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Geology of the Three Ladies Mountain/Mount Stevenson area, Quesnel Highland, British Columbia Getsinger, Jennifer Suzanne 1985

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GEOLOGY OF THE THREE LADIES MOUNTAIN/MOUNT STEVENSON AREA QUESNEL HIGHLAND, BRITISH COLUMBIA by JENNIFER SUZANNE GETSINGER A.B., HARVARD UNIVERSITY, 1974 M.S., UNIVERSITY OF WASHINGTON, 1978 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF GEOLOGICAL SCIENCES We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA APRIL 1985 © JENNIFER SUZANNE GETSINGER, 1985 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of G&ot-o &i cfrk "Sd^jces The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date ABSTRACT In late Proterozoic to early Paleozoic time, continent-derived c l a s t i c sediment and minor carbonate of the Snowshoe Group were deposited in a continental slope to shelf environment, and shallow marine e l a s t i c s and carbonates of the Cariboo Group were deposited nearer to the shore of North America. The Snowshoe Group is divided into a lower sequence of micaceous quartzite, p e l i t e , and minor amphibolite, a l l interlayered with quartz d i o r i t e sheets; and an upper sequence of micaceous quartzite, p e l i t e , and carbonate with minor c a l c - s i l i c a t e and amphibolitic rocks. Early i s o c l i n a l (F1A) and NE-verging tight folds (F1B) formed together with a metamorphic f o l i a t i o n . Tight to normal, c y l i n d r o i d a l second phase (F2) folds, characterized by SW-vergence and NW plunge, formed during the mid-Jurassic Columbian orogeny at about the same time as accretion of suspect terranes southwest of the map area. Prograde metamorphism in the Barrovian series of amphibolite facies was synkinematic to postkinematic to F2 folding, with maximum metamorphic r e c r y s t a l l i z a t i o n outlasting deformation. Garnet-biotite geothermometry indicates temperatures of 525 ± 20°C for p e l i t e s near the kyanite to s i l l i m a n i t e zone isograd. Garnet-aluminosilicate-ilmenite geobarometry l i m i t s P to less than 7 kb. Grossular-anorthite-aluminosi1icate geobarometry gives P = 5.5 ± 0.7 kb. i i Retrograde metamorphism and F3 kink-folding occurred during u p l i f t , followed by broad warping (F4) with NE trend. The low-angle, postmetamorphic L i t t l e River Fault emplaced c h l o r i t e to b i o t i t e zone p h y l l i t e and carbonate of the Cariboo Group, in the hanging wall, against staurolite-kyanite to s i l l i m a n i t e schists and gneisses of the Snowshoe Group, in the footwall, with l a t e s t movement of the hanging wall in an ESE d i r e c t i o n . A Rb-Sr model depositional age of approximately 750 Ma, assuming an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.708, was obtained for Snowshoe Group metasedimentary rocks. Paleozoic plutonism i s indicated by a Rb-Sr whole-rock isochron date of 530 ± 94 Ma with i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.706, and U-Pb dates on zircon, indicating a minimum age of 335 Ma and maximum age of about 450 Ma, for quartz d i o r i t i c gneiss intrusive into the Snowshoe Group early in i t s deformational history, pre-F1B folding. Late- to post-metamorphic pegmatite cooled through 400-500°C at 86 ± 3 Ma. The age of the L i t t l e River Fault is bracketed between intrusion of pegmatite and a Miocene(?) erosion surface. Table of Contents ABSTRACT i i LIST OF TABLES v i i LIST OF FIGURES . . v i i i LIST OF PLATES x i i i ACKNOWLEDGEMENT xiv 1. INTRODUCTION 1 1.1 LOCATION AND OBJECTIVES 1 1 . 2 PREVIOUS WORK 4 1.3 REGIONAL GEOLOGIC SETTING 6 2. LITHOSTRATIGRAPHY 8 2.1 LITHOSTRATIGRAPHY OF SNOWSHOE GROUP 8 2.2 LITHOSTRATIGRAPHY ABOVE THE LITTLE RIVER FAULT ..18 3. STRUCTURE 25 3.1 DEFORMATION: FOLD PHASES " 25 3.1.1 FOLD NOTATION 25 3.1.2 OUTLINE OF FOLD PHASES 28 3.1.2.1 F 1 A 28 3.1.2.2 F1B 28 3.1.2.3 F 1A and F1B 31 3.1.2.4 F2 34 3.1.2.5 F3 36 3.1.2.6 F4 40 3.1.3 FOLDS ABOVE LITTLE RIVER FAULT 41 3.2 STEREOGRAPHIC PROJECTIONS OF STRUCTURAL ELEMENTS 43 3.2.1 DOMAINS 43 3.2.2 DEFINITION OF TERMS 45 iv 3.2.3 SUMMARY OF STEREOGRAPHIC PROJECTIONS 46 3.2.4 SUMMARY OF FOLDING 7 0 3.3 DEFORMATION: FAULTING 71 3.3.1 LITTLE RIVER FAULT 71 3.3.2 DISCUSSION OF LITTLE RIVER FAULT 81 3.3.3 OTHER FAULTS 89 4. METAMORPHISM 91 4.1 INTRODUCTION • 91 4.2 METAMORPHIC ZONES IN PELITES 95 4.2.1 INTRODUCTION 95 4.2.2 METAMORPHIC ZONES 97 4.3 GARNET ZONING 122 4.3.1 DESCRIPTION OF ZONING PROFILES 122 4.3.2 DISCUSSION OF GARNET ZONING 134 4.4 STAUROLITE BREAKDOWN 137 4.5 GARNET-BIOTITE GEOTHERMOMETRY 144 4.6 GEOBAROMETRY: GRAIL 150 4.7 GEOBAROMETRY: GROSSULAR-ANORTHITE-ALUMINOSI LICATE-QUARTZ 155 4.8 CONCLUSION 1 62 5. IGNEOUS ACTIVITY 170 5.1 QUARTZ DIORITIC GNEISS 170 5.2 GRANODIORITlC GNEISS 172 5.3 PEGMATITE 1 73 5.4 QUARTZ VEINS 175 5.5 ECONOMIC GEOLOGY 176 5.6 VOLCANIC ROCKS 177 6. GEOCHRONOLOGY 179 v 6.1 GEOCHRONOLOGY ABSTRACT 179 6.2 INTRODUCTION 180 6.3 GEOLOGIC SETTING OF DATED SAMPLES 183 6.4 Results and interpretation 185 6.5 Conclusions 196 7. SUMMARY 197 8. CONCLUSION 201 REFERENCES 208 APPENDIX I: METAMORPHIC MINERALS 215 APPENDIX II: MICROPROBE ANALYSES 227 APPENDIX IIIA: ANALYTICAL METHODS FOR GEOCHRONOLOGY 236 APPENDIX IIIB: GEOCHRONOLOGY SAMPLE DESCRIPTIONS 237 vi $0 DR=621 $5.47, $5.57T LIST OF TABLES TABLE I. MINERAL DATA 141 TABLE II . GARNET-BIOTITE TEMPERATURES 147 TABLE I I I . GROSSULAR-ANORTHITE-ALUMINOSILICATE GEOBAROMETRY 160 TABLE IV. RUBIDIUM-STRONTIUM ANALYTICAL DATA 187 TABLE V. U-PB ZIRCON DATA FROM QUARTZ DIORITIC GNEISS INTRUSIVE INTO SNOWSHOE FORMATION 188 LIST OF FIGURES Figure 1 . Location map 2 Figure 2. Lithostratigraphy of Snowshoe Group 10 Figure 3. Lithostratigraphy above L i t t l e River Fault. . 19 Figure 4. Diagrammatic summary of fo l d phases 26 Figure 5. A. Refolded i s o c l i n e (F1A, F1B) in quartzite. B. Quartzite layers (Q) preserve F1 folds 29 Figure 6 A and B. Quartzitic layers (Q) preserve F1 is o c l i n e s 30 Figure 7. Tectonic p r o f i l e through Three Ladies Mountain. 33 Figure 8. A. SW-verging F2 fo l d in quartzite (Q) flanked by p e l i t e (P). B. I s o c l i n a l l y folded quartz segregation (Q) and quartzo-feldspathic layer (QF) in kyanite-grade gneiss. 35 Figure 9. A, B, C. Generalized cross sections through Service Mountain ("6287'") showing geology and relationships among F2, F3, and F4 folds 37 Figure 10. Sketch of sample showing F3 crenulation cleavage crosscutting an F1 fold 39 Figure 11. A. Weak f o l i a t i o n in micaceous quartzite (Q) p a r a l l e l to NE-dipping a x i a l plane of SW-verging fold (F2) . B. Thin layers in grey marble at "6287'" are subparallel to regional NE-dipping F2 a x i a l plane, but l o c a l l y kinked by F3 folds with SE-dipping a x i a l planes. 42 v i i i i Figure 12. Structural domain map of the Three Ladies Mountain area 44 Figure 13. Equal area projections for Domain II 50 Figure 14. Equal area projections for Domain III 52 Figure 15. Equal area projections for Domain IV 54 Figure 16. Equal area projections for Domain VIII and Domain X. . 56 Figure 17. Equal area projections for Domains I, V, and VII, f o l i a t i o n s and lineations 58 Figure 18. Equal area projections for combinations of structural elements from several domains 60 Figure 19. Equal area projections of pre-F2 f o l d elements for combinations of domains; andF2 fold elements in Domain IX, Mount Stevenson area 62 Figure 20. Equal area projections for Domains VI and IX. Mount Stevenson area 64 Figure 21. Equal area projections of fault surfaces, slickensides, and j o i n t s 66 Figure 22. Equal area projections of str u c t u r a l elements in hanging wall of L i t t l e River Fault, Domain XI 68 Figure 23. A. View of Three Ladies Mountain from Service Mountain. B. Sketch of view in A showing trace of L i t t l e River Fault and exhumed tectonic denudation surface (stippled) 72 Figure 24. Structures below but related to L i t t l e River Fault; for location see Figure 23 B 77 ix Figure 25. Pressure/temperature diagram i l l u s t r a t i n g AFM assemblages for metamorphic zones in the Three Ladies Mountain/Mount Stevenson area in re l a t i o n to bathozones as defined by Carmichael (1978), and modified by Archibald et a l . (1983) 96 Figure 26. Garnet zone (2), sample 82-376. Main metamorphic f o l i a t i o n (S f) of b i o t i t e and muscovite, wrapping around garnet, was subsequently folded by F3 folds 99 Figure 27. Staurolite zone (3), sample 81-344. Large s t a u r o l i t e porphyroblast includes i d i o b l a s t i c garnet and t r a i l s of r u t i l e and quartz (St-) p a r a l l e l to S e(S 2) 102 Figure 28. Staurolite-kyanite zone (4), sample 79-13. Staurolite-kyanite-garnet-muscovite schist contains c h l o r i t o i d (CD) inclusions in 'snowball' (rotational) garnet synkinematic with F2 folding 106 Figure 29. Staurolite-kyanite zone (4), sample 81-234. Garnet rims, s t a u r o l i t e , and kyanite are a l l h e l i c i t i c , including folded t r a i l s of graphite. Sj-is p a r a l l e l to Se 107 Figure 30. St a u r o l i t e - k y a n i t e - s i l l i m a n i t e zone (5), sample 81-279. A t o l l , or ring-shaped, garnet 110 Figure 31. S t a u r o l i t e - s i l l i m a n i t e zone (6), sample 80-30. F i b r o l i t i c s i l l i m a n i t e forming at the expense of staurol i t e 115 Figure 32. S i l l i m a n i t e zone (7), sample 80-31. Tiny garnets outline a former garnet porphyroblast 118 Figure 33. Garnet zoning p r o f i l e for Garnet 2 from sample 82-389, st a u r o l i t e zone 125 Figure 34. Garnet zoning p r o f i l e for Garnet 3 from sample 81-279, staurolite-kyanite zone 126 Figure 35. Garnet zoning p r o f i l e for Garnet 4 from sample 81-278, s t a u r o l i t e - k y a n i t e - s i l l i m a n i t e zone 127 x Figure 36. Garnet zoning p r o f i l e for Garnet 1 from sample 80-33, s t a u r o l i t e - k y a n i t e - s i l l i m a n i t e zone 128 Figure 37. Garnet zoning p r o f i l e for Garnet 1 from sample 80-119, s t a u r o l i t e - k y a n i t e - s i l l i m a n i t e zone 129 Figure 38. Garnet zoning p r o f i l e for Garnet 2 from sample 80-19, s t a u r o l i t e - s i l l i m a n i t e zone 130 Fi.gure 39. Garnet zoning p r o f i l e for Garnet 3 from sample 80-31, s i l l i m a n i t e zone 131 Figure 40. Garnet zoning p r o f i l e s for Garnets 3.1 and 2 from sample 81-325, kyanite zone 132 Figure 41. Garnet zoning p r o f i l e s for Garnets 4 and 1 from sample 81-325, kyanite zone 133 Figure 42. Calculated s t a u r o l i t e breakdown reactions using thermodynamic data from Lang and Rice (1985) 142 Figure 43. Pressure/temperature diagram of "GRAIL" geobarometer (modified from Bohlen et a l . 1983). .. 151 Figure 44. Sketch of photomicrograph (sample 81-344) i l l u s t r a t i n g how r u t i l e i s stable as inclusions in st a u r o l i t e , but unstable r e l a t i v e to ilmenite where in contact with quartz 153 Figure 45. Schematic cross sections of Three Ladies Mountain (3LM) area showing rela t i o n s h i p between metamorphic isograds (symbols as on Plate I I I ) , f o l i a t i o n (dashed l i n e s ) , and L i t t l e River Fault (LRF). 166 Figure 46. Summary diagram showing va r i a t i o n of temperature, pressure (or depth), and "in t e n s i t y of deformation" through time, plotted on logarithmic scale. 168 Figure 47. Sample l o c a l i t y map for dated samples 186 xi Figure 48. Rb-Sr isochrons for metasedimentary rocks, quartz d i o r i t i c gneiss, granodioritic gneiss, and pegmatite 189 Figure 49. U-Pb zircon concordia for quartz d i o r i t i c gneiss 190 Figure 50. Schematic tectonic history of the Omineca Belt in the v i c i n i t y of the Three Ladies Mountain/Mount Stevenson area 202 LIST OF PLATES ALL PLATES ARE SEPARATE MAP SHEETS. PLATE I. BEDROCK GEOLOGY OF THREE LADIES MOUNTAIN/MOUNT STEVENSON AREA, QUESNEL HIGHLAND, BRITISH COLUMBIA PLATE I I . GEOLOGIC CROSS SECTIONS, THREE LADIES / MOUNTAIN/MOUNT STEVENSON AREA [ PLATE I I I . METAMORPHIC ZONES AND SAMPLE LOCALITIES, THREE i LADIES MOUNTAIN AREA / PLATE IV. FOLIATION AND BEDDING I PLATE V. F l AND F2 FOLDS PLATE VI. F3 AND F4 FOLDS; JOINTS, FAULTS, AND SLICKENSIDES ACKNOWLEDGEMENT I am sincerely indebted to my supervisor, Dr. Hugh • Greenwood, for a l l his encouragement and support throughout the duration of thi s project. I am grateful to the other members of my advisory committee — Dr. J.V. Ross, Dr. R.L. Armstrong, and Dr. K.C. McTaggart — for their e f f o r t s toward helpful and educational c r i t i c i s m , discussion, and advice. This work could not have been done without the patient help of those who assisted me in the f i e l d and in the lab, and the help of numerous colleagues and friends who kept me going. The'work was supported by grant EMR-65-1677 from the Geological Survey of Canada and NSERC grant 67-4222 to H.J. Greenwood, research supervisor. The 1981 f i e l d season was partly supported by the Geological Society of America, grant 2831-81. Electron microprobe work was done under the supervision of John Knight; and Rb-Sr a n a l y t i c a l work was done under the supervision of K. Scott, in R.L. Armstrong's laboratory at the University of B r i t i s h Columbia. P. van der Heyden did the U-Pb dating of the zircon separate. R.L. Armstrong suggested dating methods and possible interpretations of the data. xiv 1. INTRODUCTION 1.1 LOCATION AND OBJECTIVES The Three Ladies Mountain/Mount Stevenson area i s located along the west side of the north arm of Quesnel Lake, within the Quesnel Highland part of the Cariboo Mountains. It is e n t i r e l y within the Omineca Belt, although the suture with the Intermontane Belt l i e s only about 10 km south of Mount Stevenson (see Figure 1). The present study involved detailed mapping at a scale of 1:25,000 in the Three Ladies Mountain/Mount Stevenson area during the 1979-1982 f i e l d seasons, covering an area of approximately 200 km2 (see Geologic Map, PLATE I ) . This rugged t e r r a i n i s covered with rainforest at Quesnel Lake (725 m) to alpine meadows on Three Ladies Mountain (2187 m) and Mount Stevenson (2243 m), a r e l i e f of over 1500 m. Objectives of this study were to sort out the internal stratigraphic sequence, and s t r u c t u r a l and metamorphic history of the Snowshoe Group, which was previously poorly defined in t h i s area. The Snowshoe Group l i e s within an area of metasedimentary rocks between sediments derived from North America which l i e to the east and volcanic and sedimentary rocks belonging to suspect terranes to the west. This area has recently been referred to as the "suspect" Barkerville terrane (Struik 1984, Monger and Berg 1984). The d e t a i l s of i 1 2 Figure 1. Location map showing relationship of study area (stippled pattern) to terranes of the Canadian C o r d i l l e r a . Cretaceous and/or Tertiary granitoic bodies are in flecked pattern; Paleozoic orthogneisses are in squiggle pattern. Abbreviations: IB = Intermontane Belt, OB = Omineca Belt, RMFTB = Rocky Mountain Fold and Thrust Belt, NAm = North America, CC = Cache Creek Terrane, QN = Quesnellia, SM = Slide Mountain Terrane, BV = Barkerville Terrane, KO = Kootenay Terrane, CB = Cariboo Terrane, SH = Shuswap Complex, MO = Monashee Complex, MG = Malton Gneiss; FR = Fraser River, QL = Quesnel Lake, TR = Thompson River, RMT = Rocky Mountain Trench, CR = Columbia River. References: Monger and Berg (1984); Tipper et. a l (1981). 3 the structural and metamorphic his t o r y of t h i s area are therefore important in restraining large-scale tectonic hypotheses and speculations. 4 1.2 PREVIOUS WORK Reconnaissance g e o l o g i c work in the area northwest of Quesnel Lake was done in the 1950's and e a r l y 60's by the B.C. Department of Mines (Holland 1954; Sutherland Brown 1957, 1963) and the G e o l o g i c a l Survey of Canada (Campbell 1961, 1963), p a r t l y in response to i n t e r e s t i n g o l d mines around Wells and B a r k e r v i l l e , B.C. H o l l a n d (1954) f i r s t d e s c r i b e d and named the Snowshoe Formation and assigned i t to the upper part of the Cariboo Group, as d i d Sutherland Brown (1957, 1963) and Campbell (1961, 1963). More recent work has p l a c e d the Snowshoe Group e i t h e r below the Cariboo Group along with the Kaza Group (Campbell et a l . 1973; Campbell 1978; S t r u i k 1979, 1981a, 1981b); or as part of the Cariboo Group west of the Pleasant V a l l e y F a u l t but not east of i t , and p o s s i b l y p a r t l y l a t e P a l e o z o i c i n age ( S t r u i k 1982); or as a separate group of rocks of l a t e P r o t e r o z o i c and/or P a l e o z o i c age ( i n " B a r k e r v i l l e terrane") juxtaposed with the Cariboo Group ( i n "Cariboo t e r r a n e " ) along f a u l t s ( S t r u i k 1983, 1 9 8 4 b ) . Many d e t a i l e d s t u d i e s by graduate students and t h e i r s u p e r v i s o r s have c o n c e n t r a t e d on v a r i o u s aspects of r e g i o n a l g e o l o g i c problems such as s t r a t i g r a p h i c c o r r e l a t i o n s , s t r u c t u r a l h i s t o r y , a p p a r e n t l y steep metamorphic g r a d i e n t s , and the nature of the boundaries of the Shuswap Metamorphic 5 Complex (K.V. Campbell 1971; Fletcher 1972; Pigage 1977, 1978; Fletcher and Greenwood 1979; Poulton and Simony 1980; Klepacki 1980, 1981; Rees 1981; P e l l and Simony 1981, 1982; P e l l 1982; Murphy and Journeay 1982; Getsinger 1982, 1983a, 1983b; Murphy and Rees 1983; Raeside and Simony 1983; Rees and F e r r i 1983; Dechesne et a l . 1984; Murphy 1984). 6 1.3 REGIONAL GEOLOGIC SETTING The Three L a d i e s Mountain/Mount Stevenson area l i e s c o m p l e t e l y w i t h i n the Omineca C r y s t a l l i n e B e l t , which i s the e a s t e r n of two major exposures of metamorphic and p l u t o n i c i n f r a s t r u c t u r a l r o c k s i n the Canadian C o r d i l l e r a . T h i s b e l t of p r i m a r i l y metamorphic r o c k s l i e s between the l e s s metamorphosed and unmetamorphosed s u p r a s t r u c t u r a l r o c k s of the Rocky Mountain F o l d and T h r u s t B e l t t o the e a s t and the Intermontane B e l t t o the west (Monger et a l . 1982). R e g i o n a l metamorphism and i n t e n s e d e f o r m a t i o n i n the Omineca C r y s t a l l i n e B e l t took p l a c e i n m i d - J u r a s s i c time d u r i n g c o l l i s i o n of N o r t h America w i t h a w e s t e r n composite t e r r a n e composed of f o u r v a r i o u s l y d i s p l a c e d t e r r a n e s which were amalgamated by the end of the T r i a s s i c ( E a s t e r n Assemblage or T e r r a n e , Q u e s n e l l i a , Cache Creek T e r r a n e , and S t i k i n i a or S t i k i n e B l o c k ) (Monger et a l . 1982). The S t i k i n e B l o c k s u b s e q u e n t l y u n d e r t h r u s t p a r t of the r e s t of the composite t e r r a n e t o the n o r t h and e a s t d u r i n g the m i d - J u r a s s i c , thus c o n t r i b u t i n g to i n c r e a s e d d e f o r m a t i o n (Monger et a l . 1982). The B a r k e r v i l l e T e r r a n e , which i n c l u d e s m a i n l y r o c k s of the Snowshoe Group, l i e s e a s t of a l l known a c c r e t e d t e r r a n e s , and may be c o n s i d e r e d t o be p a r a u t o c h t h o n o u s i n r e l a t i o n t o N o r t h America. In the Three L a d i e s Mountain/Mount Stevenson a r e a , the Snowshoe Group c o n s i s t s of a t h i c k s e c t i o n of c o n t i n e n t a l s l o p e and s h e l f e l a s t i c s and minor c a r b o n a t e s of p r o b a b l e l a t e P r o t e r o z o i c age which have been metamorphosed under 7 amphibolite facies conditions. It is overlain by more shallow marine rocks of the late Proterozoic to Paleozoic Cariboo Group, which are metamorphosed to greenschist f a c i e s . Both groups of rocks were folded and metamorphosed during the mid-Jurassic Columbian orogeny. The present contact between the Snowshoe Group and the Cariboo Group in the study area i s a low-angle postmetamorphic fault known as the L i t t l e River Fault. 2. LITHOSTRATIGRAPHY 2.1 LITHOSTRATIGRAPHY OF SNOWSHOE GROUP The rocks in the map area have not been p o s i t i v e l y correlated with those in any other area of the C o r d i l l e r a . The high grade metamorphic rocks west of the North Arm of Quesnel Lake, consisting of folded metasediments and various intrusive phases, have been c a l l e d Snowshoe Group and assigned a late Proterozoic to Paleozoic age by Campbell (1978) and other workers. Fletcher (1972) divided the same rocks into Isaac and Kaza Formations on the basis of a s i m i l a r i t y to his own f i e l d area at Penfold Creek, east of the North Arm of Quesnel Lake. Snowshoe Group rocks at Three Ladies Mountain may share some s i m i l a r i t i e s with the Snowshoe Group near Wells as mapped by Struik (1981), and resemble parts of the Horsethief Creek Group as mapped by P e l l (Pell and Simony, 1981) near Blue River. F o s s i l s and sedimentary structures showing d e f i n i t e facing directions have not been found. O r i g i n a l bedding is oblite r a t e d by cleavage and metamorphism, but l i t h o l o g i c units that can be distinguished are shown on the map (Plate I ) . The Snowshoe Group in the Three Ladies Mountain map area may be divided into a lower s t r u c t u r a l sequence, up to 6 km thick, underlying most of the area from Mount Stevenson to the North Arm of Quesnel Lake, and an upper st r u c t u r a l sequence best defined in the Three Ladies Mountain area, where i t is approximately 3 km thick. Much of the thickness 8 9 may be due to structural r e p e t i t i o n . Thicknesses may have been d i f f e r e n t before deformation (see Figure 2). The lower sequence consists of interlayered, f o l i a t e d grey micaceous quartzite, brownish-weathering quartz-rich biotite-muscovite schist, and minor impure carbonate, c a l c - s i l i c a t e , and amphibolitic gneiss. Grain size of the psammitic layers i s r e l a t i v e l y constant and does not exceed granule size; the coarse granule g r i t and conglomerate c h a r a c t e r i s t i c of the Kaza Group elsewhere are conspicuously lacking. Sedimentary structures other than l i t h o l o g i c layering and possible graded bedding were not observed. P e l i t i c schist containing aluminosilicate index minerals i s rare in the lower sequence. Much of the schist is semi - p e l i t i c , containing quartz, b i o t i t e , muscovite, plagioclase, and garnet, with no indicator for distinguishing kyanite zone from s i l l i m a n i t e zone. Amphibolite and carbonate occur as small lenses and are not associated with each other; amphibolite pods tend to occur near quartz d i o r i t i c s i l l s , whereas small carbonate units may be found dispersed within the schist. Greenish-grey, hornblende-biotite gneiss intercalated with the paragneiss is interpreted as quartz d i o r i t i c s i l l s , and may constitute up to 25% of the f o l i a t e d rocks. A l l are i r r e g u l a r l y intruded by quartz-plagioclase-muscovite pegmatite, which l o c a l l y constitute 50% of the t o t a l rock mass. Mixed metasedimentary gneisses and quartz d i o r i t i c s i l l s crop out from the western shore of the North Arm of Quesnel 10 UNIT Rock Type BRALCO MARBLE Coarse white calcite marble Grey, micaceous quartzite with interlayered pelite Pelite Carbonate/Amphibolite Pelite with impure carbonate Thinly-layered micaceous quartzite and pelite LOWER SEQUENCE Amphibolite lenses Quartz dioritic gneiss Thinly-layered micaceous quartzite and pelite (Covered) Micaceous quartzite, pelite, rare carbonate, and migmatitic gneiss SNOWSHOE FORMATION UPPER SEQUENCE F i g u r e 2. L i t h o s t r a t i g r a p h y of Snowshoe G r o u p i n t h e T h r e e L a d i e s Mountain/Mount S t e v e n s o n a r e a . 11 Lake south of Grain Creek to Mount Stevenson, and are p r a c t i c a l l y i n d i v i s i b l e s t r a t i g r a p h i c a l l y at the map scale, although some s i l l s have been mapped over several kilometres. Alternating l i g h t e r and darker gneisses resembling " l i t - p a r - l i t " migmatite, in which d i s t i n c t i o n between orthogneisses and paragneisses is uncertain, are c h a r a c t e r i s t i c of the lower part of the section, especially near Quesnel Lake. The uppermost part of the lower sequence consists mostly of thinly-layered, b i o t i t e - r i c h micaceous quartzite and semi-pelite; layers are 5 to 10 cm thick and flaggy where exposed. This flaggy micaceous quartzite and p e l i t e i s t r a n s i t i o n a l to the upper sequence. The upper boundary of the lower sequence is a r b i t r a r i l y chosen at the top of the f i r s t continuously mappable hornblende-bearing quartz d i o r i t i c s i l l , which i s subparallel to the intruded metasedimentary rocks. The upper sequence, in contrast, contains major carbonate and c a l c - s i l i c a t e , amphibolite, and common p e l i t i c s c h i s t . It lacks quartz d i o r i t i c s i l l s , and is not as extensively intruded by pegmatite. Internal stratigraphy i s variable due to discontinuous layering and possible repetition caused by complex f o l d i n g . The lowermost subdivision of the upper sequence consists of rusty-weathering p e l i t i c s c h i s t , micaceous quartzite, minor marble, and some magnetite-bearing schist. Between Welcome Mountain and Three Ladies Mountain the p e l i t i c schist contains large porphyroblasts of garnet, s t a u r o l i t e , and 1 2 kyanite. Whitish quartzite in d i s t i n c t i v e , massive layers about 1 m thick is interlayered with p e l i t e . Some flaggy, micaceous quartzite occurs in the lowermost part of this section, t r a n s i t i o n a l to similar rocks in the lower sequence. This mainly p e l i t i c unit averages about 500 m thick, but is thickened by~leucocratic granodioritic gneiss s i l l s north of Welcome Mountain. Overlying the lower p e l i t e is the most d i s t i n c t i v e subdivision of the upper sequence, containing a variety of rock types including hornblende amphibolite, carbonate, and c l a s t i c metasedimentary rocks. Most of the unit, which varies in thickness from 10 to over 150 m, weathers recessively to reddish-brown s o i l , d i s t i n c t i v e both on a e r i a l photographs and in the f i e l d . Thick, massive amphibolite layers are more resistant, as on the north face of Three Ladies Mountain, where a sheer 60 degree dip-slope of amphibolite drops 500 m from the summit to the valle y . The amphibolite unit i t s e l f appears to be a combination of amphibolitic metasedimentary rocks formed in reaction between p e l i t i c and carbonate units during regional metamorphism, and metamorphosed d i o r i t i c s i l l s . Reaction amphibolite includes nearly monomineralic layers of metamorphic hornblende grains up to several centimetres long, as well as more common c a l c - s i 1 i c a t e gneiss which may contain hornblende, diopside, garnet, epidote, plagioclase, and c a l c i t e . Hornblende in layered amphibolite and c a l c - s i l i c a t e rocks tends to be aligned along metamorphic 1 3 lineations (usually F2 fo l d a x i a l trends), or else in radiating habit on f o l i a t i o n planes (garbenschiefer); i t may be either synkinematic or postkinematic to F2 structures. Other amphibolite, occurring in discontinuous pods, is unfoliated, with coarse-grained, blocky hornblende and plagioclase, resembling d i o r i t e in hand specimen. Examination of thin sections shows that the amphibole is pseudomorphic after former blockier-shaped pyroxene, although no r e l i c t pyroxene was i d e n t i f i e d . Other rock types within the carbonate-amphibolite unit include impure c a l c i t e marble up to 20 m thick, minor c a l c - s i l i c a t e zones between p e l i t e and carbonate layers, flaky quartz-biotite schist, black s i l t i t e , and green, p y r i t i c quartzite (with up to 20 % pyrite l o c a l l y ) . The' black s i l t i t e i s a d i s t i n c t i v e rock type, occurring as discontinuous layers up to 5 m thick of fine-grained, hard, black, graphitic quartz s i l t i t e with a s l i g h t p h y l l i t i c sheen, and l o c a l "pencil cleavage", interfolded with porphyroblastic p e l i t i c schist (staurolite-kyanite grade), limy s c h i s t , and amphibolite. The black s i l t i t e i s not a graphite s c h i s t , as i t i s not schistose. The carbonate-amphibolite unit i s more highly fractured than the surrounding p e l i t e and quartzite, and has apparently suffered more hydrothermal a l t e r a t i o n . Crosscutting rusty zones with sparse s u l f i d e mineralization are associated with minor, late , near-vertical f a u l t s , and the nearby p e l i t i c schist i s more severely retrograded to 1 4 c h l o r i t e than that farther from the carbonate-amphibolite. The carbonate-amphibolite unit is generally overlain by another staurolite-kyanite schist with minor magnetite-bearing layers, and a thick section (up to 2 km) of grey, micaceous quartzite interlayered with quartzose, garnetiferous mica s c h i s t , which continues up to the base of a coarsely c r y s t a l l i n e white marble. The marble overlying the Snowshoe Group near Maeford Lake was previously mapped as Cunningham Formation (?) (Campbell 1978; Klepacki 1981), and has recently been assigned to the Snowshoe Group as the Bralco Succession marble by Struik (1983). The marble is up to 500 m thick near Maeford Lake, forms light-grey to buff-weathering c l i f f s , and can be traced for several kilometres to the northwest. It i s a coarsely c r y s t a l l i n e , massive, r e l a t i v e l y pure white c a l c i t e marble, with l o c a l s i l i c e o u s dolomite containing radiating needles of white tremolite. Layering is discontinuous, lensoidal in places, and is marked by f a i n t grey streaks and rare rootless i s o c l i n e s implying folding and possible transposition of bedding. Typical specimens display c r i s s - c r o s s i n g f o l i a t i o n s which may be interpreted as cleavages from various f o l d phases or, in some places, as a c a t a c l a s t i c f a b r i c , or as merely fractures. The base of the marble i s apparently conformable on Snowshoe Formation p e l i t e and quartzite. Although there are some thin, garnet and hornblende-bearing c a l c - s i l i c a t e i 1 5 layers in the schist below the contact, the Snowshoe Group is not gradational across the contact with the marble. There are no marble units within the Snowshoe near the contact. The gentler dip of the marble, in contrast to a steeper dip in the underlying p e l i t e , may be a result of measuring d i f f e r e n t f o l i a t i o n s in the two units, or evidence for a d u c t i l i t y contrast or tectonic contact. The contact is folded along with the rocks on both sides, but there i s no convincing evidence that i t is a fault contact. The environment of deposition of the Snowshoe Group was offshore from the margin of a continent which contributed fine p e l i t i c and coarser quartz-rich c l a s t i c detritus to a r e l a t i v e l y stable but subsiding slope or basin. The thick section of q u a r t z i t i c and p e l i t i c rocks exposed p a r t i c u l a r l y in the Mount Stevenson area (Snowshoe Group "lower sequence") could have been deposited off the continental slope in a r i s e environment, whereas the carbonate and limy p e l i t e higher in the section must have been deposited in water shallower than about 200 m. The Snowshoe Group i s t y p i c a l of sediments included in the late Proterozoic package of the Cordilleran miogeosyncline; i t is characterized by voluminous g r i t s and shale deposited in a thick c l a s t i c wedge, with some bank and offbank carbonate, and minor mafic volcanics (Poulton and Simony 1980). The stratigraphic position of the Snowshoe Group has long been unclear; no f o s s i l s have been reported. The Snowshoe Group was believed by early workers to o v e r l i e the 1 6 Cariboo Group, and is shown as "Lower Cambrian or l a t e r " on maps by R.B. Campbell (1961, 1963). Later work by Campbell et a l . (1973) suggested that the Snowshoe Group was "a western facies of the Kaza Group" (p. 32), and thus of late Proterozoic Windermere age. Getsinger (1982) suggested that the Snowshoe Group resembles parts of the late Proterozoic Horsethief Creek Group, rather than the Kaza Group. The sedimentology of the Snowshoe Group in the Three Ladies Mountain area is similar to that described by Poulton and Simony (1980) for the Horsethief Creek Group of late Proterozoic age. Struik proposed that one of the uppermost units of the Snowshoe Group (northwest of Wells, B.C.), i s late Paleozoic in age, based on uncertain l i t h o l o g i c c o r r e l a t i o n with a nearby unit consisting of marble, slate, and c r i n o i d a l limestone which yielded Mississippian conodonts (Struik 1982). However, i t would be pure speculation to try to correlate t h i s unit with any unit in the Three Ladies Mountain area, as further uncertainty would be involved in making l i t h o l o g i c c o r r e l a t i o n across 100 km (even along strike) of u n f o s s i l i f e r o u s rocks which themselves cannot be traced with complete confidence d i r e c t l y into the Three Ladies Mountain—Mount Stevenson area. This study suggests that the age of at least most of the Snowshoe Group in this area i s late Proterozoic, by comparison with descriptions from other areas, as well as according to a preliminary Rb-Sr whole rock date of 1 7 approximately 750 Ma for the time of sedimentation (see section on geochronology). 18 2.2 LITHOSTRATIGRAPHY ABOVE THE LITTLE RIVER FAULT The stratigraphy of the low-grade marble and p h y l l i t e above the L i t t l e River Fault is not yet uniquely defined. However, the rocks in thi s area have been assigned to some part of the late Proterozoic to lower Cambrian Cariboo Group (Campbell 1978; Klepacki 1981; Struik 1983) (see Figure 3). The lowermost units in the upper plate of the L i t t l e River Fault are exposed at Limestone Point, just east of where the fault disappears into Quesnel Lake. Below the bluff is a sequence of interlayered grey p h y l l i t e and thinly-bedded quartzite, most of which is pinkish-colored from iron-staining, and which is kinked with chevron folds. Bedding i s s t i l l distinguishable from cleavage in some places. B i o t i t e porphyroblasts are common in muscovite-chlorite p h y l l i t e below the limestone, but do not occur north of Service Creek. This unit resembles a description of the uppermost part of the Isaac Formation, "grey p h y l l i t e with sparsely interbedded s i l t s t o n e and limonitic fine-grained sandstone" in Campbell et a l . (1973, p. 34). Limestone Point is named for the massive, white c r y s t a l l i n e marble which forms a prominent c l i f f about 50 m high v i s i b l e from the whole length of the North Arm of Quesnel Lake. It resembles the Bralco marble in appearance, but occupies a di f f e r e n t structural and stratigraphic p osition. Bedding i s d i f f i c u l t to distinguish, but may be indicated by small, thin, sandy layers in the midst of more 19 50 m Rock Type Grey marble (Service Ridge) Greenish phyMite and minor quartzite Intercalated grey marble and calcareous phy Mite (Covered) Green and grey phyllite (Covered) White marble (Limestone Point) Pink and grey quartzite and phyllite, with biotite F i g u r e 3 . L i t h o s t r a t i g r a p h y above L i t t l e R i v e r F a u l t . 20 massive layers. Most of the marble is pure c a l c i t e , but some dolomite and more si l i c e o u s layers are also present. Carbonate breccia with brownish-weathering lenses (2-10 cm) in a whiter matrix is f o l i a t e d p a r a l l e l to the base of the marble unit. It is unclear whether the contact between the marble and the underlying p e l i t e and quartzite i s sedimentary or tectonic. The marble is d i r e c t l y overlain by thick g l a c i a l d r i f t ; i t s upper contact is not exposed. The marble at Limestone Point may be correlated with the Cunningham Formation, as shown on Quesnel Lake map sheet (Campbell 1978). The next section of metasedimentary rocks to the north, and s t r u c t u r a l l y higher than the marble of Limestone Point, is a sequence of interlayered marbles and limy, s i l i c e o u s p h y l l i t e , which i s exposed up to the top of the ridge above Service Creek. From Quesnel Lake north of Service Creek, at least 100 m of grey-green, s i l v e r y p h y l l i t e i s overlain by alternating c l i f f s of grey, c r y s t a l l i n e c a l c i t e marble and poorly-exposed, recessively-weathered layers of limy p h y l l i t e . There are at least six layers of marble of nearly uniform thickness (10-15 m) and similar l i t h o l o g y , varying s l i g h t l y in color from grey to pinkish with black streaks. Similar alternating marble and p h y l l i t e near the saddle where the L i t t l e River Fault crosses the ridge are probably co r r e l a t i v e with those on the slope above Quesnel Lake. Poor exposure prevents certain determination of whether the apparent repetition of rock types i s stratigraphic or i 21 st r u c t u r a l , or both. This sequence (which i s at least 500 m thick) has been mapped by Klepacki (1981) as Yankee Belle Formation, and by Struik (1983) as Cunningham Formation. It resembles the description of the "rhythmic limestone-siltstone-shale facie s " of the Yankee Belle Formation in Campbell et a l . (1973). Exposure i s better near the top of the ridge, where another greenish-grey p h y l l i t e with thin intercalated quartzite layers underlies a thick, grey marble, which forms the peak labelled "6287'" (geodetic survey point), informally referred to as Service Mountain or Service Ridge. It is possible to correlate t h i s p h y l l i t e unit with the Midas Formation, which is also recessive and incompetent where folded, and underlies a resistant grey marble (Campbell et a l . 1973). The Cariboo Group succession may have been thinner in this area than in the type sections. No unit resembling Yanks Peak Formation quartzite was ident i f ied. The grey marble at "6287'" i s about 50 m thick and dips gently to the northeast except where thickened and distorted by major folds, as at the top of the ridge. The grey marble i s the most resistant unit in the area and forms the tops of ridges; i t s upper contact was not observed in t h i s area, so i t may be thicker than 50 m. It i s a fine-grained, s t y l o l i t i c , s i l i c e o u s , grey c a l c i t e marble with white and grey streaky layering, and l o c a l p y r i t e , and i s inhomogeneous in texture. It is less coarsely c r y s t a l l i n e 22 than the Bralco marble, and could be c a l l e d a limestone, although i t has been r e c r y s t a l l i z e d in greenschist facies conditions. Clots of more resistant white carbonate blebs and oval rings about 1 cm across in one sample suggest r e c r y s t a l l i z e d organically-produced textures, but no recognizable f o s s i l s were found. Thin, slabby layers of grey marble about 5 cm thick, which look l i k e bedding on outcrop scale, dip 30-50° to the northeast. These layers intersect the more shallowly east-dipping bedding and contacts of the mappable unit at a high angle. These layers are interpreted as bedding transposed into F2 a x i a l plane or ientat ion. Locally near Service Mountain the grey marble i s crosscut by nearly v e r t i c a l zones of buff to pink calcite/dolomite breccia trending north to northeast. Ca l c i t e c r y s t a l s up to 5 cm and pink c a l c i t e marble form angular c l a s t s in a breccia with boxwork-1ike, p i t t e d weathering. These areas are interpreted as hydrothermally altered zones along faul t s or fractures within the grey marble, rather than as separate units. Struik (1983) divided the grey marble into an upper unit on Service Mountain, which he correlated with the Mural Formation, and the rest, which he correlated with the Cunningham Formation. This study interprets the grey marble as a single, continuous, but complexly folded marble unit probably c o r r e l a t i v e with the Lower Cambrian Mural Format ion. 23 The entire section of rocks above the L i t t l e River Fault is assigned here, with some uncertainty, to undivided Cariboo Group. The environment of deposition of the Cariboo Group was mainly nearshore, shallow-water marine. According to Campbell et a l . (1973), the conditions varied from a f l u v i a l d e l t a i c environment to l i t t o r a l , shallow shelf, or r e s t r i c t e d marine environments to deeper marine depths, but a l l less than 200 m. These conditions are reasonable for the types of sediments observed, although there i s l i t t l e evidence to support an a l l u v i a l environment in this area because of lack of coarse e l a s t i c s or recognizable sedimentary structures. The late Proterozoic to Cambrian Cariboo Group was deposited closer to shore and in shallower water than the Snowshoe Group. Either the Cariboo Group was l a t e r a l l y approximately time-equivalent to the Snowshoe Group, and their present juxtaposition i s due to telescoping of the continental margin during deformation; or the Cariboo Group was deposited conformably upon and overlying the Snowshoe Group, and their present juxtaposition is due to greater u p l i f t of the deeper rocks with a normal displacement along the L i t t l e River Fault. A combination of these interpretations i s preferred. If the co r r e l a t i o n of t h i s section of rocks in the hanging wall of the L i t t l e River Fault with apparently uninterrupted Cariboo Group i s correct, then the Pleasant 24 Valley Thrust, as mapped by Struik (1983) in the upper plate of the L i t t l e River Fault, cannot be a major terrane boundary. Other structural implications of th i s c orrelation are discussed in the following section on structure. 3. STRUCTURE 3.1 DEFORMATION: FOLD PHASES 3.1.1 FOLD NOTATION Four d i s t i n c t phases of fording have been distinguished on the basis of orientation of a x i a l surfaces, associated metamorphic fabrics, and f i e l d r elations of refolded folds and intrusive rocks. These fold phases are designated as F1, F2, F3, and F4 (see Figure 4). Metamorphic r e c r y s t a l l i z a t i o n was synkinematic to postkinematic to F2 folding, with c r y s t a l l i z a t i o n outlasting deformation. Time re l a t i o n s h i p s . i n the f i e l d are determined by observations of refolded folds, crosscutting relationships, intrusive contacts, and overprinting of metamorphic f a b r i c s . Figures 5 to 11 are sketches of folds; Figures 12 to 22 show domains and stereographic project ions. Pre-F2 folds have been divided into two subphases (F1A and F1B) of i s o c l i n a l and tight folds which may be related and are commonly indistinguishable in the f i e l d except in a few places. Two l i n e s of evidence led to their separation. Some i n t r a f o l i a l i s o c l i n e s were refolded p r i o r to F2 folding (see Figure S4 A). Near Mt. Stevenson, quartz d i o r i t i c gneiss s i l l s l o c a l l y crosscut i s o c l i n a l fold limbs and are i s o c l i n a l l y and 25 26 F1 A ISOCLINAL AXIAL PLANES PARALLEL TO FOLIATION SVV^. NE F1 B NORTH- & EAST-VERGING AXIAL PLANES 084/48N or VARIABLE AXES WNW ^ ^ ^ ^ ^ ^ F2 WEST-VERGING AXIAL PLANES 118/60NE AXES NW 315-340/30-45 F3 VARIABLE AXIAL PLANES SW- or E-DIPPING AXES NW or SE F4 UPRIGHT WARPS AXIAL PLANES 040/90 AXES NE 020-050/10-20 Figure 4. Diagrammatic summary of fo l d phases. 27 t i g h t l y f o l d e d t h e m s e l v e s ; t h e s e s t r u c t u r e s a r e o v e r p r i n t e d by F 2 f a b r i c s . i 28 3.1.2 OUTLINE OF FOLD PHASES 3.1.2.1 F1 A True i s o c l i n e s , p a r t i c u l a r l y i n t r a f o l i a l i s o c l i n e s and those refolded by later fold phases, are designated F1A (see Figures 5 and 6). Limbs are attenuated such that some have rootless hinges. Fold geometry tends to approximate that of similar folds (Type II, Ramsay 1967). Axial planes are a l l p a r a l l e l to the dominant f o l i a t i o n (S,), p a r a l l e l to compositional layering ( S 0 ) , except l o c a l l y in the carbonate layers, where they are variable. Fold axes were not easy to measure in most i n t r a f o l i a l i s o c l i n e s ; they show no p a r t i c u l a r pattern but tend to l i e in the northwest quadrant; some also plunge northeast. The inconsistent trends are interpreted as consistent with an e a r l i e r fold episode. 3.1.2.2 F1B Tight to i s o c l i n a l , asymmetrical folds which fold f o l i a t i o n (S,) and are also overprinted by crosscutting F2 a x i a l plane crenulation cleavage (S 2) are assigned to phase F1B. They tend to verge to the north and east, with SW-NW dipping a x i a l planes, unlike the more obvious F2 folds which verge west with NE-dipping a x i a l planes. Phase F1B folds are rarely preserved, most commonly in competent quartzite layers in the lower sequence, as near Welcome Mountain and Mount Stevenson; the quartz d i o r i t i c s i l l s also show thi s style of folding. These 29 Figure 5. A. Refolded isocline (F1A, F1B) in quartzite; hammer is p a r a l l e l to dominant regional f o l i a t i o n . 2 km east of Three Ladies Mountain. • B. Quartzite layers (Q) preserve F1 i s o c l i n a l and NE-verging (FIB) folds whereas p e l i t i c layers (P) are refolded into SW-verging F2 folds. 3 km east of Mount Stevenson. 30 Figure 6 A and B. Q u a r t z i t i c layers (Q) preserve F1 i s o c l i n e s whereas surrounding p e l i t e (P) i s crenulated and i n c i p i e n t l y transposed along NE-dipping F2 a x i a l planes. 3 km south of Mount Stevenson. 31 more competent layers folded by buckling, whereas the less competent p e l i t i c layers show thickening in hinge zones, as well as a tendency to become transposed by later folding. F1B folds were recognized l o c a l l y by clear crosscutting relationships in which the F1B Z-folds had retained their geometry in the more competent layers whereas the p e l i t i c layers had developed crenulation cleavage a x i a l planar to superimposed F2 S-folds on the same outcrop (see Figure 5 B) . On most outcrops, F1B f o l d axes are nearly coaxial with F2 fold axes. On outcrops where a quartz rod lin e a t i o n l i e s at an angle to the dominant l i n e a t i o n , the quartz rod li n e a t i o n i s p a r a l l e l to F1B fold axes, whereas the dominant li n e a t i o n (a combination of intersection and mineral orientation lineations) l i e s p a r a l l e l to F2 f o l d axes. This implies that most quartz rod lineations observed were formed during early folding. 3.1.2.3 F1A and F1B Both F1A and F1B appear to fold previous f o l i a t i o n s and compositional layering. The dominant f o l i a t i o n (S,) seen in outcrop in the Three Ladies Mountain/ Mount Stevenson area i s a x i a l planar to F1A and F1B folds; variation in limb attitudes l i e s within the spread of values for each limb and the a x i a l planes, showing that they are e s s e n t i a l l y p a r a l l e l . 32 No pre-F2 folds larger than outcrop scale have been p o s i t i v e l y i d e n t i f i e d in the f i e l d but i t is suspected that they may be there in the form of repetition of compositional layering. A possible closure near Three Ladies Mountain (between middle and south peaks) defined by narrowing and pinching out of an amphibolite unit, with porphyroblastic, aluminous p e l i t e on either side, is folded around an F2 synform (about 3 km across), ending in the hinge zone of the corresponding antiform. It i s possible that t h i s structure i s not an F1 closure but the regular succession truncated by a tectonic s l i d e near-parallel to the F2 a x i a l plane (see Figure 7 and Plate I ) . The more monotonous quartzites and p e l i t e s of the lower sequence east of Mount Stevenson to Quesnel Lake may contain several repetitions of o r i g i n a l stratigraphy, but even more extensive fieldwork would not be expected to show th i s as there are no clear marker units and the sequence i s thickened by various granitoid intrusions. If pre-F2 folds do exist on a regional scale, they could be very large and e s s e n t i a l l y i s o c l i n a l , with limbs possibly exceeding 10 km in length. The reasons for separating F1 folds into F1A and F1B are that (1) in some places, mid-Paleozoic quartz d i o r i t i c gneiss, which appears to crosscut e a r l i e r i s o c l i n e s and f o l i a t i o n , i s folded by tight folds which are overprinted by F2 a x i a l planar f a b r i c , and (2) some 33 Figure 7. Tectonic p r o f i l e through Three Ladies Mountain projected to a surface with orientation 060/45 SE, approximately perpendicular to F2 f o l d axis (330/45), with datum (X-X') at 1970 m (6000'). 34 isoc l i n e s are refolded by nearly i s o c l i n a l folds which are in turn folded by F2 folds. Such relationships are not conclusive, but rather suggestive evidence hinting at a possible Paleozoic deformation. Because the evidence for separating F1A from F1B is sparse, and because i t is possible to explain these structures in more than one way, they are designated as subphases to one episode of F1 or "pre-F2" folding. 3.1.2.4 F2 Folds assigned to phase F2 are the most prominent penetrative structures seen in the Three Ladies Mountain/Mount Stevenson area (see Figures 5 B, 6, 8, 9 A, 11, 13-15, and 19). They vary in scale from microscopic to macroscopic. A large F2-fold with wavelength about 5 km has been mapped on Three Ladies Mountain (see map Plate I and Figure 7). Hinge zones l i e in large cirques, in which steep north faces are limb dipslopes. F2 folds are tight to normal, inclined plunging, asymmetrical S-folds that f o l d the dominant f o l i a t i o n (S,). Some thickening of layers in fold hinges occurs, especially in the more p e l i t i c layers. They are mainly flattened f l e x u r a l s l i p folds (Type IC and II, Ramsay 1967). Peak metamorphic conditions accompanied F2 folding, implying a warm, d u c t i l e environment of format ion. 35 F i g u r e 8. A. SW-verging F2 f o l d i n q u a r t z i t e (Cj) i s f l a n k e d by p e l i t e (P) showing N E - t r e n d i n g F4 c r e n u l a t i o n s ( l e f t ) and i n t e r f e r e n c e p a t t e r n s ( r i g h t ) . 2 km e a s t of Three L a d i e s Mountain. B. I s o c l i n a l l y f o l d e d q u a r t z s e g r e g a t i o n (Q) and q u a r t z o - f e l d s p a t h i c l a y e r (QF) i n k y a n i t e - g r a d e g n e i s s are r e f o l d e d by SW-verging F2 f o l d . Large f e l d s p a r p o r p h y r o b l a s t s form p e g m a t i t i c s e g r e g a t i o n ( p a t t e r n ) a l o n g F2 a x i a l p l a n e , s u g g e s t i n g p a r t i a l melt o r i g i n f o r pegmatite d i k e s . 3 km s o u t h e a s t of Three L a d i e s Mountain. 36 Probably over 75% of folds d e f i n i t e l y i d e n t i f i e d as F2 folds verge southwest. They have consistently NE-dipping a x i a l planes with average attitude 118/60 NE. Fold axes plunge 20 to 60 degrees to the NNW and NW, varying from more steep at Three Ladies Mountain to less steep at Mount Stevenson (see stereographic projections). Well-defined mineral lineations (L 2) are p a r a l l e l or subparallel to F2 fold axes. Fold axes and lineations are perpendicular to the plane containing poles to folded f o l i a t i o n , demonstrating the generally c y l i n d r i c a l nature of the fo l d i n g . F2 folds are most obvious in contrasting and competent compositional layers such as quartzites and carbonates. Grenulation cleavage (S 2) i s developed p a r a l l e l to F2 a x i a l planes in p e l i t e s , and i s completely transposed p a r a l l e l to the F2 a x i a l plane in some places, p a r t i c u l a r l y in large hinge zones. P a r a s i t i c folds on the large S-shaped f o l d at Three Ladies Mountain are also dominantly west-verging, although some reversals in vergence were noted. Where both S-shaped and Z-shaped folds are seen in the same outcrop, the Z-folds are overprinted by crenulation cleavage a x i a l planar to the S-folds (see Figure 5 B). 3.1.2.5 F3 Local kinking and crenulation folds which are c l e a r l y post-F2 have been assigned to F3 (see Figure 9 ) . 37 NW < F3 F4 SE 1 6287 ' "* ^ F3 r n ^ ^ ^ _^ _ — 500 m Figure 9. A, B, C. Generalized cross sections through Service Mountain ("6287'") showing geology and relationships among F2, F3, and F4 folds, m = marble; p = p h y l l i t e and micaceous quartzite. The small "B" in A locates the outcrop shown in B. 38 Many F3 folds are coaxial or nearly so with F1 and F2 folds plunging NW, and some are east-verging with southwest-dipping a x i a l planes. Axial planes of F3 folds tend to be r e l a t i v e l y upright except for one group, which appears to be p a r a l l e l to joint planes perpendicular to F2 fo l d axes (see Figure 18 C). These lower angle features may be due to s l i g h t l y l a ter f a u l t - r e l a t e d deformation. F3 deformation had the effect of tightening and steepening F2 structures in the Three Ladies Mountain area, and may have i n i t i a t e d s l i d i n g along steepened F2 ax i a l planes. Kinks and chevron folds are p a r t i c u l a r l y well-developed in p h y l l i t e s in the hanging wall of the L i t t l e River Fault, but less v i s i b l e in the more highly metamorphosed Snowshoe Group sch i s t , where F3 folds are observed mainly in p e l i t i c units. Rare sets of conjugate kinks were observed. Variable f o l d attitudes and sparse occurrence account for the irregular and spread-out pattern on the stereographic projections (see Figure 18 C and D). In one case, east-verging, NW-plunging F3 crenulation folds folding F2 a x i a l plane cleavage in p e l i t e are c l e a r l y superimposed on an east-verging WNW-plunging F1B f o l d preserved in a f e l s i c layer (see Figure 10). In thi s sample, quartz-rod lineations are p a r a l l e l to the F1B fo l d axis and mica crenulations are p a r a l l e l to the F3 fo l d axis, they diverge by about 25°. Quartz-Biotite-Muscovite Schist Sample 81-286 F1 Axis Figure 10. Sketch of sample 81-286, a quartz-biotite muscovite schist from Station 104, showing F3 crenulation cleavage in p e l i t e crosscutting an F1 fold in a f e l s i c layer. 40 Specific examples of crosscutting and superposition of fold styles such as these have allowed for d i s t i n c t i o n of fold elements that plot as' overlapping d i s t r i b u t i o n s because of the v a r i a b i l i t y in each data set. Pegmatite intruded along F2 a x i a l planes shows poorly-defined f o l i a t i o n p a r a l l e l to F3 trends in some outcrops. In thin section, green c h l o r i t e is aligned along F3 a x i a l planes, and appears to be retrograde. Retrograde metamorphism i s believed to be contemporaneous with or later than F3 deformation. 3.1.2.6 F4 Large to small, upright open folds with v e r t i c a l a x i a l planes and shallow NNE to NE plunge, accompanied by minor crenulations in p e l i t i c layers, are assigned to fold episode F4 (see Figure 8 A, 9 C). Large F4 folds are concentric buckle folds of moderate amplitude with wavelength on the order of a few kilometers. These folds may account for the wide va r i a t i o n in plunge of F2 axes and linea t i o n s , including the southeast dip in parts of the map area. Quartz veins near Mount Stevenson are commonly nearly v e r t i c a l with NE s t r i k e , p a r a l l e l to F4 a x i a l plane fractures. Some of the curvature in the L i t t l e River Fault surface may be due to warping by F4 deformation, or perhaps both f a u l t i n g and F4 folding occurred during the same extensional event. i 41 3.1.3 FOLDS ABOVE LITTLE RIVER FAULT Fold history in Cariboo Group rocks above the L i t t l e River Fault is similar to that in the Snowshoe Group below i t , but only one possible first-phase, i s o c l i n a l f o l d was observed. In the hanging wall of the L i t t l e River Fault, the grey marble has near-horizontal to gently east-dipping bedding ( S 0 ) , but i s broken into slabs (average 5 cm thick) p a r a l l e l to F2 a x i a l planes (S 2) except where refolded into F3 kinks and chevron folds (see Figure 11 B). The p e l i t e , which is a c h l o r i t e grade p h y l l i t e , exhibits either F2 a x i a l plane cleavage (S 2) or i s transposed l o c a l l y to F3 a x i a l plane cleavage ( S 3 ) ; both cleavages may be developed in the same outcrop. Quartzite layers in the p e l i t i c units tend to retain pre-F2 layering (S 0?), although micas in fold hinges are reoriented on F2 a x i a l planes (S 2) (see Figure 11 A). F3 kinks and chevron folds plunge 080 to 100 (approximately E-W) and plunge up to 30° to the west or up to 15° to the east, depending on the limb of the F4 warp. Axial planes of kink folds trend around 010/30E l o c a l l y , p a r a l l e l to minor fault surfaces related to the L i t t l e River Fault. This suggests that movement related to the L i t t l e River Fault took advantage of F3 a x i a l plane surfaces, or that there is some genetic relationship between fa u l t i n g and kink fol d i n g . Figure 11. A. Weak f o l i a t i o n in micaceous quartzite (Q) is p a r a l l e l to NE-dipping axial plane of SW-verging fold (F2), whereas cleavage of interlayered p h y l l i t e (P) dips southeast (F3 ax i a l plane), p a r a l l e l to the face of the outcrop. Near Service Mountain ("6287'"). B. Thin layers in grey marble at "6287'" are subparallel to regional NE-dipping F2 axial plane, but l o c a l l y kinked by F3 folds with SE-dipping axial planes. 43 3.2 STEREOGRAPHIC PROJECTIONS OF STRUCTURAL ELEMENTS Domains are shown on Figure 12 and stereographic projections for various s t r u c t u r a l elements in eleven domains are shown in Figures 13 to 22. 3.2.1 DOMAINS Domains for p l o t t i n g s t r u c t u r a l elements were chosen on the basis of large structures and l i t h o s t r a t i g r a p h i c boundaries in the f i e l d . Domains I through VII are in Snowshoe Group "upper sequence"; Domains VIII, IX, and X are in Snowshoe Group "lower sequence"; and Domain XI is in Cariboo Group rocks above the L i t t l e River Fault. Domains I, V, VII, and IX include areas of generally homoclinally dipping f o l i a t i o n (on a large scale); Domains II, III, IV, and X include known hinge zones of large F2 folds; Domain VI contains a fault which l o c a l l y t i l t s f o l i a t i o n ; and Domain VIII spans an area believed to be affected by movement on the L i t t l e River Fault. Collected data are more numerous in some domains than in others, and for some structural elements (such as f o l i a t i o n ) . Where data are sparse, or where stereograms plotted separately were e s s e n t i a l l y the same, data from two or more domains have been combined in one stereogram. Centers of concentrations of points were calculated by computer p l o t t i n g program or by eye; where both methods were used there was good agreement between them. 44 Figure 12. Structural domain map of the Three Ladies Mountain area showing domains referred to on stereographic projections. Refer to Plate I for geology and to Plates IV-VI for structural d e t a i l . 45 3.2.2 DEFINITION OF TERMS Structural elements plotted on stereographic projections include those shown on Plate I, and Plates IV, V, and VI. Planar f e a t u r e s a r e plotted on 7r-diagrams as poles to surfaces. Bedding is plotted only where i t can be distinguished from metamorphic layering, as in the low-grade rocks above the L i t t l e River Fault. For the high-grade schist and gneiss, compositional layering and f o l i a t i o n are combined as " f o l i a t i o n " , as they are generally p a r a l l e l in the f i e l d and when plotted separately their d i s t r i b u t i o n s overlap e n t i r e l y . " F o l i a t i o n " refers to the dominant metamorphic f o l i a t i o n in an outcrop, which is generally p a r a l l e l to compositional layering, and may include f o l i a t i o n related to either F1 or F2 structures. Where f o l i a t i o n could be i d e n t i f i e d as a x i a l planar to a p a r t i c u l a r f o l d phase, i t i s plotted in the category of "axial planes". "Axial planes" are either measured a x i a l surfaces, a x i a l planar f o l i a t i o n s , or crenulation cleavage, or approximations to a x i a l planes of folds with no e a s i l y measurable a x i a l surfaces. "Fold axes" are measured hinge l i n e s of folds, or approximated by measuring lineations apparently p a r a l l e l to hinge l i n e s . Where fol d phases could be distinguished, a x i a l planes and f o l d axes are separated into groups of F1, F2 (mainly westerly-overturned f o l d s ) , F3, F4, and "pre-F2" folds 46 (including easterly-overturned F 1 B folds as well as i s o c l i n a l and rootless F1A f o l d s ) . "Lineations" measured in the f i e l d include intersection lineations (usually a combination of F1 and F2 structures), crenulations, and mineral orientation lineations such as quartz rods, preferred dimensional orientation of micas, and alignment of elongate hornblende. Quartz rod lineations are p a r a l l e l to F1 fold axes where they can be distinguished from other l i n e a t i o n s . Kinks and crenulations are F3 or F4 structures. Most lineations are p a r a l l e l to F1 or F2 f o l d axes, which are nearly coaxial in most areas. "Fault surfaces" are surfaces on which movement appears to have taken place, as indicated by offset or slickensides. "Slickensides" are not divided into s i i c k e n f i b r e s , s i i c k e n l i n e s , or slickensurfaces; these are meaningless d i s t i n c t i o n s here as the slickensides observed appear to consist of smoothed-over, fibrous-looking l i n e s that form a generally smooth and shiny surface. "Joints" refer mainly to prominent fracture planes in the rocks, most commonly those perpendicular to F2 fold axes. 3.2.3 SUMMARY OF STEREOGRAPHIC PROJECTIONS In the Three Ladies Mountain area, or Domains I-V, f o l i a t i o n is generally p a r a l l e l to compositional layering and probably related to to F1 folds. Limbs of F2 folds can be divided into a dominant limb at 075/44N 47 and a short limb of 135/70N. (Combination stereograms were constructed from averages for concentrations from each domain and used to determine the average trends given here.) Other concentrations in Domain IV indicate possible F1 limbs, one of which trends north and dips moderately west, and the other of which trends southeast and dips steeply southwest. Average F2 f o l d axis l i e s at 334/42, approximately perpendicular to the great c i r c l e which contains poles to f o l i a t i o n , indicating that F2 folding was c y l i n d r o i d a l and has not been disturbed by any major deformational event. The wide spread in points on the stereograms can be accounted for by F3 and F4 folding. F3 folding is d i f f i c u l t to define from s t r u c t u r a l data, and may only be apparent as changes in dip from one area to another, such as the steeper dip of f o l i a t i o n s at Three Ladies Mountain, shallower to the north and south. F4 folds, which are broad warps with a NE trend, have the most eff e c t on the plunge of F2 axes to NW or SE. In Domains VI-X, including the Mount Stevenson and Quesnel Lake area, the f o l i a t i o n s are less steep than at Three Ladies Mountain, but not much, with dominant attitude of 073/36N. There is no evidence for extending the Lightning Creek Anticlinorium (Campbell et a l . 1973, Fletcher 1972) through t h i s area, as f o l i a t i o n dips mainly north everywhere from the L i t t l e River (north boundary of map area) to Grain Creek (southwest of Mount 48 Stevenson). Axes of pre-F2 folds and F1 lineations are d i f f i c u l t to distinguish from F2 structures, but i t appears that in the Three Ladies Mountain area the most northerly axes and lineations belong to F1, whereas near Mount Stevenson there is a d i s t i n c t WNW grouping belonging to F1 structures. Easterly-overturned, tight (not truly i s o c l i n a l ) F1B folds occur mainly in the Mount Stevenson and Welcome Mountain areas. F1A i n t r a f o l i a l i s o c l i n e s occur in every domain (except XI*). F2 a x i a l planes from the entire area average at 118/58N, and are remarkably consistent considering the later phases of folding. F2 f o l d axes from the "upper sequence" rocks, or Domains I-IV and VII, average at 332/42, whereas F2 fold axes from the "lower sequence" rocks, or Domains V, VI, and VIII-X, average a l i t t l e more westerly and shallower at 315/24. F o l i a t i o n s , lineations, and f o l d axes in Domain II, which contains the north face of Three Ladies Mountain, are unusually steep. This may be due to l a t e r folding or f a u l t i n g . A large, asymmetrical F2 S-fold is well exposed in the Three Ladies Mountain area; the middle limb extends from the middle peak of Three Ladies Mountain down Marten Creek, and has v e r t i c a l f o l i a t i o n as well as f o l i a t i o n dipping to both sides of v e r t i c a l , suggesting a refolded F1 f o l d . 49 Structural elements assigned to F3 folding are inconsistent within and between domains, attesting to the r e l a t i v e subordinate nature of these structures, compared with F2 folds. As kinks and crenulations do not occur on every outcrop, there are fewer data points for F3 elements, and no consistent patterns emerged from the stereonets. Lineations are dominantly F2, p a r a l l e l to F2 fold axes. For Domains II-IV, lineations group at 340/45; for Domains IV-X, at 315/30; and for Domains I and X, at 336/28 (these are interpreted as having been rotated to lower angles by fault s related to the L i t t l e River F a u l t ) . Quartz rod lineations are associated with F1 folds, p a r t i c u l a r l y easterly-overturned F1B folds in the Mount Stevenson area, where quartz rod lineations average 288/20. See captions of stereographic projections for more d e t a i l s . 50 F i g u r e 1 3 . E q u a l a r e a p r o j e c t i o n s f o r D o m a i n I I . T h r e e L a d i e s M o u n t a i n , n o r t h e a s t r i d g e . I n c l u d e s h i n g e z o n e o f a w e s t e r l y - o v e r t u r n e d F 2 s y n f o r m . a . F o l i a t i o n s a r e f o l d e d a b o u t a c a l c u l a t e d a x i s o f 3 3 5 / 4 8 ; t h e d o m i n a n t f o l d l i m b i s 0 7 0 / 4 8 N , w i t h t h e s h o r t e r l i m b a t 1 2 4 / 6 6 N , s u b p a r a l l e l t o t h e a x i a l p l a n e ( c o m p a r e ( e . ) ) . b . , d . , a n d f . L i n e a t i o n s p l o t a t 3 3 5 / 4 4 , f o l d a x e s a t 3 3 2 / 5 6 , a n d F 2 f o l d a x e s a t 3 3 2 / 5 6 , a l l c o m p a r a b l e w i t h c a l c u l a t e d f o l d a x i s s h o w n i n ( a . ) . T h i s i m p l i e s t h a t F 2 f o l d i n g w a s c y l i n d r o i d a l a n d t h a t m o s t o f t h e f o l i a t i o n s a r e F 1 s t r u c t u r e s . c . P o l e s t o a x i a l p l a n e s o f f o l d s o f u n c e r t a i n a f f i n i t y s h o w a g r o u p i n g a r o u n d 0 9 5 / 6 4 N i n t e r p r e t e d a s F 1 a x i a l p l a n e s . e . P o l e s t o F 2 a x i a l p l a n e s a r e s p r e a d b r o a d l y a r o u n d 1 2 5 / 6 6 N ; s p r e a d m a y b e d u e t o l a t e r b r o a d w a r p i n g w i t h N E t r e n d ( F 4 ) . 51 52 F i g u r e 1 4 . E q u a l a r e a p r o j e c t i o n s f o r D o m a i n I I I . T h r e e L a d i e s M o u n t a i n n o r t h p e a k a n d e a s t c i r q u e . I n c l u d e s F 2 a n t i f o r m a n d p o s s i b l e F 1 c l o s u r e s . a . F o l i a t i o n s a r e f o l d e d a b o u t a c a l c u l a t e d a x i s o f 3 5 2 / 5 3 ? d o m i n a n t l i m b i s a t 0 7 8 / 4 5 N , w i t h s u b o r d i n a t e l i m b a t 1 3 7 / 6 8 N . b . , d . , a n d f . L i n e a t i o n s p l o t a t 3 4 0 / 5 0 , f o l d a x e s a t 3 4 0 / 5 0 , a n d F 2 f o l d a x e s a t 3 3 8 / 4 5 . D o m a i n I I I h a s s t e e p e s t a x e s a n d l i n e a t i o n s i n e n t i r e a r e a . c . N o n - F 2 a x i a l p l a n e s p l o t i n 3 g r o u p s : 0 8 8 / 3 8 N , i n t e r p r e t e d a s F l ; 1 5 0 / 3 4 N , i n t e r p r e t e d a s p o s s i b l e F 3 ( ? ) ; a n d 0 3 5 / 8 5 N , i n t e r p r e t e d a s F 4 . e . F 2 a x i a l p l a n e s s h o w g r e a t e r s p r e a d t h a n i n s o m e o t h e r a r e a s ; s p r e a d m a y b e d u e t o l a t e r b r o a d w a r p i n g w i t h N E - t r e n d ( F 4 ) . 53 54 F i g u r e 15. E q u a l a r e a p r o j e c t i o n s f o r Domain IV. Three L a d i e s Mountain m i d d l e and s o u t h peaks. I n c l u d e s l a r g e synform i n c a r b o n a t e - a m p h i b o l i t e u n i t . a. P o l e s t o f o l i a t i o n show 4 c o n c e n t r a t i o n s . Limbs at 083/44N and 124/67N a r e i n t e r p r e t e d as F2 and g i v e a c a l c u l a t e d f o l d a x i s of 323/40, p a r a l l e l t o l i n e a t i o n s and f o l d a xes, i n d i c a t i n g t h a t F2 f o l d i n g was d o m i n a n t l y c y l i n d r o i d a l . Limbs at 018/46W and 142/76S ( o v e r t u r n e d e q u i v a l e n t of 124/67N l i m b ) s u g g e s t c o n t r o l from p r e v i o u s F1 ant i form. b. L i n e a t i o n s c e n t e r around 325/40, p a r a l l e l t o f o l d axes ( d . , f.) and t o c a l c u l a t e d a x i s i n ( a . ) . Wide s p r e a d of l i n e a t i o n s s u g g e s t s two groups of F1 l i n e a t i o n s now a t 350/54 and 287/26. c. Non-F2 a x i a l p l a n e s f a l l i n t o g r e a t c i r c l e of p o l e s t o f o l i a t i o n f o l d e d by F2 f o l d s ; i n t e r p r e t e d as m a i n l y F1 a x i a l p l a n e s . d., f . F o l d axes and F2 f o l d axes both p l o t a t 324/38, p a r a l l e l t o t h e dominant l i n e a t i o n . e. F2 a x i a l p l a n e s have o r i e n t a t i o n of 109/52N. 55 56 F i g u r e 16. Equal area p r o j e c t i o n s f o r Domain V I I I , Marten Creek to L i t t l e R i v e r F a u l t ; and Domain X, Quesnel Lake from Devoe Creek to L i t t l e R i v e r F a u l t . Both c o n t a i n F2 a n t i f o r m i n Marten Creek. a. Domain V I I I . F o l i a t i o n shows up i n 3 groups. Limbs of a n t i f o r m : 072/42N and 125/42S. F o l i a t i o n r o t a t e d by drag on f a u l t s r e l a t e d to L i t t l e R i v e r F a u l t : 170/20E. b. Domain V I I I . L i n e a t i o n s and f o l d axes show c o n c e n t r a t i o n at 310/33 but a few near Quesnel Lake plunge SE i n d i c a t i n g e f f e c t of F4 f o l d i n g and/or drag on L i t t l e R i v e r F a u l t . c. Domain X. F o l i a t i o n s f a l l i n t o 4 groups. Limbs of Marten Creek F2 a n t i f o r m : 081/27N and 124/42S. F o l i a t i o n s at 120/58N are approximately p a r a l l e l to F2 a x i a l planes. A f o u r t h group l o o s e l y c e n t e r e d at 000/12E i s i n t e r p r e t e d as f o l i a t i o n s r o t a t e d by L i t t l e R i v e r F a u l t movement. d. Domain X. L i n e a t i o n s and f o l d axes show a broad spread with a dominant c o n c e n t r a t i o n at 314/22. Spread to NNW and WNW i s a t t r i b u t e d to f o l d e d F1 l i n e a t i o n s . Southeast-plunging l i n e a t i o n s and axes near Quesnel Lake are f o l d e d by F4 f o l d s and/or r o t a t e d by L i t t l e R i v e r F a u l t . e. Domain X. A x i a l planes are e i t h e r F1, which f a l l i n two groups s u b p a r a l l e l t o limbs of F2 f o l d s at 077/37N and 136/46S; or F2, at 134/66N. f. Domain X. F a u l t s u r f a c e s c o u l d be d i v i d e d i n t o two groups, one at 171/21E, i n t e r p r e t e d as r e l a t e d to L i t t l e R i v e r F a u l t ; and one at 099/23N, which i s p a r a l l e l to some f a u l t s at Limestone P o i n t , which may or may not be r e l a t e d to L i t t l e R i v e r F a u l t . 57 58 F i g u r e 17. Equal area p r o j e c t i o n s f o r Domains I, V, and V I I , f o l i a t i o n s and l i n e a t i o n s . A l l c o n t a i n areas of r e l a t i v e l y h o m o c l i n a l l y d i p p i n g f o l i a t i o n s . a. Domain I. F o l i a t i o n s c e n t e r at 072/32N, much l e s s steep than i n Domains II and I I I . b. Domain I. L i n e a t i o n s c e n t e r at 333/28, a l s o not as steep as i n Domains II and I I I . c. Domain V I I . F o l i a t i o n s p l o t at 070/46N. These rocks appear very s i m i l a r to those i n Domain I and may be continuous with them; t h i s would imply a l a r g e S - f o l d with a middle limb of about 124/67N (analogous to Domain IV) between Ishkloo Creek and Three L a d i e s Mountain, an area which i s not w e l l exposed. d. Domain V I I . L i n e a t i o n s and f o l d axes p l o t at 320/40, s i m i l a r to F2 s t r u c t u r e s i n Domain IV. e. Domain V. F o l i a t i o n s c o n c e n t r a t e at 082/42N, forming the dominant limb of the f o l d shown i n Domain IV. f. Domain V. L i n e a t i o n s p l o t at 317/34, with a few more n o r t h e r l y and w e s t e r l y l i n e a t i o n s i n t e r p r e t e d i n the f i e l d as F1 l i n e a t i o n s . 59 60 F i g u r e 18. E q u a l a r e a p r o j e c t i o n s f o r c o m b i n a t i o n s of s t r u c t u r a l elements from s e v e r a l domains. a. Domain V. F2 a x i a l p l a n e s p l o t a t 110/47, a p p r o x i m a t e l y p a r a l l e l t o those i n Domain IV. b. Domains I-V, IX. P o l e s t o c o n t a c t s of p e g m a t i t e d i k e s p l o t around 121/62N, s u b p a r a l l e l t o F2 a x i a l p l a n e s . c. Domains I-V, IX, and X. F3 a x i a l p l a n e s . Three groups emerge: one a t 048/56S, a p p r o x i m a t e l y p e r p e n d i c u l a r t o F2 f o l d axes and p a r a l l e l t o the dominant j o i n t s e t ; one at 158/61S; and one a t 166/61E. These might be i n t e r p r e t e d as c o n j u g a t e k i n k s , but a c t u a l f i e l d r e l a t i o n s of t h e s e two s e t s a r e unknown. d. Domains I I - V I , V I I I , and IX. C r e n u l a t i o n l i n e a t i o n s p l o t a l l over the NE quadrant of the s t e r e o n e t c e n t e r i n g a t 054/36. They c o u l d be i n t e r p r e t e d as r e l a t e d t o e i t h e r F3 or F4 f o l d s . e. Domains I-V, V I , and IX. P o l e s t o c o n t a c t s of p l a n a r q u a r t z v e i n s f a l l i n t o two groups; one a p p r o x i m a t e l y p a r a l l e l t o metamorphic f o l i a t i o n , and one a t 050/90, s u b p a r a l l e l t o F4 a x i a l p l a n e s . f . Domains I-V, V I , IX, and X. F4 f o l d axes plunge 035/16. F4 a x i a l p l a n e s were too few t o p l o t but average around 040/90. 62 F i g u r e 19. Equal area p r o j e c t i o n s of pre-F2 f o l d elements fo r combinations of domains; and F2 f o l d elements i n Domain IX, Mount Stevenson a r e a . a. Domains II-IV, VI, and X. A x i a l planes of pre-F2 f o l d s p l o t i n two groups. E a s t - v e r g i n g F1B f o l d s p l o t around 177/29W, whereas other F1 f o l d s i n c l u d i n g F1A i s o c l i n e s are p a r a l l e l t o f o l i a t i o n at 084/48N. b. Domains I I , I I I , and V. F1 f o l d axes show a spread from WNW to NNW and NE. c. Domain IX. A x i a l planes of pre-F2 f o l d s can be d i v i d e d i n t o two groups which are roughly concordant with f o l i a t i o n (see F i g u r e 20 ( c . ) ) , one at 041/24N ( e a s t e r l y overturned) and one a t 143/43S. d. Domains VI and IX. F1 f o l d axes are c o n c e n t r a t e d at 295/16, more we s t e r l y and shallower than F2 axes. e. Domain IX. F2 a x i a l planes p l o t at 116/54N, s i m i l a r to those near Three L a d i e s Mountain. f. Domain IX. F2 f o l d axes p l o t at 314/25. F2 s t r u c t u r e s near Mount Stevenson are c o n s i s t e n t with but l e s s s t e e p l y d i p p i n g than those near Three L a d i e s Mountain. 64 F i g u r e 20. Equal area p r o j e c t i o n s f o r Domains VI and IX. Mount Stevenson area. a. Domain VI. F o l i a t i o n s are concentrated mainly at 076/36N except near f a u l t i n upper Long Creek. b. Domain VI. L i n e a t i o n s and f o l d axes p l o t around 315/16 except those plunging SE where f o l d e d adjacent to f a u l t i n upper Long Creek. c. Domain IX. F o l i a t i o n s p l o t mainly at 073/29N with some SW-dipping f o l i a t i o n s from south of Mount Stevenson. d. Domain IX. L i n e a t i o n s p l o t in 3 groups. F1 l i n e a t i o n s p l o t at 251/16; F2 l i n e a t i o n s at 314/22; and F4 l i n e a t i o n s at 028/05. These are a l l more g e n t l y plunging than at Three Ladies Mountain. e. Domain IX. F o l i a t i o n i n quartz d i o r i t i c g n e i s s i s g e n e r a l l y p a r a l l e l to f o l i a t i o n in other rocks; the spread of poles i n d i c a t e s a c a l c u l a t e d a x i s of 314/25, i n d i c a t i n g that the quartz d i o r i t i c g n e i s s i s f o l d e d c y l i n d r o i d a l l y by F2 f o l d s . f. Domain IX. F o l d axes (not F2) p l o t at 311/27. 65 6 o m n i a 9 rxiflnot IN OMIT o i w m c ocisa aw IN 3 r X Q OT3 66 F i g u r e 21. Equal area p r o j e c t i o n s of f a u l t s u r f a c e s , s l i c k e n s i d e s , and j o i n t s . a. Domains I-V. Three Ladies Mountain. F a u l t s u r f a c e s d i p s h a l l o w l y e a s t , and are i n t e r p r e t e d as r e l a t e d to the L i t t l e R i v e r F a u l t . b. Domains I-V. Three Ladies Mountain. S l i c k e n s i d e s p o i n t mostly toward 118 with low plunge*- f i e l d evidence such as displacement and drag i n d i c a t e s movement of top down to SE. c. Domains VI and IX, Mount Stevenson. F a u l t s u r f a c e s d i p s h a l l o w l y i n every d i r e c t i o n . d. Domains VI and IX. S l i c k e n s i d e s p o i n t to NE, SE, and SW. e. Domains I-V, V I I I - X I . F a u l t s u r f a c e s are g e n e r a l l y s h a l l o w l y - d i p p i n g , and i f r e l a t e d , c e n t e r around 178/16E, which i s p a r a l l e l to part of the L i t t l e R i v e r F a u l t near Quesnel Lake. Because the L i t t l e R i v e r F a u l t i s curved, a l l of these f a u l t s u r f a c e s may be r e l a t e d to i t . f. Domains I-V, V I I I , and IX. Poles to j o i n t s u r f a c e s are co n c e n t r a t e d i n the northwest quadrant, i n d i c a t i n g that most j o i n t s measured are p e r p e n d i c u l a r to F2 f o l d axes ( p e r p e n d i c u l a r to 324/45) at 054/45S. Some o v e r l a p with f a u l t s u r f a c e s i s expected as some of these j o i n t planes show displacment and drag i n d i c a t i n g movement of top down to southeast. 68 F i g u r e 22. Equal area p r o j e c t i o n s of s t r u c t u r a l elements i n hanging w a l l of L i t t l e River F a u l t , Domain XI. a. Bedding and c o m p o s i t i o n a l l a y e r i n g i n marble p l o t at 097/21N. b. Bedding and c o m p o s i t i o n a l l a y e r i n g i n p h y l l i t e and q u a r t z i t e p l o t at 108/24N, not too d i f f e r e n t from bedding i n marble. c. F o l i a t i o n i n p h y l l i t e / q u a r t z i t e i s a x i a l p l a n a r to "F2" f o l d i n g at 139/60N. d. F o l d axes and l i n e a t i o n s p l o t mainly at 342/12, more shallow than i n the u n d e r l y i n g Snowshoe Group s c h i s t . These are mainly "F2" s t r u c t u r e s . Spread to SE i n d i c a t e s change of plunge due to NE-trending F4 warping. e. A x i a l planes of F3 kinks in p h y l l i t e p l o t at 174/60N, whereas F2 a x i a l planes in marble p l o t at 118/69N. A d d i t i o n a l measurements taken i n 1982 were not p l o t t e d on these s t e r e o n e t s but co n f i r m F2 a x i a l planes c o n s i s t e n t with those i n the Snowshoe Group; and the presence of a broad F4 a n t i c l i n e t r e n d i n g NE on S e r v i c e Ridge, which i n t e r s e c t s with an F2 a n t i f o r m at "6287'", forming a domal i n t e r f e r e n c e p a t t e r n . 7 0 3.2.4 SUMMARY OF FOLDING Metamorphic rocks in the Three Ladies Mountain area were folded f i r s t by folds which are now tight to i s o c l i n a l and subparallel with F2 folds; quartz rods developed during early folding. F2 folding was accomplished mainly by a combination of buckling in more competent layers and shear folding in less competent layers, producing flattened flexure folds t r a n s i t i o n a l between true similar folds and pure f l e x u r a l s l i p folds Dominant southwesterly-overturned F2 fold geometry across the entire study area suggests maximum compressive s t r a i n in a northeast-southwest d i r e c t i o n , and a sense of shear implying northeast over southwest t r a n s l a t i o n . Alignment of minerals p a r a l l e l to fold axes implies elongation in a northwest-southeast di rect ion. The nearly coaxial orientation of F1 and F2 folds, and possibly F3 folds, implies f a i r l y constant orientation of st r a i n d i r e c t i o n s and maximum compressive stress during progressive deformation over a time period which spanned an entire metamorphic episode. F4 folds, which are perpendicular to dominant e a r l i e r trends, may have formed late during u p l i f t . 71 3.3 DEFORMATION: FAULTING 3.3.1 LITTLE RIVER FAULT One major fault zone ( L i t t l e River Fault) and ' numerous minor faults were i d e n t i f i e d in the Three Ladies Mountain/Mount Stevenson area. The L i t t l e River Fault, which separates higher grade schist and gneiss in the footwall from lower grade metasedimentary rocks in the hanging wall, was mapped by Klepacki (1981) near Maeford Lake, immediately north of the present map area. In th i s study, the continuation of the fault was traced from the saddle west of Service Mountain south down Service Creek, and thence west of Limestone Point into Quesnel Lake. Klepacki (1981) found that in the Maeford Lake area the L i t t l e River fault dips shallowly to the east, whereas toward Quesnel Lake the dip is more variable, although s t i l l generally east-dipping. Where i t intersects Quesnel Lake, the orientation of the fault i s about 170/20 E. The fault surface appears to be convexly curved rather than planar. This curvature i s seen in the shape of the rounded ridge east of Three Ladies Mountain, believed to have formed as the fault surface was exhumed by erosion (see Figure 23). The fault trace i t s e l f is poorly exposed. Where i t crops out at Quesnel Lake, i t i s marked by a b r i t t l e zone several tens of meters wide featuring fractured, B. S k e t c h of view i n A showing t r a c e of L i t t l e R i v e r F a u l t and exhumed t e c t o n i c d e n u d a t i o n s u r f a c e ( s t i p p l e d ) . A and B r e f e r t o F i g u r e 24 A and B, r e s p e c t i v e l y . 73 non-cohesive, phacoidal, graphitic rocks barely recognizable as o r i g i n a l s c h i s t , quartzite, or pegmatite. Microscopic s t r a i n markers were not investigated. The footwall rocks include coarse-grained Bralco marble (Struik 1983) and underlying metamorphic rocks of Snowshoe Formation (late Proterozoic to Paleozoic) and probable Paleozoic granitoid gneisses. The hanging wall rocks, some of which were mapped by Klepacki (1981) as Yankee Belle Formation, consist of a series of marble layers intercalated with p h y l l i t e and fine-grained quartzite. These are mapped as late Proterozoic to Cambrian Cariboo Group, undivided. Although the ages of the two sets Of rocks may be s i m i l a r , the Snowshoe Formation and Cariboo Group cannot be correlated (Struik 1982) . The L i t t l e River Fault crosscuts complex fold structures on both sides of the f a u l t . The Snowshoe Group rocks in the footwall were folded at least once and intruded by at least two types of granitoid s i l l s p r i or to F2 folding, whereas the hanging wall rocks show very l i t t l e evidence for pre-F2 structures. Only a few i s o c l i n a l folds were observed in the marble on Service Mountain p a r a l l e l to layering, and bedding i s s t i l l i d e n t i f i a b l e in some of the p h y l l i t i c units, crosscut by l a t e r (F2 and F3) cleavages. Although the structural and metamorphic history in the footwall rocks appears to have begun e a r l i e r and is more complex than in the hanging wall rocks, the s i m i l a r i t i e s in F2 and later fold episodes are notable considering the difference in metamorphic grade and proposed stratigraphic packages across the f a u l t . The designation "F2" i s used for folds which are characterized on both sides of the fault by northeast-dipping a x i a l planes, consistent southwest vergence, and an association with maximum metamorphic r e c r y s t a l l i z a t i o n . Usage of F2 in th i s sense equals F1 folds as mapped by Klepacki (1981). Plunge of F2 axes in the footwall rocks i s steep to the northwest except near Quesnel Lake, where i t i s near horizontal or to the southeast; plunge of F2 axes in the hanging wall is generally shallower, to the northwest west of the ridge, and to the southeast or variable near Quesnel Lake (see Figure 9, and Plates I and IV). These reversals in plunge are due to a combination of drag on the L i t t l e River Fault and superimposed F4 folding. In the footwall, prograde metamorphism associated with F2 folding reached amphibolite facies, with staurolite-kyanite zone on the ridge, and s i l l i m a n i t e zone at Quesnel Lake. F3 kink-folding was accompanied or followed by retrograde metamorphism indicated by growth, perhaps mimetic, of c h l o r i t e along F3 a x i a l planes. Prograde metamorphism associated with F2 folding reached greenschist facies in the hanging 75 w a l l with the development of r e c r y s t a l l i z e d marble and c h l o r i t e - s e r i c i t e p h y l l i t e on the r i d g e , and c h l o r i t e - s e r i c i t e - b i o t i t e p h y l l i t e at Quesnel Lake. R e c r y s t a l l i z a t i o n during F3 f o l d i n g was at approximately the same grade, r e s u l t i n g in two d i f f e r e n t cleavages o u t l i n e d by c h l o r i t e in some p h y l l i t e s . "Retrograde" metamorphism a s s o c i a t e d with F3 f o l d i n g was c h a r a c t e r i z e d in both f o o t w a l l and hanging w a l l rocks by growth of c h l o r i t e , suggesting that both packages of rock were i n approximately the same metamorphic P/T regime around the time of completion of F3 f o l d i n g . At l e a s t some r e l a t i v e displacement b r i n g i n g the lower grade rocks c l o s e r to the higher grade rocks had t h e r e f o r e occurred e i t h e r postkinematic to or, more l i k e l y , synkinematic with F3 f o l d i n g . On the eastern f l a n k of S e r v i c e Mountain, a x i a l planes of some F3 f o l d s t rend around 010/30 E, approximately p a r a l l e l to minor f a u l t s u r f a c e s b e l i e v e d to be r e l a t e d to the L i t t l e R iver F a u l t . Evidence from kinematic i n d i c a t o r s i n the Snowshoe Group (below the p r o j e c t i o n of the t r a c e of the L i t t l e R i v e r F a u l t toward Three Lad i e s Mountain) i n d i c a t e s t h at movement in the hanging w a l l was east- s o u t h e a s t r e l a t i v e to the f o o t w a l l . Minor s t r u c t u r e s b e l i e v e d to be r e l a t e d to movement along the L i t t l e R i v e r F a u l t , such as s l i c k e n s i d e s (see s t e r e o g r a p h i c p r o j e c t i o n s and P l a t e VI) and drag f e a t u r e s a s s o c i a t e d with repeated f a u l t 76 surfaces, show consistent orientation toward the southeast. Many minor fault zones are found near-parallel to the unusual rounded meadow slope east of Three. Ladies Mountain to Quesnel Lake, which could be an exhumed fault surface related to the L i t t l e River Fault (see figure S10) . Minor numerous faults are observed with various orientations ranging from p a r a l l e l to joint planes which are perpendicular to F2 fold axes, to northeast dipping to eastward dipping. It looks as though any available surface--joint planes, f o l i a t i o n planes, and a x i a l planes--took up part of the movement. Slickensides on j o i n t - p a r a l l e l f a u l t s show movement to have been down toward 118 (ESE), confirmed by drag of F2 fold axes and lineations in the same d i r e c t i o n , near the joint surfaces showing the displacement Small displacements are common, no more than ten meters. However, on surfaces spaced less than one meter to ten meters apart for at least 500 m below what may have been the o r i g i n a l , major fault surface (now just above Three Ladies Mountain), the combined displacement could be very large. The j o i n t - p a r a l l e l f a u l t s are p a r t i c u l a r l y evident in the carbonate-amphibolite unit that makes up the north slope of Three Ladies Mountain. In some places, marble has been remobilized and smeared into fractures perpendicular to F2 fo l d axes and lineations (see Figure 24). The marble consists of fine-grained, finely-laminated c a l c i t e , with thin graphitic horizons 77 F i g u r e 24. S t r u c t u r e s below but r e l a t e d to L i t t l e R i ver F a u l t ; f o r l o c a t i o n see Figure 23 B. A. M u l t i p l e j o i n t s p e r p e n d i c u l a r to F2 f o l d axes i n f o l d e d q u a r t z i t e and p e l i t e showing drag f e a t u r e s of top down to southeast. C l i f f h eight i s about 20 m. B. 150 m below A. Marble (m) (1-2 m t h i c k ) i s smeared along f r a c t u r e p e r p e n d i c u l a r to dominant f o l d s t r u c t u r e s i n amp h i b o l i t e u n i t ( a ) . C. Schematic block diagram showing how marble (m; s t i p p l e d ) i n t e r c a l a t e d w i t h a m p h i b o l i t e (a) could have been smeared along f a u l t surface as upper block was d i s p l a c e d t o southeast. 7 8 which are folded such that the axial planes are perpendicular to F2 fo l d axes and p a r a l l e l to the shear zone. Closer to Quesnel Lake, these minor fault surfaces show variable attitudes, but retain ESE slickensides and drag features. The average attitude of fa u l t s in the whole area is about 170/20 E, p a r a l l e l to some fault surfaces measured in the fault zone, and consistent with the attitude of the fault trace near Limestone Point. Displacement on the L i t t l e River Fault i s unknown. The most recent movement was hanging wall down toward the east and southeast r e l a t i v e to the footwall. This could have been merely the latest episode of displacement due to d i f f e r e n t i a l u p l i f t . The metamorphic difference across the trace of the L i t t l e River Fault could be as much as 200°C and 2 kb, indicating a possible throw of as much as about 6 km. This is based on an assumption of a t y p i c a l Barrovian P-T path, with the kyanite/sillimanite zone rocks at approximately 5 to 7 kb and 500° to 600°C, and the adjacent c h l o r i t e and b i o t i t e zone rocks at approximately 3 to 4 kb and 300° to 400°C (see section on metamorphic petrology). The continuation of the L i t t l e River Fault from where i t disappears into Quesnel Lake i s unknown, although the shape of i t s surface suggests i t continues south along the North Arm of Quesnel Lake, at least as far south as Marten Creek. 79 The difference in metamorphic grade across the North Arm of Quesnel Lake has been ascribed to a late normal fault (Campbell and Campbell 1969), but i t i s probably due to e a r l i e r movement on the L i t t l e River Fault. Topographic evidence, such as the contrast in elevation between Three Ladies Mountain and Mount Watt, suggests that i f there is a late normal fault in the North Arm of Quesnel Lake, i t has downdropped the west side by a few hundred meters, not u p l i f t e d i t , as implied by Campbell and Campbell ( 1 9 6 9 ) . No d e f i n i t e premetamorphic faults were i d e n t i f i e d in the Three Ladies Mountain/Mount Stevenson area. Premetamorphic movement on the L i t t l e River Fault i t s e l f would be d i f f i c u l t to establish. There i s no evidence for a r e c r y s t a l l i z e d mylonitic zone near the L i t t l e River Fault. Two possible tectonic s l i d e s , p a r a l l e l to f o l i a t i o n , have been mapped along attenuated limbs of F2 folds where marker units (carbonate-amphibolite) are truncated by apparently continuous layers of f o l i a t e d micaceous quartzite and p e l i t i c schist (see map Plate I ) . Northeast of Three Ladies Mountain, the amphibolite plus carbonate unit i s folded synformally, and then lost in the p e l i t i c plus q u a r t z i t i c unit that continues a l l the way up to the base of the Bralco marble. Near Three Ladies Mountain middle peak, i t looks as though a band of p e l i t e has been transposed into the a x i a l plane of the large F2 f o l d , and crosscut the adjacent layer of i 80 amphibolite plus carbonate. An alternative explanation for these structures is that they represent hinge zones of F1 folds, refolded by the F2 folds. 81 3.3.2 DISCUSSION OF LITTLE RIVER FAULT Struik (1982) proposed a premetamorphic f a u l t , c a l l e d the Pleasant Valley Thrust, separating known Cariboo Group stratigraphy on the northeast side (hanging wall) from Snowshoe Group units on the southwest side (footwall). The Pleasant Valley Thrust is said to delineate the eastern margin of Snowshoe Group rocks from north and east of Wells and Barke r v i l l e to east of the North Arm of Quesnel Lake. It i s defined mainly on the basis of contrast between major l i t h o s t r a t i g r a p h i c packages, and is d i f f i c u l t to id e n t i f y in the f i e l d , although i t is supposed to be a major terrane boundary. Struik (1983) proposes that the northern contact of the Bralco marble at Maeford Lake is a high-angle fault which may be part of the Pleasant Valley Thrust. The rocks to the north of this fault have been mapped by Klepacki (1981) as schist and marble of Cariboo Group, also in the footwall of the L i t t l e River Fault. Mapping in th i s study interpreted this fault as a late normal fault with l i t t l e displacement, which has resulted in a small topographic scarp (on the order of tens of meters) at Maeford Lake; rocks on the north (down) side are obsured by cover, or outside the l i m i t s mapped here. Although t h i s fault appears to intersect the L i t t l e River Fault on the ridge west of Service Mountain, i t is not clear from f i e l d relations whether either fault o f f s e t s the other. However, 82 present topography suggests that the L i t t l e River Fault surface was raised s l i g h t l y to the southwest, and has been eroded away. It is possible that this small normal displacement is a reactivation of an older f a u l t . Struik (1983) shows the continuation of the Pleasant Valley Thrust into the hanging wall of the L i t t l e River Fault north of Limestone Point. Here the Pleasant Valley Thrust trace is mapped (by Struik) along the lower part of Service Creek between the marble and c l a s t i c rocks of Limestone Point (footwall) and the p h y l l i t e s and marbles above (hanging wall). Mapping in the same area revealed mainly sparse outcrops of nondistinctive calcareous p h y l l i t e , a difference in metamorphic grade from b i o t i t e zone to c h l o r i t e zone, and minor fault surfaces near-parallel to bedding and f o l i a t i o n in breccia in the marble at Limestone Point (110/25 N). Such evidence might support s l i g h t syn- or postmetamorphic movement on a surface p a r a l l e l to layering somewhere in the v i c i n i t y of Limestone Point, but does not necessarily confirm or deny the existence of the Pleasant Valley Thrust as mapped by Struik (1983). Geometrically i t would be d i f f i c u l t to j u s t i f y continuing a high-angle fault from the footwall into nearly adjacent hanging wall rocks, considering the di r e c t i o n and possible large magnitude of movement on the L i t t l e River Fault which i s supposed to postdate the higher-angle f a u l t . 83 The c h l o r i t e - b i o t i t e zone rocks of the Cariboo Group (?) were presumably at higher lev e l s in the crust at the time of metamorphism than were the s t a u r o l i t e to s i l l i m a n i t e zone rocks of the Snowshoe Group, unless the isotherms were steeply in c l i n e d to the earth's surface. Structural evidence indicates that the low-grade rocks in the hanging wall of the L i t t l e River Fault s l i d over or down re l a t i v e to the higher-grade rocks in the footwall, implying that the hanging wall was s t r u c t u r a l l y higher than the footwall prior to the latest movement on the L i t t l e River Fault. This evidence requires that nearly unmetamorphosed late Proterozoic to Cambrian sediments must have been emplaced above prefolded late Proterozoic to Paleozoic sediments which had been intruded by quartz d i o r i t i c and granodioritic bodies in the early Paleozoic. Although the Cariboo Group may in fact be younger than the Snowshoe Group, i t i s older than the orthogneisses intruded into the Snowshoe Group. The Cariboo Group (?) was not intruded by these bodies, so was probably not immediately adjacent to the Snowshoe Group in the early Paleozoic. Rocks of the Cariboo Group (?) could have been thrust over rocks of the Snowshoe Group from east to west (or Snowshoe Group underthrust from west) at some time prior to peak metamorphism, probably synkinematic with F2 foldi n g , as suggested by Struik (1982). 84 This type of east-over-west movement could have occurred partly on the proposed surface of the Pleasant Valley Thrust, along which the Cariboo Group and Snowshoe Group are now juxtaposed (Struik 1982). In the Three Ladies Mountain area, the dominant southwest vergence of F2 fold structures, lack of vergence reversals over a large area, and l a y e r - p a r a l l e l tectonic slides along attenuated limbs of F2 folds attest to a strong component of progressive simple shear strain in a northeast over southwest d i r e c t i o n during prograde metamorphism. These observations are consistent with the observations and interpretations of Murphy (1984) and Brown (1978), who propose that northeastward underthrusting during accretion of terranes in the early mid-Jurassic caused consistently southwest-verging structures (F2 folds) in the continental margin sequence being underthrust from the southwest. Large l i t h o s t r a t i g r a p h i c packages, including continental margin sediments of Kaza Group, Cariboo Group, and Snowshoe Group, formed large nappes separated by f a u l t s and/or d u c t i l e shear zones, which together define "a broad shear zone accommodating shear strains associated with northeastward underthrusting" (Murphy 1984). The Pleasant Valley Thrust (Struik 1982) may have been of th i s type, active during F2 folding and metamorphism; the "premetamorphic mylonite zone" (Fletcher 1972) at Penfold Creek and other portions of "Campbell's 85 Enigmatic Line" may also have been active during the underthrusting proposed by Murphy (1984). The L i t t l e River Fault, however, is not a thrust f a u l t , at least in i t s latest stage, but rather a low-angle normal fault along which the hanging wall rocks s l i d down r e l a t i v e to the footwall, as suggested by the minor structures already mentioned. Movement displacing the Cariboo Group rocks down re l a t i v e to the Snowshoe Group rocks probably began around the time of F3 folding and associated retrograde metamorphism. Movement continued through u p l i f t , most l i k e l y in the Tertiary, as i t postdated cooling of late Cretaceous (86 Ma) pegmatite (see section on geochronology' (Chapter I I I ) ) . Most of the u p l i f t in. the Columbia Mountains (which include the Cariboo Mountains and Quesnel Highland) was completed by Miocene time (Wheeler and Gabrielse 1972). In the area which i s now the Intermontane Belt, a widespread erosion surface with low r e l i e f (about 500 m) was covered by Miocene lavas. A similar erosion surface covered much of the Quesnel Highland at higher elevations, and i s now block-faulted to various l e v e l s . Although this surface has been deeply dissected by g l a c i a l erosion, i t i s s t i l l recognizable as an upper elevation l i m i t in each block. The Three Ladies Mountain/Mount Stevenson area i s a single block raised above the le v e l of the surrounding h i l l s west of the North Arm of Quesnel Lake. As 86 evidence, a l l three peaks of Three Ladies Mountain and Mount Stevenson reach a height close to 2200 m, whereas in the Goose Range, across Ishkloo Creek to the west and Grain Creek to the southwest, a l l of the "B" peaks (Barker, Borland, Browntop, Badger, and Brew) are within 15 m of 2045 m. The peaks to the southeast of the North Arm of Quesnel Lake are much higher, such as Mount Watt (2529 m). North of the L i t t l e River is a plateau at about 1800 m which ri s e s gently to the east, u n t i l i t i s abruptly terminated by the Mathew River Fault and the high Cariboo Mountains beyond (2600 m). The topography near the top of Three Ladies Mountain, which appears to approximate the le v e l of the old erosion surface, has been strongly influenced by the L i t t l e River Fault surface and minor structures associated with i t in the underlying rocks. If this interpretation is correct, then most of the movement on the L i t t l e River Fault was pre-Miocene, and post-Late Cretaceous. The timing of events involved in the formation of metamorphic core complexes elsewhere in the C o r d i l l e r a was also late Cretaceous (post-Laramide orogeny) to middle Tertiary (Coney 1980). Metamorphic core complexes, according to Crittenden et a l . (1980), are characterized by cores of older metamorphic and plutonic basement overprinted by gently-dipping gneissic f a b r i c , and overlain by less-metamorphosed, younger, commonly attenuated rocks. i 87 These are separated from the basement rocks by "decollement" zones characterized by apparently steep metamorphic gradients, brecciation, and structures indicative of detachment involving low-angle (commonly l i s t r i c ) normal faulting (Coney 1980). These detachment zones formed during u p l i f t following major compressional events such as the Cretaceous to Tertiary Sevier and Laramide orogenies in the United States and the Jurassic Columbian orogeny in Canada, but prior to extensional events such as Basin and Range style f a u l t i n g and r i f t i n g in the mid to late T e r t i a r y . In a descriptive sense, the L i t t l e River Fault in the Three Ladies Mountain area displays c h a r a c t e r i s t i c s that are similar to detachment surfaces of t y p i c a l metamorphic core complexes. These features, as well as the timing of the movement on the L i t t l e River Fault, between late Cretaceous pegmatite and Miocene erosion, suggest a relationship to the tectonic processes which led to formation of other metamorphic core complexes. However, many differences between the Three Ladies Mountain area and the t y p i c a l core complexes of the southwestern United States caution against lumping the L i t t l e River Fault with other detachment zones. For instance, the rocks in the hanging wall are not as young as Tert i a r y , and may be approximately the same age as the footwall rocks (late Proterozoic to Paleozoic), nor are they undeformed nor completely unmetamorphosed. 88 There are also no mylonitic fabrics in the footwall schists and gneisses that can be proven to be related to Tertiary movement on- the L i t t l e River Fault. Mylonitic fabrics have been observed in the footwall carbonate by Klepacki (1981). Ductile st r a i n occurred during compressional tectonic events in the mid-Jurassic, whereas b r i t t l e features related to the L i t t l e River Fault are c l e a r l y younger than mid-Jurassic folds, and are also postmetamorphic. Sl i d i n g of hanging wall rocks down along the L i t t l e River Fault occurred during u p l i f t and t i l t i n g of the underlying footwall rocks, but did not involve much rotation, as crosscut F2 structures retain similar orientations on either side of the f a u l t . Variations in orientation of pre-fault and f a u l t - r e l a t e d structures are due to later northeast-trending folds (F4) and normal f a u l t s . The la t e s t f o l d phase (F4) was accompanied by lo c a l l y - o c c u r r i n g , axial-plane mica crenulations with near-vertical a x i a l planes and gentle northeast plunge. F4 folds may imply minor compression in a northwest-southeast d i r e c t i o n , but la t e r northeast-trending normal f a u l t s , which appear to displace a Miocene erosion surface, imply extension in the same northwest-southeast d i r e c t i o n . 89 3.3.3 OTHER FAULTS Other than low-angle f a u l t s believed to be related to the L i t t l e River fault system, various higher-angle faults occur throughout the area. Most of the high-angle faults in the Three Ladies Mountain area are discontinuous and show r e l a t i v e l y small displacements; they probably formed during u p l i f t . Late, east-west s t r i k i n g , steeply north-dipping faults were observed in the Three Ladies Mountain area, none of which showed any more than a few meters of i d e n t i f i a b l e displacement; however, as they are nearly p a r a l l e l to f o l i a t i o n , and l i t h o l o g i e s are similar over hundreds of meters of thickness, actual displacement could be much greater. Some northeast-trending f a u l t s , p a r a l l e l to F4 a x i a l planes, near Mount Stevenson, are associated with quartz veins; sparse occurrences of molybdenite, pyrite, and chalcopyrite occur in small rusty zones. North-northeast-trending fractures and f a u l t s , p a r a l l e l to F4 axial planes, are common and influence topographic features including U-shaped g l a c i a l valleys such as Cariboo Lake, Ishkloo Creek Valley, and the North Arm of Quesnel Lake. Displacement on these faults is mostly down to the west, probably on the order of a few hundred meters, deduced from the assumption of a faulted pre-Miocene erosion surface. The large metamorphic displacement across the North Arm of Quesnel Lake i s due mainly to e a r l i e r movement down to the 9 0 southeast along the L i t t l e River Fault. The age of the north and northeast-trending faults is inferred to be late Tertiary, and pre-Pleistocene. Northwest-trending, high-angle faults with displacement of unknown di r e c t i o n but magnitude less than a few hundred meters also appear as topographic lineaments; they may follow F2 axial plane f o l i a t i o n . If there i s any northwest-trending, high-angle fault in Long Creek, i t i s of t h i s type. Continuity of l i t h o l o g i c units, f o l i a t e d gneiss bodies, and structural trends across the lower Long Creek valley absolutely rules out the p o s s i b i l i t y of major s t r i k e - s l i p f a u l t i n g in t h i s location. No evidence for strike s l i p f a u l t i n g was observed anywhere in the study area. 4. METAMORPHISM 4.1 INTRODUCTION Most of the high-grade metamorphic rocks in the southern Omineca Belt are included in the Shuswap Metamorphic Complex, which is loosely defined as a metamorphic and plutonic complex bounded by the s i l l i m a n i t e isograd. The Shuswap Complex has been divided by Okulitch (1984) into three major parts: (1) the Monashee Complex, which contains gneisses older than 2 Ga, surrounded by the Monashee Decollement; (2) the northwestern Shuswap Complex, which is an area of s i l l i m a n i t e grade paragneiss of late Proterozoic to early Paleozoic depositional age., and possible minor mid-Paleozoic granitoid gneiss; and (3) the Okanagan Plutonic and Metamorphic Complex. Although not formally part of the Shuswap Metamorphic Complex, and separated from i t by about 20 km, the area of high-grade metamorphic rocks of the Snowshoe Group west of the North Arm of Quesnel Lake can be viewed as a window into the "northwestern Shuswap Complex" as defined by Okulitch (1984). Struik (1984) includes the Snowshoe Group in the "suspect" Barkerville Terrane, which l i e s between d e f i n i t e North American strata such as the Kaza Group and Cariboo Group to the northeast, and the suture zone between the Omineca Belt and Quesnellia. The suture crosses Quesnel Lake about 10 km south of Mount Stevenson (Okulitch 1984). The rela t i o n s h i p between Barkerville Terrane and the Shuswap 91 92 Metamorphic Complex has not yet been c l a r i f i e d . The s i l l i m a n i t e isograd in the Three Ladies Mountain area as well as the s i l l i m a n i t e isograd defining the western boundary of the Shuswap Complex do not coincide with major l i t h o l o g i c or tectonic contacts, but are part of a normal metamorphic progression from low grade to high grade rocks within an apparently continuous l i t h o s t r a t i g r a p h i c succession. This implies that rocks in the Bark e r v i l l e Terrane are t r a n s i t i o n a l into the Shuswap Metamorphic Complex at least on i t s west side. This is not true of the northeastern boundary of the northwestern Shuswap Complex, where steep metamorphic gradients have been shown to coincide with faults or shear zones (Fletcher 1972; Pigage 1978; Engi 1984). The northeastern boundary of the s i l l i m a n i t e zone in the Three Ladies Mountain area i s also a fault zone. Here, the L i t t l e River Fault crosscuts metamorphic isograds at a high angle rather than p a r a l l e l s them, and places c h l o r i t e and b i o t i t e zone rocks d i r e c t l y on staurolite-kyanite and s i l l i m a n i t e zone rocks. The L i t t l e River Fault i s not continuous along str i k e with the northeastern boundary of the Shuswap Metamorphic Complex, whereas the Matthew Fault, which l i e s about 10 km to the northeast, probably i s . The L i t t l e River Fault i s discussed in a separate section. The regional metamorphic progression in the Three Ladies Mountain/Mount Stevenson area follows a t y p i c a l Barrovian sequence from greenschist facies to amphibolite fac i e s , with 93 maximum metamorphic r e c r y s t a l l i z a t i o n synkinematic and postkinematic to second-phase deformation during the mid-Jurassic Columbian Orogeny. Timing of formation of the oldest metamorphic f o l i a t i o n i s unclear. Possible evidence for pre-Jurassic deformation and/or metamorphism rests e n t i r e l y on a few observations that quartz d i o r i t i c s i l l s folded by first-phase folds apparently truncate metamorphic f o l i a t i o n in the adjacent metasedimentary rock. Prograde metamorphism involving growth of index minerals such as garnet, s t a u r o l i t e , kyanite, and s i l l i m a n i t e (and rarely preserved c h l o r i t o i d ) took place primarily during and after second-phase folding in one r e l a t i v e l y continuous metamorphic-deformational cycle. R e c r y s t a l l i z a t i o n of muscovite accompanied later .folding, followed by retrograde growth of c h l o r i t e . Metamorphic isograds mapped in the Three Ladies Mountain/Mount Stevenson area follow the general pattern shown by Fletcher (1972) and Campbell (1978) with a few differences resulting from subdivision of metamorphic zones and recognition of the L i t t l e River Fault (see Plate I I I ) . P e l i t i c rocks of the Snowshoe Group in t h i s area are mainly in the staurolite-kyanite zone of the Barrovian series in amphibolite fac i e s , with s l i g h t l y lower grade rocks to the west near Ishkloo Creek and f i r s t s i l l i m a n i t e zone rocks to the east near the North Arm of Quesnel Lake. To the northwest, metamorphic grade in the Snowshoe Group decreases to garnet and then b i o t i t e zone a few kilometers west of 94 Ishkloo Creek. Metamorphic grade also decreases to the northeast, southeast, and southwest; the o r i g i n a l d i s t r i b u t i o n of isograds is disrupted by f a u l t s in the northeast and southeast, and is obscured by retrograde metamorphism and cover in the southwest toward Grain Creek. The relationship of metamorphism to the suture zone between the Snowshoe Group rocks and Quesnellia has been studied by Rees (1981). 4.2 METAMORPHIC ZONES IN PELITES 95 4.2.1 INTRODUCTION Ten metamorphic subdivisions or zones have been defined in the Three Ladies Mountain/Mount Stevenson area on the basis of metamorphic textures and mineral assemblages in p e l i t e s . Plate III is a map showing metamorphic zones and sample l o c a l i t i e s . Zones (1) through (8) are in p e l i t e s of the Snowshoe Group, and represent d i v i s i o n s of the Barrovian series in amphibolite f a c i e s . Zones (9) and (10) are in p h y l l i t e s of the hanging wall of the L i t t l e River Fault, and are d i v i s i o n s of greenschist f a c i e s . For each zone, d i s t i n c t i v e mineral textures are summarized as supporting evidence for a suggested metamorphic history. I l l u s t r a t i o n s showing representative textures accompany the text (Figures 26 to 32). Schematic AFM diagrams for zones (3) through (8) are shown on a P-T petrogenetic gri d in Figure 25. The minerals that characterize the zones are referred to by their abbreviations, which are given in Appendix I. i 96 BATHOZONES 8- Zone ( 8 ) KY 1-+ QUARTZ + MUSCOVITE + PLAGIOCLASE + H20 T -r 400 500 600 700 F i g u r e 25. P r e s s u r e / t e m p e r a t u r e d i a g r a m i l l u s t r a t i n g AFM a s s e m b l a g e s f o r metamorphic zones i n t h e T h r e e L a d i e s Mountain/Mount Stevenson' a r e a i n r e l a t i o n t o b a t h o z o n e s as d e f i n e d by C a r m i c h a e l ( 1 9 7 8 ) , and m o d i f i e d by A r c h i b a l d e t a l . (1983) . 97 4.2.2 METAMORPHIC ZONES ZONE (1). RETROGRADE GARNET ZONE QZ-BI-MS-PL-GT (GT replaced by CH) ± CH, OP, AP, RU, ZR, CA, EP, SP Garnets are completely retrograded to c h l o r i t e . No other index minerals are present. Garnets contain vague straight S. p a r a l l e l to S, but not to S 2. Fo l i a t i o n (S,) defined by muscovite and perhaps b i o t i t e was overgrown by garnet. Further metamorphic c r y s t a l l i z a t i o n formed S 2, defined by b i o t i t e . Renewed deformation cracked garnets and kinked b i o t i t e , also kinked and smeared muscovite, which appears to be later than b i o t i t e . Retrograde reactions involve b i o t i t e and garnet a l t e r i n g to c h l o r i t e and s e r i c i t e . Red-brown, titanium-rich b i o t i t e i s replaced by pale-green, r u t i l a t e d (sagenitic) c h l o r i t e with prominent pleochroic haloes from zircon. Highest metamorphic grade achieved was at least garnet-biotite zone, and could have been much higher. 98 ZONE (2). GARNET-BIOTITE ZONE QZ-BI-MS-PL-GT ± CH, OP, ZR, RU, SP, KS, CA Garnets range from 2 to 6 mm in diameter, with possible zoning and some rotational texture. Most are severely retrograded to c h l o r i t e on the rims, and to c h l o r i t e plus s e r i c i t e between rim and core (see Figure 26). Cores show straight inclusion pattern, with some rotational texture (S-pattern) toward the rims. Inclusions are quartz, opaques, and K-feldspar, which is interpreted as r e l i c t c l a s t i c grains. Some retrograded garnets have deformed c h l o r i t e rims, with undeformed inner retrograde zones. Garnet grew on layering or f i r s t f o l i a t i o n ( S T ) , was then rotated while growing synkinematically with formation of S 2 , which is defined by muscovite and b i o t i t e . Large, postkinematic porphyroblasts of red-brown b i o t i t e are kinked and crosscut by muscovite, which grows along kink direction and is either also kinked or mimetic on polygonal arcs. Some retrograde c h l o r i t e i s also kinked, and crosscut by muscovite, and later retrograde c h l o r i t e i s undeformed within garnet outlines. 99 Figure 26. Garnet zone (2), sample 82-376. Main metamorphic f o l i a t i o n (Sg) of b i o t i t e and muscovite, wrapping around garnet, was subsequently folded by F3 folds (S 3 with minor additional muscovite r e c r y s t a l l i z a t i o n ; retrograde c h l o r i t e and s e r i c i t e after garnet are postkinemat i c . 1 00 ZONE (2a). "GARNET" ZONE Subset of Zone (3) or Zone (4) QZ-BI-MS-PL-GT-TO ± CH, OP, ZR, AP, SP, RU This zone is distinguished on the basis of garnet textures and mineral association, and is not recognizable as a separate zone in the f i e l d . The samples were co l l e c t e d from within the s t a u r o l i t e and staurolite-kyanite zones. Garnets range from 2 to 14 mm, with S^  p a r a l l e l to S^ ( S 2 ) , defined by straight t r a i l s of quartz and opaques. These samples escaped the rotation and further porphyroblast growth so common in other p e l i t e s in the same areas. It i s possible that they were less p e l i t i c i n i t i a l l y and so fewer reactants (less A l 2 0 3 , H 20) were available to r e c r y s t a l l i z e at the higher grades. Most of these samples are associated in the f i e l d with the uppermost Snowshoe Formation quartzite and p e l i t e , and close to outcrops with s t a u r o l i t e and/or kyanite. This texture may be compositionally controlled. 101 ZONE (3). STAUROLITE ZONE QZ-BI-MS-PL-GT-ST-TO ± CH, OP, ZR, RU, AP Five subgroups are based on textures in garnets: (a) Garnets are about 5 mm and have textural zoning. Straight S;- in core is defined by quartz and opaques, surrounded by a rotational zone with S-shaped inclusion t r a i l s , and a clear rim free from inclusions. (b) Garnets have straight S^  in core, but no obvious rotational zone; however, they have been rotated after cessation of growth. (c) Garnets contain folded or crenulated S- , so are h e l i c i t i c , but not rot a t i o n a l . (d) Garnets are broken and retrograded, as is st a u r o l i t e . (e) Staurolite contains tiny, i d i o b l a s t i c garnets, with no garnets in matrix (see Figure 27). A l l the garnets in this zone are te x t u r a l l y zoned, indicating at least two periods of growth, or changing conditions during growth. Most cores contain only straight Sf- of dominantly quartz. Rapid growth during renewed shear and rotation led to syndeformational, "snowball" textures in the intermediate zone of the garnet. F i n a l l y , some garnets continued to c r y s t a l l i z e without inclusions, resulting in clear rims. Some 102 Figure 27. Sta u r o l i t e zone (3), sample 81-344. Large s t a u r o l i t e porphyroblast includes i d i o b l a s t i c garnet and t r a i l s of r u t i l e and quartz (S,- ) p a r a l l e l to S e ( S 2 ) . 103 s t a u r o l i t e s , as well as garnets, show S-shaped inclusion patterns, indicating synkinematic rotation during growth. S;- may be p a r a l l e l to Sg , either at edges of grains showing rotational textures or a l l through postkinematic grains. Staurolite growth outlasted that of garnet in some rocks. It contains garnets and folded ( h e l i c i t i c ) internal t r a i l s of opaques. Garnet rims appear to be contemporaneous with b i o t i t e c r y s t a l l i z a t i o n f o l i a t i o n (S 2) in some places. Later f o l i a t i o n (S 3) i s defined by muscovite and is at a sli g h t angle to S 2; i t may represent flattened and kinked S 2 that has been only partly r e c r y s t a l l i z e d . Both s t a u r o l i t e and plagioclase porphyroblasts contain inclusion t r a i l s related to S 2. F i n a l l y , some b i o t i t e and most garnet rims have retrograded to c h l o r i t e . Presence of s t a u r o l i t e may be partly controlled by composition of the host p e l i t e . One l o c a l l y mappable unit contains the largest percentage of s t a u r o l i t e porphyroblasts (>10%), as well as large plagioclase porphyroblasts, uncommon in other p e l i t e s (for example, samples 80-69, 80-135, 82-344). 1 04 ZONE (4). STAUROLITE-KYANITE ZONE QZ-BI-MS-PL-GT-ST-KY-TO ± CH, OP, GR, ZR, AP, RU, SP, EP P e l i t i c schists in this zone show the greatest variety of textures which record the c r y s t a l l i z a t i o n history of the rocks. Garnet, s t a u r o l i t e , and kyanite reach their maximum size and modal abundance in this zone, and the recorded deformational history is more complex than for the other zones. This may be a consequence of increased reaction rates and shearing due to increasing temperatures during progressive deformation, leading to rapid c r y s t a l l i z a t i o n and large size of porphyroblasts, which tend to preserve the textural history. Garnets range in size from 1 to 15 mm, averaging 2 to 5 mm, and make up 5-25% (average 10%) of a sample of p e l i t i c s c h i s t . They are i d i o b l a s t i c to subi d i o b l a s t i c , rounded, or ske l e t a l , or atoll-shaped, and contain up to 40% inclusions. Minerals included in garnet are quartz, ilmenite and other opaques, graphite, and minor c h l o r i t o i d , s t a u r o l i t e , c h l o r i t e , and tourmaline. It is possible that some c h l o r i t e and/or s t a u r o l i t e included in garnet have replaced former c h l o r i t o i d inclusions. 105 A l l garnets show some degree of rotation, with S^  not p a r a l l e l to S e . Most garnet textures in the staurolite-kyanite zone are similar to those in the st a u r o l i t e zone. Patterns of inclusion t r a i l s and textures within garnet may be categorized as follows (number of examples for each shown in parentheses): - Straight S/ (3) - Straight S;- only in core (8) - H e l i c i t i c (folded or crenulated S^) (2) - Snowball or rotational (S-shaped pattern) (9) - Texturally zoned (9) - A t o l l (3), mostly near ST-KY-SIL t r a n s i t i o n zone (5) (see Figures 28, 29, and 30) •Chloritoid occurs only as inclusions in a large garnet -(Figure 28), between the inner core and the outer zone, within the rotational zone of the garnet, implying that folding was taking place during c h l o r i t o i d growth. This rock, which is now in the staurolite-kyanite zone, must have c r y s t a l l i z e d along a prograde P-T path that included formation of c h l o r i t o i d , such as a t y p i c a l medium to high-pressure Barrovian sequence. The c h l o r i t o i d inclusions in t h i s sample are large enough to determine complete o p t i c a l properties, and to distinguish them from c h l o r i t e inclusions in the same garnet (see Appendix I ) . Many c h l o r i t e inclusions in garnet may be replacements of e a r l i e r c h l o r i t o i d grains. In one rock (81-315), abundant small inclusions of st a u r o l i t e in a garnet may have replaced 106 Figure 28. Staurolite-kyanite zone ( 4 ) , sample 7 9 - 1 3 . Staurolite-kyanite-garnet-muscovite schist contains c h l o r i t o i d (CD) inclusions in 'snowball' (rotational) garnet synkinematic with F2 folding, indicating prograde metamorphic growth of garnet. Green c h l o r i t e (CH) is included in garnet core. Staurolite and kyanite are postkinematic to S 2 but prekinematic to S 3, and some kyanite is replaced by retrograde s e r i c i t e . 107 Figure 29. Staurolite-kyanite zone (4), sample 81-234. Garnet rims, s t a u r o l i t e , and kyanite are a l l h e l i c i t i c , including folded t r a i l s of graphite. S-is p a r a l l e l to S 1 08 former c h l o r i t o i d , as this texture is unusual for st a u r o l i t e , which is usually porphyroblastic. Staurolite continues into the staurolite-kyanite zone, where i t is as important a porphyroblast as kyanite. Staurolite forms i d i o b l a s t i c to subidioblastic porphyroblasts up to 3 cm long by 1 cm wide, but is commonly < 1 cm. It includes grains of quartz, graphite, ilmenite, r u t i l e , b i o t i t e , kyanite, garnet, tourmaline, and zircon. Inclusion t r a i l s may be p a r a l l e l to the outside f o l i a t i o n , folded ( h e l i c i t i c ) , or rotational (S-shaped). Some sta u r o l i t e s are texturally zoned, showing two growth periods, that are d i s t i n c t in terms of deformation. The cores contain many inclusions that define a vague f o l i a t i o n , and the rims are clear, and apparently post-kinematic. Staurolite may include kyanite, or kyanite may include s t a u r o l i t e , in some cases in the same thin section, so st a u r o l i t e and kyanite are interpreted as contemporaneous. P o i k i l o b l a s t i c s t a u r o l i t e may include smaller s t a u r o l i t e grains as well as kyanite, attesting to a long growth period, or wide s t a b i l i t y f i e l d for s t a u r o l i t e . Kyanite commonly occurs in two diff e r e n t settings in the staurolite-kyanite zone: (1) It i s common as porphyroblasts up to 2 cm long in p e l i t i c schist along with s t a u r o l i t e and garnet. Kyanite is somewhat aligned on f o l i a t i o n and li n e a t i o n , but is neither strongly oriented nor completely random. It tends to be synkinematic with respect to F2 folding, but some is bent, kinked, and has undulose i 1 09 extinction, probably as a result of F3 or later deformation. Some grains contain S-shaped inclusion t r a i l s , and generally S^  i s p a r a l l e l to S e . Inclusions are quartz, graphite, ilmenite, r u t i l e , muscovite, b i o t i t e , and tourmaline. Near the kyanite to s i l l i m a n i t e zone t r a n s i t i o n , kyanite grains are rounded and embayed by quartz; in some cases the core has been replaced by quartz and b i o t i t e , leaving part of the rim. (2) Kyanite is more sparsely di s t r i b u t e d as large, tabular c r y s t a l s (up to 2 cm wide by 10 cm long) within unfoliated quartz-muscovite segregations occurring in p e l i t i c schist or micaceous quartzite, throughout the staurolite-kyanite and kyanite zones. In the same area, near the kyanite to s i l l i m a n i t e boundary, large garnets are also beginning to break down to " a t o l l garnets" (Figure 30). The rim of the garnet remains as a ring-shaped grain, whereas the intermediate zone, which probably contained S-shaped inclusion t r a i l s , has been replaced by quartz and b i o t i t e . In some cases the core has been the la s t part of the i n t e r i o r of the garnet to break down. The outermost zone of the garnet would s t i l l be in equilibrium with the p e l i t e matrix, whereas the i n t e r i o r would be unstable i f exposed to the matrix, and able to react with i t or with the inclusions (in some of these garnets, the mid-zone contains up to 40% inclusions). The components of the i n t e r i o r part of the garnet would then be available for further reactions, including those producing 110 Figure 30. S t a u r o l i t e - k y a n i t e - s i l l i m a n i t e zone (5), sample 81-279. A t o l l , or ring-shaped, garnet near kyanite-sillimanite isograd; i n t e r i o r of garnet has been replaced by muscovite, b i o t i t e , and quartz. 111 new garnets. Such a reaction might be that of the breakdown of s t a u r o l i t e and formation of s i l l i m a n i t e : QZ + MS + ST = BI + GT + SI + H 20 (1.1a) There must also be a garnet breakdown reaction, such as MS + GT = SI + BI + QZ. (1.2) These reactions could occur simultaneously as long as di f f e r e n t garnet compositions were involved. If the cores of the r e l i c t garnets were the l a s t part to break down, Mn would be released after the new garnets had started to grow. Then the rims of the newly regenerated garnets would be. higher in Mn than expected for s i l l i m a n i t e zone. This is the case in some of the analysed garnets (see section on garnet zoning). Most of the larger opaque inclusions in porphyroblasts in the staurolite-kyanite zone are lath-shaped ilmenite (confirmed with probe data). Other T i - r i c h minerals are rare, such as sphene, which only occurs in s l i g h t l y calcareous s c h i s t s . Rutile occurs as inclusions within s t a u r o l i t e and kyanite, but not in garnet (except in the kyanite zone). Where r u t i l e comes in contact with quartz, whether inside a grain of s t a u r o l i t e or kyanite, or at the rim next to the matrix, i t i s replaced by ilmenite. This is a common texture throughout both the s t a u r o l i t e zone and 1 1 2 staurolite-kyanite zone. Some s t r i k i n g examples occur where an elongate grain straddles the contact between the porphyroblast and the schist matrix, and the part included within the st a u r o l i t e or kyanite i s c l e a r l y r u t i l e , and the part projecting into the matrix is opaque ilmenite, with the tran s i t i o n coinciding exactly with the rim of the porphyroblast. See the section on the GRAIL geobarometer for an i l l u s t r a t i o n and interpretation of this texture. The i l m e n i t e - r u t i l e textures suggest that the st a u r o l i t e zone and kyanite zone were formed in P-T conditions below those for the pressure-sensitive reaction ALM + RUT = ILM + KY/SI + QZ, (1.3) known as the GRAIL geobarometer (Bohlen et a l . 1983). Tourmaline is commonly zoned in p e l i t e s from blue-green to oli v e green. In thin sections of p e l i t e s cut perpendicular to the dominant F 2 f o l d axis, tourmaline end-sections (0001) are more abundant than those p a r a l l e l to the c-axis, indicating some synkinematic orientation of tourmaline, implying either rotation or growth during peak metamorphism. Grain size i s too small to see tourmaline in most hand specimens. 1 1 3 ZONE (5). STAUROLITE-KYANITE~SILLIMANITE ZONE Also referred to as "Transition Zone". QZ-BI-MS-PL-GT-ST-KY-SI-TO ± CH, OP, RU, ZR, AP This zone is defined by the " s i l l i m a n i t e - i n isograd", or f i r s t appearance of f i b r o l i t e in thin sections. Kyanite and sta u r o l i t e are s t i l l present, but are apparently breaking down to s e r i c i t e , or quartz, b i o t i t e , and garnet. Tourmaline in this zone shows complex zoning with more than three concentric colour-zones, in contrast to the usual two. In sample 80-33, colour-zoning concentric to the c-axis is as follows, from core to rim: (1) bluish-green to brown, (2) olive green, (3) pale yellowish o l i v e , (4) olive green. Cation zoning in one grain of thi s sample i s shown in probe data (see Appendix I I ) . Probe data indicate a variation from higher Mg than Fe in the core, to lower Mg than Fe in the rim, as well as higher Na in the core than in the rim, but lower Ca in the core than in the rim. There is no Mn in thi s tourmaline, negligible K, and constant Ti from core to rim. This zoning pattern i s continuous with no abrupt t r a n s i t i o n s . Tourmaline in thi s zone may contain inclusions of quartz, zircon, or opaques, and may also crosscut b i o t i t e . In another sample (81-279), tourmaline inclusions inside kyanite have only one zone, and contain inclusions that are p a r a l l e l to inclusion t r a i l s in the kyanite, which are 1 1 4 p a r a l l e l to the outside f o l i a t i o n . Tourmaline in the matrix of this rock shows two colour-zones, implying that the rim of the matrix tourmaline is contemporaneous with kyanite growth. Garnet textures are markedly d i f f e r e n t from those in the staurolite-kyanite zone. Although some r e l i c t garnets up to 3 mm remain, most garnets are now < 1 mm in diameter and comprise less than 5% of the rock. Larger, r e l i c t garnets contain straight S;- perpendicular to Sgl are zoned, and include kyanite, s t a u r o l i t e , b i o t i t e , and r u t i l e in the outer portions. Small, new garnets have few inclusions, and are associated with f i b r o l i t e near embayed s t a u r o l i t e (see Figure 31). These textures have preserved evidence for two uncompleted reactions: KY = SIL (1.4) QZ + MS + ST = BI + GT + SI + H 20. (1.1a) P-T conditions were probably near the intersection of these reactions. See the section on s t a u r o l i t e breakdown for estimated temperatures and pressures of t h i s intersection. 115 Figure 31. Staurolite-si11imanite zone (6), sample 80-30. F i b r o l i t i c s i l l i m a n i t e forming at the expense of s t a u r o l i t e . Note small size of garnets (<1 mm). 1 1 6 ZONE (6). STAUROLITE-SILLIMANITE ZONE QZ-BI-MS-PL-GT-ST-SI ± CH, OP, ZR, RU, AP, KS, EP Staurolite is s t i l l present, but skeletal and embayed; kyanite i s no longer present. F i b r o l i t e surrounds s t a u r o l i t e , which appears to be breaking down. Garnets are tiny, < 2 mm, look new, and are associated with new b i o t i t e . A few r e l i c t garnets are broken and embayed. That kyanite has disappeared before s t a u r o l i t e i s suggestive of a P/T path in Bathozone 4 (Carmichael 1978). 1 1 7 ZONE (7). SILLIMANITE ZONE QZ-BI-MS-PL-GT-SI ± CH, OP, RU, ZR, SP, ST This corresponds to the c l a s s i c " f i r s t s i l l i m a n i t e zone". As muscovite is present in a l l p e l i t e s , conditions are below those of the reaction MS + Q = SI + KS + H 20. Garnets are small (< 1 mm) except r e l i c t s (3 to 6 mm), and comprise 1-5% of the rock. They tend to be blocky to rounded, sub i d i o b l a s t i c , or oval. Garnets contain less than 5% inclusions, which are quartz, opaques, and b i o t i t e . S i l l i m a n i t e i s in the form of f i b r o l i t e knots, which are bundles of tiny, acicular s i l l i m a n i t e grains, commonly associated with b i o t i t e . 118 Figure 32. S i l l i m a n i t e zone (7), sample 80-31. Tiny garnets outline a former garnet porphyroblast replaced by quartz, plagioclase, b i o t i t e , and muscovite. Similar textures, involving both garnet and kyanite, were observed in the kyanite zone (8). 1 19 ZONE (8). KYANITE ZONE QZ-BI-MS-PL-GT-KY ± CH, OP, RU, ZR, AP, ST, TO, KS Garnets 2-5 mm are r e l i c t , embayed, and include quartz, opaques, r u t i l e , b i o t i t e , muscovite, and s t a u r o l i t e . Garnets < 2 mm have rare inclusions of quartz, opaques, b i o t i t e , and r u t i l e . A l l are very small and in some places appear to be growing in an old garnet shape, as a ring of l i t t l e garnets (see Figure 32). In some rocks, tiny garnets contain no inclusions. Kyanite in thi s zone is also tiny, occurring along layers with garnets. Big kyanites are pseudomorphed by s e r i c i t e ; s t a u r o l i t e i s reacting out. This is the only zone in which garnets include b i o t i t e or r u t i l e . Textures suggest regenerated forms of garnet, b i o t i t e , and kyanite in this zone, although a l l were also present in Zone (4). Such textures are consistent with an interpretation that the discontinuous reaction QZ + MS + ST = BI + GT + KY + H 20 (1.1b) has taken place in Bathozone 5 (Carmichael 1978). 1 20 ZONE (9). BIOTITE-CHLORITE ZONE QZ-MS-CH-BI-OP This zone i s r e s t r i c t e d to the hanging wall of the L i t t l e River Fault near Limestone point, in p h y l l i t i c rocks interlayered with with carbonates. F o l i a t i o n i s mainly defined by muscovite and c h l o r i t e . B i o t i t e i s rare; where i t occurs i t is red-brown and porphyroblastic, postkinematic to the main muscovite-chlorite f o l i a t i o n , but also kinked and retrograded to c h l o r i t e . 121 ZONE (10). CHLORITE ZONE This zone also occurs in the hanging wall of the L i t t l e River Fault, but s t r u c t u r a l l y higher, on Service Mountain, in p h y l l i t i c rocks interlayered with carbonates. The following assemblages are present: Q-MS-CHL ± CA, TO, ZR, OP (IL, HE, MG) Q-MS ± GP, OP Q-MS-CA-GP ± OP, PY The p e l i t i c rocks in this zone may show two d i s t i n c t p h y l l i t i c cleavages, a x i a l planar to F2 and F3 folds. Either one of these cleavages may be dominant in a par t i c u l a r outcrop, which makes them d i f f i c u l t to distinguish with certainty, as both were formed in c h l o r i t e grade greenschist facies conditions, and are defined by muscovite-chlorite c r y s t a l l i z a t i o n f o l i a t i o n . The more quartz-rich units tend to preserve the F2 a x i a l planar cleavage, whereas interlayered p h y l l i t e may show F3 ax i a l planar cleavage nearly perpendicular to i t (see Figure 9). 1 22 4.3 GARNET ZONING 4.3.1 DESCRIPTION OF ZONING PROFILES Garnets from eight samples were analyzed on the electron microprobe (data shown in Appendix I I ) . A l l garnets analyzed can be characterized by the mineral formula (Fe,Mg,Ca,Mn) 3Al 2Si 30, 2. They are dominantly almandine (28 to 37 Wt.% FeO) with lesser amounts of pyrope (1 to 4 Wt.% MgO), spessartine (0.1 to 4.5 Wt.% MnO), and grossular (1 to 8 Wt.% CaO). Zoning p r o f i l e s of garnets are shown in Figures 33 to 41. The s t a u r o l i t e zone (Zone 3) i s represented by sample 82-389; the staurolite-kyanite zone (Zone 4) by sample 81-279; the s t a u r b l i t e - k y a n i t e - s i l l i m a n i t e zone (Zone 5) by samples 80-33, 80-119, and 81-278; the s t a u r o l i t e - s i l l i m a n i t e zone (Zone 6) by sample 80-19; the s i l l i m a n i t e zone (Zone 7) by sample 80-31; and the kyanite zone (Zone 8) by sample 81-325. Although the number of analyzed samples is limited, certain patterns have emerged that may have general s i g n i f i c a n c e . Observations and interpretations made here have been supplemented by textural evidence from additional samples. 1 23 In the samples analyzed, Fe is lower in the core of the garnet and increases toward the rim in a l l but one sample. The maximum difference between core and rim is 5 Wt.% (32 to 37) in 81-279. The exception i s sample 81-325, in which the garnets are very small (<0.5 mm); Fe is r e l a t i v e l y constant with s l i g h t l y higher values in the core ( a l l within 1 Wt.% (34 to 35 Wt.%). Some of the larger garnets near the kyanite to s i l l i m a n i t e t r a n s i t i o n show a drop in Fe values near the rim (81-278, 81-279). Mg varies less strongly than does Fe. In the lower grade samples i t is lowest in the core of the garnet, and higher near the rim, with a sl i g h t drop right at the rim (81-278, 81-279, 82-389). Mg is highest in the core of garnets in rocks from the high-grade side of the kyanite to s i l l i m a n i t e t r a n s i t i o n (80-19, 80-119, 80-33, and 80-31) and in the kyanite zone (Zone 8) (81-325). Mg and Fe zoning patterns in garnet are related in that in the lower grade rocks Mg and Fe are both lower in the core and higher in the rim, but in s i l l i m a n i t e zone (Zones 6 and 7) they are opposite, with Mg higher in the core whereas Fe is s t i l l lower. In kyanite zone (Zone 8) both Fe and Mg show s l i g h t l y higher values in the core of the garnet, but the variation i s small. Mn zoning patterns also vary from the low-grade to the high-grade rocks. The larger garnets from the staur o l i t e and staurolite-kyanite zones (82-389, 81-279, 1 24 81-278) have higher Mn in the core, lower Mn toward the rim, and higher Mn on the rim i t s e l f . This kind of pattern is f a i r l y t y p i c a l of garnets that retain prograde textural zoning. The higher Mn on the rim of these large, zoned garnets may be due to retrograde metamorphism of the rim of the garnet. This i s probably not the case in some of the higher-grade garnets, however, in which the Mn zoning p r o f i l e s show lower Mn in the cores grading out to higher Mn in the rims (80-19, 80-119, 81-325). Not a l l of the higher-grade garnets follow t h i s pattern; small garnets from samples 80-31 and 80-33 show normal zoning patterns with Mn higher in the core, and Fe lower in the core. Ca zoning patterns in garnets from rocks in the s t a u r o l i t e and staurolite-kyanite zone (82-389, 81-279) show extreme zoning with higher Ca in the core and lower Ca in the rim. This may be compositionally controlled or r e f l e c t a prograde metamorphic cation preference. Ca values in the higher-grade rocks are f a i r l y low and constant. Presence of Ti in garnets i s d i r e c t l y related to nearby ilmenite inclusions and i s considered to be ins i g n i f icant. 125 Figure 33. Garnet zoning p r o f i l e for Garnet 2 from sample 82-389, stau r o l i t e zone. 126 Figure 34. Garnet zoning p r o f i l e for Garnet 3 from sample 81-279, staurolite-kyanite zone. 127 81-278 GARNET (Fe,MgfCa,Mn)3AI2Si30 Figure 35. Garnet zoning p r o f i l e for Garnet 4 from sample 81-278, st a u r o l i t e - k y a n i t e - s i l l i m a n i t e zone. 128 Figure 80-33, 36. Garnet zoning staurolite-kyanite p r o f i l e for - s i l l i m a n i t e Garnet 1 from sample zone. 129 80-119 GARNET 1 Figure 37. Garnet 80-119, s t a u r o l i t e zoning p r o f i l e f o r Garnet - k y a n i t e - s i l l i m a n i t e zone. 1. from sample 130 F i g u r e 38. Garnet z o n i n g p r o f i l e f o r Garnet 2 from sample 80-19, s t a u r o l i t e - s i l l i m a n i t e zone. 80-31 GARNET 3 131 Figure 39. Garnet zoning p r o f i l e for Garnet 3 from sample 80-31, s i l l i m a n i t e zone. Figure 40. Garnet zoning p r o f i l e s for Garnets 3.1 and 2 from sample 81-325, kyanite zone. 133 81-325 GARNET 4 GARNET 1 F i gure sample 41. Garnet zoning p r o f i l e s 81-325, kyanite zone. for Garnets 4 and 1 from 1 34 4.3.2 DISCUSSION OF GARNET ZONING As mentioned in the section on the staurolite-kyanite zone, textures in garnets from rocks near the kyanite to s i l l i m a n i t e t r a n s i t i o n suggest breakdown of larger, t e x t u r a l l y zoned garnets and formation of smaller, apparently homogeneous garnets. A t o l l garnets attest to the breakdown of the intermediate zone of the garnet f i r s t , s tarting with the area richest in inclusions, which i s the rotational zone of the garnet. Both a t o l l garnets and whole garnets from sample 81-279 appear to be of the same generation because of similar size and inclusion patterns, but they represent different stages of breakdown. The whole "garnet shown in Figure 34 contains more Ca and Mn in the core than the rim, and i s normally zoned for a garnet which has retained some of i t s prograde history into staurolite-kyanite zone conditions. It exhibits a rotational pattern in the mid-zone of the garnet, which is t y p i c a l for garnets in t h i s metamorphic zone. An a t o l l garnet observed in the same sample shows complete breakdown of this mid-zone to b i o t i t e , quartz, and muscovite, and incipient breakdown of the core, whereas the rim i s clear of inclusions and apparently stable. Rims of some a t o l l garnets appear to be in the process of breaking up into smaller, more idiomorphic grains. A similar texture has been observed in the kyanite zone (Zone 8) where tiny garnets are arranged around a ring i 1 35 as i f mimicking the former rim of a larger garnet which has broken down (see Figure 32). The i n t e r i o r of larger garnets may be unstable in the presence of inclusions. A l t e r n a t i v e l y , the breakdown of garnet of appropriate composition may be due to a combination of reactions, including MS + GT = 2 SIL + BI + QZ. (2.4) The inner cores are the l a s t part of the garnet to break down, thus releasing Ca and Mn--the Ca going into plagioclase rather than garnet, and the Mn going to the rims of the new garnets that have nucleated on the r e l i c t grains of the old rim, producing reverse Mn zoning in the small garnets. Farther from t h i s t r a n s i t i o n , the higher grade garnets show normal zoning. The new garnets may also be acquiring components from the breakdown of s t a u r o l i t e , QZ + MS + ST = BI + GT + KY/SIL + H 20 (2.1) which i s consistent with textures observed in thin section. Whatever combination of reactions i s responsible, the staurolite-out isograd is accompanied by a d i s t i n c t change in garnet textures, as well as changes in the stable aluminosilicate. Below the staurolite-out 1 36 isograd, garnets are large, texturally and compositionally zoned and retain evidence of their prograde metamorphic and structural history. Above the staurolite-out isograd, small, clear garnets appear homogeneous but show reverse Mn zoning. At Three Ladies Mountain, kyanite porphyroblasts become unstable along with large garnets, f i b r o l i t e appears, and s t a u r o l i t e persists for a short distance into the s i l l i m a n i t e zone. East of Mount Stevenson, on the other hand, large garnet and kyanite porphyroblasts are replaced above the staurolite-out isograd by much smaller grains of kyanite (rather than f i b r o l i t e ) and tiny garnets with reverse Mn zoning. Such evidence i s consistent with a change from Bathozone 4 to Bathozone 5 (Carmichael 1978). defined by the intersection between the reactions KY = SIL (1.4) and QZ + MS + ST = BI + GT + KY/SIL + H 20. (2.1) This intersection is discussed in the section on s t a u r o l i t e breakdown, and would be located in the f i e l d somewhere near lower Long Creek between Three Ladies Mountain and Mount Stevenson. 1 37 4.4 STAUROLITE BREAKDOWN For defining the bathograd between bathozones 4 and 5, Carmichael (1978) has chosen a s t a u r o l i t e breakdown reaction ST + MS + QZ = GT + BI + KY/SIL + H 20 (2.1) from f i e l d and textural evidence, and located i t on a P/T diagram within reasonable conditions for the host p e l i t e s based on other indications of metamorphic grade. This reaction i s a reasonable representation of changes in assemblages observed across the staurolite-out isograd in rocks from the Three Ladies Mountain/Mount Stevenson area. Carmichael (1978) shows the intersection of reaction (2.1) with KY = SI at 4.75 kb and 540°C, but states that the experimental data for s t a u r o l i t e are inconsistent with these conditions of s t a u r o l i t e s t a b i l i t y , and would indicate a higher temperature for the breakdown of s t a u r o l i t e . Archibald et a l . (1983) place the intersection at about 5.5 kb and 580°C, having revised Carmichael's curve using an analysis of experimental data on s t a u r o l i t e by Pigage and Greenwood (1982). Higher temperatures for s t a u r o l i t e breakdown were calculated by Pigage and Greenwood (1982) for the reaction 6ST + 12.5QZ = 4ALM + 23KY/SIL + 6H20. (2.2) Experimental data for s t a u r o l i t e are inconsi stent with f i e l d 1 38 evidence for the location of t h i s reaction in P/T space. According to Pigage and Greenwood (1982), experimental data for s t a u r o l i t e with a composition of F e 2 A l 9 S i 3 7 5 0 2 2(OH) 2 are i n t e r n a l l y consistent, but s t i l l put the intersection, between this reaction (2.2) and KY = SIL at 6.5 kb and 680°C. As this temperature i s approaching that of granite melt, i t appears high for breakdown of s t a u r o l i t e . Lang and Rice (1985) give values for AH°f and S ° 2 9 8 for s t a u r o l i t e which are consistent with the range of best data chosen by Pigage and Greenwood (1982), so these values have been used in calculations in t h i s study. Staurolite in rocks from the Three Ladies Mountain area is a binary solution of F e 2 + and Mg end-members, with XFe =* 6.85. The reaction ST + MS + QZ = GT + BI + KY/SIL + H 20, (2.1) described by Carmichael (1978) as defining the bathograd between bathozones 4 and 5, involves six components and seven phases, and so cannot be calculated in the Fe end-member system. Data for Mg-staurolite are poorly known, and have not yet been found to be consistent with other sets of data. Instead of c a l c u l a t i n g reaction (2.1), two other s t a u r o l i t e breakdown reactions involving Fe end-member phases were considered: 1 39 25QZ + 3ST = 46KY + 8ALM + 12H20 (2.2) 17ALM + 25MS + 3ST = 96KY + 25BI + 12H20. (2.3) (Note that s t a u r o l i t e formula of F e 2 A l 3 S i 3 . 7 5 0 2 2 ( O H ) 2 has been multiplied by four in order to have an integral value for Si for computing purposes, so c o e f f i c i e n t s appear unusually high.) These reactions were calculated and then displaced assuming the oversimplification that measured values of XFe in st a u r o l i t e , b i o t i t e , and garnet approximate a c t i v i t i e s of these phases, and that a l l other phases are pure, and using the thermodynamic data compiled by Lang and Rice (1985) (Table I).. 1 40 Expressions for a c t i v i t y assume ideal s i t e mixing, as follows: Almandine (Fe 3A1 2 S i 30 , 2 ) a f l / m = <X^) 3 B i o t i t e (KFe 3Si 3A10, o (OH) 2 ) afc|- = (xj?'g) Staurolite ( F e B A l 3 6 S i 1 5 0 8 8 ( O H ) 8 ) ast = (X)%) 8 AG, = AG r° + RT i n Kdi5pl (2.4) where ( _ \ 8 ( y C ' \ 2 « K f 2 2) = = o (2.2a) ( a ^ ) 3 - ( X ) % ) " ( a 6 f ) a s (Xfe)'» K. . = = — — (2.3a) Results are presented graphically in Figure 42. Both reactions are displaced to lower temperatures as expected. Reaction (2.2) i s only displaced by about 20°C, whereas reaction (2.3), involving muscovite and b i o t i t e , as well as breakdown of garnet, i s displaced by over 250°C, so the effect of Mg in b i o t i t e is s i g n i f i c a n t . Temperatures and pressures of intersection of these reactions with KY = SIL (P = 7-7.3 kb, T =670-690°C) s t i l l appear too high, but are beginning to approach reasonable values. Water a c t i v i t y <1 would displace these reactions to lower temperatures. However, reduction of XH20 to 0.88, assuming equilibrium 141 TABLE I. MINERAL DATA MINERAL AH" f (vl/mol ) FORMULA S' 298K (d/m-K) (O/m-K) ABBREVIATION b (J/m-K' ) c (J/m-K) Vs ( J / b a r ) ••MINERALS** A-OUARTZ S I ( 1 ) 0 ( 2 ) -910648.00 41.33800 A-QTZ 46.94000 .034309 ALMANDINE F E ( 3 ) A L ( 2 ) S I ( 3 ) 0 ( 1 2 ) ALM -527720O.00 298.700 408.15 .14075 FE-BIOTITE -5155504. MUSCOVITE -5972275. K ( 1 ) F E ( 3 ) A L ( 1 ) S I ( 3 ) 0 ( 1 2 ) H ( 2 ) FE 398 . 3 445.30 1246 MS 1 1037 KYANITE -2581097. SILLIMANITE -2573574. FE-STAUROLITE -47497890. ANORTHITE GROSSULAR **GASES** STEAM K( 1 ) A L ( 3 ) S I ( 3 ) 0 ( 1 2 ) H ( 2 ) 287.9 408.19 A L ( 2 ) S I ( 1 )0(5) KY 83.68 173. 189 .02852 A L ( 2 ) S I ( 1 )0(5) SIL 96.776 167.46 .030922 F E ( 8 ) A L ( 3 6 ) S I ( 1 5 ) 0 ( 9 6 ) H ( 8 ) -1129700. -7837000. -BIO -8079000. - 10644000. -5389900. -4884400. FE-ST 1770.0 3467 .6 .6704 CA( 1 ) A L ( 2 ) S I ( 2 ) 0 ( 8 ) 205.4 264.83 C A ( 3 ) A L ( 2 ) S I ( 3 ) 0 ( 1 2 ) 254.7 435.207 -241818. H(2)0( 1 ) 188.72 H20 30.54 AN .061898 GROS .071182 .01029 -93880000. -6460100. -11429900. 23.2400 115.28 154.32 140.71 44 .09 49 .90 897.6 100.79 125 . 3 Data from Lang and R1ce (1985) A c t i v i t i e s used f o r d i s p l a c e d r e a c t i o n s a r e as f o l l o w s . XFe ST = (0.85) • = -0.56465 XFe BI = (0.54) 1 = -0.8028 XFe ALM = (0.79) 1 = -0.30712 142 Figure 42. Calculated s t a u r o l i t e breakdown reactions using thermodynamic data from Lang and Rice (1985) and measured s t a u r o l i t e XFe = 0.85, garnet XFe = 0.79, and b i o t i t e XFe = 0.54. Dashed curves indicate Fe end-member reactions. Intersection of reaction (A) with KY = SIL is displaced from 7224 bars and 683°C to 6985 bars and 670°C; intersection of reaction (B) with KY = SIL i s displaced from 11825 bars and 945°C to 7333 bars and 689°C. Reduction of XH20 to 0.88 displaces both curves by only 10°C. 1 43 with graphite, only displaces these reactions by about 10°C. Use of a more comprehensive and consistent database (including data for Mg-staurolite) and less s i m p l i f i e d a c t i v i t y models for s o l i d solution phases may result in a more r e a l i s t i c petrogenetic gri d involving s t a u r o l i t e react ions. Analysis of rocks from the Three Ladies Mountain area indicates that s t a u r o l i t e breakdown is accompanied by textural and compositional changes in garnet (suggesting discontinuous reaction involving garnet) and also involves muscovite and b i o t i t e . Therefore, neither reaction (2.2) nor (2.3) alone i s completely adequate for describing s t a u r o l i t e breakdown in these rocks, However, according to the displacement of the reactions shown in Figure 42, they would occur nearly simultaneously near the kyanite to s i l l i m a n i t e t r a n s i t i o n . Reaction (2.1), suggested by Carmichael (1978), cannot be balanced exactly with the minerals of this study, as the XFe (St) i s almost equal to the XFe (Gt). However, simultaneous equilibrium among muscovite, quartz, garnet, s t a u r o l i t e , b i o t i t e , and kyanite must also s a t i s f y e q u i l i b r i a (2.2) and (2.3). 1 44 4.5 GARNET-BIOTITE GEOTHERMOMETRY Ferry and Spear (1978) f i r s t introduced a calibr a t e d geothermometer based on the Fe-Mg cation exchange reaction between garnet and b i o t i t e . Subsequently, several authors have offered corrections for taking into consideration the effe c t s on calculated temperature of Ca and Mn components in garnet (Newton and Haselton (1981); Pigage and Greenwood (1982); Ganguly and Saxena (1984)). Calculated temperatures can vary by over 100°C depending on which solution models are used, especially those involving manganese. Engi (1984) and Lang and Rice (1985) favor the correction by Newton and Haselton (1981) as giving the most r e a l i s t i c and consistent temperatures. Eight samples were chosen from those least affected, by retrograde metamorphism. One garnet-biotite temperature was calculated for each sample using averaged analyses of several points on garnet rims and b i o t i t e grains. Averaged microprobe analyses are presented in Appendix II; note that standard deviations among di f f e r e n t b i o t i t e grains in the same section are approximately the same as those from one grain ( b i o t i t e from sample 80-19 i s given as an example). Garnet-biotite temperatures were calculated for garnet rims and also for cores paired with matrix b i o t i t e in some samples; temperatures were also calculated for garnet with b i o t i t e inclusions from sample 80-119. The methods used were those of Ferry and Spear (1978; T1), Newton and Haselton (1981; T2), and Ganguly and Saxena (1984; T3). 145 The following formulas were used for cal c u l a t i n g garnet-biotite temperatures (Lang and Rice 1985): 2089 + 0.00956*P (bar) T1 (K) = 0.7820 - In K Ferry and Spear (1978) (1661 - 0.755*T1 (K ) )x£* + 2089 + 0.00956*P (bar) T2 (K) = — 0.7820 - ln K Newton and Haselton (1981) „ ( K ) . »W% - XMS - °-8)/R 0.7820 - ln K 1510*(X^ + Y^n) + 2089 + 0.00956*P (bar) 0.7820 - ln K Ganguly and Saxena (1984) where ^ F e M g = 11.34 kJ and R = 8.314 JK- 1mol" 1 1 46 Table II (A and B) shows the temperatures given by these three methods at 5000 and 6000 bars. Four of the samples (80-119, 80-33, 81-278, 81-279) were chosen from as close to the k y a n i t e - s i l l i m a n i t e t r a n s i t i o n as possible. Average rim temperatures for samples from the area as a whole as well as from the kyanite-sillimanite isograd are shown. Garnet rim temperatures using the method of Newton and Haselton (1981) average around 525 ± 18°C at 5 kb and 528 ± 18°C at 6 kb for the whole area and 521 ± 25°C at 5 kb and 525 ± 25°C at 6 kb for the kyanite to s i l l i m a n i t e t r a n s i t i o n zone. Uncorrected Ferry and Spear (1978) gives temperatures about 35°C lower, whereas the method of Ganguly and Saxena (1984) gives temperatures about 35° higher. Note that v a r i a t i o n of temperature among samples from a single mapped isograd exceeds variation across the whole area from st a u r o l i t e zone to s i l l i m a n i t e zone. If the s i 11imanite-in isograd corresponds to the reaction KY = SI, then the best f i t temperature and pressure are those given by Ganguly and Saxena (1984) at 557°C and 5 kb. Note that there is not much difference in rim temperature between samples from the s t a u r o l i t e zone, kyanite zone, and s i l l i m a n i t e zone, but that core "temperatures" (using matrix b i o t i t e ) are lower than rim temperatures in the s t a u r o l i t e zone sample, and higher than rim temperatures in the s i l l i m a n i t e zone, kyanite zone, and kyanite to s i l l i m a n i t e t r a n s i t i o n zone. TABLE IIA. GARNET-BIOTITE TEMPERATURES SAMPLE 82-389 82-389 81-279 81-278 80-33 80-33 80-119 80-119 80-119 RIM CORE RIM RIM RIM CORE RIM INC1 INC2 MET. ZONE ST ST ST-KY ST-KY-SI ST-KY-SI ST-KY-SI ST-KY-SI ST-KY-SI ST-KY-SI XMgGt O.100 0.081 0.087 0.089 0.085 0.095 0.090 0.123 0.119 XFeGt 0.690 0.657 0.806 0.770 0.801 .0.779 0.811 0.778 0.799 XCaGt 0.175 0.188 0.075 0.078 0.061 0.047 0.063 0.067 0.050 XMnGt 0.035 0.O73 0.032 0.064 0.053 O.080 0.036 0.033 0.033 SUM 3.030 3.063 3.028 3.030 2.966 2.958 3.006 2.992 3.012 Mg/Fe Gt 0.145 0.124 0.108 0.115 0.106 0.121 0.112 0.157 0.149 Mg/Fe Bi 1.253 1.253 0.760 0.978 0.785 0.785 0.761 1.213 1.143 K1 0.116 0.097 0.142 0.118 0.135 0.155 0.147 0.130 0.130 T C O at P 5 kb T1 (FS') T2 (NH') T3 (GS«) 454 521 562 416 486 544 508 537 567 459 488 532 494 517 555 533 552 605 517 542 573 44 1 466 49 1 484 504 528 T (*C) at P = 6 kb T1 (FS') T2 (NH1) T3 (GS 4) 458 524 565 419 489 547 51 1 541 570 462 491 535 497 521 559 537 555 609 521 546 576 444 469 495 488 507 532 1K = (Mg/Fe Gt)/(Mg/Fe BI) 'FS = Ferry and Spear (1978) 1NH = Newton and Haselton (1981) 'GS = Ganguly and Saxena (1984) 4 ^ . TABLE IIB. GARNET-BIOTITE TEMPERATURES SAMPLE 80-19 80-19 80-31 80-31 RIM CORE RIM CORE MET. ZONE ST-SI ST-SI ST-SI ST-SI XMgGt 0 089 O. 131 0.094 0. 106 XFeGt 0.784 0. 753 0.806 0.772 XCaGt 0.07G 0. 091 0.058 0.047 XMnGt 0.051 0. 026 0.041 0.076 SUM 3.005 3. 025 3.025 3.030 Mg/Fe Gt 0.114 0. 173 0.117 0. 138 Mg/Fe BI 0.856 0. 893 0.823 0.823 K 1 0. 133 O. 194 0. 142 0. 167 T C C) at P = 5 kb T1 (FS<) 489 609 509 558 T2 (NH1) 5 19 647 532 577 T3 (GS') 558 682 564 630 T C C) at P = 6 kb T1 (FS') 493 613 513 562 T2 (NH3) 522 651 536 581 T3 (GS*) 561 686 568 634 1K = (Mg/Fe Gt)/(Mg/Fe BI) 'FS = Ferry and Spear (1978) 'NH = Newton and Haselton (1981) 'GS = Ganguly and Saxena (1984) 'Average of 4 samples at KY=SI Isograd: 80-33, 80-119, 81-325 81-325 AVERAGE AVERAGE RIM CORE RIMS RIMS KY KY ST-KY-SI* ALL ZONES O.108 O.122 0.758 0.758 0.085 0.082 0.049 0.038 3.040 3.055 0.142 0.160 1.003 1.003 O.142 O.160 508 542 582 544 577 613 494 (+26) 52 1 (+25) 557 (+18) 492 (+24) 525 (+18) 562 (+15) 512 545 586 548 580 617 498 (+26) 525 (+25) 560 (+18) 496 (+24) 528 (+18) 565 (+15) -278, 81-279 149 Overall, garnet-biotite geothermometry estimates temperatures between about 490° and 570°C at pressures between 5 and 6 kb. Presence of s i l l i m a n i t e in the rocks indicates that either the pressure was somewhat lower (4 kb?) at the same T, or that the temperatures given by a l l of these methods, except perhaps Ganguly and Saxena (1984) are too low at the kyanite-sillimanite isograd by approximately 50°C. No temperature gradient from sta u r o l i t e zone to s i l l i m a n i t e zone was demonstrated using garnet-biotite geothermometry. Assuming that changes in metamorphic assemblages in p e l i t e s are due to changes in pressure and temperature conditions during metamorphism, either the garnet-biotite geothermometer does not adequately estimate metamorphic temperatures, or the temperature gradient required between s t a u r o l i t e zone and s i l l i m a n i t e zone is smaller than the resolution possible with t h i s method so far, or less than about 50°C. 1 50 4.6 GEOBAROMETRY: GRAIL A new geobarometer referred to as "GRAIL" has been calibrated by Bohlen et a l . (1983) for the reaction 3 IL + SI/KY + 2 QZ = AM + 3 RU. (3.1) The almandine plus r u t i l e assemblage i s on the high-pressure side of the reaction (see Figure 43). This geobarometer can only be applied i n d i r e c t l y to the metamorphic rocks of the Three Ladies Mountain area as the univariant assemblage ilmenite + aluminosilicate + quartz + almandine + r u t i l e i s nowhere observed to be stable. However, textures involving subsets of this assemblage have been useful in l i m i t i n g the pressure conditions over most of the area to those pressures below the GRAIL reaction. Of thirteen p e l i t e samples containing both ilmenite and r u t i l e , seven are in staurolite-kyanite zone (Zone 4). In these seven samples, r u t i l e i s only present as small inclusions within porphyroblasts of s t a u r o l i t e and/or kyanite,,whereas ilmenite i s present both as inclusions in porphyroblasts such as garnet, s t a u r o l i t e , and kyanite, and as individual laths in the matrix p e l i t e . The same textures were noted in one sample from the s t a u r o l i t e zone (Zone 3) and two samples from the s t a u r o l i t e - k y a n i t e - s i l l i m a n i t e zone (Zone 5). Where r u t i l e comes in contact with quartz, whether within a porphyroblast or near the rim, i t i s replaced by ilmenite. This texture i s i l l u s t r a t e d in 151 "GRAIL" GEOBAROMETER 400 500 J°Q 600 700 F i g u r e 43. P r e s s u r e / t e m p e r a t u r e diagram of "GRAIL" geobarometer ( m o d i f i e d from Bohlen e t a l . 1983). Shaded a r e a shows approximate P/T c o n d i t i o n s i n d i c a t e d by assemblages observed i n r o c k s from Three L a d i e s M o u n t a i n / Mount Stevenson a r e a . 1 52 Figure 44, where a t r a i l of r u t i l e inclusions in s t a u r o l i t e is continuous with a t r a i l of ilmenite grains in the matrix. Elongate grains straddling the rim of the porphyroblast are r u t i l e within the s t a u r o l i t e (or in other places kyanite) and ilmenite where projecting into the matrix. The assemblage ST + RU + QZ appears to be—less stable than ST + RU without QZ. Ilmenite is stable with garnet, quartz, kyanite, s t a u r o l i t e , and the other minerals of the p e l i t e matrix. The remaining three samples containing both ilmenite and r u t i l e are from the apparently higher-pressure kyanite zone (Zone 8 ) . They are the only samples in which r u t i l e occurs both as inclusions in garnet and as individual grains in the matrix without ilmenite rims, but i t must be noted that r u t i l e i s a very minor constituent of these rocks. Rutile inclusions in kyanite s t i l l show replacement by ilmenite where in contact with quartz. Perhaps pressures approximating those for the "GRAIL" reaction have been approached. In s t a u r o l i t e to staurolite-kyanite zone rocks near Three Ladies Mountain, lack of r u t i l e inclusions in garnet suggests a pressure below that of the "GRAIL" reaction. In the same rocks, textures involving r u t i l e , quartz, and s t a u r o l i t e suggest temperatures compatible with the reaction ST + RU + QZ = KY + IL + H20, (3.2) 153 SAMPLE 81-344 STAUROLITE ZONE S2 st - staurolite g - garnet q - quartz r - rutile - - ilmenite - biotite - muscovite 5 mm Figure 4 4 . Sketch of photomicrograph (sample 8 1 - 3 4 4 ) i l l u s t r a t i n g how r u t i l e i s stable as inclusions in st a u r o l i t e , but unstable r e l a t i v e to ilmenite where in contact with quartz, as in p e l i t e matrix. Note that garnet, which predates s t a u r o l i t e , contains only ilmenite inclusions, and no r u t i l e . This sample is from the st a u r o l i t e zone near the kyanite-in isograd, and may be evidence for the reaction ST + RU + Q = KY + IL + H 20 as shown in Figure 4 3 . 1 54 which is stable at temperatures between about 550° and 650°C, and below pressures for the "GRAIL" reaction (3.1) (see Figure 43). A representative value of 0.78 was chosen (using microprobe data) for XFe in garnet in order to estimate displacement of the "GRAIL" curve to lower pressure (about 7 kb) than that for the same reaction involving pure phases (about 10 kb). Non-garnet phases are assumed to be pure (a = 1). Displacement i s estimated using curves from Bohlen et a l . (1983). Because of assumptions and generalizations made here, and because accurate P/T conditions for those reactions involving s t a u r o l i t e are incompletely known, the "GRAIL" curve and associated invariant point cannot be exactly located in P/T space. P/T conditions in the Three Ladies Mountain area can be i n d i r e c t l y estimated from the "GRAIL" geobarometer and associated reactions as below about 7 kb and between about 550° and 650°C. These conditions are reasonably consistent with those estimated from other reactions (see Figure 25) for staurolite-kyanite zone rocks near Three Ladies Mountain, considering the uncertainty associated with s t a u r o l i t e . 1 55 4.7 GEOBAROMETRY: GROSSULAR-ANORTHITE-ALUMINOSILICATE-QUARTZ Estimates of pressure can be made using a geobarometer suggested by Ghent (1976) based on the reaction 3 AN = GR + 2 KY/SI + QZ (4.1) Other authors including Newton and Haselton (1981) and Lang and Rice (1985) have refined the method. Newton and Haselton (1981) present a c t i v i t y models for grossular and plagioclase, methods of c a l c u l a t i o n , and discussion of the thermodynamics of the geobarometer. Following Newton and Haselton (1981), equation (4.1) is rewritten with grossular as C a A l 2 / 3 S i O a , such that 3 AN = 3 GR + 2 KY + QZ 3 AN = 3 GR + 2 SI + QZ Their formulation i s given as follows: &G° + RT l n ( a /a f l f l) 3 + PAV, ^ 0 (4.2) (Newton and Haselton 1981, p. 132). (4.1a) (4.1b) 156 AG° (at 1 bar) can be approximated by -P°AV°, where P° is the pressure of the end-member equilibrium at T and AV° is the end-member volume change. The end-member reaction can be represented in P/T space by the line P = -7111.9 + 22.77 T (for kyanite) (4.3) (Lang and Rice 1985). To modify equation (4.3) for s i l l i m a n i t e , the following method was used. A similar linear approximation for the reaction SI = KY was obtained (from Rice, pers. comm., modified from Ghent 1976): P = P = -6367.2 + 20.23 T°C or -11893 + 20.23 T (K) (4.4) 1 57 Pressure terms from reactions (4.3) and (4.4) are multiplied by AV° to give energy terms, AG° . AV of reaction is -6.57 j/bar for (4.1a), -5.40 J/bar for (4.1b), and -0.581 J/bar for SI = KY. P is in bars, and AG0 is in joules. Subtracting 2*AG° for SI = KY from AG° (4.1a) gives AG° (4.1b). 3 AN = 3 GR + 2 KY + QZ (4.1a) - (2 SI = 2 KY) 3 AN = GR + 2 SI + QZ (4.1b) AG° for reaction (4.1b) is then divided by AV for reaction (4.1b) to give the expression shown in (4.5). Thus, for s i l l i m a n i t e , P = -6093.6 + 23.35 T (for s i l l i m a n i t e ) (4.5) P i s in bars; T in Kelvins. AV i s the p a r t i a l molar volume change at 1 bar. P a r t i a l molar volume of garnet depending on composition can be read d i r e c t l y from the graph given in Newton and Haselton (1981, p. 138). D i f f e r e n t i a l compressibilities are neglected. Volume of anorthite was used for V of anorthite component in plagioclase. 1 58 A c t i v i t y models used for grossular in garnet and anorthite in plagioclase are those given by Lang and Rice (1985), which are modified from Newton and Haselton (1981). agr =  XCa'1gr (where Gr = CaAl 2 / 3 S i O a ) agr = x g ^ e x p ( W c ^ [ ( X ^ ) 2 + X % ' ^ ] } ( 4 . 6 )  g r L Q RT where ^ C a M g = 13807 - 6.3 T (J,K) aan = — * e x P [ ( 1 0 3 2 + 4 7 2 6 X a « ) ] ( 4 ' 7 ) Putting t h i s a l l together and i s o l a t i n g P on one side of the equation, pressure may be calculated using -the following formula: 159 Seven of the eight microprobe samples from the Three Ladies Mountain area contain the required assemblage garnet-plagioclase-aluminosilicate-quartz. Plagioclase rim composition ranges from An 2 2 to An 3 8, and is consistent within each sample, but does not show any consistent pattern of d i s t r i b u t i o n across the area; but systematic changes with P and T would only be expected in maximum-phase assemblages. The aluminosi1icate is kyanite only in two samples (81-279 and 81-325), and s i l l i m a n i t e only in two samples (80-19 and 80- 31). The remaining three samples (80-33, 80-119, and 81- 278) contain both kyanite and s i l l i m a n i t e , as they are taken from the s t a u r o l i t e - k y a n i t e - s i l l i m a n i t e zone, representing the KY = SI isograd. Pressure estimates for these samples have been calculated using the geobarometer outlined above and the temperatures at 6 kb given by garnet-biotite geothermometry using the method of Newton and Haselton (1981) (see section on garnet-biotite geothermometry, Table I I ) . Pressure estimates using the grossular-anorthite geobarometer range from 4.6 kb to 6.6 kb (Table I I I ) , but do not vary systematically across metamorphic isograds in the area. No matter how the group of samples i s subdivided on reasonable geologic grounds, the pressure estimates cannot be narrowed down any further or forced to show any s i g n i f i c a n t gradient. The two closest samples in the f i e l d , 81-278 and 81-279, found within a few hundred meters of one another, give a pressure estimate difference of nearly 2 kb between them. TABLE III. GROSSULAR-ANORTHITE-ALUMINOSILICATE GEOBAROMETRY Sample 81-325 80-19 81-279 81-278 80-33 80-119 80-31 MET. ZONE KY ST-SI KY-ST ST-KY-SI ST-KY-SI ST-KY-SI SI XAn 0.374 0.381 0.228 0.307 0.263 0.277 0.217 XFeGt 0.758 0.784 0.806 0.770 0.801 0.811 0.806 XMgGt 0.108 0.089 0.087 0.089 0.085 0.090 0.094 XCaGt 0.085 0.076 0.075 0.079 O.061 0.063 0.058 XMnGt 0.049 0.051 0.032 0.064 0.031 0.036 0.041 V.gr ' 12.67 12.61 12.60 12.63 12.51 12.52 12.50 AV KY -6.43 - -6.50 -6.47 -6.59 -6.58 AV SI - -5.32 - -5.30 -5.42 -5.41 -5.43 AV r KY -6.57 - -6.57 -6.57 -6.57 -6.57 AV r SI - -5.40 - -5.40 -5.40 -5.40 -5.40 T C O ! at 6kb 545 522 541 491 520 546 536 P (bar) KY 5559 6610 4695 4906 5391 SI 4687 4946 507 1 5552 6144 'V in J/bar. 'T2, method of Newton and Haselton (1981). 161 Using only one sample per metamorphic zone may not be s u f f i c i e n t to produce s i g n i f i c a n t results, and the sim p l i f i e d a c t i v i t y models may make the results sensitive to mineral composition. In conclusion, garnet-plagioclase pressures for a l l samples considered average 5.5 ± 0.7 kb. Temperatures calculated for these rocks using the method of Newton and Haselton (1981) for P = 6 kb average around 530°C. These P/T conditions are approximately consistent with the KY = SI isograd, but not with the P/T conditions calculated from s t a u r o l i t e breakdown reactions. 1 62 4.8 CONCLUSION P e l i t i c rocks of late Proterozoic Snowshoe Group were metamorphosed to greenschist to amphibolite facies conditions during the mid-Jurassic Columbian Orogeny, and can be divided into zones corresponding to c l a s s i c d i v i s i o n s of the Barrovian series. The d i s t r i b u t i o n of metamorphic zones as defined by mineral assemblages and textures in amphibolite facies p e l i t i c schist in the Three Ladies Mountain/Mount Stevenson area implies an increase in metamorphic grade over a distance of 10 km from garnet zone in the northwest to s i l l i m a n i t e zone in the southeast. Textural zoning and s t a t i c overgrowths of porphyroblasts are due to metamorphic c r y s t a l l i z a t i o n outlasting deformation (high temperatures outlasting active strain) and to discontinuous reactions involving s o l i d - s o l u t i o n minerals as expected during conditions of increasing T and P. There is no unequivocal evidence to support more than one episode of prograde metamorphism involving r e c r y s t a l l i z a t i o n of garnet, s t a u r o l i t e , or kyanite. Discontinuous reactions involving garnet and s t a u r o l i t e breakdown interpreted from microscopic textures, along with garnet zoning p r o f i l e trends, suggest a metamorphic temperature increase from west to east, with a pressure increase from north to south. The intersection of the s t a u r o l i t e breakdown reaction with KY = SI defining the bathograd between bathozones 4 and 5 (Carmichael 1978) should correspond to the intersection of the staurolite-out isograd with the s i l l i m a n i t e - i n isograd 1 63 in the f i e l d , thus establishing a pressure-temperature point somewhere near Long Creek south of Welcome Mountain (see Plate I I I ) . Unfortunately, estimates of t h i s point of intersection in P/T space vary from Carmichael (1978), who places i t empirically at about 4.75 kb and 540°C; to Archibald et a l . (1983), who place i t at about 5.5 kb and 580°C; to calculations in this study which only approximate the appropriate reaction and place the intersection at about 7-7.3 kb and 670-690°C. The l a t t e r is c l e a r l y too high to be reasonable. Maximum pressure estimated using the GRAIL geobarometer (Bohlen et a l . 1983) i s about 7 kb; compatibility of kyanite, ilmenite, s t a u r o l i t e , and quartz also suggests temperatures between 550° and 650°C. These are higher than garnet-biotite temperatures but lower than temperatures estimated from other s t a u r o l i t e breakdown reactions. Estimates of metamorphic conditions based on reactions involving s t a u r o l i t e tend to give higher temperatures than other methods. Further experimental and thermodynamic work on s t a u r o l i t e i s required for resolution of t h i s problem. Geothermometry and geobarometry results show no s i g n i f i c a n t temperature or pressure gradient across the map area corresponding to the variation in metamorphic grade as indicated by mapped isograds. Unfortunately, no samples from the garnet zone were suitable for garnet-biotite geothermometry. Garnet-biotite geothermometry using the correction-method of Newton and Haselton (1981) gives 1 64 average peak metamorphic temperatures of around 525 ± 18°C at 5 kb and 528 ± 18°C at 6 kb for the whole area, including the kyanite-sillimanite t r a n s i t i o n zone. Using these temperatures, pressures calculated according to the grossular-anorthite-aluminosi1icate-quartz geobarometer average 5.5 ± 0.7 kb with no systematic variation in d i s t r i b u t i o n across the study area. These temperatures and pressures are reasonable for the staurolite-kyanite zone rocks which cover most of the Three Ladies Mountain/Mount Stevenson area. However, the lack of P or T gradients demonstrated by these geothermometric and geobarometric methods does not r e f l e c t the gradients in conditions which are represented by progressive changes in mineral assemblages, even though these changes correspond well with the t y p i c a l Barrovian series of metamorphic zones in p e l i t e s . The resolution of these methods as used here seems to be no better than 50°C for garnet-biotite temperatures and about 1-2 kb for garnet-plagioclase pressures. Temperature and pressure gradients required for metamorphic changes from s t a u r o l i t e zone to f i r s t s i l l i m a n i t e zone may be smaller than these values. Although the mapped changes in metamorphic assemblages are r e a l , they may not indicate large changes in P and T. For instance, presence of s t a u r o l i t e may be more strongly controlled by bulk composition of the host p e l i t e than by measurable gradients in temperature and pressure within the large s t a b i l i t y f i e l d 1 65 for s t a u r o l i t e , although i f the isograd reactions were tr u l y univariant, this effect would be eliminated. The large area covered by a comprehensive staurolite-kyanite zone (8 km wide between garnet zone and s i l l i m a n i t e zone, and at least 25 km long in a northeast direction) indicates that the dip of s i g n i f i c a n t metamorphic isograds such as KY = SI must be r e l a t i v e l y shallow to the northwest, crosscutting the s l i g h t l y more steep and northerly regional dip of the metamorphosed strata (see Figure 45). There i s no unusually steep metamorphic gradient in t h i s area (nor any inverted isograds), although there i s s i g n i f i c a n t post-metamorphic displacement along the L i t t l e River Fault. Given peak metamorphic conditions of about 5-6 kb and 500-600°C, metamorphism occurred at a depth of about 15-20 km with a geothermal gradient of about 30°C/km. This is a normal geothermal gradient so no special temperature and pressure conditions need to be invoked to explain the regional metamorphism. However, metamorphism could not have been simply by b u r i a l . If the apparent bathograd shown in Figure 45 i s rotated to horizontal, the isotherms appear to outline a thermal dome under Three Ladies Mountain. Structural conditions necessary for regional metamorphism are (1) subsidence to 20 km with the required thickness of overlying rock, (2) accompanying compressive deformation, and (3) subsequent u p l i f t to present position. 166 Figure 45. Schematic cross sections of Three Ladies Mountain (3LM) area showing relationship between metamorphic isograds (symbols as on Plate I I I ) , f o l i a t i o n (dashed l i n e s ) , and L i t t l e River Fault (LRF). 1 67 If the Snowshoe Group rocks, which are presently at least 6 km thick, are late Proterozoic Windermere-equivalent strata, then they were deposited in one of the thickest sections of the Cordilleran miogeosyncline, which formed during nearly continuous subsidence of the North American continental margin from about 1500 Ma to about 200 Ma. The entire Paleozoic miogeosynclinal section could have been deposited above the Snowshoe Group prior to metamorphism, amounting in the thicker sections to well over 10 km of overlying sediment. Additional thickness could have been caused by pile-up of accreted terranes during c o l l i s i o n with North America in the mid-Jurassic (Monger et a l . 1982). This event was also responsible for the intense deformation which accompanied regional metamorphism. U p l i f t and thrusting during the Cretaceous removed much of the overlying rock either t e c t o n i c a l l y or by erosion (see Figure 46) . Plagioclase-quartz-muscovite pegmatite formed a n a t e c t i c a l l y in deep-seated metamorphic rocks during regional metamorphism and intruded kyanite and s i l l i m a n i t e zone rocks in the Three Ladies Mountain/Mount Stevenson area. Renewed heating in the Cretaceous reset the muscovite age in the pegmatite, giving a cooling date of 86 Ma (see section on geochronology). Third phase folding, which affected the pegmatite, was accompanied in p e l i t e s by muscovite r e c r y s t a l l i z a t i o n . This "thermal event" is not considered to be a separate episode of prograde regional i time, Ma Figure 46. Summary diagram showing v a r i a t i o n of temperature, pressure (or depth), and " i n t e n s i t y of deformation" through time, p l o t t e d on logarithmic scale. 169 metamorphism in this area, but rather an intermediate thermal high during a general u p l i f t and cooling trend which was characterized metamorphically p r i m a r i l y b y retrograde c h l o r i t e . 5. IGNEOUS ACTIVITY Igneous a c t i v i t y occurred at several intervals in the history of the Snowshoe Group in the Three Ladies Mountain/Mount Stevenson area. See the section on geochronology for a discussion of timing of igneous episodes. 5.1 QUARTZ DIORITIC GNEISS Quartz d i o r i t i c s i l l s , containing b i o t i t e , hornblende, and epidote, are common in the lower micaceous quartzite and p e l i t e of the Snowshoe Group. They constitute about a 20% of the lower sequence, thus accounting for some of the great thickness of this unit. The s i l l s vary in width; one that has been mapped for several kilometers averages.about 50 m thick. Amphibolite lenses tend to be concentrated at or near the contacts with the adjacent quartzite and p e l i t e . They are small, irregular, and contain minor su l f i d e s , and are d i f f e r e n t from the amphibolites near Three Ladies Mountain, in that they are not associated with carbonates. The quartz d i o r i t i c gneisses display well-developed f o l i a t i o n subparallel to that in the adjacent metasedimentary rocks; they appear to be folded by east-verging phase F1B folds, so their f o l i a t i o n may have been formed p a r a l l e l to a x i a l planes of F1A i s o c l i n e s . In a few places the gneisses crosscut limbs of e a r l i e r , nearly i s o c l i n a l , i n t r a f o l i a l folds in the metasedimentary rocks. The f o l i a t i o n in the gneiss is p a r a l l e l to the ax i a l plane 170 171 of the i n t r a f o l i a l folds and also to the general f o l i a t i o n in the micaceous quartzite. It could be that t h i s kind of feature i s caused by later shearing along the a x i a l plane f o l i a t i o n of folds involving both quartz d i o r i t i c gneisses and the rocks they intrude, and that i t represents small-scale tectonic slides rather than a truly crosscutting relationship. The d i s t i n c t i o n between fold phases F1A and F1B i s d i f f i c u l t and i t may in some cases be meaningless. Mineral lineations in the quartz d i o r i t i c gneiss are generally p a r a l l e l to those in the surrounding rocks; some trending WSW and W are p a r a l l e l to f o l d axes of phase F1B, but most trend within the spread of values for F2 axes. The quartz d i o r i t i c layers s u p e r f i c i a l l y resemble metamorphosed greywackes, and in places are d i f f i c u l t to d i s t i n g u i s h from intercalated micaceous quartzites. However, crosscutting relationships and r e l a t i v e l y homogeneous composition that i s more mafic than that of the common psammite in the area, as well as idiomorphic hornblende and igneous accessory minerals such as zircon and a l l a n i t e , are convincing evidence for an igneous o r i g i n . Because these s i l l s are involved in pre-F2 folding and appear to be r e s t r i c t e d to the lower sequence of the Snowshoe Group, they are included in the l i t h o l o g i c column (see Figure 2 ) . Isotopie dates on igneous zircons from the quartz d i o r i t i c gneiss give a possible minimum age for deposition of the Snowshoe Group of approximately 335 Ma (see section on geochronology). 1 72 5.2 GRANODIORITlC GNEISS Granodioritic and quartz monzonitic s i l l s , more leucocratic than the quartz d i o r i t i c group, are also found in the Snowshoe Group. The largest bodies occur on the slope toward Quesnel Lake, as indicated by abundance of angular g r a n i t i c float boulders, but cannot be mapped in d e t a i l due to heavy vegetation. These gr a n i t i c gneisses were mapped by Campbell (1963, 1978) as "Kg", or Cretaceous granite. Between Three Ladies Mountain and Welcome Mountain, granodioritic gneiss layers are p a r a l l e l to compositional layering and general f o l i a t i o n , and folded by F2 folds. F o l i a t i o n and mineral lineations in the granodioritic s i l l s are p a r a l l e l to F2 trends. Apophyses of these bodies crosscut the schist as dikes connecting two s i l l s on the ridge north of Welcome Mountain. Although the granodioritic gneiss seems to be related to nearby dikes and irregular bodies of pegmatite, s t r u c t u r a l relations demonstrate that the granodioritic gneiss must be an e a r l i e r igneous phase. Whereas the granodioritic gneiss s i l l s must have been intruded prior to the end of F2 deformation, the pegmatite dikes are mainly post-kinematic. Intruded sediments are not baked or contact-metamorphosed. Locally, the granodioritic gneiss contains large grains of magnetite up to 1 cm across. 1 73 5.3 PEGMATITE Large areas of pegmatite are exposed on the west side of the North Arm of Quesnel Lake, southeast of Mount Stevenson, and near Welcome Mountain. The larger bodies, located mainly in the lower sequence of the Snowshoe Group, are irregular masses of coarse-grained, leucocratic pegmatite consisting of quartz, plagioclase, and muscovite with minor garnet, but no K-feldspar. Grain size varies from 0.5 to 5 cm. Locally, mesoscopic graphic texture may be observed. The pegmatites are compositionally homogeneous, with no unusual minerals, not even tourmaline. These are regarded as forming from melting of metamorphic rocks at depth. Higher in the section where the pegmatite bodies are smaller they tend to occur in the form.of dikes p a r a l l e l to the F2 a x i a l plane. This i s shown by the close correspondence on a stereographic projection of the poles of F2 a x i a l planes and the poles of pegmatite dike contacts (Figure 18 B). Some pegmatites have intruded along other surfaces as well. The largest pegmatite dike in the study area is about 150 m thick and extends from the northwest flank of Mount Stevenson for at least 10 km to the ESE where i t joins a larger pegmatite body and i t s contacts become obscured in the forest. The larger bodies of pegmatite contain up to 30 % xenoliths of schist and quartzite, most of which retain s t r u c t u r a l attitudes consistent with those of the surrounding metasedimentary rocks. There i s some rotation of s c h i s t o s i t y especially near the contacts with the larger 1 74 dikes. Crude f o l i a t i o n in the larger bodies and folds in the pegmatite dikes are believed to be related to F3 deformat ion. Pegmatites l i k e these may have formed from p a r t i a l melting of paragneisses during metamorphism at upper s i l l i m a n i t e zone conditions. It is also possible that they are late-stage d i f f e r e n t i a t e s of the same magma that produced the granodioritic gneisses, but more K-spar would be expected. The pegmatites contain l i t t l e or no K-feldspar. A Rb-Sr date of 86 Ma on a muscovite separate has been obtained and i s thought to represent cooling through 400° to 500°C. This i s interpreted as indicating slow cooling from mid-Jurassic metamorphism or reheating during the late Cretaceous. 1 75 5.4 QUARTZ VEINS Quartz veins averaging about one meter thick are present but not common. Those observed are roughly planar and crosscut e a r l i e r structures. Whereas some occur along joint surfaces probably related to the L i t t l e River Fault, others are p a r a l l e l or at a low angle to compositional or metamorphic layering. Some quartz veins p a r a l l e l to f o l i a t i o n are the resul t of early segregation of quartz in the rocks during formation of f o l i a t i o n ; these are common features in metamorphic rocks of this grade. Many quartz veins in the Mount Stevenson area are nearly v e r t i c a l and northeast-trending, p a r a l l e l to F4 ax i a l planes. This is an example of how quartz veins in areas of simple f o l d systems with steeply dipping a x i a l surfaces tend to occur p a r a l l e l to the axi a l planes. Rusty a l t e r a t i o n zones along their borders, some associated with l o c a l f a u l t s , contain sparse molybdenite, pyrite, and chalcopyrite. 1 7 6 5.5 ECONOMIC GEOLOGY The Snowshoe Group i n the Three L a d i e s Mountain/Mount Stevenson a r e a i s m o s t l y b a r r e n of economic m i n e r a l d e p o s i t s . Teck C o r p o r a t i o n examined a s m a l l m o l y b d e n i t e p r o s p e c t at Mount Stevenson, but has abandoned i t . Many s u l f i d e - b e a r i n g a m p h i b o l i t e s have been s t a k e d i n s m a l l a r e a s , but no work appears t o have been done on these p r o s p e c t s . S e v e r a l samples of p y r i t i c q u a r t z i t e and s i m i l a r r o c k s c o l l e c t e d d u r i n g mapping near Three L a d i e s Mountain were assayed f o r g o l d a t the U n i v e r s t i y of B r i t i s h C olumbia; g o l d v a l u e s were below l i m i t s of d e t e c t i o n ( A . J . S i n c l a i r , p e r s . comm.). Where the Snowshoe Group i s i n g r e e n s c h i s t f a c i e s , as i n the W e l l s and B a r k e r v i l l e a r e a t o the n o r t h w e s t , q u a r t z v e i n s t r e n d i n g n o r t h and n o r t h e a s t have y i e l d e d r i c h g o l d mines i n the p a s t . Other m i n e r a l showings are minor and can be found on maps showing m i n i n g d i s t r i c t s . 177 5.6 VOLCANIC ROCKS Olivine basalt flows of limited extent were discovered on the floor of a cirque at 1800 m elevation, 3 km east of Mt. Stevenson. They are small flows no more than 3 m thick, nearly horizontal, and lying in two adjacent cirques along with g l a c i a l deposits. The basalt exhibits crude columnar joi n t i n g , is vesicular and d i k t y t a x i t i c , and is uniform grey in colour, with pale green o l i v i n e phenocrysts (2 mm) in an aphanitic groundmass. At contacts with g l a c i a l clay, pillow structures with cooled, glassy margins have developed, with baked clay embedded in the cracks. Basalt boulders are g l a c i a l l y s t r i a t e d . The evidence suggests that the lava was erupted into wet g l a c i a l clay, or into a tarn, or beneath an alpine glacier occupying the cirque at the time of eruption. The cirques where the lava flows were found are east-facing and form part of the headwaters of Devoe Creek, which l i e s in a deep U-shaped va l l e y . More o l i v i n e basalt flows and basaltic v o l c a n i c l a s t i c rocks located at the mouth of Devoe Creek l i e in a.horizontal p i l e about 250 m thick above lake l e v e l , r e s t r i c t e d to a small area. The top of t h i s section of volcanic rocks i s l e v e l with g l a c i a l terraces in outwash next to i t and elsewhere along the lake shore. These lavas probably had a l o c a l source vent near lower Devoe Creek, and pooled up against a large glacier occupying the North Arm of Quesnel Lake at that time. The previously unmapped lavas higher up in the cirques probably had their own source vents. Similar lavas are located on the west side of the junction of the North Arm and main arm of Quesnel Lake, and along Grain Creek. South of Grain Creek near Tasse Lake, a small conical h i l l with a central depression looks l i k e a small volcano covered with trees. It has been mapped as a Recent or I n t e r g l a c i a l cinder cone (Campbell 1978). These o l i v i n e basalts are probably related to the Anahim volcanic belt that extends a l l the way across B.C. into Wells Gray Park (Hickson and Souther 1984, Fiesinger and Nicholls 1977). 6. GEOCHRONOLOGY 6.1 GEOCHRONOLOGY ABSTRACT In the Three Ladies Mountain area, Quesnel Highland, B r i t i s h Columbia, the Snowshoe Group consists of s i l i c e o u s p e l i t e , micaceous quartzite, impure carbonate, and minor amphibolitic rocks, which have been metamorphosed to upper amphibolite facies and multiply deformed. A Rb-Sr model depositional age of approximately 750 Ma, assuming an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.708 for the Snowshoe Group c l a s t i c metasedimentary rocks, supports correlations with late Proterozoic strata. Early to mid-Paleozoic plutonism is indicated by a Rb-Sr whole-rock isochron date of 530 ± 94 Ma with i n i t i a l 8 7 S r / 8 6 S r ratio of 0.706 for quartz d i o r i t i c gneiss intrusive into the Snowshoe Group early in i t s deformational history (pre- or synkinematic to F1 foldi n g ) , and by U-Pb dates on zircon from the same quartz d i o r i t i c gneiss, which indicate a minimum age of 335 Ma and a maximum age of about 450 Ma. No Rb-Sr isochron could be determined for younger, syndeformational (pre-F2 folding), S r - r i c h granodioritic gneiss. Late to post-metamorphic (post-F2 folding) pegmatite cooled through 400-500°C at 86 ± 3 Ma, consistent with Jurassic metamorphism inferred from regional geology. 1 79 180 6.2 INTRODUCTION This isotopie dating study i s part of a PhD thesis on the structural and metamorphic history of the Snowshoe Group in the Three Ladies Mountain/Mount Stevenson area. The Snowshoe Group (Campbell et a l . 1973) comprises quartz-rich p e l i t i c metasedimentary rocks, impure marbles, minor amphibolites, and associated orthogneisses of inferred late Proterozoic to Paleozoic age. It occurs in a fault-bounded western part of the Omineca Belt. To the west is the eugeosynclinal Intermontane Belt (Quesnellia) and to the east are North American sediments such as the Kaza Group. The study area i s included in the B a r k e r v i l l e terrane (Struik 1984). Monger and Berg (1984) suggest equivalence of the Bark e r v i l l e and Kootenay terranes. Polyphase deformation in the B a r k e r v i l l e terrane was accompanied by regional metamorphism of the Barrovian type. Rocks of the Snowshoe Group range in metamorphic grade from lower greenschist facies northwest of Wells, B.C., to s i l l i m a n i t e zone of amphibolite facies in Quesnel Highland and Wells Gray Park, where they are a northwest part of the Shuswap metamorphic complex. The stratigraphic position of the Snowshoe Group has long been unclear; no f o s s i l s have been reported. The Snowshoe Group was believed by early workers to overlie the Cariboo Group, and i s shown as "Lower Cambrian or l a t e r " on maps by R.B. Campbell (1961, 1963). Later work by Campbell et a l . (1973) suggested that the Snowshoe Group was "a i 181 western facies of the Kaza Group" (p. 32), and thus of late Proterozoic Windermere age. Getsinger (1982) suggested that the Snowshoe Group resembles parts of the late Proterozoic Horsethief Creek Group, rather than the Kaza Group. Struik proposed that one of the uppermost units of the Snowshoe Group (northwest of Wells, B.C.), i s late Paleozoic in age, based on an uncertain l i t h o l o g i c correlation with a nearby unit consisting of marble, s l a t e , and c r i n o i d a l limestone which yielded Mississippian conodonts (Struik 1982). Further uncertainty would be involved in making a l i t h o l o g i c c o r r e l a t i o n across 100 km (even along strike) of unfossi1iferous rocks which themselves cannot be traced with complete confidence d i r e c t l y into the Three Ladies Mountain—Mount Stevenson area: Previous dating in the Quesnel Highland has been limited to studies of the Quesnel Lake Gneiss, a heterogeneous suite of g r a n i t i c intrusions and augen gneiss associated with the Snowshoe Group near Quesnel Lake. It is exposed about 5 km south of Grain Creek, and was not included in the mapped study area. Blenkinsop (1972) reported a whole-rock Rb-Sr date of 752 ±76 Ma on Quesnel Lake Gneiss c o l l e c t e d by C.J.N. Fletcher from the East Arm of Quesnel Lake. K-Ar dates on the East Arm gneiss, c o l l e c t e d by Okulitch, are 153 ± 8 Ma on hornblende (metamorphic age) and 108 ± 4 Ma on b i o t i t e (reset by Cretaceous plutonism) (Stevens et a l . 1982). Okulitch et a l . (in preparation) report a U-Pb date of 350 Ma on zircons from the East Arm gneiss, 182 with inherited Precambrian lead. Okulitch (written communication, 1981) also suggested a preliminary U-Pb date of about 350 Ma on zircons from Quesnel Lake Gneiss which I collected in 1981 at the junction of the North Arm of Quesnel Lake (see Appendix 11 IB, samples JSG-80-QLGNW and JSG-81-166). K-Ar and Rb-Sr dates on post-metamorphic plutons in Wells Gray Park are 143 ± 14 Ma and 163 ± 7 Ma, respectively (Wanless et a l . 1965; Pigage 1977). 183 6.3 GEOLOGIC SETTING OF DATED SAMPLES In the study area, west of the North Arm of Quesnel Lake, the Snowshoe Group is primarily in staurolite-kyanite zone and s i l l i m a n i t e zone. The c l a s t i c metasedimentary rocks include grey micaceous quartzites and quartz-rich, garnet-bearing two-mica schists with s t a u r o l i t e , kyanite, and/or s i l l i m a n i t e . Impure carbonates have r e c r y s t a l l i z e d as c a l c - s i l i c a t e marbles and minor amphibolite. These rocks experienced at least two phases of penetrative deformation (F1 and F2) during prograde metamorphism, with nearly coaxial folds plunging northwest. Metamorphic temperatures reached a maximum after F2 folding. Retrograde metamorphism accompanied b r i t t l e deformation (F3) characterized by kink folds and crenulations with variable a x i a l planar attitudes and northwest or southeast plunging axes. Post-metamorphic, low-angle normal faulting ( L i t t l e River Fault) was followed by gentle warping (F4), with fold axes plunging shallowly to the northeast. Mapping in the Three Ladies Mountain—Mount Stevenson area indicates several episodes of igneous intrusion during the s t r u c t u r a l and metamorphic history of the Snowshoe Group. Four groups of rock types have been selected for dating from samples c o l l e c t e d during 1979-1982 f i e l d seasons. Relative ages, from oldest to youngest, are: metasediments, quartz d i o r i t i c gneiss, granodioritic gneiss, and pegmatite. This order i s based on the following f i e l d observations. Quartz d i o r i t i c gneiss sheets intrusive into 184 layered paragneisses are involved in east-verging F1 folds. In a few places, the orthogneiss appears to crosscut an e a r l i e r f o l i a t i o n in the paragneiss, implying intrusion syndeformational to F1 folding. Younger, granodioritic dikes and s i l l s are folded only by F2 structures, and are f o l i a t e d and lineated along F2 trends only. The granod i o r i t i c gneisses and F2 structures are crosscut by pegmatite dikes, many of which have been intruded p a r a l l e l to a x i a l planes of F2 folds. Rocks metamorphosed during and after F2 folding contain pegmatitic segregations in some places, and are c l e a r l y crosscut by pegmatite dikes elsewhere, implying a late to post-metamorphic r e l a t i v e age for the pegmatitic intrusions. 185 6.4 RESULTS AND INTERPRETATION Analy t i c a l methods in use at UBC are summarized in Appendix IIIA. Sample locations are shown in Figure 47, and rock descriptions in Appendix IIIB. Analytical results are presented in Table IV (Rb-Sr) and Table V (U-Pb), Rb-Sr isochrons rn Figure 48, and concordia plot in Figure 49. The calculated regression for the Snowshoe Group metasediment isochron (Fig. 2) gives a date of 418 ± 54 Ma and an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.726 (method of York 1967). The scatter of points exceeds that of experimental error alone, and therefore the interpretation of the date i s heavily dependent on the model used to calculate i t (Cameron et a l . 1981). Rb-Sr whole rock analyses of c l a s t i c metasedimentary rocks give isochrons that may range from age of p r o t o l i t h to age of metamorphism (Spanglet et a l . 1978; Chaudhuri 1976). In general, isochrons from unmetamorphosed p e l i t e s approximate time of deposition or diagenesis, while those from psammites approach the age of provenance. Metamorphism causes rotation of i n i t i a l isochrons toward the metamorphic age, in some cases nearly completely, as a result of r e d i s t r i b u t i o n of radiogenic strontium, es p e c i a l l y during deformation. The calculated i n i t i a l 8 7 S r / 8 6 S r r a t i o for such metasedimentary rocks i s usually high, suggesting rotation of the isochron during metamorphism. The resulting date may not necessarily be assumed to represent the age of metamorphism, and may be meaningless (Field and Raheim 1969). However, an estimate of the age of deposition can be 186 T H R E E L A D I E S M O U N T A I N A R E A 121°10'W j (mineca Crystalline Belt ^ — in British Columbia E X P L A N A T I O N p P e g m a t i t e g G r a n o d i o r i t i c g n e i s s q d Q u a r t z d i o r i t i c g n e i s s € c C a r i b o o G r o u p ( ? ) , u n d i v i d e d m M a r b l e s S n o w s h o e F o r m a t i o n , u n d i v i d e d • 245 G e o l o g i c a l c o n t a c t F a u l t S a m p l e l o c a l i t y a n d n u m b e r Figure 47. Sample l o c a l i t y map for dated samples. 187 Table IV. Rubidium-strontium a n a l y t i c a l data Sample Description Sr Rb Rb 87 Sr 87 (ppm) (ppm) Sr 86 Sr 86 Metasedimentary Rocks 80-42 Gar-bio-ms s c h i s t 122 134 3 .19 0.7356 79-15 Gar-bio s c h i s t 85 228 7 .81 0.7728 80-67 Gar-staur s c h i s t 237 133 1. .63 0.7393 80-80 Micaceous q u a r t z i t e 193 90 1 .36 0.7349 80-96 Gar-bio-ms s c h i s t 83 161 5 .68 0.7580 82-375 Ms-bio-gar s c h i s t 114 137 3 .50 0.7471 81-245 Amphibolite 408 74 0 .53 0.7075 80-21 B i o t i t e gneiss 240 115 1 .39 0.7350 80-22 Muscovite s c h i s t 85 282 9 .66 0.7470 81-171 B i o t i t e gneiss 283 118 1 .21 0.7224 81-281 Gar-bio-chl s c h i s t 74 134 5 .28 0.7562 81-328 Kyanite s c h i s t 96 172 5 .25 0.7687 Orthogneiss 81-X1 Quartz d i o r i t i c gneiss 495 52 0 .30 0.7093 81-X2 Quartz d i o r i t i c gneiss 355 138 1 .13 0.7139 81-X3 Quartz d i o r i t i c gneiss 1082 67 0 .18 0.7070 81-X4 Quartz d i o r i t i c gneiss 1037 80 0 .22 0.7066 81-X5 Quartz d i o r i t i c gneiss 508 118 0 .67 0.7108 81-X6 Quartz d i o r i t i c gneiss 612 94 0 .45 0.7099 80-99 Gra n o d i o r i t i c gneiss 1879 60 0 .09 0.7081 80-136A Gra n o d i o r i t i c gneiss 1463 11 0 .02 0.7084 81-205 Grano d i o r i t i c gneiss 862 52 0 .18 0.7051 81-327 Grano d i o r i t i c gneiss 137 183 3 .88 0.7484 Pe; ?matite 80-36-WR Pegmatite - whole rock 752 35 0 .13 0.7150 80-36-MUS Muscovite from pegmatite L15 298 7 .50 0.7239 80-36-PL Plagioclase from pegmatite 7 68 8.0 0 .03 0.7146 Table's?. U-Pb zir c o n data from quartz d i o r i t i c gneiss i n t r u s i v e into Snowshoe Formation Sample Number JSG-81-X (A) JSG-81-X (B) Size Fraction (fxm), handpicked 74-149 44-74 Weight (mg) 9.8 6.3 U (ppm) 357.8 581.2 Pb (ppm) 21.6 34.7 Abundances:^ Pb Pb 208^ Pb 204,,, Pb 100 5.596 16.30 0.0067 100 5.462 17.41 0.0007 206 Observed Pb/ 204^ Pb 3823 12989 Mole % Blank Pb 0.5 0.4 Radiogenic Pb/(Radiogenic + „ 0.996 0.999 Common Pb) Atomic r a t i o s : 2 0 6 P b / 2 3 8 u 2 0 7 P b / 2 3 5 u 2 0 7 p b / 2 0 6 p b 0.05760 0.4366 0.05498 0.05646 0.4244 0.05452 3 Dates (Ma): (+ 2cr) 2 0 6 P b / 2 3 8 u 2 0 7 P b / 2 3 5 u 361.0 + 3 367.9+3 .1 .3 354.1+3 359.2 + 3 .1 .2 207^.206^ Pb/ Pb 411.3 + 3 .2 392.4 + 2 .6 *Corrected for blank Pb. Total blank =0.5 ng. Isotopie composition of common lead i s that of 345 Ma old Pb on the growth curve of Stacey and Kramers (1975). 3 238 -9 -1 235 -9 -1 238 235 Decay constants: U, 0.155125 x 10 a ; U, 0.98485 x 10 a ; U/ U = 137.88. 0.7001 i i i i i i « 1 1 L_ 0 5 10 Figure 48. Rb-Sr isochrons for metasedimentary rocks ( t r i a n g l e s ) , quartz d i o r i t i c gneiss (hexagons), g r a n o d i o r i t i c gneiss ( s o l i d c i r c l e s ) , and pegmatite (squares). 191 made using an an assumed i n i t i a l 8 7 S r / 8 6 S r r a t i o and the centre of gravity of the sample d i s t r i b u t i o n , around which the isochron is presumed to have rotated by metamorphism (Peterman 1966; Spanglet et a l . 1978; Cameron et a l . 1981). A reasonable i n i t i a l 8 7 S r / 8 6 S r r a t i o for sediments deposited in early Paleozoic or late Proterozoic seawater is 0.708 (Faure and Powell 1972, p. 78). Using t h i s r a t i o , and a centre of gravity based on an arithmetic average of the data points, a date of 753 Ma was calculated (a meaningful error cannot be calculated). This late Proterozoic date supports previous correlations of Snowshoe Group with part of the Windermere Supergroup. Six whole-rock samples of quartz d i o r i t i c gneiss from one c o l l e c t i n g were s i t e were analysed for Rb-Sr and combined to produce a zircon concentrate for U-Pb dating. Rb-Sr whole rock isochrons in meta-igneous rocks tend to preserve age of c r y s t a l l i z a t i o n (Faure and Powell 1972). The quartz d i o r i t i c gneiss isochron gives a date of 530 ± 94 Ma with an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.706. This implies a Cambrian or older age for the Snowshoe Group into which the gneiss intrudes, but the uncertainty i s large. Zircons from the quartz d i o r i t i c gneiss are t y p i c a l l y colourless to pale pink, subhedral prisms with rounded edges and terminations. Handpicked zircons consist of clear, uncracked, whole c r y s t a l s of two size f r a c t i o n s : 74-140 um (100-200 mesh) and 44-74 Mm (200-325 mesh). On a 2 0 6 P b / 2 3 8 U vs. 2 0 7 P b / 2 3 5 U concordia graph, these points l i e just below 192 the concordia curve near 360 Ma (Figure 49; Table V). The a n a l y t i c a l errors are small enough that the points are c l e a r l y discordant. Two possible interpretations are suggested: a) If a straight chord is plotted through the two points, i t intersects the concordia curve around 335 Ma. This "reverse discordance" could indicate that the gneiss has a minimum age of Mississippian, and inherited a small amount of older lead from the country rocks. Mesozoic metamorphism causing lead loss could bring the lower intercept below the true value for i n i t i a l c r y s t a l l i z a t i o n of the gneiss. b) These points could be part of a discordant series of points from a gneiss which was older (perhaps Ordovician, around 450 Ma) and lost lead during Mesozoic metamorphism (around 175 Ma). With only two nearly overlapping points, either interpretation i s possible. In either case, an early to mid-Paleozoic minimum age of intrusion is indicated. Early Paleozoic intrusions have only recently been recognized in l o c a l i t i e s scattered through the C o r d i l l e r a , and are probably more common than generally realized. Cambrian-Ordovician plutonism in east-central Idaho was reported by K.V. Evans (1981). Mortensen (1983) defined four plutonic suites intrusive into a sequence of metasedimentary rocks in the Yukon-Tanana terrane that are l i t h o l o g i c a l l y similar to metasedimentary rocks of the 193 Snowshoe Group in Quesnel Highland. Dates on the intrusions are 359 ± 6 Ma, 349 ±5 Ma, and 342 ± 12 Ma, with a suggested maximum age of 400 Ma. The Quesnel Lake Gneiss and similar orthogneisses in the Omineca Belt have been dated by A.V. Okulitch at about 350 Ma (Okulitch et a l . 1975; Wanless and Okulitch 1976; Okulitch, written communication, 1981; Okulitch et a l . , in preparation). It is unclear as yet whether the Quesnel Lake Gneiss belongs to the same intrusive suite as the quartz d i o r i t i c gneiss dated here. Although some of the quartz d i o r i t i c gneiss occurs along Quesnel Lake near the Quesnel Lake Gneiss, no contact i s exposed. These two gneisses are d i s t i n c t l y d i f f e r e n t 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 . The Quesnel Lake Gneiss i s a large body (over 30 km long and 2-4 km wide) of coarse-grained, leucocratic, gneissose granite with potassium feldspar megacrysts. It is generally concordant with layering in the Snowshoe Formation as well as p a r a l l e l to the border of the Intermontane Belt; i t is unclear whether the contact with the Snowshoe Group is intrusive or tectonic (Blenkinsop 1972; Rees 1981). In contrast, the quartz d i o r i t i c gneiss dated here i s more mafic (30%), finer-grained, and lacks potassium feldspar, either as megacrysts or in groundmass. It i s intimately interlayered with and intrusive into metamorphic rocks of the Snowshoe Group and was c l e a r l y folded and metamorphosed along with the metasedimentary rocks. This quartz d i o r i t i c gneiss occurs in s i l l s no wider than a few hundred meters 1 94 within the Snowshoe Group from Mount Stevenson to Quesnel Lake. Although the quartz d i o r i t i c gneiss dated here and the Quesnel Lake Gneiss may be similar in age, i t is suggested that they belong to two d i f f e r e n t intrusive sui tes.. Four whole-rock samples of granodioritic gneiss were analysed for Rb-Sr. Two of them (80-99, 80-136A) are from closely related s i l l s connected by dikes which crosscut F1 structures in the surrounding schist, but which are folded, f o l i a t e d , and lineated by F2 folding and metamorphism. These syndeformational intrusions should approximate the age of F2 deformation and metamorphism. Both samples contain extremely high concentrations of strontium (over 1000 ppm; see Table IV) and have i n i t i a l r a t i o s of about 0.708, regardless of their exact age. In order to establish an age for this granodioritic gneiss, samples with a wider range of Rb/Sr ra t i o or zircons for U-Pb dating are required. Sample 81-327 plots within the Snowshoe paragneiss f i e l d ; this suggests either an old age or synmetamorphic eq u i l i b r a t i o n with the metasedimentary rocks. Sample 81-205 stands isolated with a rather low 8 7 S r / 8 6 S r r a t i o of 0.705. This sample was c o l l e c t e d from an isolated outcrop in an area largely covered with vegetation; because f i e l d relations are obscure, no conclusions can be drawn. One t y p i c a l specimen of pegmatite was separated into mineral f r a c t i o n s . A plagioclase-muscovite-whole rock isochron gives a date of 86.4 ± 3.3 Ma approximating 1 95 post-metamorphic cooling through about 400-500°C (Jager et a l . 1967; Harrison et a l . 1978). The cooling date is consistent with dates from similar post-metamorphic pegmatites elsewhere in the Omineca Belt (Parrish 1979). Time of metamorphism has been dated by other workers as mid-Jurassic (Wanless et a l . 1965; Wheeler 1970; Pigage 1977). Mid-Cretaceous cooling ages imply either a very gradual cooling from a middle Jurassic metamorphism (about 100-200°C in 70 Ma, or about 2°C/million years), or renewed heating during the mid-Cretaceous. 1 96 6.5 CONCLUSIONS The time of deposition of the Snowshoe Group is approximately 750 Ma (late Proterozoic) assuming an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.708. Quartz d i o r i t i c gneiss intruded early in the deformational history of the Snowshoe Group gives a Rb-Sr date of 530 ± 94 Ma with an i n i t i a l 8 7 S r / 8 6 S r ratio of 0.706. U-Pb dating of zircons from the same gneiss suggests Paleozoic intrusion with a minimum age of 335 Ma and a maximum age around 450 Ma. Such gneisses in the Snowshoe Group and other parts of the C o r d i l l e r a indicate widespread early to mid-Paleozoic deformation and metamorphism, now almost completely overprinted by Jurassic orogeny. Younger, syndeformational granodioritic gneiss with high strontium concentrations and i n i t i a l 8 7 S r / 8 6 S r ratio of 0.708 was not successfully dated. Coarse muscovite, from pegmatite intruded after F2 folding and metamorphism, was dated by Rb-Sr as 86 ±3 Ma, consistent with Jurassic metamorphism and subsequent gradual cooling through 400-500°C. 7. SUMMARY The geologic history of the Three Ladies Mountain/Mount Stevenson area is summarized b r i e f l y , starting with the e a r l i e s t events that can reasonably be interpreted from the rocks, continuing up to the present time. LATE PROTEROZOIC TO EARLY PALEOZOIC Deposition of sediments on continental margin of western North America (Kaza, Snowshoe, Cariboo Groups) L i t h i f i c a t i o n of sandstone, mudstone, shale, limestone, marl; bedding Subsidence, heating, diagenesis Beginning of prograde metamorphism, quartz segregations and mica 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 , muscovite) Possible deformation, formation of f o l i a t i o n EARLY TO MID-PALEOZOIC Intrusion of quartz d i o r i t i c s i l l s and amphibolite p r o t o l i t h into lower sequence (possibly a x i a l planar to i s o c l i n e s although also folded) Quartz d i o r i t i c gneisses display WSW and W to NW lineations (L1) Asymmetrical tight to i s o c l i n a l folding (F1B), east-verging; folds f o l i a t i o n and quartz d i o r i t e , WNW to NW plunge (L1); some ax i a l planes SW-dipping, some p a r a l l e l to f o l i a t i o n ; preserved mainly in competent quartzite layers; F2 a x i a l plane cleavage (S2) i s superimposed across many F1 folds in p e l i t i c hinge zones 1 97 198 Dominant f o l i a t i o n (S1B) i s formed p a r a l l e l to a x i a l planes of F1B folds PALEOZOIC TO MESOZOIC Prograde metamorphism continues into amphibolite facies; early garnet growth Granitic s i l l s and dikes intruded l o c a l l y (granodiorite, quartz monzonite); they look l i k e finer-grained precursors of pegmatite intrusions which come la t e r and with which they are associated in the f i e l d ; they show F2 structures (S2, L2) MID-JURASSIC F2 folding: west-verging, asymmetrical, tight to normal, inclined plunging folds, with NW plunge and NE-dipping ax i a l planes (very consistent); a x i a l plane cleavage with complete transposition in p e l i t i c layers i s common especially in large f o l d hinges; refolds F1A and F1B folds; lineations (L2) nearly coaxial with F1 lineations (L1); F2 a x i a l planar f o l i a t i o n (S2) is developed most strongly in p e l i t i c schist in F2 hinge zones Peak of prograde metamorphism; growth of porphyroblasts; synkinematic garnets, postkinematic kyanite and st a u r o l i t e ; s i l l i m a n i t e JURASSIC TO CRETACEOUS Intrusion of pegmatite dikes in F2 a x i a l planes LATE CRETACEOUS U p l i f t and cooling 199 LATE CRETACEOUS TO EARLY TERTIARY F3 kink-folding and crenulation folding, with varying attitudes including NW-trending, SW-dipping a x i a l planes (S3); best developed in p e l i t e ; seen superimposed on S2 and on F1B folds; some development of crude f o l i a t i o n in pegmatite; c h l o r i t e growth on a x i a l planes Major f a u l t i n g , formation of L i t t l e River Fault system; slickensides and drag indicate upper plate moved down toward ESE Fault surfaces vary from F2 A-C joints to 170/20E Metamorphic displacement at Quesnel Lake is s i l l i m a n i t e zone adjacent to c h l o r i t e - b i o t i t e zone Retrograde metamorphism: Late muscovite replaces kinked and folded b i o t i t e ; c h l o r i t i z a t i o n of micas especially on F3 a x i a l planes; c h l o r i t i z a t i o n of b i o t i t e and garnet and s t a u r o l i t e ; l o c a l s e r i c i t i z a t i o n of kyanite and s t a u r o l i t e F4 folding: broad, open warps or flexures with gentle NE plunge; some crenulation in p e l i t e ; quartz veins are ax i a l planar; reverses plunges of F2 folds Late quartz veins injected along j o i n t s , f a u l t s , and F4 a x i a l planes, and p a r a l l e l to f o l i a t i o n Minor mineralization, including molybdenite, pyrite, and chalcopyrite MIOCENE (?) U p l i f t and erosion; formation of widespread erosion surface 200 Minor high-angle f a u l t i n g , N, NE trends, EW trends, displacement on N and NE trends down to west PLEISTOCENE (?) Glaciation; erosion by alpine and valley glaciers Extrusion of o l i v i n e basalt and v o l c a n i c l a s t i c s above Devoe Creek, at Quesnel Lake, and Tasse Lake cinder cone (subglacial and synglacial) Glaciers melt; deposition of t i l l , outwash and moraines RECENT Erosion 8. CONCLUSION Structural and metamorphic history of the Three Ladies Mountain/Mount Stevenson area may be related to larger tectonic events. Refer to Figure 50 for schematic cartoons showing reconstructions of possible tectonic settings through time. Figure 50A depicts the development of the Cordilleran miogeocline up to the early Mesozoic. Snowshoe Group sedimentary rocks were deposited in a continental slope to shelf environment, perhaps on t r a n s i t i o n a l crust outboard of cratonic basement, probably in the late Proterozoic to Paleozoic, some time between about 753 Ma and 418 Ma, according to Rb-Sr dates given in t h i s study (see Figure 47). Late Proterozoic to Cambrian Cariboo Group shallow marine e l a s t i c s and carbonates are shown closer to the shore of North America, but partly time-equivalent and partly overlapping the Snowshoe Group rocks. Sediments overlying the Snowshoe Group may have been as thick as 10-15 km by the early Mesozoic, so some metamorphic r e c r y s t a l l i z a t i o n may have begun during the Paleozoic. Mafic s i l l s (now quartz d i o r i t i c gneisses and minor amphibolitic bodies) were intruded into the Snowshoe Group in the mid-Paleozoic; minor deformation may have accompanied intrusion. These quartz d i o r i t i c gneisses y i e l d a tentative Rb-Sr date of 530 ± 94 Ma and somewhat discordant U-Pb zircon dates of around 335 Ma to 450 Ma (see Figures 47 and 48). A west-dipping subduction zone may have existed off the edge of the continent. Island arc rocks, parts of 201 202 EARLY JURASSIC MID-JURASSIC TERTIARY to QUATERNARY Figure 50. Schematic tectonic history of the Omineca Belt in the v i c i n i t y of the Three Ladies Mountain/Mount Stevenson area. Not to scale. SG = Snowshoe Group, CG = Cariboo Group, SM = Slide Mountain Terrane, QN = Quesnellia, PVT = Pleasant Valley Thrust, LRF = L i t t l e River Fault, 3LM = Three Ladies Mountain, MT = Matthew River Fault. 203 Quesnellia and Slide Mountain terranes, accumulated in arc and ocean environments to the west. By the early Jurassic, plate convergence bringing in island arcs from the west had begun to affe c t the miogeocline, which had been r e l a t i v e l y stable for over a b i l l i o n years (1 Ga). Figure 50B shows continued west-dipping subduction as rocks of the Slide Mountain terrane and Quesnellia were obducted to the east over continental margin sediments of the Bar k e r v i l l e and Cariboo terranes. Folding was mainly easterly-directed; large nappes may have developed in some areas at thi s time. Northeasterly-overturned F1B folds, some of which involve quartz d i o r i t i c s i l l s in the Three Ladies Mountain/Mount Stevenson area, may be related to this event. As Slide Mountain terrane and Quesnellia p i l e d up, the crust was t e c t o n i c a l l y thickened, causing further subsidence of the miogeoclinal rocks to depths of 20-25 km. Thickening of the crust may have been accompanied by a reversal of subduction in the mid-Jurassic, as i t was around this time that island arc and oceanic rocks from the west, and perhaps some of the deeper parts of the miogeocline, began underthrusting the continental margin from southwest to northeast (see Figure 50C). This may have been the cause of the second, and strongest, phase of deformation recorded in the rocks of the Three Ladies Mountain/Mount Stevenson area. A marked asymmetry, indicating shearing in a northeast over southwest d i r e c t i o n , characterizes second phase structures 204 throughout the Omineca Belt. F2 folds in the Three Ladies Mountain/Mount Stevenson area are dominantly southwesterly-verging. Their similar to flattened f l e x u r a l s l i p style and microscopic f a b r i c s indicate formation in warm, d u c t i l e conditions, accompanied by maximum metamorphic r e c r y s t a l l i z a t i o n . Metamorphic assemblages synkinematic to postkinematic to F2 folding indicate conditions of pressure at 5.5 ± 0.7 kb and temperature around 525 ± 20°C, corresponding to a depth of around 18-20 km with a normal geothermal gradient of about 30°/km (see Figure 46). Parts of the Cariboo Group may have been thrust over the Snowshoe Group to the west along the east-dipping Pleasant Valley Thrust, which is a pre- to synmetamorphic reverse f a u l t , according to Struik (1982). Although some telescoping of these units has c l e a r l y occurred during deformation, the Pleasant Valley Thrust may not be a major terrane boundary. The B a r k e r v i l l e terrane i s considered here to be parautochthonous rather than suspect. There i s no evidence in the Three Ladies Mountain/Mount Stevenson area for the existence or necessity of having isolated blocks of continental crust move in from west of the miogeoclinal basin in order to explain c r u s t a l thickening and southwesterly-directed folding, although this may have been the case for the Monashee Complex, which is cored by much older rocks (Read and Brown 1983). Northeastward underthrusting during progressive convergence of oceanic terranes with North America has also been 205 proposed by Murphy (in preparation) to explain westerly-verging F2 structures in the Premier Range, in North American rocks of the Kaza Group. This general view is consistent with structures observed in the Three Ladies Mountain/Mount Stevenson area, but a more s p e c i f i c kinematic model such as that proposed by Murphy (in preparation) is beyond the scope of t h i s study. Tectonically loaded continental crust consisting of deformed and metamorphosed geosynclinal rocks to the west of the North American craton would have been anomalously thick following the mid-Jurassic, resulting in r e l a t i v e l y rapid u p l i f t . P a r t i a l melting of lower c r u s t a l rocks formed pegmatites and other plutonic bodies which intruded deformed rocks higher in the crust. Continued, but less severe, convergence from the west during u p l i f t caused tightening and/or overturning of F2 folds by F3 structures. Third phase deformation occurred in a more b r i t t l e regime in the Three Ladies Mountain/Mount Stevenson area than did F2 folds, as indicated by sparsely-distributed F3 kink-folding and minor r e c r y s t a l l i z a t i o n of retrograde minerals, such as c h l o r i t e , which indicate lower metamorphic temperatures. Postmetamorphic pegmatite was intruded along F2 a x i a l planar surfaces in kyanite and s i l l i m a n i t e zone rocks of the Snowshoe Group pr i o r to F3 deformation. One Rb-Sr mineral separate date was obtained for such a pegmatite, indicating cooling through 400°C at 86±3 Ma. This has been interpreted as representing a late Cretaceous heating event rather than 206 merely extremely slow cooling after mid-Jurassic metamorphism. High heat flow in the Cretaceous and thickened crust contributed to u p l i f t and erosion of deep-seated metamorphic rocks in the Omineca Belt in early Tertiary time. Tectonic denudation, represented by low-angle normal faults such as the L i t t l e River Fault in the Three Ladies Mountain area, contributed to more u p l i f t and erosion (see Figure 50D). The L i t t l e River Fault is approximately contemporaneous with many decollement zones associated with metamorphic core complexes, having moved at some time between the late Cretaceous and mid-Tertiary. Structures in the Snowshoe Group schists and gneisses suggest that i t i s a lowTangle, extensional fault associated with u p l i f t of the high grade rocks forming the core of the Three Ladies Mountain/Mount Stevenson block, and that the low-grade rocks of the hanging wall (Cariboo Group) s l i d down toward the east-southeast, at least near the present trace of the L i t t l e River Fault east of Three Ladies Mountain. Later broad warping (F4 folding) along a northeast trend, and high-angle block-faulting along north, northeast, and northwest trends, occurred with minor disruption of e a r l i e r structures. There i s no evidence in the Three Ladies Mountain/Mount Stevenson area for s t r i k e - s l i p f a u l t i n g . Minor basaltic volcanic rocks are associated with Pleistocene g l a c i a l deposits and geomorphologic features i 207 suggesting approximately synglacial eruption. These o l i v i n e basalts may have a mantle source, but there is no evidence of the structural control of their present d i s t r i b u t i o n . REFERENCES Archibald, D.A., Glover, J.K., Price, R.A., Farrar, E., and Carmichael,D.M. 1983. Geochronology and tectonic implications of magmatism and metamorphism, southern Kootenay Arc and neighbouring regions, southeastern B r i t i s h Columbia. Part I: Jurassic to mid-Cretaceous. Canadian Journal of Earth Sciences, 20, pp. 1891-1913. Blenkinsop, J. 1972. 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APPENDIX I: METAMORPHIC MINERALS ABBREVIATIONS AND MINERAL NAMES AB = ALBITE AM = ALMANDINE AN = ANORTHITE AP = APATITE BI = BIOTITE CA = CALCITE CH = CHLORITE CD CHLORITOID Dl = DIOPSIDE DO = DOLOMITE EP = EPIDOTE GT = GARNET GP GRAPHITE GR = GROSSULAR HE = HEMATITE HB - HORNBLENDE IL = ILMENITE KS = K-FELDSPAR KY = KYANITE LI = LIMONITE MG = MAGNETITE MC = MICROCLINE MS = MUSCOVITE OL = OLIVINE OP = OPAQUES (MG, IL, GP, PY) OR = ORTHOCLASE PL = PLAGIOCLASE PY = PYRITE PR = PYROPE QZ = QUARTZ RU = RUTILE SC SCAPOLITE SI = SILLIMANITE SS = SPESSARTINE SP = SPHENE ST = STAUROLITE TO = TOURMALINE TR = TREMOLITE ZR = ZIRCON ZO = ZOISITE 215 216 DESCRIPTIONS OF MINERALS SECTION I. PELITES Note: Sj- and Sg refer to internal and external S-surfaces r e l a t i v e to porphyroblasts in thin section, respectively. QUARTZ Quartz is ubiquitous in p e l i t e s (5-50%). U(+). Grain size varies even within same rock. Xenoblastic. Boundaries may be granoblastic to granoblastic polygonal; mosaic; sutured; dentate. Combinations common in many rocks. Commonly occurs in quartz segregation layers subparallel to f o l i a t i o n and i n t e r s t i t i a l to other minerals, and as inclusions in other minerals. Undulatory extinction i s common, as well as ribbon quartz, and some mortar texture. Fine-grained quartz defines internal S-surfaces in garnets, p a r t i c u l a r l y in the cores where S^  is straight. PLAGIOCLASE (+) Rel i e f ; cleavage (010) and (100); 2V high (-) to high (+). Occurs in small amounts in most p e l i t e s . Twins: a l b i t e , p e r i c l i n e Zoning may be normal, patchy, reverse Composition mostly An25 to An35, may be as high as An55, low as An22; commonly c a l c i c oligoclase to sodic andesine. An content i s s l i g h t l y higher in s i l l i m a n i t e zone. Deformation twins in some rocks; rusty along cracks and grain boundaries. Usually associated with quartz in matrix of p e l i t e , but in some places plagioclase forms porphyroblasts or glomeroblasts. Rotational t r a i l s of inclusions indicate porphyroblast growth at same time as s t a u r o l i t e (80-69a, 80-135). Commonly altered to s e r i c i t e or other fine-grained minerals (saussuritization does not appear to be common in p e l i t e s ) . Not much newly r e c r y s t a l l i z e d feldspar; much looks overgrown by mica f o l i a t i o n . 217 K-FELDSPAR Occurs in only a few p e l i t e s , <5% in those. (-) Relief; untwinned to hazy twins; possibly p e r t h i t i c in places. P o i k i l o b l a s t i c both inside and outside garnet (82-380). Occurs also in 81-322, 325 (kyanite zone) and 80-19 ( S i l . zone). K-spar occurs in orthogneisses of granodioritic to quartz monzonitic composition, but is rare in pegmatite (plagioclase is dominant). CHLORITE Pale green to dark green pseudo-uniaxial(-) or (+)? Mica-like habit and form. Anomalous blue to brown birefringence. Occurs as foliation-forming mineral with muscovite in low-grade zones, c h l o r i t e zone and b i o t i t e zone; may occur stably in some higher-grade rocks, but i t is d i f f i c u l t to t e l l . Most commonly c h l o r i t e i s a retrograde mineral over most of the area, in some l o c a l i t i e s more than others. It replaces garnet rims and may replace the garnet completely; forms from breakdown of b i o t i t e ; forms in a x i a l planes of F3 kinks and crenulations after muscovite; may be kinked by F3 or F4 folds as well; may also replace s t a u r o l i t e . 218 MUSCOVITE Nearly u b i q u i t o u s i n p e l i t e s from c h l o r i t e zone to s i l l i m a n i t e zone; formed at v a r i o u s stages throughout rock h i s t o r y . C o l o r l e s s , though may appear greenish in hand specimen. (-)2V 40-45; 6 = 0.038 (2° b l u e ) . Mica habit and appearance. Formed duri n g the f o l l o w i n g stages of c r y s t a l l i z a t i o n h i s t o r y : 1. O r i g i n a l f o l i a t i o n , i n c h l o r i t e zone and higher zones. 2. A s s o c i a t e d with b i o t i t e i n c r y s t a l l i z a t i o n f o l i a t i o n (S1,S2). or c h l o r i t e i n lower zones. 3. Postkinematic and p o s t - b i o t i t e p o r phyroblast growth i n zone 2, and l o c a l l y elsewhere; as l a r g e , c r o s s c u t t i n g g r a i n s e i t h e r i n v o l v e d i n l a t e kinks or mimetic on polygonal a r c s of l a t e c r e n u l a t i o n cleavage ( n o n - p e n e t r a t i v e ) . Replaces b i o t i t e in some rocks. 4. As replacement of unstable a l u m i n o s i l i c a t e m i n e r a l s and s t a u r o l i t e as pseudomorphs of k y a n i t e , s t a u r o l i t e , and garnet; muscovite here i s f i n e - g r a i n e d , s e r i c i t i c ; may a l s o r e p l a c e f e l d s p a r in form of s e r i c i t e . Muscovite i s s t a b l e over e n t i r e f i e l d area to Quesnel Lake. Second s i l l i m a n i t e zone i s never reached; k-spar i s r a r e . Some muscovite postdates r e t r o g r a d e c h l o r i t e . SERICITE F i n e - g r a i n e d , random white mica r e p l a c i n g unstable minerals such as ky a n i t e , s t a u r o l i t e , r a r e l y garnet, and p l a g i o c l a s e . 219 BIOTITE Olive to green, a = brownish yellow; mica form and habit. Defines dominant f o l i a t i o n in magnetite-bearing schists. Birefringence is too high for c h l o r i t e . Some is bent, some might be mimetic. Reaction: GT to BI to CH (79-1, 79-27) Brown and reddish-brown; mica form and habit; pseudo-uniaxial(-). Commonly involved in c r y s t a l l i z a t i o n f o l i a t i o n , but may be mimetic around polygonal arcs and also occurs as l a t e , postkinematic porphyroblasts. Can be bent or kinked, or undeformed; found in pressure shadows; crosscutting e a r l i e r f o l i a t i o n . Replaces garnet in some rocks; i s in equilibrium with new garnets in kyanite and s i l l i m a n i t e zones; may be i n t e r s t i t i a l to kyanite. B i o t i t e is commonly retrograded to c h l o r i t e , and in some places has gone to form r u t i l a t e d c h l o r i t e (79-22). B i o t i t e contains zircon inclusions outlined by pleochroic haloes; they are more obvious in the l i g h t e r colored c h l o r i t e . Postkinematic b i o t i t e porphyroblasts in zone (2) are kinked and crosscut by even later muscovite. B i o t i t e i s seen only as inclusions in garnet in kyanite zone or s i l l i m a n i t e zone, not in staurolite-kyanite or s t a u r o l i t e zone. j i 220 GARNET Idi o b l a s t i c to s u b i d i o b l a s t i c , some s k e l e t a l , some a t o l l garnets. Isotropic, pale pink (not color-zoned), high r e l i e f . Composition: 2/3 almandine, the rest subequal parts of pyrope, spessartine, and grossular (see probe data). Zoning patterns in composition are seen in probe data. Textural zone patterns reveal much about the history. Size of garnets ranges from 0.1 mm to 15 mm; bimodal in that most garnets below s i l l i m a n i t e grade are 2-15 mm; s i l l i m a n i t e and kyanite zone garnets are <2 mm. The largest garnets (10-15 mm) are in the ST-KY zone. Garnets are commonly p o i k i l o b l a s t i c except in the highest grades. T y p i c a l l y they contain internal S surfaces defined by t r a i l s of quartz, and/or opaques; other inclusions tend not to be aligned on d e f i n i t e layers as not abundant enough. Internal S may be straight or curved, in a few places folded, many cases rotational (snowball). Patterns of inclusions show textural zoning. In general, toward higher grades garnets get bigger and more complexly zoned, then appear to be breaking down, become embayed and a t o l l , and f i n a l l y they become small r e l i c t s which then provide nucleation s i t e s for new garnets coming in with the the aluminosilicate after breakdown of s t a u r o l i t e . Garnets are stable across the entire f i e l d area, but in d i f f e r e n t forms. The area has been divided into 10 metamorphic zones based on mineral assemblages combined with d i s t i n c t i v e textures of porphyroblasts, p a r t i c u l a r l y garnet, s t a u r o l i t e , and kyanite. CHLORITOID Pleochroic formula: a = pale green, /3 = pale slate blue, 7 = c o l o r l e s s ; a = j3 > 7; 5 = grey; ( + )2V med.-high; r>v. Occurs as a few grains included in a large garnet (79-13 only), between inner core and outer zone, within rotational zone of garnet, implying that folding was taking place during c h l o r i t o i d growth. Rock i s now in ST-KY zone. C h l o r i t o i d in t h i s rock i s d i s t i n c t l y d i f f e r e n t from c h l o r i t e inclusions in same garnet; c h l o r i t e i s pale green to dark green, and has much lower 2V. Some c h l o r i t e inclusions in garnet (and in 81-315 perhaps also sta u r o l i t e ) may be replacements of e a r l i e r c h l o r i t o i d grains. 221 STAUROLITE Low 6; r>>v; (+)2V 70-80; p a r a l l e l extinction. Pleochroic formula: a - pale yellow, /3 = yellow, 7 = golden yellow; 7>j3>a. Idioblastic to subidioblast ic porphyroblasts up to 3 cm long by 1 cm wide. Rarely twinned. Mode is 2-5% of rock except in one unit where i t i s 10-25% (80-69, 80-135). Inclusions are: ilmenite, r u t i l e , quartz, tourmaline, graphite, garnet, kyanite, b i o t i t e , zircon. Some S- = S , some straight, some curved ( h e l i c i t i c ) or rotational S^  . Retrograde reactions: ST to CH + MS + QZ. Ilm inclusions l e f t over. Some sta u r o l i t e s are completely pseudomorphed by fine-grained c h l o r i t e and s e r i c i t e . Rocks with abundant st a u r o l i t e also have plagioclase porphyroblasts. Staurolite may be included in big garnets as small xenoblastic grains (perhaps after c h l o r i t o i d (in 81-315)) or near the rim of a garnet. Staurolite may include i d i o b l a s t i c garnets as in 80- 69 and 80-135. Staurolite includes some kyanite, but kyanite surrounds end of s t a u r o l i t e , so must be about same age. P o i k i l o b l a s t i c s t a u r o l i t e may contain smaller s t a u r o l i t e and kyanite, also r u t i l e (79-28). Rutile inclusions in st a u r o l i t e become ilmenite at rim of s t a u r o l i t e where r u t i l e is in contact with quartz (best example i s 81-344). Synkinematic, rotational s t a u r o l i t e i s seen in 80-45; later embayed. Some sta u r o l i t e s show two d i s t i n c t growth periods: 1. Abundant inclusions, vaguely f o l i a t e d ; 2. Post-kinematic, clear (80-95). Staurolite including folded S i , or h e l i c i t i c (80-69, 81- 234). Staurolites in s i l l i m a n i t e zone are embayed, breaking down. Staurolite is c l e a r l y being replaced by f i b r o l i t e in transition zone ( ST-KY-SIL zone). 222 KYANITE Low 6, c o l o r l e s s , high r e l i e f , length slow; p a r a l l e l extinction and good cleavage seen looking at (010), (001). Extinction angle = 30° on (100); (-)2V = 82. Cleavage {100}, poor {001} v i s i b l e on (010). Up to 2 cm long in schist; in isolated ky-qz-ms segregations kyanite is up to 2 cm by 10 cm. Commonly tabular, pale blue to grey in hand specimen. Kyanite may form up to 10% of rock. Inclusions: Ilmenite, muscovite, r u t i l e , quartz, tourmaline, b i o t i t e . A lters to : s e r i c i t e (± c h l o r i t e ) ; c h l o r i t e , b i o t i t e may be i n t e r s t i t i a l to cracked kyanite. Kyanite i s somewhat aligned on f o l i a t i o n and l i n e a t i o n , but not e n t i r e l y oriented. Kyanite tends to be syn- to postkinematic with respect to F2 folding; but some i s bent, kinked, and has undulose extinction. Some contains ro t a t i o n a l inclusions, is synkinematic (79-22). Generally S;. i s p a r a l l e l to Sg . Kyanite gets more embayed closer to t r a n s i t i o n to s i l l i m a n i t e zone. In Kyanite-only zone near Mt. Stevenson, kyanites are very tiny and aligned on f o l i a t i o n , and may be confused with s i l l i m a n i t e , although they do not occur in the same rock, and kyanite properties are d i s t i n c t i v e in thin section. These small kyanites appear to come in with the reaction ST + MS + QZ = KY + BI + GT, along with small garnets and new b i o t i t e . Older, larger kyanites are gone, except as large segregations; older, larger garnets are gone, except as embayed r e l i c t s . Rutile inclusions trapped in kyanite are OK as long as they do not come in contact with quartz. If they do, they go to ilmenite; outside the kyanite, or where r u t i l e comes in contact with quartz, r u t i l e goes to ilmenite; t h i s i s the same relationship as seen in s t a u r o l i t e with r u t i l e inclusions. Kyanite i s contemporaneous with s t a u r o l i t e for the most part, although in some rocks c r y s t a l l i z a t i o n of s t a u r o l i t e both pre- and postdates kyanite growth. SILLIMANITE Length-slow, acicular prisms in fibrous masses ( f i b r o l i t e ) . B i r e f . i s f i r s t order; no c r y s t a l s are big enough to determine regular o p t i c a l properties. S i l l i m a n i t e i s associated with b i o t i t e ; replaces s t a u r o l i t e ; also associated with small garnets. Clumps of s i l l i m a n i t e may be in cores of folds (80-30). Knots of s i l l i m a n i t e are v i s i b l e on outcrop on Quesnel Lake(80-31). S i l l i m a n i t e does not replace kyanite d i r e c t l y . Kyanite in the t r a n s i t i o n zone i s embayed and replaced by s e r i c i t e , quartz, and b i o t i t e . S i l l i m a n i t e appears to be replacing s t a u r o l i t e : ST + MS + QZ = BI + GT + SIL + H 20 . 223 OPAQUES Opaque dust--commonly interpreted as GRAPHITE. May be i d e n t i f i e d in hand specimen in some rocks (81-234). Defines t r a i l s of inclusions in porphyroblasts, but is not common in matrix, except in some schists, especially those within 1 km of contact with Bralco Limestone. Fault rocks have a very high percentage of graphite. Blocky, probably cubic: may be MAGNETITE, PYRITE. MAGNETITE may be determined with magnet in some specimens; some schists are magnetic enough to swerve the Brunton 90° or so, but have no effect outside of about a meter from the rock surface. Magnetite-bearing schists are most common near the amphibolite unit, and contain green rather than red-brown b i o t i t e ; they are interlayered with other schists so do not appear to be at a d i f f e r e n t metamorphic grade. Some elongate opaques are also iron oxides as shown by probe, but may be retrograded ilmenites. PYRITE may be recognized in hand specimen, and is indistinguishable from magnetite, etc. in transmitted l i g h t . It occurs in some schists and many micaceous quartzites, p a r t i c u l a r l y those interlayered with amphibolite unit, and carbonates. ILMENITE Opaque, elongate, platy, forms along s c h i s t o s i t y or random. Nearly stoichiometric composition confirmed by probe data, although some opaques turned out to be pure iron oxide. Lath-shaped to s k e l e t a l ; defines Si in some porphyroblasts such as garnet, s t a u r o l i t e , kyanite; in some rocks i t i s c h a r a c t e r i s t i c a l l y included in mainly certain zones of the porphyroblast, or p r e f e r e n t i a l l y so. Some garnets have more quartz inclusions in the core, then more ilmenite in the rotational and outer zones as well. In places ilmenite is surrounded by sphene or r u t i l e . In ST-KY zone, ilmenite i s stable as inclusions in garnet and in matrix, or in ST or KY. Rutile inclusions in the same KY or ST react to form ilmenite where in contact with quartz. Ilmenite i s common in a l l p e l i t e s , 1-2%. 224 RUTILE Very high r e l i e f and high absorption, reddish brown, more orange than sphene, and less euhedral, and less red than hematite. Occurs as l i t t l e ovoid blobs rarely in matrix of p e l i t e , more commonly trapped inside kyanite or s t a u r o l i t e where not in contact with quartz. In ST-KY zone, r u t i l e inclusions with an abrupt change from one end of the mineral grain to the other, or else the r u t i l e is surrounded by ilmenite. Rutile is not commonly associated with garnet, except in the kyanite or s i l l i m a n i t e zones (81-283, 81-298, 81-310, 80-119). Most of the study area is below the react i on ALM + RUT = ILM + ALS + QZ. TOURMALINE Trigonal shape; U(-); i d i o b l a s t i c , small. a » e ; u> = blue-green to o l i v e ; e = c o l o r l e s s to pale green. Zoned, concentric to c-axis, not lengthwise, most commonly blue-green to brown-green, but also more complex zoning, as in 80-33: (1) bluish-green to brown, (2) oliv e green, (3) pale yellowish o l i v e , (4) oli v e green. In many p e l i t e s , end-sections of tourmaline are very common in thin sections cut perpendicular to lineation (= fold axis) implying that tourmaline growth i s synkinematic. Inclusions seen in tourmaline: QZ, ZR (80-33), opaques (80-148), May crosscut b i o t i t e . In 81-279, tourmaline inclusions inside kyanite show one zone, with inclusions S^. (to) = S;. (ky) = Sg ; tourmaline outside kyanite has two zones. Possibly outer zone of tourmaline grew at same time as kyanite. Tourmaline occurs in lowest grade p h y l l i t e s also, and is zoned there. Tourmaline with the most zones is in ST-KY-SI zone. No tourmaline noted in Kyanite zone or S i l l i m a n i t e zone. Large tourmalines (up to 1 cm) occur in massive veins with c h l o r i t e near contact of amphibolite unit and p e l i t e on east side of middlepeak of Three Ladies Mountain, where the rocks look as though they have undergone p a r t i a l melting or hydrothermal a l t e r a t i o n . Most tourmalines in p e l i t e s are too small to see in hand specimen, <0.1 mm. 225 ACCESSORY MINERALS APATITE U(+), grey 5, small prisms, length fast, med.-high r e l i e f . Minor constituent of most p e l i t e s . HEMATITE Bright red absorption. Late v e i n - f i l l e r ; in tiny fractures and grains, esp. in retrograde schists (ex. 80-123). Probably from oxidation of magnetite or ilmenite. SPHENE High r e l i e f , sphene shape, brownish 5 and absorption. Rare in p e l i t e s (81-340, 82-382, 81-295, 80-123, 81-333, 79-36, 79-38, 81-204), and a very minor constituent of these. ZIRCON Tetragonal shape; U(+); very high r e l i e f and 6. Common as inclusions in b i o t i t e , surrounded by pleochroic haloes. Also included in c h l o r i t e which has replaced b i o t i t e ; haloes very obvious in paler c h l o r i t e . May be up to 1% of rock but commonly much less in p e l i t e s . 226 SECTION I I . CALC-SILICATES C a l c - s i l i c a t e rocks vary in composition from nearly pure marbles to amphibolites. Assemblages were not as useful as those in p e l i t e s for distinguishing metamorphic zones. A l l green amphiboles are hornblende rather than a c t i n o l i t e as determined by very dark green color, association, and o p t i c a l properties. The metamorphic grade is at least amphibolite facies. There i s no a l b i t e . Minerals suggestive of higher metamorphic grades in c a l c s i l i c a t e rocks are diopside, z o i s i t e , scapolite, and k-feldspar, but are not r e s t r i c t e d to one metamorphic zone, except for z o i s i t e . C a l c - s i l i c a t e s from Quesnel Lake in the s i l l i m a n i t e zone are d i s t i n c t i v e , containing c a l c i t e , plagioclase, quartz, k-feldspar, diopside, epidote (zoned p i s t a c i t e , high (-)2V, yellow), z o i s i t e (/3-zoisite, near pseudo-uniaxial(+)), opaques, ± garnet, sphene, apatite, or scapolite . Sphene and scapolite do not occur in the same rock. Textures indicate many reaction relationships involving diopside and garnet, plagioclase and epidote, two phases of K-feldspar, and fe l d s p a r - z o i s i t e intergrowths. The only occurrence of scapolite that is not in the s i l l i m a n i t e zone is in a vein in calcareous schist near sample 79-28, in d e f i n i t e staurolite-kyanite zone. Diopside and K-feldspar-bearing c a l c - s i l i c a t e s also occur in the Staurolite-Kyanite zone, but not below i t ; many samples in th i s zone containing K-feldspar were co l l e c t e d adjacent to gra n o d i o r i t i c gneisses. As most of the c a l c - s i l i c a t e and amphibolitic units occur within the staurolite-kyanite zone, most of the samples are of the same metamorphic grade, although their compositions vary widely. i APPENDIX II: MICROPROBE ANALYSES The electron microprobe at the Department of Geological Sciences, University of B r i t i s h Columbia, was used for mineral analyses. Accelerating potential = 15 KV Specimen current on Al = 40 namps Aperture size = 200 u Count time = 10 s or 20 s. Analyses of minerals are shown on the following tables. There are three tables with garnet analyses, two tables with b i o t i t e analyses, one table with plagioclase analyses, and one table with s t a u r o l i t e analyses. 227 228 Standards used for electron microprobe analyses are from the reference standards c o l l e c t i o n (number in parentheses) at the Department of Geological Sciences, University of B r i t i s h Columbia. A l l minerals were analyzed using the same standards, except as noted. Element Standard Number Or igin F F-Phlogopite (24) (synthet ic) Na Albite (20) (Oregon) Mg For s t e r i t e (22) (synthet ic) Al Andalus i te (26) (Brazil) Si Wollastonite (21 ) (New York) K Orthoclase (96) (New York) Ca Wollastonite (21 ) (New York) Ba Benitoite (35) (California) Ti Rutile (13) (synthet ic) Mn Pyroxmang i te (245) (Japan) Fe Fayali te (250) (synthet ic) Some garnets were also analyzed using: Mg Pyrope garnet (235) (New Zealand) Al Pyrope garnet (235) (New Zealand) Si Pyrope garnet (235) (New Zealand) GARNET A N A L Y S E S S a m p l e 8 0 - 1 9 R I M 8 0 - 1 9 CORE 8 0 - 3 1 R I M A n a l y s e s 9 7 10 WEIGHT % ( o ) MgO 2 . 2 2 ( 0 . 1 7 ) 3 . 3 3 ( 0 . 3 0 ) 2 . 36 ( 0 . 12 ) A l ,03 21 . 15 ( 0 . . 6 2 ) 21 . 16 ( 0 . 4 5 ) 2 0 . 74 ( 0 . 3 7 ) S 1 0 . 37 . 2 1 ( 0 3 0 ) 37 .61 ( 0 6 0 ) 36 97 ( 0 . 4 1 ) CaO 2 . 6 4 ( 0 . 2 0 ) 3 0 4 ( 0 18) 2 . 0 3 ( 0 . 12) T i O i 0 . 0 2 ( 0 . 0 1 ) 0 . 12 ( 0 . 3 2 ) 0 .01 ( 0 . 0 2 ) MnO 2 . 2 7 ( 0 . 1 2 ) 1 36 ( 0 . 3 1 ) 1 . . 8 2 ( 0 . 2 5 ) FeO 3 5 . 0 4 ( 0 2 8 ) 34 21 ( 0 . 3 1 ) 3 5 . 9 3 ( 0 . 7 3 ) T o t a l 100 . 5 5 ( 0 . 5 4 ) 1 0 0 . 8 3 ( 0 . 6 4 ) 9 9 8 6 ( 0 . 8 4 ) FORMULA ( „ ) MgO 0 2 6 7 ( 0 . 0 1 8 ) 0 . 3 9 5 ( 0 . 0 3 5 ) 0 . 2 8 6 ( 0 . 0 1 5 ) A l i 0 , 2 . 0 0 3 ( 0 . 0 4 8 ) 1 . 9 8 5 ( 0 . 0 3 7 ) 1 9 8 3 ( 0 . 0 3 3 ) S 1 0 . 2 , . 9 9 2 ( 0 . 0 3 1 ) 2 . 9 9 3 ( 0 . 0 3 5 ) 2 9 9 9 ( 0 . 0 2 9 ) CaO 0 . . 2 2 7 ( 0 . 0 1 7 ) 0 . 2 7 6 ( 0 . 0 4 7 ) 0 . 176 (O . 0 1 1 ) T 1 0 * 0 . 001 ( 0 . 0 0 1 ) 0 . 0 0 7 ( 0 . 0 1 9 ) 0 . 0 0 1 ( 0 . 0 0 1 ) MnO 0 . . 155 ( 0 . 0 0 8 ) 0 . 0 7 7 ( 0 . 0 3 0 ) 0 . 125 ( 0 . 0 1 6 ) FeO 2 3 5 6 ( 0 . 0 2 7 ) 2 2 7 7 ( 0 . 0 2 7 ) 2 . 4 3 8 (O . 0 4 5 ) T o t a l 8 . 0 0 1 8 . 0 1 0 8 . 0 0 7 ( 0 . 0 1 9 ) 8 0 - 3 1 CORE 8 0 - 3 3 R I M 8 0 - 3 3 CORE 7 2 0 1 1 2 . 6 6 ( 0 . 0 4 ) 2 . 1 0 ( 0 . 15) 2 . 3 4 ( 0 . 2 1) 2 0 . 7 6 ( 0 . 3 6 ) 2 1 . 6 5 ( 0 . 2 6 ) 2 1 . 8 9 ( 0 . 4 4 ) 3 6 . 8 9 ( 0 . 5 4 ) 3 6 . 9 8 (O . 4 7 ) 3 7 . 0 2 ( 0 . 5 9 ) 1 . 6 2 ( 0 . 0 3 ) 2 . 1 1 ( 0 . 14 ) 1 . 6 0 ( 0 . 1 5 ) 0 . 0 0 ( 0 . 0 0 ) 0 . 0 5 ( 0 . 13) 0 . 0 1 ( 0 . 0 1 ) 3 . 34 ( 0 . 4 1) 2 . 3 1 ( 0 . 13 ) 3 . 4 8 ( 0 . 4 3 ) 3 4 . 4 5 ( 1 . 2 0 ) 3 5 . 3 2 ( 0 . 5 0 ) 34 3 5 ( O . 4 2 ) 9 9 . 7 2 ( 1 . 4 4 ) 1 0 0 . 5 2 ( 0 . 9 5 ) 1 0 0 . 6 9 ( 1 . 0 1 ) 0 . 3 2 2 ( 0 . 0 0 6 ) 0 . 2 5 1 ( 0 . 0 1 6 ) 0 . 2 8 0 ( 0 . 0 2 3 ) 1 . 9 8 6 ( 0 . 0 3 4 ) 2 . 0 5 2 ( 0 . 0 2 8 ) 2 . 0 6 9 ( 0 . 0 2 6 ) 2 . 9 9 4 ( 0 . 0 3 6 ) 2 . 9 7 4 ( 0 . 0 1 7 ) 2 . 9 6 9 ( 0 . 0 1 5 ) 0 . 1 4 1 ( 0 . 0 0 3 ) 0 . 1 8 2 ( 0 . 0 1 2 ) 0 . 138 ( 0 . 0 1 4 ) O.OOO ( 0 . 0 0 0 ) 0 . 0 0 3 ( 0 . 0 0 8 ) 0 . 0 0 1 ( O . O O D 0 . 2 2 9 ( 0 . 0 2 9 ) 0 . 158 ( 0 . 0 0 9 ) 0 . 2 3 7 ( 0 . 0 2 9 ) 2 . 3 3 8 ( 0 . 0 5 8 ) 2 . 3 7 6 ( 0 . 0 2 5 ) 2 . 3 0 4 ( 0 . 0 4 2 ) 8 . 0 0 9 ( 0 . 0 2 7 ) 7 . 9 9 6 ( 0 . 0 1 3 ) 7 . 9 9 6 ( 0 . 0 1 6 ) ro "n S -) O c/i > s (0 3 — 0) — — IQ o o o o o - o . O •n 3 -I o t/l > s (0 3 — OJ - —(Q o o o a o ~ a ~ o o o UI o b u M O O O M U O •U — O — (0 O fo u o O a a ^  >i a> m ro co cn -* ro o o o o o o o b b < -j -j ro ut O CD O > O QO O O O •>0 u u u u o 30 s c ~ 8 u CJ to <n O ro —. ro — ui O O -» ro ui co A u) CJ cn cn O O O O O O O ui ro o x» £>•-•• -* cn co o cn - J - J 8 o b 10 ro O O O ro ro o U O - <D O U ro Q o (D Co ro cn ~J O CJ ID CJ) - J O O O O O O O b bb b b b b cj O O -* ro CJ ro — to CJ O ui O O en co O cn CJ CJ ro x* O ro -~ CJ 01 k O u t> O 01 CO Ol CJ 00 00 O O O O O O O IO -» O -» 01 CJ -» co cj & ro -j ui O O io o O O ro ro O o to o tn cn CJ cn cn co — o ui a> CD o o o o o o o b b b b b b b MOO-'-'-O O - * - » CJ O - J 8 b ro co CJ CJ to ui -» o — cn -* ro cn ^ o - J - J co cn x* — & cn CJ CB O o O O O O O ui o o - t - o b. ro -•. cj A -J CJ O b UI M O O O w u O cj Q ro co O ro cj to o CJ oo o cn ro & ro cn CJ o to O O O O O O O b b b o b b b t t u - -ro x» ro co o cn CJ O o cj ro CJ oo CJ cj M X* ro O ro cn — ro ui oo o - J to O ro cj & CJ ro cn ro CJ O O O O O O O ui ro o ui -•• oi -» ro cn o x* — M O O O r o - O x» O O ro to to ro x>. to O ro to to cn ro ~i CJ cn o O cj O O O O O O p o o b b b b o u o O U U M -> 00 to CJ ui ro ui CD O cj CJ ro O cn o ro cn o ro — x» o cn to oo — O ro ui — oo oo oo - o o o o o o o ui oi ui — o x* cn x* — co — x>. co ro x» GARNET ANALYSES Sample 81-325 RIM 81-325 CORE 82-389 RIM 82-389 CORE Ana 1yses 25 13 13 4 WEIGHT 5 4 U) MgO 2 . 72 (0 18) 3 .09 (0 .08) 2 .55 (0 50) 2 . 10 (0. 42) Al 1 O 1 20 .85 (0. 45) 20 .87 (O 38) 21 .24 (0. 59) 20 . 74 (0. 37) S10> 36 .85 (0 47) 37 OO (O 40) 37 36 (O 56) 37 . 77 (0. 55) CaO 2 .98 (0. 14) 2 .90 (0. 10) 6 .21 (0 86) 6 78 (0. 64) TIO, 0 01 (0. 02) 0 .04 (0. 09) 0. .06 (0 05) 0 09 (0. 04) MnO 2 16 (0. 26) 1 .70 (0. 10) 1 55 (0. 48) 3 . . 32 (O. 39) FeO 34 . 02 (0. 38) 34 . 37 (0. 39) 31 . 32 (0. 68) 30. 28 (0. 39) Total 99. 59 (0. 88) 99 97 (0 62) 100. 29 (O. 87) 101 . 08 (0. 45) FORMULA (<j) MgO 0 .328 (0.021) 0 .372 (0 .010) 0 . 304 (0.059) 0. . 249 (0 .048) Al I O I 1 . 990 (0.033) 1 . .983 (0 .028) 1 .998 (0.055) 1 , ,944 (0 .046) S10; 2. 985 (0.020) 2 982 (0 .014) 2 982 (0.033) 3 003 (0 .029) CaO 0. 259 (0.012) 0 . 250 (0 .009) 0. ,531 (0.074) 0 577 (0 .054) T10, 0. .001 (0.001) 0 .002 (0 .005) 0. .004 (0.003) 0 006 (0 .003) MnO 0. 148 (0.017) 0 , 1 16 (0 .006) 0 . 105 (0.033) O, . 224 (0, 028) FeO 2 . 305 (0.032) 2. ,317 (0 .035) 2 .090 (0.038) 2. 014 (0 .027) Total 8 . 017 (0.011) 8 . 022 (0. 013) 8. 013 (0.013) 8 . 016 (0, .012) BIOTITE ANALYSES Sample 80-19 80-19 B 80-31 A n a l y s e s 9 (d1f. g r . ) 16 (same g r . ) 6 WEIGHT % (a) F 0 . 37 (0 . 15) 0 29 (0.14) 0. 24 (0 08) Na>0 0 .21 (0 03) 0 22 (0.04) 0. 38 (0 05) MgO 9 .35 (0 47) 9 65 (0.19) 8 66 (0 25) Al 703 18 .31 (0 61) 18 .25 (0.60) 19 26 (0 . 18) S10» 35 .61 (0 81) 36 .08 (0.75) 35 56 (0 87) K,0 8 .93 (0 . 36) 8 .49 ( 1 1 5 ) 8 24 (0 07) CaO 0 .00 (0. 01) 0 01 (0.01) 0. 01 (0 01) BaO 0 .36 (0. 08) 0. 32 (0.07) 0. 20 (0 07) T10r 1 98 (0. 31) 1 . 76 (0.09) 2 81 (0 .23) MnO O 03 (O 03) 0 03 (0.02) 0. 03 (0 02) FeO 19. .48 (1 .04) 19. 23 (0.60) 18. .77 (0 31) T o t a l 94 63 (1. 12) 94. 43 ( 1 42) 94 . 16 (0. 69) FORMULA Oxygens 22.000 22.000 22.000 S1 5.484 5.535 5.442 Al IV 2.516 2.465 2.558 Sum IV 8.000 8.000 8.000 Al VI 0.807 0.835 0.915 TI 0 230 0.203 0.324 F e ! * 2.508 2.466 2.403 Mn 0.004 0.004 0.004 Mg 2.147 2.203 1.977 Sum 5.696 5.711 5.623 Ca OOOO 0.002 0.001 Na 0.063 0.092 0.113 K 1.754 1.659 1.609 Ba 0.021 0.018 0.012 Sum 1.838 1.771 1.735 OH 3.937 3.908 3.883 F 0.063 0.092 0.117 80-33 14 81-278 10 8 1-279 8 0.39 (O.13) 0.31 (0.06) 9.00 (0.38) 19.09 (0.82) 35.42 (0.83) 8.74 (0.30) 0.00 (0.01) 0.11 (0.05) 2.22 (0.34) 0.04 (O.OI) 20.42 (1.16) 95.74 (0.98) 0.35 (O.13) 0.29 (0.07) 10.11 (0.35) 19.30 (0.53) 35.42 (0.67) 8.42 (0.01) 0.01 (0.01) 0.07 (0.04) 1.70 (0.19) 0.04 (0.02) 18.45 (0.56) 94.16 (1.07) 0 31 (0. 16) 0 23 (0. 03) 8 .81 (0 85) 19. 59 (0. 47) 35. 54 (0. 71 ) 8 . 06 (0. 33) 0 03 (0. 07) 0 . 16 (0. 04 ) 1 .70 (0 . 33) 0 .04 (0. 05) 20 .62 (1 40) 95 .09 (O 53) 22.000 5.397 2.603 8.000 0.825 O. 254 2 .604 0.006 2 .044 5.733 0.001 0.091 1 .699 0.006 1 . 797 3.810 0. 190 22.000 5.409 2.591 8 .OOO 0.894 0. 199 2 . 365 0.005 2.313 5.776 0.002 0.088 1 .646 0.005 1 .741 3.835 0. 165 22 OOO 5 .424 2 . 576 8.000 0.947 O. 196 2 .634 0 0 0 6 2 .003 5 . 786 0.005 0.069 1 .570 0.010 1 .654 3.850 O. 150 BIOTITE ANALYSES Sample 80-119 80-119 INC1 Analyses 17 3 WEIGHT % (a ) F 0 . 28 (0 10) 0 .47 (0 15) Na>0 0 . 30 (0 05) 0 .44 (0. 03) MgO 8 .72 (0 47) 1 1 .24 (0 13) Al , 0 i 19 .27 (0 .90) 18 . 17 (0 28) S10, 35 . 20 (0 43) 36 .24 (0 94) K , 0 8 .44 (0 19) 8 . 33 (0. 05) CaO O .03 (0. .04) 0 OO (0. 01) BaO 0 . 12 (0. 04) 0 .09 (0. 03) T107 2 .38 (0 42) 2 .23 (0. 03) MnO 0 .04 (0 02) 0 .03 (0. 02) FeO 20 .42 (0. .95) 16 .52 (0. 18) Total 95 .20 (0. 57) 93 .76 (0. 46) FORMULA Oxygens 22.000 22.000 S1 5.382 5.523 Al IV 2.618 2.477 Sum IV 8.000 8.000 Al VI 0.854 0.788 T1 0.273 0.256 Fe'* 2.612 2.106 Mn 0.005 0.003 Mg 1.988 2.555 Sum 5.732 5.708 Ca O.O04 0.001 Na 0.089 0.130 K 1.646 1.619 Ba 0.007 O.005 Sum 1.746 1.755 OH 3.864 3.771 F 0.136 0.229 80-119 INC2 81-325 82-389 3 15 7 0.30 ( O H ) 0.37 (O 03) 11 .05 (0.23) 19.75 (O.15) 35.59 (O.10) 7.86 (0.43) 0.00 (0.01) 0.09 (0.04) 1 .86 (0.13) 0.02 (0.02) 17.24 (0.56) 94.13 (1.05) 0. . 37 (O. 14) 0 40 (0. 04) 10 09 (0. 15) 18. . 68 (0. 36) 36 . 15 (O. 32 ) 8 . 45 (0. 20) 0. 02 (0. 04 ) 0 . 13 (0. 05) 2. .06 (0. 19) 0. 06 (0. .06) 17 . 93 (0. 50) 94 34 (0. 55) 0. .50 (0. 17) 0 25 (0. 10) 1 1 . 77 (0 52) 18 . 05 (0 . 18) 36 58 (0 .23) 8 84 (0 .40) 0 01 (0. .01) 0. 07 (0 04 ) 1 4 1 (0. 08) 0. 03 (0 .02 ) 16 . 75 (0 42) 94 26 (0 .24) 22.000 5.392 2 .608 8 .000 0.919 0.212 2 . 184 0.002 2 .496 5.813 0.001 0. 109 1 .519 0.005 1 .634 3.857 O. 143 22.OOO 5 . 509 2.491 8 OOO 0 864 0. 236 2 . 286 0.OO7 2 . 291 5.684 0.003 0. 1 19 1 .643 0.008 1 . 773 3 . 822 0. 178 22.000 5.563 2 .437 8 .000 0.797 0.161 2. 130 0.004 2 .669 5.761 0.002 0.073 1 .741 0.005 1 .821 r-o 4.000 y X > 3 CO 09 CD CO CO o b n l - i i i n x w M Z - n (D 3 — 0) 0> » — —IQ g> o o o a o o o ~ a -- o o O O O O O O M - O O O 8 O O O C 0 O c n c 0 O 9 > O O O C J ^ O M B O O O r o o o o c n . u K j — o - J c n O O O O o o o o o o o g b g b o g b b g b b O O i cn — £• co O ao o a X3 > CO 00 CO 01 - n X - t O O O X c / j & S Z - n IB 3 — o> n> •« — — iQ 0) o o o o o o o - o -~ o o cn ro O O O O - o O - i c n O c n o 882 O oo co Ul CS CO & 8S2 o o o o o o o o o o o — O O O i n O - J c n O ' o O ' O O U t > u - ' - > 0 - > ^ ID I > 3 0) < CO CD Ul l/l Dl 3 T3 — CO oo O i > o o > cn > z > r--< in ui b cn o b 0 0 0 0 0 0 1 0 — 0 0 0 o • o • 8 ro o ~J M O - J O - O ^ M O B O O O O O O O O O O O O 88888822828 0 0 0 - ( D - . m t n o O - J co co ro cn ro O O O O < t * O r o u O < 0 O O O O O U - > M U O O O u o - ' U > i o u a s o u i u O O O O O O O O O O O O *> O O O O — O C O . U O — O CO — O — U1CO — 0 0 O — CJ.U 00 O o Ul CO 0 Ul O O O O O O r o — O O O ro cn CJ b CO ro Q O O O M O - l M O - J O O O O Q - J O r o c n o c n o a s O o O — 01 — cn — j *cn O O O O O O O O O O O O 019) O O O O O O O O O O O O O O O r o O — — O r o — u i O O O — ro — co — O r o O Ul O O O O O O M - O O O 8 0 0 0 r o o c n c o o ^ i o Q O Q c o o c n c d O u i o — O O O — O O - I O C D O O O O Ul o o 03 cn ro O O O O m o — & 0 0 0 0 U O O Q J 1 - - - 0 - 1 O — 0 - . O C O O c o - . r o * . * . .O O O O O O O O O O O O — O O 0 * - O u i u i O — O — — O O CO ro — — coco CO cn ro o o o o c n o c o u i o o o o O Q O Q — O C 0 U 1 O 0 3 O r o O O o — -J — u i o r o o ao O u co ao O 0 Ul O O O O O O i o - O O O CO 0 b ro Ul O O Q O c o o c n c o o c n o O O Q Q — o c n c o o c o o u i o o o o & u i c n o c n o o O O O O O O O O O O O O 038) .008) .057) .OOO) .014) .005) OOI ) .017) .000) OOO) OOO 003) co ui ro • co o o o o c n o c o u i o - i O — O 0 0 * - O t o r o o c o o co — — — c o - 4 r o u o c o c n — O O O O O O O O O O O O O O O M O - I U O - J O C O M — r o c o r o u i c n — o t n 0 Ul O O O O O O r o — O O O ro ro 00 b ro CO Q O O O W O - J M O - J O O g o o u o u m o ^ o coooo — A-oocn — -0 co O O O O O O O O O O O O 019) O O O O O O O O O O O O O O O — O — — O c o o — — 0 0 ~ J — roco — — cn O 01 ro — O O O O * - O r o i i O C 0 O to O O O O C O O U i m O - O ui ~i — — — c o o o * . c o O c n r o O O O O O O O O O O O O O O O U O M U O U O •u — - . r o c o — o o ^ o - ^ c n ro CO o CO O O O O O O r o — O O O CO CO o ro O O O ' O Q O 1 *• O O 1 co o cn co o cn 0 cn o *• cn o co o co *• O — ro 01 01 O O O O O O O O O O O O O O O O O O O O O O O O O — — O r o o O O < ui O 1 O ^ i — cocncoroco co CO co O cn 1^ Ul M O O O O - J O c o c n o c n o - O O O ^ O O - i O H O U - 0 0 < H ^ 0 U M U I & O O O O O O O O O O O — O O O — O C O f O O r o O ro — — — oo ro co cn ui O cn co 10 cn KZ STAUROLITE ANALYSES Sample 82-389 81-279 81-278 Ana 1yses 8 1 1 WEIGHT % , ( « ) F 0 .04 (0. .07) 0 .00 0 .05 Na*0 0 .01 (0. .01) 0 .00 O .03 MgO 1 . 76 (0. 21) 1 . 58 1 . 36 A l 70, 51 .85 (0. 48) 55 . 42 52 . 35 S10* 27 .53 (0. 38) 27 . 73 27 . 34 K*0 0 .01 (0. 01) 0 .00 0 .02 CaO 0 .01 (0 .01) 0 .00 0 .00 BaO 0 .02 (0. 03) 0 .00 0 .00 T10J 0 .64 (0. 04) 0 . 29 0 . 58 MnO 0 . 13 (0. 02) 0 .07 0 . 16 FeO 13 . 27 (0. 20) 14 . 10 14 . 34 T o t a l 95 . 25 (0. 42) 99 .21 96 . 23 FORMULA Oxygens 23 .00 23 .00 23 .00 S1 3 .92 3. 79 3 87 A l 8 . 70 8 . 94 8 . 74 T1 0 .03 O. .03 0. 06 Sum 8 . 73 8 . 97 8. 80 F e ! * 1 . 58 1 . 61 1 . 70 Mg 0 38 0. 32 0. 29 Mn 0 .02 0. 01 0. 02 Sum 1 .98 1 . 94 2 . 01 T o t a l 14 .64 14 . 70 14 . 68 OH 2 .00 2 . OO 1 . 98 F 0 00 0. 00 0. 02 XFe 0 . 8 0 0 .83 0 .85 S t a u r o l i t e F o r m u l a : ( F e , M g ) ( 2 ) A l ( 9 ) S 1 ( 3 . 7 5 ) 0(22) 0H(2) 80 -33 1 0 . 0 0 0 . 0 3 1 . 39 53 .84 27 .43 0.01 0 . 0 0 0 .02 0 . 5 0 0.11 14 . 59 97 . 92 23 . 00 3 . 82 8 .83 0 . 0 5 8 . 88 1 . 70 0 . 29 0.01 2 .00 14 . 70 2 .00 O.OO 0 .85 APPENDIX IIIA: ANALYTICAL METHODS FOR GEOCHRONOLOGY Anal y t i c a l Techniques. Rb and Sr concentrations were determined by replicate analysis of pressed powder p e l l e t s using X-ray fluorescence. U.S. Geological Survey rock standards were used for c a l i b r a t i o n ; mass absorption c o e f f i c i e n t s were obtained from Mo K-a Compton scattering measurements. Rb/Sr rat i o s have a precision of 2% (1a) and concentrations a precision of 5% (1O). Sr isotopie composition was measured on unspiked samples prepared using standard ion exchange techniques. The mass spectrometer, V.G. Isomass 54R, has data acquisition d i g i t i z e d and automated using a H.P. 85 computer. Experimental data have been normalized to a 8 6 S r / 8 8 S r r a t i o of 0.1194 and adjusted so that the NBS standard SrC0 3 (SRM987) gives a 8 7 S r / 8 6 S r r a t i o of 0.71020 ± 2 and the Eimer and Amend Sr a rat i o of 0.70800 ± 2. The precision of a single 8 7 S r / 8 6 S r r a t i o is <0.00010 ( l o ) . Rb-Sr dates are based on a Rb decay constant of 1.42 x I 0 " 1 1 a _ 1 . The regressions are calculated according to the technique of York (1967). Zircon separate was prepared using standard grinding and mineral separation procedures for heavy minerals. Dissolution of zircons and i s o l a t i o n of U and Pb were done using a procedure based on that of Krogh'(1973). The sample was analysed using single Re filament and s i l i c a gel techniques on an Isomass 54R mass spectrometer in the Department of Geological Sciences, University of B r i t i s h Columbia, by P. van der Heyden. Krogh, T.E. 1973. A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopie age determinations. Geochimica et Cosmochimica Acta, 37, pp. 485-494. 236 APPENDIX 11 IB: GEOCHRONOLOGY SAMPLE DESCRIPTIONS Sample descriptions. JSG-81-X1 to X6. 52°40.5' N. Lat., 121°05.7' W. Long. Foliated quartz d i o r i t i c gneiss with plagioclase(An 2„. 3 0) (35%), quartz (15%), hornblende (10%), and reddish-brown b i o t i t e (20%) and epidote (15%) from metamorphism of hornblende, and sphene, minor apatite, a l l a n i t e , and zircon. C r y s t a l l i z a t i o n f o l i a t i o n i s defined by oriented- b i o t i t e and hornblende, and incipient segregation of mafic from f e l s i c layers. Staurolite-kyanite zone. JSG-81-X1 to X6 were combined for zircon analyses. Zircons are co l o r l e s s to l i g h t pink, t y p i c a l l y subhedral prismatic with rounded edges and terminations. Length to width r a t i o varies from 2 to 6. About 50% of the coarser (100-200 mesh) grains are broken and inte r n a l l y cracked. Bulk sample contains 1-2% non-zircon material. Handpicked fractions consist only of clear, crack-free, unbroken c r y s t a l s . Quesnel Lake Gneiss (for comparison with JSG-81-X): JSG-80-QLGNW. 52°33.0' N. Lat., 121°04.4' W. Long. From junction of north and west arms of Quesnel Lake. Leucocratic, medium-grained, f o l i a t e d granite with microcline (40%), quartz (35%), plagioclase (15%), and ch l o r i t e plus muscovite (10%), minor opaques(<2%). F o l i a t i o n i s defined by mortar texture: fine-grained mica, quartz, and feldspar between broken grains of feldspar and quartz with sutured boundaries and undulose extinction. JSG-81-166. 52°33.5' N. Lat., 121°01.1' W. Long. From junction of north and east arms of Quesnel Lake. Weakly f o l i a t e d , coarse-grained, pink and green K-feldspar granite with p e r t h i t i c microcline megacrysts (20%). Groundmass i s microcline (30%), quartz (30%), epidote and green b i o t i t e (10%), and saussuritized feldspar, c h l o r i t e , sphene, and muscovite (10% together), and minor a l l a n i t e . F o l i a t i o n i s c a t a c l a s t i c rather than metamorphic. 237 238 JSG-80-36. 52°41.75' N. Lat., 120°55.8' W. Long. Pegmatite with quartz (40%), plagioclase (50%), and muscovite (10%); average grain size is >1 cm. Intruded as late to post-metamorphic dikes. JSG-80-99. 52°43.3' N. Lat., 120°58.8' W. Long. Foliated granodioritic gneiss with large (up to 1 cm) magnetite grains l o c a l l y . Staurolite-kyanite zone. JSG-80-136A. 52°43.3' N. Lat., 120°59.8' W. Long. Foliated granodioritic gneiss with plagioclase (50%), quartz (30 % ) , p e r t h i t i c potassium feldspar (10%), b i o t i t e (10%), and minor opaque oxides. Staurolite-kyanite zone. JSG-81-205. 52°40.6' N. Lat., 120°59.2' W. Long. Weakly f o l i a t e d granodioritic gneiss. S i l l i m a n i t e zone. JSG-81-327. 52°37.85' N. Lat., 121°03.85' W. Long. Foliated granodioritic augen gneiss, with f o l i a t i o n subparallel to layering in adjacent paragneiss. Augen up to 1 cm are feldspar (45%); other minerals are quartz (30%), b i o t i t e (20-25%), and minor oxides. Kyanite zone. JSG-80-42. 52°47..2' N. Lat., 120°59.55' W. Long. Graphitic garnet-kyanite(?)-bearing biotite-muscovite schist. Garnets up to 1 cm; quartz segregation layers p a r a l l e l s c h i s t o s i t y . Staurolite-kyanite zone. JSG-79-15. 52°46.15' N. Lat., 120°58.0' W. Long. Quartz-rich schist with garnet, b i o t i t e , muscovite, and plagioclase. Staurolite-kyanite zone. JSG-80-67. 52°43.5' N. Lat., 120°59.3' W. Long. Well-foliated biotite-muscovite schist with porphyroblasts of s t a u r o l i t e , garnet, and plagioclase. Staurolite zone. JSG-80-80. 52°44.2' N. Lat., 120°57.9' W. Long. Well-1ineated micaceous quartzite with quartz, b i o t i t e , and plagioclase. Kyanite to s i l l i m a n i t e zone. JSG-80-96. 52°43.4' N. Lat., 120°58.1' W. Long. Folded, lineated quartz-biotite-muscovite schist with garnet porphyroblasts. Staurolite-kyanite zone. JSG-82-375. 52°45.7' N. Lat., 121°04.6' W. Long. Crenulated quartz muscovite schist with b i o t i t e and garnet porphyroblasts. Metamorphosed to at least garnet zone. JSG-81-245. 52°45.5' N. Lat., 121°00.05* W. Long. Coarse-grained, hornblende-plagioclase amphibolite with some quartz and iron oxides. Staurolite-kyanite zone. 239 JSG-80-21. 52°42.0' N. Lat., 120°55.4' W. Long. Foliated gneiss with b i o t i t e , quartz, plagioclase, and some muscovite. S i l l i m a n i t e zone. JSG-80-22. 52°41.95' N. Lat., 120°55.5' W. Long. Crenulated muscovite schist with b i o t i t e , quartz, plagioclase. S i l l i m a n i t e zone. JSG-81-171. 52°41.7' N. Lat., 120°56.0' W. Long. Weakly f o l i a t e d gneiss with medium to coarse-grained quartz, plagioclase, and b i o t i t e . S i l l i m a n i t e zone. JSG-81-281. 52°40.05' N. Lat., 121°05.2' W. Long. Garnet-biotite schist with plagioclase and c h l o r i t e . Staurolite-kyanite zone. JSG-81-328. 52°37.85' N. Lat., 121°03.9' W. Long. Coarse quartz-muscovite-biotite schist with garnet and kyanite (20%). Kyanite zone. i 

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