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Structure and metamorphism in the Niagara Peak area, western Cariboo Mountains, British Columbia Garwin, Stephen Lee 1987

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STRUCTURE AND METAMORPHISM IN THE NIAGARA PEAK AREA, WESTERN CARIBOO MOUNTAINS, BRITISH COLUMBIA by STEVEN LEE GARWIN B.Sc. Geology, Stanford University, 1984 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE D E G R E E OF MASTER OF SCIENCE in THE F A C U L T Y OF G R A D U A T E STUDIES Geological Sciences We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA 16 September 1987 ® Steven Lee Garwin, 1987 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at The University of British Columbia, I agree that the Library shall make 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. Geological Sciences The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date: 16 September 1987 ABSTRACT A more than 2000 m . thick sequence of Hadrynian to Paleozoic Snowshoe Group metasedimentary rocks of the Omineca Belt (OMB) is exposed near Niagara Peak in the western Cariboo Mountains, central British Columbia. This package contains the northern extremity of the Shuswap Metamorphic Complex and lies 30 km northeast of the accretionary boundary with Intermontane Belt (1MB) Mesozoic sedimentary and volcanic rocks (Quesnellia Terrane) and Upper Paleozoic (?) ophiolitic and sedimentary rocks (Slide Mountain Terrane). Four phases of folding (Di-D4) are recognized. D! consists of isoclinal folds and transposed compositional layering. D3 commonly forms southwest verging, open to close folds with subhorizontal axes and moderately northeast dipping axial sufaces. In the eastern part of the area, divergent fanning of D2 axial surfaces and a reversal of vergence direction occur about a map-scale synform characterized by greater strain, bimodal fold style and a locally penetrative axial planar cleavage. D3 and D4 form orthogonal upright open buckles with respective northwest and northeast trending axes. Steeply dipping normal and minor reverse faults crosscut all fold structures, displaying minor offsets. Prograde regional metamorphism reached greenschist grade late in D\. Staurolite and kyanite growth accompanied D2> followed by postkinematic sillimanite generation under conditions of approximately 635° C and 5 kb. D3 associated sericite-chlorite retrogression of porphyroblasts occurs in sub-sillimanite grade rocks in the western part of the area. Synmetamorphic veins represent polyepisodic hydraulic fracture development during progressive dewatering of a sedimentary pile by prograde metamorphism. Eastward obduction of Quesnellia and Slide Mountain Terranes onto the ii Omineca Belt took place in the Middle Jurassic. Shortly following this event, the 1MB-OMB tectonic suture was deformed, forming map-scale folds of cuspate/lobate geometry. iii TABLE OF CONTENTS 1. INTRODUCTION 1 2. STRATIGRAPHY 6 2.1. Metasedimentary rocks 9 2.2. Igneous rocks 22 2.3. Interpretation and correlation 24 3. STRUCTURE 27 3.1. Mesoscopic structure 28 3.1.1. Phase one 31 3.1.2. Phase two 38 3.1.3. Phase three 49 3.1.4. Phase four 52 3.1.5. Faulting '. 57 3.2. Microscopic structure 59 3.2.1. Phase one 60 3.2.2. Phase two 62 3.2.3. Subsequent deformation 76 3.3. Discussion 78 4. METAMORPHISM 82 4.1. M l - Jurassic prograde metamorphism 84 4.2. M l Metamorphic reactions - metapelites 98 4.3. Conditions of metamorphism 104 4.4. M2 - Post Jurassic retrograde metamorphism I l l 4.5. M2 metamorphic reactions 115 4.6. Calc- silicate reactions 118 4.7. Summary 122 5. STRUCTURAL A N D METAMORPHIC CORRELATIONS 123 5.1. Synmetamorphic veins 124 5.2. Hydraulic fracture origin of synmetamorphic veins 135 5.3. Vein mineral deposition - relationship to 1 + e, 137 5.4. Synmetamorphic veins and deformational style 140 5.5. Summary 144 6. REGIONAL CORRELATION A N D INTERPRETATION 145 6.1. Regional correlation 146 6.2. Discussion and interpretation 152 7. REFERENCES 159 APPENDIX A - METAMORPHIC MINERALS 167 APPENDIX B - ELECTRON MICROPROBE ANALYTICAL PROCEDURE A N D RESULTS 169 iv List of Figures 1. Five major tectonic belts of the Canadian Cordillera 2 2. Generalized geology of the Quesnel Lake region 3 3. Lithologic units of the Niagara Peak area 7 4. Mesoscopic D i - D 4 fold structures 29 5. Distribution of structural domains 30 6. D j isoclinal folds 32 7. Dt fold profiles in quartzite and marble 34 8. Equal-area projections of D j data 36 9. D2 refolding of D i isoclinal fold 39 10. D2 fold profiles in quartzite, schist and marble 40 11. Equal-area projections of D 2 data 41 12. D2 folds characteristic of the D2 synformal zone hinge regions 46 13. D2 folds characteristic of D2 synformal zone limb regions 47 14. D3 refolding of D2 fold 50 15. Equal-area projections of D3 data 51 16. D 4 fold profiles in various lithologies 53 17. Equal-area projection of D4 data 54 18. D4 refolding of D2 fold - dome and basin structure 55 19. Imbricate normal faults offset unit 2 calcite marble 58 19. Microscopic Si-S,/S0 fabric relationships 61 21. D2 crenulations 64 22. D2 dissolution-type cleavage 66 23. D2 differentiated cleavage 67 24. S2 mica foliation - penetrative cleavage 68 25. Photomicrograph of D2 pressure shadow 70 26. Photomicrographs of microfractures in garnet and quartz 72 27. Photomicrographs of strained quartz and calcite ..73 28. Grain boundary migration in quartz 75 29. D4 mica crenulations and buckled quartz veins 77 30. D2 fold shear direction in structural domain one 80 31. Distribution of metamorphic zones 83 32. Relationships between metamorphic mineral growth and deformation 85 33. Summary chart - timing of metamorphic mineral growth with respect to deformation 86 34. Two populations of plagioclase 88 35. Two generations of garnet 90 36. Staurolite coronas around First generation garnet 93 38. Resorbed staurolite admist fibrolite mats 94 38. Kyanite masses adjacent to first generation garnet 96 39. Fibrolite within muscovite pseudomorph after kyanite 97 40. Schematic AFM assemblages and Bathozones 105 41. Pressure-Temperature conditions for pelitic rocks 110 42. Sericite-chlorite retrogression of staurolite 112 43. Fibrous chlorite within microfracture of garnet 114 44. Zoisite replaces embayed plagioclase in calc-silicate gneiss 119 45. Distribution of synmetamorphic veins 125 46. Summary chart - relative timing of vein formation with respect to deformation 126 47. Pressure solution along quartz grain boundaries in quartz vein 127 v 48. Tremolite veins 129 49. Equal-area projection of tremolite veins and related structural elements 131 50. Kink bands and twinning in kyanite internal to a kyanite vein 132 51. Trondhjemite sills and dikes - relationships to D2 134 52. Development of hydraulic fractures 136 53. Tremolite vein relationships - 1 + d 138 54. Evolution of D2 synformal zone with respect to synmetamorphic vein development 143 55. Eureka and Pundata Thrusts define 1MB-OMB tectonic boundary 154 56. Model for 1MB-OMB boundary development 155 vi L I S T O F T A B L E S 1. Unit 3 carbonate lithology types 17 2. Metamorphic temperatures and pressures for metapelites 108 3. Metamorphic mineral abbreviations and formulae 167 4. Plagioclase compositions in metapelites 168 5. Microprobe standards for garnet and biotite analyses 170 6. Microprobe sample locations 171 7. Microprobe analyses of garnet and biotite 172 vii LIST OF PLATES •All plai-es—ate-Ja—baek--rmF=puortret. Plate 1: Geology of the Niagara Peak area, Cariboo Mtns., British Columbia. Plate 2: Phase one and two folds and mineral lineations. / "J- "~ £f Plate 3: Phase three and four folds, faults and slickensides. Plate 4: Geologic cross sections. viii ACKNOWLEDGEMENTS Field and laboratory costs were provided for by NSERC grant 58-2134 to Dr. J. V. Ross, whose enthusiasm and insight for Cariboo geology are greatly appreciated. Discussions with P. Lewis served as an ever expanding foundation for thought Special thanks go to J. Fillipone, J. Montgomery, M. Bloodgood, D. McMullin, J. K. Russell, D. Murphy, R. Berman and J. Radloff for ellucidating conversations, geologic and otherwise. Michelle Desjardins assisted in collection of field data during the summer of 1985. Kay McQueen aided in preparation of the manuscript and kept my spirits high. ix 1. INTRODUCTION The Canadian Cordillera may be separated into five major belts (Figure 1) and numerous tectonic terranes (Wheeler and Gabrielse, 1972). Accretion of Intermontane Belt (1MB) terranes to the Omineca Belt (OMB) of parauthochonous North America occurred during the Mid-Jurassic (Monger et al, 1982). Upper Paleozoic (?) ophiolitic and sedimentary rocks of the Slide Mountain Terrane and Triassic to Jurassic marine sedimentary and volcanic rocks of the Quesnellia Terrane were thrust eastward over a Hadrynian to Permian continental shelf sedimentary and intercalated volcanic rock sequence (Monger et al., 1982; Struik, 1986). Jurassic deformation and Barrovian metamorphism of both parauthochon and allocthon are related to this obduction process. Metamorphic peak conditions within the OMB marked the development of the Shuswap Metamorphic Complex (SMC), as defined by the sillimanite isograd (Reesor, 1970; Okulitch, 1984). Map- area During the summer of 1985 approximately 40 km2 of OMB amphibolite grade metamorphic rocks were mapped at a scale of 1:15,840 in the vicinity of Lynx Creek and Niagara Peak (2400 m) in the western Cariboo Mountains, British Columbia. This area contains the most northerly occurence of the SMC and lies 30 km northeast of the 1MB-OMB tectonic suture (Figure 2). Situated between the north and east arms of Quesnel Lake, at the headwaters of Roaring River, this area lies 110 km northeast of the town of Williams Lake. Timberline occurs at 1700 m with rock outcrops, alpine meadows and glaciers dominating the higher elevations. The area is best accessed by helicopter or boat along the east arm of Quesnel Lake. For ease of location, in the text that follows, the western part of the map-area will be termed the Lynx Creek area, while the eastern part will be referred to as the Niagara Peak area. 1 INTRODUCTION / 2 FIGURE 1. The five major tectonic belts of the Canadian Cordillera, including the location of the Shuswap Metamorphic Complex (SMC) and the Quesnel Lake region (Figure 2) (after Wheeler and Gabrielse, 1972). INTRODUCTION / 3 120*30' -h 52*45' SNOWSHOE Qf»OUP QUARTZ-MICA SCmBT. OUAHTIiTE. UAHBLE *KD CALC-9ILICATE IN AMPHI80LITC *»CIIS Q* UE TAMORFHISU F IGURE 2. Generalized geology of the Quesnel Lake region, central British Columbia (legend modified from Ross et al., 1985). INTRODUCTION / 4 Previous Work Initial geologic mapping of the Cariboo Mountains was published by Bowman in 1889 whose interest was in regional placer and lode gold deposits. Early regional mapping programs were carried out by Holland (1954) and Sutherland Brown (1957, 1963) of the British Columbia Department of Mines and by Campbell (1961, 1963) of the Geological Survey of Canada. Rocks originally mapped by Holland (1954) at Yanks Peak-Roundtop Mountain area were termed the Snowshoe Formation and assigned to the uppermost Cariboo Group. Subsequent work in Antler Creek and Cariboo River areas by Sutherland Brown (1957, 1963) and the Quesnel Lake region by Campbell (1961, 1963) followed this classification. Campbell (1978) redesignated the Snowshoe Formation to be a distall equivalent of the Kaza Group and to underlie Cariboo Group rocks. The regional stratigraphic corelations of Struik (1982, 1983, 1984, 1986) have led him to define the Cariboo and Barkerville terranes, the former structurally juxtaposed above the latter along a west directed thrust fault, the Pleasant Valley Thrust. Struik reclassified the Snowshoe Formation as Snowshoe Group and tectonically separated it from the Cariboo Group by assigning the former and latter to the Barkerville and Cariboo terranes, respectively (Struik, 1982). During the past decade many University of British Columbia thesis workers and professors have focused on the complex deformation and metamorphic history of the Snowshoe Group rocks in the vicinity of the 1MB-OMB tectonic suture in the Quesnel Lake region. Four major phases of folding and a regional Barrovian metamorphism have been outlined (Engi, 1984; Fillipone, 1985; Getsinger, 1985; Ross et al., 1985). INTRODUCTION / 5 Purpose The primary objectives of this study were: to better document the geometry of the oldest recognizable phase of deformation in the OMB, by working further from the suture than have other workers, where these features are less obscured by subsequent strain, to relate metamorphic processes associated with the development of the SMC to local deformation mechanisms in order to better understand their interdependence, to access the validity of the Pleasant Valley Thrust as a structural discontinuity versus a statigraphic division between two distinct lithologic sequences, to correlate local stratigraphy, structure and metamorphism with that of other Quesnel Lake workers in order to further develop a regional framework. 2. STRATIGRAPHY The lithologies present in the map-area consist generally of a broadly folded northwest trending sequence of metamorphosed siliciclastic and carbonate rocks (Figure 3 and Plate 1). A granodiorite orthogneiss crops out in the central part of the map-area, while trondhjemite pegmatite sills and dikes are common in rocks east of Peak 8100'. Sedimentary structures and textures have been obscured completely during amphibolite grade metamorphism and polyphase deformation. The lithologic thicknesses reported refer to present thicknesses, resulting from the superposition of deformational processes on primary stratigraphy. The metasedimentary lithologies have been previously mapped by Campbell (1978) and included in the Hadrynian to Paleozoic (?) Snowshoe Formation and as undifferentiated rocks of the Shuswap Metamorphic Complex. Struick (1984) redesignated the same stratigraphic package to belong to the Snowshoe and Cariboo Groups, of inferred Hadrynian to early Paleozoic age. Correlations by both workers are based on gross lithologic appearance, as fossil control is lacking. 6 FIGURE 3. a) Distribution of rock types in the Niagara Peak area, showing the major lithologic units and the location of the lithologic columns drawn in 3b. b) Variation and con-elation of map-area rock types. 1 2 3 4 5 3c M i M i m i n i i l U 3b 3a WrWrWi I V I V I ' I V I 1111111 11111111 1111111 11111111 1111111 11111111 ' I ' I ' I I ' I ' I ' I ' i ' i ' i ' i ' i ' i ' i ' i ' i ' i 1111111  i § 3d / / quartzose schist politic schist 100m C/5 2.1. METASEDIMENTARY ROCKS STRATIGRAPHY / 9 Map-area metasedimentary lithologies may be grossly separated into three distinct units, each composed of individual mappable members (figure 3). The lower succession, unit 1, is composed of dominantly quartzose schist and quartzite, but includes lesser amounts of pelitic schist The middle unit, unit 2, consists of a massive grey crystalline calcite marble that thickens towards the central part of the area, where it includes pelitic schist and impure marble interlayers and lenses. The upper sequence, unit 3, is a well layered heterogeneous sequence of discontinuous marble marker units (3a-d), pelitic schist, and quartzo-feldspathic rocks, including minor amphibolite horizons. The contacts between these three units are sharp and well defined. Thus, the above structural succession may be viewed simplistically as a medial marble separating lower siliciclastic rocks from an upper heterogeneous carbonate-rich sequence of laterally variable composition. The total observed thickness exceeds 2000 m. STRATIGRAPHY / 10 UNIT 1 This siliciclastic unit is a minimum of 700 m thick, as the base is not exposed. Throughout the western and central part of the area this unit consists of two distinct lithologic members: member la, a lower sequence of quartzose schist and micaceous quartzite, and member lb, an upper more micaceous sequence of pelitic schist and quartzite (Figure 3, column 1). Unit lb thins from west to east across the area and is indistinguishable from la east of Peak 8100'. General lithologic trends in both members include the increase in the abundance of pelitic schist and in the degree of lithologic heterogeneity from the homogeneous relatively quartzose rocks of the western and eastern regions of the area towards its center. la. Micaceous quartzite and quartzose schist Thickness>500 m. Dark grey to black weathering well jointed angular cliff and ledge forming micaceous quartzite and lesser quartzose schist comprise this sequence. The schist component increases from Lynx Creek and Niagara Peak regions towards the area's center and to a lesser extent up section within this member. Lithologic alternation is best defined in the central portion of the study area, where compositional banding occurs on the order of 10-200 cm. There, resistant layers of light grey to white quartzite contrast with recessive bands of dark brown schist The abundance of schistose alternations increases from 25% to 60% up section. Micaceous quartzite is fine grained and commonly contains 75-85% quartz, 10% biotite, 3-10% feldspar and small amounts of muscovite and garnet. Local variation includes mm scale interlayers of coarse grained mica, producing discrete cm spaced parting surfaces. Quartzose schist is medium to coarse grained and variably composed, containing quartz, muscovite, biotite, feldspar, and garnet in order of decreasing abundance. Quartz to mica ratio varies from 3:1 to 1:1. Feldspar abundance increases STRATIGRAPHY / 11 from 3% to 15% from west to east, paralleling increasing metamorphic grade. Staurolite and kyanite porphyroblasts are present in the more pelitic lithologies of the central part of the map-area, but are absent elsewhere. The contact with the overlying lithologic member, lb, is generally gradational over a distance of 2-4 m. Where observed, it is defined by a break in slope as lower cliffy outcrops give way to upper more recessive slopes and ledges. lb. Quartzite and pelitic schist Thickness=0-200 m. This member crops out as dark greenish grey moderately rounded ledges and rusty brown weathering slopes. The lithology consists of variable amounts of fine to medium grained quartzite and medium grained pelitic schist. Where both rock types are present, the scale of alternation is on the order of several to tens of cm with quartzite layers commonly more thick and abundant than those of schist Quartz veins, mm to tens of cm thick, occur only in the schist, locally comprising 5-15% of the rock. The amount of schist within this member increases from approximately 35% in the Lynx Creek area to almost 100% near Peak 8100'. The quartzite contains 90% quartz, 5% biotite and small amounts of muscovite and garnet The pelitic schist is composed of 40-50% biotite and muscovite, 35% quartz, 10-15% feldspar, 5% garnet and rarely staurolite and kyanite porphyroblasts. Color index, as defined by Williams, Turner and Gilbert (1982), ranges from 15 to 30. The contact with the overlying marble of unit 2 is well defined and marked by an abrupt steepening in slope as lower recessive schist outcrops meet upper marble ledges or cliffs. STRATIGRAPHY / 12 UNIT 2 This unit consists dominantly of a crystalline calcite marble which includes lesser amounts of pelitic schist, impure marble, calc-silicate gneiss and minor amounts of amphibolite locally. The thickness of this unit increases from 5 m of calcite marble in the western most part of the area to exceed 220 m of calcite marble, schist and impure marble near its center (Figure 3, columns 1 and 2). In the eastern part of the area, this unit, composed of calcite marble, impure marble, calc-silicate gneiss and minor amounts of amphibolite, thins rapidly to pinch out beneath Niagara Peak (figure 3, column 5). The dominant lithology of unit 2, calcite marble, weathers to form massive light to medium grey, well jointed ledges and cliffs. It provides the most continuous and distinct lithologic marker within the map-area. It is cream white and medium to coarse grained equigranular. Calcite constitutes approximately 98% of the rock, along with minor amounts of fine grained white and brown mica, quartz, tremolite, and spinel. Compositional layering is defined by sparse mm thick white mica interlayers, forming 5-50 cm spaced parting surfaces. Rust to dark brown pelitic schist interlayers and lenses account for less than 15% of unit 2 in the central part of the area. There, they are localized in the upper part of the unit over a 30 m interval (Figure 3, column 2). Quartz veins, mm to cm thick, account for approximately 5-10% of this lithology. The schist is composed of quartz, muscovite, biotite, feldspar, 3-8 mm diameter red garnet, staurolite and kyanite in order of decreasing abundance. Color index ranges from 15 to 30. Grain size is fine to medium with the exception of medium to coarse grained muscovite. Impure marble, characterized by interlayered micaceous marble and biotite quartzose schist, forms recessive horizons between calcite marble ledges and cliffs STRATIGRAPHY / 13 locally. In the center of the area, this layered lithology forms an up to 30 m thick sequence of buff-orange to tan colored ledges and slopes in the lower part of unit 2. The abundance of these lithologies increases from 15% in this region to make up over 50% of the unit near Niagara Peak. Interlayering of approximately equal amounts of these two rock types occurs distinctly on a scale of 2-10 cm and more gradationally on a larger 30-90 cm scale with internal cm thick alternations. This gives the outcrop a strongly banded appearance with recessive buff-yellow marble alternating with resistant black quartzose schist The micaceous marble is fine to medium grained and contains 70-90% calcite, with mica interlayers, 1-5 mm thick, distributed at cm intervals. The quartzose schist is very fine grained, composed dominantly of quartz and biotite with lesser feldspar, with a color index of approximately 30. Calc-silicate gneiss interlayers and amphibolite lenses, both one to tens of cm thick, are associated with the layered impure marble. These amphibole-rich rocks, where observed, occupy 10-20% of the outcrop spaced on the order of 2-5 m apart The calc-silicate gneiss is similar to that which occurs in unit 3, described in Table 1, although it contains more calcite (up to 15%). The amphibolite consists of 90% dark green to black, medium to coarse grained, amphibole and 10% medium grained plagioclase. As much as 20% black biotite and rare pink 3-10 mm diameter garnets occur locally. The abundance of both calc-silicate gneiss and amphibolite increases near Niagara Peak. The contact with the overlying rocks of unit 3 is always sharp and marked by the change from lower marble ledge or cliff to upper, more recessive, ledge or slope. STRATIGRAPHY / 14 UNIT 3 Unit 3 comprises a heterogeneous sequence of siliciclastic and carbonate rocks exceeding 1200 m thick, with the top of this unit not being exposed. Discontinuous marble layers of variable composition serve as mappable marker units within otherwise uniform siliciclastics throughout much of the area. The abundance of these marker units increases laterally and up section from the dominantly siliciclastic lithologies exposed in the Lynx Creek and Niagara Peak regions to account for approximately 30% of the succession in the area's center (Figure 3). Siliciclastic sequence The siliciclastic rocks consist dominantly of pelitic schist, quartzose schist, lesser quartzite, with minor marble interlayers and amphibolite intercalations locally. Near Lynx Creek, approximately equal amounts of well jointed, dark vitreous cliff forming quartzose schist and brown slope forming pelitic schist alternate on a 20-50 m scale (Figure 3, column 1). The contact between these two lithologies is gradational over a distance of 1-3 m. Where this alternation is less distinct, discontinuous buff impure marble ledges, 0-20 m thick, locally separate overlying quartzose rocks from underlying pelitic schists. Over 300 m of unit 3 siliciclastics are exposed in this area. In the central region of the map-area the siliciclastic sequence is composed of roughly 50-60% pelitic schist, 30-40% quartzose schist, 5-10% micaceous quartzite, with local amphibolite and marble intercalations (Figure 3, columns 2, 3, and 4). The individual lithologies are discontinuous and poorly defined, ranging from 3-30 m thick. Resistant quartzose lithologies crop out as small ledges, while the recessive pelitic schists weather to form moderate slopes. Pelitic and quartzose schist abundance appears relatively constant throughout the section, whereas the micaceous quartzite is more STRATIGRAPHY / 15 common in the intermediate levels of the exposed succession. Amphibolite lenses, 30-60 cm thick, are concentrated within the quartzose schists near pelitic schist contacts and account for 10-15% of the outcrop locally. Lithologic contacts with the marble marker units which lie within this siliciclastic sequence are sharp and commonly interdigitated. Near Niagara Peak, the rock type is much more homogeneous and quartzo-feldspathic than observed elsewhere in unit 3 (Figure 3, column 5). There, dark grey to brown, flaggy ledge and cliff forming quartzo-feldspathic schist and micaceous quartzite alternate gradationally on the scale of several to several tens of m with no apparent periodicity. A calcite bearing quartzite unit and an impure marble provide the only stratigraphic markers within unit 3 in this area. Medium to coarse grained pelitic schist, the most abundant lithology of unit 3, contains 35-50% biotite and muscovite, 30% quartz, 10-25% staurolite, kyanite, and garnet porphyroblasts and 5-15% feldspar. Color index ranges from 20 to 30. An increase in feldspar content coincides with a decrease in muscovite abundance from west to east across the area. The abundance of quartz veins, 5-10 mm thick, increases from about 5% to 15% from the western to the central part of the area. In glaciated regions, biotite alters to stain the schist outcrops a rusty red to brown color. Pelitic schist is not observed in the Niagara Peak area. Quartzose schist is fine to medium grained, composed of 60% quartz, 15-30% biotite and muscovite, 5-15% feldspar and minor garnet Where associated with marble marker units, this lithology commonly lacks garnet, containing small amounts of muscovite and 5-15% calcite. The composition is commonly homogeneous, but layering may be defined by micaceous horizons, 2-5 mm thick, alternating with saccharoidal quartzo-feldspathic layers to produce 2-4 cm spaced parting surfaces. Quartz veins make up approximately 5% of this lithology. STRATIGRAPHY / 16 Fine to medium grained micaceous quartzite contains 75-80% quartz, 10% feldspar, 10% biotite, 0-5% muscovite and minor <3mm diameter garnet The composition is generally homogeneous, but in the eastern part of the area 1-10 mm thick micaceous interlayers occur spaced with quartzo-feldspathic intervals on a 1-5 cm scale. Several impure marble intervals, 1-20 m thick, are included in the siliciclastic sequence. They are not discussed as separate units due to their thin and discontinuous nature, however, they are included on the geologic map (Plate 1). These marble intervals are distinct from and should not be confused with the marble marker units which make up much of unit 3 in the central part of the map-area. The mineralogy of these dark grey to buff marble intervals consists of variable amounts of fine to medium grained calcite, mica, quartz, feldspar, and calc-silicates in order of approximate decreasing abundance. The amphibolite lenses and intercalations present in unit 3 have a very similar mineralogy to those described to lie within the layered impure marble of unit 2 and will not be further discussed here. Marble marker units These laterally variable discontinuous marble-rich marker units occur in the central part of the map-area (units 3a, 3b and 3c), with the exception of a quartzite/marble unit near Niagara Peak (unit 3d). These marbles thin eastward and pinch out along a northwest front west of Niagara Peak (Figure 3). These marble-rich units are composed of numerous lithology types which are described in table 1. TABLE 1: UNIT 3 CARBONATE LITHOLOGY TYPES LITHOLOGY TYPE COLOR : f resh/weath. MINERALOGY TEXTURE O C C U R R E N C E / C O M M E N T S calcite marble c ream w h i t e / 9 0 - 9 5 % ca lc i te , 2 - 5 % wh i te medium grey m ica , minor quartz, fe ldspar , pyrite medium to coa rse gra ined, crysta l l ine, massive or layered, m i ca - r i ch bands out l ine fo l iat ion mass ive marble o c c u r s in 3 a , the basal part o f 3 b , 3 c and 3 d , layered marble is res t r i c ted to 3 c h o m o g e n e o u s c ream y e l l o w / 8 0 % ca lc i te , 10% wh i te and impure marble bu f f to tan b r o w n mica , <10% quartz and fe ldspar , minor ca l c - s i l i ca tes . f ine to medium gra ined, fr iable mass ive to fo l iate southern part o f 3 b , variable in mineral con ten t layered impure marble cream y e l l o w / bu f f - o range wh i te / grey marble comp . ; 8 5 - 9 0 % ca lc i te , 1 0 % whi te mica , minor quartz and feldspar s i l ic ic last ic comp . ; 8 0 - 9 0 % quartz,' 5 - 1 0 % fe ldspar , 3 - 1 0 % biot i te medium gra ined, fol iate marble and very f ine gra ined, massive s i l i c i c las t i c inter layered on 5 - 5 0 m m scale 3 b , w h e r e s i l i c i c , c o m p . = 2 5 - 5 0 % , middle and upper part o f 3 c , w h e r e s i l i c i c , c o m p . = 2 5 - 3 0 % and is local ly absent calc-si l icate mot t led geen 5 0 % hbl. and ep ido te , 3 0 % medium to c oa r s e gra ined, basal s e c t i o n s o f 3b and gneiss and grey quartz, 10% fe ldspar , 5 % gne issos i ty de f i ned by 1-4 3 c , c o m m o n l y marking the sphene, minor ca lc i te , b r o w n mm interlayering o f m a f i c s bo t tom con tac t m ica , Fe o x i d e s and f e l s i c s STRATIGRAPHY / 18 3a. Calcite marble Thickness=0-120 m. A ledge and cliff forming grey calcite marble is 120 m thick in the northern part of the map-area, but thins towards the south to pinch out north of Peak 810CT (Figure 3, columns 2 and 3). In the north, unit 3a is composed almost entirely of pure calcite marble, while at its most southern exposure the marble includes 5% 20-30 cm thick schist interlayers and 10% 4-15 cm thick amphibolite intercalations. In the south, the marble contains 5 mm thick pyrite-rich laminae spaced on the order of 30-40 cm apart. Top and bottom contacts with surrounding schists and quartzites are sharp. 3b. Layered and homogeneous impure marble, calcite marble, calc-silicate gneiss, lesser pelitic and quartzose schist interlayers and lenses Thickness=0-110 m. Unit 3b consists of two individual markers. Neither is totally continuous throughout the map-area, but the small amount of overlap observed places the southern marker only 10 m structurally above the northern marker. Since this distance is comparable to the scale of alternation observed within the unit 3 siliciclastic sequence and given the similar compositions of both markers, they are considered to represent the same lithologic unit, including interlayers and lenses of siliciclastic rock. The lithologies of this sequence are the most variable of the marble units in the area. In the southern part of the map-area, the sequence consists of buff craggy cliffs and pinnacles of relatively homogeneous impure marble with internal elongate lenses of dark schist and minor amphibolite (Figure 3, column 2). The base of this sequence is marked commonly by a one m thick zone of amphibole-epidote calc-silicate gneiss lenses. Towards the north, these lithologies pinch out laterally into a well layered impure marble sequence. This sequence consists of alternating layers of recessive medium grey to brown marble, 5-50 mm thick, and more resistant, 3-30 mm STRATIGRAPHY / 19 thick, grey quartzo-feldspathic rock. Near the center of the area, where both 3b marble markers overlap, the layered impure marble sequence includes a basal grey calcite marble (Figure 3, column 3) which increases in thickness towards the north to account for much of the unit Horizons containing abundant amphibolite lenses, continuous for less than 10 m along strike, comprise 5-10% of the marble locally, commonly marking its top contact with overlying impure marble and schist In the northern part of the map-area, unit 3b is more siliciclastic in nature, composed of layered impure marble, quartzose schist, micaceous quartzite, and calcite marble (Figure 3, column 4). There, the top contact of the unit is marked by a 10m interval of rusty brown recessive layered impure marble; the bottom contact is defined by a more than 40 m thick sequence of grey quartzite ledges and medium gray to brown calcite marble, partially vegetated, slopes. The rock types intermediate to these bounding intervals, from top to bottom, consist of quartzose schist, layered impure marble, and calcite marble with less than 5% quartz-rich interlayers, 5-10 mm thick. Amphibolite lenses are not present in this region. Internal unit 3b contacts throughout the area are gradational between impure marble and siliciclastic rock, but are sharp when separating calcite marble from other lithologies. Upper and lower unit contacts were placed at the extreme occurrences of marble lithologies. 3c. Layered calcite marble, layered impure marble, massive calcite marble, lesser pelitic and quartzose schist lenses Thickness=0-150 m. This member consists of numerous discontinuous marble-rich intervals that interdigitate with and pinch out into siliciclastic rocks. The thickness reported includes both siliciclastic and marble lithologies, displaying an increase from 30 to 150 m STRATIGRAPHY / 20 northward along strike through the map-area (Figure 3). Overall, unit 3c is composed of about 40-50% layered calcite marble, 30% layered impure marble, 20% massive calcite marble, local calc-silicate gneiss and pelitic and quartzose schist lenses all of which interdigitate with one another. Layered and massive calcite marbles commonly comprise the basal part of the sequence, forming buff to grey ledges and cliffs as high as 35 m. Layered impure marble forms recessive rust brown slope and ledge successions up to 25 m thick and is most abundant in the central part of the area. Layering in the banded calcite marble is defined by alternating medium grained gray and medium to coarse grained white calcite intervals. The grey bands are approximately 3 cm wide, containing 1-2 mm thick pyrite seams, while the white interlayers are only 5-10 mm thick. Very fine grained dark grey calcite intervals, 5 cm thick, and 1-10 mm thick white mica interlayers occur additionally, but comprise less than 10% of this calcite marble. At the southern most exposure of unit 3c, east of Peak 8100', its basal contact is gradational over 10 m, marked by homogeneous impure marble similar to that of unit 3b in this region, while the top contact is not exposed. Immediately north of this area, both top and bottom contacts are well defined by calcite and layered impure marbles, with only local interlayering with overlying and underlying siliciclastics. In the central and northern parts of the map-area, this unit is separated into several thin, 10-20 m thick, discontinuous marble layers which are observed to merge and interdigitate with one another and siliciclastic rocks locally (Figure 3, columns 3 and 4). All three dominant lithologies, layered and massive calcite marbles and layered impure marble, occur at the northern extreme of the map-area, where calc-silicate gneiss is common near the unit's base. STRATIGRAPHY / 21 Contacts between the three dominant lithologies of the unit are commonly sharp or, more rarely, gradational over several m. Contacts with overlying and underlying siliciclastic rocks are well defined and marked rarely by amphibolite lenses, 20-60 cm thick. 3d. Calcite bearing quartzite and minor calcite marble Thickness=0-50 m. An orange-brown stained, ledge forming, sequence of interlayered quartzite and minor calcite marble comprise unit 3d. Quartzite constitutes about 85% of this unit and weathers resistantly relative to the marble. Lithologic alternation between quartzite and marble is well defined on the scale of 10-50 cm. In contrast to the rocks of units 3b and 3c, unit 3d consists of a relatively uniform interval rather than an interlayered sequence of marbles and siliciclastics. Within the map-area, this unit occurs only near Niagara Peak (Figure 3, column 5); it pinches out towards the west The dominant lithology, fine to medium grained quartzite, commonly contains 80-85% quartz, 10% intergranular calcite and minor amounts of mica and feldspar. The calcite marble includes 1-10 mm thick intervals of white mica and minor quartz spaced on the order of 1-10 cm apart The distinct weathering color of this unit and the sharp lithologic contacts with overlying and underlying siliciclastics distinguish it as a map unit STRATIGRAPHY / 22 2.2. IGNEOUS ROCKS Two types of igneous rocks are recognized on the map scale. Leucocratic granodiorite orthogneiss and trondhjemite pegmatite account for less than 5% of the exposed surface of the map-area (Figure 3). Granodiorite gneiss Thickness> 100 m. This unit crops out in the saddle west of Peak 8100' beneath unit 1 micaceous quartzite and quartzose schist The gneiss weathers to light colored well rounded outcrops and ledges several m high. It is of fine to medium grained hypidiomorphic-granular texture, containing 5-15% muscovite, 5-10% biotite, and variable amounts of Carlsbad twinned orthoclase augen up to 2-3 cm long. A gneissic foliation is defined by mica orientation and the 2-5 mm scale alternation of mica-rich and quartzo-feldspathic layers. The contact with overlying unit 1 siliciclastics, where exposed, is sharp and parallel to foliation within both lithologies. Trondhjemite pegmatite Coarse grained to pegmatitic trondhjemite sills and minor dikes, 5 cm--20 m thick, occur in the siliciclastic rocks of units 1 and 3 in the central and eastern parts of the map-area. The rock consists of variable amounts of plagioclase and quartz, with 10% muscovite, 3% euhedral 1-3 mm diameter red garnet, and black tourmaline locally. In the eastern most part of the map-area, as much as 3% potassium-feldspar is observed in this assemblage. The intergrowth of quartz rods in plagioclase outlines a graphic texture, while muscovite books reach up to 6 cm in diameter. The distribution of muscovite and garnet rarely define a poor layering. STRATIGRAPHY / 23 Sill and dike contacts are sharp and commonly marked by 5-20 mm thick black tourmaline-rich zones within adjacent schist wall rock. This occurrence is interpreted to represent metasomatism of the surrounding wall rock during trondhjemite emplacement 2.3. INTERPRETATION AND CORRELATION STRATIGRAPHY / 24 Sedimentary protoliths for the described structural succession probably include shale, sandstones, limestone and dolomite. The lower sequence, unit 1, was originally interlayered shale and silty sandstone containing rare local limestone lenses. The medial marble horizon, unit 2, was a massive limestone unit with local shale interbeds. The amphibolite lenses within this lithology and those associated with the carbonates of unit 3 correspond most likely to dolomite horizons present commonly at shale-limestone contacts or could represent the derivation of iron and magnesium from nearby shale. The upper sequence, unit 3, probably represents a variable succession of shale, limestone, sandstone, and rare dolomite. The overall spatial distribution of marble layers within this unit suggests the past existence of facies changes which have been modified by subsequent folding. The most notable lithologic transition in unit 3 occurs west of Niagara Peak, across a northwest front. There, a heterogeneous sequence, composed of several marble layers, pelitic and quartzose schist, interdigitates with a relatively homogeneous succession of quartzo-feldspathic schist and micaceous quartzite. This transition is gradual over a distance of about 2-3 km and believed to represent largely a facies change from limestone and shale on the v/est to sandstone in the east Similar facies changes occur to the northwest in the vicinity of Mt Wotzke and to the southeast along the east arm of Quesnel Lake (McMullin, oral comm. 1987). The depositional environment was probably miogeoclinal, with earlier offshore slope and basin siliciclastics succeeded by a shallow water, carbonate rich, shelf sequence. The heterogeneous nature of unit 3 may indicate that periods of sedimentation were separated by times of local quiescence and associated carbonate deposition. Frequent sea level transgression coupled with tidal and/or river controlled STRATIGRAPHY / 25 influx of terrigenous elastics is one plausible mechanism for the formation of such a varied stratigraphy (Wilson, 1975). The transition within unit 3 from limestone and shale to sandstone, upon several structural levels, may indicate the superposition of terrigenous clastic depositional fronts throughout geologic time. The inferred composition and geometry of unit 3 strata bears some resemblance to the terrigenous-carbonate shelf cycles present in the Lower Carboniferous Yoredale Series of the British Isles and the Pennsylvanian to Wolfcampian shelf cyclothems of parts of the Midwestern United States. Modern depositional analogs include parts of the western North Atlantic and the Eastern Gulf of Mexico shelves (Ginsberg and James, 1974). Struik (1984, 1986) has equated units 1 and 2 with the lower Paleozoic Downey and Bralco successions of the Snowshoe Group, respectively. Based solely on the abundance of marble layers in unit 3, correlation of this sequence is made with the Hadrynian Isaac and Cunningham Formations of the Cariboo Group (Struick, 1984, 1986). The Pleasant Valley Thrust is defined to place Cariboo terrane upon Barkerville terrane and regionally to follow individual units in both the Cariboo and Snowshoe Groups for numerous km (Struik, 1982 and 1986). Struick (1984) has mapped this thrust through the central part of the map-area, structurally above the calcite marble of unit 2, to truncate compositional layering at a low angle. Detailed mapping and observations do not show any structural discontinuities of the type proposed by Struik (1982, 1984, 1986) to exist in the map-area and suggest that such "truncations" are largely sedimentary rather than tectonic. All map-area lithologies are correlated to those of the Snowshoe Group of Struik (1982). The unit 2 marble serves as a regional marker unit and may be followed towards the east into the Penfold Creek area and towards the west beneath Mount Watt. Correlation is made with the calcite marble south of Maeford Lake, designated STRATIGRAPHY / 26 Bralco by Getsinger (1985) and Struik (1984), on the basis of similar rock type, structural position, and stratigraphic relations with respect to underlying units. Therefore, the lithologies of these areas occupy approximately the same structural level and are constrained to lie above the lithologies described in the Ogden Peak area (Lewis, 1987). 3. STRUCTURE Four phases of folding (D^D,) and subsequent brittle faulting are observed in the Niagara Peak map-area. Recognition and superposition of the structures and fabrics associated with these deformational events are determined by crosscutting structural elements and by their orientation and spatial distribution. Structural elements include refolded axial surfaces and cleavage (S) and deformed fold axes and mineral lineations (L). The temporal relationships of deformational phases with respect to metamorphic mineral and foliation development further aided in the delineation of folding events. Structural studies include observations on both the mesoscopic and microscopic scales. The correlation of mesoscopic deformational style with microscopic fabric leads to a better understanding of the mechanisms responsible for the strain accomodated in these rocks. 27 3.1. MESOSCOPIC STRUCTURE STRUCTURE / 28 Figure 4 illustrates the general outcrop appearance of D]-D4 mesoscopic strucures throughout the map-area. Di consists of isoclinal folds and transposed compositional layering. D2 commonly forms southwest verging, open to close folds with sub-horizontal axes and moderately northeast dipping axial surfaces. In the eastern part of the area, divergent fanning of D2 axial surfaces and a reversal of vergence direction occur about an upright map-scale synform characterized by greater strain, bimodal fold style and a locally penetrative axial planar cleavage. D3 and D 4 form orthogonal upright open buckles with respective northwest and northeast trending axes. Only D2 and D3 folds are recognized on the map scale. Plates 2 and 3 show the spatial distribution and orientation of these mesoscopic structures. High angle normal and lesser reverse faults of two common attitudes crosscut all fold structures. Four northwest trending structural domains are delineated on the basis of structural element orientation and distribution, and by their location with respect to mapr scale fold axial traces (Figure 5). STRUCTURE / 29 PHASE ONE PHASE FOUR MESOSCOPIC SCALE FIGURE 4. Mesoscopic character of D,-D, fold strucures and associated axial planar foliadons. STRUCTURE / 30 FIGURE 5. Distribution of structural domains with respect to map-scale fold axial traces and map-area geography. STRUCTURE / 31 3.1.1. Phase one Dj structural elements consist of Si axial surfaces, Lj fold axes, and a regional foliation that parallels compositional layering. This mica foliation represents the transposition of original bedding into the Sj plane and is designated Si/S0. Evidence for transposition includes: Present parallelism of compositional layering with observed Si axial surfaces. Si parallel growth of mica across compositional layering in the hinge regions of Di folds. The ubiquitous presence of rootless isoclinal quartz veins contained within Sj/So. The size of observed D! fold stuctures is limited to mesoscopic and lesser scales. No map-scale Dj folds are recognized within the study area. Plate 2 shows the distribution and orientation of Dt fold structures throughout the map-area. Di folds are commonly isoclinal with variably inclined axial surfaces and curvilinear fold axes which display over 120 degrees local variation in orientation. Fold style varies according to the lithology deformed. In pelitic schists, quartz veins outline dismembered isoclines with axial surfaces contained within the local mica foliation (Figure 6). Where the enveloping surfaces to these veins are observed, they commonly parallel Si/S0, but locally form moderate angles with this foliation (Figure 6). In the more competent lithologies, Dx folds are outlined by compositional layering. Impure marbles display isoclinal folds with rounded hinges (Figure 6) while folds in quartzose rocks are tight to isoclinal, with angular hinges and planar limbs. Si forms angles as large as 10-20° with compositional layering in the quartzites of domain one. Where determined, Di folds are similar in style (class 2, Ramsay, 1967. Figure 7). Vergence direction in the pelitic schists and impure marbles are indeterminable due to greatly THE QUALITY OF THIS MICROFICHE IS HEAVILY DEPENDENT UPON THE QUALITY OF THE THESIS SUBMITTED FOR MICROFILMING. LA QUALITE DE CETTE MICROFICHE DEPEND GRANDEMENT DE LA QUALITE DE LA THESES SOUMISE AU MICROFILMAGE. UNFORTUNATELY THE COLOURED ILLUSTRATIONS OF THIS THESIS CAN ONLY YIELD DIFFERENT TONES OF GREY. MALHEUREUSEMENT, LES DIFFERENTES ILLUSTRATIONS EN COULEURS DE CETTE THESES NE PEUVENT DONNER QUE DES TEINTES DE GRIS. STRUCTURE / 32 FIGURE 6. Dj isoclinal folds, a) intrafolial veins of quartz in unit 3, b) a series of dismembered quartz vein isoclines, displaying an enveloping surface which forms a moderate angle with local schistosity (S/So) in unit 3, c) isoclinal fold outlined by compositional layering in an impure marble of unit 3b. STRUCTURE / 33 STRUCTURE / 34 t/'- « plot of Di isoclinal folds in quartzite and marble illustrating their similar style (class 2, Ramsay, 1967). STRUCTURE / 35 attenuated fold limbs and the lack of paired hinges. Folds within quartzose rocks customarily display a westerly vergence with amplitudes ranging from 10-80 cm. Enveloping surfaces to these folds make a large angle with Si indicating great amounts of local structural thickening. The spatial distribution and orientation of D[ structures is illustrated in plate 2. Si, Li and compositional layering (Si/S0) are grouped according to domains based on the disposition of later deformational structures (Figure 8). The majority of Si and Lj data occur in domain one, the Lynx Creek area, where the D2 structures are less abundant and poorly developed with respect to other domains. In domain one, the curvilinear nature of Li is best displayed by its distribution upon a great circle which coincides with the resonably consistent orientation of Si for the area (Figure 8). L, data appear to become better grouped in orientation towards a subhorizontal northwest direction in domains two, three and four, roughly coincident with D2 and D3 structural trends. Si/S0 orientation changes from being shallowly north dipping in the Lynx Creek area to dip variably towards both the southwest and northeast in the structural domains to the east STRUCTURE / 36 FIGURE 8. Equal-area projection of Dj data by structural domain, a) fold axial surfaces and fold axes, b) transposed compositional layering (Si/S0). Values refer to number of data points. STRUCTURE / 37 STRUCTURE / 38 3.1.2. Phase two D2 forms the most obvious and abundant structures in the study area. Structural elements related to D 2 include S2 axial surfaces and a locally associated penetrative cleavage, U fold and crenulation axes and mineral lineations. The scale of phase two structural elements ranges from microscopic to map-scale. Modification of phase one folds results in distorted Sj and curvilinear Li (Figure 9). Distribution, orientation and intensity of D2 structural elements are heterogeneous throughout the area (Plate 2). The tightness and asymmetry of D 2 folds and the occurrence of an S2 parallel mica foliation indicate a localization of strain within a one km wide, northwest trending, synformal zone west of Niagara Peak. D2 fold style varies somewhat according to locality and lithology, but generally approximates a modified parallel fold, with thickened hinges with respect to limbs, close to similar in profile (IC and 2, Ramsay, 1967. Figure 10). Fold geometry and orientation vary systematically across structural trend with reversals in S2 dip and fold vergence directions centered about the synformal zone. Progressive tightening of D2 folds occurs towards the core of this synformal region. Equal area projections of S2 and 1^  data have been separated into four domains according to position of mesoscopic folds with respect to map-scale D2 and D3 fold axial traces (Figure 11). Domain one, the Lynx Creek area, lies to the west of a broad D3 antiform. Folds are strongly asymmetric towards the southwest with long limbs approximately five times longer than short limbs. Fold hinges are rounded with dihedral angles of 70-110°. Axial surfaces dip shallowly towards the east containing sub horizontal fold axes (Figure 11). STRUCTURE / 39 FIGURE 9. Near coaxial superposition of southwesterly inclined D 2 fold upon D : isocline, outlined by interlayered unit 3 quartzite and pelitic schist STRUCTURE / 40 FIGURE 10. t'-<* plot of D2 folds in quartzite, schist and marble, illustrating Class IC and 2 geometry (after Ramsay, 1967). STRUCTURE / 41 .sl * L 2 - 4 0 domain one L 2 - 9 3 domain two solid c i r c les -ax ia l sur faces open c i rc les- fo ld axes L 2 - 2 6 domain three L 2 - 2 8 domain four (a) FIGURE 11. Equal-area projection of D2 data by structural domain, a) fold axial sufaces and fold axes, b) quartz-biotite mineral lineations, c) staurolite^kyanite mineral lineations. Open circles denote mineral lineations while solid circles represent the S ^ S o from which the measurements were obtained. Contour intervals are 6%, 17%, 23%, 29% and 35% for domain one and 4%, 7%, 18%, 23% and 35% for domain two (contoured after method of Kamb, 1959). STRUCTURE / 42 STRUCTURE / 43 domain two staurolite-95 (O STRUCTURE / 44 kyanite-61 staurolite-111 domain three STRUCTURE / 45 Domain two is east of domain one and west of the map-scale synformal region. D2 folds are more abundant and tighter than those of the Lynx Creek area. Fold asymmetry decreases to become nearly symmetric in the eastern part of the domain while dihedral angles range from 60-90°. As S2 steepens to dip moderately to steeply towards the northeast, S2 and 1^  trends are shifted towards the west with respect to those of domain one (Figure 11). S2 forms approximately a 30-35° angle with Si/S0 in both domains one and two. Domain three is defined by the map-scale D2 synformal zone about which divergent fanning of S2 and a reversal in vergence direction occur. Map-scale folds within this domain have amplitudes and wavelengths of 20-100 and 100-800 m, respectively. The enveloping surface to these folds is subhorizontal. Two populations of mesoscopic folds are observed in this region. Fold type one is found commonly upon, but not restricted to, the hinge regions of local map-scale folds. These folds are tight to nearly isoclinal, displaying little sense of asymmetry (Figure 12). Fold hinges are subangular with a dihedral angle of about 20°, and fold profiles are close to similar in style (1C and 2, Ramsay, 1967). Amplitudes range from 10-100 cm. Fold type two occurs exclusively upon the limb regions of the same map-scale folds. These folds are disharmonic. Drastic thickening of schist within fold hinges and the local detachment of compositional layering are common on the mesoscopic and outcrop scales (Figure 13). The style of some of the mesoscopic folds approximates that of a parallel fold while others appear close to similar in profile (IB and 1C, Ramsay, 1967). These folds are variably asymmetric to symmetric, composed of rounded hinges and dihedral angles of 40-70°. Amplitudes range from 5-200 cm. Local enveloping surfaces to these folds are outcrop scale gentle to open buckles with amplitude to wavelength ratios of about 1:10. a STRUCTURE / 46 FIGURE 12. Symmetric, tight to isoclinal, D 2 folds characterize domain three synformal zone hinge regions, a) outcrop scale isocline with numerous parasitic structures, b) mesoscopic fold with attenuated southwestern limb. STRUCTURE / 47 F IGURE 13. Disharmonic D 2 folds characterize D 2 synformal zone limb regions, a) outcrop scale buckles are detached from overlying, more tightly folded, mesoscopic structures (the outcrop is 10 m tall), b) layer-parallel slip and the development of flat bottomed box folds on the mesoscopic scale (large fold amplitude is 1 m). STRUCTURE / 48 S2 and Lj orientation are the same for both fold types in this domain. S2 is subvertical to steeply southwest dipping with plunging shallowly towards both the northwest and southeast The transition from one fold population to another is abrupt and not marked by any feature other than hinge and limb locations. A locally penetrative cleavage, defined by the orientation and distribution of biotite and muscovite, parallels D2 "axial surfaces, representing transposed Si/S0. This occurrence is more commonly associated with the tight folds characteristic of the hinge regions, but not restricted to these localities. Lj fold axes are rarely curvilinear with doubly plunging axial trends which range throughout 30° within an uniformly oriented axial surface. Transitions in structural style from the surrounding areas into the folds of domain three is gradational and coincident with the appearance of the S2 mica foliation. Domain four lies to the east of the synformal zone and includes Niagara Peak. The folds of this region appear as a "mirror image" of those observed in domain two reflected about the intermediate synformal zone. Asymmetry is well pronounced towards the northeast with long to short limb ratios of about 5:1. Hinge regions are rounded with dihedral angles of 50-90°. Eastward across the domain, S2 surfaces shallow to dip moderately towards the southwest and the S2 mica foliation characteristic of domain three loses distinction. S2 forms commonly a 30° angle with local enveloping surfaces to folded Si/So. Mineral lineations associated with phase two structures include elongate quartz and calcite grains, the edges of fine grained mica, and the orientation of tabular staurolite and kyanite porphyroblasts. These lineations parallel local fold and crenulation axes and are contained within S^ So. Quartz and mica lineations are more tightly grouped than those of kyanite and staurolite (Figure 11). STRUCTURE / 49 3.1.3. Phase three The structures associated with D} include S3 axial surfaces "and L, fold and crenulation axes. These structural elements exist on all scales, with those related to crenulations and map-scale buckles more common than those associated with mesoscopic folds. D3 refolds nearly coaxial phase two folds (Figure 14) and buckles S^ So. A broad D3 antiform-synform pair, spaced two km apart, controls the map pattern and present attitude of many earlier structures in domains one and two. In the vicinity of domain three, the disposition of phase one and two structures reflects the superposition of D2 and D3 strain. Mesoscopic D3 folds appear as gentle buckles with dihedral angles of 110-140°. Amplitudes range from 5-30 cm, while wavelengths approximate several m. Cm-scale mica crenulations are commonly parasitic to these enveloping buckles. Axial surfaces trend northwest and dip steeply towards both directions with subhorizontal to shallowly northwest plunging axial directions (Figure 15). Map-scale fold attitudes are similar to those of mesoscopic structures. D3 folds and crenulations are common in domains one and two, but almost absent entirely from the two most easterly domains, where D2 structures are more abundant The distribution and orientation of S3 outline poorly a convergent fanning of axial surfaces about the D3 antiform which separates domain one from two (Figure 15 and Plate 3). STRUCTURE / 50 FIGURE 14. Near coaxial superposition of southwesterly inclined D3 fold upon tight D 2 structure in layered impure marble of unit 3b. STRUCTURE / 51 L 3 - 4 0 domain one L 3 - 2 8 domain two solid circles-axial surfaces open circles-fold axes L 3 - 0 6 domains three and four FIGURE 15. Equal-area projection of D3 fold axial surfaces and fold axes by structural domain. STRUCTURE / 52 3.1.4. Phase four S4 axial surfaces and L4 axial directions define D4. The scale of deformational structures includes crenulations, mesoscopic and outcrop size folds. Regions of localized phase four deformation, characterized by abundant mesoscopic folds, modify regional Si/So, but do not form map-scale strucures. D4 folds are abundant in the two most westerly domains, while uncommon in domains three and four, a trend which roughly parallels increasing D2 strain. The style of mesoscopic D 4 folds approximates that of a hinge thickened parallel fold (1C Ramsay, 1967; Figure 16), but is dependant on lithologic type and thickness of the deformed layer. The folds are open to tight, with dihedral angles commonly being 100°. Axial surfaces dip steeply towards the northwest and southeast containing northwest trending fold axes (Figure 17). Amplitudes range from 5-40 cm, while wavelengths average 1 m. Sense of fold asymmetry is slight Both senses are observed and commonly oppose S4 dip direction. Cm-scale mica crenulations are parasitic to these mesoscopic folds. In areas of relatively large D4 strain a spaced cleavage is defined by the similar orientation and coplanar distribution of short limb regions of tight asymmetric crenulations. Outcrop-scale open buckles serve as the enveloping surface to mesoscopic and crenulation structures. The orthogonal orientation of phase four with respect to earlier fold structures locally produces a classic dome and basin interference pattern (type 1, Ramsay, 1967; Figure 18). S4 attitude trends northeast, dipping moderately to steeply towards both directions throughout the study area (Figure 17 and Plate 3). L 4 plunges variably towards the northeast and southwest, dependant on the dip direction of the enveloping STRUCTURE / 53 FIGURE 16. t/-<* plot of D, folds in quartzite, pelitic and quartzose schist, illustrating Class IC geometry (after Ramsay, 1967). STRUCTURE / 54 S 4 - 3 5 L 4 - 4 3 all domains solid c i rc les-ax ia l surfaces open circ les-fold axes FIGURE 17. Equal-area projection of D 4 fold axial surfaces and fold axes for all structural domains. STRUCTURE / 55 FIGURE 18. Dome and basin interference structure produced by the orthogonal superposition of northeasterly trending D 4 buckles on southwesterly asymmetric D 2 folds in unit la. S T R U C T U R E / surfaces to folded S i / S o and the orientation of S 4 (Plate 3). STRUCTURE / 57 3.1.5. Faulting Numerous map-scale faults offset map-area lithologies (Figure 19). These faults may be divided into two sets based on orientation and spatial distribution (Plate 3). Both sets are well expressed topographically, commonly occupying valleys, saddles and small chasms. The faults clearly cut all phases of fold-forming deformation. Observed fault offsets are small, on the order of 10-100 m. The first set of faults strikes northwest, coincident with the trends of D2 and D3 structures. Fault planes are subvertical, commonly dipping steeply towards the southwest Slickensides, mullions and stratigraphic offsets indicate approximately dip-slip normal movements, with reverse offsets being less common. The occurrence of these faults is limited to domains two, three and four, regions characterized by numerous D2 mesoscopic folds. The second fault set trends northeast and dips moderately to steeply towards the southeast, parallel to the majority of D„ axial surfaces. Fault offsets are normal with the exception of an oblique-slip reverse fault which may be traced for over three km through the northern part of the study area. The majority of these faults exist in domain one, where phase three and four structures are relatively abundant Timing relationships between fault sets is ambiguous. STRUCTURE / 58 FIGURE 19. Imbricate, down to the southeast, normal faults offset unit 2 calcite marble on the order of tens of meters, west of Peak 8100. STRUCTURE / 59 3.2. MICROSCOPIC STRUCTURE Microscopic fabric and structure, defined by mineral distribution and orientation, consist of compositional layering, schistosity, porphyroblast inclusions, crenulations and associated cleavages, pressure shadows and mineral lineations. D! and D2 microstructural elements form the most penetrative fabrics observed in the map-area. Microstructures are correlated with mesoscopic structures through observed field relations and microscopic analysis of thin sections oriented with respect to local S surfaces, axial directions and mineral lineations. Deformation mechanisms involving grain boundary solution, microfracturing and dislocation glide played a large role in the development of mesoscopic deformational style and the manner in which the total strain was accomodated. STRUCTURE / 60 3.2.1. Phase one Recognizable T)x microstxuctural elements consist primarily of the Si/S0 compositional layering and schistosity. This fabric is defined by biotite and muscovite orientation and commonly also by 1-5 mm thick alternating bands of mica and elongate quartz and feldspar grains. The distribution and orientation of very fine grained elongate opaques within all matrix minerals coincide with Sj/So. No evidence is observed for the existence of an earlier relict foliation, nor of crenulations internal to compositional layering. Hence, this fabric does not represent a crenulation cleavage, but rather a primary foliation. Porphyroblast Inclusions The distribution and/or orientation of included minerals, termed "inclusion trails", within garnet, staurolite, kyanite and plagioclase porphyroblasts commonly define S^So, as well. Inclusions consist of quartz, opaques, mica, feldspar, sphene and zircon in order of decreasing abundance. Inclusion grain size varies from 0.1 to 1.0 mm, finer grained than identical matrix mineral phases. The nature of the internal fabric (Si) outlined by these "inclusion trails" and its relation to the matrix foliation are variable, dependant on the intensity, style and timing of post-D! deformation with respect to porphyroblast growth. Figure 20 illustrates the common Si-Sj/So fabric relationships observed. The discordance observed commonly between Si and S/So indicates noncoaxial strain and shear of matrix about porphyroblast during and/or after Si development In domain three, the D2 synformal zone, the parallelism of Si to the enveloping surface of crenulated Sj/So may indicate local coaxial strain conditions. STRUCTURE / 61 FIGURE 20. Schematic illustration of the relationships between porphyroblast inclusion trails (Si) and compositional layering-foliation (Si/S0), a) planar Si parallels Si/S0, b) planar Si is discordent with undulose and crenulated Si/S0> c) folded Si and undulose Sj/So, d) planar Si is discordent with Sj/So, but parallels the enveloping surface of D2 crenulations, e) folded Si follows the form of D2 crenulations. STRUCTURE / 62 3.2.2. Phase two Phase two microstructural elements are ubiquitous throughout the rocks of the map area. Mineral distribution, grain shape and orientation well define S2 and Lj in thin section. Microscopic evidence for increasing D2 strain towards the D2 synformal zone, domain three, is observed in the progressive increase of the penetrative character of S2 in pelitic rocks. Axial Planar Fabrics Three major types or gradations of S2 fabric are recognized. They are listed in order of increasing penetrative nature: 1) The axial surfaces of microfolds or crenulations define a nonpenetrative cleavage in domain one and the western part of domain two. 2) A penetrative, differentiated, cleavage (Williams, 1972) occurs in rocks proximal to and within the D2 synformal zone. 3) A penetrative cleavage defined by foliate mica is observed in regions of intense mesoscopic folding, commonly within domain three. 1) Crenulations Crenulation or microfold axial surfaces provide the only microscopic expression of S2 in the western part of the map-area. S2 is outlined commonly by anastamosing surfaces of kinked and broken mica grains within crenulation hinges. Both microfold amplitude and wavelength range from 5-30 mm. The spatial variation of crenulation geometry and style is identical to that observed for the D 2 mesoscopic folds of this region. Crenulations were studied in several thin sections, collected from domains one STRUCTURE / 63 and two, oriented perpendicular to both Sj/So and S2 surfaces. Limb regions consist dominantly of biotite and muscovite, while quartz and feldspar are more common in the hinge regions of these crenulations (Figure 21). Crenulation wavelength decreases by more than a factor of two from west to east across these two domains. Coincident with the tightening of these microfolds, the (quartz+feldspar): mica ratio increases in hinge regions and decreases in limb regions. The aspect ratio of quartz and feldspar, measured parallel to the trace of Sj/So, increases from 3:1 to about 5:1 in crenulation limbs while remaining approximately constant at 1:1 in hinge areas in this same direction. In these microfolds, mica grains are kinked and broken indicating that little recovery or recrystallization of Sj/So schistosity has occurred after crenulation development The trend towards enhanced mineral differentiation and quartz and feldspar grain shape distortion with the progressive tightening of microfolds indicates the activity of a dissolution mechanism (Durney, 1972; Williams, 1972). In this process, the more soluble grains, quartz and feldspar, are dissolved in high stress regions and transported in solution to precipitate in areas of lower stress (pressure solution: de Boer, 1977). High stress regions are commonly intergrain contact points and crenulation limbs, where compositional layering makes a large angle with the compressive stress direction. As solution transfer removes material from limb regions, the relatively insoluble mica grains rotate passively towards the crenulation axial surface (Robin, 1978). The combined activity of dissolution and mica rotation creates the observed mineral differentiation S2 layering. A dissolution-type cleavage (Durney, 1972) depicted by insoluble residue, often marks solution pathways. Such a cleavage defines S2 in domain two proximal to the synformal zone, outlined by very fine grained opaques, mica and rare fibrous quartz STRUCTURE / 64 FIGURE 21. Plane light photo negative of D 2 crenulations in thin section illustrating the preferential distribution of quartz and feldspar in hinge regions and biotite and muscovite in limb regions, sample 262, unit 3. Bar scale is 5 mm. STRUCTURE / 65 (Figure 22). This cleavage forms dark anastomosing seams, less than 50 microns wide, which branch locally to form zones up to 0.4 mm thick within mica-rich limb regions of tight crenulations. These opaque-rich seams truncate mica and isolate hinge from limb domains (Figure 22). Cleavage seams are discontinuous and often en echelon on the mm scale, displaying a consistent sense of stepping. 2) Differentiated Cleavage The term differentiated cleavage (Williams, 1972; Hobbs et al., 1980) corresponds to the occurrence of distinct mineralogic domains, in this case parallel to S2. These domains are composed of alternating quartzo-feldspathic and micaceous layers which correlate to the hinge and limb regions of the previously described crenulations (Figure 23). Si/S0 schistosity is truncated by domain boundaries, perhaps representing advanced stages of the dissolution process. Spacing of this differentiated cleavage ranges from 1 to 5 mm, largely a function of relict crenulation wavelength and rock mineral proportions. Sj parallel mica flakes crosscut Si/S0 foliation locally and further define S2 in both quartzo-feldspathic and micaceous cleavage domains (Figure 23). These "cross-micas" are unstrained relatively and of equal or larger grain size than Si/S0 mica grains. The crystallization of these micas is interpreted to be synkinematic with respect to the development of S2. 3)S2 Mica Foliation An S2 mica foliation, representing transposed S]/S0 schistosity, occurs locally within pelitic schists displaying intense mesoscopic folding adjacent to and within the synformal zone (Figure 24). Microscopic examination shows this fabric to be composed of a series of tightly spaced microlithons, 0.3-1 mm thick (Figure 24). The STRUCTURE / 66 FIGURE 22. Photomicrographs of D2 dissolution-type cleavage, outlined by anastamosing seams containing very fine grained opaques, and mica in sample 244, unit 3, a) plane light, b) crossed nicols. Bar scale is 2 mm. STRUCTURE / 67 a FIGURE 23. D 2 differentiated cleavage is defined by alternating quartzo-feldspathic and micaceous domains and mica orientation in sample 393, unit 3, a) photomicrograph (crossed nicols), b) labelled sketch. Bar is 2 mm. Note the discontinuity of foliation between mineralogic domains and the distribution of S2-parallel mica across S,/So mica foliation. STRUCTURE / 68 FIGURE 24. S2 mica foliation in sample 289, unit 3, a) plane light photo negative of a thin section illustrating the penetrative nature of this cleavage, bar scale is 5 mm, b) photomicrograph (crossed nicols) showing S2 to be transpositional in character, composed of a series of tightly spaced microlithons, bar scale is 2 mm. STRUCTURE / 69 microlithons consist of alternating mica domains of two distinct populations: 1) Mica flakes oriented parallel to domain boundaries and 2) flakes canted 20° to these S2 parallel surfaces. Fine grained mica in the more quartzo-feldspathic regions of these rocks show a strong S2 orientation as well. The nature of this transposition is so intense locally that the only observed relict S/So is limited to "inclusion trails" internal to porphyroblasts. Pressure Shadows In rocks containing mechanically competent objects uneven strains are often set up around the site of competency contrast Pressure induced dissolution and crystallization of soluble mineral species, such as quartz and feldspar, may occur in an identical manner described in the discussion of dissolution-type cleavage. If the boundary between the competent object and the rock matrix is mechanically weak, the matrix may be pulled away from the object in a direction parallel to the local maximum elongation (1 + eO direction. In this site, fibrous growth often occurs coincident with the direction of dilatancy (Durney and Ramsay, 1973). Pressure shadows composed dominantly of quartz, with or with out feldspar and mica are observed adjacent to porphyroblasts in thin sections oriented normal to Sj/So, both parallel and perpendicular to 1^  (Figure 25). Two sets are recognized: 1) parallel to 1^  and 2) normal to L2, coincident with the S2 dip direction. L? parallel pressure shadows are ubiquitous throughout the area, while down-dip pressure shadows occur proximal to and within the synformal zone, associated with differentiated cleavage and/or S2 mica foliation. Both sets formed during D2 and may be coeval, although the timing of growth of one with respect to the other is uncertain. Pressure shadow mineral grain size is less than that of corresponding matrix mineral phases and often increases outward from porphyroblast walls, consistent with STRUCTURE / 70 FIGURE 25. A symmetr ic D 2 pressure shadow, composed of quartz and mica , occurs adjacent to garnet para l le l to Lj i n sample 38, uni t 3, a) p lane l ight, b) crossed nicols Bar scale is 1 m m . STRUCTURE / 71 growth in that direction (Durney and Ramsay, 1973). Pressure shadow quartz is fibrous rarely, showing undulose extinction and polygonal grain boundaries, indicating strain and recrystallization subsequent to pressure shadow formation. Maximum length exceeds 2 cm, adjacent to garnet porphyroblasts of similar diameter. Pressure shadows are both symmetrically and asymmetrically disposed, with those parallel to Lj indicating a northwest over southeast sense of shear of matrix with respect to porphyroblast (Figure 20 c). Microfractures Microfractures in porphyroblasts, oriented normal to Lj, are associated with pressure shadows commonly and filled with the identical mineral phases (Figure 26). Fractures range from 0.1 to 1 mm wide and have sharp irregular to planer boundaries. Internal quartz is strained, lacking recognizable fibrous growth. Fracture initiation was likely in response to the competency contrast set up between porphyroblast and matrix, where rather than the physical separation of one from the other, as required for the development of a pressure shadow, the porphyroblast itself was brittlely extended. Fracture propagation is inferred to have been driven by the availabity and activity cf dissolved species to the leading edge of the fracture, as discussed in the theory of stress corrosion cracking (Anderson and Grew, 1977). Mineral lineations The most penetrative mineral lineations are defined by the orientation of elongate quartz and calcite grains. Quartz grains have axial ratios up to 7:1 while those of calcite exceed 10:1. Deformation lamellae and kink bands are well developed in quartz and mechanical twinning and kinking are observed in calcite (Figure 27). The kink bands in quartz form high angles with grain boundaries and are often STRUCTURE / 72 FIGURE 26. Photomicrographs (crossed nicols) of microfractures in garnet and quartz, a) microfracture in garnet is filled with quartz, feldspar and minor mica, sample 395, unit 3. Bar scale is 2 mm. b) healed microfractures in quartz outlined by the planar distribution of spherical and cylindrical voids in sample 367, unit 3. Bar scale is 0.5 mm. STRUCTURE / 73 FIGURE 27. Photomicrographs (crossed nicols) of strained quartz and calcite, a) deformation lamellae in quartz form high angles to kink band boundaries in sample 506, unit 3, bar scale is 0.5 mm, b) kinked twin lamellae in calcite porphyroclast in marble mylonite, sample 241, unit 3b, bar scale is 2 mm. STRUCTURE / 74 associated with healed microfractures, outlined by cylindrical and spherical voids (Figure 26) (Wanamaker and Evans, 1985). The dominant mechanism associated with grain elongation is inferred to be that of dislocation glide. The local presence of small quartz neoblasts within quartz grains and along sutured grain boundaries (Figure 28) indicates dynamic recrystalliztion has occurred by bulge nucleation (Nicolas and Poirier, 1976). In this process, grain boundary migration proceedes in the direction of highest dislocation density so as to reduce the maximum amount of internal strain energy (Nicolas and Poirier, 1976). STRUCTURE / 75 FIGURE 28. Photomicrograph (crossed nicols) of neoblasts in strained quartz and along grain boundaries, illustrating that dynamic recrystallization has occurred by grain boundary migration. Sample 506A, unit 3. Bar scale is 0.5 mm. 3.2.3. Subsequent deformation STRUCTURE / 76 Microstrucural elements related to D3 and D 4 consist primarily of open to tight crenulations of Si/S0 schistosity, which reorient earlier deformational fabrics. The mica grains involved are kinked and fractured, displaying little sign of post kinematic polygonalization. The detachment of D4 mica crenulations from quartz-rich compositional layering and S^So parallel quartz veins occurs where D4 is well developed (Figure 29). Microfractures of porphyroblasts, less than 1 mm wide, are oriented normal to L2/L3 and filled with chlorite and/or fine grained white mica. These microfractures crosscut D 2 pressure shadows locally and are inferred to be related to D3. STRUCTURE / 77 FIGURE 29. D 4 mica crenulations in sample 007, unit 3, a) crossed nicols photo negative of D 4 crenulations and buckled quartz veins in thin section, bar scale is 1 mm, b) photomicrograph (crossed nicols) displaying detachment of D 4 crenulation from folded quartz vein, bar scale is 2 mm. STRUCTURE / 78 3.3. DISCUSSION Si/S0 compositional layering and schistosity and D) intrafolial folds are reported by numerous workers to represent the earliest recognizable deformational event in the Omineca Belt rocks of the Quesnel Lake region (Ross et al., 1985; Fillipone, 1985; Elsby, 1985; Montegomery, 1985; Ross et al., 1987). The nature of this regional deformation is poorly understood due to the obscurring and modification of Di structures by subsequent deformation. In the Niagara Peak map-area, Di folds are most widely recognized in domain one, where D2 fold structures are less well developed than in other structural domains. Di isoclines, outlined by quartz veins in pelitic schist and compositional layering in impure marble, are transposed into and dismembered along Si. This transposition and dismemberment, coupled with the curvilinear distribution of Li fold axes within Si, indicate the reactivation of this surface by shear during subsequent deformation (Hobbs et al., 1980). The systematic variation in D2 mesoscopic fold style and S2 fabric intensity across the map-area implies the localization of strain in the D 2 synformal zone west of Niagara Peak. This zone is characterized by upright map-scale D2 folds, bimodal mesoscopic fold style, a locally penetrative S2 cleavage and "down-dip" S2 pressure shadows. The profiles of the tight to isoclinal folds common in map-scale fold hinge regions are interpreted to indicate their modification by homogeneous flattening (de Sitter, 1958; Ramsay, 1962 and 1967). Upon the limbs of these same map-scale structures, the nature and profile of disharmonic folds and the presence of layer-parallel detachments imply buckle fold formation by a flexural-slip mechanism (Ramsay, 1967). The role of superimposed flattening in the development of these folds is inferred to be smaller than that for the first fold type. STRUCTURE / 79 This contrasting fold style is inferred to indicate coaxial deformation of hinge regions, producing high flattening strain, and noncoaxial deformation of limb regions, giving way to layer-parallel shear. These fold styles likely also indicate variations in lithologic viscosity contrast throughout D2 deformation, where low viscosity contrasts allow homogeneous flattening and high contrasts favour buckling (Parrish et al, 1976). Thus the pressure, temperature and fluid activity condtitions of the D2 synformal zone rocks probably played a large role in the localization of strain and the style of deformation in this area. L 2 parallel mineral lineations and pressure shadows imply the coincidence of local l+e^ direction of the finite strain ellipsoid with D 2 fold axes. This direction need not represent regional l + e!, but more likely indicates the projection of this vector, into S^ So, due to compositional constraints. By assuming that S2 approximates the XY-plane of the finite strain ellipsoid (Ramsay, 1967; Williams, 1976; Gray and Dumey, 1979), the intersection of this surface with S]/S0 will not only define L,, but 1 + ej as well. An independent indicator of the phase two 1 + ej direction is provided by the determination of a2, the shear direction, for the formation of similar style D2 folds (Ramsay, 1960 and 1967). The orientation of the a-direction determines l + e! in the rocks deformed; these two vectors parallel one another. In domain one, the locus of deformed Li fold axes lie about a great circle zone which coincides with local Si/S0. An a2 direction of 08/350 is determined from the intersection of this locus and the average of S2 for this domain (Figure 30). This direction is coincident with domain one l^. indicating the local parallelism of the X-direction with D 2 fold axes during deformation. D2 microstructure indicates that deformation was dominated by mechanisms of STRUCTURE / 80 L 1 - 5 0 domain one FIGURE 30. Determination of D 2 fold shear direction (a2) in structural domain one. Lj locus is constructed by eye to fit the data contoured after the method of Kamb (1959). Contour intervals are 15%, 25%, 30% and 35%. STRUCTURE / 81 grain boundary solution, microfracturing, dislocation glide, with dynamic recrystalliztion and mineral growth playing a minor role. The direction of pressure-induced solution transport, inferred from pressure shadow and crenulation cleavage orientation, was contained within S2, coinciding with both the strike and dip directions of this surface. The open profiles of D3 and D4 mesoscopic folds, the presence of D3 microfractures and the layer-parallel detachment of D4 mica crenulations indicate semi-brittle deformation conditions characterized by high viscosity contrast The lack of recognizable D3 and D4 strucures in domains three and four, where D2 structures are abundant, may indicate the geometric hardening of these regions during phase two. In the case of D3, this spatial relationship probably indicates the tightening of earlier structures by fold reactivation, instead of the development of phase three folds and crenulations. The small displacements of late brittle normal and minor reverse faults indicate that only minor amounts of differential uplift took place after D4 in the map-area. The imbricate nature of some of these faults (Figure 19) likely indicates that they are associated with a detachment surface(s) at depth, upon which extension of the structurally overlying package occurred. The mechanical anisotropy necessary to form such a detachment surface is likely a function of lithologic heterogeneity, and perhaps associated with the contact between overlying metasedimentary rocks and an underlying basement of granitic gneiss. 4. METAMORPHISM Lithologies of the Niagara Peak map-area have been affected by two major metamorphic events. The earlier event (Ml) is Barrovian and responsible for the generation of the sillimanite isograd used to define the boundary of the Shuswap Metamorphic Complex (SMC) (Reesor, 1970; Okulitch, 1984). The origin of this event has been ascribed to the middle Jurassic accretion of the Intermontane Belt (Monger et al., 1982; Okulitch, 1984). The later event (M2) is more restricted in nature and is characterized by extensive retrogression of the prograde Ml mineral assemblage in the western part of the map-area. Mineral assemblages within pelitic rocks affected by these two metamorphic events define five northwest trending zones (Figure 31). Previous work on the SMC in this vicinity has been performed along its eastern flank. Fletcher (1972) mapped adjacent to the map-area in the Penfold Creek region; Engi (1984) worked approximately ten km to the south, east of Niagara Creek; and Pigage (1976) carried out his studies in the vicinity of Azure Lake. The northern part of the SMC is one of three metamorphic culminations present in the Quesnel Lake region. Sillimanite isograds are mapped in the Three Ladies Mountain-Mount Stevenson and Boss Mountain areas (Getsinger, 1985; Fillipone, 1985) and may represent erosional windows into the SMC 82 METAMORPHISM / 83 FIGURE 31. Distribution of the five metamorphic zones in the Niagara Peak area, including the locations of microprobe samples used to estimate metamorphic temperature and pressure. M2 sericite-chlorite retrogression of Ml porphyroblasts is restricted to the western part of the map-area, southwest of the dotted line. METAMORPHISM / 84 4.1. M l - JURASSIC PROGRADE M E T A M O R P H I S M Four major prograde metamorphic zones are defined by the presence or absence of staurolite, kyanite and sillimanite within metapelitic rocks. The zones include: kyanite-staurolite, sillimanite-kyanite-staurolite, sillimanite-kyanite and sillimanite from southwest to northeast (Figure 31). The three easternmost zones represent the breakdown of garnet, staurolite, kyanite and muscovite in favor of sillimanite. Isograds separate these zones from one another, with the exception of the "staurolite-out" line, as it most likely represents changes in rock composition in addition to the breakdown of staurolite. Prograde regional metamorphism is inferred to have reached greenschist grade late in Staurolite and kyanite growth accompanied D2 followed by post kinematic sillimanite generation. The timing of metamorphic mineral growth with respect to deformation is inferred from textural relationships and is summarized in Figures 32 and 33. Biotite In metapelites biotite is medium to coarse grained, lepidoblastic, and defines Si/S0 schistosity. S]/S0 is bent and wrapped about all porphyroblast phases. Biotite occurs also in D2 pressure shadows and as fine grained overgrowths of porphyroblasts which clearly post-date pressure shadow formation. In sillimanite grade rocks, fine to medium grained biotite crosscuts Si/S0, paralleling local S2 surfaces. These textural relationships indicate biotite growth was coeval with the first two phases of deformation and outlasted D2. Biotite occurs with porphyroblast and matrix mineral phases in rocks of all metamorphic grades. The extent to which biotite replaces garnet, staurolite, kyanite and plagioclase, increases with increasing metamorphic grade. PHASE DEFORMATIONAL FABRIC MINERAL GROWTH D1 SYN: Biotite / Muscovite schistosity P O S T : S 1 / S 0 _ — ' Plagioclase D2 MW 'S2 SYN: Bkrtite/Muscovite ~~, / S2 - — s v s o -7 [ > K & $ ^ ^ POST - Kyan'rte/StauroBte ' Kyanite/Stauroite / / / S2 ^mw^ ^M$mw 7 S 2 S2, *y\tfty Plag D3 ^ • " T " ^ ^ — ' " s i / S O 1 S3 | S Y N : SencHe A r - 0 ^ ^ _ s3smx3E23s&&z>— y " — • s i /so — — T S1/S0 FIGURE 32. Schematic illustration of the relationships between metamorphic mineral growth and deformation in the pelitic rocks of the Niagara Peak area. METAMORPHISM / 86 Mineral D1 syn post D2 syn post D3 syn post Chlorite Muscovite Biotite (Sericite) Plagioclase Garnet Staurolite Kyanite Sillimanite observed — inferred FIGURE 33. Summary of the relative timing of metamorphic mineral growth with respect to deformation in the pelitic rocks of the Niagara Peak area. METAMORPHISM / 87 Muscovite Medium to coarse grained lepidoblastic muscovite occurs interleaved with biotite, also defining S^So. S2 parallel flakes and nonorientated laths which cross cut local S2 surfaces occur more commonly in higher grade rocks. Muscovite growth is inferred to have been synchronous to that of biotite. Muscovite decreases in abundance with increasing metamorphic grade. In the kyanite-staurolite zone, muscovite occurs in contact with all minerals, with the exception of garnet, comprising 25-30% of the assemblage. In the sillimanite zone, muscovite is a minor constituent (<5%), distributed in the rock matrix separate from all porphyroblasts and as overgrowths of staurolite, kyanite and/or plagioclase. Plagioclase Two populations of plagioclase are recognized based on textural relations and composition. 1) Helicitic plagioclase occurs as 3-10 mm long xenoblastic crystals including quartz, opaques and mica, which define planar S^So. Plagioclase occurs in D 2 presssure shadows and as embayed fragments surrounded by biotite and muscovite (Figure 34). Growth is inferred to have been continuous througout D i and D 2 . Optically determined composition ranges from An3J.,7 (Table 4, Appendix A) in the same thin section, regardless of metamorphic grade. Several of the larger grains are reversely zoned with rim compositions of An56.65, representing growth under prograde conditions ( D e e r , Howie and Zussman, 1981). 2) Idioblastic and/or equant, finely twinned, 0.3-1 mm diameter plagioclase is associated with garnet, staurolite and kyanite (Figure 35). Plagioclase occurs in sillimanite grade rocks with matrix quartz, mica and fibrolite, where the composition is albitic (An8.12) (Table 4). The growth of idioblastic plagioclase overlaps in time with METAMORPHISM / 88 FIGURE 34. Two populations of plagioclase are recognized on the basis of size, occurrence and crystal form as illustrated by these photomicrographs (crossed nicols). a) Helicitic plagioclase porphyroblast is embayed by biotite and muscovite in sample 456, unit 3. Bar is 2 mm. b) very fine grained equant albitic plagioclase occurs with quartz and biotite adjacent to resorbed garnet in sample 370A, unit 3. Bar is 1 mm. METAMORPHISM / 89 that of the first population, but extends past the development of D2 microstructures. Plagioclase abundance increases from 5% to 20% from west to east accross the area. Garnet Two populations of garnet are recognized on the basis of crystal form and textural relationships. 1) First generation, dark red, xenoblastic garnet (5-10 mm in diameter) includes quartz, opaques and biotite. The size of inclusions increases from core to rim, indicating growth under prograde conditions (Spry, 1983). Trails of inclusions outline surfaces, kinks and open microfolds of relict Si/S0. Displaced schistosity and D2 pressure shadows occur adjacent to this porphyroblast Garnet growth is inferred to be pre and syn D2, since it includes both planer and deformed Si/So. In the kyanite-staurolite zone, garnet appears partially resorbed and replaced by the assemblage: biotite, quartz, plagioclase, minor muscovite and opaques. In rocks of sillimanite grade, this assemblage includes abundant fibrolite. The preferential replacement of garnet cores leaves relict rims ("atoll garnet") (Figure 35). Garnet consumption becomes more complete with increasing metamorphic grade. 2) Second generation garnet occurs as blocky overgrowths on first generation garnet and as tiny 0.5-2 mm diameter isolated idioblastic crystals (Figure 36). Both overgrowths and small crystals lack inclusions and do not displace S^So mica. This lack of displacement and the overgrowth and replacement of first generation garnet by this population, implies that growth occurred during the late stages of, or after, D2. Isolated second generation garnet is part of the replacement assemblage after xenoblastic garnet and staurolite pophyToblasts and is associated with matrix biotite, muscovite and sillimanite in the semi-pelitic rocks of the sillimanite-kyanite and METAMORPHISM / 90 FIGURE 35. Photomicrographs (plane light) and sketch showing the two generations of garnet observed in metapelitic rocks, a) first generation resorbed garnet exhibiting "atoll garnet" texture in sample 395, unit 3. Bar is 2mm. b) second generation blocky garnet overgrowth of first generation garnet in sample 370A, unit 3. Bar is 2 mm. c) second generation idioblastic garnet occurs in the site of replaced first generation garnet with biotite, fibrolite and quartz in sample 262, unit 3. Bar is 0.2 mm. METAMORPHISM / 91 METAMORPHISM / 92 sillimanite zones. Second generation garnet occurs only in rocks of sillimanite grade. Staurolite Staurolite is brown to rootbeer orange and occurs as 4-60 mm long tabular to hexagonal prisms. Crystal orientation moderately defines Lj. D2 pressure shadows and displaced schistosity occur adjacent to staurolite in numerous localities, but are absent in others. Inclusion trails outline planar Si/S0 and D2 microfolds. Many of these folds show amplitude/wavelength ratios less than those displayed by D2 crenulations of the matrix schistosity. Staurolite crosscuts S2 crenulation cleavage surfaces locally. Thus staurolite growth occurred throughout and after the formation of D2 microstructures. Both staurolite and kyanite are kinked about D3 crenulations in the kyanite-staurolite zone. Staurolite occurs with all minerals and forms coronas about resorbed garnet locally (Figure 36). Mineral abundance decreases through the sillimanite- kyanite- staurolite zone to become exceedingly rare in higher grade rocks. Coincident with this trend is the progressive increase in the extent of staurolite replacement by biotite, muscovite and in rocks of sufficient grade, fibrolite. This replacement has nearly gone to completion in some samples, where ragged isolated staurolite grains are enclosed in fibrolite-muscovite mats (Figure 37). Kyanite Pale sky blue prismatic blades of kyanite reach lengths up to 50mm. Kyanite has the same relationship to deformation as described for staurolite. Kyanite and staurolite porphyroblasts enclose garnet. Polycrystalline masses of 0.5 mm long idioblastic kyanite are associated with quartz, bioite and muscovite adjacent to partially resorbed plagioclase and garnet porphyroblasts in sample 262 (Figure 38) and METAMORPHISM / 93 FIGURE 36. Coronas of staurolite, biotite and quartz partially enclose first generation garnet in sample 130, unit 3, a) photograph, b) labelled sketch. Bar is 1 cm. METAMORPHISM / 94 FIGURE 37. Resorbed staurolite admist dense fibrolite-rich mats in sample 442, unit 3, a) photomicrograph (plane light), b) labelled sketch. Bar is 1 mm. METAMORPHISM / 95 occur with all matrix minerals in the hinge regions of D 2 crenulations. Epitaxial growth of kyanite with its (100) face adjacent to the (010) face of staurolite is observed rarely. The replacement of kyanite porphyroblasts by muscovite and minor biotite increases toward the east In the sillimanite zone, resorbed remnants of kyanite are surrounded by biotite, quartz, sillimanite, plagioclase and minor opaques. Sillimanite Sillimanite is found with biotite in the post D2 mineral assemblage that mantles resorbed porphyroblasts. It occurs along kink bands in S/So muscovite and lies across S2-parallel mica flakes. Sillimanite growth took place after the deformational peak of D2, likely under relatively "static" conditions. The L 2 mineral lineation defined by prismatic needles in the high grade rocks near Niagara Peak is believed to be mimetic upon a mica edge lineation. It is not taken as evidence for synkinematic growth. With increasing metamorphic grade fibrolite replaces first generation garnet, staurolite and kyanite and becomes more abundant within matrix mineral phases. Complete replacement of garnet and staurolite by massive fibrolite is observed locally with only small amounts of muscovite and/or biotite present. The first occurrence of fibrolite in the matrix of metapelites is in the eastern sillimanitv"- kyanite- staurolite zone. There, it forms . needles in biotite and muscovite adjacent to porphyroblasts (Figure 39), and nearly monomineralic "horsetails" in the presence of quartz, albitic plagioclase and minor mica. METAMORPHISM / 96 FIGURE 38. Masses of tiny, idioblastic, kyanite associated with quartz., biotite and muscovite adjacent to first generation garnet in sample 262, unit 3, a) plane light, b) crossed nicols. Bar is 1 mm. METAMORPHISM / 97 FIGURE 39. Fibrolite-opaque mineral mat within muscovite adjacent to xenoblastic kyanite porphyroblast in sample 456, unit 3, a) plane light, b) crossed nicols. Bar is 2 mm. METAMORPHISM / 98 4.2. Ml METAMORPHIC REACTIONS - METAPELITES Mineral reactions have been deduced for each metamorphic zone based on the observed mineral relations. Many of these reactions are written employing the scheme of Carmichael (1969), where sub-reactions are constructed assuming that the metamorphic system is open to mass transfer on the scale of a thin section, allowing cation exchange from one sub-reaction domain to another. The following balanced reactions represent the sum of these sub-reactions or reaction mechanisms and assume end member compositions in the system KjO-FeO-AljOj-SiOj-HjO (KFASH after Thompson, 1976), unless otherwise indicated. Both balanced reactions and sub-reactions are unique in terms of the participating mineral phases, but not for the reaction coefficients and mobile cations that appear in these equations. General mineral formulae and abbreviations are listed in Table 3 (Appendix A). Kyanite and staurolite are inferred to have formed at the expense of chlorite, muscovite and garnet The generation of sillimanite is dependant on several reactions that lead to the sequential destruction of garnet, staurolite, muscovite and kyanite. The reaction mechanism(s) for the formation of second generation garnet is somewhat uncertain, but is perhaps related to the consumption of garnet and/or staurolite porphyroblasts. METAMORPHISM / 99 Kyanite-Staurolite Zone Assemblage: QZ, MS, BI, PL, GT, ST, KY, CHL, SER, OP ± TO, SP, AP, ZI. Textural relationships indicating staurolite and kyanite forming reactions are lacking; however, the presence of both phases within muscovite-rich zones and their local distribution adjacent to partially resorbed first generation garnet al.lows for the construction of some possible reactions. The absence of prograde chlorite in kyanite-staurolite schists and the occurrence of these porphyroblasts in Sj/So mica is consistent with staurolite and kyanite formation by two possible chlorite consuming reactions: 1) 41MS + 31CHL = 8ST + 41BI + 33QZ + 108 H 20 (modified from Hoscheck, 1969; Thompson and Norton, 1968). 2) 5MS + 3CHL = 8KY + 5BI + 1QZ + 12H20 The abundance of muscovite in kyanite-staurolite schists indicates that chlorite, not muscovite, served as the limiting factor to the extent of these reactions. Coronas of staurolite, biotite and quartz about garnet in sample 130 (Figure 36) suggest a reaction relationship. These coronas presently exclude the garnet from contact with matrix minerals. A reaction that may explain this texture is: 3) 9MS + 4CHL + 5GT = 2ST + 9BI + 13QZ + 12H20 Thus as reaction 3 proceeds toward chlorite exhaustion, garnet is preserved by coronas composed of product mineral phases. Idioblastic sub-0.5 mm kyanite masses occur with quartz, biotite and muscovite adjacent to biotite embayed garnet (Figure 38) in the rocks proximal to the sillimanite isograd. These textures suggest the activity of two possible sub-reactions: 4a) 1GT + 3QZ + 6H20 + 2K+ + 3Fe2t = 2BI + 8H* 4b) 1BI + IMS + 8H+ = 2KY + 4QZ + 6H20 + 2K+ + 3Fe2+ METAMORPHISM / 100 These reaction mechanisms are constructed to balance mass and charge, conserve aluminium in the solid phases and approximate constant volume as reactants form products (Carmichael, 1969). The role of plagioclase in these reactions is not texturally clear and hence it has been excluded. The cations and H 20 produced from one sub-reaction serve as the reactants for the other. When combined, these sub-reactions represent the closed system reaction: 4) IMS + 1GT = 2K.Y + 1BI + 1QZ No realiable textural evidence exists for the generation of kyanite through the consumption of staurolite. In this respect, the Ml metamorphism outlined in this region differs from that described for a typical Barrovian type facies series (Winkler, 1979). Sillimanite-Kyanite-Staurolite Zone Assemblage: QZ, BI, MS, PL, GT, KY, ST, SILL, OP ± CHL, SER, TO, SP, AP, ZI. Textural evidence suggests that sillimanite was generated by several reactions involving the breakdown of garnet and staurolite. The first appearance of fibrolitic sillimanite in the biotite-rich mineral assemblage which embays garnet and the progressive decrease of muscovite throughout this metamorphic zone is consistent with the reaction: 5) IMS + 1GT = 2SILL + 1BI + 1QZ This reaction has been proposed by numerous workers for sillimanite formation within the metapelitic rocks of the SMC and outlying metamorphic culminations (Engi, 1984; Fletcher, 1972; Getsinger, 1985; Pigage, 1978). Although the form of this reaction is identical to that of equation 4, with the exception of the aluminosilicate polymorph, the reaction mechanisms are different and more complex. Plagioclase occurs not only as a product phase in the replacement of garnet, but as a reactant where it is embayed METAMORPHISM / 101 by matrix biotite (Figure 34). It is necessary for the amount of plagioclase generated in one sub-reaction to balance that consumed in another, for plagioclase can not appear in the final equation due to the limited amounts of Na20 and CaO that are tied up in the other participating mineral phases. In the eastern part of this metamorphic zone, the replacement of staurolite by fibrolite, biotite and minor muscovite and the distribution of sillimanite in Si/S0 muscovite may indicate the staurolite consuming reaction: 6) 8MS + 17QZ + 6ST = 62SILL + 8BI + 12H20 The local persistance of staurolite in rocks of higher metamorphic grade may be due to the chemical isolation of these grains from reactant matrix muscovite by fibrolite-rich mantles. Tiny idioblastic second generation garnet occurs in the presence of fibrolite in the sites of resorbed first generation garnet and staurolite. The occurrence of this idioblastic garnet is limited. Its distribution indicates that it may have formed at the expense of earlier garnet porphyroblasts, by a modification of reaction 5, and/or from a staurolite breakdown reaction similar to that of reaction 6. A common garnet forming reaction involving the breakdown of staurolite is: MS + QZ + ST = SILL + GT + BI + H 20 (Thompson and Norton, 1968; Carmichael, 1969). This reaction involves seven phases and is described by a minimum of five oxide components. It is therefore invariant and does not satisfy the phase rule for a univariant reaction in the KFASH system. An alternative reaction which explains the observed textures is: 7) 25QZ + 6ST = 46SILL + 8GT + 12H20 The lack of reactant muscovite in this equation may reflect its consumption by other coeval sillimanite generating reactions. The combination of reactions 6 and 7 produces METAMORPHISM / 102 the assemblage for which the Thompson and Norton (1968) reaction is written. Sillimanite-Kyanite Zone Assemblage: QZ, BI, PL, SILL, MS, KY, GT, OP, ± ST, TO, SP, AP, ZI. Sillimanite forms by the break down of muscovite in this zone. The abundance of sillimanite within Si/S0 mica in the quartzo-feldspathic and pelitic schists of the sillimanite-kyanite zone increases from west to east This trend and the parallel decrease in muscovite abundance is consistent with the production of sillimanite by the classic reaction: MS + QZ = SILL + K.SP + H 20 This reaction requires the production of potassium feldspar to balance the equation in terms of the K 20 in muscovite, but potassium feldspar is absent from these rocks. The first appearance of very Fine grained idioblastic albitic plagioclase (An ,_12) with sillimanite in the presence of Si/S0 mica suggests the alternate reaction with Na end member phases: 8) 1QZ + lParagonite = 1SILL + 1ALB + H 20 The measured paragonite component of white mica ranges from 15-20%. Fletcher (1972) has proposed the identical reaction to explain the decrease in the paragonite content of the muscovite solid solution in his sillimanite grade rocks. Guidotti (1970) documents a greater than 10% drop in the paragonite content from his lower sillimanite zone to the sillimanite-potassium feldspar zone of Evans and Guidotti (1966) in Northwestern Maine, which led him to suggest the reaction: 8a) Na Muscovite + QZ = SILL + ALB + K-richer Muscovite + H 20 This reaction involves K 20 and is balanced in terms of the muscovite solid solution. It explains the same textures on which reaction 8 is based. METAMORPHISM / 103 Sillimanite Zone Assemblage: QZ, PL, BI, SILL, OP, MS ± KY, ST, TO, SP, AP, ZI. The replacement of kyanite by sillimanite in the pelitic rocks proximal to the boundary between the sillimanite- kyanite and sillimanite zones indicates the polymorphic transformation: 9) IKY = 1SILL Sillimanite is found rarely to be in contact with kyanite, but is more often associated with the muscovite pseudomorphs after kyanite (Figure 39). This texture is consistent with the combination of sub-reactions: 9a) 3KY + 3QZ + 3H20 + 2K+ = 2MS + 2H+ 9b) 2MS + 2H+ = 3SILL + 3QZ + 3H20 + 2K+ (Carmichael, 1969). As in reaction 4, the cations and H 20 generated from one sub-reaction serves as the reactant phases for the other. 4.3. CONDITIONS OF METAMORPHISM METAMORPHISM / 104 Estimates of temperature and pressure may be made by employing the bathozone scheme of Carmichael (1978), based on the observed metamorphic mineral assemblages. Figure 40 illustrates the schematic AFM assemblage of all four metamorphic zones and their Bathozone designation in P-T space. These metamorphic mineral assemblages are consistent with Bathozones 4 and 5. Bathozone boundaries are defined by the invariant points generated by a series of reactions that take place commonly in metapelitic rocks. Placement of the two dehydration reactions in Figure 40 is dependant on the relative activity of reactant and product phases. Thus bathozone locations are only approximations based on experimental and theoretical calibrations. The assemblage garnet-biotite-kyanite-sillimanite-quartz-ilmenite is used to estimate the temperature and pressure conditions attending metamorphism. The following experimentally calibrated equilibria were used to calculate metamorphic temperature and pressure: la) Kyanite = Sillimanite lb) Andalusite = Kyanite lc) Sillimanite = Andalusite (Holdaway, 1971). 2) Annite + Pyrope = Phlogopite + Almandine (Ferry and Spear, 1978). 3) Almandine + 3Rutile = Al 2Si0 5 + 3Ilmenite + 2Quartz (Bohlen et al., 1983). Both 2 and 3 involve the calculation of garnet activity based on chemical composition. A number of garnet solid solution models have been proposed (Ganguly and Kennedy, 1974; Ganguly, 1979; Newton and Hasselton, 1981; Ganguly and Saxena, METAMORPHISM / 105 ' 1 1 1 1 1 1 1 500 600 700 T ( ° C ) FIGURE 40. Schematic illustration of the AFM assemblages for each metamorphic zone and their Bathozone designation in P-T space (after Carmichael, 1978; Archibald et al., 1983). METAMORPHISM / 106 1984). Those formulated by Ganguly (1979) and Ganguly and Saxena (1984) incorporate Ca and Mn solid solutions in addition to that of Fe and Mg. Newton and Hasselton (1981) allow for Ca-Fe-Mg mixing and ignore that of Mn. Calculations by Engi (1984) and Lewis (1987) illustrate that the temperatures estimated by the Newton and Hasselton (1981) modification of the Ferry and Spear (1978) garnet-biotite geothermometer, equilibrium 2, are more consistent with observed metamorphic mineral assemblages than other modifications. Temperatures calculated for three samples collected from the map-area using the equations of Ferry and Spear (1978), Ganguly and Saxena (1984) and Newton and Hasselton (1981) support the findings of Engi (1984) and Lewis (1987). For this reason, and since analyzed garnet rims have very little manganese (2-4 weight %), the Newton and Hasselton (1981) Ca-Fe-Mg mixing model was chosen to give the best estimate of metamorphic temperature. Equilibrium 3, the GRAIL geobarometer, has been modified using this same mixing model, so as to make the calculations of equilibria 2 and 3 internally consistent with one another. Electron microprobe analyses of garnet and biotite were performed on samples from the kyanite-staurolite (sample 94), sillimanite-kyanite-staurolite (sample 262) and sillimanite (sample 370A) zones (see Figure 32 and Table 6, Appendix B, for sample locations and Tables 5 and 7, Appendix B, for analytical procedures and results). Garnet is compositionally zoned commonly, with an increase of Fe, Mg and Mg/Fe at the rim and greater concentrations of Ca and Mn in the core. Second generation garnet overgrowths from the sillimanite zone have rim compositions identical to those of neighboring first generation garnets. Several garnets in the sillimanite zone show only negligible compositional differences from rim to core and may have been homogenized by intracrystalline diffusion (Tracey, 1982). To circumvent problems associated with the chemical zonation of garnet, only analyses of co-existing garnet METAMORPHISM / 107 rim-biotite pairs were used in the determination of temperature and pressure. The values obtained for these metamorphic conditions are inferred to correspond only to the very late growth and/or equilibration of these phases. Temperature Temperature estimates from the map area range from 510+17° C (one sigma from a mean of N data points) in the kyanite -staurolite zone to 632±25°C for second generation garnet in the sillimanite zone, assuming a pressure of 5 kb. Mean metamorphic temperatures are reported in Table 2. These temperature estimates further constrain the metamorphic conditions previously determined by Bathozone designation in Figure 40. The temperatures calculated for first generation garnet rim-biotite pairs in the sillimanite zone may represent: 1) the continued post-metamorphic equilibration of biotite with garnet rims on the down-grade side of the P-T path or 2) pre-metamorphic peak conditions correlative to the formation of second generation garnet by the discontinuous metamorphic reactions proposed in the previous section. Pressure The equilibrium assemblage garnet-rutile-aluminosilicate-ilmenite-quartz can be used to establish metamorphic pressure (GRAIL geobarometer, Bohlen et al., 1983). Rutile is not observed in the samples studied. This lack of rutile expands the stability of the low pressure assemblage (the right side of equilibrium 3). Therefore, the calculation provides maximum metamorphic pressures only. At the temperatures calculated, these pressures range from 6 to 7 kb (Table 2). These pressure estimates exceed those defined in P-T space by the Bathozone 4/5 transition and the sillimanite-kyanite univariant line of Holdaway (1971) for the calculated temperatures of the sillimanite-bearing samples, 262 and 370A. Sillimanite is absent from sample 94 METAMORPHISM / 108 TABLE 2 ZONE SAMPLE T(°C)±o (5Kb)1 P(Kb)2 N KY-ST 94 510±17 6.1 12 SILL-KY-ST 262 535120 7.1 11 SILL 370A 1*) 550+16 6.7 10 2*) 632+25 7.0 12 1 - one STD from a mean of N data points 2 - maximum pressure caculated from mean temperature 1*- first generation garnet 2"- second generation garnet METAMORPHISM / 109 and thus the GRAIL geobarometer places an upper constraint on the pressure of equilibration. P-T bounds determined by equilibriums 1, 2, and 3 are plotted for all three samples in Figure 41. Summary The samples analyzed from three of the four metamorphic zones indicate an increase in metamorphic temperature with increasing metamorphic grade. The "metamorphic peak" defined by the formation of sillimanite and second generation garnet in the sillimanite zone occurred at a minimum temperature of 635±25°C and at a pressure less than that of the Bathozone 4/5 transition (5.5 kb). METAMORPHISM / 110 8 7 ^ P(Kb) 5 -2\ / / / K y a n i t e ft' / ,1.1 / SSSmanite \ Andalus i te 5 0 0 6 0 0 TCC) 7 0 0 2 6 2 8 • 7 - j ^ 0 3 3 _ -6 • P(Kb) 5 • 4 • K y a n i t e Sfflmanite 3 . 2 • 1 • Andalus i te \ 5 0 0 6 0 0 TCC) 7 0 0 3 70A jgoae^-— ) A / .'< i i Mr V / B. • - 1 • J i i / W K y a n i t e ^ Mil. I1!1!' S B m a n i t e Andalus i te \ \ 5 0 0 6 0 0 TCC) 7 0 0 FIGURE 41. Pressure-Temperature bounds for the pelitic rocks of the Niagara Peak area determined from the three equilibria discussed in the text Sample numbers appear in upper right hand corner of each figure. METAMORPHISM / 111 4.4. M2 - POST JURASSIC RETROGRADE METAMORPHISM A retrograde metamorphic event has affected the westernmost rocks of the map-area (Figure 32). The partial replacement of Ml biotite, garnet staurolite and kyanite by sericite and chlorite characterize this region. Textural relationships indicate that the formation of these retrograde minerals occurred during and after phase three deformation (Figures 33 and 34). Timing of mineral growth with respect to D 4 is uncertain. Sericite Fine grained white mica (<0.1 mm) replaces staurolite and kyanite (Figure 42). The extent of replacement is in various degrees, from sub 1 mm thick rims to complete. Opaque inclusion trails are preserved in the white mica pseudomorphs after porphyroblasts. Relatively coarse grained white mica, less than 1 mm long, fringe fine grained mica mats and form internal zones locally. As much as 5% Fe-rich tourmaline (schorl, CJ = greenblue or green- brown, e = colorless or pale yellow) and sphene are included within sericite pseudomorphs and may be related to the replacement process. Chlorite Anomalous blue, length-slow chlorite forms rims around garnet and has replaced over 70% of the porphyroblasts locally. The preferential replacement of garnet core regions is observed in places. Masses of dominantly anomalous brown, length-fast chlorite mantle the sericite mats which enclose staurolite (Figure 42) and replaces matrix biotite along and across cleavage. Where chlorite has replaced biotite an anomalously high proportion of opaques occur as inclusions. Randomly oriented white mica laths, up to 1 mm long, lie within both compositions of chlorite about embayed METAMORPHISM / 112 Sericite-chlorite retrogression of staurolite. Masses of abnomalous brown chlorite mantle sericite replacement mats which enclose staurolite in sample Oil, unit 3, a) plane light, b) crossed nicols. Bar is 2 mm. METAMORPHISM / 113 porphyroblasts. Fibrous chlorite fills microfractures in garnet and staurolite. Fibers are oriented nearly normal to microfracture walls and appear symmetric about a medial surface (Figure 43). METAMORPHISM / 114 FIGURE 43. Fibrous chlorite within microfracture of garnet Chlorite fibers are oriented normal to fracture walls and symmetrically distributed about a medial surface characterized by extremely fine grained mineral phases in sample 506, unit 3, a) plane light, b) crossed nicols. Bar is 0.2 mm 4.5. M2 METAMORPHIC REACTIONS METAMORPHISM / 115 The observed textures are consistent with a number of coupled reaction mechanisms involving the generation of sericite and chlorite at the expense of kyanite, staurolite, garnet, biotite and perhaps muscovite. Sub-reactions are constructed following the methods previously outlined. Quartz is not always present in the described assemblage and hence what is written as Si02 in these reactions may represent an aqueous species. The presence of muscovite laths with chlorite surrounding garnet indicates the mechanism: 1) 8GT + 24H20 + 2K+ + Fe+ = 5CHL + 2SER + 3Si02 + 4H+ Sericite and chlorite shells surrounding embayed staurolite are consistent with the possible sub-reaction: 2) 16ST + 216H20 + 82K+ + 41Fe+ + 57Si02 = 82SER + 21CHL + 164H* Sericite pseudomorphs after kyanite may be explained by the mechanism: 3) 24KY + 16H+ + 24Si02 +24H20 = 16SER + 16H+ These proposed reactions satisfy the observed textural relationships, but require the exchange of K, Fe, and H cations in order to balance reactant and product assemblages. H 2 0 is a necessary reactant in all three sub-reactions. Rather than call on a metasomatic front to drive these reactions by supplying K and Fe and removing H, the construction of another mechanism to close the reaction system provides a preferred solution. The replacement of matrix biotite by chlorite may be modeled as: 4) 2BI + 4H+ = 1CHL + Fe+ + 2K+ + 3Si02 When this sub-reaction is combined with reactions 1 and 2 the following two balanced METAMORPHISM / 116 reactions are formed: 5) 4GT + 1BI + 12H20 = 3CHL + 1SER + 3Si02 6) 8ST + 41BI + 33Si02 + 108H2O = 41 SER + 31CHL Both these reactions are retrograde analogs to commonly proposed prograde metamorphic reactions (Thompson and Norton, 1968; Hoscheck, 1969; Winkler, 1979). Due to the lack of an Fe-bearing mineral phase in the kyanite breakdown reaction 3, sub-reaction 4 is unable to close the cation exchange system. However, if this sub-reaction is allowed to contain only minor amounts of reactant Ml muscovite in regions where kyanite replacement has occurred, the equation may be rewritten as: 4a) 15BI + IMS + 16H+ + 12H20 = 9CHL + 16K.+ 21Si02 This conserves Fe within the solid phases and allows the exchange system between the two sub-reactions to be closed. The balanced equation formed by the combination of 3 and 4a : 7) 8KY + 5BI + ISiOj + 12H20 = 3CHL + 5SER represents the reversal of reaction 2 proposed in the Ml metamorphic reaction section. Discussion Similar styles of retrogression are documented in the Three Ladies-Mount Stevenson and Ogden Peak areas (Getsinger, 1985; Lewis, 1987). Widespread retrogression of kyanite-staurolite grade rocks is observed on both sides of the east arm of Quesnel Lake and in the Mount Watt vicinity (McMullin, oral comm. 1987). Thus, the M2 metamorphic event responsible for the replacement of Ml minerals appears has affected a majority of the rocks of sub-sillimanite grade west of the northern SMC. The origin of this event is perhaps related to D3 and D„(?) mobilization of intergranular fluid within kyanite-staurolite grade rocks. This fluid may have been retained as a grain boundary film representing residual connate "water" or METAMORPHISM / 117 the already once mobilized fluid derived from the dehydration of sillimanite grade rocks. METAMORPHISM / 118 4.6. CALC-SILICATE REACTIONS Calc-silicate rock types commonly mark the contacts between calcite marble and siliclastic lithologies and occur as lenses within micaceous marble. The thickness of these zones range from 5 to 300 cm. Calc-silicate rocks occur throughout the map-area in all metamorphic zones, but are far more abundant in the central region where marbles are common and the metapelitic rocks are of sillimanite-kyanite-staurolite grade. The calc-silicate assemblage includes variable amounts of quartz, plagioclase ( A i U s - A n j s ) , potassium feldspar, white and brown mica, calcite, sphene and opaques. Zoisite or epidote, Ca-amphibole, diopside and Ca-scapolite are additional phases locally. These minerals commonly define a Si/S0 parallel mineral banding, or gneissosity, 3 - 3 0 mm thick. Inferred Reactions and Discussion The occurrence of prismatic zoisite within and adjacent to embayed plagioclase (Figure 44) in the kyanite-staurolite zone is consistent with the reaction: 1) ICC + 3AN + 1H20 = 2ZO + 1C02 Ferry (1976) proposed this reaction to explain a similar texture observed in staurolite-cordierite-andalusite grade rocks in South-central Maine. The same reaction was proposed to progress in the opposite direction for the Northwest Himalaya (Misch, 1964). Thus there is some dispute as to which Ca-aluminosilicate is the high grade phase. The slope of reaction 1 is steeply negative in T-XC0 2 space with the formation of zoisite favored at small values of XC0 2 (Winkler, 1979; Ferry, 1976). The reaction may proceed towards zoisite or anorthite dependant on if the univariant line is approached from the low or high XC0 2 side, respectively (Ferry, 1976). METAMORPHISM / 119 FIGURE 44. Prismatic zoisite replaces embayed plagioclase in calc-silicate gneiss, sample 143B, unit 3b. Bar is 0.5 mm. METAMORPHISM / 120 Textures indicating the breakdown of Ca-amphibole to form diopside are not observed, but the coincident spatial occurrence of one and absence of the other suggests a reaction relationship. Ca-amphibole is observed with calcite, quartz, white and brown mica in the micaceous marbles of the kyanite-staurolite zone, but is absent from the calc-silicate gneisses and marbles of sillimanite grade. Diopside, exceedingly rare in the kyanite-staurolite zone, is abundant in the higher grade rocks, where it forms up to 2 cm long poikiloblasts, enclosing quartz, zoisite, epidote and calcite. To explain these spatial and textural relations, the reaction: 2) 1TR + 3CC + 2QZ = 5DIOP + 3C02 + 1H20 (Winkler, 1979; Turner, 1981) is proposed to have occurred in calc-silicates proximal to the sillimanite isograd. The approximate spatial coincidence of this reaction with the first occurrence of sillimanite in the map-area is consistent with the distribution of the diopside isograd one km on the down-grade side of the sillimanite isograd in South-central Maine (Ferry, 1976) and as a culmination in sillimanite grade rocks of the Penfold Creek area (Fletcher, 1972). In the sillimanite-kyanite-staurolite zone, xenoblastic scapolite, up to 3 mm long, occurs in polymineralic bands with zoisite, epidote, quartz, diopside, sphene and small amounts of plagioclase and calcite. It is also recognised as discrete altered grains, replaced by fine grained brown mica along irregular fractures, admist calcite, quartz and zoisite. The generation of Ca-scapolite in sillimanite grade calc-silicate gneisses is attributed to the prograde reaction: 3) 3AN + ICC = 3 CaAl2Si208 • CaC03 (Meionite) (Orville, 1975; Ferry, 1976; Goldsmith and Newton, 1977). Scapolite is observed in rocks of inferred sillimanite grade in South-central Maine (Ferry, 1976), in sillimanite METAMORPHISM / 121 grade rocks of the Penfold Creek area (Fletcher, 1972) and the sillimanite-kyanite zone of the Boss Mountain area (Fillipone, 1985). Reaction 3 involves a continuous cation exchange between plagioclase and scapolite, both minerals exhibiting a complete solid solution between Na and Ca end-members. This reaction is independant of XC0 2 and pressure and hence has potential as a geothermometer (Winkler, 1979; Berman, oral comm. 1987). Pigage and Berman (unpublished data) have calibrated this geothermometer for the compositions of co-existing scapolite and plagioclase based on the compilation of experimental and empirical data. In sample 385C, scapolite and plagioclase compositions are estimated optically to be Meonite7J and An50, respectively (Deer, Howie and Zussman, 1981). These compositions indicate a metamorphic temperature of about 565° C for the sillimanite-kyanite-staurolite zone. This estimate is reasonably consistent with the 535+20° C value obtained for this region from the Ferry and Spear (1978) garnet-biotite geothermometer. METAMORPHISM / 122 4.7. SUMMARY Two metamorphic events have affected the rocks of the Niagara peak map-area. A Middle Jurassic Barrovian regional metamorphism, previously ascribed to the accretion of the Intermontane Belt (Monger et al., 1982; Okulitch, 1984), has been overprinted by a retrograde event west of the northern Shuswap Metamorphic Complex. Metamorphic peak conditions attending the prograde event are indicated by the generation of sillimanite at the expense of kyanite and muscovite and are estimated to be approximately 630° C at a pressure of 5.5 kb (Bathozone 4/5 transition). The genesis of the chlorite grade M2 event may be related to the D3 and D4(?) remobilization of fluids driven off from higher to lower grade rocks during Ml metamorphism. 5. STRUCTURAL AND METAMORPHIC CORRELATIONS The relative timing of deformational events with respect to metamorphic mineral growth and associated metamorphic grade is determined in the previous chapter. Further structural and metamorphic correlations are implied by the temporal and spatial relationships of synmetamorphic veins to deformation and metamorphism. The geometry and composition of these veins indicate the activity of a hydraulic fracture mechanism during and following the first two phases of deformation at metamorphic conditions ranging from that of chlorite (?) to sillimanite grade. The cyclic opening and sealing of these hydraulic fractures, reflecting fluctuations in pore fluid pressure and rock permeability, is inferred to affect the manner by which strain was localized in the D2 synformal zone and other regions in the map-area. The fibrous nature of some of the synmetamorphic veins provides insight into the noncoaxial deformational history of these rocks. 123 STRUCTURAL AND METAMORPHIC CORRELATIONS / 124 5.1. SYNMETAMORPHIC VEINS Four types of synmetamorphic veins are recognized on the basis of internal mineral assemblage. They are referred to as: 1) quartz, 2) tremolite, 3) kyanite and 4) trondhjemite. The general spatial distribution of these veins with respect to structural domains and metamorphic zones appears in Figure 45. Vein composition indicates the minimum metamorphic grade during which vein formation took place. The timing of vein development with respect to deformation is determined by crosscutting mesoscopic and microscopic relationships and is illustrated in Figure 46. 1) Quartz Quartz veins, 3 mm-100 cm thick, comprise 5-15% of the pelitic rocks throughout the map-area. These veins, composed of quartz and minor biotite, parallel and crosscut local S^So compositional layering. Contacts with the enclosing wall rocks are sharp, marked by mica selvages. Grain size exceeds that of wall rock quartz by a factor of 5-10. Vein quartz is strained and exceptionally fibrous, with fiber lengths oriented approximately normal to vein walls. The constant enveloping surfaces to sutured blocky grain boundaries and the serrated nature of suture teeth indicate the activity of a pressure solution mechanism. Hence, the teeth form by the same process as do stylolites, directed towards the maximum compressive stress direction(o 3) (Choukroune, 1968) (Figure 47). Deformation lamellae, kink bands and healed microfractures are common; neoblasts formed by dynamic (syntectonic) recystallization and polygonal subgrains developed by grain boundary rotation are observed as well. Many quartz veins outline rootless isoclinal folds internal to S]/S0, indicating their transposition into this surface (Figure 6, Chapter 3), while less deformed quartz veins crosscut Dj and D2 folds. These and other relationships indicate vein formation STRUCTURAL AND METAMORPHIC CORRELATIONS / 125 , . Lynx Creek . 52*37 - f - - j - 52*37' 120*38' 120°34" FIGURE 45. Distribution of the four synmetamorphic veins common in the Niagara Peak area with respect to structural domains and metamorphic zones. STRUCTURAL AND METAMORPHIC CORRELATIONS / 126 Vein composit ion D1 syn post D2 syn post Quartz T r - c c - ( w t mica) Ky-qz- (p l ) - (mu) Trondhjemite observed — — — — inferred FIGURE 46. Summary of the relative timing of synmetamorphic vein development with respect to deformational phases in the Niagara Peak area. STRUCTURAL AND METAMORPHIC CORRELATIONS / 127 Photomicrograph (crossed nicols) showing constant enveloping surface to sutured blocky quartz grain boundaries in a quartz vein in sample 007, unit 3. Suture teeth form in response to a pressure solution mechanism and are directed towards the maximum compressive stress direction( o3). Bar is 0.5 mm. STRUCTURAL AND METAMORPHIC CORRELATIONS / 128 before, during and after Dx and D2. Metamorphic grade during this period ranged from that of chlorite (?) to sillimanite. 2) Tremolite Tremolite veins, 5-100 mm thick, make sharp contacts with the calcite marble of Unit 2 in the eastern kyanite-staurolite zone (domain two) and the kyanite-sillimanite zone (domain four). These veins are commonly planar and often occur in parallel sets, spaced 10-70 cm apart, and comprise less than 15% of the outcrop locally. The veins are composed dominantly of slender tremolite needles, up to 12 mm long. Tremolite forms a nearly monomineralic assemblage, or occurs within a matrix of fine grained calcite and minor white mica porphyroblasts. The distribution of tremolite needles, slightly kinked and often replaced by calcite along microfractures, outlines straight or curved trails that form moderate to high angles with vein walls. Many of these veins contain medium to coarse grained calcite marble wall rock fragments. Randomly oriented tremolite rosettes and very fine grained calcite overgrow calcite kink bands and grain boundaries in these included marble fragments. Two major vein types are recognized on the basis of internal tremolite needle geometry. One set contains consistently oriented needles forming a 50-60° angle with vein walls (Figure 48), while the other is of a more complex nature. The second vein type is distinguished by the sigmoidal distribution of tremolite, oriented nearly normal (70-80°) to the wall contacts in vein interiors and 40° to these contacts at the vein/wall rock interface (Figure 49). A secondary vein, characterized by the rectilinear distribution of tremolite at approximately 30° to vein walls, often occupies the center of and parallels this vein type. The orientation of both vein types approximates 315/85 NE, crosscutting local Si/S0 at large angles. Figure 49, a stereonet plot, illustrates the angular relationships STRUCTURAL AND METAMORPHIC CORRELATIONS / 129 FIGURE 48. Common tremolite vein types, a) simple vein, characterized by the reasonably straight distribution of tremolite needles oblique to vein walls in sample 231A, unit 2 calcite marble. Bar is 1 cm, b) Complex vein, characterized by both sigmoidal and rectilinear distributions of tremolite needles (outlined) in sample 206A, unit 2 calcite marble. Bar is 1 cm. STRUCTURAL AND METAMORPHIC CORRELATIONS / 130 between vein orientation, compositional layering and slickenside striae within the Unit 2 calcite marble of domain two. Tremolite veins truncate Si/S0 and outline D 2 folds locally. Vein mineralogy and timing relationships indicate formation under the conditions of greenschist grade. 3) Kyanite Kyanite veins, 5-60 cm thick, form moderately sharp contacts with pelitic rocks of kyanite-staurolite and sillimanite grade. Vein abundance is low, with the maximum occurrence of these veins (<5% of the local outcrop) existing in the sillimanite-kyanite-staurolite zone, in the vicinity of the D2 synformal zone. Kyanite and quartz characterize the vein mineral assemblage with plagioclase (andesine), poikiloblastic muscovite and rare idioblastic garnet present in rocks of sillimanite grade. Grain size ranges from coarse to pegmatitic with kyanite blades up to 10 cm long. Mineral textures indicate the crystallization sequence: garnet-kyanite-plagioclase-muscovite -quartz, in veins that are composed of these phases. Both kyanite and muscovite display simple and conjugate kink bands at a high angle to (100) and (001) cleavages, respectively (Figure 50), indicating dislocation glide along these cleavage surfaces. Kyanite veins parallel and crosscut Si/S0; no evidence for transposition is recognized. They are deformed by phase two, commonly outlining elongate boudins parallel to L?. Internal kyanite blades are oriented infrequently towards this direction as well. These relations indicate vein formation before and during D2 at kyanite-staurolite metamorphic grade. The observed spatial changes in vein mineral assemblage confirms that during vein development metamorphic grade increased from west to east 4) Trondhjemite Tourmaline-garnet-muscovite trondhjemite pegmatite sills and dikes, described in STRUCTURAL AND METAMORPHIC CORRELATIONS / 131 solid circles-tremolite veins (34) open c i rc les- slickenside striae (09) tr iangles-S"l/SO (10) FIGURE 49. Equal-area projection of tremolite veins, slickenside striae and local compositional layering (S^So) in unit 2 calcite marble of structural domain two. STRUCTURAL A N D METAMORPHIC CORRELATIONS / 132 FIGURE 50. Conjugate kink bands form high angles to the (100) twin plane of kyanite in sample 543, a kyanite vein in the core of the domain three synformal zone, a) plane light, b) crossed nicols. Bar is 2 mm STRUCTURAL AND METAMORPHIC CORRELATIONS / 133 chapter 2, form sharp contacts marked by tourmaline schist selvages, with the siliciclastic rocks of domains two, three and four. Trondhjemite comprises less than 5% of the surface area in these domains. Plagioclase composition is oligioclase (An10.20), while as much as 3% potassium feldspar occurs in the mineral assemblage of trondhjemites east of Niagara peak. Many trondhjemite sills and dikes were deformed by D2 (Figure 51) and commonly contain an internal S2 fabric defined by muscovite distribution and orientation. Several planar dikes crosscut D2 folds (Figure 53) and show internal D2 crenulations of garnet and muscovite-rich horizons locally. Neither sills nor dikes are observed to crosscut the other, although both occur in the same outcrop. Trondhjemite emplacement is inferred to be syn- to post-kinematic with respect to D2, spanning the growth period of staurolite, kyanite and sillimanite. STRUCTURAL AND METAMORPHIC CORRELATIONS / 134 FIGURE 51. Trondhjemites sills and dikes, a) D : boudin train outlined by sill (facing west), b) dike cuts across soutwesterly asymmetric reclined D 2 fold. STRUCTURAL AND METAMORPHIC CORRELATIONS / 135 5.2. HYDRAULIC FRACTURE ORIGIN OF SYNMETAMORPHIC VEINS Vein geometry, internal fabric, included wall rock fragments and sharp external contacts indicate a hydraulic fracture mechanism of formation (Etheridge, 1983). In this model, the veins occupy Mode I extension fractures oriented normal to the local minimum compressive stress direction (a 0 (Secor, 1965; Fyfe et al., 1978; Etheridge, 1983; Etheridge et al., 1984). Fracture or crack initiation occurs when the pore fluid pressure exceeds the sum of a i and the tensile strength of the rock (Secor, 1965). At elevated temperatures the tensile strength, or fracture toughness, of the rock may be greatly reduced due to subcritical crack growth by stress corrosion cracking (Atkinson, 1984). The dilatant crack serves as the site to which fluids migrate as long as the crack fluid pressure is less than that of the surrounding wall rock (Etheridge et al., 1984). Fracture dilatantcy will decrease the pore fluid pressure, due to the local increase in volume and permeability created by cracking (Murrell and Ismail, 1976). As the fracture is sealed by mineral deposition, solution migration stops and permeability drops. Reduced permeability leads to the progressive increase in pore fluid pressure until the same fracture is reopened (crack-seal: Ramsay, 1980) or a new one is initiated. Figure 52 schematically illustrates the cyclic nature of this hydraulic fracture initiation/propagation model. STRUCTURAL AND METAMORPHIC CORRELATIONS / 136 initiation of new fracture 3.fluid migration FIGURE 52. Schematic illustration of the cyclic development of hydraulic fractures with respect to rock tensile strength (T) and the fluctuations in pore fluid pressures distributed throughout the rock (Pf rock) and localized in the fracture sites (Pf frac). STRUCTURAL AND METAMORPHIC CORRELATIONS / 137 5.3. VEIN MINERAL DEPOSITION - RELATIONSHIP TO l+E, Mineral deposition within synmetamorphic veins took place along vein medial surfaces (syntaxial) or at the exterior walls (antitaxial), probably derived from the adjacent wall rock by mechanisms of dissolution and solution transport (Durney, 1972; Durney and Ramsay, 1973; Ramsay, 1980). The observed oriented growth of vein mineral fibers likely coincided with the direction of vein dilatantcy, thus recording the incremental maximum elongation direction (1 + eO throughout vein formation. The curved paths outlined by tremolite needles in tremolite veins are inferred to represent the rotation of the incremental 1 + d direction away from a i during noncoaxial deformation (Durney and Ramsay, 1973). Tremolite vein geometry, mineral orientation and included wall rock fragments indicate that the veins are antitaxial, in that minerals grew outward from the center of the vein through time. The relationship of tremolite needles and vein wall orientations to slickenside striae contained within wall rock S^So, for both vein types previously described, is illustrated in Figure 53. The relationships depicted for 206A indicate about 30° of counterclockwise rotation of 1 + ei away from a i within the S]/S0 plane during the formation of the primary vein (25/025 to 20/350) and an additional 10° of rotation prior to the formation of the internal secondary vein, the latter being characterized by the straight distribution of tremolite needles towards 15/340. The relatively consistent orientation of tremolite needles towards 10/352 in 206B marks the incremental 1 + ej direction at an intermediate point in the strain history recorded by the tremolite needles of 206A. The First increment of tremolite growth for both vein types is inferred to have been oblique (50-80°) to vein walls, implying that at the onset of vein deposition, 1 + ei was oblique to this surface. This obliqueness may S T R U C T U R A L A N D M E T A M O R P H I C C O R R E L A T I O N S / 1 3 8 FIGURE 5 3 . Relationships between tremolite vein and internal tremolite needle orientation with respect to exterior wall rock slickenside striae and compositional layering ( S i / S 0 ) . In each case, vein tremolite needles are contained within the plane of the page. STRUCTURAL AND METAMORPHIC CORRELATIONS / 139 indicate: 1) dilation oblique to vein walls and/or 2) tremolite growth only after several increments of vein and wall rock rotation. STRUCTURAL AND METAMORPHIC CORRELATIONS / 140 5.4. SYNMETAMORPHIC VEINS AND DEFORMATIONAL STYLE The temporal and spatial relationships of synmetamorphic veins to deformational style indicate the cyclic activity of strain hardening and softening processes. The enhanced permeability created by crack dilatantcy reduces local pore fluid pressure and leads to the increase in the effective compressive principal stresses, making the rock stronger (dilatantcy hardening: Murrell and Ismail, 1976; Ismail and Murrell, 1976). This dilatantcy hardening process serves to homogenize deformation and the distribution of pore fluid, while the high pore fluid pressure and activity preceding embrittlement weaken the rock, localizing strain and fluids in discrete zones (Murrell, 1985). Many workers cite prograde metamorphic dehydration reactions as the primary cause of this elevated pore fluid pressure and subsequent reduction in rock strength (Rubey and Hubbert, 1959; Heard and Rubey, 1966; Etheridge et al., 1983; Murrell, 1985). Pore fluid enhancement of D t is indicated by the high proportion of quartz veins which crosscut compositional layering, but are folded by this earliest phase of deformation. The localization of these veins within pelitic schist implies that the schistor-ity of these rocks served as a relatively impermeable barrier to fluid transport across S^So, while fluid migration through more homogeneous quartzose lithologies was likely less restricted. The variable wavelength and amplitude of these deformed quartz veins imply episodic vein deposition throughout ongoing phase one deformation and Si fabric development The nature and distribution of tremolite veins in the over 100 m thick Unit 2 calcite marble of domain two and the prevalence of quartz veins in the underlying pelitic schist probably indicate this marble's role as a relatively impermeable "cap-rock" after Di and prior to D 2 folding. As fluid was initially supplied to the marble, STRUCTURAL AND METAMORPHIC CORRELATIONS / 141 perhaps by the decomposition of minerals in the surrounding siliclastic rocks, its transport through the granoblastic matrix of this "cap-rock" was highly restricted. Fluid entrapment increased pore fluid pressure leading to strain localization, brittle failure and vein mineral deposition. The inferred rotation of veins and wall rock in the compositional layering (Si/S0) plane may indicate the resolution of regional principal stress directions tangential and normal to this relatively rigid marble block. The slickenside striae record at least one increment of this rotation and perhaps were formed in the early stages of vein development, as indicated by the coincidence of the striae with tremolite needle orientation, near vein medial surfaces (Figure 53). Tremolite vein-outlined D2 folds imply that following vein formation dilatantcy hardening took place, distributing strain relatively homogeneously throughout the marble unit The increased abundance of kyanite veins and the presence of trondhjemite sills and dikes in the vicinity of the D2 synformal zone may indicate that strain localization in this region was enhanced by the high pore fluid pressure and chemical activity attending amphibolite grade metamorphism. Other possible strain softening mechanisms include: 1) recrystallization, 2) reaction softening and 3) fabric softening (White and Knipe, 1978; White et al., 1980). Evidence for dissolution and solution transport mechanisms, dissolution-type cleavage, differentiated layering and down-dip pressure shadows, indicates that local fluid migration was along grain boundaries parallel to the S2 dip-direction. In this region, the high angle between vein enveloping surfaces and subvertical S2 indicates the restriction of this near vertical fluid transport by subhorizontal, relatively impermeable, lithologic anisotropies. The cyclic activity of pore fluid enhanced strain softening and dilatantcy hardening is inferred from the variations in phase two deformational style in this zone. Mesoscopic fold styles indicate the activity of both low viscosity contrast homogeneous STRUCTURAL AND METAMORPHIC CORRELATIONS / 142 flattening and high viscosity contrast flexural-slip mechanisms (Chapter 3, Structure). The high pore fluid pressures and chemical activities preceding vein formation would serve to lower lithologic viscosity contrast, while the reduction of these fluid properties by dilatantcy hardening would increase the viscosity contrast of the same rock. Thus, with polyepisodic vein formation, one might envision the surrounding rocks to undergo alternating homogeneous flattening and flexural-slip folding due to cyclic fluctuations of local viscosity contrast. Although the majority of the D2 synformal zone may have undergone this cyclic strain process, flexural-slip folds and detachment surfaces are recognized only on the limbs of map-scale folds, perhaps reflecting the significant amount of Sj/So layer parallel shear in these regions. The highly flattened folds, characteristic of hinge regions, do not record this shear strain, indicating the containment of the maximum shortening direction (l + e3) within the subhorizontal enveloping surface to these mesoscopic folds. This inferred spatial relationship is consistent with the subvertical orientation of local S2 fabric and map-scale fold axial surfaces, assuming these surfaces are nearly coincident with the XY-plane of the finite strain ellipsoid (Ramsay, 1967; Williams, 1976; Gray and Durney, 1979). Thus, during dilatantcy hardening, the mesoscopic folds parasitic to map-scale hinge zones likely underwent further flattening and tightening. The model illustrated schematically in Figure 54 represents the idealized evolution of the D2 synformal zone with respect to synmetamorphic vein development STRUCTURAL AND METAMORPHIC CORRELATIONS / 143 5m i i fluid migration i 1 5m FIGURE 54. Idealized evolution of the D2 synformal zone with respect to synmetamorphic vein development Early and late veins are denoted by open and closed forms, respectively. STRUCTURAL AND METAMORPHIC CORRELATIONS / 144 5.5. SUMMARY The mineralogy of synmetamorphic veins reflects progressive dewatering of a sedimentary pile during prograde metamorphism, ranging from chlorite (?) to sillimanite grade. Cyclic fluctuations in pore fluid pressure and chemical activity led to alternating high and low lithologic viscosity contrasts. Consequently, D2 folds developed by the episodic activity of flexural-slip and homgeneous flattening mechanisms. This deformation was accomplished under semi-brittle conditions dominated by pressure induced grain boundary solution and hydraulic fracturing. Dislocation glide and dynamic recrystallization were active under these conditions as well, but are suggested to have played a secondary role in the accommodation of strain in the rocks of the map-area. 6. REGIONAL CORRELATION AND INTERPRETATION The Niagara Peak map-area straddles the northern Shuswap Metamorphic Complex and lies approximately 30 km northeast of the present exposure of the Intermontane Belt (1MB)-Omineca Belt (OMB) boundary (Figure 2, Chapter 1). This boundary is a suture zone of crustal proportions. In the vicinity of Quesnel Lake, this suture outlines a series of shallowly northwesterly plunging lobate antiforms cored by metasedimentary rocks of the western OMB. In the following discussion, these OMB rocks are referred to as autochthonous, while the structurally overlying sequence of late Paleozoic (?) to Mesozoic rocks of the Slide Mountain and Quesnellia Terranes are termed allochthonous. The purpose of this chapter is correlate the stratigraphy, structure and metamorphism of the Niagara Peak area with other areas in the Quesnel Lake region. These correlations will be the foundation from which interpretations are made concerning the development of the 1MB-OMB tectonic boundary. 145 REGIONAL CORRELATION AND INTERPRETATION / 146 6.1. REGIONAL CORRELATION The map-area lithologies comprise the upper exposed levels of the over 10 km thick, structurally unbroken succession of Hadrynian to Paleozoic Snowshoe Group rocks outlined by Ross et al. (1987) west of the Matthew Fault to structurally underlie the 1MB-OMB suture. This sequence contains quartzite, marble, pelitic and amphibole schist, and calc-silicate gneiss. It is separated into upper and lower successions based on several criteria, including the presence of major marble and amphibole schist marker units near the base of the upper succession. These markers may be traced for over 70 km across the Quesnel Lake region. Nowhere within this succession is the Pleasant Valley Thrust of Struik (1982, 1984, 1986) recognized. The map-area biotite granodiorite orthogneiss represents the highest of three distinct orthogneiss structural horizons present within Snowshoe Group rocks. The Boss Mountain and Mount Perseus Gneisses comprise the lowest structural level; they are equated on the basis of similar composition, similar stratigraphic relations to adjacent marbles and identical position with respect to the IMB-OMB suture (Campbell, 1971; Elsby, 1985; Fillipone, 1985). The Quesnel Lake Gneiss is compositionally distinct from these two gneiss bodies and structurally overlies them, forming the intermediate orthogneiss horizon (Campbell, 1971; Montgomery, 1985). Both the Boss Mountain and Quesnel Lake Gneisses yield Late Devonian to Early Mississippian U-Pb dates from xenocrystic zircon (Mortensen et al., 1987). The four phases of folding outlined in the Niagara Peak map area are correlated to the deformational phases described by other workers elsewhere in the Quesnel Lake region. Dj intrafolial folds and transposed compositional layering, Sj/So, are recognized throughout the autochthon, but are absent from the allocthon (Ross et REGIONAL CORRELATION AND INTERPRETATION / 147 al., 1985). Di map-scale folds are not observed anywhere in the Quesnel Lake area. All orthogneiss bodies are deformed by this earliest phase of deformation and thus place a maximum age constraint on D^ Northwesterly trending phase two folds observed in the Niagara Peak area are commonly west verging, but reverse their vergence direction through a synformal zone. This synformal zone is characterized by the divergent fanning of fold axial surfaces and a reversal in the direction of fold assymetry. Close to the tectonic boundary, Ross et al. (1985) describe a second phase of deformation denoted by east verging folds which have axial surfaces parallelling the suture surface locally. The region separating these fold vergence populations is exposed in the Mount Perseus vicinity. Nowhere are folds of one vergence sense observed to interfere with those of another; hence, their correlation is uncertain. Ross et al. (1985) equate east verging second phase autochthon strucures with the first phase recognized in the allochthon on the basis of similar orientation and style. The absolute timing of this deformation is bracketed to be between the Early Jurassic, the age of the youngest 1MB rocks deformed and 163±7 m.y., the Rb-Sr whole rock data obtained by Pigage (1978) from a postkinematic granodiorite-aplite body in the Azure Lake vicinity. Shallowly northwesterly plunging phase three folds display variable wavelength/amplitude relationships dependent on structural position with respect to the tectonic suture. In the Niagara Peak area, D3 folds have an upright, symmetric and gentle to open form. With increasing proximity to the 1MB-OMB boundary, phase three folds tighten as they become westerly verging, displaying a cuspate/lobate geometry (Ross et al., 1985). These changes in geometry indicate that the localization of D3 strain near the tectonic suture was controlled by the viscosity contrast between autochthon and allochthon (Ross et al, 1985). This third autochthon fold set is REGIONAL CORRELATION AND INTERPRETATION / 148 considered to be coeval with the second phase of allochthon deformation and is responsible for the regional fold pattern of the 1MB-OMB tectonic suture (Ross et al., 1985). Northeasterly trending, variably asymmetric, upright D4 folds of the Niagara Peak area are correlated with the fifth phase of autochthon deformation recognized adjacent to the suture (Fillipone, 1985; Elsby, 1985). Fillipone (1985) and Elsby (1985) describe a fourth phase of deformation in the Boss Mountain and Mount Perseus areas, characterized by northwesterly trending, east verging, reclined buckle folds, which is absent from the rocks of the Niagara Peak area. Map-scale structures associated with either of these deformational phases are not common, except for the series of northeasterly trending folds outlined by an upper Snowshoe Group marble marker unit in the vicinity of Bouldery Creek, north of Mount Isosceles. All phases of major deformation are inferred to have taken place prior to 104+ 3 m.y.a. on the basis of Rb-Sr whole rock analyses of the undeformed Raft Batholith which transects the 1MB-OMB boundary south of Quesnel Lake (Jung, 1986). Metamorphism in the Quesnel Lake region ranges from chlorite to sillimanite grade (Campbell, 1978; Ross et al., 1987). Metamorphic zone boundaries, determined by the occurrence of diagnostic indicator minerals, transect the 1MB-OMB contact, are discordant to lithologic trends and outline phase three map-scale folds. The distribution of these metamorphic zones defines three thermal culminations: 1) the northern Shuswap Metamorphic Complex, 2) the Boss Mountain area and 3) the Three Ladies Mountain-Mount Stevenson area (Campbell, 1978; Getsinger, 1985; Fillipone, 1985). The metamorphic peak conditions obtained in these three culminations are estimated to be approximately 5-6 kb and 600° C (Fletcher, 1972; Getsinger, 1985; Fillipone, 1985, this study). REGIONAL CORRELATION AND INTERPRETATION / 149 The bulk of metamorphism is inferred to have been synkinematic with respect to the development of phase two autochthon structures; however, the metamorphic peak generation of sillimanite followed this event It is not certain, nor assumed, that this. metamorphism took place at the same time throughout the Quesnel Lake region. The absolute timing of metamorphism in the Mount Isosceles area is approximately 174±4 m.y. on the basis of U-Pb analyses of metamorphic sphene from the Quesnel Lake Gneiss (Mortensen et al., 1987). The Cretaceous dates yielded from Rb-Sr analyses of a granodiorite-aplite body near Azure Lake and the Quesnel Lake Gneiss (77 ±20 m.y. and 119± 11 m.y., Pigage, 1978; 108 m.y., Mortenson, pers. comm., 1986) are inferred to indicate post metamorphic disturbance of Rb-Sr and are not viewed to represent metamorphic ages. The 114 and 117 m. y. ages obtained from the U-Pb analyses of monazite in the Boss Mountain Gneiss are anomalous, perhaps representing a later metamorphic event and/or a lower blocking temperature for monazite than the previously inferred 550-600°C (Wagner et al., 1977; Koppel et al., 1980). Widespread retrogression of metamorphic porphyroblasts to sericite and chlorite occurs locally within sub-sillimanite grade rocks in the Niagara Peak area and elsewhere in the Quesnel Lake region. These retrograde zones form asymmetric mantles about the thermal culminations and occur in close prcximity to the tectonic boundary (Getsinger, 1985; Fillipone, 1985; this study). These zones transect metamorphic zone boundaries, parallelling the strikes of local lithologic units commonly. Timing of retrogression is inferred to be synkinematic with phases three and four (?) of autochthon deformation. Synmetamorphic veins of variable composition, pervasive throughout both the autochthon and allochthon, are suggested to have formed by a hydraulic fracture mechanism. Veins that are inferred to have developed under relatively low grade REGIONAL CORRELATION AND INTERPRETATION / 150 conditions are composed commonly of quartz, calcite, chlorite and tremolite, while those formed at higher grades either bear kyanite or are granitic in composition. Both antitaxial and syntaxial vein relationships are observed. Vein localization is recognized in the cuspate Eureka Peak Syncline (Bloodgood, 1987), in areas of extensive retrogression, especially close to the 1MB-OMB boundary (Fillipone, 1985) and in zones of localized strain (Fillipone, 1985; this study). These relationships may indicate the enhancement of deformation and metamorphism by increased pore fluid pressure and chemical activity. Three map-scale faults transect and offset metamorphic zone boundaries in the Quesnel Lake region. The Little River Fault System (LRFS) dips moderately towards the east and shows more than 6 km of near down-dip normal movement, placing chlorite-biotite grade rocks on those of sillimanite grade near the extremity of the north arm of Quesnel Lake (Getsinger, 1985). The northwesterly trending Matthew Fault is nearly vertical at its present levels of exposure in Penfold Creek and juxtaposes chlorite upon garnet or higher grade (Campbell, 1978). A fault is inferred along the north arm of Quesnel Lake to explain the present distribution of sillimanite grade rocks along the west shore and sub-garnet grade rocks on the east shore (Campbell, 1978). This fault may be part of the LRFS, although due to its lack of exposure, this is uncertain. The relative and absolute timing relationships of these faults are poorly constrained to postdate the attainment of metamorphic peak conditions and in the case of the LRFS, to succeed 86±3 m.y., the inferred emplacement age of granitic pegmatite in the Three Ladies Mountain-Mount Stevenson area (Getsinger, 1985). Getsinger (1985) suggests that movement on the LRFS ceased prior to the development of northeasterly trending D4 folds. The Eocene and Late Cretaceous dates obtained from K-Ar analyses of muscovite from syn and post-D2 granitic pegmatites in the Niagara Peak and Three Ladies Mountain-Mount Stevenson areas (51 ±2 m.y., REGIONAL CORRELATION AND INTERPRETATION / 151 57±2 m.y. and 69+2 m.y., Mortensen, pers. comm., 1986) may represent the cooling of these bodies through 350°±30°C due to uplift along these post metamorphic faults or may simply indicate the reheating of these areas. REGIONAL CORRELATION AND INTERPRETATION / 152 6.2. DISCUSSION AND INTERPRETATION In recent years, the geologic setting of the Quesnel Lake region has been inferred to be that of a convergence zone between the island arc related Quesnellia Terrane of the 1MB and the western OMB rocks of parautochtanous North America (Monger et al., 1982; Ross et al., 1985; Struik, 1986). Quesnellia Terrane lithologies consist of a 3-4 km thick, internally faulted, sequence of basal Middle to Late Triassic Quesnel River Group sedimentary rocks and overlying Late Triassic to Early Jurassic Takla Group volcanics (Struik, 1985; Bloodgood, 1987) In the vicinity of Eureka Peak, the petrography and geochemistry of the Takla Group volcanics show them to be of island arc and marginal basin affinities (Bloodgood, 1987). The Crooked amphibolite, a unit consisting mainly of amphibolite with lesser amounts of serpentine and tectonized ultramafic bodies, lies structurally in between OMB Snowshoe Group and Quesnellia Terrane lithologies (Struik, 1986). It is 15-250 m thick and rests upon the Eureka Thrust of Struik (1986), in fault contact with the overlying Quesnel River Group sedimentary rocks. The Crooked amphibolite is discontinuous along the Eureka Thrust, which places the Quesnellia Terrane directly on OMB rocks in its absence. This amphibolite has been correlated with the Mississipian to Permian Antler Formation of the Slide Mountain Terrane, although these two units are not laterally continuous (Struik, 1986). The Crooked amphibolite, alternatively, may represent a slice of the oceanic basement to the Quesnellia Terrane and be early Mesozoic in age. The characteristic wavelength of the map-scale fold pattern of the Eureka Thrust (1MB-OMB boundary) in the Quesnel Lake region approximates 20 km. Directly northwest of this area, the Eureka Thrust outlines an easterly reclined anticline/syncline REGIONAL CORRELATION AND INTERPRETATION / 153 pair east of Spanish Lake (Campbell, 1978; Rees, 1981; Rees and Ferri, 1983) (Figure 55). The Pundata Thrust, presently exposed in the Wells vicinity, has been postulated to represent the eastward extension of the Eureka Thrust, placing the 1MB-OMB boundary more than 35 km northeast of its present exposure at Quesnel Lake (Campbell, Mountjoy and Struik, 1986; Struik, 1986) (Figure 55). In this area, the Pundata Thrust outlines a syncline which passes into the Black Stuart Synclinorium, characterized by the divergent fanning of fold axial surfaces, in the structurally underlying OMB rocks. This synclinorium has been mapped towards the south to the north arm of Quesnel Lake, approximately 10 km northwest of the Niagara Peak map-area (Campbell, 1978), directly on trend with the map-area synformal zone. Hence, at one time the Eureka Thrust/Pundata Thrust may have structurally overlain the rocks of the Niagara Peak area. The present absence of this thrust surface from this region may indicate the preferential uplift of OMB rocks of high metamorphic grade with respect to the western sub-garnet grade 1MB lithologies. Given the location of Niagara Peak with respect to the synclines exposed at Eureka Peak and near Spanish Lake, 20 km and 35 km towards the east, respectively, and the 20 km characteristic wavelength of the folded Eureka Thrust for the region, a third syncline may have been positioned structurally above the synformal zone near Niagara Peak. Thus, the character of this synformal zone may represent the autochthon manifestation of the large Finite strain associated with this past eroded synclinal closure. Convergence on the tectonic boundary was initiated in Middle Jurassic time (Monger et al., 1982), followed shortly by the eastward obduction of the Quesnellia and Slide Mountain terranes over western OMB rocks (Ross et al, 1985) (Figure 56). As the autochthon was overridden, it became variably metamorphosed, reaching up to sillimanite grade locally (Monger et al., 1982; Okulitch, 1984). Thermal modelling by REGIONAL CORRELATION AND INTERPRETATION / 154 1 FIGURE 55. Simplified regional map of the 1MB-OMB tectonic boundary as defined by the Eureka and Pundata Thrusts. SMC=Shuswap Metamorphic Complex; RAFT=Raft Batholith (104±3 m. y. a.); Tr-Jr=Triassic and Jurassic volcanic rocks, includes Takla Group; UTr=Triassic sedimentary and volcanic rocks, includes Quesnel River Group; SM=Slide Mountain Group, includes Crooked Amphibolite; QLG=Quesnel Lake Gneiss; HPS=Snowshoe Group; HPK=Kaza Group (modified from Tipper et al., 1978). REGIONAL CORRELATION AND INTERPRETATION / 155 1. Middle Jurassic FIGURE 56. Schematic model for the evolution of the 1MB-OMB boundary in the Quesnel Lake region, a) accretion of the 1MB and obduction of the Slide Mountain (coarse supple) and Quesnellia Terranes (fine stipple and V), b) locking-up of the thrust surface, subsequent deformation of the stuture and further eastward thrusting of the Takla volcanics (V) (modified from Ross et al., 1985). In both illustrations, the large arrow points towards the direction of successive deformation and/or metamorphism REGIONAL CORRELATION AND INTERPRETATION / 156 England and Thompson (1984) suggests that a metamorphic temperature front induced by tectonic burial would lag behind and follow allochthon emplacement, in this case, as it was thrust eastward. The estimated depths of burial of Snowshoe Group rocks approximate 15-20 km on the basis of the metamorphic mineral assemblage of kyanite and sillimanite grade rocks (Fillipone, 1985; Getsinger, 1985; this study). How much of this depth is due to tectonic versus sedimentary burial is not certain. The present 4 km thickness of the allochthon (Ross et al., 1987) places only a minimum constraint on the contribution of obduction to OMB metamorphism, since the amount of postmetamorphic peak allochthon erosion is unknown. The east verging structures of the first phase of deformation common to both allocthon and autochthon developed contemperaneously with obduction, but preceded the attainment of metamorphic peak conditions (Ross et al., 1985; Ross et al., 1987). The lack of east verging folds east of Mount Perseus may indicate the remote position of this area with respect to the tectonic boundary. The present distribution of 1MB and OMB rocks in the Quesnel Lake region indicates that at least 70-80 km of shortening was accomodated during obduction. The synmetamorphic west verging folds east of Mount Perseus do not fit well into this obduction model, perhaps indicating the activity of another mechanism for their formation. As the transport surface locked up, perhaps due to the attainment of a critical allochthon length to thickness ratio, obduction ceased and deformation of the suture began (Figure 56). The viscosity contrast between autochthon and allochthon created an instability that led to the development of the syn- to post-metamorphic west verging map-scale cuspate/lobate folds of the tectonic suture, presently exposed in the western part of the region, adjacent to the 1MB-OMB boundary (Ross et al., 1985). The west verging synmetamorphic folds, termed phase two, east of Mount Perseus, removed from REGIONAL CORRELATION AND INTERPRETATION / 157 the suture, may indicate the propagation of the suture fold front from the leading edge of the allochthon westward throughout time. Thus, the development of synmetamorphic structures in this eastern region may be coeval with and related to the folding of the terrane boundary. The observed temporal relations with respect to metamorphism of these two west verging fold populations may reflect the relative timing and opposing direction of the metamorphic and suture folding deformational fronts which are inferred to have swept through the Quesnel Lake area. The similar deformational style of the Eureka Peak Syncline and the synformal zone near Niagara Peak supports the formation of these structures by similar mechanisms, involving the folding of the terrane boundary, but at vastly different structural levels. The upright post metamorphic folds east of Mount Perseus may represent the waning of suture folding deformation, as this area occupied higher structural levels, characterized by reduced metamorphic conditions. Thus, both syn- and post-metamorphic folds east of Mount Perseus may have developed as part of a deformational continuum. An alternative explanation of the synmetamorphic west verging folds east of Mount Perseus is found in the "back folding" and "back thrusting" models proposed for the southeastern Canadian Cordillera (Monger and Price, 1979; Archibald et al, 1983; Brown et al, 1986; Price, 1986). These models call for westerly directed thrusting and folding during and/or shortly after the eastward obduction of allochthonous terranes. The mechanisms proposed for the development of these antithetic structures involve: 1) crustal delamination and the wedge-like insertion of the allochthon along conjugate shear zones (Archibald et al, 1983; Price, 1986) and 2) the eastward underthrusting of attenuated continental crust beneath cratonal North America (Brown et al, 1986). Although both these models may provide some of the solutions for large regional-scale structural problems, neither satisfactorily explains the lack of interference REGIONAL CORRELATION AND INTERPRETATION / 158 of synmetamorphic east and west verging folds in the Quesnel Lake region, nor are any westerly directed thrusts or fold nappes recognized in this vicinity. The cuspite synclines of the deformed terrane boundary are inferred to have acted as channelways for fluids generated by the dehydration of the autochthon (Ross et al., 1985). The abundance of synmetamorphic veins and pressure solution features in the Eureka Peak Syncline support this synkinematic localization of fluid. The elevated pore fluid pressures of the allochthon in these regions and elsewhere along the terrane boundary may have served as a buoyant force in the detachment and eastward transport of the Takla Group volcanics over the underlying Quesnel River Group sedimentary rocks (Ross et al., 1985). 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APPENDIX A - METAMORPHIC MINERALS TABLE 3: Mineral Abbreviations and Formulae Mineral name Abbreviation General formulae albite ALB NaAlSi308 anorthite AN CaAISijO, apatite AP Ca5(P04)3(OH) biotite BI K(Fe,Mg)3AlSi3O10(OH)2 calcite CC CaC03 chlorite CHL (Fe,Mg)5Al2Si3O10(OH), diopside DIOP CaMgSi206 garnet GT (Fe,Mg,Mn,Ca)3Al2Si3012 kyanite KY Al 2Si0 5 meonite ME 3 CaAl2Si208 • CaC03 muscovite MS KAl3Si3O10(OH)2 opaques OP plagioclase PL (Na,Ca)AlSi3Os potassium feldspar KSP KAlSi308 quartz QZ Si02 sericite SER KAl3Si3O10(OH)2 sillimanite SILL Al 2Si0 5 sphene SP CaTiSi04(OH) staurolite ST (Fe,Mg)4Al18Si7 50 4 4(OH) 4 Fe-tourmaline TO Na(Fe,Mn)3Al6B3Si6027(OH)4 tremolite TR Ca2Mg5Si8022(OH)2 zircon ZI ZrSiO, zoisite ZO Ca2Al3Si3012(OH) 167 TABLE 4 : Plagioclase Compositions in Metapelites / 168 Sample Number Metamorphic Zone Composition Comment 120 KY-ST A n 6 0 - 6 5 porphyroblast 147 KY-ST An35 262 SILL-KY-ST Aa,, 289 SILL-KY-ST An38-44 247 SILL-KY-ST An42 porphyroblast 395 SILL-KY-ST An45 xenoblastic 395 SILL-KY-ST An57 porphyroblast 442 SILL-KY An09 idioblastic 456 SILL-KY An! 2 idioblastic 370 SILL An36.42 porphyroblast 370 SILL An08 idioblastic 367 SILL An32 in mica-quartzite APPENDIX B - ELECTRON MICROPROBE ANALYTICAL PROCEDURE AND RESULTS Microprobe analyses of coexisting garnet and biotite pairs were performed upon carbon coated polished circular thin sections using the four channel automated Cameca SX-50 microprobe at the University of British Columbia. Three to six garnet rim-biotite analyses were made for each of the several garnets present in the three pelitic schist samples examined. Spot analyses of garnet cores were done to ascertain rim-core chemical zonation. All garnet-biotite pairs examined were in contact with one another or separated by less than 1 mm exceptionally. Natural and synthetic minerals from the U.B.C. collection were used as standards for the microprobe analyses. Instrument operating conditions were an acceleration potential of 15 kV, a specimen current (on aluminum) of 38-40 nanoamps, an electron beam diameter of 3-5 microns, and a counting time of 10 seconds. All iron reported for both minerals analyzed is considered to be FeO and H 20 in biotite is assumed to be stoichiometric. 169 / 170 Table 5: Microprobe Standards for Garnet and Biotite Analyses Element Standard ID Number Source Si hornblende 229 New Zealand Al orthoclase 96 New York, U.SA. Fe fayalite 250 synthetic Mg forsterite 275 Arizona, U.S.A. Mn pyroxmangite 245 Japan Ca hornblende 229 New Zealand Na anorthoclase 365 location unkown K orthoclase 96 New York, U.S.A. Ti hornblende 229 New Zealand Si,Al,Mg,Cat garnet 278 South Africa t - for garnet analyses Table 6: Microprobe Sample Locations / 171 Sample Latitude(N) Longitude(W) Metamorphic zone 94 52° 37' 30" 120° 35' 00" KY-ST 262 52° 40' 00" 120° 35' 00" SILL-KY-ST 370A 52° 40' 45" 120° 32' 30" SILL GARNET; SAMPLE 94 TABLE 7: MICROPROBE ANALYSES OF GARNET AND BIOTITE Oxide 94-1.2 94-1.3 94-2.1 94-2.2 94-3.1 94-3.2 94-3.4 94-3.5 94-3.7 SiO, 36.40 36.87 37.06 36.89 37.06 36.93 37.23 36.42 37.16 Al,0, 20.83 20.86 21.04 21.01 21.10 21.51 21.14 20.74 21.02 FeO 33.38 32.13 33.81 33.64 31.72 31.1 1 33.57 33.61 33.14 MgO 2.43 2.45 2.49 2.62 2.39 2.35 2.77 2.71 2.95 MnO 4.04 3.61 4.01 3.97 3.61 2.44 3.71 2.78 3.73 CaO 2.49 4.33 2.39 2.42 4.63 5.73 2.32 2.67 2.40 TiO, 0.01 0.04 0.01 0.00 0.00 0.00 0.01 0.00 0.01 TOTAL 99.57 100.28 100.81 100.54 100.51 100.08 100.75 98.93 100.40 Element 94-1.2 94-1.3 94-2.1 94-2.2 94-3.1 94-3.2 94-3.4 94-3.5 94-3.7 Si 5.932 5.943 5.957 5.944 5.948 5.923 5.968 5.917 5.972 Al 4.001 3.962 3.987 3.991 3.990 4.067 3.995 3.971 3.981 Fe 4.550 4.330 4.545 4.533 4.257 4.1 73 4.501 4.566 4.453 Mg 0.590 0.588 0.597 0.629 0.572 0.561 0.663 0.656 0.706 Mn 0.557 0.493 0.546 0.542 0.490 0.331 0.503 0.520 0.508 Ca 0.434 0.747 0.4 12 0.418 0.796 0.985 0.398 0.465 0.413 Ti 0.001 0.004 0.001 0.000 0.000 0.000 0.001 0.000 0.001 TOTAL 16.064 16.069 16.045 16.058 16.054 16.041 16.030 16.095 16.034 GARNET; S A M P L E 262 Oxide 262-2.2 262-2.3 262-3.1 262-3.2 262-3.5 262-4.1 262-4.2 262-5.1 262-5.2 S i O , 37.20 36.34 36.13 35.81 35.59 36.95 36.05 36.58 36.04 A i p , 21.34 20.7 1 20.64 20.51 20.60 20.88 20.72 20.78 20.54 FeO 35.61 35.51 35.59 36.01 36.30 36.07 35.85 36.16 36.17 MgO 2.76 2.71 2.63 2.80 2.78 2.64 2.79 2.79 2.73 M n O 1.73 1.56 1.56 1.59 1.72 1.72 1.56 1.77 1.65 CaO 2.48 2.78 3.02 2.32 2.57 2.54 2.76 1.99 2.36 TiOj 0.03 0.02 0.04 0.00 0.02 0.01 0.02 0.02 0.00 T O T A L 101.15 99.69 99.60 99.03 99.57 100.80 99.76 100.08 99.50 Element 262-2.2 262-2.3 262-3.1 262-3.2 262-3.5 262-4.1 262-4.2 262-5.1 262-5.2 Si 5.944 5.914 5.896 5.886 5.837 5.947 5.878 5.934 5.898 Al 4.0 18 3.984 3.970 3.973 3.981 3.960 3.981 3.72 3.962 Fe 4.759 4.832 4.858 4.949 4.978 4.855 4.888 4.905 4.950 Mg 0.657 0.658 0.640 0.687 0.679 0.634 0.678 0.674 0.666 Mn 0.233 0.215 0.216 0.221 0.239 0.235 0.2 18 0.243 0.229 Ca 0.424 0.484 0.528 0.408 0.451 0.437 0.482 0.345 0.414 Ti 0.004 0.003 0.005 0.000 0.002 0.001 0.002 0.003 0.000 T O T A L 16.040 16.089 16.1 12 16.1 24 16.168 16.069 16.127 16.075 16.1 19 —1 GARNET: S A M P L E 370A Oxide 370A-1.2 3 70A-1.5 370A-2.1 370A-2.5 370A-3.1 370A-3.4 370A-4.1 370A-4.4 370-5.1 S i O , 37.63 36.90 37.35 37.31 36.77 36.12 36.59 37.44 36.66 A l , 0 , 21.68 20.80 21.10 21.21 21.15 21.08 20.93 21.33 2 1.27 FeO 35.00 34.57 34.79 35.07 34.83 35.64 35.30 35.59 35.01 MgO 3.1 1 3.56 3.22 3.04 3.00 3.38 3.16 3.05 3.36 M n O 2.03 2.22 2.24 1.92 2 26 2.27 2.17 1.95 2.40 CaO 2.06 1.72 2.17 2.37 2.25 1.53 2.22 2.26 2.07 TiOj 0.02 0.00 0.00 0.03 0.03 0.00 0.01 0.01 0.00 T O T A L 101.52 99.78 100.85 100.95 100.29 100.02 100.37 101.63 100.76 Element 370A-1.2 370A-1.5 370A-2.1 370A-2.5 370A-3.1 370A-3.4 370A-4.1 370A-4.4 370-5.1 Si 5.963 5.963 5.97 1 5.962 5.926 5.861 5.907 5.951 5.886 Al 4.049 3.961 3.975 3.994 4.017 4.031 3.983 3.996 4.025 Fe 4.637 4.672 4.651 4.687 4.695 4.835 4.766 4.730 4.701 Mg 0.735 0.857 0.766 0.725 0.721 0.816 0.760 0.723 0.804 Mn 0.272 0.304 0.303 0.259 0.308 0.312 0.296 0.262 0.326 Ca 0.350 0.298 0.37 1 0.405 0.388 0.266 0.384 0.384 0.357 Ti 0.002 0.000 0.000 0.004 0.004 0.000 0.001 0.001 0.000 T O T A L 16.008 16.054 16.039 16.035 16.059 16.121 16.098 16.048 16.099 4^  BIOTITE: SAMPLE 94 Oxide 94-1.2 94-1.3 . 94-2.1 94-2.2 94-3.1 94-3.2 94-3.4 94-3.5 94-3.7 SiO, 35.02 35.84 35.73 36.25 35.36 35.31 35.75 35.01 35.60 Al p , 18.91 19.13 18.97 19.67 18.58 18.74 19.6 1 18.87 18.80 Ti0 2 1.55 1.35 1.44 1.54 1.53 1.43 1.97 1.56 1.59 MgO 1 1.17 1 1.29 10.64 1 1.36 1 1.12 10.78 10.79 1 1.41 1 1.56 FeO 17.70 17.38 18.03 18.19 18.19 18.73 1 7.24 17.92 17.87 MnO 0.1 1 0.08 0.12 0.05 0.1 1 0.03 0.05 0.10 0.08 CaO 0.01 0.01 0.04 0.00 0.02 0.02 0.03 0.00 0.00 Na,0 0.16 0.24 0.25 0.21 0.20 0.20 0.22 0.23 0.25 K,0 8.64 8.36 8.42 8.70 8.77 8.43 8.97 8.72 8.60 H,0 3.90 ' 3.94 3.92 4.02 3.91 3.90 3.97 3.91 3.95 TOTAL 97.16 97.62 97.54 99.98 97.78 97.56 98.62 97.72 98.29 Element 94-1.2 94-1.3 94-2.1 94-2.2 94-3.1 94-3.2 94-3.4 94-3.5 94-3.7 Si 5.387 5.455 5.464 5.406 5.420 5.423 5.399 5.364 5.408 Al 3.429 3.432 3.419 3.458 3.355 3.392 3.491 3.408 3.367 Ti 0.1 79 0.154 0.166 0.172 0.1 77 0.165 0.224 0.179 0.182 Mg 2.560 2.562 2.425 2.526 2.540 2.468 2.430 2.606 2.619 Fe 2.277 2.2 13 2.307 2.268 2.33 1 2.405 2.1 78 2.297 2.270 Mn 0.014 0.010 0.016 0.006 0.014 0.004 0.007 0.013 0.010 Ca 0.001 0.002 0.006 0.000 0.003 0.003 0.005 0.000 0.000 Na 0.048 0.07 1 0.074 0.060 0.060 0.066 0.065 0.067 0.072 K 1.696 1.623 1.642 1.655 1.714 1.651 1.729 1.705 1.667 TOTAL 15.592 15.522 15.519 15.550 15.613 15.571 15.528 15.638 15.596 OH 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 F/(F+MX%) 47.2 46.5 48.9 47.4 48.0 49.4 47.3 47.0 46.6 —a BIOTITE: SAMPLE 262 Oxide 262-2.2 262-2.3 262-3.1 262-3.2 262-3.5 262-4.1 262-4.2 262-5.1 262-5.2 SiO, 34.82 34.82 34.91 36.10 34.78 35.34 36.28 34.72 34.98 A I A 18.74 19.31 18.81 19.10 18.87 18.62 19.75 18.53 18.15 TiO z 1.80 1.99 2.04 1.71 1.94 1.94 2.09 1.92 2.68 MgO 10.96 10.60 9.48 10.64 10.28 10.12 10.03 10.65 9.35 FeO 18.69 18.42 20.53 18.24 18.55 18.92 17.93 18.59 19.34 MnO 0.03 0.04 0.07 0.07 0.08 0.02 0.06 0.02 0.04 CaO 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 Na;0 0.39 0.18 0.21 0.22 0.26 0.32 0.35 0.24 0.31 K,0 8.32 8.46 8.44 8.50 8.52 8.28 8.60 8.63 8.75 H,0 3.90 3.91 3.90 3.96 3.88 3.90 3.99 3.88 3.87 TOTAL 97.64 97.73 98.40 98.54 97.14 97.47 99.07 97.17 97.48 Element 262-2.2 262-2.3 262-3.1 262-3.2 262-3.5 262-4.1 262-4.2 262-5.1 262-5.2 Si 5.352 5.335 5.366 5.464 5.373 5.435 5.450 5.369 5.415 Al 3.394 3.487 3.408 3.407 3.435 3.375 3.497 3.378 3.312 Ti 0.207 0.229 0.236 0.194 0.225 0.224 0.236 0.223 0.312 Mg 2.510 2.422 2.173 2.401 2.367 2.321 2.246 2.454 2.158 Fe 2.402 2.360 2.640 2.309 2.396 2.434 2.253 2.404 2.504 Mn 0.004 0.005 0.008 0.009 0.01 1 0.002 0.007 0.003 0.005 Ca 0.000 0.000 0.003 0.000 0.000 0.000 0.000 0.000 0.000 Na 0.1 15 0.054 0.063 0.064 0.076 0.096 0.102 0.072 0.093 K 1.631 1.654 1.654 1.642 1.679 1.625 1.648 1.702 1.727 TOTAL 15.616 15.546 15.553 15.491 15.562 15.513 15.440 15.605 15.528 OH 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 F/(F+MX%) 49.0 49.4 54.9 49.1 50.4 51.2 50.2 49.5 53.8 BIOTITE: S A M P L E 370A Oxide 370A-1.2 370A-1.5 370A-2.1 370A-2.5 370A-3.1 370A-3.4 370A-4.1 370A-4.4 370-5.1 S i O z 35.97 37.24 34.56 35.49 33.88 35.30 35.14 35.82 34.93 A I A 18.32 20.33 18.53 19.78 19.90 18.17 18.62 18.85 18.39 T i O , 1.89 2.17 1.62 2.48 1.25 2.84 1.92 2.38 2.20 MgO 10.88 1 1.41 9.38 8.77 1 1.03 . 9.34 9.72 9.67 9.52 FeO 18.13 15.69 22.00 18.51 • 19.88 20.93 19.95 19.66 20.35 M n O 0.04 0.03 0.06 0.06 0.08 0.1 1 0.09 0.1 1 0.09 CaO 0.06 0.03 0.00 0.00 0.05 0.03 0.01 0.00 0.02 Na,0 0.30 0.34 0.19 0.30 0.26 0.22 0.30 0.32 0.30 K 2 0 8.38 8.39 8.05 8.74 7.03 8.22 8.50 8.56 8.48 H ; 0 3.93 4.08 3.87 3.93 3.89 3.93 3.90 3.96 3.89 T O T A L 97.91 9.69 98.26 98.05 97.23 99.09 98.14 99.32 98.1 7 Element 370A-1.2 370A-1.5 370A-2.1 370A-2.5 370A-3.1 370A-3.4 370A-4.1 370A-4.4 370-5.1 Si 5.485 5.478 5.352 5.418 5.222 5.393 5.405 5.425 5.386 Al 3.291 3.526 3.382 3.558 3.615 3.27 1 3.375 3.365 3.341 Ti 0.217 0.240 0.188 0.284 0.145 0.327 0.222 0.271 0.255 Mg 2.474 2.502 2.164 1.994 2.534 2.127 2.227 2.182 2.188 Fe 2.312 1.930 2.849 2.362 2.562 2.674 2.566 2.491 2.624 Mn 0.006 0.004 0.008 0.008 0.0 10 0.014 0.012 0.014 0.012 Ca 0.009 0.004 0.000 0.000 0.008 0.005 0.00 1 0.000 0.003 Na 0.090 0.097 0.057 0.089 0.077 0.064 0.088 0.093 0.089 K 1.630 1.574 1.591 1.701 1.381 1.603 1.668 1.653 1.667 T O T A L 15.512 15.355 15.592 15.4 14 15.555 15 478 15.564 15.494 15.566 OH 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 F/(F + MX%) 48.4 43.6 56.9 54.3 50.4 55.8 53.7 53.4 54.6 


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