STRUCTURAL RELATIONS OF THE SOUTHERN QUESNEL LAKE GNEISS, ISOSCELES MOUNTAIN AREA, SOUTHWEST CARIBOO MOUNTAINS, BRITISH COLUMBIA by JOHN R. MONTGOMERY A.B. OCCIDENTAL COLLEGE, 1982 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE 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 June, 1985 e John R. Montgomery, 1985 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the The University of .British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geological Sciences The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date: ft^y /9S>S^ ABSTRACT The southern extension of the Quesnel Lake Gneiss lies approximately 10 km northeast of the Intermontane-Omineca Belt tectonic contact in the southwestern Cariboo Mountains, British Columbia. The aim of this thesis is the investigation of the structural development and style at a deep structural level relative to the 1MB-OB contact, and to determine the nature origin of the southern extension of the Quesnel Lake Gneiss. Omineca Belt rocks in the Quesnel Lake region are the Late Proterozoic to Late Paleozoic Snowshoe Group metasediments. The Snowshoe Group rocks in this study area comprise a package of variably micaceous schist, quartz-biotite gneissose schist, calcareous metasandstone, marble and amphibolite which represent deformed and metamorphosed continental margin deposits. The Quesnel Lake Gneiss is a predominately subalkaline granodioritic intrusive into these sediments that has been modified by subsequent deformation and metamorphism. High Sr content, low initial S7Sr/uSr ratios and an alkalic component imply a mantle source although possible Pb inheritance in zircons and regional Sr data suggest a certain amount of assimilated continental crust A U-Pb zircon age on the Quesnel Lake Gneiss indicates intrusion in Mid-Paleozoic, probably Devonc—Mississippian time. A regional metamorphic event affecting the entire sedimentary and intrusive package is interpreted to have occurred in the Middle-Jurassic as suggested by sphene U-Pb geochronometry and regional stratigraphic relations. The structural sequence observed in this area is composed of five phases of folding followed by a brittle fracturing and faulting phase. The entire sequence of deformation is seen in both the Snowshoe Group and the Quesnel Lake Gneiss. A pervasive metamorphic foliation defines the compositional layering (S0/1) and is axial planar to isoclinal first phase folds in both rock packages. Syn-metamorphic second phase deformation is evidenced as tight similar-style folds with an axial surface penetratively developed at a low angle (10-15°) to the compositional layering. Syn- to ii post-metamorphic third phase deformation produced southwest verging folds with only locally penetrative axial surfaces developed at approximately 40° to SO/1 compositional layering and northwest plunging fold axes nearly coaxial with F2 folds. The Quesnel Lake Gneiss shows a lack of F3 macroscopic folds. Fourth and fifth phase folds are brittle, broad warps that are only locally developed in the more micaceous units. A series of t' vs. a plots on second and third phase folds in both rock types indicates a ductile regime associated with high shear strain during F2 deformation with decreasing shear strain and less ductile behavior during the third phase of deformation. This change in behavior corresponds with the waning of metamorphism. At least one regional metamorphic episode has affected this area in association with the deformational sequence outlined above. The metamorphic peak occurs post-F2 and pre- to syn-F3 deformation producing Barrovian-type assemblages of the amphibolite facies. Metamorphic temperatures of approximately 590° C at 5.5 kb were determined by garnet-biotite geothermometry in sillimanite-bearing schists northeast of the Quesnel Lake Gneiss. A tectonic history for the rocks in this map area began with the deposition of the Snowshoe Group sediments in a continent margin basin from the Late Proterozoic to the Early Mississippian. Intrusion into this package by the Quesnel Lake granitic body occurred between 317 and 400 Ma ago. The first phase of deformation recognized in the Snowshoe Group and Quesnel Lake Gneiss is absent in the Quesnellia and Slide Mountain rocks and may also be of Paleozoic age. The accretion of Quesnellia onto the continental margin in Early Jurassic time is inferred to have initiated the subsequent deformation and regional metamorphism. iii Tahle of Contents I. INTRODUCTION 1 Location 1 Regional Geology 1 Thesis Area 3 II. STRATIGRAPHY 6 Introduction 6 Snowshoe Group 8 Caput Mountain Area 8 Isosceles Mt. Area 12 Southwest Gneiss Contact 15 Quesnel Lake Gneiss 15 Major and Minor Element Chemistry 17 Geochronometry 25 Summary and Discussion 28 III. STRUCTURE 30 Introduction 30 Snowshoe Deformation F1-F5 30 Quesnel Lake Gneiss FI-F5' 40 Correlation of Deformation 48 Summary and Discussion 50 IV. METAMORPHISM 55 Introduction 55 Mineral Relations 55 Garnet- Biotite Geothermometry 66 Summary 68 V. SUMMARY 72 REFERENCES CITED 78 APPENDIX A '. 82 APPENDIX B 85 APPENDIX C 87 iv List of Figures and Plates FIGURE PAGE 1. Location map 2 2. Regional map 4 3. Map of study area 7 4. Structural succession of Caput Mtn. area 11 5. Structural succession of Isosceles Mtn. area 13 6. Harker diagram 19 7. ACF diagram 20 8. Alkaline/subalkaline discriminant diagram 22 9. Rb vs. Y + Nb discrimant diagram 24 10. Rb-Sr diagram 26 11. Ll and Ll' lineations 32 12. Isoclinal Fl fold ....33 13. S2 axial surfaces 35 14. L2 lineations 36 15. F3 fold : 37 16. F3 axial surfaces 38 17. L3 lineations 39 18. S4 and S4' axial surfaces and L4 and L4'lineations 41 19. F4 open fold 42 20. F5 open fold 43 21. S5 and S5' axial surfaces and L5 and L5'lineations 44 22. Fl' folded vein 46 23. F2' fold 47 24. t' vs o diagram 52 25. a3 slip direction 54 26. S0/1 fabric 56 27. F3 crenulation of micas 57 v 28. Composite garnet 58 29. Garnet with kyanite inclusion 60 30. Staurolite over- F2 crenulation 61 31. Kyanite over F2 crenulation 62 32. Sillimanite (fibrolite) 64 33. Metamorphic mineral zones 65 34. ASK phase diagram 67 35. Garnet-biotite geothermometry 69 36. Mineral growth vs. deformation 70 Plate 1: Geologic map of the Isosceles Mountain area Plate 2: Geologic cross sections A-A' and B-B' ( vi Acknowledgements J.V. Ross, thesis supervisor, suggested the project and provided support, supervision and advice throughout the research. Valuable assistance in laboratory analyses was provided by J. Knight and K. Scott. K. Scott also provided Rb-Sr analyses. B. Cousens provided elemental analyses. J. Mortensen enthusiastically contributed to this project with his zircon and sphene analyses. L. Erdman helped with data reduction and other computer problems. Discussions with J. Fillipone, D. Parkinson, J. Mortensen, R.L. Armstrong, H.J. Greenwood and S. Garwin helped clarify numerous issues and problems. E. Bomer provided able assistance in the Field. J. O'Brien was a source of patience and moral support National Research Council of Canada grant number A-2134 to J.V. Ross provided funding for this project vii I. INTRODUCTION Location The study area is located at the southwestern edge of the Cariboo Mtns., central B.C., approximately 140 km east of Williams Lake, B.C. and 5-10 km south of the east arm of Quesnel Lake (Fig. 1). Headwaters of the Horsefly River divide the study area with Caput Mountain to the south and Isosceles Mountain to the northeast A total of 40 km2 was studied during the summer of 1983. Regional Geology Five major tectonic provinces are defined in the Canadian Cordillera of which the Omineca Belt (OB) and the Intermontane Belt (1MB) are the most important to this study (see Fig. 1). Rocks within the Omineca Belt in the Quesnel Lake region are Upper Proterozoic and Paleozoic psammitic and pelitic metasedimentary rocks and bodies of granitic gneiss (Campbell and Campbell, 1970; Campbell, 1978). This package represents a miogeoclinal wedge of sediments that was deposited along the western edge of the North American craton from Proterozoic through Late Paleozoic time (Wheeler and Gabrielse, 1972). Intermontane Belt rocks are composed of Upper Paleozoic ophiolitic rocks (Montgomery, 1978), overlain by Triassic and Lower Jurassic sedimentary and volcanic rocks (Campbell and Campbell, 1970). These two packages are a part of the Slide Mountain and Quesnel terranes, respectively (see Monger, 1977; Monger and Price, 1979; Monger et al., 1982). The contact between the 1MB and OB is tectonic and is thought to record the accretion of an allochthonous oceanic/arc terrane (1MB) with the cratonic margin (OB) (Monger et al., 1972; Ross et al., 1985). In the Quesnel Lake region the Omineca Belt rocks are composed of the recently redefined Snowshoe Group of Late Proterozoic to Late Paleozoic age (Struik, 1983). These miogeoclinal sediments also contain elongate bodies of granitic gneiss, the most prominent being a northwest-trending body occurring adjacent to Quesnel Lake 1 Figure 1: Location map of the Quesnel Lake and Crooked Lake map areas in central British Columbia. 3 (Campbell and Campbell, 1970). Northwest of Quesnel Lake and between the north and east arms of the lake, the gneiss has been interpreted to be a metamorphosed intrusive (Rees, 1983; Struik, 1983). Structures in the region are dominated by northwest-plunging anticlinoria and synclinoria.