet + pyroxene i n ec l o g i t e . 49: assemblage i s glaucophane + lawsonite + sphene + white mica + acmitic pyroxene. 205: the assemblage glaucophane + white mica + aragonite + c h l o r i t e occurs as fracture f i l l i n g s i n an acmite + sphene + lawsonite + quartz rock. 197 F i g . 26 Element v a r i a t i o n i n zoned g l a u c o p h a n e . Note i r o n e n r i c h m e n t a t c o r e s and margins o f glaucophane c r y s t a l s w i t h A I 2 O 3 and MgO showing an a n t i p a t h e t i c v a r i a t i o n . Oxide p e r c e n t a g e s a r e c o n s i d e r e d s e m i q u a n t i t a t i v e . 198 Optical properties are given i n Table 6. C e l l dimensions for three glaucophanes are given i n Table 7. C e l l volumes are c h a r a c t e r i s t i c of the "high pressure" polymorph, glaucophane II (Ernst, 1963) rather than the lower pressure higher temperature glaucophane I which has not been recognized i n nature. Microprobe analyses for three sodic amphiboles are given i n Table 8 together with host rock l i t h o l o g y and mineral assemblage. Atomic proportions of elements were not calculated on account of the absence of v o l a t i l e analyses 3+ 2+ and the i n a b i l i t y of the probe to d i s t i n g u i s h Fe and Fe A glaucophane from a metachert was traversed with the electron-probe giving the zonation i l l u s t r a t e d i n F i g . 26. Of p a r t i c u l a r i n t e r e s t i s the antipathetic v a r i a t i o n of MgO and A^O^ with FeO, the l a t t e r oxide being concentrated i n the cores and the margins. The weight per cent oxide values are to be considered semiquantitative as absorption corrections were not made, and an unsatisfactory standard was used for s i l i c o n . The significance of t h i s zonal d i s t r i b u t i o n i s discussed on p. 83. Brown and green amphiboles At two l o c a l i t i e s , j u st west of Pinchi Mine (Map VII, No. 36) and on the Darbar claim group (Map VII, No. 201), dark radiating sprays of amphibole C l cm max length) were 199 TABLE 9 ELECTRON MICROPROBE ANALYSES OF RELICT PYROXENES Px-163 Px-165 s i o 2 51.0 50.0 A1 20 3 2.7 2.3 T i 0 2 0.96 0.81 F e 2 ° 3 - -FeO 9.7 8.1 MnO 0.14 0.13 MgO 15.2 17.5 CaO 19.6 19.4 Na 20 0.7 0.3 Total 100.0 98.5 Numbers of ions on the basis of 6 oxygens S i 1.90 1.89 A 1 I V • 0.10 0.10 T i 0.03 0.02 A 1 V I 0.03 -F e 2 + 0.30 0.25 Mg 0.85 0.98 Mn - 0.01 Na 0.05 0.02 Ca 0.78 0.78 Total 4.04 4.05 200 noted occurring along fractures. The amphibole coexists with coarse grained aragonite and i n places forms rims on glaucophane. Optical properties (Table 6) suggest that i t i s either kataphorite (Na 2CaFe^ + (Fe 3 +Al) S i 2 A l 0 2 2 (OH ,F) 2) or an arfvedsonitic amphibole. Green amphibole was observed at only one l o c a l i t y , as a minor constituent i n a large boulder one mile northeast of Pinchi Mine (Map VII, No. 155). I t coexists with lawsonite and i s l o c a l l y rimmed with glaucophane. Optical properties are given i n Table 6. Pyroxenes Five types of pyroxene are present i n the area: r e l i c t augites, sodic-augites, acmite-jadeites, j a d e i t i c pyroxenes and omphacitic pyroxenes. Pyroxene nomenclature i s i l l u s t r a t e d on an acmite-jadeite-Ca(Mg,Fe)Si 2Og triangular diagram i n F i g . 27 (after Essene et al. 3 1967). R e l i c t igneous augites are found i n most metabasic rocks. Metamor-phic sodic-augites are found only i n the Pinchi Mountain greenstones and acmite-jadeites are found i n lawsonite bearing metabasic rocks. J a d e i t i c pyroxene i s present i n lawsonite bearing metagreywackes, and omphacitic pyroxene i s observed only i n eclogite boulders. Optical properties of pyroxenes are given i n Table 6 and c e l l parameters of a j a d e i t i c pyroxene i n Table 7. 201 TABLE 10 ELECTRON MICROPROBE ANALYSES OF METAMORPHIC PYROXENES Px-49 Px-205 Px-48 Px-74 Px-103 s i o 2 53.9 57.3 56.1 58.1 53.2 A 1 2 ° 3 8.4 13.3 7.5 18 .5 6.7 T i 0 2 0.10 0.34 0.41 0.20 0.08 F e 2 ° 3 16.2* 14.0* 19.6* 4.2* 4.8* FeO 2.2 0.4 0.9 0.8 6.6 MnO 0.20 0.03 0.04 0.03 0.07 MgO 2.0 0.5 1.5 1.7 8.0 CaO 4.5 1.1 2.7 3.0 16.3 Na 20 12.1 14.3 12.8 13.4 6.2 K 20 0.01 0.00 0.00 0.00 0. 01 Total 99.6 101.3 101.5 99.9 102.0 Numbers of ions on the basis of 6 oxygens S i 1.99 2.01 2.02 2. 01 1.94 A 1 I V 0.01 - - - 0.06 T i - 0. 01 0.01 0.01 -A 1 V I 0.35 0.55 0 .32 0.75 0.23 F e 3 + 0.45 0.37 0.53 0.11 0.13 F e 2 + 0.07 0.01 0.03 0.02 0.20 Mg 0.11 0.03 0.08 0.09 0.43 Mn 0.01 - - - -Na 0.87 0.97 0.89 0.90 0.44 Ca 0.18 0.04 0.11 0.11 0.64 Total 4.04 3.99 3.99 4.00 4.07 TABLE 10 (continued) (a) Approximate values of Fe20.j and FeO were obtained from t o t a l Fe aft e r assuming charge balance, n e g l i g i b l e Mn and no A l 1 ^ . In this case _ 2+ M 2+ .• _ 2+ Ca = Mg + Fe (b) Standard deviations for Px-74 are given i n Table 14. Specimen Data Px-4 9: assemblage: glaucophane + lawsonite + acmitic pyroxene + sphene + white mica Px-2 05: assemblage: acmitic pyroxene + sphene + lawsonite + quartz Px-4 8: assemblage: lawsonite + acmitic pyroxene + sphene + c h l o r i t e ; glaucophane i s present but may not be i n equilibrium with the assemblage. Px-74: metagreywacke; assemblage: j a d e i t i c pyroxene + quartz + white mica + lawsonite + sphene + glaucophane + p y r i t e + carbonaceous material. Px-103: glaucophane-lawsonite eclogite; assemblage: omphacitic pyroxene + garnet. Specimen locations are given i n Map VII. 203 F i g . 27a: diagram i l l u s t r a t i n g nomenclature (Essene and Fyfe, 19671 and compositions of Franciscan and Sanbagawa clinopyroxenes (Ernst et al. , 19701. Franciscan t e r r a i n • : j a d e i t i c pyroxene from metagreywackes (Bloxam, 1956, 1959; Coleman, 1965; Ernst et al.t 19701. • : omphacitic pyroxene from "greenstones" and eclogites (Switzer, 1945; Bloxam, 1959; Coleman et al. 3 1965; Essene and Fyfe, 19671. Mt.B. : compositional range of jadeite-acmite from Mount Boardman (Essene and Fyfe, 1967) Sanbagawa • : clinopyroxenes from ec l o g i t e schlieren i n ultramafic rocks (Miyashiro and Seki, 1958; Shido, 1959; Ernst et al.3 1970). • : clinopyroxenes from s i l i c e o u s metasedimentary schists (Banno, 1959, 1964; Iwasaki, 1963). F i g . 27b: diagram i l l u s t r a t i n g compositional range of clinopyroxenes from Pinchi Lake. 74 : j a d e i t i c pyroxene from metagreywacke 103 : omphacitic pyroxene from eclogite 48 49 {: jadeite-acmites from lawsonite bearing 205 metabasic rocks P.M.G. : compositional range of sodic augites from Pinchi Mountain greenstones Note (a) End member proportions were calculated employing procedure outlined by Banno (1959). (Na+K) 100A1 V I (Na+K) 100Fe + 3 J d = • rv= 7-^— ; Ac = (Na+K+Ca) ( A l V I + F e + 3 ) ' (Na+K+Ca) ( A l V I + F e + 3 Ca ( j M g , F e l S i 2 0 6 = 100 - (Jd + Ac) 204 PLOT OF PYROXENE COMPOSITIONS 205 F i g . 27b (continued! : Spec. Jadeite (Jd). Acmite CAc) Ca (Mg,Fe) S i 2 O g No. ("Augite"! Px-74 77.9 11.3 10.8 Px-103 26.0 14.8 59.2 Px-49 36.7 46.4 16.9 Px-48 33.5 55.9 10.6 Px-205 57.5 38.5 4.0 Note (b) 98% of cations are represented on the diagram. The remaining 2% i s comprised of A 1 I V , Mn and T i . 206 R e l i c t igneous augites within the Pinchi Mountain greenstones have been metamorphosed to blotchy sodic-augites which coexist with a l b i t e and have small a^c. Microprobe analyses of two r e l i c t augites are given i n Table 9 together with atomic proportions. Reproduceable analyses of sodic-augites were not obtained but they show 3+ an increase i n Na, Fe (Fe ?) and A l and an antipathetic decrease i n Ca and Mg with respect to the r e l i c t pyroxenes which they commonly replace. The approximate values for Na 20 (2-3 wt.%), CaO (13-17 wt.%) and A l 2 0 3 (2.5-4 wt.%) obtained from p a r t i a l probe analyses indicate that Px-163 and Px-165 p l o t i n the sodic-augite f i e l d i n the system acmite-jadeite-Ca (Mg ,Fe) Si 2Og (Fig. 27b). Analyses of three acmite-jadeites, a j a d e i t i c and an omphacitic pyroxene are given i n Table 10. The t o t a l s range between 99.6 and 102 wt.% and cation t o t a l s between 3.99 and 4.07. Approximate values of F e 2 0 3 and FeO were obtained from t o t a l Fe after making the reasonable assumptions of charge balance, n e g l i g i b l e Mn and A l I V . In t h i s case, 2+ 2+ 2+ Ca = Mg + Fe This enabled c a l c u l a t i o n of atomic proportions of elements and of end-member proportions aft e r Banno (1959). Compositions are i l l u s t r a t e d i n F i g . 27b. The compositional range of clinopyroxenes from the Franciscan (Ernst et al. 3 197 0) i s indicated for comparison with the Pinchi analyses. The j a d e i t i c pyroxene (Px-74) 207 and the omphacitic pyroxene (Px-103), obtained respectively from a metagreywacke and an ec l o g i t e at Pinchi, are similar to j a d e i t i c and omphacitic pyroxenes from the Franciscan. The acmite-jadeites (Px-48,49,205) are compositionally similar to Franciscan pyroxenes from the Mount Boardman area analyzed by Essene et al., (1967). They may be similar to the "omphacitic" pyroxenes from metavolcanic rocks i n the Pacheco Pass area (Ernst et al.3 1970) but chemical analyses were not performed because of fine grain s i z e . At Pacheco Pass these metavolcanics are considered to have formed under P-T conditions similar to the j a d e i t i c pyroxene bearing metagreywackes with which they are c l o s e l y associated. This i s also the case at Pinchi. The contrast i n compositions i s accounted for by Fe-Mg enrichment i n metavolcanics. Ch l o r i t e C h l o r i t e i s widespread i n the greenstones of Pinchi Mountain and the glaucophanitic rocks. I t i s generally pleochroic i n greens, but may be colourless, i n which case i t i s pseudoisotropic. Almost a l l samples studied are length slow ( i . e . o p t i c a l l y negative) but a few specimens were found containing both length slow and length f a s t c h l o r i t e . Interference colours are low f i r s t order and produce anomalous blue or brown t i n t s i n some samples. 208 TABLE 11 ELECTRON MICROPROBE ANALYSES OF CHLORITES Specimen Number Chi-38 Chl-48 Chl-165 s i o 2 31.6 30.0 29.3 Al 203 15.9 15.3 15.9 FeO* 19.1 26.5 27.4 MnO 0.27 0.33 0.25 MgO 22.4 17.4 15.8 H20** 11.0 11.0 11.0 Total 100.3 100.5 99.7 * Total Fe calculated as FeO ** based on approximate water content of c h l o r i t e s according to Deer, Howie and Zussman (1963), H„0 content for most c h l o r i t e s ranges between 10.3 and 12 Numbers of ions on basis of 2 8 oxygens Specimen Number 38 48 165 S i 6.30 6.11 6.11 Al!V 1.70 1.89 1.89 AlVI 2.04 1.78 2.00 F e 2 + 3.20 4.56 4.78 Mg 6.62 5.33 4.89 Total 19.86 19.67 19.67 Specimen Data Chl-38: c h l o r i t e blebs i n lws + acm + sph + c h l ± wh m + glph ± s t i l p matrix Chl-48: assemblage: lws + acm + c h l + sph ± late(?) glph Chl-165: assemblage: chl + acm + ab + sph; Pinchi Mt. Greenstones 209 Chemical analyses are given i n Table 11. C h l o r i t e formulae were calculated on the basis of 28 oxygen equiv-alents and a l l iron was assumed to be ferrous, as Ernst, (.197 0, p. 163). states that three Franciscan c h l o r i t e s , analyzed by wet chemical methods, contain an average of 1.57 wt.% Fe2G-2« The analyses p l o t within the pycnochlorite or diabantite f i e l d s (Hay, 19541. According to Winchell (1936) and Hey (1954). , A l poor c h l o r i t e s are o p t i c a l l y negative (length slow) and A l r i c h are o p t i c a l l y p o s i t i v e (length f a s t ) . I t follows that the predominance of o p t i c a l l y negative c h l o r i t e s at Pinchi suggests that they are compositionally similar to those of the C a l i f o r n i a n Coast Ranges. In a comparison of C a l i f o r n i a n and Japanese blueschists, Ernst (.1970) notes a tendency for the A l content of c h l o r i t e s to increase with r i s e i n metamorphic temperatures. Aragonite Aragonite, the high pressure polymorph of c a l c i t e , i s commonly found i n the metasediments and metabasic rocks of the Pinchi Mountain greenstones and the glaucophanitic rocks. L o c a l i t i e s are shown i n F i g . 28. I t occurs as veins or blebs i n mafic rocks and i s an important constituent of the limestones north of Pinchi Lake. Almost invariably, p a r t i a l inversion to c a l c i t e may be. noted on grain margins or along cleavages. 210 2 miles 3 2 km F i g . 2 8 Aragonite occurrences • aragonite ± inverted c a l c i t e • c a l c i t e only \ f a u l t i i 3-2 km. F i g . 29 Prehnite, pumpellyite and selected glaucophane-lawsonite occurrences • pumpellyite • prehnite ± lawsonite + glaucophane 1 ^ boundary of lawsonite + glaucophane ..bearing f a u l t black 211 TABLE 12 CARBONATE MINERALOGY AND SAMPLE DISTRIBUTION Mineralogy Ca) arag + dol + cc ± qtzCtr): spec. nos. 77, 78, 79, 82, 215, 217, 221, 224, 227. Cb) arag + d o l : spec. no. 213. Cc) arag + cc ± q t z ( t r ) : spec. nos. 80, 222, 223, 225, 230. Cd) dol + cc ± qtz: spec. nos. 216, 226, 234. Ce) cc + qtz + ba r i t e : spec. no. 81. Cf) cc ± qtz ± plag: spec. nos. 219, 220, 228, 229, 231, 232, 233, 235. Sample d i s t r i b u t i o n Ci) samples from carbonates associated with lawsonite-glaucophane rocks: 77, 78, 79, 80, 81, 82, 215, 216, 217,218 221, 222, 223, 224, 225, 226, 227, 234. Cii) samples from carbonates interbedded with Pinchi Mt Green-stones: 82, 218. Ci i i ) samples from Mt. Pope b e l t : 228, 235, 229, 232. Civ) samples from Takla Group limestones: 219, 220, 230, 231, 233. Note: Ca) Specimen locations are given on F i g . 28. Cb) cc = c a l c i t e ; dol = dolomite; arag = aragonite; qtz = guartz; plag = plagioclase; t r = trace. 212 Argonite can be e a s i l y i d e n t i f i e d i n thin section by i t s c h a r a c t e r i s t i c negative 2V of 18°, st r a i g h t e x t i n c t i o n on i t s prismatic cleavage, low r e l i e f with a p a r a l l e l to the po l a r i s e r and {110} twinning on basal sections. T h i r t y X-ray diffractometer traces of limestones from the area were also made. Carbonate mineralogy and sample d i s t r i b u t i o n are given i n Table 12. White mica White mica i s most abundant within glaucophanitic metacherts and s c h i s t s . Greenstones and glaucophanitic metabasic rocks commonly contain disseminated s e r i c i t e (<5y) with coarser grained v a r i e t i e s near fractures or as cluste r s i n amygdules. The white mica appears greenish to the unaided eye and shows f a i n t pleochroism i n thin section with a-colourless and 3 = y -very pale green. Micas examined i n two specimens (151, 212) are u n i a x i a l negative but some show a negative 2V of up to 10°. Deer, Howie & Zussman (Vol. 3, p. 22) state that small 2Vs are occasionally seen i n white micas. The reason i s uncertain, but i t may be caused by anomalous o p t i c a l e f f e cts r e s u l t i n g from superimposition of mica f l a k e s . X-ray examination of three micas indicated that they were the normal 2M^ polytype. P a r t i a l microprobe analyses of micas from a glauco-213 TABLE 13 PARTIAL ELECTRON MICROPROBE ANALYSES OF. PHENGITES AND CELADONITE Ph-151 Ph-205 Cel-196 A B s±o2 - - - 50.50 46.50 A1 20 3 26 21 6.1 20.57 29.82 T i 0 2 0.1 0.1 tr 0.76 0.18 F e 2 0 3 - - 6.95 1.50 FeO 3.3* 5.3* 16.4* 0.00 1.86 MnO tr t r 0. 05 0.82 0.02 MgO 4.4 4.7 6.1 5.68 3.97 CaO 0.00 0.04 0.09 0.26 0.23 Na 20 tr t r 0.00 t r 0.37 K 20 - - - 10.95 9.15 H 20 — - - 2.74 4.77 * Total Fe calculated as FeO = not determined t r = trace Specimen data Ph-151: metachert, Pinchi Lake; assemblage: glaucophane + lawsonite + quartz + phengite. Ph-205: metabasic rock; assemblage: glaucophane + phengite + aragonite + c h l o r i t e (occurs as fracture f i l l i n g s i n acmite + sphene + lawsonite + quartz rock). Cel-196: assemblage: a l b i t e + c h l o r i t e + pumpellyite + sphene + celadonite. Prehnite, c a l c i t e and white mica are also pre-sent sporadically i n blebs (Pinchi Mt. Greenstone). A: ferriphengite from quartz + a l k a l i feldspar + green b i o t i t e + c a l c i t e + epidote gneiss (Plas, 1959) . B: phengite from glaucophane + c h l o r i t e + aragonite + j a d e i t i c pyroxene s c h i s t (Ernst, 1963, Table 1, No. 10). 214 TABLE 14 STANDARD DEVIATIONS FOR SELECTED MINERAL ANALYSES Px-74 a Gl-49 a s ± o 2 58 .1 ± 1.0 56.2 ± 0.4 A l 2 ° 3 18 .5 ± 0.6 7.4 ± 0.8 T i 0 2 0. 20 ± 0.20 0.1 3 ± 0. 06 F e 2 ° 3 5. 1* ± 0.81 • - -FeO - - 17. 2** + 0.2 MnO 0. 03 - 0. 19 ± 0 .07 MgO 1. 7 ± 0.2 8. 4 ± 0. 4 CaO 3. 0 ± 0.4 1. 0 ± 0. 4 Na 20 13. 4 ± 0.3 6. 8 ± 0. 1 K 20 0. 00 - 0. 07 + -t o t a l Fe calculated as Fe 20 ** t o t a l Fe calculated as FeO 215 p h a n i t i c m e t a c h e r t (Ph-151) and f r o m v e i n s i n a m e t a b a s i c r o c k (Ph-2051 a r e g i v e n i n T a b l e 13. T h e s e r e s u l t s a r e u n c o r r e c t e d f o r a b s o r p t i o n a n d f l u o r e s c e n c e and s h o u l d be c o n s i d e r e d s e m i q u a n t i t a t i v e . The MgO v a l u e s C4.4, 4.7 wt.%) and t h e FeO v a l u e s C3.3, 5.3 wt.%) show t h a t t h e m i c a s a r e p h e n g i t e s o r f e r r i p h e n g i t e s . C e l a d o n i t e C e l a d o n i t e i s s p o r a d i c a l l y p r e s e n t w i t h i n t h e P i n c h i M o u n t a i n g r e e n s t o n e s , t y p i c a l l y o c c u r r i n g i n r a d i a t i n g a g g r e g a t e s . I t i s e a s i l y r e c o g n i z e d by i t s b r i g h t g r e e n p l e o c h r o i s m Cot-fawn, B = Y - b l u e - g r e e n ) , and i t s l e n g t h s l o w c h a r a c t e r . O p t i c a l i d e n t i f i c a t i o n was c o n f i r m e d by X - r a y d i f f r a c t i o n . A p a r t i a l s e m i q u a n t i t a t i v e a n a l y s i s i s g i v e n i n T a b l e 13. A l b i t i c p l a g i o c l a s e A l b i t e o c c u r s w i t h i n t h e P i n c h i M o u n t a i n g r e e n s t o n e s as r e p l a c e m e n t s o f p r i m a r y p l a g i o c l a s e m i c r o l i t e s and p h e n o c r y s t s , and as v e i n s . I n t h e g l a u c o p h a n i t i c r o c k s i t i s e x c e e d i n g l y r a r e , o c c u r r i n g o n l y as l a t e v e i n s i n m e t a b a s i c r o c k s (No. 49, 58) and g r a p h i t i c c h e r t s CNo. 118). B l a s t o p o r p h y r i t i c a l b i t e s a r e g e n e r a l l y c h a r g e d w i t h i n c l u s i o n s C s e r i c i t e , p u m p e l l y i t e , c h l o r i t e and unknowns) and r e l i e f d e t e r m i n a t i o n s a r e d i f f i c u l t . Where d e t e r m i n e d 216 t h e r e l i e f i s l o w w i t h t h e M i c h e l - L e v y m e thod g i v i n g compo-s i t i o n s o f AnQ_j-. V e i n a l b i t e i n t h e g l a u c o p h a n i t i c r o c k s h a s p o s i t i v e s i g n , a n d 3 1 o r y" = 1.536. P u m p e l l y l t e P u m p e l l y l t e i s f o u n d o n l y w i t h i n t h e g r e e n s t o n e s o f P i n c h i M o u n t a i n ( f o r l o c a l i t i e s s e e F i g . 29). I t o c c u r s a s m a t t e d a c i c u l a r a g g r e g a t e s i n v e i n s , f e r r o m a g n e s i a n p s e u d o m o r p h s a n d b l a s t o p o r p h y r i t i c a l b i t e s . O p t i c a l p r o p e r -t i e s a r e o b t a i n e d o n l y w i t h d i f f i c u l t y b e c a u s e o f f i n e g r a i n s i z e . T h e s e a r e : a = y - f a w n , 3 - g r e e n ; 8 Ac = 5°; a n o m a l o u s b l u i s h o r b r o w n i s h i n t e r f e r e n c e t i n t s . O p t i c a l i d e n t i f i c a t i o n was c o n f i r m e d b y X - r a y d i f f r a c t i o n . S t i l p n o m e l a n e S t i l p n o m e l a n e t y p i c a l l y o c c u r s a s l a t e v e i n s i n g l a u c o p h a n i t i c r o c k s a n d a l s o a s a n a l t e r a t i o n p r o d u c t o f g a r n e t i n a n e c l o g i t e b o u l d e r . The g r e e n p l e o c h r o i c v a r i e t y , f e r r o s t i l p n o m e l a n e , ( a - c o l o u r l e s s , $ = y - p a l e g r e e n ) o c c u r s w i t h t h e more common b r o w n s t i l p n o m e l a n e . L a w s o n i t e M e t a b a s i c r o c k s , m e t a s e d i m e n t s a n d e c l o g i t e b o u l d e r s a l l c o n t a i n l a w s o n i t e . The m i n e r a l was a l s o t e n t a t i v e l y i d e n t i f i e d i n o n e t h i n s e c t i o n f r o m t h e g r e e n s t o n e s o f 217 Pinchi Mountain. Within the eclogite boulder and some schi s t s i t commonly occurs as poly s y n t h e t i c a l l y twinned porphyroblasts, but otherwise i t i s recognized by i t s tabular habit, s t r a i g h t e x t i n c t i o n and negative elongation. Prehnite Colourless sheaf-like aggregates of prehnite were noted i n the Pinchi Mountain greenstones at the west end of Murray Ridge i n association with pumpellyite, celadonite, c h l o r i t e , a l b i t e and c a l c i t e (Fig. 29). The mineral also occurs with quartz i n amygdules i n basic volcanics of presumed Upper T r i a s s i c age. Garnet An eclogite boulder contains subhedral garnets Cl mm max diameter) p a r t l y replaced by stilpnomelane. Reconnaissance probe work indicates that they are chemically similar to those found elsewhere i n eclogites associated with blue-schists (Coleman,.at.., 1965) . They are s l i g h t l y zoned with the cores having the composition almandine (60%), grossular (.30%) and spessartine + pyrope (10%) . Deerite A specimen of d r i l l core taken from the Pinchi Mine area contains dark brown rad i a t i n g sprays up to 1 cm i n 218 diameter of deerite (Agrell et al. 3 1965). The mineral appears late i n the paragenetic sequence and cross-cuts glaucophane, lawsonite and sodic pyroxene. I t i s s l i g h t l y pleochroic on thin edges (dark brown to black), has s t r a i g h t extinction and an amphibole-like cross section (Fig. 3 4). X-ray examination of a deerite concentrate using a KBr i n t e r n a l standard and P h i l l i p ' s diffractometer revealed a f a i r l y good correspondence with the calculated cl^kl obtained from the c e l l parameters published by A g r e l l . The well defined (020) and (110) r e f l e c t i o n s yielded d-spacings of 9.42 ±0.02 A and 9.04 ± 0.02 A. The difference from the o calculated values (9.42 and 9.18 A respectively) can be attributed to s h i f t of peak positions due to s o l i d solution. Deerite peaks at higher 26 values were d i f f i c u l t to index because of contamination by r i e b e c k i t e , sodic pyroxene and lawsonite. Opaques Within metabasic rocks, opaque minerals other than r e l i c t ilmenomagnetites are very fine grained (0.3 mm max). Grains studied with the r e f l e c t i n g microscope are composite, with creamy yellow p y r i t e forming the nucleus, surrounded by magnetite Cor maghemite). This i n turn i s rimmed by hematite (Fig. 30). P y r i t e i s widespread within graphite s c h i s t s . 219 F i g . 30 T e x t u r e s i n opaque m i n e r a l s 220 Carbonaceous material Carbonaceous material from schists and cherts was separated employing the method of French (1964) by digestion with HF and HCl and X-rayed using a P h i l l i p ' s diffractometer tCuK^ rad i a t i o n ; scan-rate - 2°/min; chart speed - 5 x 240 mm/hr). After digestion, p y r i t e and r a l s t o n i t e were the only phases present i n addition to carbonaceous material. One sample out of the f i v e studied gave a broad d i f f u s e peak at 26°, suggesting the presence of nearly amorphous graphitic material (graphite-d^ according to the c l a s s i f i c a t i o n of Landis, 1971). Four samples yielded featureless diffractometer charts i n d i c a t i v e of amorphous material. APPENDIX III BULK CHEMICAL ANALYSES Representative homogeneous samples of thirteen metabasic rocks and two metagreywackes were analysed by the Geological Survey of Canada laboratory i n Ottawa. Analyses for MnO, T i 0 2 , CaO, K 20, S i 0 2 and A l 2 0 3 were obtained by X-ray fluorescence methods; Fe ( t o t a l ) , FeO, Na 20, P 2°5' C 0 2 a n d H 2 ° ( t o t a l ) ' bY "rapid chemical a n a l y t i c a l techniques" and MgO by atomic absorption. E s t i -mated errors quoted by the Geological Survey are given i n Table 15. The analyst also noted that some of the high values for T i 0 2 were outside the normal range of the method used. Precision may be estimated by inspection of the duplicate specimens (36a and 36b). Totals range from 97.8% to 101.2% by weight. Metabasic rocks Prior to norm c a l c u l a t i o n for metabasic rocks, two adjustments were made to the a n a l y t i c a l data as suggested by Irvine and Baragar (1971). This involved r e c a l c u l a t i o n of the f e r r i c - f e r r o u s r a t i o to correct for oxidation during metamorphism, and r e c a l c u l a t i o n of the analyses to 100% omitting H„0 and CO». O r i g i n a l analyses and oxidation 222 TABLE 15 ACCURACY OF BULK CHEMICAL ANALYSES Oxide Range i n Value one a s i o 2 30-75% ± 1.2 A1 20 3 0-20% ± 0.7 F e t o t C F e 2 ° 3 > 0-15% ± 0.5 CaO 0-40% ± 0.3 MgO 0-40% ± 1.0 K 20 0-5% ± 0.1 T i 0 2 0-2% ± 0.05 MnO 0-1% ± 0.02 FeO 0-15% ± 0.2 Na 20 0-10% ± 0.15 P2°5 0-1% ± 0.04 c o 2 ± 0.1 H 20 ± 0.1 Note: errors are those reported by the analyst of the Geological Survey of Canada. TABLE 16a CHEMICAL ANALYSES OF METABASALTS FROM THE PINCHI LAKE AREA GREENSTONES OF PINCHI MT. LAWSONITE-GLAUCOPHANE BEARING METABASALTS MASSIVE ROCKS FOLIATED S i 0 2 T i 0 2 A1 20 3 ?e2Oj FeO KnO MgO CaO Na20 K 20 p 2 o 5 c o 2 H20t Total Oxidn. ra t i o 1?0 163 43.8 44.6 4.35 2.72 14.7 13.1 3.9 5.0 ' 9.6 6.7 O.13 0.16 4.5 7.2 6.5 9.5 4.8 2.0 0.1 2.6 O.56 0.28 0.8 0.6 4.7 4.3 98.4 98.8 26 40 162 44.2 3.36 18.1 3.1 6.8 0.18 3.0 8.6 5.1 0.3 0.36 0.7 4.6 98.4 29 31 55 46.7 47.5 1.49 3.64 14.1 14.6 5.1 3.6 7.0 7.6 0.14 0.13 45 36a 36b 48 61 202 23 37 45.6 33.I 32.3 42.9 47.9 40.2 42.7 44.8 3.22 1.75 1.63 5.51 1.49 2.45 1.20 4.00 13.7 9.1 9.2 10.5 14.4 12.2 13.8 16.4 5.6 5.2 5.0 4.2 4.1 7.5 4.4 5.2 6.3 4.3 4.3 8.1 7.8 5.7 5.5 5.2 0.17 0.20 0.21 0.11 0.14 0.14 0.10 0.06 5.4 6.2 6.0 7.3 5.6 11.1 7.0 4.4 9.8 20.0 21.0 11.1 7.9 8.8 13.8 8.8 2.6 3.2 3.1 2.9 2.9 2.7 1.1 2.7 0.8 1.3 1.4 0.1 0.2 0.1 0.2 1,7 0.37 1.29 2.14 0.08 0.13 0.27 0.11 0.17 0.11 0.46 0.3 9.2 10.4 0.8 1.2 0.2 4.0 0.1 1.3 0.1 4.5 3.9 3.8 4.8 5.1 6.4 6.0 5.7 6.6 5.0 98.4 98.7 100.5 98.2 98.9 97.8 99.9 99.1 [101.2 100.5 44 51 52 32 32 54 41 47 39 30 7.5 9.2 1.9 0.1 4.9 3.9 3.3 0.8 A 49.20 3.40 12.87 4.94 12.04 0.27 4.94 8.54 3.09 0.31 0.36 B 44.10 2.70 12.10 3.20 9.60 0.20 13.00 11.50 .1.90 0.70 0.30 C 45.40 3.60 14.70 4.10 9.20 0.20 7.80 10.50 3.00 1.00 0.40 D 49.16 2.29 13.33 1.31 9.71 0.16 10.41 10.93 2.15 0.51 0.16 27 23 29 11 A 1 Franciscan greenstonei Coleman & Lee, (1963, Table 2, no. 60-804). B 1 Average Hawaiian ankaramitei Macdonald (1968, Table 8). C • Average Hawaiian a l k a l i - o l i v i n e basalti Macdonald (1968, Table 8) D 1 Olivine t h o l e i i t e , Kilauea volcano, Hawaii) Yoder & Tilley,(1962, Table 2, no,14). Note 1 f o r mineral assemblages, see Table 22. 224 TABLE 17 C. I . P. W. NORMS FOR ANALYSED METABASALTIC ROCKS GREENSTONES OF LAWSONITE-GLAUCOPHANE BEARING METABASALTS PINCHI KT. MASSIVE ROCKS FOLIATED 1?0 163 162 45 36 43 61 202 23 37 31 55 quartz - - - 2.43 - 3.U - - 0.94 2.85 1.85 orthoclase 0.65 16.37 1.77 5-32 0.59 1.18 0.59 1.18 10.75 0.65 4.96 alb i t e 41.85 17.00 33.07 23.69 - 26.49 26.57 20.08 10.15 24.45 17.26 29.28 anorthite 19.68 20.33 27.56 24.77 - 16.60 27.83 22.88 35.93 29.54 31.83 23.74 nepheline 0.98 0.51 7.20 - - - 2.88 - -. - -diopside 9.05 22.37 12.82 19.68 - 32.94 11.24 18.66 31.99 12.47 13.37 15.65 hypersthene - - - 9.38 1.97 21.99 - 12.34 5.95 26.07 10.70 olivine 11.43 10.57 4.96 - - 3.39 - 22.83 1.11 - - -magnetite 6.09 6.52 4.83 7.32 6.73 4.68 6.23 4.35 5.74 4.65 5.47 ilmenite 8.89 5.51 6.86 6.53 - 11.30 3.06 5.13 2.54 8.13 3.04 7.24 apatite 1.39 0.70 0.90 0.93 0.21 0.32 0.70 0.28 0.42 0.28 1.11 Note 1 a) norms i n wt.?S TABLE 16b "ADJUSTED" CHEMICAL ANALYSES OF METABASALTS GREENSTONES' OF PINCHI MT. 170 I63 162 LAUSONITE-GLAUCOFHANE BEARING METABASALTS MASSIVE ROCKS 45 36a 36b 48 61 202 23 37 FOLIATED 31 55 sio 2 T102 A1203 Fe 20 3 FeO MnO KgO CaO Na,0 K20 p 2o 5 47.1 47.6 47.5 4.68 2.90 3.61 15.8 14.0 19.4 4.2 4.5 3.3 10.3 7.9 7.3 0.14 0.1? 0.19 4.8 7.7 3.2 7.0 10.1 9.2 5.2 2.1 5.5 0.1 2.8 0.3 0.60 0.30 0.39 48.8 38.7 37.5 46.3 51.8 44.3 47.5 48.0 3.44 2.04 1.89 5.95 1.61 2.70 1.34 4.28 14.7 10.7 10.7 11.3 15.6 13.4 15.4 17.6 5.1 3.8 3.6 4.6 3.2 4.3 3.0 5.6 7.6 7.1 7.0 8.8 9.5 9.8 7.9 .5.6 0.18 0.23 0.24 0.12 0.15 0.15 0.11 0.06 5.8 7.3 7.0 7.9 6.1 12.2 7.8 4.7 10.5 23.4 24.4 11.9 8.6 9.7 15.4 9.4 2.8 3.8 3.6 3.1 3.1 3.0 1.2 2.9 0.9 1.5 1.6 0.1 0.2 0.1 0.2 1.8 0.40 1.51 2.49 0.09 0.14 0.30 0.12 0.18 50.1 49.8 1.60 3.81 15.1 15.3 3.2 3.8 9.6 8.0 0.15 0.14 8.1 5.1 9.9 9.3 2.0 3-5 0.1 0.8 0.12 0.48 The analyses were a) recalculated to 100# omitting H20 and CO, and b) corrected for oxidation during metamorphism by adjusting the ferric-ferrous ratlo(assuming % F e 2 0 3 = % T10 2 + 1.5) - after Irvine and Baragar (1971). 225 FIG. 31 A-F-M DIAGRAM ILLUSTRATING BASALT COMPOSITIONS F FIG.t 32 ALKALI-SILICA VARIATION DIAGRAM FOR BASALTS S i 0 2 ( w t % ) 226 r a t i o s are given i n Table 16a and adjusted analyses i n Table 16b. C.I.P.W. norms are presented i n Table 17. R e l i c t mineralogy and textures show that the majority of the basic rocks were o r i g i n a l l y volcanics p r i o r to the formation of the metamorphic minerals (Appendix IV). Two rocks (Nos. 23 and 48) are considered to have been basic i n t r u s i v e s . The b a s a l t i c composition of the analysed rocks i s i l l u s t r a t e d i n an A.F.M. diagram (Fig. 31) with the area encompassing Franciscan basalts shown for comparison (Ernst et a l . , 1970). Relevant information r e l a t i n g to the c l a s s i f i c a t i o n of the primary basalt type i s presented i n Table 18 and i n the a l k a l i - s i l i c a v a r i a t i o n diagram (Fig. 32). The Pinchi Mountain greenstones (Specs. 162, 163 and 170) possess the chemical c h a r a c t e r i s t i c s of a l k a l i basalts. Normative hypersthene i s absent and on the a l k a l i - s i l i c a v a r i a t i o n diagram, the basalts p l o t i n the a l k a l i f i e l d as given by MacDonald (1968). The high T i 0 2 values (>2.9) are thought to be representative of the primary basalt, as titanium i s generally considered immobile during metamorphism. Engels et al*3 C1965) present the following average T i 0 2 values for basalts; (a) oceanic t h o l e i i t e s (.1.49 wt.% T i 0 2 ) ; (b) a l k a l i basalts (2.87 wt.% T i 0 2 ) . The range of T i 0 2 values i n the Pinchi Mountain greenstones (.4.7, 2.9, 3.6 wt.%) suggests an a f f i n i t y with the a l k a l i basalt type. Within the glaucophane-lawsonite bearing metabasic rocks, both types of basalt appear to be represented. Three 227 TABLE 18 SUMMARY OF SIGNIFICANT CHEMICAL CHARACTERISTICS OF METABASALTS Spec, no. Normative hyp. qtz. Na20 + K 20 T102 Basalt type 170* - 5.2 + 0.1 4.7 a l k a l i , basalt 163* - 2.1 + 2.8 2.9 a l k a l i basalt 162* - 5-5 + 0.3 3.6 a l k a l i basalt 45 9.4 2.4 2.8 + 0.9 3.4 ? 36 highly carbonated sample 48 2.0 3.1 + 0.1 6.0 a l k a l i basalt 61 22.0 3.1 3.1 + 0.2 1.6 t h o l e l l t i c basalt 202 - 3.0 + 0.1 2.7 a l k a l i basalt 23 12.3 1.2 + 0.2 1.3 t h o l e l l t i c basalt 37 6.0 0.9 2.9 + 1.8 4.3 ? 31 26.1 2.9 2.0 + 0.1 1.6 t h o l e l l t i c basalt 55 10.7 1.9 3.5 + 0.8 3.8 ? * Pinchi Mountain greenstones Oxide values obtained from Table 16b 228 out of eight analyzed rocks have normative hypersthene and quartz, have low TiC^ values (1.3 - 1.6 wt.%) and p l o t i n the t h o l e i i t i c f i e l d on an a l k a l i - s i l i c a v a r i a t i o n diagram. One basalt (No. 202) has an analysis s i m i l a r to a Hawaiian ankaramite (Table 16a) and possesses the c h a r a c t e r i s t i c s of an a l k a l i basalt. The four remaining basalts are a l k a l i n e i n that the T i 0 2 content i s high and they p l o t i n the a l k a l i basalt f i e l d i n the v a r i a t i o n diagram. However, they also contain normative hypersthene and quartz which, according to Yoder and T i l l e y i s c h a r a c t e r i s t i c of t h o l e i i t i c basalts. The c o n f l i c t i n g evidence as to the basalt type suggests either titanium and a l k a l i enrichment of a t h o l e i i t i c basalt or s i l i c a enrichment of an a l k a l i basalt during metamorphism. Con-sidering the probable immobility of titanium during metamorphism, the l a t t e r suggestion i s preferred. I t i s therefore concluded that both t h o l e i i t i c and a l k a l i basalts were present i n the rock sequence p r i o r to metamorphism. Metagreywackes Analyses for two metagreywackes are shown i n Table 19. Also included for purposes of comparison are the average of 21 Franciscan greywackes (Bailey et al., 1964, Table 2, No. 1). The Pinchi metagreywackes have lower 229 TABLE 19 CHEMICAL ANALYSES OF METAGREYWACKES Specimen Number 71 70 A B s i o 2 50.5 57.0 67.5 65.0 T i 0 2 0.82 0.68 0.5 0.69 A 1 2 ° 3 19.8 17.6 13.5 14.14 F e 2 0 3 3.4 2.7 1.2 0.58 FeO 4.4 4.0 3.0 5.44 MnO 0.16 0.12 0.1 0.08 MgO 3.2 3.1 2.2 3.44 CaO 5.7 3.3 2.4 2.28 Na 20 3.7 5.4 3.6 2.29 K 20 1.9 1.4 1.7 2.24 P2°5 0.28 0.23 0.1 0.14 c o 2 0.1 0.1 0.8 0.01 H2°t 5.4 2.1 2.9 4.02 Total 99.4 97 .7 99.5 100.35 71: metagreywacke, Pinchi Lake (analysed by G.S.C.) 70: metagreywacke, Pinchi Lake (analysed by G.S.C.) A: average of 21 Franciscan metagreywackes (Bailey et al.} 1964, Table 2, No. 1). B: j a d e i t i c pyroxene bearing metagreywacke (Ernst, 1965, Table 12, No. 190). 230 SiC^ and higher Al^O^ and CaO values. Presumably t h i s r e f l e c t s either less quartz or a higher anorthite content i n the i n i t i a l sediment. APPENDIX IV PETROLOGY Greenstones of Pinchi Mountain Metamorphic and r e l i c t minerals present i n the green-stones of Pinchi Mountain are given i n Table 20. C h l o r i t e , a l b i t e , sodic pyroxene and sphene are the main minerals with celadonite, quartz, pumpellyite, prehnite, f e r r o -stilpnomelane, aragonite and white mica sporadically d i s -tributed. A few small grains of lawsonite and glaucophane were also observed. O p t i c a l properties and mineral composi- . tions are covered i n Appendix I I . The r e l i c t mineralogy and bulk chemistry (Appendix III) suggest that the primary rocks were a l k a l i basalts consisting of augite, plagioclase, ilmenite and o l i v i n e (?) exhibiting p o r p h y r i t i c , t r a c h y t i c and amygdaloidal textures. Domains within i n d i v i d u a l thin sections are of f i v e textural types: matrix, r e l i c t s , blebs, pseudomorphs and veins. Metamorphic mineral assemblages c h a r a c t e r i s t i c of each domain are given i n Table 21 and are i l l u s t r a t e d i n F i g . 33. The d i s t i n c t i o n between blebs and pseudomorphs i s somewhat a r b i t r a r y and dependant on the l a t t e r possessing a euhedral or subhedral o u t l i n e . Blebs may have originated 232 TABLE 20 MINERAL ASSEMBLAGES IN THE GREENSTONES OF PINCHI MOUNTAIN Spec No. 160 161 162 163 164 165 166 169 170 171 172 173 193 195 196 197 203 204 alb i t e X 60 20 X 55 - - 40 X X X X X X X X X chlorite X X 20 3 X 10 X X 35 X X X X X X X X X quartz X - - 7 - - X X - - - X - - - - - -sodic pyx. X X 8 12 X t r X X 11 X X - X X - - X X pumpellyite - X 3 - - 3 - - - - - - X X X X X -white mica X - 2 7 - - - - 5 - - - X - - - X -sphene X X 8 2 X 5 X X 5 X X X X X X X X X aragonite X - - - X - X - 4 X X X celadonite 7 prehnite X X - -c a l c i t e X X - -augite (r) X X - 35 - 27 - X X X - X X X X -ilraenite ( r i - - - 5 - - - - - - - - - - - - - -* = trace of lawsonite x = mineral present + = trace of glaucophane r = r e l i c t mineral Modes are approximate. 233 TABLE 21 TEXTURAL DOMAINS WITHIN THE PINCHI MOUNTAIN GREENSTONES Size range: r e l i c t s 0.5 -.1.5 mm matrix a l b i t e m i c r o l i t e s CO.2 mm), c h l o r i t e , sphene (.0.01 mm average). , acmitic pyroxene (0.05 mm average) blebs 0.5 - 5 mm pseudomorphs 2 - 8 mm veins 0.1 - 3 mm R e l i c t minerals: augite, ilmenite Matrix assemblages: ab + Na px + c h l + sph ± wh m ± celad ± arag ± lws qtz + c h l + Na px + sph ± wh m ± arag ab + c h l + pump + sph ± prehn ± cc ± celad ± wh m R e l i c t microgranular augite may also be present i n the matrix Blebs: c h l (radiating aggregates) pump ± c h l ± celad ± prehn ± wh m arag ± celad ± pump ce l a d Pseudomorphs: plagioclase — ab ± ser ± c h l ± pump o l i v i n e (?) — chl — cc — chl ± pump ± celad ± prehn clinopyroxene — b l o t c h y sodic pyroxene 234 TABLE 21 (continued). Veins: pump + qtz ± wh m early qtz + Na px + arag arag + qtz ± ab l a f c e ab Abbreviations given on p, 74. 235 Figure 33 Textures i n Pinchi Mountain Greenstones (a) Early quartz-pumpellyite vein i s cross-cut by an aragonite-quartz vein rimmed with a l b i t e . Micro-granular matrix consists of a l b i t e + sodic pyroxene + sphene (Spec. 204). (b) C h l o r i t e blebs and r e l i c t augites occur i n matrix consisting of a l b i t e m i c r o l i t e s , sodic pyroxene, sphene and c h l o r i t e (Spec. 165). Cc) Plagioclase phenocrysts are pseudomorphed by a l b i t e , pumpellyite and s e r i c i t e . Matrix contains a l b i t e laths, c h l o r i t e and sphene C S p e c . 162). (d). R e l i c t augites are p a r t l y replaced by a brownish sodic pyroxene with straight e x t i n c t i o n . Note celadonite blebs, ilmenite p a r t l y replaced by sphene and white mica + quartz + lawsonite matrix C S p e c . 163). Ce) Large bleb contains aragonite p a r t l y inverted to c a l c i t e and rimmed by ferrostilpnomelane and c h l o r i t e . Matrix consists of a l b i t e + sodic pyroxene + sphene + white mica (Spec. 170) . Cf) C h l o r i t e and pumpellyite replace ferromagnesian phenocrysts ( o l i v i n e ? ) . A l b i t e and augite c r y s t a l s l i e i n a microgranular matrix (Spec. 196) . 236 FIG. 33 TEXTURES IN PINCHI MT. GREENSTONES 237 either as phenocrysts or amygdules which were la t e r subject to deformation and r e c r y s t a l l i z a t i o n . C h l o r i t e blebs are by far the most common type. Hornblende or o l i v i n e are the most l i k e l y precursors of some blebs, but the l a t t e r i s preferred on account of the basic composition of the rocks and absence of aci c u l a r pseudomorphs. Prehnite occurs only i n the greenstones at the west end of Murray Ridge, coexisting with a l b i t e , c h l o r i t e , pump-e l l y i t e , celadonite and c a l c i t e . Lawsonite-Glaucophane Bearing Rocks  Metabasic rocks Metamorphic mineral assemblages and r e l i c t minerals observed i n 39 thin sections are given i n Table 22. Modal analyses were performed for four representative rocks using a Zeiss micrometer eyepiece. Rough estimations were made for a l l other modes. Sample locations are given i n Map VII. The metabasic rocks have been subdivided into two groups: f o l i a t e d glaucophanitic rocks and massive rocks containing jadeite-acmite pyroxene. Gradations between each of these rock types are, however, common. Fo l i a t e d metabasic rocks are characterized by a high glaucophane content, a dearth of r e l i c t minerals and f a i r l y uniform f a b r i c . T y p i c a l l y they contain: glaucophane 238 TABLE 22 MINERAL ASSEMBLAGES IN LAWSONITE-GLAUCOPHANE BEARING METAVOLCANICS 24 2? 28 29 31 32 33 34 36 37 38 39 40 42 44 45 46 47 48 49 10 8 - 5 10 - - - - - - - 2 - tr - x 6 x x lo 1 x 15 10 20 x 5 - tr tr 10 10 5 30 15 x 27 x x l l 1 x 5 5 20 x 16 15 - 65 5 45 52 35 41 x 48 x x 22 10 x 35 20 32 x 50 46 50 22 39 27 - 12 12 x 6 x x 11 3 x 10 8 13 x 17 23 - 10 1 5 - tr - - 1 tr t r t r - -15 - - - 5 x x 37 70 x 5 27 5 - 12 16 30 tr 25 7 8 - 2 x t r x x 9 - x - - - - - - t r t r - 4 - 2 0 x 7 - - - - x 15 10 - x - - - tr - -- t r - - - - - - - - - - tr - - - -3 3 - - t r - t r - - t r - - - - - - - - -10 - - - - - - - - - - - - - - - - - - -- tr - - - - - - - - - - - - - - - - -- 20 5 x - x x 9 7 x 10 10 10 x - - 20 3 15 aragonite chlorite glaucophane lawsonite sphene quartz acmitic px. white mica pyrite stilpnomelane^ brown hbl. ^ magnetite # hematite # deerite # dolomite # calcite # augite (r) opaques (r) 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 201 202 205 aragonite chlorite glaucophane lawsonite sphene quartz acmitic px. white mica pyrite stilpnomelane^ brown hbl, # magnetite # hematite ^ deerite ^ dolomite & calcite ^ augite (r) opaques (r) x x 5 X X x 15 X X X X X X - 15 X - X - X t r x 8 5 4 x x - - x 2 x x 68 x 65 31 x x 57 - x 40 x x x x x x 30 x x x l 0 - l l l 5 x x l 5 x x 8 x x x x x x l 0 x - x - x - - - x - - x l 0 x x x - 1 5 x - - - x l 8 x x l 0 x x 2 - x - - x -x x X X X - X - - X X X x 15 x X x x 11 tr tr 15 30 :ote a) epproxlmc.te melon £ lven l n Korae c a E e s , b) * rnodsl a n a l y s i s o b t n i n e d u s i n g a Z e i s s a l c r o m e t c r e y e p i e c e o:.d c o u n t i n g o v e r 500 p o l n t f l , c) " M " ratw.p t r a c e o f m i n e r a l l r . p r e s e n t , J) ( r ) = r e l i c t m l n e r n l , c:) // = re t r o g r e s K l ve o r n l t';r'« t l o n s i l rsnrnl tmd c) 7. = m i n e r a l p r e s e n t . 1 239 TABLE 23 TEXTURAL DOMAINS IN LAWSONITE-GLAUCOPHANE BEARING METABASIC ROCKS Matrix: jd-acm + lws + sph + c h l ± glph ± wh m ± s t i l p ± qtz ± py R e l i c t minerals: ilmenite, augite Coften altered to jadeite-acmite pyroxene or glaucophane) Blebs and pseudomorphs: lws + wh m + c h l (plagioclase pseudomorphs) lws + c h l + arag + glph + s t i l p (plag. pseudomorphs) arag + c h l + wh m ± s t i l p (plag. pseudomorphs) lws ch l ± s t i l p (olivine pseudomorphs ?) jd-acm + lws ± wh m ± glph arag ± sph (rims) (amygdules) Vein assemblages and paragenesis: Time jd-acm lws + c h l ± wh m arag + c h l ± wh m glph + qtz ± s t i l p -? s t i l p ab + magn -? arag + brown hbl - ? ' c a l c i t e R e l i c t textures: p o r p h y r i t i c , t r a c h y t i c , amygdaloidal, glomeroporphyritic. Note: Abbreviations given on p. 74 . 240 PIG. .34 TEXTURES IN LAWSONITE-GLAUCOPHANE BEARING METABASIC ROCKS acm-jd rims 025 0-5mm a) Plagioclase phenocrysts pseudomorphed by tabular lawsonite and c h l o r i t e i n sphene 4- Jadeite-acmite matrix; cross-cutting a l b l t e vein (Spec. 6l)« b) R e l i c t augite zoned to jadeIte-acmlte and sporadically rimmed by riebeckite l n lawsonite + c h l o r i t e matrix; l a t e deerite sprays (Spec. 2 3 ) . c) Aragonite/calcite blebs rimmed with sphene i n lawsonite + sphene + c h l o r i t e + glaucophane T jadeite-acmite matrix; r e l i c t augite with acmite-jadeite on rims and along fractures; bleb contains c h l o r i t e + stilpnomelane (Spec.38) d) glaucophane + quartz + lawsonite £ c h l o r i t e veins cross-cutting jadeite-aomite vein l n lawsonite + Jadeite-acmite + sphene matrix (Spec. 57) • 241 + lawsonite + sphene ± c h l o r i t e ± stilpnomelane. Sporadic grains containing hematite rimming p y r i t e and/or magnetite may also be present. I d i o b l a s t i c lawsonites are set i n a f i n e l y woven matrix of lineated glaucophane with sphene occurring i n stringers p a r a l l e l to the f o l i a t i o n . Massive rocks display an abundance of r e l i c t igneous textures similar to the greenstones of Pinchi Mountain. Mineral assemblages c h a r a c t e r i s t i c of thin section domains are given i n Table 23: textures are i l l u s t r a t e d i n F i g . 34. The t y p i c a l assemblage contains jadeite-acmite, lawsonite, sphene and c h l o r i t e with glaucophane, aragonite and white mica as possible add i t i o n a l phases. Glaucophane i s commonly found f i l l i n g fractures or immediately adjacent to fractures (Fig. 34}. In rocks containing both acmite-jadeite and glaucophane, textures indicate that the pyroxene formed pr i o r to the glaucophane. P y r i t e , rimmed by magnetite and hematite (Fig. 30) constitutes about 0.1% of a l l thin sections. Dolomitic carbonates The main minerals present within the dolomitic lime-stones are aragonite, c a l c i t e and dolomite (Table 12). Disseminated carbonaceous material i s a minor accessory i n most limestones and b a r i t e and quartz are found l o c a l l y . Limestones have been subdivided into two categories: 242 PIG. 35 TEXTURES IN CARBONATES rial Spec. 77 , ( |:| scale) Spec. 213 (scale I-l ) Note : samples were stained f o r CaCO^ using a l i z a r i n e red solution and for aragonite using F i e g l ' s solution (Friedman, 1959) j i d e n t i f i c a t i o n of carbonate mineralogy was confirmed by optics and X-ray d i f f r a c t i o n . 243 massive l i m e s t o n e s w h i c h c o n s t i t u t e most exposures and f o l i a t e d l i m e s t o n e s o c c u r r i n g e i t h e r as t h i n l a y e r s i n v o l c a n i c s o r as f o l i a t e d zones i n m a s s i v e l i m e s t o n e s . A t y p i c a l specimen o f m a s s i v e l i m e s t o n e ( F i g . 3 5 a ) c o n t a i n s 5 0 % e u h e d r a l d o l o m i t e rhombs CO.2 mm) c l o s e l y a s s o c i a t e d w i t h g r a n u l a r , opaque carbonaceous m a t e r i a l i n an a r a g o n i t e - c a l c i t e m a t r i x . I n t h i n s e c t i o n , a r a g o n i t e shows no s i g n s o f d i s e q u i l i b r i u m w i t h d o l o m i t e and i s p r e f e r e n t i a l l y r e p l a c e d a l o n g c l e a v a g e s and margins by f i n e g r a i n e d C 8 0 y ) c a l c i t e . L a t e i r r e g u l a r f r a c t u r i n g and v e i n i n g by w h i t e s p a r r y c a l c i t e CO.4 mm) tends t o g i v e the r o c k a b r e c c i a t e d appearance. A c o n c e n t r a t i o n o f c a r b o n -aceous g r a n u l e s i s commonly seen a t t h e margins o f t h e s e l a t e c a l c i t e v e i n s . A f o l i a t e d l i m e s t o n e i s i l l u s t r a t e d i n F i g . 35b. I t c o n t a i n s c o n c o r d a n t s t r i n g e r s o f carbonaceous a r a g o n i t e , w h i t e a r a g o n i t e and d o l o m i t e w h i c h a r e c r o s s - c u t by an a r a g o n i t e v e i n . There i s no i n v e r s i o n o f a r a g o n i t e t o c a l c i t e , the l a t t e r m i n e r a l b e i n g found o n l y i n l a t e f r a c t u r e s . I n summary, i t appears t h a t a carbonaceous l i m e s t o n e w i t h v a r i a b l e d o l o m i t e c o n t e n t was metamorphosed i n t h e b l u e s c h i s t f a c i e s t o a r a g o n i t e + d o l o m i t e . The d o l o m i t e -a r a g o n i t e l a m i n a e w i t h i n f o l i a t e d c a r b o n a t e s may i n d i c a t e t h a t the metamorphism was s y n k i n e m a t i c d u r i n g the F^ d e f o r m a t i o n . T h i s was f o l l o w e d by a r a g o n i t e v e i n i n g w h i c h 244 TABLE 24 MINERAL ASSEMBLAGES IN METAGREYWACKES s p e c no. 69 70 76 75 74 71 72 j a d e i t i c px. X X X X X - X glaucophane X X X X X 3 X lawsonite X X X X X 38 X quartz X X X X X 7 X white mica X X X X X 10 X c h l o r i t e X X - - - 5 X stilpnomelane X X sphene - - X - - 2 X aragonite X - - X - - X p y r i t e - - X X X - -opaques X X X X - -c l a s t s carbonaceous - ~ - X X — TABLE 25 MINERAL ASSEMBLAGES IN CHERTS AND CHERTY GRAPHITE SCHISTS spec. no. 116 122 12? 143 13tt 151 H8 123 148 150 98 90 65 x x x x x x x 2 5 t r x x - - - -5 x x x x x - -10 x t r x x x x x 3 - x - - x x x - - - - - - x x quartz glaucophane lawsonite white mica carb.granules p y r i t e acmitlo px. #? aragonite sphene stilpnomelane a l b i t e § magn.Aem. # t r 4 2 t r t r t r t r x x X note i a) modes are approximate b) " t r " means trace. c) # ! retrograde mineral d) x : mineral present 245 cross-cuts the f o l i a t i o n and presumably took place i n a d i f f e r e n t stress regime from that e x i s t i n g during the F^ deformation. Sparry c a l c i t e veins and inversion of aragon-i t e to c a l c i t e occurred during a la t e r deformation (F^7) presumably at a higher s t r u c t u r a l l e v e l . Metagreywackes The metagreywackes r e t a i n many sedimentary features despite having a thoroughly r e c r y s t a l l i z e d f a b r i c . The o r i g i n a l sediment appears to have been poorly sorted and consisted predominantly of plagioclase with minor quartz, ilmenite and carbonaceous c l a s t s ; clay minerals and carbonates probably constituted much of the matrix. D r i l l core shows fine-grained carbonaceous interbeds and a close association with graphitic cherts. Metamorphic re c o n s t i t u t i o n of the greywacke produced the mineral assemblage: j a d e i t i c pyroxene + lawsonite + white mica + quartz + glaucophane + c h l o r i t e + stilpnomelane + aragonite + sphene. Apparent equilibrium assemblages with modes are given i n Table 24. Fabric elements can be considered under four headings: matrix, pseudomorphs, r e l i c t minerals and veins. The matrix consists of microgranular quartz, white mica, c h l o r i t e , j a d e i t i c pyroxene, sphene, and lawsonite. Aragonite may be present. Rectangular c l a s t s , thought to have been plagio-246 PIG. 36 TEXTURES IN METASEDIMENTS a) Dolomite rhombs i n aragonite/inverted c a l c i t e matrix (dolomitic limestone-spec. 82). b) Pseudomorphed d e t r i t a l grains i n metagreywacke; plagioclase i s replaced by lawsonite aggregates and Jadeite 4- quartz grains appear to replace d e t r i t a l pyroxene, amphibole or p l a g i o c l a s e . Note d e t r i t a l quartz and unfoliated f a b r i c (Spec. 71) . c) Fractured glaucophane i n metachert (assemblage : glaucophane + quartz + lawsonite + phengite + magnetite + carbonaceous granules); a l b l t e grains appear to have c r y s t a l l i z e d a f t e r the deformation which fractured the glaucophanes (i.e.F 2)(Spec . 1 5 3 ) . d) Contorted phengite i n metachert (Spec . 153) . 247 clase p r i o r to metamorphism, contain lawsonite aggregates (Fig. 36b).. These generally show a preferred o r i e n t a t i o n , presumably controlled by the atomic structure of the pre-e x i s t i n g a l b i t e (?\. J a d e i t i c pyroxene and quartz also appear to replace r e l i c t d e t r i t a l grains which by t h e i r morphology could have been pyroxene, amphibole or plagio-clase. However, as lawsonite commonly replaces plagioclase, pyroxene or amphibole are considered the most l i k e l y precursors. Other r e l i c t c l a s t i c grains include quartz, p y r i t i c carbonaceous material and s k e l e t a l ilmenite pseudomorphed by sphene. Aragonite veins were observed i n a few t h i n sections. Glaucophane c r y s t a l s are i d i o b l a s t i c and generally comprise only a small proportion of the rock. Retrograde metamorphism or a l t e r a t i o n gave r i s e to stringers of leucoxene, yellow or white i n r e f l e c t e d l i g h t and veining a l l metamorphic minerals. The stringers are commonly associated with hematite granules. Metacherts Typical metacherts consist of 1 mm to 2 cm q u a r t z i t i c beds separated by t h i n laminae of glaucophane + white mica + lawsonite ± carbonaceous material ± sphene ± p y r i t e . Metacherts grade into graphite s c h i s t s . Individual mineral assemblages are given i n Table 25. Quartz grain size varies between 20 and 200]i. Grain boundaries are generally polygonal but may be sutured i n specimens where undulose 248 e x t i n c t i o n i s observed. I d i o b l a s t i c , fractured glaucophanes, averaging 1 mm in. length (Fig. 36c) show a strong preferred orientation generally with the c axes sub-parallel to the (L^l crenulate l i n e a t i o n . A microprobe traverse of a glaucophane i n specimen 151 showed the c r y s t a l s to be zoned with Fe r i c h cores and margins CFig. 26). Phengitic mica i s generally contorted (Fig. 36d) and wraps around lawsonite CO.5 mm) and glaucophane porphyroblasts. Carbonaceous granules commonly occur as inclusions i n the mica. Retrogressive minerals appear to have formed during a post-metamorphic period of deformation {F^ ?)• Glauco-phanes are broken, with the fractures f i l l e d with quartz or occasionally a l b i t e (Fig. 36c). Within g r a p h i t i c cherts post-tectonic p y r i t i c quartz veins are common and contain a l b i t e at intersections with micaceous layers. Magnetite and hematite replace p y r i t e and form as inclusions i n glaucophane. Lawsonite tablets contain microgranular aggregates and possess cross-fractures f i l l e d by quartz veins. Some samples (e.g. 134) contain aggregates of 50 to 70y acmitic (?) pyroxene. Whether t h i s pyroxene i s part of the i n i t i a l equilibrium assemblage or i s retrogressive i s uncertain. I t i s concluded that the a l b i t e , hematite, magnetite, leucoxene, the quartz-pyrite veins and possibly the acmitic pyroxene formed during retrogressive metamorphism which was possibly associated with the F. 0 deformation. 249 Quartz-carbonate rocks and schists In the v i c i n i t y of the mine, where exposures are p l e n t i f u l , the following assemblages were noted i n l i t h o l o g i e s interbedded with cherts or limestones: Ci) quartz + aragonite + white mica C i i ) quartz + white mica + glaucophane + lawsonite + c h l o r i t e . The f i r s t assemblage consists of equigranular C3Ou) quartz and aragonite containing occasional aragonite porphyroblasts CO.7 mm). Within the second assemblage lawsonite forms euhedral porphyroblasts Cl mm max) exh i b i t i n g polysynthetic twinning and i s commonly fractured and a l t e r e d . In general, phengitic mica and glaucophane display t e x t u r a l relationships similar to those i n metacherts. Most thin sections from the mine v i c i n i t y show signs of a pervasive a l t e r a t i o n which preceded the mercury mineral-i z a t i o n and post-dated the formation of metamorphic minerals. C a l c i t e , dolomite, ankerite, quartz and limonite are commonly found as replacements or veins i n most rocks, l o c a l l y obscuring the primary metamorphic mineralogy. APPENDIX V ECLOGITE BOULDERS Glaucophane bearing eclogite boulders were found at two l o c a l i t i e s i n the course of t h i s study. The f i r s t (Fig. 37, spec. 103) i s located 9 km east of Pinchi Lake Mercury Mine on the Tezzeron Lake logging road. I t measures 12 x 4 x 3 m and i s apparently embedded i n g l a c i a l t i l l overlying Takla Group rocks. The boulder contains f o l i a t e d and lineated blocks up to 30 cm i n diameter con-s i s t i n g of green pyroxene and garnet a l t e r i n g to stilpnomelane. Glaucophane and lawsonite occupy i n t e r s t i c e s between blocks and also permeate them. Late cross-cutting fractures are f i l l e d with c h l o r i t e and a brown amphibole. The second l o c a l i t y (Fig. 37, spec. 214) l i e s outside the thesis area 24 km east-southeast of Fort St. James beside the Beaver Lake logging road. Two boulders are present, measuring 4 x 2 x 2 m and 7 x 5 x 3 m; both are embedded i n t i l l and possess g l a c i a l s t r i a e on t h e i r surfaces. The main lithology i s dark blue massive glaucophanitic rock containing zones r i c h i n garnet, white mica, pyroxene, lawsonite and p y r i t e . Quartz veins are also present. In thin section the white mica appears undeformed. A K-Ar radiometric date on the white mica (Appendix VI) gave an age of 218 ± 7 m yrs. 251 Tezzeron Lake ^^y'LS^' • eclogite locality % inferred source area 0 5 kms. F i g . 37 E c l o g i t e l o c a l i t i e s and source areas Cas i n f e r r e d from g l a c i a l transport d i r e c t i o n ! 252 The i n f e r r e d s o u r c e a r e a f o r t h e s e b o u l d e r s i s i l l u s t r a t e d i n F i g . 37. Because t h e d i r e c t i o n o f i c e move-ment d u r i n g t h e l a t e s t g l a c i a t i o n was towards t h e e a s t -n o r t h e a s t (Armstrong, 19491 i t i s assumed t h a t t h e b o u l d e r s were d e r i v e d from t h e west. E c l o g i t e has n o t been found w i t h i n t h e glaucophane l a w s o n i t e b e a r i n g r e g i o n b u t c o n s i d e r i n g t h e h i g h p r e s s u r e o r i g i n of e c l o g i t e s (Chapter IV) and t h e i r a s s o c i a t i o n w i t h glaucophane, i t i s b e l i e v e d t h a t t h e y o r i g i n a t e d from w i t h i n t h e P i n c h i F a u l t zone. I n C a l i f o r n i a , e c l o g i t e specimens have been o b t a i n e d from i s o l a t e d t e c t o n i c b l o c k s w i t h i n glaucophane s c h i s t t e r r a i n . Coleman et al., (1965) c o n s i d e r : T hat t h e s e e c l o g i t e s a r e n o t i n p l a c e and have been t r a n s p o r t e d t o t h e i r p r e s e n t p o s i t i o n as i n c l u s i o n s i n ' d x a p i r i c s e r p e n t i n e s or i n shear zones o f major f a u l t s . A s i m i l a r o r i g i n i s proposed f o r t h e P i n c h i e c l o g i t e s . APPENDIX VI POTASSIUM-ARGON RADIOMETRIC DATES Sample locations are given i n Map VII (237, 124, 151) and F i g . 37 (214). Specimens 237 and 124 (quartz + white mica + lawsonite + glaucophane schist) were c o l l e c t e d from the open p i t at Pinchi Lake Mercury Mine. There i s con-siderable carbonatization i n the area but the micas appear to be fre s h and unaltered i n thin section apart from the presence of minute a c i c u l a r r u t i l e (?) needles. Glaucophane, lawsonite and white mica are deformed by F^. Sample 151 (quartz + glaucophane + phengite + magnetite metachert) was colle c t e d 3.5 km northwest of Pinchi Mine'. Phengitic muscovites are deformed by F^ and contain minor amounts of carbonaceous material. Approximately 10% of the micas are s l i g h t l y i r o n stained. Sample 238 was obtained outside the thesis area 24 km east-southeast of Fort St. James, from an eclogite boulder containing glaucophane + lawsonite + pyroxene + garnet + p y r i t e and white mica occurring i n sehlieren. In thin section, the white mica i s undeformed. After crushing and sieving, e s s e n t i a l l y pure white mica separates were obtained using a water column and heavy l i q u i d s . Potassium-argon analyses were ca r r i e d out by J.E. Harakal and V. Bobik i n the laboratories of the 254 TABLE 26 ANALYTICAL DATA FOR POTASSIUM-ARGON ANALYSES Sample no. 237 124 151 214 Lo c a t i o n : l a t . 54 ° 38' 5" 54 ° 38' 5 " 54 ° 39' 9" 54 ° 24* 30" long. 124° 26' 15" 124° 2 6 ' " l 5 " 124° 28• 48" 123 ° 53* 00* Hock type m i c a - s c h i s t m i c a - s c h i s t metachert e c l o g i t e M i n e r a l muscovite muscovite muscovite muscovite Mesh s i z e 28-48 48-65 48-65 60-80 K % ± <r 8.63 ± 0.05 8.36 ± 0.04 7.60 + 0.04 7.96 + 0.02 !*0Ar rad* 40 Ar t o t a l 0.95 O.96 0.93 0.94 4<>Ar rad (10~5cc STP/gm) K40 7.810 1.337 x I O - 2 7.375 1.303 x I O - 2 6.823 1.326 x I O - 2 7.285 1.352 x 1 0 - 2 Apparent age 216 + 7 m.y. 211 + 7 m.y. 214 + 7 m.y. 218 + 7 m.y. ** Potassium analyses by J.S. Harakal, and V. Boblk u s i n g KY and KY-3 flame photometers; Q- = standard d e v i a t i o n . * Argon analyses by J.E. Harakal using MS-10 mass spectrometer. Constants used l n model age c a l c u l a t i o n s : \ = 0.585 x l O - l O y " 1 , \ - 4.72 x 10-1V 1 . S 0 K / K = 1 > l 8 1 x 1 Q _ 4 e . Specimen l o c a t i o n s are given I n ' F i g . 43(237, 124, 151) and F i g . 42 (214). 255 Geology and Geophysics Departments at the University of B r i t i s h Columbia using procedures and equipment previously described by White et al. 3 (1967). A n a l y t i c a l data and apparent ages are given i n Table 26. APPENDIX V I I CALCULATION OF EQUILIBRIUM:CONSTANT FOR REACTION: PLAGIOCLASE = JADEITIC PYROXENE + QUARTZ Sin c e AG = -RT, In K • Jl 8 A G A T T and = A V _ 31nK - A V then = whence l n K 2 - I n K± ="|| ( P 2 - P.^ Now, i f standard s t a t e i s taken a t P^ then l n = 0 and- i n K ^ = = ^ (P, - P,) (A) A c c o r d i n g to McConnell and McKie C 1 9 6 0 ) most d i s o r d e r -i n g i n a l b i t e takes p l a c e between 575° and 625°C. T h e r e f o r e , i n the c a l c u l a t i o n of A V , data f o r low a l b i t e has been used. Employing t h e o r e t i c a l and experimental d a t a , Newton and Smith (J.966) produced a breakdown curve f o r low a l b i t e = j a d e i t e + q u a r t z . Two p o i n t s on t h i s curve a r e : (a) T = 773°K, P^ = 14.3 kb (b) T = 473°K, P o = 8.2 kb 257 Equation (A) can be solved for several d i f f e r e n t values of 3 -1 -1 K at each temperature using R = 83.14 bar cm deg mol and AV = 16.98 cm mol . At T = 773°K a l n K (P b - 14300) and at T = 473°K, c l n K = ^ CPd - 8200) Results are tabulated v/below;,. and i l l u s t r a t e d on F i g . 8b, K P b P d .05 2.96 1.26 .1 5.58 2.87 .2 8.21 4 .47 .3 9.74 5.41 .4 10.81 6 .08 .5 11.68 6.59 .6 12.37 7.02 .7 12.95 7.37 .8 13.45 7.68 .9 13.90 7.95 Assuming unit a c t i v i t y for quartz and mole f r a c t i o n (X) equal to a c t i v i t y , K = X?* / X ^ ^ g . 258 APPENDIX VIII SPECIMEN NUMBERING SYSTEM Note: locations of most specimens are given i n Map VII using "thesis number". Thesis Hand Specimen Thesis Hand Specimen Number Number Number Number 1 P-17-69 39 P-81-68 2 P-66-69 40 P-91-68 3 P-218-69 41 P-93-68 4 P-234-69 42 P-94-68 5 5-71P-5A 43 P-79-68 6 8-71P-4 44 P-27-69 7 P-70-69 45 P-30-69 8 P-19-69 46 P-31-69 9 P-60-69 47 P-33-69 10 P-68-69 48 P-92-69 12 P-116-69 49 P-93-69 13 P-117-69 50 P-96-69 14 P-200-69 51 P-101-69 15 P-210-69 52 P-105-69 16 P-214-69 53 P-106-69 17 P-225-69 54 P-119-69 18 P-232-69 55 P-121-69 19 P-242-69 56 P-131-69 20 P-243-69 57 P-138-69 21 S-6-69 58 P-140-69 22 P-122-68 59 P-142-69 23 420-1589 60 H-C-5 24 420-1579 61 P-188-69 25 420-1581 62 P-195-69 26 P-2 CH-67 63 P-196-69 27 P-8-67 64 P-249-69 28 P-9-67 65 HM-10-604 29 P-6-68 66 HM-8-149 30 P-18-68 67 HM-200-417 31 P-23-68 68 P-131-135 32 P-23-68 69 P-7-68 33 P-29-68 70 P-99-68 34 P-30-68 71 P-191-69 35 P-44-68 72 P-207-69 36 P-45-68 73 HM-20-651 37 P-78-68 74 HM-11-344 38 P-80-68 75 HM-I1-185 259 Thesis Hand Specimen Thesis Hand Specimen Number Number Number Number 76 HM-8-610 126 P-76-68 77 P-l-67 127 P-82-68 78 P-19-68 128 P-88-68 79 P-58-68 129 P-89-68 80 P-75-68 130 P-135-68 81 P-98-68 131 P-10-69 82 P-148-68 132 P-12-69 83 P-90-68 133 P-13-69 84 P-65-68 134 P-25-69 85 P-70-68 135 P-48-69 86 P-72-68 136 P-53-69 87 P-73-68 . 137 P-73-69 88 P-152-68 138 P-77-69 89 P-153-68 139 P-83-69 90 P-154-68 140 P-88-69 91 P-165-69 141 P-90-69 92 P-2-70 142 P-143-69 94 P-28-68 143 P-146-69 95 P-38-68 144 P-157-69 96 P-40-68 145 P-170-69 97 P-56-68 146 421-155 98 P-62-68 147 HM-10-79 99 P-104-68 148 HM-10-458 100 P-105-68 149 HM-10-690 101 P-110-68 150 HM-11-613 102 5-71P-9E 151 P-220-69(1) 103 5-71P-9E 152 P-220-69 (2) 104 5-71P-9E 153 P-220-69(3) 105 5-71P-9E 154 P-152-69 106 P-59-69 155 P-152-69 107 5-71P-3 156 P-154-69 108 11-71P-1 157 r Stuart Lake 109 16-71P-3 158 110 6-71P-5B 159 Antimony Mine 111 8-71P-3 160 P-112-68 112 8-71P-2A 161 P-145-68 113 8-71P-2B 162 P-146-68 115 P-3-67 163 P-39-69 116 P-5-68A 164 P-162-69 117 P-5-68B 165 P-189-69 118 P-8-68 166 HM-2-54 119 P-14-68 167 HM-4-102 120 P-15-68 168 HM-3-94 121 P-16-68 169 HM-5-308 122 P-21-68 17 0 P-37-69 123 P-48-68 171 P-100-68 124 P-50-68 172 P-101-68 125 P-49-68 173 P-161-69 260 Thesis Hand Specimen Thesis Hand Specimen Number Number Number Number 174 HC-1 210 P-87-68 175 HC-2 211 P-6-68 176 HC-3 212 P-224 177 HC-4 213 P-112-69 178 P-275-69 214 24-72P-1 179 P-110-69 ' 215 P-2-68 180 P-282-70 216 P-12-68 181 2020-169. 217 P-25-68 182 1-Nat Cu 218 P-102-68 183 2-Nat Cu 219 P-123-68 184 2022-362 220 P-65-69 185 HMP-11-120 221 P-94-69 186 HMP-12-140 222 P-97-69 187 P-68-68 223 P-116-69 188 P-216-69 224 P-118-69 191 S-71P-7 225 P-124-69 192 10-71P-6 226 P-131-69 193 P-147-68 227 P-247-69 194 HMP-8-100 228 S-7-69 195 3-71P-7A 229 1-71P-2 196 9-71P-2A 230 4-71P-1C 197 9-71P-2B 231 4-71P-2 198 6-71P-4A 232 6-71P-3 199 P-2 (.2) -67 233 7-71P-2 200 10-71P-4B 234 13-71P-1 201 10-71P-4C 235 P-201-69 202 P-282 236 P-4-67 203 10-71P-SC 237 15-71P-4 204 P-252-69 239 P-109-68 205 P-85-68 240 S-11-69 206 P-85-68 241 15-71P-2 207 P-283-69 242 15-71P-3 208 P-721 243 P-87-68 209 P-185-69 244 P-278-69 261 LIST OF PLATES 1. View of the Pinchi Lake area looking northwest from the summit of Mount Pope. 2. Laminated s i l t s t o n e s and sandstones i n the Takla Group. 3. Northwesterly plunging mullions i n quartz-mica-carbonate s c h i s t . Note prominent j o i n t surfaces perpendicular to mullions. 4. Angular limestone cobble conglomerate i n Takla Group. "M" points to Monotis s u b e i r e u l a r i s occurring within cobbles and i n matrix. 5. Northerly dipping compositional layering i n s i l i c a -carbonate rocks. Note layers of rough weathering ferroan magnesite veined by quartz, enclosing layer of pure white magnesite. 6. Primary (?) two phase f l u i d i n c l u s i o n i n metachert. 7. Late pyroxenite layers p a r a l l e l to dunite r i c h layers i n harzburgite. 8. Late discordant pyroxenite layer cross-cutting i r r e g u l a r dunite. To the l e f t , off the photograph, t h i s pyroxenite cross-cuts the dunite harzburgite contact and also an early pyroxenite layer. Note s l i g h t o f f s e t of pyroxenite by fracture cleavage. 9. Smooth weathering dunite layer p a r a l l e l to f o l i a t i o n i n harzburgite outlined by v a r i a t i o n i n o l i v i n e / pyroxene r a t i o . Note the chromite stringer i n the dunite layer. 10. Folded early pyroxenite layer.. 11. Folded dunite layer i n harzburgite. Note the weak f o l i a t i o n which p a r a l l e l s j o i n t s on r i g h t side of outcrop. 262 L E G E N D CRETACEOUS OR LOVER TERTIARl USLIKA FORMATION ( ? ) i conglomerate UPPER TRIASSIC 4 LOWER JURASSIC TAKLA GROUP 10) greywacke, s l l t s t o n e i 11) limestone 12) basic rocks PENNSYLVANIAN & PERMIAN CACHE CREEK CROUP MOUNT POPE BELT |H PI 7) llnestonesi 8) chertt 9) volcanic breccia. CACHE CREEK CROUP (?) GREENSTONES OF PINCHI MOUNTAIN 5) basalti 6) limestone BASIC ROCKS SOUTH 0? PINCHI LAKE gabbro, diabase, greenstone GLAUCOPHANE BEARING ROCKS OP PINCHI LAKE TERTIARY (?) carbonatlsed serpentinite.(l6a), sediments (16b) JURASSIC (?) MaW| 1) metabasic rocksi 2) chert, sch i s t , greywacke 3) dolomitic limestone i Ei eclogite |^  13 j hornblende d i o r i t e PERMO-TRIASSIC TREMBLEUR INTRUSIONS jfrofiVttl harzburclte, minor dunite and pyroxenite (14a) serpentinite (lUb) GEOLOGY OF THE PINCHI LAKE AREA IOOO 2000 3000 4000 m»rr»» STUART LAKE SYMBOLS strike and dip of bedding strike and dip of bedding (tops known) strike and dip of overturned bedding f o l i a t i o n (SjJ pyroxenite layering ln ultramafites l i n e a t i o n (J-2) axial trace of folds y1^// faults (defined, approximate, assumed) inferred dip of fault plane S^, geologic contact (defined, approximate) —». l i m i t of exposure •P-2 f o s s i l l o c a l i t y Note i a) exposure Is poor l n l i g h t l y coloured areas b) extrapolation beneath d r i f t cover Is based on topographic and magnetic expression of rock units and sparse d r i l l hole Information. o) geology by I.A. Paterson (1968, 1969i 1971) MAP II CROSS SECTIONS OF TS£ PINCHI LAKE A R E A AEROMAGNETIC PROFILES OBTAINED FROM M A P HI MAP / MAP III JT S A M BBBMJ MAP IV f o l i a t i o n ( S j ) l i n e a t i o n ( L ? ) and f o l d axea ( P " 2 ) s f o l d axes ( P j ) \ ^ I n f e r r e d P 2 antlforma/synforma '"X " ^ • ^ I n f e r r e d P j antlforras/synforms l i m i t of exposure approximate g e o l o g i c a l c o n t a c t '•"» l i m i t of a l t e r a t i o n l n nine area 100 2 00 3 0 0 4 0 0 500 Metres See M a p I f o r m o p l o c a t i o n * * I b r e c c i a zone MESOZOIC UPFEB TRIASSIC & LOWER JURASSIC greywacke ( T a k l a Croup) FERKO- TRIASSIC s e r p e n t i n i t e (Trembleur I n t r u a l o n s ) LATE PALEOZOIC CACHE CREEK GROUP (?) greenstones of F l n c h l Mountain, minor limestone. -lawsonite-glaucophane be'arlng metabasic rocks massive d o l o m i t i c l i m e s t o n e s metacherts, g r a p h i t e s c h i s t s , quartr-caxbonate s c h i s t s metagreywacke MAP V MAP V GEOLOGY OF AN AREA ON THE NORTHWEST SHORE OF PINCHI LAKE « 0 & 7 5 f < - o ilk OUTCROP C L I F F MARSH T A L U S ROAD-S T R E A M FOL IATED M E T A V O L C A N I C MASS IVE METAVOLCAN IC MASS IVE L I M E S T O N E F O L I A T E D L I M E S T O N E fj r FAULT ( A P P R O X I M A T E ) ^ — > CONTACT (OEF1NEO, APPROX- ) * V . FOL IAT ION (S,) >% C L E A V A G E (Sj) ""V ' JO INT /^j? L INEAT IONS < L , , L a , L r > MINOR FOLD A X E S /// I ' 2 ' 3 ' A P P R O X I M A T E AXIAL TRACE i3 M E T A C H E R T rupj S«t F-ty, 3 for mop locat ion MAP VI M A P VII PINCHI LAKE M A P II PINCHI PINCHI MOUNTAIN FAULT MURRAY RIDGE PINCHI FAULT CROSS SECTiONS OF T*tE PINCHI L A* E AREA AESOMAGNETIC PROFILES OSTAiNEO FROM MAP III r c a L AND LEG MAP l M A P III MAP IV f o l i a t i o n (Sj) lineation (L 2) and fold axes (F^) fold axes (Fj) \ Inferred Fj antlforms/sjnfoiiaa " " \ * \ inferred Fj antlforai/i/nfonu l i m i t of expoaura approximate geological contact H a l t of alteration l n alne area 1,\ ^ faulta (defined, approximate, assumed) S C A L E : 4 0 0 8 0 0 1200 1 6 0 0 Feat 100 2 0 0 3 0 0 4 0 0 5 0 0 Mitret See Map f for mop location HESOZOIC JffFEB TBIASSIC * 10VEB JliBASSIC greywacke (Takla Group) FERMO-TRIASSIC . serpentinite (Treaoleur Intrusions) .. LATE PALEOZOIC CACHE CHEEK GBODP (») rpm greenstones of Flnchl (fountain, nlnor llnestone. -laaaonlte-glaucophane bearing metabasic rocks • BBSI ITO dolomitic line8tone• 1 I metachert*, graphite schists, quartz-carbonate schists [s^ lc^ lf netagrejwacke N MAP V MAP VI * M A P VII 

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