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Mesozoic ductile shear and paleogene extension along the eastern margin of the central Gneiss Complex,… Heah, T. S. T. 1991

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MESOZOIC DUCTILE SHEAR AND PALEOGENE EXTENSION ALONG THE EASTERN MARGIN OF T H E CENTRAL GNEISS COMPLEX, COAST BELT, SHAMES RIVER AREA, NEAR TERRACE, BRITISH COLUMBIA by T.S.T. HEAH B.Sc, University of British Columbia, 1982 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF GEOLOGICAL SCIENCES We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April, 1991 (c) Thomas S.T. Heah, 1991 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. Department of The University of British Columbia Vancouver, Canada DE-6 (2/88) i i ABSTRACT Near Terrace, British Columbia, the eastern margin of the Central Gneiss Complex (CGC) is a 3-4 km thick, gently northeast dipping, ductile-brittle shear zone with northeast movement of the upper plate. Along Shames River, deformed amphibolite-facies rocks to the west are juxtaposed against lower greenschist to amphibolite facies units to the east along the steep, east side down, brittle Shames River fault (SRF). Gentle to moderate northwest and northeast dips west of SRF contrast with steep southeast dips to the east. Lineations plunge gently northeast and southwest. West of SRF, the Shames River mylonite zone (SRMZ) separates granitoid rocks below from less deformed granitoid rocks, orthogneiss and metasedimentary rocks above. West of Exstew River, the moderately northeast dipping, ductile Exstew River fault, juxtaposes the SRMZ against metamorphic rocks and granitoids of the CGC. The SRMZ is cut by anastomosing brittle-ductile shear zones. Most kinematic indicators show northeast directed shear. Heterogeneous strain in SRMZ accommodates a minimum upper plate movement of 25 km to the east-northeast. Hornblende geobarometry indicates a structural omission of 13.4 km across SRMZ. East of SRF, amphibolite and greenschist facies supracrustal and plutonic rocks of Lower Permian and older Stikine Assemblage are thrust above greenschist facies volcanic strata correlated with Telkwa Formation of the Lower to Middle Jurassic Hazelton Group. Foliation in late synkinematic, 69 Ma granodiorite which intrudes this thrust package dips steeply southeast. Stikine Assemblage is comprised of lower greenstone, granitoid rocks, volcanic breccia and flows overlain by fusulinid-rich marble. A deformed intrusive rock in Stikine Assemblage has a minimum Pb-Pb date of 317 + 3 Ma. Hazelton Group contains lower andesitic and upper dacitic to rhyolitic packages comprised of agglomerate, volcanic breccia, tuff, and plagioclase porphyry flows. The earliest recognised metamorphism and deformation in the SRMZ, at upper amphibolite grade, affects 188 + 8 Ma orthogneiss, and occurred before intrusion of a garnet-biotite granite dated by Woodsworth et al. (1983) at 83.5 Ma. Early fabrics are overprinted by i i i Campanian to Paleocene ductile deformation and a second metamorphism. The second deformation waned during intrusion of three granitic intrusions with concordant U-Pb zircon crystallization dates of 68.7 - 69 Ma. A late to post-kinematic granite dyke in the SRMZ has a U-Pb zircon crystallization date of 60 ± 6 Ma. The second phase of metamorphism began before, and outlasted ductile deformation. The SRF and other high angle normal faults cut 69 Ma granodiorite, but do not significantly offset Eocene (46.2-52.3 Ma) K-Ar biotite cooling isothermal surfaces. The 60 Ma granite is deformed by low angle semi-brittle faulting with upper plate movement to the northeast. A 48 ± 3 Ma synkinematic granite dyke in the footwall of SRMZ was intruded during this deformation, which ended before 46.2 - 46.5 ± 1.6 Ma, the K-Ar biotite cooling dates from SRMZ. The entire region is deformed by post-ductile open, upright, east-northeast plunging folds. K-Ar biotite dates for granitoid rocks range from 51.1 Ma in the upper plate to 46.2 Ma in SRMZ, indicating downward progression of cooling. North-northwest trending brittle faults and lamprophyre dykes cut the SRMZ, and are therefore younger than mid-Eocene. Thermobarometry of pelitic and granitoid rocks indicates increasing metamorphic grade with increasing structural depth. Al-j; in hornblende geobarometry indicates slightly lower pressure of crystallization for the interior than the margin of a granodiorite body east of SRF.In the upper plate of SRMZ, west of SRF, sillimanite-staurolite-garnet schist records ductile deformation and metamorphism at 3.8 + 1.6 kbar and 570 + 50°C. The schist is intruded by orthogneiss cut by 68.7 Ma granodiorite. The granodiorite crystallized at 3.4 ± 1 kbar, and was deformed at 2.2 ± 1 kbar at 68.7 Ma. In SRMZ, hornblende in pre-kinematic, 188 ± 8 Ma granodiorite crystallized at 5.5 ± 1 kbar. Deformation and synkinematic metamorphism occurred at 4.9 ± 1 kbar, between 83.5 and before 60 + 6 Ma. East of SRF, greenschist conditions prevailed, except near the southern margin of the 69 Ma granodiorite body, where amphibolite facies was stable during ductile deformation. A metapelitic sample gives near-peak metamorphic conditions of 4.9 + 1.6 kbar and 700 + 50°C, and contact metamorphic conditions of 2.9 + 1.6 kbar and 610 + 50°C during intrusion of late syn-kinematic, 69 Ma granodiorite. i v P-T-time paths for the upper plate of SRMZ west of Shames River indicate initial rapid, near-isothermal decompression beginning before 69 Ma, continuing to 69 Ma, followed by rapid cooling to 0.9-1.1 kbar, at 51.1 Ma. Paleogene to middle Eocene deformation was probably extensional in nature. It occurred in a vigorous magmatic arc, in response to, and possibly coeval with, crustal thickening. V TABLE OF CONTENTS ABSTRACT ii LIST OF TABLES vii LIST OF FIGURES viii LIST OF PLATES ix ACKNOWLEDGMENTS x I. INTRODUCTION 1 A. LOCATION AND ACCESS 1 B. REGIONAL SETTING 2 II. LITHOLOGIES 10 A: METAMORPHOSED VOLCANIC AND PLUTONIC ROCKS EAST OF SRF 10 B. DEFORMED METAMORPHIC ROCKS WEST OF SHAMES RIVER FAULT 12 1. Upper Plate of SRMZ 12 2. SRMZ and Lower Plate of Exstew River fault (ERF) 15 C. LAMPROPHYRE AND MAFIC DYKES 18 D. CORRELATION 18 m. GEOCHRONOMETRY 31 A. INTRODUCTION 31 B. U-PB ZIRCON GEOCHRONOMETRY 31 1. Upper Plate of SRMZ 31 2. Lower plate and SRMZ 33 C. K-AR GEOCHRONOMETRY 35 D. SIGNIFICANCE OF U-PB AND K-AR DATES 36 IV. METAMORPHISM 43 A. INTRODUCTION 43 B. EASTWARD DECREASE IN METAMORPHIC GRADE 44 1. Western Block of SRF 44 a. Metamorphosed granitoid rocks and gneisses 44 b. Metamorphosed supracrustal rocks 45 2. Eastern Block of SRF 50 a. Mineralogy 50 b. Metamorphic grade 51 C. THERMOBAROMETRY 53 1. Thermobarometers Used 53 a. Garnet-biotite geothermometry 53 b. Garnet-homblende geothermometry 54 c. Garnet-plagioclase-quartz-Al2Si05 (GASP) geobarometry 54 d. Alj in hornblende geobarometry 54 2. Precision and Accuracy 55 3. Closure temperature 57 4. Results 58 a. Garnet-biotite and garnet-hornblende thermometry and GASP barometry 58 b. A1T in hornblende geobarometry 60 v i D. SUMMARY AND P-T-TIME PATHS 61 V. STRUCTURE 66 A. GENERAL STRUCTURAL CHARACTERISTICS 66 B. DEFORMATION WEST OF SHAMES RIVER FAULT 68 1. Upper plate of SRMZ 69 2. Lower plate and SRMZ 70 3. Folding 71 C. DEFORMATION EAST OF SHAMES RIVER FAULT 72 1. Description of fabrics 73 2. Folding 74 D. KINEMATIC INTERPRETATION 74 E. NATURE OF STRAIN 77 F. SUMMARY AND TIMING OF DEFORMATION 77 VI. SUMMARY AND TECTONIC EVOLUTION 96 A. SUMMARY 96 B. TECTONIC EVOLUTION 99 VH. REFERENCES 104 APPENDK 1. THIN SECTION DESCRIPTIONS OF SELECTED SAMPLES 117 APPENDIX 2. GEOCHRONOMETRIC ANALYTICAL TECHNIQUES 121 APPENDIX 3. ELECTRON MICROPROBE ANALYTICAL TECHNIQUES, DATA REDUCTION PROCEDURES AND THERMOBAROMETRIC RESULTS 147 v i i LIST OF TABLES MAIN TEXT: 1. Structural and metamorphic terminology used 9 2. Sequence of metamorphic mineral growth, metamorphic, intrusive and deformation events 63 3. Pressures obtained from Al-p in hornblende geobarometry 65 4. Geological correlation chart 102 APPENDIX: 1.1. Thin section mineralogy and shear sense 117 2.1. U-Pb zircon isotopic data and dates 122 2.2. K-Ar analytical data and dates 140 3.1. Microprobe standards used 149 3.2. Representative microprobe hornblende analyses 151 3.3. Representative microprobe garnet rim analyses 153 3.4. Representative microprobe biotite rim analyses 154 3.5. Representative microprobe plagioclase rim analyses 155 LIST OF FIGURES Frontispiece. View to the east at Mt. Remo xi 1. Regional geology and location of study area 8 2. Sketch of Shames River area, looking north 19 3. Schematic stratigraphic column of Shames River area showing major map units 20 4. Metavolcanic rock of unit IPSi 21 5. Looking east at vertical beds of marble of unit l P s m 22 6. Marble of unit lPsm with 1 cm long, pink fusulinids 23 7. Looking north-northwest at phyllitic foliation (XY) surface of unit UHa 23 8. View to northwest of gently northeast dipping bedding contact 24 9. Volcanic breccia of unit Uj j a 24 10. Looking northeast at SRMZ 25 11. Brittle-ductile shear between granite of unit T and folded mylonitic hornblende-biotite schist 26 12. Folded, sheared sillimanite-biotite-quartz-plagioclase mats 26 13. Banded mylonitic granodiorite of unit EJ„g of SRMZ 27 14. Looking north at roadcut in mylonitised biotite > hornblende granodiorite of unit EJgg 28 15. Flattened quartz in unit EJgg defining an S-fabric inclined at up to 25° to C-planes 29 16. Mylonitised hornblende-biotite orthogneiss of unit EJgg cut by mylonitic bands 29 17. Late synkinematic, faintly foliated granite dykes cutting mylonitised hornblende gabbro and biotite-hornblende granodiorite orthogneiss 30 18. U-Pb concordia diagrams for samples TH89179, TH89207c and TH89404d 39 19. Concordia diagram for sample TH89265 40 20. Concordia diagram for sample TH89257c 40 21. Concordia diagram for sample TH89253e 41 22. Concordia diagram for sample TH89131 41 23. Temperature-time plot for granitoid rocks in Shames River area 42 24. P-T-time path for sample 84WW82-3 collected south of Mt. Morris 64 25. P-T-time path for sample TH89044 collected east of Shames River 64 26. Structural domains of the Shames River area, showing lower hemisphere projections of lineations and poles to foliation 80 27. Rootless, isoclinal folds (F^) of calc-silicate layers 81 28. Looking north-northeast at tight, F m t folds which warp mylonitic foliation 81 29. Lower hemisphere projections of shear band (C) fabrics 82 30. Progressive deformation of granitoid rocks towards SRMZ west of SRF 83 31. Recrystallized quartz ribbon in unit EJgg 85 32. Looking west-southwest at S-shaped, isoclinal recumbent folds in granodiorite of unit LK2 85 33. View to northwest of well developed S-C and C fabrics 86 34. View northwest at sigma type porphyroclasts in granodiorite of domain III 86 35. Northeast directed (right) shear 87 36. Detail of Fig. 35, showing pulled apart sphene with flattened ribbon quartz 87 37. View to northwest of delta-type porphyroclast of plagioclase in deformed granodiorite 88 38. Sheared boudin showing northeast directed shear in deformed granodiorite of unit EJgg 88 39. Sketch from photomicrograph of S-C fabric and rotated porphyroclast 89 40. Conjugate ductile shears in hornblende orthogneiss of footwall of SRMZ 89 41. Lower hemisphere plots of fold axes and axial planes 90 42. Separation arc for fold axis vergence reversals west of SRF 90 43. View to northwest of sheared, recrystallized hornblende in SRMZ 91 44. Brittle - ductile fault in mylonitic orthogneiss in SRMZ 91 45. Semi-ductile listric normal fault rooting to the southwest in mylonitic layers 92 46. View to northwest of late shear bands cutting previously formed S-C fabrics 92 47. Steep ductile normal shears indicating sinistral movement in granite of unit LKj 93 48. View to west of gently warped biotite hornblende schist and granodiorite orthogneiss of unit EJgg 94 49. Conjugate ductile shears in granodiorite of unit LK3 94 50. View to northwest of late, sigmoidal quartz-feldspar tension gashes 95 51. View to northwest of mylonitised granodiorite of LK3 95 52. Hypothetical east-west cross-section of Canadian Cordillera during Paleocene time 103 LIST OF PLATES 1. Geology of Shames River area in pocket 2. Cross-sections A-A', B-B' and C-C in pocket 3. Sample locations in pocket 4. Location of kinematic indicators and dykes in pocket ACKNOWLEDGMENTS x I thank R.L. Armstrong for excellent supervision and discussion throughout this thesis. I also thank G.J. Woodsworth for introducing me to the area, and for providing moral support and insight. Together with G.J. Woodsworth and P. van der Heyden, J.A. Roddick provided much appreciated initial impetus to the project. Excellent field assistance was provided by C J . Green. J.M. Journeay's insight helped elucidate many ideas on the structural complexities of the area. The manuscript greatly benefitted from discussions with R.L. Armstrong, R.M. Bustin, S.A. Gareau, J.M. Journeay, K. Wilks, P.D. Lewis, T.A. Richards, J.K. Russell, P. van der Heyden, and G.J. Woodsworth. R.M. Friedman spent many hours elucidating the fine art of U-Pb zircon dating, and gave much appreciated moral support. P. van der Heyden, J. Gabites and D.K. Ghosh also made the U-Pb lab more "user friendly" for me. D. Runkle instructed me in mineral separation in the K-Ar lab, and did the K analyses. P. van der Heyden and J. Beekman kindly supplied field notes, ideas and photographs. J. Knight spent many long hours acquainting me with the intricacies of the electron microprobe. Fellow graduate students at University of British Columbia, particularly T.A. Delaney, J. Hunt, B. James, P.D. Lewis, and S. Taite gave moral support and valuable discussions. University of British Columbia technicians M. Baker, B. Cranston and Y. Douma are also thanked for technical advice. The project was funded by the Geological Survey of Canada, an NSERC grant to R.L. Armstrong, and a NSERC graduate fellowship. Frontispiece. View to the east at Mt. Remo. Rocks are predominantly grey, rounded weathering 69 Ma granodiorite which lies in upper plate of Shames River mylonite zone. Shames River is deep valley in centre of photograph. 1 /. INTRODUCTION In the Shames River area (Fig. 1), a thick, regionally extensive mylonite zone separates a lower plate of highly deformed amphibolite facies stratified and plutonic rocks of the Central Gneiss Complex (CGC) from less deformed granitoid rocks, orthogneiss and minor metasedimentary rocks above. East of this predominantly gneissic and granitoid package, and separated from it by the Shames River fault (SRF) are greenschist to amphibolite facies deformed volcanic and sedimentary rocks of the Hazelton Group, marble and metavolcanic rocks of the early Permian and older Stikine Assemblage, and deformed plutonic rocks. Rocks bothe west and east of SRF are intruded by the Late Cretaceous to Eocene Ponder pluton. Mylonitic fabrics along the east flank of the CGC and Coast Plutonic Complex (van der Heyden, 1982, 1989, 1990; Woodsworth etal., 1985; Sisson, 1985; Friedman and Armstrong, 1988; Rusmore and Woodsworth, 1988, 1989; Journeay, 1989, 1990) have recently attracted considerable attention. The nature, timing and kinematic significance of these fabrics in the Terrace area (Woodsworth et al., 1985; Heah, 1990), however, had not been studied in detail prior to this work. This paper describes the geology, metamorphism and kinematics of deformation in the Shames River area, gives new U-Pb and K-Ar geochronometric data which constrain the timing of deformation and metamorphism, and offers a model for the tectonic evolution of the eastern boundary of the CGC in the Terrace region. A summary of the structural and metamorphic terminology used in the text is shown in Table 1. A. LOCATION AND ACCESS The study area is centered about 25 km southwest of Terrace, along the eastern margin of the Central Gneiss Complex (Fig. 1). Skeena River flows from east to west through the southern part of the map area, while the Shames River drains into the Skeena River from the north, across the central part of the map area. Access to the lower elevations is from Highway 16, which follows the north bank of Skeena River in the study area. Well maintained logging 2 roads along the Shames and Exstew rivers, the south bank of Skeena Paver, and rough logging roads near Dasque, Amesbury and Delta creeks, give access to the middle slopes (Plate 1). Higher elevations are reached either by helicopter from Terrace, or by foot through steep bush and timber. Exposure is excellent on the ridge tops, river banks and road cuts, and poor along the bushy hill sides. B. REGIONAL SETTING The core of the Coast Plutonic Complex in the Prince Rupert-Terrace region is composed of amphibolite- to granulite-facies rocks of the Central Gneiss Complex (CGC) (Hutchison, 1982). CGC rocks extend southward from southeast Alaska to near Bella Coola. Deformed migmatitic layers in CGC indicate that ductile shearing postdated or was synchronous with high grade metamorphism. Protoliths of the CGC are arc- and continent-derived plutonic, volcanic and sedimentary rocks of Paleozoic and Mesozoic age (Armstrong and Runkle, 1979; Hutchison, 1982; van der Heyden, 1982, 1989; Hill, 1984, 1985; Barker and Arth, 1984; Gareau, 1988, 1989; Currie, 1990). Based on the occurrence of crinoid stems similar to those in Lower Permian limestones of Stikinia and presence of metamorphosed equivalents of the Upper Jurassic Bowser Lake Group, Hill (1985) and Hill et al. (1985) suggested that some of the supracrustal rocks in the eastern part of the CGC represent a highly metamorphosed portion of Stikinia. A. 139 Ma U-Pb zircon date from leucogneiss in the Redcap Mountain area (Hill, 1984) gives a minimum age for the protolith, which is thought to be similar to silicic volcanic rocks of Stikinia. Other supracrustal sections of the CGC may belong to Gravina-Nutzotin (Douglas, 1983) and Nisling terranes (Currie, 1990; Gareau, pers. comm., 1990; Gehrels et al., 1990; Samson et al., 1990). In the Yukon, the Upper Proterozoic to Lower Paleozoic Nisling Terrane (NT) can be traced into the lower part of the composite Yukon-Tanana Terrane (YTT) (Hansen, 1990). NT is considered to represent para-autochthonous North American margin (Hansen, 1990; Samson et al., 1990; Gehrels et al., 1990). The CGC is separated from kyanite zone rocks to the west by a prominent topographic and structural feature, the Work Channel lineament (Fig. 1). According to Crawford and 3 Crawford (1990), this lineament marks the approximate position of the suture between the Insular and Intermontane superterranes. In most areas, the Insular-Intermontane suture has largely been obliterated by mid-Jurassic to Tertiary plutons (van der Heyden, 1989). West of Work Channel, K-Ar biotite and hornblende cooling dates of 70-110 Ma are recorded in the Ecstall pluton (Harrison et al., 1978; Gareau, pers. comm., 1991). Crawford et al. (1987) proposed that the CGC was rapidly uplifted at up to 1 mm/yr during 62-48 Ma along the Work Channel lineament. Underplating of the continental crust by intermediate magmas is postulated as one cause of uplift (Hollister, 1982; Douglas, 1983). Concordant U-Pb zircon dates of 60-57.1 Ma were obtained by Armstrong and Runkle (1979) and Gareau (1988, 1989) for Quottoon pluton, emplaced during amphibolite- to granulite-facies metamorphism of the CGC (Kenah, 1979; Kenah and Hollister, 1982). Metamorphic conditions in the CGC progressed from high-P, moderate T to low-P and high-T (Crawford et al., 1987). K-Ar cooling dates of 50-43 Ma obtained from the CGC and associated plutons by Harrison et al. (1978), Wanless et al. (1979), Hutchison (1982), Hill (1985) and van der Heyden (1989) are consistent with Tertiary denudation and/or widespread intrusion. In the Scotia-Quaal area to the west, the sudden metamorphic grade increase eastward across Work Channel lineament that exists to the north is not present (Gareau, 1990). Instead, rocks which equilibrated at 8 kbar are found in a broad zone of intense folding and no significant faulting (Gareau, pers. comm.). Late Cretaceous K-Ar cooling dates suggest that significant deformation of this western belt was over by 70 Ma. Tertiary deformation appears to have been concentrated within the Quottoon pluton to the east. East of the CGC is the Stikine Terrane, formed in one early Mesozoic arc that was accreted to North America in Middle Jurassic time (Rusmore and Woodsworth, in press). Stikinia is comprised of a distinctive package of Lower Permian platformal carbonates overlain unconformably by Late Triassic arc volcanic rocks typified by Stuhini Group. In northwest B.C. and southeast Alaska, Stikine Terrane volcanic rocks have E N d = +3 to +7, similar to modern intraoceanic volcanic island arc rocks (Samson et al., 1990). 4 West and south of Terrace, the eastern boundary of the CGC with Stikine Terrane is largely engulfed by Late Cretaceous to Eocene calc-alkaline plutons, including the Ponder pluton (Hutchison, 1982; Woodsworth etal., 1983, 1985; Sisson, 1985). East of this boundary, Late Mesozoic to Paleogene structures are north- to east-verging thrust faults involving rocks as young as the Lower Cretaceous Skeena Group (Woodsworth et al., 1985). Jurassic to mid-Cretaceous uplift in one or more stages is indicated by westerly derived sediments in the Late Jurassic Bowser Lake Group (Tipper and Richards, 1976), Early Cretaceous Skeena Group (Bassett, 1991) and in the latest Cretaceous, non-marine Brian Boru Formation (Richards, pers. comm., and 1990). East dipping, late Cretaceous thrusts interleaving Permian carbonate with Jurassic volcaniclastic rocks, are also reported in the Zymoetz River area, 28 km southeast of Terrace (Mihalynuk, 1987). The informally-named "Kitselas volcanics" of the Terrace map area, which are exposed west and north of Terrace, are of unknown age, and in parts resemble volcanic rocks of the Gamsby Complex, a Middle to Late Jurassic metavolcanic-tectonite complex exposed in the Whitesail Lake area, 130 km to the southeast (Woodsworth, 1979; van der Heyden, 1982, 1989). Kitselas volcanics may in part represent metamorphosed Hazelton Group (Woodworth et al., 1985). South of Skeena River, and east of Dasque Creek in the Shames River map area, van der Heyden,(1982) obtained a whole rock Rb-Sr isochron of 150 ± 47 Ma with an initial ratio of 0.70504 from felsic and intermediate volcanic rocks. In the Whitesail Lake area, the Gamsby Complex, intensely deformed in the late Jurassic, is brittlely thrust over the Hazelton Group. The northeast-directed, mid- to Late Cretaceous brittle thrusts may form part of a regionally extensive thrust system extending from Alaska to southern B.C. (van der Heyden, 1982, 1989, 1990; Rusmore and Woodsworth, 1988, 1989; Crawford et al., 1987; Rubin et al., 1990). At Deep Creek, 10 km north of Terrace, the base of the Kitselas volcanics is in ductile fault contact with deformed granodiorite and orthogneiss of the CGC. Kinematic indicators suggest upper-plate-to-the-northeast directed shear. The timing and nature of this ductile 5 shearing is equivocal. Nowhere are gneisses similar to the CGC found above Kitselas volcanic rocks in the area. Near the Shames River area, foliations in the CGC have gentle to moderate northeasterly and northwesterly dips, and contain lineations which plunge gently northeast or southwest (Hutchison, 1970, 1982; Kenah, 1979; Woodsworth et al., 1985). In Stikine Terrane to the east, foliations dip steeply southeast, but lineations are concordant with those found in the CGC, leading Woodsworth et al. (1985) to postulate a common origin for structures in the two diverse lithologic packages. In the Shames River area, a 3-4 km thick zone of highly deformed granitoid and metasedimentary rocks, and minor migmatite, herein termed the Shames River mylonite zone (SRMZ), separates a lower plate of orthogneisses, migmatites, schists and undeformed granitic rocks of Early Jurassic to Eocene age (Woodsworth et al., 1984, 1985) from an upper plate of mildly to highly deformed granitoid rocks, orthogneisses, and minor metasedimentary schists tentatively correlated with the Late Jurassic Bowser Lake Group. The steep Shames River brittle normal fault (SRF) separates the deformed orthogneiss-granitoid-metasedimentary sequence on the west from volcanic and sedimentary rocks tentatively correlated with the Early to Middle Jurassic Hazelton Group, and lesser plutonic rocks, limestone and metavolcanic rocks of the Early Permian and older Stikine Assemblage (Woodsworth et al., 1985). These rocks were studied by Duffel and Souther (1964), and more recently by Tipper and Richards (1976), Woodsworth et al. (1985), Mihalynuk (1987) and Richards (1990) near Terrace. Granodiorite of the Ponder Pluton intrudes the eastern and western blocks of the SRF. Although previously thought to be entirely Eocene in age, based on K-Ar dating (Hutchison, 1970, 1982), the Ponder Pluton is now considered to be composed of several plutonic bodies ranging in age from Late Cretaceous to Eocene (Harrison et al., 1978; Woodsworth et al., 1984; Sisson, 1985; UBC unpublished data; van der Heyden, 1989). Overprinted on portions of the Ponder Pluton are the regionally extensive, gently east-northeast plunging lineations discussed earlier, suggesting a deformation episode which postdates mid to Late Cretaceous thrusting and intrusion. This episode of deformation was 6 postulated to be extensional in nature by van der Heyden (1989), based on observations in the Whitesail Lake area, and the eastward younging of K-Ar biotite cooling dates in the CGC from the belt of rocks west of the Work Channel lineament. In this interpretation, the CGC was uplifted during Early Tertiary time as an extensional metamorphic core complex, in response to regionally extensive early Tertiary magmatism and crustal collapse which postdates mid- to Late Cretaceous thrusting and crustal thickening (Heah, 1990). Uplift was facilitated and accompanied by the presence of abundant anatectic melt (Crawford et al., 1987). Late Cretaceous to Paleocene thrusting in the Skeena Fold Belt to the northeast has also been genetically linked to uplift of the Coast Plutonic Complex (Evenchick, 1989, in press). Volcanic events in the Intermontane Belt to the east may be related to igneous and metamorphic activity in the CGC (Crawford et al., 1987). Regionally extensive, Late Cretaceous to Eocene volcanism in the Skeena Arch occurred in block faulted complexes (Macintyre, 1985; Richards, 1990 and pers. comm.). In addition, Paleocene-age Moricetown sedimentary rocks are preserved in graben s. In the Iskut River area to the north, Anderson (1990) documents Eocene brittle extensional faulting related to bimodal plutonism. There, Eocene granitoid rocks are undeformed, and provide a younger limit on the age of extensional deformation. Eocene brittle extensional faulting is also documented in the Queen Charlotte area to the west (Lewis, 1991). The early Tertiary was a time of changing plate-tectonic interactions between the Kula and Farallon Plates and North America, from predominantly orogen-normal subduction to orogen-parallel transpression and transtension (Engebretson et al., 1984, 1985; Lonsdale, 1988; Stock and Molnar, 1988). According to Lonsdale (1988), oblique subduction began abruptly at 57 Ma. The onshore expression of this plate tectonic change may be the development of an extensive series of north-northwest trending transtensional faults, some of which have been correlated with the formation of extensional metamorphic complexes (Price and Carmichael, 1986; Friedman and Armstrong, 1988; van der Heyden, 1990). Extension in these core complexes was triggered by post-compressional collapse in a vigorous magmatic arc which weakened the crust (Friedman and Armstrong, 1988; Armstrong and Ward, in press). Richards (pers. comm., 1991) envisions the Late Cretaceous to early Tertiary block fault pattern in the Intermontane Belt as having formed within a brittlely deformed block subjected to dextral shear during extensional faulting. The latest structural features in the area are north west-trending, high angle brittle normal faults. Similar faults have been recognised in the Whitesail Lake, Bella Coola and Anahim Lake areas (van der Heyden, 1982, 1990). Some faults, such as those along the Kitsumkalum-Kitimat graben, are the sites of recent tectonism, as evidenced by the 250-year old extrusion of alkali-olivine basalt, the presence of hot springs and seismic activity near Terrace, and present day structurally-controlled topography. 8 | | Plutonic Rocks Coast Belt west of Work Channel : Alexander Terrane V/.\ Gneisses and schists Central Gneiss Complex K ' ->| Orthogneisses and schists Y / \ Paragneisses and schists Stikine Terrane Lower and Middle Jurassic | v v | Hazelton Group Devonian-Permian h! i ! \ Asitka Group Overlap Assemblages: Upper Cretaceous | A A A | Brian Boru Formation Cretaceous I*. *»l Skeena Group Middle and Upper Jurassic .1 Bowser Lake Group • 47±3 Biotite K-Ar ages from Hutchison (1982) — — Normal fault —fc— Thrust fault Figure 1. Regional geology and location of study area (modified after Wheeler and McFeely, 1987; Gareau, 1989, Runkle, 1978; Armstrong and Runkle, 1979, and present mapping). Plutonic rock ages: ET, early Tertiary; KT, Cretaceous-Tertiary; LK, Late Cretaceous; mK, mid-Cretaceous; JK, Jurassic-Cretaceous; mJ, mid-Jurassic; J, Jurassic; EJ, Early Jurassic; S, Silurian. Biotite K-Ar dates are reported with 2a error. 9 Table 1. Structural and metamorphic terminology used in text. Symbol Description Deformation: D N D F D B D m P r e - D m Metamorphism: M2 M l Folds: FF 1 mi Fmt High angle brittle normal faulting. Open map scale and mesoscopic folding of Middle Eocene and younger age. Low angle detachment faulting of Paleocene to Middle Eocene age. Ductile deformation of Late Cretaceous to Paleocene (83.5-60 Ma) age. Late Cretaceous (83.5 Ma) and older ductile deformation. Amphibolite grade contact and regional dynamothermal metamorphism of Late Cretaceous to early Eocene age. Late Cretaceous and older amphibolite grade regional metamorphism. Open upright map scale and mesoscopic folds related to Dp . Isoclinal, recumbent, rootless sheath folds with hinge lines parallel to elongation lineations. Restricted to most deformed and layered parts of S R M Z , and related to D m . Tight, recumbent to reclined folds which may warp F ^ folds. Hinge lines at low to high angles to elongation lineations and F ^ hinge lines. Related to D m . 10 II. LITHOLOGIES The Shames River area is underlain by two diverse lithological assemblages separated by the north west-trending, high angle Shames River fault (SRF, Plate 1). To the east, rocks of the Stikine Terrane are comprised of deformed granitic and lower amphibolite facies metamorphosed volcanic rocks which lie structurally above less metamorphosed volcanic and sedimentary rocks (Fig. 2). West of the SRF are slightly to highly deformed granitoid rocks, migmatite, orthogneiss, schist and metasedimentary and calc-silicate rocks (Fig. 3). A. METAMORPHOSED VOLCANIC AND PLUTONIC ROCKS EAST OF SRF The units east of SRF are composed of epidote-actinolite to amphibolite facies deformed volcanic, plutonic and minor metasedimentary rocks. These are intruded by granodiorite of unit LK3 in the north. A sharp eastward decrease in strain and metamorphic grade across steep, north northeast trending brittle faults characterizes the eastern block of SRF. Deformed metavolcanic and granitoid rocks comprise unit lPgi- Textures and mineral compositions indicate that the volcanic rocks were originally andesitic to rhyolitic flows and pyroclastic rocks. Most of the volcanic rocks are dark, dense greenstones comprised of fine grained mixtures of plagioclase (An > 2o), actinolitic hornblende, chlorite, epidote, quartz and fine grained biotite (Fig. 4). East of Delta Creek, a thin layer of phyllite containing rounded elongate clasts with gentle northeast plunge is present. The clasts are composed of a fine grained, foliated mass of biotite, stilpnomelane, quartz and plagioclase. The protolith of this rock is thought to be a conglomerate with volcanic or fine grained granitic clasts. Foliation, where present, is defined by layers of actinolite, actinolitic hornblende, chlorite and plagioclase. Euhedral actinolite needles are usually aligned parallel to elongation lineation, but some cross-cut it (Fig. 4). Plagioclase phenocrysts in flows, and lithic fragments in pyroclastic rocks, are stretched parallel to well developed, predominantly southeast-dipping, mylonitic foliations. Granitoid rocks are brittlely to ductilely sheared, frequently mylonitic, heavily chloritized and epidotized quartz diorite and hornblende diorite. Contacts between units are 11 brittle shears. In thin sections, the hornblende in granitic rocks is altered to actinolite, epidote, albite and chlorite. Unit lPsm (Fig- 5), whose lower contact with unit lP$i is brittlely sheared, consists of medium to coarse grained, white and grey, crystalline and argillaceous marble. Bedding, which dips steeply north and south, has been transposed by at least two periods of tight to isoclinal folding. Foliation is axial planar to upright second phase folds. Pink, ovoid fusulinids up to 4 mm long were observed at one outcrop east of Shames River (Fig. 6). Similar rocks found to the east belong to the Early Permian Stikine Assemblage with which unit lPsm * s tentatively correlated (Mihalynuk, 1987; Woodsworth et al., 1984, 1985). In places, thin layers of medium and dark green, phyllitic and deformed volcanic rocks are present (Fig. 5). Argillaceous layers contain actinolite, chlorite, incipient biotite and epidote. West of Delta Creek, this unit is overlain along a sheared, disconformable, contact by andesitic volcanic rocks of unit Una- m t n e Zymoetz River area east of Terrace basal conglomerate of Hazelton Group lies with angular unconformity above Early Permian limestones (Mihalynuk, 1987). Unit Ujja consists of dark green-grey andesitic breccias, tuffs, agglomerates, flows, and metasedimentary rocks. Although similar in composition and texture to parts of lP§i, this unit is distinguished from IPsi by its lower metamorphic grade, presence of amygdaloidal and pillow textures, and lesser amounts of greenstones. Vesicular, porphyritic and amygdaloidal textures are common in pillow-shaped fragments, many of which are rimmed by a light medium pistachio green rind comprised of epidote, chlorite and plagioclase. In places, subangular fragments interpreted as either pillows or bombs up to 10 cm across are enclosed in a dark green phyllitic matrix composed of actinolite-chlorite-quartz-feldspar-epidote. The fragments are commonly elongate and plunge gently northeast, parallel to actinolite lineations (Fig. 7). East of Amesbury Creek, light green to maroon-green angular fragments up to 1 m across are set in a dark green, lithic lapilli to lapilli ash tuff matrix (Fig. 8, 9). West of Amesbury Creek, a thin bed containing subrounded, elongate clasts in a phyllitic matrix crops out along logging roads above Skeena River. 12 West of Delta Creek, unit Ujia is overlain by dacitic to rhyolitic flows, tuffs and breccias of unit Und- Siliceous clasts and phenocrysts are elongate and plunge gently northeast. East of Amesbury Creek, pyroclastic textures are well preserved. There, angular breccia fragments from 2 mm to 1 m across are porphyritic andesite to rhyolite set in a dacitic tuff matrix. Bedding dips 25°-45° northeast. Minerals in the tuffaceous matrix consist of very fine-grained mixtures of plagioclase, quartz, actinolite, chlorite, and epidote. Unit LTjjd may be similar to the "upper member" of the Telkwa Formation of the Hazelton Group, which, from bottom to top, contains: lapilli ash tuff, quartz bearing tuff, welded crystal tuff, lithic ash tuff and vitric ash tuff and flows (Mihalynuk, 1987). B. DEFORMED METAMORPHIC ROCKS WEST OF SHAMES RIVER FAULT The SRMZ is a 3-4 km thick, regionally extensive mylonitic zone that separates an upper plate of slightly to highly deformed and metamorphosed plutonic, orthogneissic and minor metasedimentary rocks from a lower plate of deformed orthogneiss, migmatite, granitoid rocks, and minor schist. The rocks are amphibolite grade, and intruded by granite, tonalite and granodiorite dykes and sills. West of Exstew River, the SRMZ is truncated by the moderately northeast dipping Exstew River fault (ERF). 1. Upper Plate of SRMZ SRMZ upper plate rocks form the ridges and cliffs north of Skeena River and east of the Exstew River logging roads. The major upper plate units are: lowermost, and interfingering with higher units, faintly foliated, late to post-kinematic granite (unit T), then moderately deformed granodiorite (LK2) with screens of orthogneiss and schist, and pendants of metapelitic rocks possibly correlative with the Bowser Lake Group (UJR), and uppermost, slightly to moderately deformed hornblende granodiorite and minor orthogneiss with schistose lenses (LK3)(Fig. 10). 13 Unit T, the youngest mappable unit in the Shames River area, is best exposed on the ridges east of Exstew River and south of Skeena River. It consists of unfoliated to faintly foliated hornblende- and biotite-granite and granodiorite dykes and sills which cut mylonitic fabrics in the SRMZ, and variably deformed granitoid rocks in the upper and lower plates of SRMZ. On a ridge east of Exstew River, a semi-brittle fault, with upper-plate to the northeast movement, juxtaposes granite of unit T against folded and deformed biotite-hornblende schists of unit E J u m (Fig. 11). South of Skeena River, a different, coarser grained, body of unit T forms the footwall to the SRMZ. The upper contact of this body with the SRMZ, however, was not observed. Along Highway 16, 1 km west of Shames River, faintly foliated granite dykes intrude the SRMZ but are themselves dragged into parallelism with brittle-ductile shear zones. One sample has a U-Pb zircon crystallization date of 60 + 6 Ma, the end of ductile deformation (D m , Table 1), and the beginning of semi-brittle detachment faulting (Dg). Unit L K 2 , best exposed on the ridges south of Mt. Morris, is characterised by grey to rusty-beige weathering, coarse grained, mylonitized biotite granodiorite with or without hornblende and sphene. Abundant screens of gneissic and schistose material are common, many of which are isoclinally folded about gently northwest- and north-dipping axial surfaces. The rocks are cut by numerous ductile normal shear zones, some of which affect unfoliated pegmatite veins. Plagioclase commonly displays oscillatory zoning, indicating its igneous origin, and ranges from A n 2 0 to A n 3 u . Plagioclase and quartz are commonly dynamically recrystallized. Unit L K 3 , which forms the rounded ridge tops and peaks both east and west of Shames River (Frontispiece, Fig. 2), intrudes unit LK2 along ridges south of Mt. Morris. It consists of hornblende > biotite granodiorite with sphene. Foliations are weak to moderate. Near unit LK2, foliations are moderately well developed. East of SRF, rocks of unit L K 3 are less foliated than those to the west. In addition, lineation is absent in LK3 east of SRF. Mafic enclaves locally comprise up to 35% of the rock. In thin sections, the rock is hypidiomorphic, and contains zoned plagioclase, quartz, K-feldspar, hornblende, biotite, 14 epidote, sphene, apatite, minor zircon and opaques. Two concordant U-Pb zircon crystallization dates of 68.7-69.0 + 1 Ma have been obtained from west of Shames River (Plate 3). Plagioclase (An25_38) shows both normal and oscillatory zoning. In many places, it displays undulose extinction, and is dynamically recrystallised. Euhedral laths of plagioclase and hornblende define a well developed mineral lineation which plunges gently northeast. The concordance of lineations in this unit with those of underlying, more deformed units, indicates that unit LK3 was emplaced during late stages of deformation. Myrmekite is present in many samples as globular and platy masses. Hornblende is common as euhedral and subhedral grains, and is in places parallel to plagioclase laths. In general, there is little retrograde alteration of the minerals except for very minor sericitisation of feldspars and saussuritisation of hornblende. Biotite is fresh, and undeformed. East of the SRF, unit L K 3 intrudes and metamorphoses volcanic rocks of unit Ujj(j along a 100-150 m wide agmatite zone containing angular xenoliths up to 4 cm across. The granodiorite is fine grained and foliated sub-parallel to the contact, which dips moderately southeast. Metamorphosed pelitic screens within the granodiorite contain garnet-sillimanite-biotite mats which are tightly folded and sheared (Fig. 12). Six kilometres northeast of that contact, a 150 m wide mylonite zone, dipping steeply southeast and trending northeast, is found in the granodiorite. On Shames Mountain, west of SRF, a discrete, 200 m long, 10-15 m wide mylonite zone dipping steeply and variably northwest and southeast occurs within undeformed, lineated granodiorite. There, biotite granite and pegmatite dykes cut mylonitic fabrics at high angles but are themselves mylonitized parallel to the fabric of the country rocks. 15 2. SRMZ and Lower Plate of Exstew River fault (ERF) The lower plate of the SRMZ consists of coarse grained, undeformed biotite granodiorite south of Skeena River (Plate 1). The transition from the footwall to the SRMZ is not exposed. West of the Exstew River, the footwall of the ERF consists of orthogneiss, migmatite, and hornblende-biotite granodiorite and tonalite sheets. The hanging wall consists of the SRMZ and its upper plate. The transition from the footwall to the hanging-wall of ERF is marked by a strain gradient, expressed by an increase in X:Z length ratios and in the amount of granitic rather than mafic amphibolite rich boudins. In addition, there is a decrease in grain size and amount of partial melt and migmatite, particularly along ductile shear zones, and a decrease in the inter-shear angles of conjugate ductile shears as the hangingwall of ERF is approached from below. North of Skeena River, the ERF is gently folded, with gentle north and east plunge, and is drawn at the lowermost part of a medium to coarse grained, ductilely deformed, gneissic garnet-biotite granite unit (LK\). Below the ERF, conjugate ductile shears typically have inter-shear angles of 110° or more, decreasing to about 80-100° in SRMZ, and 65-80° in the upper plate. The rocks in the SRMZ are deformed granitoid rocks and orthogneiss, with minor pelitic and calc-silicate layers (Fig. 13). Unit m forms the footwall of ERF west of Exstew River, and consists of migmatite, biotite and hornblende-biotite granodiorite orthogneiss and tonalite sheets cut by abundant ductile shears. Elongate schlieren of fine grained biotite-hornblende-feldspar schist are common parallel to foliation. Gneissic compositional layering is defined by leucosomes of foliated to unfoliated tonalite interlayered with hornblende- and biotite-rich melanosomes. Foliated granitic sheets up to 1 m thick are common. Oscillatory zoning in plagioclase is observed in many rocks, indicating its original igneous nature. In other rocks, the feldspars are completely annealed, some to strain free grains, and others to grains which show undulose extinction. The An-content in plagioclase is 25-40%. Quartz rods are present in some rocks. Most of the quartz form dynamically recrystallized grains. Rusty metasedimentary and amphibolite layers are rare. Lineation is defined by streaked out aggregates of quartz and 16 feldspar. In thin-sections cut parallel to the X-Z plane, X:Z ratios may reach 10:1. The amphibolite layers are composed of plagioclase and hornblende, and have a granoblastic texture. The layered and deformed plutonic rocks are cut by hypidiomorphic granodiorite or tonalite sheets, felsic veins, and pegmatites. South of Skeena River, white, undeformed, coarse grained, biotite granite (unit T) forms the lower plate of SRMZ. The contact between unit T and SRMZ is not exposed, but is inferred to be a low angle ductile fault (Plate 2). Unit LK4, a medium to coarse grained, ductilely deformed and gneissic, biotite-garnet granite, is best exposed along a railway cut east of the confluence of the Exstew and Skeena rivers. Angular xenoliths of migmatite and granodioritic orthogneiss are common near its lower contact with unit m. Garnet occurs as phenocrysts up to 1 mm across, and molybdenite occurs as rare flakes up to 5 mm across. Textures grade from hypidiomorphic to strongly foliated. The rocks are cut by abundant ductile normal shears (Fig. 47), many of which are invaded by moderately foliated tonalite. Toward the upper and lower margins of this unit, grain size decreases, and textures become mylonitic. In thin-sections, garnet comprises up to 3% of the rocks, is euhedral to subhedral, and light flesh pink in colour. Inclusions are rare, and the garnet rims are unaltered. No internal foliation is present. Plagioclase commonly shows oscillatory zoning and undulose extinction, and has an An-content of 25-35%. Plagioclase is dynamically recrystallized to strain-free-grains. Rare muscovite is fresh, and forms interlocking grains with plagioclase, and may be primary in nature. The distinctive, cliff forming rocks of unit EJgg are best exposed in road cuts on Highway 16, about 1 km west of Shames River (Fig. 14). This unit consists of banded, mylonitized biotite> hornblende granodiorite orthogneiss (Fig. 13) with a U-Pb date of 188 + 5 Ma, hornblende gabbro and diorite, and subordinate metasedimentary rocks of unknown affinity. Metasedimentary units are biotite-quartz-feldspar+garnet+epidote schist and gneiss, and garnet-diopside-calcite gneiss. Peculiar to this unit are sheared felsic boudins of aplitic and granitic material in contrast to amphibolitic boudins west of Exstew River in the footwall of 17 the ERF. This difference may reflect the lower ductility and/or higher shear strain rates of the SRMZ compared to rocks in the footwall of ERF. Biotite-hornblende granodiorite orthogneiss of EJgg is medium to coarse grained, and may contain sphene and pyrite. Plagioclase is commonly An25_3g, and forms dynamically recrystallized grains and porphyroclasts. In parts, oscillatory zoning is preserved in the plagioclase. Quartz forms dynamically recrystallized grains and elongate rods which form a pronounced elongation lineation. In some rods and quartz aggregates, annealing has led to strain-free subgrains with only slight optical misorientation. Well developed S-C fabrics are common (Fig. 15). Concordant and discordant, dark grey to black, mylonitic bands consisting of broken feldspars up to 4 mm set in a fine grained, granular matrix cut the rocks (Fig. 16). In places, mylonitic bands completely engulf sheared granodiorite boudins. Other dark grey layers cut and are themselves cut by granodiorite, and are interpreted to be deformed syn-plutonic mafic dykes. These layers commonly occur as detached slivers of biotite-hornblende-feldspar schist within the granodiorite. Elongate, mafic schlieren may represent metamorphosed, deformed volcanic xenoliths. Some of these are altered to epidote-chlorite-feldspar-quartz masses. Interlayered with the granodiorite are minor layers of mylonitized hornblende gabbro and diorite bodies. These units are commonly cut by anastomosing shear zones which sometimes cuts earlier-formed S-C fabrics. The unit is intruded by unfoliated to faintly foliated, biotite-muscovite granite dykes and sills (Fig. 17), and late, unfoliated, biotite lamprophyre dykes trending 330-350°. Compositional layering in orthogneiss is defined by alternating leucosomes composed of medium grained, deformed granodiorite, and melanosomes of fine grained, hornblende-quartz-plagioclase schist. The melanosomes have a granoblastic texture, and form continuous layers in both the XZ and YZ planes. In the leucosomes, deformed granodiorite forms a strong L-tectonite fabric, in which XZ planes display streaky mineral textures and X/Z ratios of up to 15:1, and YZ planes display hypidiomorphic textures and Y/Z ratios of up to 2:1. Unit EJgg grades upwards with decreasing grain size into well foliated biotite-hornblende-quartz-feldspar schist and orthogneiss of unit EJum, which forms recessive units on 1 8 the middle slopes east of Exstew River. Biotite-hornblende-quartz-feldspar schists are seen to be in shallow, semi-brittle fault contact with faintly foliated granite of unit T on a ridge 2 km east of Exstew River (Fig. 11). Tight S-folds in schists underlying T plunge gently north, and indicate movement of the upper block to the northeast along gently plunging slickensides. C. LAMPROPHYRE AND MAFIC DYKES Unfoliated and steeply dipping lamprophyre dykes containing biotite phenocrysts, basalt and andesite dykes trend northwest. They intrude, and are common in all units mapped (Plate 4). Hutchison (1982) obtained a whole-rock K-Ar date of 41.1 + 5.6 Ma from a mafic dyke west of the area. Other lamprophyre dykes in the region are less than about 35 Ma (Harrison et al., 1978; van der Heyden, 1989). D. CORRELATION Correlation of supracrustal units is hampered by the scarcity of isotopic dates and fossils, and geochemical data in the map area. Greenschist and higher grade metamorphism and strong deformation also obscures contact relationships and original textures and features. Unit lPsm * s tentatively correlated with the Lower Permian and older Stikine Assemblage (Monger, 1977), based on the presence of fusulinids, and similarities to Stikine Assemblage rocks described by Tipper and Richards (1976), Monger (1977), Woodsworth et al. (1985), and Mihalynuk (1987). A metaplutonic rock of IPsi, thrusted over unit lPsm> B n < ^ intrusive into surrounding volcanic units, has a minimum U-Pb zircon date of 317+3 Ma. The metamorphosed volcanic and sedimentary rocks of units Ujj a and Ujjd ^ tentatively correlated with Early Jurassic Telkwa Formation of the Hazelton Group. The higher grade of regional dynamothermal metamorphism in the map area has obscured most contact relationships and original sedimentary and volcanic textures. Figure 2. Sketch of Shames River area, looking north, showing Shames River fault separating volcanic-plutonic rocks to the east from deformed granitoid rocks, orthogneiss, migmatite and metasedimentary rocks to the west. 20 Figure 3. Schematic stratigraphic column of Shames Paver area showing major map units. See Plate 1 and text for description of units. Not drawn to scale. Figure 4. Metavolcanic rock of unit lPcf, consisting of euhedral actinolitic hornblende mats defining foliation, and epidote, quartz, feldspar, biotite and chlorite forming groundmass. Figure 5. Looking east at vertical beds of marble of unit LP S m . Thin partings of argillaceous material contain actinolite, chlorite, epidote and fine grained, incipient biotite. Figure 6. Marble of unit lPsm with 1 cm long, pink fusulinids of possible Lower Permian age. Figure 7. Looking north-northwest at phyllitic foliation (XY) surface of unit U H a . Volcanic fragments of dark green-grey andesite and light grey dacite are subangular and elongate parallel to X, the elongation direction, which plunges gently northeast (right). 24 Figure 8. View to northwest of gently northeast dipping bedding contact between rhyolite breccia (above) and andesite breccia and flows (below) east of Amesbury Creek. Figure 9. Volcanic breccia of unit UHa> containing angular clasts of rhyolite, hornblende basalt and hornblende quartz diorite. 25 o c/5 BO c o u N I-§ c+-> O O K 2 $ If S2 x s a t-t — r t c si C/5 ll <U | on C T3 38 £ Is CO £ rt I, . O co h i-H N a> "C C o 3 x: MX) Figure 11. Semi-brittle shear between granite of unit T (above) and folded mylonitic hornblende-biotite schist of unit F J u m (below). Upper plate to the northeast (right) movement indicated by fold vergence of the lower plate. Figure 12. Folded, sheared sillimanite-biotite-quartz-plagioclase mats in pelitic screens within granodiorite of unit L K 3 . 27 Figure 13. Banded mylonitic granodiorite of unit E J g g of SRMZ showing darker, finer grained mylonite bands wrapping around lighter, coarse grained granodiorite. Upper plate to the northeast (right) movement is indicated. Figure 14. Looking north at roadcut in mylonitised biotite > hornblende granodiorite of unit EJgg. Compositional banding defined by lighter coloured layers of medium to coarse grained granodiorite alternating with fine grained hornblende-biotite schist. White dykes have U-Pb (zircon) crystallization date of 60 ± 6 Ma, and cut orthogneiss dated at 188 ± 8 Ma. ro 29 Figure 15. Flattened quartz in unit UGg defining an S-fabric inclined at up to 25° to C-planes, which are defined by biotite and hornblende crystals. Long axis of quartz rods are parallel to direction of maximum infinitesimal elongation. S is rotated into parallelism with C-planes at boundaries during progressive deformation. Figure 16. Mylonitised hornblende-biotite orthogneiss of unit UGg cut by mylonitic bands (bottom and top). Note streaked out quartz-feldspar aggregates forming detached slivers, and more resistant plagioclase porphyroclasts. Well developed shear band (C) foliation in upper right and bottom of photograph. Figure 17. Late synkinematic, faintly foliated granite dykes (locally concordant) cutting mylonitised hornblende gabbro and biotite-hornblende granodiorite orthogneiss of unit EJgg These dykes have been dated at 60 Ma (U-Pb, zircon). 31 ///. GEOCHRONOMETRY A. INTRODUCTION Variably metamorphosed intrusive and volcanic rocks have been selected for U-Pb (zircon) and K-Ar (biotite, hornblende, and whole rock) dating in order to determine the times of magmatism and to bracket the times of metamorphic-deformational events. In U-Pb systems, concordant dates are interpreted to represent crystallization ages for simple igneous systems (Wetherill, 1956; Faure, 1986), and the date of cooling below approximately 800°C (Mattinson, 1978). Discordant dates may be due to lead loss (or uranium gain) and inheritance, or a combination of both. Conventional K-Ar dates, on the other hand, represent the time the minerals dated passed through their respective closure temperatures (Dodson, 1979; Harrison and McDougall, 1980; Harrison, 1981). They are "cooling dates" which may, in the case of biotite systems, significantly post-date peak metamorphic episodes (Armstrong, 1966). Analytical techniques are described and results (la error) listed in Appendix 2. U-Pb dates are reported with 2a error in the figures and text that follow. K-Ar dates are reported with Iff error. B. U-PB ZIRCON GEOCHRONOMETRY Zircons from seven intrusive and metamorphosed intrusive rocks have crystallization ages from Early Pennsylvanian to Eocene. The samples chosen include rocks interpreted as pre- to post-mylonitic (Dm), and pre- to syn-low angle detachment (DR) deformation in relative age. 1. Upper Plate of SRMZ East of Shames River, and northwest of Amesbury Creek, a Latest Maastrichtian age was obtained from a granodiorite from unit LK3. A quartz diorite of unit IPJJJ crystallized 32 during Early Pennsylvanian. In the upper plate of the SRMZ, west of Shames River, two samples from unit LK3 yield Latest Maastrichtian crystallization ages. East of Shames River, granodiorite of unit LK3 intrudes volcanic and sedimentary rocks with a well defined contact. Sample TH89179 was obtained from a small summit north of the southern intrusive contact of L K 3 . This sample is a medium grained, unfoliated and nonlineated hornblende granodiorite with identical composition to sample TH89207c (below). In thin section, the minerals are fresh, and show very mild to no undulose extinction. Zircons are euhedral, colourless and clear, except for the finest magnetic fractions, which contain fine grained black inclusions. Two abraded, nonmagnetic, coarse and intermediate fractions have concordant ages of 69 + 1 Ma (Fig. 18). A third nonmagnetic coarse fraction barely overlaps concordia at 68 Ma with a 2a error. A 69 ± 1 Ma crystallization age is likely for this rock. East of Shames River, along Highway 16, sheared granitoid and volcanic rocks of unit lP§i are thrust eastward above recrystallized, fusulinid-rich limestones and marbles interpreted to have an Early Permian age, and greenschist grade volcanic rocks of the Hazelton Group. Sample TH89265 is representative of the granitoid rocks, and was collected for U-Pb dating. This rock is brittlely sheared, chloritised biotite quartz diorite. It is locally folded into north and northeast verging Z-folds. Chlorite and epidote are found with quartz and feldspar in foliation-parallel stringers and veins. Two abraded, coarse, nonmagnetic, zircon fractions plot, within 2a sigma error, on concordia at 298 and 312 Ma (Fig. 19). A York (1967) fit line through the two discordant fractions with present-day Pb-loss gives a minimum age of 317 + 3 Ma. A free-fit York regression through the two discordant points gives upper and lower intercepts of 331 +62/-46 and 226 +148/-160 Ma, respectively. The upper intercept represents a maximum, Late Mississippian crystallization age, while the lower intercept may indicate Late Triassic Pb-loss. A unique interpretation of the age of this rock is not possible, but minimum and maximum ages of crystallization are 317 and 331 Ma, respectively. Sample TH89207c is a faintly to moderately lineated, medium grained hornblende granodiorite located west of Shames Mt., and representative of unit LK3. South of Mt. Morris, this unit is observed to intrude granodiorite orthogneiss of unit LKrj. Lineation, 33 defined by euhedral hornblende and plagioclase laths, is concordant with regional stretching and mineral lineations associated with D m deformation. Plagioclase and quartz show moderate undulose extinction. This sample is interpreted to have intruded during the late stages of regional ductile deformation. Zircons are euhedral, light pink to colourless, and clear. Most zircons are doubly terminated. The three coarsest, pyrite-abraded fractions are concordant, within 2 a error, at 68.7 ± 0.4 Ma (Fig. 18). East of Shames Mt. sample TH89404d (Plate 3) is a beige, medium grained, mylonitised biotite granite dyke that intrudes hornblende granodiorite represented by sample TH89207c described above. Mylonitic foliation within the dyke is concordant to that within the granodiorite which it intrudes, but discordant to the contact, indicating that mylonitisation post dates intrusion of the granite dyke. Feldspars are dynamically recrystallized, and S-C fabrics pervade the rock. Euhedral sphene is pulled apart along northeast directed shear bands. Earlier formed 5-shaped porphyroclasts of plagioclase are often modified to later a-shaped porphyroclasts during continued, less severe, deformation. Zircons obtained are subhedral to anhedral, showing their deformed nature (Parrish and Roddick, 1985; Friedman, 1988). Two abraded, coarse fractions are concordant at 69+ 1 Ma (Fig. 18). From the field relationships noted above, this crystallization age corresponds to late stages of mylonitic deformation, since the contemporaneous country rocks it intrudes are only mildly foliated and lineated, implying that mylonitic deformation waned around 69 Ma in the hanging wall of SRMZ west of Shames River. 2. Lower plate and SRMZ Three samples of granitoid rocks were obtained for U-Pb dating to bracket the ages of deformation in the SRMZ. Sample TH89257c is characteristic of unit EJgg in the SRMZ. It is a medium to coarse grained, ductilely deformed, biotite-hornblende granodiorite orthogneiss. Distinctive are abundant screens of hornblende-biotite-plagioclase schist and gneiss. Feldspars, predominantly plagioclase, retain igneous oscillatory zoning, and are dynamically recrystallized. The fabrics 34 pre-date ductile D m deformation. Mildly foliated anatectic melts invade the numerous ductile shears which cut this rock. Three zircon fractions were obtained from this sample. A concordant date from one fraction is 188 ± 9 Ma (Fig. 20). A second, coarse, nonmagnetic fraction is slightly discodant, and has a 2 0 o p b / 2 3 8 T J date of 188 ± 1 Ma. A third fraction overlaps concordia at 183 Ma, but has extremely large errors due to problems in measuring 2 0 4 Pb and high common Pb in the sample, and is not listed or shown. An Early Jurassic crystallization age of 188±8 Ma is tentatively assigned to this rock. In order to constrain the younger limits of ductile deformation, and the older limits of brittle detachment faulting, two samples of biotite granite were dated, TH89253e and TH89131. TH89253e was obtained from within the SRMZ, 1 km west of Shames River. This sample is a fine grained, mildly foliated, biotite granite dyke which cuts granodiorite orthogneiss represented by sample TH89257c above. Foliation within this sample is very weak to absent. Where present, it is concordant with foliation in the country rock. In places, the rock is ductilely sheared along discrete foliation planes. This rock is thus considered to be late-to post-kinematic with respect to D m deformation. Identical rocks are found in semi-brittle fault contact with schists of EJum east of Exstew River (Plates 1 and 2). The crystallization date of this unit thus marks the maximum age of the ductile to brittle faulting transition. Four zircon fractions were obtained for analysis, and results plotted in Fig. 21. A non-magnetic, abraded, -149+134/im fraction plots on concordia at 59 + 1 Ma. Early Triassic (244 Ma) inheritance is shown by a chord through a second, discordant fraction. A free-fit York-fit line gives a lower intercept of 60+6 Ma, the inferred crystallization age of this rock. Two other zircon fractions with very high common Pb plot with large uncertainties below concordia, and are not listed or shown. Both have Early Eocene (45-53 Ma) 2 0 6 p b / 2 3 8 U dates. The large uncertainties preclude any meaningful interpretation of these two discordant fractions. 35 A final sample, TH89131, is a white, fine grained, mildly to moderately foliated, syn-Dg kinematic, biotite - hornblende + garnet granite dyke found in the lower plate of Exstew River fault west of Exstew River. In places, rocks represented by this sample cut ductilely deformed granitoids, while in others, it is found as nebulitic screens which grade into gneissic biotite - garnet dated at 83.5 Ma by Woodsworth et al. (1983). Four zircon fractions show a normal discordance pattern (Fig. 22), with upper and lower intercepts, representing inheritance and crystallization ages, respectively, of 45 ± 6 and 211 +49 Ma. A nonmagnetic, abraded, intermediate zircon fraction plots, with small errors, on concordia at 50 + 1 Ma, while a fine, unabraded magnetic fraction falls slightly above concordia at about 45 Ma. A crystallization age, with errors encompassing the ages of the lower intercept of the York fit line, and the concordant date of the nonmagnetic intermediate fraction, of 48 + 3 Ma is assigned to this rock. C. K-AR GEOCHRONOMETRY K-Ar dates were determined for six hornblende, biotite and whole rock samples from the SRMZ and its upper plate, and both east and west of the SRF. Previous K-Ar studies of the area are reported in Harrison et al. (1978); Wanless et al. (1979), Hutchison (1982), Hill (1985) and van der Heyden (1989). K-Ar dates represent the time of cooling below the closure temperatures of biotite, hornblende, and whole rock mineral aggregates. Harrison and McDougall (1980) and Harrison (1981) report closure temperatures of 530 + 40°C and 280 + 40°C for hornblende and biotite, respectively, for reasonably rapid uplift rates. The new K-Ar dates reported allow construction of a metamorphic cooling history for the area (Fig. 23). All but one of the K-Ar dates are Eocene, between 46.2 and 52.3 Ma. Two K-Ar (biotite) dates from within the SRMZ (samples TH89253e and TH89257c) are 46.2 and 46.5 + 1.6 Ma respectively. Sample TH89253e is a weakly foliated biotite granite dyke which intrudes TH89257c, a mylonitised biotite granodiorite. A K-Ar (biotite) date of 51.1 + 1.8 36 Ma was obtained for sample TH89404d, with a U-Pb date of 69 ± 1 Ma. Located east of Mt. Morris, sample TH89363b is a mildly deformed biotite granodiorite from the upper plate of SRMZ; it has a K-Ar (biotite) date of 48.0 ± 1 . 6 Ma. East of Amesbury Creek, a whole rock K-Ar date of 52.3 ± 1.8 Ma was obtained from a lower greenschist metamorphosed andesitic flow (sample TH89286b). A single older date of 73.1 ± 2.6 Ma is from hornblende separated from a weakly lineated hornblende granodiorite (sample TH89207c) located in the upper plate of the SRMZ. This sample has a concordant U-Pb date of 68.7 + 0.4 Ma. D. SIGNIFICANCE OF U-PB AND K-AR DATES The U-Pb and K-Ar dates show a complex magmatic, deformational and metamorphic history for the area. U-Pb dates indicate that the earliest magmatism in the area is Early Pennsylvanian or older. Middle Eocene magmatism is the youngest extensive intrusive event dated in the present study. Mafic dykes cut Middle Eocene bodies, but have not been dated in the present study. Ductile and brittle deformation events have been bracketed between 188 Ma to 46.2 Ma. The oldest date obtained in this study is an Early Pennsylvanian-Late Mississippian (>317 ± 3 Ma and <331+62/-46 Ma) U-Pb date from brittlely sheared quartz diorite structurally overlying marble correlated with Stikine Assemblage (Monger, 1977; Woodsworth et al., 1985). The date confirms field observations that the quartz diorite and volcanic rocks were thrust over Lower Permian and younger units. These intrusive rocks cut greenstone units east of Shames River, implying minimum ages of Early Pennsylvanian for the volcanic rocks. The Dala River pluton to the south of the Shames River area has a 331 +8 Ma crystallization age, assuming present-day Pb-loss (van der Heyden, 1989). Paleozoic plutons have only been recently recognised in the Coast Plutonic Complex (van der Heyden, 1989; Gareau, 1989). Early Jurassic (~ 188 Ma) crystallization dates obtained from a deformed orthogneiss pre-dates all ductile deformation in the area. Some early deformation, herein referred to as pre-Dm, pre-37 dates intrusion of deformed, gneissic, 83.5 Ma granite, dated by Woodsworth et al. (1983), which includes xenoliths of migmatite and orthogneiss. Within the upper plate of SRMZ, 68.7-69 Ma crystallization dates from late syn-kinematic granitoid rocks show that ductile deformation waned around this time. A 60 ± 6 Ma crystallization date from a late to post kinematic granite dyke marks the end of ductile deformation. Early Paleocene intrusive bodies are cut by semi-brittle detachment faulting, which culminated in intensity during intrusion of 48 + 3 Ma synkinematic granite, and ended by Middle Eocene (46.2-46.5 Ma), when the SRMZ passed through the K-Ar (biotite) cooling isotherm. The upper plate of SRMZ passed through this isotherm at 51.1-48 Ma (Fig. 23). For rocks in the upper plate of the SRMZ, about 20 Ma separates the U-Pb (zircon) from the K-Ar (biotite) dates. For sample TH89207c, the U-Pb and K-Ar (hornblende) dates are concordant at 2 a. This concordance of cooling and crystallization ages may be explained by one or a combination of the following: a. Addition of excess Ar in hornblende (Brewer, 1969); b. Incomplete re-setting during metamorphism; c. Extremely rapid cooling after crystallization, followed by ambient metamorphic conditions which did not exceed the closure temperature for Ar-retention in hornblende, and d. Similar daughter loss for both isotopic systems during metamorphism. From field observations, and detailed metamorphic studies presented below, alternative c. appears most plausible. The addition of excess Ar cannot be ruled out, since abundant xenoliths exist. In general, there is a slight downward decrease in K-Ar dates west of Shames River. A temperature-time plot (Fig. 23) shows rapid but slightly different cooling histories for the SRMZ and its upper plate. The upper plate of SRMZ cooled more slowly, and well before, the SRMZ. The nearly concordant U-Pb and K-Ar (hornblende) dates in the upper plate of the SRMZ suggest rapid cooling during Late Maastrichtian time. Two K-Ar biotite dates from above the SRMZ are 48.0 -51.1 Ma. Within the SRMZ, two K-Ar dates are slightly younger, at 46.2 - 46.5 Ma. This slight apparent decrease in age with increasing structural depth suggests that the upper plate of SRMZ remained at higher crustal levels during Paleocene to Middle Eocene deformation and metamorphism than the SRMZ and its lower plate. A simple 38 calculation, dividing the 2 km distance separating TH89404d in the upper plate from TH89253e and TH89257c in the SRMZ by 4.9 Ma gives an average isotherm movement rate of 0.4 mm/yr, from 51.1 to 46.2 Ma, which is slower than the 1-2 mm/yr uplift rate, from 62 - 48 Ma calculated by Hollister (1982) for the CGC to the west. The Eocene K-Ar cooling dates reported above correspond with those obtained for Canadian and U.S. Cordilleran metamorphic core complexes (Armstrong, 1982, 1988; Friedman and Armstrong, 1989; Armstrong and Ward, in press). The dates also correspond to K-Ar dates inferred to represent the crystallization age of parts of the Ponder pluton (Wanless et al., 1979; UBC unpublished data; Hill, 1985; van der Heyden, 1989; Anderson, 1990) and the Eocene magmatic front (Armstrong, 1988). In the Coast Plutonic Complex, there exists a general eastward younging in K-Ar biotite cooling dates from Cretaceous in the west, to early Cenozoic in the east (Hutchison, 1982; Woodsworth et al., 1985; Armstrong, 1988). This eastward younging of metamorphic dates has been explained by (not mutually exclusive): a. An eastward migration of the magmatic front (Armstrong, 1988; van der Heyden, 1989); b. Progressive uplift of crustal blocks in the east along steep shear zones (Crawford et al., 1987; Hutchison, 1982; Woodsworth et al., 1985), and c. Uplift and cooling of a metamorphic core complex during the Eocene (van der Heyden, 1989). Early Cenozoic metamorphic culminations occur along the central and eastern parts of the Coast Plutonic Complex and in the Omineca Belt of the Canadian Cordillera (Harrison et 0 al., 1978; Parrish, 1983; Armstrong, 1988; van der Heyden, 1989; Armstrong and Ward, in press). Within the Omineca Belt, Early Cenozoic resetting is induced by an Eocene magmatic event, crustal extension and rapid tectonic unroofing of hot metamorphic core rocks (Parrish, 1984; Armstrong, 1982). In the present study, the Eocene K-Ar biotite dates mark the end of low angle semi-brittle detachment faulting (between 60.0-46.2 Ma). 0.014 0.012 0.010 0.012 -n 03 cn OJ \ -Q 0.010 Q. L0 o OJ 0.012 0.010 0.00B TH89404D N1.5H/3, +134um, RBR N2.1H/0.S, -134+74um, HBR TH89207C N1.5H/1, -74-t-44um, HBR N1H/5, +134um, HBR N2H/0.5, -134+74um, HBR TH89179 N1.5FV3, -74+44um, HBR N2R/1, +134um, HBR N2H/1, -134+74um, HBR ERRORS HRE 2 SIGMH 0.006 0.05 0.06 0.07 O.OB 2 0 7 p b / 2 3 5 u 0.09 Figure 18. U-Pb concordia diagrams for samples TH89179, TH89207c and TH89404d. 0.040 0. 024' « 1 1 L . 1 , L L_ 0.17 0.19 0.21 0.23 0.25 0.27 2 0 7 P b / 2 3 5 u Figure 20. Concordia diagram for sample TH89257c. 0 . 0 2 1 0 . 0 1 7 ZD CD cn OJ \ 0 . 0 1 3 _ Q Q_ CD O OJ 0 . 0 0 9 0. 005 1 ••• 1 ' T H 8 9 2 5 3 E , i | -1 2 0 y s -N 2 H / 1 , + I 4 9 u m , R R R / / -- i o o y y ^ y y ^ -yy 8 0 y X -N 2 R / 1 , -149+134umyy • RBR -SOy/ INTERCEPTS RT 2 4 4 + a4- and G0+6 Ma _ . . 1 , 1 ERRORS RRE 2 SIGMR I . I , -0 4 O.OG 0 . 0 8 0.10 0 . 1 2 0. 2 0 7 P b / 2 3 5 u Figure 21. Concordia diagram for sample TH89253e. 0.017 0. 015h ZD 0.013 CD m OJ ^o.on q_ co o ^ 0.009 0.007r-TH8913 1 N2R/1, +149um, RBR 0. 005 N2R/1, -149+134um, RBR 4 l 2 R / l , -74+44um HBR Ml .5R/3, -44um JL 0.04 0.0G INTERCEPTS RT 211+49 and 45+6 Ma CMSWD-0) ERRORS RRE 2 SIGMR . I • 0.08 0.10 2 0 7 P b / 2 3 5 u Figure 22. Concordia diagram for sample TH89131. 0.12 42 TEMPERATURE-TIME PLOT FOR SHAMES RIVER AREA 1,000 I 1 Figure 23. Temperature-time plot for granitoid rocks in Shames River area. 43 IV. METAMORPHISM A. INTRODUCTION A detailed study of metamorphism in the study area is limited by the scarcity of suitable metapelitic index minerals. The best qualitative indicators of metamorphic grade are obtained from 3 localities containing metasedimentary rocks in the area. Within metamorphosed igneous rocks, metamorphic grade is roughly estimated by the An-content in plagioclase (Winkler, 1979), presence of dynamic recrystallization in plagioclase (Tullis and Yund, 1977), and suitable mineral assemblages. Thermobarometry employing a variety of techniques has also been used to determine relative pressure and temperature conditions of metamorphism and granitoid rock emplacement. In general, a decrease in metamorphic grade is observed eastward across the SRF, and upwards in the SRMZ. West of Shames River, at least two phases of dynamothermal metamorphism (Ml and M2, Tables 1 and 2) reached or exceeded upper amphibolite facies. East of SRF, volcanic units are generally metamorphosed to greenschist facies, except near the Late Cretaceous to early Tertiary hornblende granodiorite body, unit LK3, where aligned and folded sillimanite needles co-exist with garnet, quartz, plagioclase, muscovite and biotite, indicating pre- to post-Dm contact metamorphism at upper amphibolite facies. South of the contact aureole, the lack of hornblende, and the presence of actinolite-chlorite-epidote assemblages in the Hazelton Group meta-volcanic units indicates that metamorphic temperatures did not exceed about 500°C (Winkler, 1979). Further south, metamorphosed volcanic and plutonic rocks of Stikine Assemblage (Plate 1) lie structurally above Hazelton Group, and contain hornblende - plagioclase (An> 1 7) - quartz assemblages diagnostic of amphibolite facies metamorphism. Together with U-Pb and K-Ar dating, mineral textures and thermobarometery of metamorphosed pelitic rocks both east and west of Shames River indicate that rapid uplift has occurred in the area. This conclusion is consistent with those of Harrison et al. (1978), Kenah (1979), Hollister (1982), Parrish (1983), Douglas (1983), Kenah and Hollister (1982), Sisson (1985), and Crawford et al. (1987). 44 In the following discussion, metamorphic conditions inferred from observed mineral equilibria are first discussed, using P,T data obtained from thermobarometry, presented later. Uplift paths for the area are proposed in a final section. Following convention in the literature, kbar units are used to report pressures. The conversion to MPa is: 1 kbar = 100 MPa. Detailed petrographic data are listed in Appendix 1, and representative electron microprobe analyses and analytical techniques are listed in Appendix 3. B. EASTWARD DECREASE IN METAMORPHIC GRADE Metamorphic grade decreases sharply across the SRF. To the west, igneous emplacement and metamorphic P, T conditions in metamorphosed granitoid rocks decrease upwards from the SRMZ, as indicated by the decreasing Abj- content in hornblende, An-content in plagioclase and inter-ductile shear band angles. East of SRF, the metamorphic grade is predominantly epidote-actinolite-chlorite facies except near the 69 Ma granodiorite body of unit LK3, where upper amphibolite facies contact metamorphism has affected the rocks. 1. Western Block of SRF a. Metamorphosed granitoid rocks and gneisses The predominant mineral assemblages in metamorphosed and deformed granitoid rocks and gneisses west of Shames River most closely resemble those of granodiorite and tonalite protoliths. They consist of amphibolite facies, quartz + plagioclase ( A n > 1 7 ) + alkali feldspar + hornblende + biotite + zircon + sphene + garnet + apatite + opaque assemblages. A first, upper amphibolite facies metamorphic event (Ml) is indicated by the presence of large, angular xenoliths of migmatite and gneiss within a garnet-biotite granite body dated at 83.5 Ma by Woodsworth et al. (1983). Fabrics in the xenoliths are indistinguishable from those in orthogneiss of unit EJgg. M l is overprinted by D m to post-Dm metamorphism (M2) and ductile fabrics which affect the 83.5 Ma granite. M2 metamorphism reached upper amphibolite facies, as indicated by the presence of foliated granite injections and migmatite along ductile shear zones, and dynamic recrystallization of plagioclase (An 1 7 . 4 8). The synchroneity of M2 metamorphism and D m 45 ductile deformation is substantiated by the presence of both cross-cutting and concordant and mylonitised, pegmatitic and granitic veins and dykes, which are interpreted as metamorphic melts. Annealed plagioclase indicates that temperatures remained high after ductile deformation. The ubiquitous stable presence of hornblende and plagioclase (An> 1 7) is generally considered to be indicative of the greenschist to amphibolite facies transition (Winkler, 1979). Hornblende commonly outlines a lineation within the foliation, and is recrystallized to subhedral to euhedral laths. Hornblende rosettes are also common along foliation planes in the SRMZ, indicating that post-Dm metamorphism has taken place. M2 metamorphism, with which hornblende recrystallization is associated, thus accompanies and post-dates D m ductile deformation. Oscillatory and normal zoning in plagioclase is common, even in the most severely deformed rocks, and is considered to reflect original igneous compositions. Retrograde alteration is rare in the granitoid rocks and orthogneisses. Rocks most affected by retrograde alteration are calc-silicate layers and biotite-rich granitoid rocks and schists within the SRMZ, where fluids may have been concentrated along discrete ductile shear zones and brittle fractures. In these areas, narrow zones of retrogression to actinolite - chlorite - epidote - albite + quartz + sericite + opaque assemblages has occurred. These minerals sometimes show cross-cutting relationships with D m minerals, and thus are thought to be synchronous with, or post-date the final stages of D m deformation. They are especially common near and along a semi-brittle fault which emplaces granite of unit T, correlated with a similar granite dyke in the SRMZ dated at 60 ± 6 Ma, above schist and gneisses of unit EJum-b. Metamorphosed supracrustal rocks Metamorphosed supracrustal rocks are rare, and found only in a pendant underlying Mt. Morris, in outcrops east of Exstew River, as isolated pendants in foliated granodiorite (unit LK2) south of Mt. Morris, and as thin layers in mylonitic granitoid rocks in the SRMZ. 46 Based on textural features described below, metamorphic minerals in metapelitic rocks crystallized before, during and after D m deformation, dated between 83.5 - 60 Ma in the upper plate of SRMZ. Pelitic assemblages suitable for pressure and temperature determinations were found only in the upper plate of SRMZ. The intersection of the staurolite + muscovite + quartz = biotite + sillimanite curve with the Ferry and Spear (1975) thermometer for peak temperature conditions (Section C. Thermobarometry, and Fig. 24) gives a minimum temperature of 610°C at 3 kbar for peak metamorphic conditions for sillimanite - staurolite - garnet - muscovite -quartz - biotite assemblages in the upper plate. Because inter-ductile shear angles and the amount of migmatite along ductile shears are greater in the SRMZ than in its upper plate, 610°C is the minimum temperature for D m deformation in the SRMZ. Mineralogy Mineral assemblages in pelitic rocks west of SRF are typical of high temperature regional metamorphism at amphibolite facies. Metapelites in a ductilely deformed pendant on Mt. Morris contain sillimanite (fibrolite) - staurolite - garnet - biotite - quartz - plagioclase -muscovite. The pendant is intruded by numerous subhorizontal, mylonitic granitoid layers (van der Heyden, pers. comm.) similar to rocks in unit LK2. South of Mt. Morris, unit LK2 is intruded by 69 Ma granodiorite of unit LK3. West of Shames Mt., a metabasite contains garnet - hornblende - plagioclase - biotite - quartz - chlorite. Calc-silicate layers and pods comprising epidote + calcite + garnet + diopside are found at two locations, 1 km west of Shames River and 0.5 km east of Exstew River. Sillimanite forms fibrolite needles in mats, and comprises up to 25% of two samples (84WW82-3 and 84WW82-4, Plate 3) from a pelitic pendant on Mt. Morris. Fibrolite mats are folded, and often wrap around garnet grains. Brown biotite is intergrown with, and reacts to, fibrolite. Fibrolite crystals also define northeast plunging lineations. Garnet is rare, and forms rounded, subhedral to euhedral grains which show no internal foliation. Garnet is sometimes seen to overgrow biotite crystals. Rims and cores are 47 infrequently partially resorbed to chlorite. Because schistosity defined by biotite flakes and fibrolite mats are deflected by garnet rims, garnet formation is thought to be pre- to syn-kinematic with respect to D m deformation. In rare instances, biotite is included by garnet. Thermobarometry on garnet cores should thus record pre-Dm, and possibly near-peak metamorphism, M l . Staurolite is fine grained and very rare. It occurs in contact with muscovite and quartz, and is characterised by anhedral, resorbed outlines. Like garnet, staurolite porphyroblasts deflect sillimanite foliation, and are thought to be pre- to syn-kinematic. Plagioclase (An;^) forms recrystallized, relatively strain free grains. Within sample TH89190, plagioclase in cores of poikilitic garnet forms randomly oriented, well twinned crystals. Muscovite is rare as a primary mineral. It occurs as fine grained intergrowths with biotite, and helps to define D m foliation. Biotite defines D m foliation, and is found around partially embayed garnets. Biotite is also crenulated and often intergrown with muscovite, suggesting a close genetic link between these two minerals. It is rarely found as inclusions in garnet. In such cases, the biotite is randomly oriented with respect to the external foliation. Chlorite is very rare, and forms along edges of biotite and hornblende. Its rarity as a retrograde product suggests rapid and/or dry cooling. It is most common in the upper plate of SRMZ, along edges of hornblende crystals and garnet rims. Hornblende is found as euhedral to subhedral, randomly oriented grains within a large, poikiloblastic garnet in a meta-basite (sample TH89190) west of Shames Mt. Electron microprobe analyses indicate alteration of hornblende rims to chlorite. Mineral equilibria Metamorphic grade in the supracrustal rocks studied may be obtained through calibrated mineral equilibria in rocks with appropriate bulk compositions. 48 The relative scarcity of chlorite and primary muscovite in thin sections may be attributed to either unsuitable bulk compositions or the following prograde reaction (Carmichael, 1970): 1. Chlorite + Muscovite = Sillimanite + Biotite + Quartz According to Hoschek (1969) and Woodsworth (1979), staurolite may form by the following reaction, which leads to a decrease in primary modal muscovite and chlorite, and provides a lower temperature bound of approximately 550°C for the formation of staurolite: 2. Chlorite + Muscovite = Staurolite + Biotite + Garnet + Quartz + H 2 0 At higher temperatures, the near absence of muscovite, and the presence of anhedral staurolite fringed by fibrolite mats suggests the following reaction (Thompson and Norton, 1968; Richardson, 1968 and Dutrow and Holdaway, 1983): 3. Staurolite + Muscovite + Quartz = Sillimanite + Biotite + H 2 0 This equilibrium intersects the andalusite-sillimanite curve at ~605°C and "2.5 kbar (Fig. 24). A second possible reaction for the formation of fibrolite involves fibrolitization of biotite (Kerrick, 1990): 4. Biotite + HC1 = Fibrolite + Quartz + KC1 + (Mg,Fe)Cl2 + H 2 0 In the above reaction, HC1 may have been derived from acidic volatiles released during late stage granitic intrusive crystallization. In the Shames River area, fibrolite formed before and during intrusion of late syn-Dm 69 Ma granodiorite. Strain-induced fibrolite has been described by Vernon (1987), who suggested that it may be concentrated by dissolution of other phases in the presence of fluids during non-coaxial shearing. The presence of aligned fibrolite needles parallel to stretching lineations defined by feldspars and quartz rods suggests that fibrolite formed before or during deformation. Because fibrolite is deflected by garnet and staurolite porphyroblasts, garnet and staurolite are considered to be pre-tectonic with respect to D m deformation. Fibrolite and sillimanite may have different stability fields, as pointed out by Holdaway (1971) and Kerrick (1990). Holdaway (1971) suggested that fibrolite may contain excess Si 49 and H 2 0 , and have Al-Si disorder. Insufficient work has been done to delimit the differences in P-T conditions of these two phases (Kerrick, 1990). In this paper, the P-T stability fields of fibrolite and sillimanite are tentatively assumed to be the same. On a first order, this assumption is corroborated by garnet -biotite and garnet - hornblende geothermometry. The absence of cordierite and K-feldspar indicates either unsuitable bulk compositions, or that the following reactions have not occurred: 5. Biotie + Sillimanite = Garnet + Cordierite (Holdaway and Lee, 1977) 6. Muscovite + Quartz = Sillimanite + K-feldspar + H2O (Guidotti, 1963; Evans and Guidotti, 1966) 7. Staurolite + Quartz = Sillimanite + Cordierite + Garnet + H 2 0 (Woodsworth, 1979) The disappearance of staurolite may take place at medium to high grade, without the production of cordierite. Staurolite of MgO/(MgO+FeO) = 0.2 to 0.4 breaks down at 2 kbar H 2 0 pressure and 575 ± 15°C and 5.5 kbar H 2 0 pressure at 675 ± 15°C (Hoschek, 1969). Staurolite may persist to high grade conditions where muscovite is absent for its decomposition. At high temperatures, where neither muscovite nor biotite are present, staurolite may break down in the presence of K-feldspar in an anatectic melt at about 700QC (Winkler, 1979): 8. Staurolite + K-feldspar = Sillimanite + Almandine + Biotite To the north, Sisson (1985) reports the association of orthopyroxene, garnet and plagioclase, indicative of intermediate granulite facies, near the western contact of the Ponder pluton. Reactions 5 and 6 define maximum temperature conditions for the pelitic pendant on Mt. Morris, while reaction 3 constrains the lower bound for prograde conditions. The presence of leucotonalite melt along ductile shear zones in SRMZ is taken to indicate temperatures in excess of 650°C (Sisson, 1985; Crawford et al., 1987). 50 Moderate retrogression of amphibole rims to chlorite is observed only with electron microprobe analysis. Amphibole cores preserve hornblende-like compositions (Table 3). In garnet, decreasing Mn-content from rim to core is also consistent with retrograde alteration and Tracy et al., 1976; Woodsworth, 1977; Tuccillo et al., 1990) 2. Eastern Block of SRF East of SRF, both regional greenschist and contact upper amphibolite grade metamorphism are observed. Contact metamorphism is confined along a 1-2 km wide contact aureole within deformed and metamorphosed volcanic rocks and greenstones. Weak foliations in 69 Ma granitoid rocks of unit LK3 are concordant with strong fabrics in the supracrustal rocks it intrudes, but discordant with the intrusive contact, indicating that intrusion of LK3 is late-synkinematic (Plate 1). South of the contact aureole, regional dynamothermal metamorphism has affected the supracrustal rocks, except locally, where igneous bodies raised local temperature. a. Mineralogy Pelitic schist (sample TH89044, Plate 3) from the southern contact of unit LK3 east of Shames River, contains the following mineral assemblage: sillimanite (fibrolite) - staurolite -almandine- biotite - muscovite - plagioclase - quartz - opaque. Large porphyroblasts of almandine up to 1 cm across are set in a fine grained, well foliated matrix. Within each porphyroblast, garnet occurs as subangular grains rimmed by biotite, plagioclase, quartz and minor muscovite. Green, unoriented, unaltered biotite is intergrown with garnet grains. Plagioclase occurs as fresh grains, and is unoriented. Fine, randomly oriented fibrolite needles are found within almandine and biotite grains. Rare staurolite occurs as resorbed grains altering to biotite, almandine and fibrolite. Staurolite is not found in the matrix surrounding the garnet porphyroblasts. The porphyroblasts are surrounded by a matrix comprised of mylonitic, tightly folded fibrolite-biotite layers. 51 Almandine and staurolite are restricted to the porphyroblasts. Fibrolitic mats are intimately intergrown with, and replace, brown biotite. They wrap around, and are found in pressure shadows of, almandine porphyroblasts. Minor unoriented fibrolite also grows at oblique angles to fibrolite foliation. The above observations indicate that a first fibrolite growth took place prior to almandine formation during static, high temperature metamorphism. A second fibrolite growth occurred after almandine growth, before, and continuing after, ductile (Dm) deformation. The second fibrolite growth is thought to be synchronous with intrusion of deformed granodiorite of unit LK2, and may have continued until intrusion by 69 Ma granodiorite of unit LK3. Within metavolcanic rocks, euhedral, aligned actinolite crystals lie in, and help to define, foliation planes. Late stage actinolite is also seen to grow randomly across foliation planes. Actinolite growth is thus thought to pre-date and outlast ductile deformation. Plagioclase in metavolcanic rocks has An content less than 15%, except in rocks belonging to unit IP$[, where An-content exceeds A n 2 0 . Phyllitic metavolcanic rocks of unit lPgj and unit Ujjd n e a r m e pluton contact contain light brown, fine grained, biotite which lies in the foliation planes. b. Metamorphic grade Metamorphic conditions have been determined from mineral assemblages in sample TH89044 above. The mineral assemblage described above imply contact metamorphic temperatures of at least 605°C at 2.5 kbar . The presence of green biotite in garnet porphyroblasts, and brown biotite in the matrix indicates that biotite is more Fe rich in the matrix than in the almandine porphyroblasts. The absence of staurolite and almandine in the matrix, and the observation that staurolite reacts to form fibrolite, almandine and biotite, suggests the following reaction (Thompson and Norton, 1968): 9. Staurolite + Muscovite + Quartz = Sillimanite + Almandine + Biotite + H 2 0 52 The above reaction partitions Fe between almandine and biotite, giving rise to less Fe-rich biotite in the porphyroblasts than in the matrix. The following reaction is thought to have produced the resorbed garnet textures, broken down earlier formed garnet porphyroblasts, and produced later D m sillimanite which nucleates against garnet, and grows oblique to the main D m schistosity: 10. Garnet + Muscovite = Biotite 4- Sillimanite + Quartz The relative scarcity of muscovite, which was previously consumed by prograde reaction 9, may explain the incomplete consumption of garnet by reaction 10. The reaction (Holdaway and Lee, 1977): Biotite + Sillimanite = Garnet + Cordierite has not been observed, and may place an upper boundary for metamorphic temperatures. Minor retrogression to phengite and chlorite in sample TH89044 may have taken place by the following reactions (Hyndman, 1972): 11. Biotite + Muscovite + Quartz + H 2 0 = Phengite + Chlorite or 12. Garnet + Biotite + H 2 o = Chlorite + Muscovite + Quartz In metabasites of Hazelton Group near the intrusive contact of unit LK3, the presence of plagioclase with An > j 7 + hornblende in hornfelsic green plagioclase porphyry flows indicates contact metamorphic temperatures exceeding 500°C at 4 kbar and 490°C at 3 kbar (Winkler, 1979). Within these rocks, a pre-existing foliation defined by actinolitic hornblende is overprinted by randomly oriented hornblende and actinolitic hornblende. South of the contact aureole, regional metamorphic grade of Hazelton Group is generally lower greenschist grade, with ubiquitous albite - actinolite - chlorite - epidote assemblages in metamorphosed volcanic rocks. Lineated hornblende and actinolitic hornblende is observed in greenstones of unit lP$i, east of Shames River. Plagioclase in these rocks generally contains A n > 2 0 , which, together with hornblende, indicates metamorphism at lower amphibolite facies (Winkler, 1979). Dynamic recrystallization of hornblende has not occurred, 53 but plagioclase shows some dynamic recrystallization, indicating maximum temperatures of 550°C, assuming that paleo-strain rates were greater than 10"^  s~* (Tullis and Yund, 1977). C. THERMOBAROMETRY Mineral compositions for thermobarometric calculations have been determined for ten samples both east and west of the SRF, and within the SRMZ and its hangingwall. A Cameca SX-50 automated electron microprobe at the University of British Columbia was used for analysis. Analytical techniques, standards and reduction procedures used, and data obtained, are listed in Appendix 3. This section discusses the thermobarometers used, and presents the results of application of thermobarometric calculations to the Shames River area. 1. Thermobarometers Used a. Garnet-biotite geothermometry Experimental and empirical calibrations of the Fe-Mg exchange reaction: Fe 3 Al 2 Si 3 0 1 2 + KMg 3 AlSi 3 O 1 0 (OH) 2 = M g 3 A l 2 S i 3 0 1 2 + KFe 3AlSi 3O 1 0(OH) 2 have been carried out by Thompson (1976), Ferry and Spear (1978), Hodges and Spear (1982), Ganguly and Saxena (1984), Indares and Martignole (1985) and others. The calibration of Hodges and Spear (1982), based on Ferry and Spear (1978), incorporates non-ideal Ca mixing in garnet, and results in higher temperature estimates than those of Ferry and Spear (1978). Non-ideal Ca, Mn and Fe-Mg mixing in garnet was used by Ganguly and Saxena (1984) and results in temperatures up to 100°C greater than that of Ferry and Spear (1978). Indares and Martignole (1985) corrected for Ti and A1 V I in the biotite solution, resulting in lower temperatures. Studies by Selverstone and Hollister (1980) show that the Thompson (1976) and Ferry and Spear (1978) calibrations give indistinguishable results. In this paper, temperatures reported are those obtained by the Ferry and Spear (1978) calibration, as these give temperatures which most closely approximate, within 2a error, those of the 54 Thompson (1976) calibration, the mineral assemblages observed, and those obtained by garnet-hornblende thermometry. b. Garnet-hornblende geothermometry Graham and Powell (1984) have carried out empirical calibrations of the Fe-Mg reaction: 4Mg3Al2Si301 2 + 3NaCa 2Fe 4Al 3Si 60 2 2(OH) 2 = 4Fe 3 Al 2 Si 3 0 1 2 + 3NaCa 2Mg 4Al 3Si 60 2 2(OH) 2 This geothermometer has been applied to a metabasite west of SRF and gives comparable results to those obtained with the garnet-biotite thermometer. c. Garnet-plagioclase-quartz-AfySiOs (GASP) geobarometry Metamorphic pressures in three meta-pelitic samples containing sillimanite were estimated using the following net transfer equilibrium (Ghent, 1976 and Ghent et al., 1979): 3CaAl 2Si 20 8 = Ca 3 Al 2 Si 3 0 1 2 + 2Al 2Si0 5 + Si0 2 The mixing models of Ghent et al. (1979) and Newton and Haselton (1981) were used to determine grossular and anorthite activities. For consistency, the calibration of Ghent et al. (1979) was used throughout in reporting simultaneous calculations of P and T. d. Alf in hornblende geobarometry The total aluminum content (A1T) in hornblende has been empirically calibrated as a geobarometer by Hammarstrom and Zen (1986) and Hollister et al. (1987). Recent experimental calibrations performed by Rutter and Wyllie (1988) and Rutter et al. (1989) at 10 kbar have confirmed the validity of empirical calibrations by Peters (1984), Hammarstrom and Zen (1986) and Hollister et al. (1987). Extensive recent experimental work by Johnson and Rutherford (1989) suggests that the original pressure estimates may be too high. This geobarometer is based on the following reaction: 2Si0 2 + 2CaAl 2Si 20 8 + K(Fe,Mg)Al 4Si 3O 1 0(OH) 2 = Ca 2(Fe,Mg) 3Al 4Si 60 2 2(OH) 2 + KAlSi 3 0 8 55 The pressure-sensitive substitution is probably in the tschermakite component, as indicated by the sympathetic increase in A1 I V + A1 V I with pressure (Hollister et al., 1987). In the present study, the calibration of Johnson and Rutherford (1989) has been used to determine pressures of igneous hornblende crystallization. The regression, employing Alp based on a 23-oxygen structural formula, gives pressures about 0.5 to 1.5 kbar lower than those obtained by Hollister et al. (1987) in the pressure range 2-5.5 kbar. The calibration curve of Johnson and Rutherford (1989) used is: P (kbar) = -3.46 + 4.23 A1 T Proponents of this geobarometer, including Anderson, (1987, 1988) and Zen (1989) suggest that it may be used to determine crystallization pressures in metaluminous rocks which lack other suitable mineral assemblages with which to determine crystallization pressures. These rocks may thus be used as "crustal nails" (Anderson et al., 1988). Moreover, Anderson et al. (1988) notes that even after severe mylonitization, hornblende cores in some igneous rocks retain their original igneous compositions. Recent experimental work by Blundy and Holland (1990) on a hornblende-plagioclase thermometer also resulted in a pressure calibration similar to that of Hollister et al. (1987) and Johnson and Rutherford (1989), but with much larger errors. These authors however, warn against the utility of Al-j as a pressure gauge due to the large errors and temperature-dependent pressure estimates. In the present study, however, pressures obtained using this geobarometer generally agree with those obtained by other methods. It is thus considered, for the purposes of this study, a reliable indicator of pressures of crystallization, as long as the pre-requisite mineral assemblage quartz, biotite, plagioclase, K-feldspar, sphene, hornblende, magnetite (or ilmenite), with or without rutile is present. Recent successful studies employing the Alj in hornblende geobarometer include Rui and Kerrich (1990) and Ghent et al. (1990). 2. Precision and Accuracy In P-T studies, it is the precision of measurements which defines the P-T-time path. According to Spear (1989), the precision of Fe-Mg exchange thermometers, based on repeated analyses, is ±20-50°C at a given pressure. For the GASP geobarometer, precision is generally 56 +0.3-2.0 kbar at a given temperature. These precision estimates are based on the assumption that analytical errors are on the order of ±3-5% (Spear, 1989). The accuracy of P-T results is harder to specify, and has been addressed by Hodges and McKenna (1987) and Hodges and Crowley (1985). According to Hodges and McKenna (1987), the accuracy of the garnet-biotite thermometer of Ferry and Spear (1978) is ± 150°C at constant pressure, and that of the GASP barometer is ±5kbar at constant temperature. These surprisingly large errors cover the majority of metamorphic conditions covered by the rocks studied, and suggests that there may be major errors associated with covariances in the calibrations (Spear, 1989). In the present study, different methods were used to estimate P-T conditions in order to achieve internal consistency. For example, simultaneous solution of the Ferry and Spear (1978) garnet-biotite Fe-Mg exchange thermometer and the Ghent et al. (1979) GASP barometer for metapelite sample TH89044 gives rim P, T of 2.9 ± 1.6 kbar and 608 ± 50°C. Using the Johnson and Rutherford (1989) calibration for the A l j in hornblende barometer, hornblende quartz diorite sample TH89042 which intrudes the above metapelite gives rim pressures of 3.2 + 1 kbar and 660°C assuming hornblende crystallized on the granodiorite solidus of Kerrick (1972), with X H 2 Q = 1.0. Thermobarometric calculations made for the Shames River area are also consistent with mineral assemblages and reactions observed, and agree with observations made by Sisson (1985) in the CGC and Ponder pluton to the north. Following Hodges and Spear (1982) 2a deviations from mean P and T of chemical analyses were calculated for the different thermobarometers. For the garnet-biotite Fe-Mg exchange thermometer, 2a errors from the analyses (including outliers) range from ±34-212°C. Excluding outliers, the range of 2a errors is reduced to ±34-50°C. For the garnet-hornblende thermometer, 2a error is ±44°C. For the garnet-plagioclase pairs, 2a in pressure at 600°C is ± 0.2-1.06 kbar. The AVj in hornblende barometer gives 2a errors in pressure of ±0.44-1.80 kbar. If outliers in 2a error are excluded, then the range in 2a errors becomes ±0.44-0.9 kbar for the Alf; in hornblende barometer. 57 In the following discussion, 2a errors for the garnet-biotite and garnet-hornblende Fe-Mg exchange thermometers of ±50°C are used. For the GASP barometer, errors are estimated to be ± 1 . 6 kbar (following Hodges and Spear, 1982). For the Alj in hornblende barometer, a 2a error estimate of ± 1 kbar is used in reporting results. 3. Closure temperature The concept of closure temperature (Tc), as used in geochronology (Dodson, 1973), may be aplied to thermobarometry (Spear, 1989). Dodson (1973) predicts that T c is a function of: 1. Activation energy; 2. Diffusion coefficient; 3. Cooling rate; 4. Geometry, and 5. Diffusion distance. By definition, the temperature recorded by a geothermometer is the closure temperature for that system (Spear, 1989). For the Fe-Mg exchange in garnet-biotite, rim analyses taken 5 fitn from the edges of adjoining grains have T c from 525-580°C for cooling rates of 10-100°C/Ma. Peak temperatures that are greater than 600°C are difficult to recover, due to diffusional re-equilibration, especially in slowly cooled terranes. Closure temperatures for different thermobarometers vary. In exchange thermometers such as the Fe-Mg exchange in garnet-biotite systems, the rate-limiting step is the intracrystalline diffusion of Fe and Mg in garnet. In net transfer reactions, such as in the GASP barometer, grain-boundary diffusion and surface kinetics may be the rate-limiting step (Spear, 1989). In retrograde terranes, the presence of fluids may also catalyse reactions. In order to obtain unique P and T, simultaneous solution of the garnet-biotite and GASP systems was applied. For the simultaneous solution of P-T conditions, it has to be assumed that Fe-Mg re-distribution between adjacent garnet and biotite, and Ca between plagioclase and garnet, stopped simultaneously at the same T c . Even in simple cases, however, Fe and Mg diffusion between garnet and biotite occurs more readily than Ca net transfer between garnet and plagioclase (Thompson and England, 1984). In such a case, the P and T obtained by simultaneous solution may not fall on the P-T loop traversed by the rock. This complication has been considered in the application of thermobarometric results below, and reflected in the choice of larger 2a errors in the expression of results. 58 One goal of thermobarometry is to construct a complete P-T-time path by defining both peak and retrograde conditions of a P-T loop (Spear and Selverstone, 1983; Piatt, 1986; Spear, 1989). In some cases, garnet-core and biotite matrix and core compositions may be used to compute Kgq representative of peak metamorphic conditions, assuming that cooling rates were rapid enough to preserve peak compositions in both the biotite matrix and garnet core. Used in combination with adjoining rim compositions of garnet and biotite, which record the closure temperature, a larger section of the P-T loop may be deduced. In the Shames River area, temperatures of mylonitization in SRMZ reached 550°C, as deduced by the pervasive dynamic recrystallization of plagioclase (Tullis and Yund, 1977). Because original igneous compositions are often retained in hornblende cores, the sub-solidus closure temperature for the hornblende barometer probably exceeds about 550°C, and/or the cooling rate was rapid and/or dry enough to prevent complete re-equilibration of hornblende cores to reflect ambient metamorphic conditions. 4. Results a. Garnet-biotite and garnet-hornblende thermometry and GASP barometry The application of the garnet-biotite, garnet-hornblende and GASP thermobarometers is complicated by the fact that all garnets analysed are chemically zoned. Decreasing Mn values from rim to core of up to 1 % indicate higher metamorphic conditions recorded in garnet cores (Tracy et al., 1976; Woodsworth, 1977; Indares and Martignole, 1990; Tuccillo et al., 1990). Garnet zonation indicates that syn-Dm temperatures did not exceed those required for garnet homogenization (Woodsworth, 1977). The temperatures recorded by garnet-biotite core, and garnet-biotite matrix compositions are invariably higher than those recorded by rim compositions. These temperatures are thought to approximate peak metamorphic conditions if the volume of garnet in the rock is small compared to that in biotite. In this case, matrix biotite will record peak metamorphic conditions in its Fe-Mg distribution coefficient, which will not be modified by exchange with garnet during retrograde alteration (Spear, 1989). 59 One metapelitic sample (84WW82-3), containing the mineral assemblage sillimanite (fibrolite) - staurolite - garnet - quartz - plagioclase - biotite - muscovite from a pendant on Mt. Morris, collected by P. van der Heyden, gives Thompson (1976) rim temperatures of 560 + 50°C and core temperatures of 620 + 50°C. At 3 kbar, Ferry and Spear (1978) temperatures for rim and core are 567 ± 50°C and 649 + 50°C, respectively. The GASP geobarometer of Ghent et al. (1979) was also applied. At 600°C, sample 84WW82-3 gives rim pressures of 4.2 ± 1.6 kbar (Fig. 24), while sample 84WW82-4 (not shown) gives rim pressures of 4.6 + 1.6 kbar at the same temperature. Simultaneous solution of the Ferry and Spear (1978) thermometer and Ghent et al. (1979) barometer for sample 84WW82-3 gives rim T and P of 570 + 50°C and 3.8 ± 1.6 kbar respectively. Structurally continuous with, or slightly below the Mt. Morris pendant, a schistose meta-basite, sample TH89190 (Plate 3) contains the assemblage garnet-hornblende-biotite-plagioclase-quartz. A Thompson (1976) rim temperature of 567 + 50°C is obtained, and, at 3 kbar, a temperature of 579 + 50°C is obtained with the Ferry and Spear (1978) calibration. Within 2cr error, these temperature estimates overlap those calculated using the garnet hornblende thermometer of Graham and Powell (1984), which yields a temperature of 632 ± 50°C. East of SRF, one metapelitic sample, TH89044 (Plate 3), containing the mineral assemblage sillimanite (fibrolite) - staurolite - almandine - plagioclase - quartz - biotite -muscovite, was sampled for thermobarometry. Simultaneous solution of the Ferry and Spear (1978) garnet-biotite thermometer and the Ghent et al. (1979) GASP barometer give near-peak conditions, interpreted to record conditions of regional metamorphism, prior to intrusion of 69 Ma granodiorite, of 4.9 ± 1.6 kbar and 700 ± 50°C. Rim P and T of 2.9 ± 1.6 kbar and 610 ± 50°C may record pressures and temperature of final equilibration during subsequent D m deformation, which waned during intrusion of the late synkinematic granodiorite. The P, T region, as constrained by thermobarometry and mineral assemblages, lies in the five-sided shaded area of Fig. 25. 6 0 b. Alf in hornblende geobarometry The total Al-content in hornblende was measured to determine crystallization pressures of six metaluminous granitoid rocks. Results are listed in Table 3 and plotted in Plate 1. As noted previously, metamorphic pressures obtained with the GASP geobarometer are indistinguishable, within 2a error, from those obtained with the Al f in hornblende geobarometer. West of SRF, there is a trend towards decreasing pressure of crystallization upwards from the SRMZ. East of SRF, a very slight decrease in emplacement pressure is observed eastward. West of SRF, the trend is towards decreasing pressures from core to rim, while to the east, the converse is true. In the footwall of ERF, a deformed granodiorite (TH89131) has hornblende cores recording pressures of crystallization of 4.8 + 1.0 kbar. Rims record syn-Dm crystallization at 4.4-4.6 ± 1.0 kbar. Within the SRMZ, deformed granodiorite (TH89257c) dated at 188 ± 5 Ma has hornblende cores which record pressures of crystallization of 5.5 ± 1.0 kbar. Rim pressures of 4.6 to 5.0 kbar record the conditions of D m ductile deformation, dated at between 83.5 - 60 Ma. In the upper plate of SRMZ, 1.7 km above sample TH89257c, a late-synkinematic hornblende granodiorite (sample TH89207c) dated at 68.7 ± 0.4 Ma, records hornblende core crystallization pressures of 3.4 + 1 kbar. Hornblende rims record final crystallization at 2.2 + 1 kbar. Assuming a barometric gradient of 3.5 km/kbar, a metamorphic omission of 6.7 km is recorded by hornblende rim AI7 values in the upper half of the SRMZ. A total metamorphic omission of about 13.4 km is estimated across the SRMZ. East of SRF, the pressures of crystallization appear to increase slightly from core to rim, indicating post-crystallization subsidence and burial and/or faulting at temperatures for the hornblende barometer to re-equilibrate. Catalytic reactions by fluids may be ruled out since no retrograde minerals are present. The converse is true with the metapelitic GASP geobarometer, which indicates decreasing pressure with age. This difference may indicate either: 61 a. Closure temperatures of the two different barometers are different, or b. Sensitivity to post-crystallization effects are different for the different systems. Near the southern margin of unit LK3, sample TH89042 is a quartz diorite containing hornblende with Al-p which records core crystallization pressures of 2.9 ± 1 kbar, and rim pressures of 3.3 + 1 kbar. GASP geobarometry on a metapelitic sample gives similar results. Granodiorite (TH89413) from the core of the. same pluton records similar conditions (2.6 kbar and 2.8 kbar for core and rim, respectively) within 2a error. East of Amesbury Creek Fault, slightly lower rim pressures of 2.0 + 1.0 kbar are recorded by sample TH89179, which has a U-Pb crystallization date of 69 ± 1 Ma. D. SUMMARY AND P-T-TJJVIE PATHS Mineral equilibria and thermobarometry yield internally consistent results. West of the SRF, in the upper plate of SRMZ, M l metamorphism pre-dates or is synchronous with, intrusion by granodiorite orthogneiss of unit LK2. Since unit LK2 is cut by 69 Ma, late-synkinematic granodiorite, 69 Ma is a minimum age for M l metamorphism. The metamorphic grade of M l exceeds the staurolite-out reaction, and reaches partial melting conditions, and temperatures in excess of about 610°C at 3 kbar. In the SRMZ, M l metamorphism is recorded by migmatitic and gneissic xenoliths in 83.5 Ma gneissic granite, which also cuts ductile fabrics in 188 Ma unit EJgg. M l metamorphism is probably older than 83.5 Ma, also the younger limit of pre-Dm deformation. In the SRMZ, M2 metamorphism probably began during or after intrusion of ductilely deformed 83.5 Ma granite. In the upper plate of SRMZ, M2 occurred at about 570 ± 50°C and 3.8 + 1.6 kbar after intrusion of deformed orthogneiss of unit LK2, and ended before intrusion of late-synkinematic, 69 Ma granodiorite of unit LK3, which has hornblende cores indicating igneous crystallization at 3.4 + 1 kbar, and rim pressures of 2.2 + 1 kbar, which may be the pressure of deformation. A deformed granodiorite in the footwall of ERF records pre-Dm crystallization at 4 .8±1 kbar, and syn-Dm crystallization at 4.4-4.6 kbar. 62 In the SRMZ, igneous crystallization and metamorphic pressures are approximately 2.2 - 2.4 kbar higher than those in the upper plate. Alj; in hornblende from the core of one sample in the SRMZ indicates igneous crystallization at mid crustal pressures of 5.5 ± 1 kbar, 188 Ma ago. Metamorphic crystallization occurred at 4.6 - 5.0 + 1 kbar, probably after intrusion of 83.5 Ma granite. A metamorphic omission of about 13.4 km is present across the SRMZ. East of Shames River, near-peak, M l conditions, are 4.9 + 1.6 kbar and 700 + 50°C in a metapelitic sample within 69 Ma, late kinematic granitoid rocks of unit LK3. Rim pressures and temperatures from the same sample are 2.9 ± 1.6 kbar and 610 + 50°C, and are thought to reflect D m metamorphic conditions which waned around 69.0 Ma. A nearby intrusive quartz diorite body gives core and rim values of 2.9 and 3.3 + 1 kbar, respectively. Retrograde metamorphism is indicated by increasing Mn values from core to rim of garnets in metapelitic samples both east and west of SRF. Pressure-temperature-time paths for the Shames River area (Figs. 24 and 25) may be constructed from the data obtained. Clockwise loops are chosen on the basis of abundant regional and local geological evidence for prior crustal thickening involving rocks of Upper Jurassic Bowser Lake Group, with which the pelitic samples analysed for thermobarometry are correlated. In the P-T-time diagrams, P-T conditions recorded by the cores were attained before intrusion of 69 Ma granitoid bodies in the upper plate of SRMZ, and likely before intrusion of highly deformed 83.5 Ma granite in SRMZ. Rim compositions record pre- to syn-69 Ma conditions of metamorphism. The P-T-time paths indicate initial rapid decompression before 69 Ma. This was followed by rapid near isobaric cooling after 69 Ma to near-surface conditions (about 0.8 - 1.1 kbar) by 46.2 - 51.1 Ma, the time of cooling below the Ar-retention temperature for biotite. Table 2. Sequence of metamorphic mineral growth, metamorphic, intrusive and deformation events in Shames River area. PERIOD/EPOCH CRETACEOUS PALEOC. EOCENE OLIGOCENE MIOCENE I N T R U S I V E E V E N T S : 8 3 . 5 6 9 i 1 6 0 1 4 8 1 D E F O R M A T I O N : D N D F D B D M P R E - D M M E T A M O R P H I S M : M 1 M 2 M I N E R A L S : F I B R O L I T E G A R N E T S T A U R O U T E B IOTITE M U S C O V I T E P L A G I O C L A S E Q U A R T Z 64 Figure 24. P-T-time path for sample 84WW82-3 collected south of Mt. Morris. Curves: 1. Aluminosilicate triple point from Holdaway (1971). 2. Granodiorite solidus, X H 2o = 1-0 (Kerrick, 1972). 3. Staurolite + Muscovite + Quartz = Biotite + Sillimanite (Dutrow and Holdaway, 1983; Richardson, 1968). 4. Biotite + Al 2 Si0 5 = Garnet + Cordierite (Holdaway and Lee, 1977). 5 and 6b. Ghent (1976) and Ghent et al. (1979) geobarometer from core and rim analyses, respectively, of garnet and plagioclase. 6a. Ferry and Spear (1978) geothermometer from rim analyses of garnet and biotite. 10 8 -J HYPOTHETICAL CRUSTAL THICKENING PATH —.6 -CO JD Jet 4 -2 -SAMPLE TH89044 •1000 - 800 - 600 CORE PRE-69MA U-Pb RIM 69MA U-Pb 51.1 MA K-Ar I 200 400 I 600 — T -800 TJ 0) 400 - 200 1,000 TCP) Figure 25. P-T-time path for sample TH89044 collected east of Shames River. Curves: 1,2, 3, 4 as in figure above. 5a. Garnet core - biotite core and matrix geothermometer (Ferry and Spear, 1978). 6a. Ferry and Spear (1978) geothermometer from rim analyses of garnet and biotite. 5b. and 6b. Ghent (1976) and Ghent et al. (1979) geobarometer from core and rim analyses, respectively, of garnet and plagioclase. Table 3. Pressures obtained from AI7 in hornblende geobarometry. Sample A1 T content Pressure o"P West of SRF: Upper plate of SRMZ: T H 8 9 2 0 7 C P 1.34 ± 0.19 2. 19 ± 0. 79 + 1. 16 H 1.35 ± 0.20 2. 22 + 0. 83 + 1. 13 c 1.61 ± 0.13 3. 35 + 0. 53 SRMZ granitoid rocks: TH89257C P 1.91 + 0.10 4. 62 + 0. 43 +0. 91 H 1.99 + 0.13 4. 96 + 0. 57 +0. 57 C 2.12 ± 0.21 5. 53 + 0. 90 Footwall of Exstew River f a u l t TH89131 P 1.90 ± 0.13 4. 57 + 0. 54 +0. 23 H 1.86 + 0.13 4. 43 + 0. 55 +0. 37 C 1.95 + 0.20 4. 80 + 0. 84 East of SRF: West of Delta Creek: TH89413 P 1.51 ± 0.10 2. 93 + 0. 43 -0. 35 H 1.47 ± 0.15 2. 75 + 0. 66 -0. 17 C 1.43 ± 0.22 2. 58 + 0. 94 TH89042 P 1.66 ± 0.07 3. 55 + 0. 31 -0. 68 H 1.58 ± 0.12 3. 22 + 0. 49 -0. 35 C 1.50 ± 0.17 2. 87 + 0. 74 East of Delta Creek: TH89179 P 1.30 ± 0.11 2. 02 ± 0. 22 H 1.31 ± 0.10 2. 04 + 0. 47 Notes: ^ Sample locations plotted in Plate 3. P = hornblende rim contact with plagioclase. H = hornblende rim in contact with minerals other than plagioclase. C = hornblende core. 2 Alj content = A l ^ + Al^* reported with la errors. 3 Pressure obtained using Johnson and Rutherford (1989) calibration, and reported with la errors in table, but with 2a errors in text. 4 Core to rim pressure difference. + indicates higher core pressures. 66 V. STRUCTURE The Shames River area is affected by at least four phases of deformation. The earliest recognisable ductile fabrics are younger than Early Jurassic plutons and Late Jurassic Bowser Lake sedimentary rocks (pre-Dm). Superimposed on pre-Dm structures are Late Cretaceous (Campanian) to Paleocene mylonitic fabrics (Dm), Eocene semi-brittle faults (DR) , map scale open folds (Dp) and Eocene and younger high angle brittle normal faults (D^); (Table 2). This section presents the results of a structural study of the Shames River area, and proposes a model for its tectonic development. Evidence for high temperature, ductile deformation in the western block of the SRF, and progressively lower grade deformation eastward and in the hangingwall of the SRMZ, is presented. The kinematics and timing of deformation will also be discussed. A. GENERAL STRUCTURAL CHARACTERISTICS The area may be divided into seven structural domains based on similar foliation and lineation trends in each domain (Fig. 26). Gently east-northeast- and west-southwest-plunging mineral and elongation lineations are found in different parts of the map area. A notable contrast is steeply southeast-dipping foliations east of SRF, and predominantly north-dipping fabrics west of SRF, first noted by Woodsworth et al. (1985). West of SRF, the SRMZ is gently warped about approximately east-northeast trending axes, forming a synform to the north of Skeena River, and an antiform to the south. East of SRF, a progressive eastward decrease in both metamorphic grade and deformational strain is observed across a series of steep, northwest-trending brittle, east-side down normal faults. East of the Amesbury Creek fault, volcanic flows, breccias and tuffs are undeformed, and experienced lower greenschist facies metamorphism during D m deformation. West of the fault, pebbles and clasts are stretched, volcanic textures are not easily recognised, and metamorphic grade reaches amphibolite facies near igneous bodies. Pre-Dm ductile fabrics have been strongly obscured by subsequent D m deformation, recrystallisation and strain recovery. The old fabrics are best preserved south of Mt. Morris, 67 east of Exstew River, near Shames Mt, and east of Shames River. South of Mt. Morris, large, angular xenoliths of folded and deformed, banded granodiorite gneisses are contained in mylonitic augen orthogneiss and late synkinematic granodiorite dated at 68.7 + 0.4 Ma (Latest Maastrichtian). East of Shames River, sillimanite (fibrolite)-garnet-biotite-quartz-feldspar schist in granodiorite of unit L K 3 contains fibrolite-rich foliation that is warped by tight folds of D m age. The folded fibrolite is inferred to be a product of shearing (see Chapter III), and associated with M2, which is synchronous with, and outlasts, D m deformation, which waned around 68.7 + 0.4 Ma. East of Shames River, greenschist to lower amphibolite facies rocks of unit lP$i sit structurally above greenschist facies rocks of unit lPsm- This older over younger imbrication is interpreted to be a result of thrusting prior to intrusion by 69 Ma granodiorite of unit LK3 (Plate 1). D m may be divided into two progressive and overlapping phases: D m l occurred during the early stages of deformation, and F m j folds associated with it are restricted to the most highly deformed parts of the area, primarily in SRMZ. F m j folds are isoclinal, rootless, sheath folds which have hinge lines parallel to the elongation lineation (Fig. 27). F m j folds are sometimes folded by tight F m t folds, which formed during D,,^, towards the latter stages of D m deformation. Penetrative axial planar mylonitic foliations are produced by both D m l and Dm2- Folds produced by D,,^ commonly, but not always, have hinge lines at high angles to elongation lineations. The progressive nature of the deformation is deduced from the mutually overprinting fabrics of folding events and mylonitic deformation, and the systematic spread along a great circle of F m fold axes. In most places, F m t folds warp mylonitic foliation, but are themselves overprinted by mylonitic fabrics parallel to axial planes (Fig. 28). Low angle semi-brittle detachment faulting (DR) truncates granite correlated with a Paleocene dyke in SRMZ dated at 60 + 6 Ma. On ridges east of Exstew River, a flat lying, semi-brittle fault juxtaposes undeformed granite above, against folded and mylonitised biotite-hornblende schist and orthogneiss below. West of Exstew River, along the base of the SRMZ, D R faulting overprints ductile fabrics along the base of the SRMZ. High angle Dp faults 68 disrupt 69 Ma granodiorite, but do not significantly offset Eocene K-Ar biotite isothermal surfaces and SRMZ fabrics. Dp deformation affects the whole area, and produces meso- and megascopic scale open folds which are coaxial with elongation lineations and F m j hinge lines. No penetrative cleavage or mineral growth is associated with Dp, which is thus thought to have occurred at higher structural levels in the crust than D m . Dj^ produces high angle north-northwest trending, brittle normal faults, which disrupt the SRMZ. Associated with this episode of faulting is the intrusion of mafic and lamprophyre dykes with the same trend. Sigmoidal shaped tension gashes in the SRMZ dip steeply west-southwest, indicating southwest directed movement of the upper plate during brittle-ductile faulting. With few exceptions, C-surfaces and shear bands (Berthe et al., 1979; Piatt and Vissers, 1980; Simpson and Schmid, 1983; Piatt, 1984; Simpson, 1986) dip gently towards the northeast and east (Fig. 29), and are associated with D m and older deformation. Northeast-directed movement along shallow, northeast-plunging lineations is shown by most kinematic indicators east of Exstew River and contrasts with both northeast- and southwest-directed normal movement farther west (Plate 4). East of SRF, kinematic indicators in metavolcanic units indicate both northeast and southwest directed shear. In some places, later, southwest-directed ductile shears cut earlier formed mylonite bands and late pegmatite veins. Ductile shears are commonly invaded by moderately foliated, syn-kinematic granite, indicating high temperature shearing. B. DEFORMATION WEST OF SHAMES RIVER FAULT West of SRF, the Exstew River fault separates a footwall of high grade orthogneiss, migmatite, granitoids and metasedimentary rocks from a hangingwall consisting of the SRMZ and its upper and lower plate. Strain associated with D m was accommodated at amphibolite facies, as indicated by the presence of granitic melt along ductile shears, and the stable mineral assemblage hornblende - biotite - plagioclase (An> } 7) - quartz in the orthogneiss, and sillimanite (fibrolite) - garnet - biotite - plagioclase ± muscovite in metapelitic rocks. A progressive increase in strain towards the SRMZ is marked by a decrease in grain size, development of preferred mineral lattice orientations and S-C fabrics (Fig. 30, 31). 69 1. Upper plate of SRMZ The upper plate of the SRMZ is mildly to strongly foliated, and may be divided into three structural domains: I, II and III (Fig. 26). Domain I, which contains the least deformed rocks, has a moderately developed magmatic flow foliation that dips moderately north-northeast and is thought to be synchronous with D m deformation using the criteria proposed by Paterson et al. (1989); Marre (1986) and Tobisch and Paterson (1988). Lineations are not observed. Poles to foliations in domain II describe a diffuse north-northwest trending girdle, which dips steeply to the west, parallel to the girdle in structurally overlying domain III. Poorly developed mineral lineations plunge gently northeast. The rocks in this domain are best described as "protomylonites" (Sibson, 1977). Most are medium to coarse grained granodiorite, which display well developed S-C and shear band fabrics, and contain tightly to isoclinally folded mylonitic foliations (Fig. 34). Kinematic indicators mostly show upper plate to the northeast movement along gently north-dipping shear bands and C-surfaces. Igneous oscillatory zoning in plagioclase is well preserved in all thin sections studied. Myrmekite is found along a few deformed plagioclase feldspar rims (Fig. 33). Recrystallized grains of feldspar and quartz are found along the maximum finite elongation axis. Many feldspars show extensive dynamic recrystallization to grains, some of which show slight undulatory extinction furthest away from the main porphyroclast. The above observations indicate that deformation probably occurred at temperatures below 550°C (Tullis and Yund, 1977; Simpson, 1986). Annealing to relatively strain-free grains indicates that metamorphism (M2) outlasted ductile deformation (Dm). South of domain I, poles to foliations and axial planes of domain III describe a moderately well defined north-northwest trending, steeply west-dipping girdle. Stretching and magmatic mineral lineations are concordant, and plot in the same field as F m t fold axes which plunge gently east-northeast. Rocks in this domain are predominantly lineated granodiorite, dated at 68.7 + 0.4 Ma, cut by rare, discontinuous mylonitic zones 15m wide and up to 150m long, and containing granite dated at 69 + 1 Ma (U-Pb, zircon; Chapter III) and slightly to 70 highly deformed pegmatitic dykes and veins possibly associated with M2 metamorphism (see Chapter IV). Magmatic flow foliations, defined by vaguely layered, euhedral plagioclase and hornblende rich layers, and elongate mafic inclusions, are parallel to mylonitic foliations. Kinematic indicators show northeast directed transport of the upper plate (Figs. 34, 35, 36, 37). 2. SRMZ and Footwall of ERF The footwall of ERF and rocks of the SRMZ appear to have similar structural trends, and are subdivided into two structural domains, IV and V (Fig. 26). A sharp, semi-brittle fault (DR ) separates the ductile SRMZ from overlying moderately foliated granite of unit T, inferred to be 60 + 6 Ma. Asymmetric folds in the underlying SRMZ indicate a northeast-directed sense of shear. Because unit T granites clearly intrude mylonitic fabrics associated with D m elsewhere in the footwall, this more brittle phase of shearing is younger than 60 Ma. Poles to foliations in domain IV are concentrated in the southwest quadrant, whereas in domain V, they are in the southeast quadrant. In domain IV, they describe a vague and steep, north- northwest trending girdle. Lineations plunge gently east-northeast in domain IV, and both northeast and southwest in different parts of domain V. The lineations fall close to the separation arc determined by fold vergence reversals (Hansen, 1971). Mesoscopic, northeast directed ductile shears (Berthe et al., 1979; Piatt and Vissers, 1980; Passchier, 1984; Piatt, 1984) are common in the SRMZ and footwall (Figs. 38, 39). Conjugate shear bands, whose origin is the subject of much debate (eg. Piatt and Vissers, 1980; Harris and Cobbold, 1984; Piatt, 1984; Behrmann, 1987; Reynolds and Lister, 1990) are also found, especially in the western part of the map area, in the lower plate of SRMZ (Fig. 40). 3. Folding Folds on all scales are rare in the map area, especially in the granitoid units, as is common in granitoid rocks elsewhere (Denis and Secor, 1990). Pre-Dm folding is observable only in xenoliths contained in Dm-mylonitised granitoid rocks south of Mt. Morris and east of 71 Exstew River. D m folds are best developed in the most deformed and layered parts of the SRMZ, where deformation has progressively re-oriented earlier formed hinge lines into parallelism with elongation lineations, and dismembered folds into intrafolial, rootless folds. Where observed, two fold generations, F m [ and F m t , may be recognised in hornblende-biotite-quartz-feldspar gneiss and migmatite. A penetrative, axial planar foliation is moderately well developed during Fm{ folding. Isoclinal F m i folds are commonly rootless, and are present only in the most deformed and previously foliated rocks of the SRMZ. They are best developed in folded calc-silicate layers and schists, where their fold axes are parallel to elongation lineations, and where their axial planes are parallel to foliation (Fig. 41). F m t folds have foliation-parallel axial planes, and hinge lines which are both subparallel and at high angles to the elongation lineations. On lower hemisphere stereonets, F m j and F m t hinge lines describe a roughly northwest trending girdle which lies in the plane of axial surfaces and mylonitic foliations, but form a concentration in the northeast quadrant. Using Hansen's (1971) slip-line technique, this fold axis concentration is the separation arc for fold vergence reversal, indicating, together with other kinematic indicators, eastward movement of the upper plate (Fig. 42). The symmetry of F m t structural elements with those of ductile shearing indicate a genetic link between folding and ductile deformation in the SRMZ. Outside the SRMZ, F m j folds are found only in shear zones. These folds are inferred to have initially formed at high angles to the elongation lineation and been progressively rotated into parallelism with it with increasing shear strain (Bryant and Reed, 1967; Bell, 1978; Berthe and Brun, 1980; Brunei, 1986). They are termed "sheath" folds by Cobbold and Quinquis (1980) and Henderson (1981). The parallelism of fold axes and elongation lineations is indicative of high strain in an environment of inhomogeneous shear (Bell, 1978; Berthe and Brun, 1980; Piatt, 1983; Cobbold, 1976). The presence of sheath folds only in zones of intense mylonitisation indicates that they may be used as qualitative indicators of shear strain (Grocott and Watterson, 1980). Their irregular distribution throughout the SRMZ indicates that strain was heterogeneous. 72 Dp upright, open, map- and mesoscopic-scale folds are coaxial with regional, east-northeast plunging lineations and F m j hinge lines. Mylonitic foliation is warped by Dp folds, which have interlimb angles of between 120-140°. Similar folds have been reported in other ductilely deformed regions (Goldstein, 1982; Reynolds and Lister, 1990). Dp folds do not have an associated penetrative cleavage, and may be due to lateral north-south directed constriction at the ends of the SRMZ during the last stages of ductile deformation or to northwest-southeast directed compression during movement along transcurrent faults. At the mesoscopic scale, Dp folds are gentle crenulations of mylonitic foliation, and are cut by late, sigmoidal quartz-feldspar tension gashes indicative of deformation in a brittle-ductile regime (Ramsay, 1980; Ramsay and Huber, 1986). C. DEFORMATION EAST OF SHAMES RIVER FAULT The Shames River fault (SRF) marks a sharp structural and lithological break between the amphibolite facies deformed crystalline units to the west from greenschist to amphibolite facies deformed volcanic and granitoid rocks to the east. Deformational strain and metamorphic grade decrease sharply eastward across a series of brittle, sub-vertical, east-northeast dipping normal faults, the largest of which is the SRF. The earliest movement along the steep, north-northwest trending faults is bracketed between 69-46 Ma, since they cut 69 Ma granodiorite and disturb foliations in the upper plate of SRMZ, but do not appreciably offset mid-Eocene K-Ar biotite cooling isotherms. These faults juxtapose rocks with markedly different metamorphic grades and strain histories. South of the map area, the southern extension of the SRF is truncated by an Eocene granitoid body (Woodsworth et al., 1985). Latest movement along the SRF and other high angle faults displace Paleocene to Eocene age semi-brittle fabrics and faults in SRMZ, and are therefore younger than Eocene. 73 1. Description of fabrics Foliations east of SRF dip steeply to the southeast, while lineations plunge gently northeast. Two structural domains, whose common boundary is an intrusive contact, may be distinguished (Fig. 26). Domain VI consists of weakly lineated and foliated hornblende > biotite granodiorite dated at 69 ± 1 Ma (U-Pb, zircon). Flow foliations defined by aligned feldspar, quartz and hornblende crystals are better developed near the intrusive contact, and are parallel to foliations in a semi-ductile shear zone east of Shames River. Foliations are also parallel to those in the igneous and sedimentary rocks the shear zone cuts to the south. Plagioclase (An 3 0 . 3 6) displays minor cataclasis and myrmekite along grain boundaries in thin sections, but are more commonly euhedral and relatively fresh, with little retrograde alteration. Magmatic lineations plunge gently northeast. The interior of this body is unfoliated. Along logging roads north of Shames River, steep, north-northeast and north-northwest trending, mesoscopic, ductile shears which flatten into horizontal ductile shears cut a moderately deformed biotite granite. In domain VII, mylonitic deformation has resulted in foliations that dip steeply to moderately southeast. Poles to foliation lie along a poorly defined, northwest trending girdle (Fig. 26). Lineations plunge gently northeast and southwest. Mineral lineations are defined by aligned, euhedral actinolite crystals, and are parallel to elongation lineations defined by stretched pebbles and clasts in volcanic rocks and rare conglomerates. Mesoscopic foliations (C-surfaces) in metamorphosed volcanic rocks are defined by closely spaced mats of aligned and acicular actinolite needles and chlorite clots. In some thin-sections, planar, preferred orientations of flattened quartz and feldspar grains define an S-foliation. These may be inclined at angles of up to 45° with the C-surfaces. The C-S obliquity has been used to infer shear sense as discussed below. In plutonic rocks, mesoscopic S-C fabrics are common, with C-surfaces being defined by actinolite, chlorite and biotite rich, quartzo-feldspathic layers, and S-surfaces by inclined and flattened plagioclase and quartz crystals. The deformed plutonic rocks east of SRF contain more actinolite, chlorite and epidote than those to the west. 74 In rare instances, a shear band fabric, C , cuts the C- and S- surfaces at low angles, curve into parallelism with C-surfaces, and have the appearance of listric normal faults. Immediately east of Shames River, and west of Amesbury Creek, rocks of Stikine Assemblage structurally overlie rocks correlated with Hazelton Group along steeply southeast dipping, brittle to ductile shear zones. Mylonitic foliations decrease in intensity eastward, disappearing altogether immediately east of the high angle Amesbury Creek fault. East of this fault, original volcanic textures and bedding are well preserved. 2. Folding East of the SRF, folds are observed at only two localities. At the first locality, west of Delta Creek, two phases of folding affect marble of unit lPsm. ^\ is isoclinal, and is folded by upright, tight to isoclinal F 2 folds. Second phase axial planes dip steeply northwest, and are parallel to foliation. Hinge lines of F 2 folds plunge steeply west, and are coaxial with FI hinge lines. At another locality (station TH90044, Plate 3), east of Shames River, fibrolite-biotite-quartz-feldspar-muscovite mats are tightly folded into S-folds about steeply southeast plunging hinge lines. Axial planes are inclined at up to 25° with respect to the foliation, and dip steeply to the south. D. KINEMATIC INTERPRETATION Microscopic and mesoscopic shear sense indicators are abundant in deformed crystalline rocks west of SRF (Plate 4). Kinematic indicators include S-C fabrics, sheared porphyroclasts, mineral "fish", lattice preferred orientations, pressure shadows, asymmetric folds and shear (C) bands (Berthe et al., 1979; Brown and Murphy, 1982; Bouchez et al., 1983; Simpson and Schmid, 1983; Brunei, 1986; Passchier and Simpson, 1986; Simpson, 1986; Choukrouneetal., 1987). 75 In general, shear sense indicators west of SRF and east of Exstew River show northeast-directed shearing, down the dip of the SRMZ (Fig. 43). West of Exstew River and east of SRF, both northeast and southwest directed shear is seen. In the western part of the map area, both east and west of Exstew River, late, ductile shear bands (C) cut pre-existing mylonitic foliation, and have both synthetic and antithetic shear sense (Fig. 36, 44, 45, 46, 47). Shear bands ( C , or extensional crenulation cleavages) with synthetic shear sense are generally at small angles to, and dip in the same direction as, C and S planes. According to Piatt (1984), shear bands form during C-plane formation and curve into them (Fig. 35). Passive markers, such as foliations, are back-rotated along these shear bands. In detail, then, C planes have geometries similar to listric normal faults, and are interpreted to have accommodated northeast directed movement during ductile shearing by transfering motion to C-planes by ramping. In domain III, conjugate shear bands (Fig. 49) have inter-shear angles of 75-85°, increasing to 90-110° in the underlying domains II and IV. This increase in inter-shear angles may be due to an increase in the flattening component in strain and in the ductility downwards in the SRMZ (Ramsay, 1980). The finite shortening strain directions inferred from these conjugate ductile shears are sub-vertical, and the finite elongation axis is sub-parallel to lineations plunging gently east-northeast. Conjugate shears at one locality west of the study area, in the footwall of ERF, show a clear ductile to brittle transition in the style of conjugate ductile shears (Fig. 40). Piatt and Vissers (1980) have suggested that conjugate shear bands indicate strong flattening strain across a shear zone. In contrast, Harris and Cobbold (1984) showed with their plasticine experiments that conjugate shears occur during simple shear if back-sliding along foliation surfaces occurs. Antithetic southwest directed shear bands which do not form conjugate pairs are present west of Exstew River. These types of shear bands have been documented in several back-dipping mylonite zones of metamorphic core complexes by Reynolds and Lister (1990), and in thrust complexes by Behrmann (1987). In metamorphic core complexes, antithetic shear bands 76 develop during upwarping as the lower plate rocks are tectonically unroofed. Antithetic shear bands with ramp-flat geometries have also been recorded in the Betic Movement Zone, and developed during thrusting of anisotropic rocks (Behrmann, 1987). Late, brittle-ductile, sigmoidal quartz-filled tension gashes (Ramsay and Huber, 1986; Choukroune et al., 1987) in SRMZ along Highway 16 west of Shames River, and ductile listric normal faults also give antithetic shear sense, recording west-southwest directed shear (Figs. 44, 45, 50) during the late stages of deformation, at higher crustal levels. The progressive decrease in ductile strain with time is also indicated by the presence of both 8- and cr-type porphyroclasts (Passchier and Simpson, 1986) in granitoid rocks west of Shames River. In some thin sections, both types of porphyroclasts are observed (Fig. 51). East of SRF, however, 6-shaped porphyroclasts are not observed. West of SRF, 5-type porphyroclasts are generally less common, and are modified into later cr-type porphyroclasts. The former are thus inferred to have formed prior to cr-type clasts, in a higher shear strain to recrystallization rate regime (Passchier and Simpson, 1986). This high shear strain rate may explain the preservation of original igneous oscillatory zoning during relatively high temperatures of deformation, and the preservation of high grade metamorphic minerals in metapelitic rocks, as discussed in Chapter IV. The high rate inferred is consistent with the relatively rapid denudation rates of about 0.5 mm/yr calculated from thermobarometry and K-Ar dates (Chapters III and IV). Strain during retrogression may also explain the presence of more brittle, antithetic shear bands and tension gashes which overprint earlier, more ductile fabrics (Figs. 44, 50). Kinematic indicators are less common in deformed volcanic and plutonic rocks east of SRF, and show both northeast- and southeast-directed shear along elongation lineations inclined obliquely to the dips of foliation planes (Plate 4). E . NATURE OF STRAIN West of SRF, heterogeneous strain was accommodated at amphibolite grade, while to the east, it occurred at lower greenschist to lower amphibolite grade. In addition, metamorphic grade and strain magnitudes west of SRF increase downwards in relative intensity towards the SRMZ and decrease east of SRF. The presence of discrete, anastomosing shear zones indicates that strain was preferentially concentrated along favourable zones which may have had abnormally high pore-fluid pressure (Carter et al., 1990). The concentration of shear strain along discrete zones may also be a normal consequence of strain softening. In domains IV and V, L-tectonised granodiorite is well preserved along roadcuts north of Highway 16 and south of Skeena River. In some places, X:Y:Z ratios for feldspar and quartz aggregates reach 15:1:1, with X defining the elongation lineation. Rocks with extremely high X/Z ratios commonly preserve relic igneous textures, a feature observed in severely mylonitised terranes elsewhere (eg. Anderson et al., 1988; Anderson, 1988). Displacement across the shear zone was roughly estimated by plotting shear strain (expressed by the angular obliquity of C and S planes) as a function of distance from the centre of the zone (Ramsay, 1980; Ramsay and Graham, 1970). A displacement of about 25 km was obtained. These estimates are considered to be minimum displacement estimates, since strain is extremely heterogeneous in the SRMZ, and post-deformation annealing has destroyed some of the original deformational fabrics. Furthermore, discrete shear zones cutting the SRMZ often have much higher X:Z ratios and almost parallel C and S planes. F. SUMMARY AND TIMING OF DEFORMATION In the present study, the U-Pb and K-Ar dates (Chapter III) indicate that pre-Dm northeast directed ductile shearing in the map area occurred after 188 Ma, and ended by 83.5 Ma. D m ductile deformation began after 83.5 Ma intrusion of gneissic granite, waned during intrusion of 69 Ma granodiorite, and ended before intrusion of late to post-kinematic, 60 Ma granite. A semi-brittle phase of deformation affected and therefore postdated emplacement of granite correlated with a 60 Ma granite dyke in SRMZ, and continued during intrusion of 78 synkinematic 48 Ma granite. Final uplift and cooling through the biotite closure temperature for Ar is constrained by K-Ar biotite dates of 46.2 -51.1 Ma. East of Shames River, Early Permian and older greenschist to amphibolite grade rocks of Stikine Assemblage structurally overlie greenschist facies, Early Jurassic Hazelton Group. This older-on-younger sequence may be a result of northeast-directed compressional deformation, along thrust systems which today trend northeast-southwest, and dip steeply southeast. These rocks are intruded by late kinematic, 69 Ma granodiorite, which is more deformed near the margins than the interior. West of Shames River, pre-D M deformation reached upper amphibolite facies, but its nature is equivocal. Before 69 Ma, metamorphic pressures are similar in the SRMZ and its upper plate (Chapter IV). If pre-D M deformation took place in a convergent tectonic setting, it is compatible with contemporaneous thrusting elsewhere in the Cordillera (van der Heyden, 1989, 1990; Rusmore and Woodsworth, 1989; Evenchick, in press). D M deformation timing overlaps Late Cretaceous to Paleocene thrusting elsewhere in the Cordillera and Late Cretaceous to Eocene extensional magmatism in the Skeena Arch. West of Shames River, the early part of D M deformation (83.5-69 Ma) may be convergent in nature, since pre-D M metamorphic pressures are similar in the SRMZ and its upper plate, as noted above. The downward increase in strain gradient, however, is also consistent with development of ductile fabrics during extension, whereby less ductile rocks above are juxtaposed against more ductile, hotter rocks below. Steep, north-northwest trending normal faults cut 69 Ma granodiorite, juxtapose rocks of very different metamorphic grades, but do not significantly offset Eocene (46-52 Ma) K-Ar biotite cooling isotherms. Low angle, semi-brittle faulting ( D R ) occurred between 60-46.2 Ma in the SRMZ. East of Exstew River, undeformed granite above is juxtaposed against mylonitic schist and orthogneiss below. D R faulting is contemporaneous and coeval with abundant granitic intrusions in the SRMZ, and with extensive extensional volcanism in the Skeena Arch to the east. Both D M and D R fabrics are affected by broad, open folds which plunge gently northeast 79 (Fig. 48). These folds may have formed either during low angle, semi-brittle faulting, or transcurrent movement along north-northwest trending strike-slip faults. Strike-slip faults have not been recognised in the study area, but affect many parts of the Intermontane Belt to the east. The youngest features in the area are north-northwest trending, steep, brittle normal faults (Djsf) which disrupt semi-brittle fabrics, and which are coeval with intrusion of mafic dykes which trend north-northwest (Plate 4). Figure 26. Structural domains of the Shames River area, showing lower hemisphere projections of lineations (squares) and poles to foliation (crosses). Figure 27. Rootless, isoclinal folds (Fmi) of calc-silicate layers in biotite-hornblende orthogneiss of unit EJgg near station TH89006 in SRMZ. View to northeast along elongation lineation. Figure 28. Looking north-northeast at tight, F m t folds north of Shames River which warp mylonitic foliation, but is itself cut by later, axial planar, shear band (C) fabrics. N=1 13 Figure 29. Lower hemisphere projections of poles to shear bands (C). 83 Figure 30. Progressive deformation of granitoid rocks towards SRMZ west of SRF. a. Coarse grained, weakly lineated hornblende biotite granodiorite from domain I north of Shames River, b. Bent plagioclase twinning in slightly deformed granodiorite of domain II. c. Polygonisation of quartz subgrains in domain IV. d. Extensive recrystallization to subgrains of quartz and feldspar, leaving relict, rounded plagioclase porphyroclasts. e. Mylonitic equivalent of d. showing extensive dynamically recrystallized quartz and plagioclase, and ribbon lenses of alternating quartz and mica-rich layers. (continued) 85 Figure 31. Recrystallized quartz ribbon in unit EJgg north of Highway 16, at station TH89006. View to northwest. Shear (C) plane defined by quartz ribbons, while S planes are defined by inclined quartz fabrics. Shear sense is top to the northeast (right). Figure 32. Looking west-southwest at S-shaped, isoclinal recumbent folds in granodiorite of unit L K 2 on ridge south of Mt. Morris. Mylonitic foliation and later intrusive sheets are folded about gently northwest dipping axial planes. Fold vergence indicates closure of synclinorium to north (right). 8 6 Figure 33. View to northwest of well developed S-C and C fabrics in plagioclase porphyroclasts of unit EJgg Lattice preferred orientations are defined by biotite and flattened recrystallised quartz. Dynamically recrystallized plagiocalse at ends of porphyroclasts are parallel to direction of maximum elongation. Myrmekite located on edges facing direction of maximum shortening. Note clockwise rotation sense for earlier S-C fabrics (centre), and anticlockwise rotation for later C fabrics (top right). Figure 34. View northwest at sheared and recrystallized zoned plagioclase sigma type porphyroclasts in granodiorite of domain III east of Shames River. Orientation of recrystallized tails, and inclination of biotite grains, indicate dextral (northeast) directed shear. Figure 35. Northeast directed (right) shear indicated by amphibole fish, rotated sigma plagioclase porphyroclasts, pulled apart sphene (top centre) and shear band (C) foliation dipping 45° to the right, and inclined and flattened quartz grains. Photo taken near station TH89404d. Figure 36. Detail of above, Fig. (35), showing pulled apart sphene with flattened ribbon quartz inclined at 40° to well developed shear band (C) foliation dipping from left to right. Also note inclined quartz ribbons being rotated into parallelism in left central portion of photomicrograph, and platy biotite on edge of zoned plagioclase (bottom centre) that face direction of maximum shortening. Sigma-type porphyroclast of plagioclase at bottom centre also indicates northeast directed shearing. Figure 37. View to northwest of delta-type porphyroclast of plagioclase in deformed granodiorite of unit LK3 on Shames Mt. Dextral (northeast directed) shear is indicated. Delta type porphyroclasts are indicative of extremely high strain raterrecrystallization rates (Passchier and Simpson, 1986). Photo taken at station TH89404d. Figure 38. Sheared boudin in SRMZ east of Exstew River, showing northeast (right) directed shear in deformed granodiorite of unit E J g 2 . Figure 39. Sketch from photomicrograph of S-C fabric and rotated porphyroclast, showing dextral, northeast directed shear in SRMZ north of Highway 16 east of Exstew River. Sheared plagioclase (top centre) also shows "bookend-type" sliding indicative of dextral shear. • • ' — • I : «. •; * . ; . ' . ; • / • - '• ' : • ' • • i • •ft " . i v i - . * . -• . : • i -l-'«r V-', i*«|8 ; . > : - •• •- • y Figure 40. Conjugate ductile shears (centre right) in hornblende orthogneiss of footwall of SRMZ. Inter-shear angles of ductile shears are 95°-110°. Ductile shears are cut by later brittle conjugate shears with inter-shear angles of 45-50° (centre left). Photo taken along Highway 16, about 2km west of station TH89131, outside study area. 90 • F o l d a x i s (n=58) • A x i a l p l a n e (n=44) Figure 41. Lower hemisphere plot of poles to fold axes (squares) and axial planes (crosses). sepa-rah'on arc « Void axis Figure 42. Separation arc for fold axis vergence reversals measured west of SRF. 91 Figure 43. View to northwest of sheared, recrystallized hornblende in SRMZ hornblende-biotite orthogneiss west of Shames River. C plane is defined by aligned hornblende parallel to base of picture, while S planes are defined by flattened hornblende, quartz and plagioclase inclined at 45° from right to left. C planes cut obliquely from middle left to bottom centre, and flatten into C planes. Figure 44. Brittle - ductile fault in mylonitic orthogneiss in SRMZ. Darker layers of amphibole rich mylonite are brittlely offset, while leucogranitic material is ductilely sheared. View is to the north. Station located on logging road east of Exstew River. Figure 45. Semi-ductile listric normal fault rooting to the southwest (bottom left) in mylonitic layers. Fault cuts grey medium to coarse grained biotite-hornblende orthogneiss and black fine grained biotite-hornblende mylonite derived from orthogneiss. S-C foliation is developed in orthogneiss, with S planes inclined at 15° from top right to bottom left, and C (shear) planes defined by dark layers. View is to northwest, and transport of upper plate to northeast is indicated. Station located on Highway 16, within SRMZ. Figure 46. View to northwest of late shear bands cutting previously formed S-C fabrics in SRMZ west of Shames River. Shear bands dip obliquely from left to right. S is defined by hornblende inclined at 40° from top right to bottom left. Figure 47. Steep ductile normal shears indicating sinistral movement in coarse grained biotite-garnet granite of unit L K i . Left side is southwest. Figure 48. View to west of gently warped biotite hornblende schist and granodiorite orthogneiss, near west edge of SRMZ on logging road west of Exstew River. Figure 49. Conjugate ductile shears in granodiorite of unit L K 3 , west of station TH90207c, dated at 68.7 ± 0.4 Ma. View is looking east. 95 Figure 50. View to northwest of late, sigmoidal quartz-feldspar tension gashes indicating southwest directed shearing which post-dates earlier northeast directed (Dm) ductile shearing. Granite dyke at bottom (crosses) is sheared towards the northeast by D m deformation. Photo taken at roadcut on Highway 16 west of Shames River. Figure 51. View to northwest of mylonitised granodiorite of LK3, showing both 5-type porphyroclasts of plagioclase (large white grains in centre and centre left) and a-type hornblende porphyroclasts (bottom right and centre). Late shear bands cut obliquely across photograph from left to right. Shear plane is approximately parallel to base of photomicrograph. All kinematic indicators show upper plate to northeast (right) shear sense. VI. SUMMARY AND TECTONIC EVOLUTION 96 The present study has documented a complex structural, metamorphic and plutonic evolution for the eastern margin of the Central Gneiss Complex (Table 4). The most prominent events recorded are Early Jurassic to Late Cretaceous, pre-Dm deformation and metamorphism, Late Cretaceous to Paleocene, northeast directed, ductile shearing (Dm), and middle Eocene semi-brittle detachment faulting (DR). Post-Dg, Dp open folding and Eocene and younger brittle normal faulting, D^, are also recognised. A. SUMMARY The Shames River area is underlain by two diverse blocks separated by the north-north west-trending, high angle, brittle Shames River fault (SRF). To the east, Lower Permian and older Stikine Assemblage granitic and greenschist to lower amphibolite facies metamorphosed volcanic rocks are thrust over Lower Jurassic Hazelton Group, lower greenschist facies volcanic and sedimentary rocks. Structures associated with this juxtaposition include a strong penetrative foliation that dips steeply southeast. These fabrics are older than a late syn-kinematic, 69 Ma granodiorite which intrudes them. A brittlely deformed quartz diorite body which intrudes volcanic rocks in Stikine Assemblage has a minimum U-Pb date of 317 + 3 Ma. West of SRF, and south of Skeena River, Tertiary-age granitoid rocks underlie moderately deformed to undeformed granitoid rocks and minor metasedimentary rocks along the northeast dipping, 3-4 km thick, Shames River mylonite zone (SRMZ) with upper plate movement to the northeast. East of Exstew River, the SRMZ is in ductile fault contact with orthogneiss, schist, migmatite and minor metasedimentary rocks of the CGC along the moderately northeast dipping Exstew River fault (ERF). Within SRMZ, a granodiorite orthogneiss has a U-Pb zircon date of 188 ± 8 Ma, and records about 25 km of ductile movement from post-Early Jurassic to Paleocene. An 83.5 Ma U-Pb zircon date obtained by Woodsworth et al. (1983) from deformed gneissic granite intruding this body brackets the younger limit of pre-Dm deformation. In SRMZ, D m deformation began after 83.5 Ma, waned during 69 Ma, and ended before 60 Ma. In the upper 97 plate of SRMZ, granitoid rocks interpreted as late-synkinematic with respect to D m have been dated by U-Pb (zircon) at 69.0-68.7 Ma. These rocks intrude pre-Dm kinematic granodiorite orthogneiss. Semi-brittle detachment faulting (DR) began after 60 + 6 Ma, and deformed a synkinematic dyke which crystallized at 48 ± 3 Ma (U-Pb zircon). Cooling through the Ar-retention temperature for biotite was accomplished by 46.2 Ma and 51.1 Ma in the SRMZ and its upper plate, respectively. Dp open folding warps mylonitic structures. Dp folds are cut by Djyi high angle normal brittle faults with north-northwest trends, (Table 4). These faults cut 69 Ma granodiorite, do not significantly offset Eocene K-Ar cooling isotherms, but juxtapose rocks of widely different metamorphic grades. Regional metamorphic grade decreases sharply eastward across the SRF. To the west, metamorphic grade reaches upper amphibolite facies. Migmatite is common in the footwall of the ERF, and in the SRMZ. In the upper plate of SRMZ, peak temperature corresponding to pre-Dm deformation, recorded by thermobarometry on a pelitic schist, is 649° ± 50°C at 5 kbar. Garnet-plagioclase and garnet-biotite rim pressures and temperatures, inferred to reflect syn-Dm conditions, are 3.8 ± 1.6 kbar and 570 + 50°C in the upper plate of SRMZ. A garnet-biotite-hornblende schist gives similar temperatures with the garnet-biotite and garnet-hornblende thermometers. A nearby late-kinematic granodiorite dated at 68.7 + 0.4 Ma gives hornblende core pressures of 3.4 + 1 kbar, for pressure of igneous crystallization, and rim pressures of 2.2 + 1 kbar, for late-Dm metamorphic and deformational conditions. In the SRMZ, A l j in hornblende geobarometry on deformed granodiorite gives pre-Dm igneous crystallization pressures of 5.5 + 1 kbar. Hornblende rims from the same rock give pressures of 4.9 + 1 kbar, which probably records syn-Dm conditions. A metamorphic omission of 13.4 km is thus indicated. East of SRF, greenschist facies is attained except in a 1-2 km wide contact aureole, where sillimanite (fibrolite) grade rocks are found. Fibrolite formed before, during and after ductile shearing and intrusion of late synkinematic granodiorite with a U-Pb zircon date of 69 + 2 Ma. Thermobarometry on garnet core-biotite inclusions and matrix, and garnet core-plagioclase cores from a pelitic sample in the contact aureole give temperatures of 700 + 50°C and pressures of 4.9 + 1.6 kbar. These conditions reflect pre-Dm deformation 98 conditions. Final equilibration recorded for D m deformation by rims are 610 ± 50°C and 2.9 ± 1 . 6 kbar. Similar values for pressure (2.9 and 3.3 kbar for core and rim, respectively) are recorded by Al f content in hornblende from a nearby quartz diorite intrusive body to the north. Core to rim Mn zoning in garnet is also indicative of falling P, T conditions. Alx in hornblende geobarometry on samples east of SRF suggest slightly lower pressures of crystallization for the core than the rim. The converse is true to the west. This relationship may indicate east-side-down tectonic burial of granitoid rocks east of the SRF. Kinematic indicators also suggest predominantly northeastward transport of hanging wall rocks. A metamorphic P-T-time path for the upper plate of SRMZ indicates initial rapid decompression beginning at about 83.5 Ma and lasting until about 68.7 Ma and 3.8 ± 1.6 kbar. At 60 Ma, the lower and upper plates of SRMZ were again intruded by calc-alkaline magmas. Rapid decompression was followed by rapid, nearly isobaric cooling to surface conditions, the upper plate rocks passing through the Ar-retention temperature in biotite at 51.1 Ma and about 1.2 kbar. Rapid cooling at 69 Ma is also indicated by the relatively short interval between U-Pb (zircon) and K-Ar (hornblende) cooling dates. Rapid and/or dry uplift and cooling may account for the lack of a ductile-brittle metamorphic overprint in the rocks as they passed through the amphibolite-greenschist transition. Between 51.1 - 46.2 Ma, an average isotherm migration rate of about 0.5 mm/yr may be calculated from differences in K-Ar (biotite) cooling ages from the SRMZ and its upper plate granitoid rocks. Brittle dip-slip movement along the north-northwest trending, Shames River and other high angle brittle faults is younger than the Eocene fabrics they cut. Movement along these faults is not very large. Lamprophyre, basalt and andesite dykes have the same orientation as these faults. 99 B. TECTONIC EVOLUTION Pre-Dm deformation is contemporaneous and coeval with early Late Cretaceous and older thrusting events elsewhere in the Cordillera (Table 4; Crawford and Hollister, 1982; Brown and Murphy, 1982; Carr et al., 1987; Crawford et al., 1987; Armstrong, 1988; Friedman, 1988; Parrish et al., 1988; Friedman and Armstrong, 1988; Gareau, 1988, 1989; van der Heyden, 1989, 1990; Rusmore and Woodsworth, 1988, 1989; Evenchick, in press). Thrusting may have been accommodated along basal detachments which underlie the Coast Plutonic Complex, Stikinia, and Omineca and Rocky Mountain Belts (Evenchick, in press). Some of this thrusting was accommodated along the SRMZ and thrust faults east of SRF. Pre-D m deformation ended by 83.5 Ma in Shames River area. Late Cretaceous to Paleocene (83.5-60 Ma) northeast directed ductile deformation (Dm) followed pre-Dm deformation, and corresponds in timing to the final stages of northeast directed thrusting in the Skeena Fold Belt to the northeast (Evenchick, in press), and in the Mt. Waddington and Whitesail Lake areas to the south (Table 4; Rusmore and Woodsworth, 1988, 1989; van der Heyden, 1982, 1989). D m was accompanied by abundant igneous intrusions. In the Intermontane Belt to the east, abundant volcanics were erupted during Late Cretaceous to Paleocene. D m deformation appears to have waned around 69 Ma, and ended before 60 Ma. East and west of SRF, thrusting ended during 69 Ma. Early ductile extension may have occurred between 69-60 Ma, along gently northeast dipping ductile shear zones. The timing of this extension corresponds to that of Ootsa Lake extensional volcanism in the Skeena Arch to the east (Richards, 1990). The slightly older age of this extension than that of core complexes in the Canadian Cordillera, invites the hypothesis that D m deformation may be related to crustal collapse initiated immediately after, or possibly during the late stages of, crustal thickening. Extension in the Shames River area may have arisen due to the gravitational instability caused by crustal thickening (Dewey and Bird, 1970; Dalziel and Molnar, 1981; Dalziel and Brown, 1988; Molnar, 1988, 1989; England and Houseman, 1985; Wells et al., 1990). According to Armstrong and Ward (in press) magmatic weakening of an overthickened crust 100 may also trigger extension. Contemporaneous compression and extension cannot be ruled out on the basis of the data obtained in this study. In the Andes and Himalayas (Molnar and Tapponnier, 1975, 1978; Tapponnier et al., 1981; Molnar, 1988, 1989; Burchfiel et al., 1989); in the Sevier thrust belt in northwestern Utah and southern Idaho (Wells et al., 1990), and in the Shuswap Complex (Brown, pers. comm., 1991), contemporaneous compression and extension were triggered by a combination of one or more of the following factors: 1. High topographic slopes due to crustal thickening and 2. Thermal erosion and removal of a lithospheric root in the middle of a thickened welt. Yin (1989) proposed a model in which contemporaneous compression and extension occur. In this model, crustal scale extension may occur due to a basal shearing traction exerted on a weak, ductile lower crust by asthenospheric drag, giving rise to low angle extensional faults which root into weak lower crust. In brittle regimes, these low angle extensional faults are the primary shears in Riedel shear couples (Yin and Kelty, 1991). Semi-brittle detachment faulting ( D R ) in Shames River area occurred between 60-46.2 Ma. The start of D R faulting corresponds in timing with east-side-up ductile faulting along Work Channel lineament and synkinematic emplacement of 62-57 Ma Quottoon Pluton east of Work Channel (Fig. 52). At 57 Ma, an abrupt change in Kula Plate motion, from orthogonal to oblique subduction occurred (Lonsdale, 1988). D R extension may have been partly triggered by this relaxation of far field stresses. The intrusion of abundant plutons during Paleocene to Eocene time may have thermally weakened the lithosphere, which was additionally softened by previous crustal thickening. Large scale crustal collapse in the Cordillera is documented in Early to Middle Eocene-age metamorphic core complexes in B.C. and the northwestern U.S. (Armstrong, 1982, 1988; Carretal., 1987; Friedman, 1988; Friedman and Armstrong, 1988; Parrish et al., 1988; van der Heyden, 1988, 1989, 1990; Armstrong and Ward, in press). A model for extension during Paleocene (-60 Ma) time is presented in Fig. 52. In this model, uplift of the CGC was accommodated along the Work Channel lineament to the west, and the SRMZ and other east dipping shear zones, on the east side. Paleocene to Eocene extensional structures, including the Exstew River and semi-brittle (DR) faults, are 101 superimposed on pre-Dm and D m compressional structures. Comparison of these extensional faults may be made with the Okanagan Lake and Columbia River faults, which are superimposed on the Monashee Decollement in the Shuswap Complex (Carr et al., 1987). As in most other mountain belts (Dalziel and Molnar, 1981; Tempelman-Kluit and Parkinson, 1986; Piatt, 1986; Brown and Journeay, 1987; Gottschalk and Oldow, 1988; Pavlis, 1989; Sacks and Secor, 1990; Wells et al., 1990; Jolivet et al., 1990), extension directions in the Shames River area are parallel with compression directions, and both are orthogonal to the trend of the belt. Tectonic events in the Shames River area may be linked with tectonic events in the Intermontane Belt, and particularly in the Skeena Arch to the east (Table 4). Latest Cretaceous and older compression in the Intermontane Belt may be related to ductile deformation along the SRMZ. Paleocene and younger extension reactivated existing thrust faults in the SRMZ, and is synchronous with extensional magmatism in the Skeena Arch. Extension occurred during Paleogene magmatic weakening of lithosphere weakened by overthickening of continental crust. Relaxation of far field stresses, due to changes in offshore Kula Plate motion during late Paleocene time, may also have triggered extension. o i TIME (MA) >-cr < F DC UJ CO o Ul o g oc o o CO CO OLIGOCENE UPPER MIDDLE MAASTRICHT CAMP AN IAN 20 40 60 QANTONIAN CENOMANIAh o CO co < CC I— z < 2 DC UJ •_ J >-(0 z z UJ 0_ ALBIAN A P T I A N BARREMIAN HAUTERMAT* VALANGINIAN I AH: MIDDLE EARLY I ATE MIDDLE EARLY LATE EARLY LATE 80 100 120 140 160 180 200 220 240 260 280 300 320 U*B DATES SHAUE8 n «U SHAMEt * AW I I 1(1 (1)1 K AR DATES V DCFOMMrtON WTfNWTT T(t> SHAMES RIVER OOT1A OKI <MOUP MOrVCEIOWN (2) STKNIA IN IN TERM. BELT (2) ENOAKO GROUP TYPE KASALKA GROUP EVENTS IN SHAMES RIVER AREA •LOCK FAULT WtfC DYKES DETACHMENT FAULTING ) DUCTILE EXTENSION AND CONTRACTION (Dm) CRUSTAL THICKENING DUCTILE DEFORMATION (PRE-Dm) REGIONAL GEOLOGICAL EVENTS (9) WOPK CHANNEL LINEAMENT (3) (5) (5) (to) (8) i—»i—»i—» J r"j oo cw vo H CP poo' p vo /~) g CD S-p^^ go " *gag. D. C/3 2 § " •« 8 vo c p ^-N y vo^ F »-* 3 o I — vo vo5)P-oo-oo VO o o 3 3 CD < CD 3 _ O O 3* Q. B 3 3-XI ^ ro vo K oo w OO o t-H 3 5 3 o o H-J= VO o g3 ON 0 0 ^ < 3 ' ° O o 2 3 O P. 3 o 3* 3 oo VO o ' o VO D. OO vi U\ < 3 5 W 3* o-r->- 3 R e . 3" • IS 00 o to 103 LEGEND Intrusive Rocks: Paleocene: Syn- and late-kinematic granitoid rocks. Upper Cretaceous to Paleocene: Pre- and syn-kinematic granitoid rocks. Supracrustal Rocks: Mid-to Upper Cretaceous: Kasalka Group and Brian Boru volcanic rocks. v v y v v Lower to Mid- Cretaceous: Skeena Group sedimentary rocks. o e Upper Jurassic: Bowser Lake Group sedimentary rocks and metamorphosed equivalents. Basement: Nisling Terrane and continental basement. ~ — Ductile lower crust Lower to Middle Jurassic: Hazelton and Gamsby Groups calc-alkaline volcanic rocks and metamorphosed equivalents. v—v-— V — v Paleozoic to Triassic: Takla-Stuhini volcanic and sedimentary rocks, and Stikine Assemblage volcanic, sedimentary and plutonic rocks, and metamorphosed equivalents. Undivided: Scotia-Quaal metamorphic belt (Gareau. 1990), Prince Rupert shear zone (Crawford et al.. 1987) and Alexander Terrane (Wheeler and McFeely, 1987). Figure 52. Hypothetical east-west cross-sections of western margin of Canadian Cordillera at the latitude of Terrace-Prince Rupert during Paleocene (~60 Ma) time. See text for details. 104 VII. REFERENCES Anderson, J.L. 1988: Core complexes of the Mojave-Sonoran Desert: Conditions of plutonism, mylonitization, and decompression; in Metamorphic and crustal evolution of the Western United States, W.G. Ernst (ed.), Rubey Volume VII, Prentice Hall, p. 503-525. 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L . , Dallmeyer, R.D. and Allmendinger, R.W. 1990: Late Cretaceous extension in the hinterland of the Sevier thrust belt, northwestern Utah and southern Idaho; Geology, v. 18, p. 929-933. Wetherill, G.W. 1956: Discordant uranium-lead ages; Transactions of AGU, v. 37, p. 320-326. Wheeler, J.O. and McFeely, P. 1987: Tectonic assemblage map of the Canadian Cordillera and adjacent parts of the United States of America; Geological Survey of Canada, Open File 1565. Winkler, H.G.F. 1979: Petrogenesis of metamorphic rocks; Springer-Verlag, new York, 348p. Woodsworth, G.J. 1979: Metamorphism, deformation, and plutonism in the Mount Raleigh pendant, Coast Mountains, British Columbia; Geological Survey of Canada, Bulletin, v. 295. 1979: Geology of Whitesail Lake map area, British Columbia; in Current Research, Part A, Geological Survey of Canada, Paper 79-1A, p. 25-29. 1978: Eastern margin of the Coast Plutonic Complex in Whitesail Lake map-area, British Columbia; in Current Research, Part A, Geological Survey of Canada, Paper 78-1A, p. 71-75. 1977: Homogenization of zoned garnets from pelitic schists; Canadian Mineralogist, v. 15, p. 230-242. Woodsworth, G.J.; Loveridge, W.D.; Parrish, R.R., and Sullivan, R.W. 1983: Uranium-lead dates from the Central Gneiss Complex and Ecstall pluton, Prince Rupert map area, British Columbia; Canadian Journal of Earth Sciences, v. 20, p. 1475-1483. Woodsworth, G.J.; van der Heyden, P., and Hill, M.L. 1985: Terrace map area; Geological Survey of Canada, Open File 1136. Yin, A. 1989: Origin of regional, rooted low-angle normal faults: A mechanical model and its tectonic implications; Tectonics, v. 8, p. 469-482. Yin, A. and Kelty, T.K. 1991: Development of normal faults during emplacement of a thrust sheet: An example from the Lewis allochthon, Glacier National Park, Montana (U.S.A.); Journal of Structural Geology, v. 13, p. 37-47 York, D. 1967: The best isochron, Earth and Planetary Science Letters, v. 2, p. 479-482. APPENDIX 1. THIN SECTION DESCRIPTIONS OF SELECTED SAMPLES Table 1.1. Thin section mineralogy and shear sense. Minerals* Shear Sense TOP Sample QZ PL KF Bl HB CH EP AC SI GA MS ST OP SP AP CA An Content To NE To SW Rock Type2 Number (%) Id X X X X X X 25-34 CS-GN 4a X X X X X X X X X DI 6b X X X X X X X CS-GN 6a X X X X X X X CS-GN 6c X X X X X X X 29-45 OGN 17 X X X X X X X X 20 GD 31a X X X X X X X X 25 OGN 32a X X X X X X X X 36-40 GD 34 X X X X X X X X 29-35 GD-OGN 39a X X X X X X X X X X 34 GD 42 X X X X X X X HB QD 44d X X X X X X X SC 49d X X X X X X AN-PHY 52a X X MA 53b X X X X X 22 OGN 55b X X X X X X X X Ambiguous OGN Minerals Shear Sense Sample Number QZ PL KF Bl HB CH EP AC SI GA MS ST OP SP AP CA An Content TOP To NE To SW Rock Type 2 260b X X X X HB DI 265a X X X X X X 22 QD 268 X X X X 23-45 DA-BX 269a X X X X X X X 21-32 SC 275d X X X X X X 25 X HB GD OGN 286a X X X X X Ambiguous AN-DA TF 286b X X X X X X X 22-30 R Y T F 289 X X X X X X X X X DA-AN 290 X X X X X X 25-30 GD OGN 292 X X X X X X X X X 24-36 Ambiguous HB-BI GD 293a X X X X X X 25-28 HB GD OGN 294c X X X X X X X 26-30 X GD OGN 350 X X X X X X X 12 GD OGN 327 X X X X X X X X X 17 BI-HB GD 377a X X X X X X X X X BI-HB GD OGN 399 X X X X X X X HB>BI GD 404d X X X X X X X HB-BI GD OGN 405a X X X X X X X X X Ambiguous HB-BI GD OGN 405b X X X X X X X Bl GR 412 X X X X X X X X X 23 X BI>HB GD 416b X X X X X X X 22-26 BI-HB GD 427a X X X X X X X X X Bl GD OGN 429c X X X X X X X 36-46 X HB-BI GD Minerals* Shear Sense TOP Sample Number QZ PL KF Bl HB CH EP AC SI GA MS ST OP SP AP CA An Content To NE To SW Rock Type2 76a X X X X X X X SC 77 X X X X X X X SC 96b X X X X X X 20 X OGN 117c X X X X X X X 20 OGN 118a X X X X X X X X X 40-55 X QD-OGN 119a X X X X X X X X X 33-36 QD-OGN 121 X X X X X X X X X 20-32 BI>HB GD 132 A X X X X X X X X SC-OGN 183a X MA 190 X X X X X X X X 40-55 X SC 200 X X X X X X X X 23 QD-OGN 207c X X X X X X X X HB>BI GD 210c X X X X X X X 24-32 X GD-OGN 218c X X X X X X X X X X HB-BI QD 219a X X X X X X 33 X HB-BI QD 231a X X X X X X 30 HB>BI DI 232b X X X X X X X 75 OGN-SC 253 X X X X X X X 28-35 X SC-OGN 253a X X X X X X X 36-48 X SC-OGN 253e X X X X X Bl GN 254 X X X X X X X X 28-30 GD 257a X X X X X X X X X 33-37 X BI>HB GD OGN 258 X X X 38 OGN Minerals* Shear Sense Sample Number QZ PL KF Bl HB CH EP AC SI GA MS ST OP SP AP CA An Content TOP To NE To SW Rock Type2 430 X X X X X HB QD 430b X X X X X X X 25-35 HB-BI GD OGN 84WW82-3 X X X X X X X X X X SC 1 Mineral abbreviations: QZ - quartz, PL - plagioclase, KF - alkali feldspar, Bl - biotite, HB - hornblende, CH - chlorite, EP - epidote, AC - actinolite, SI -sillimanite, GA - garnet, MS - muscovite or sericite, ST - staurolite, OP - opaques, SP - sphene, AP - apatite, CA - calcite. 2 Rock type: GD - granodiorite, QD - quartz diorite, DI - diorite, GR -granite, TN - tonalite, BA - basalt, AN - andesite, RY - rhyolite, DA - dacite, TF -tuff, BX -breccia, PHY - phyllite, SC - schist, OGN - orthogneiss, CS-GN - calcsilicate gneiss, MA - marble. APPENDIX 2. GEOCHRONOMETRIC ANALYTICAL TECHNIQUES AND DATA 2.1 U-PB ANALYTICAL TECHNIQUES The procedures followed are described in detail in van der Heyden (1989). Zircon samples are first crushed with a jaw crusher and pulverised with a disc mill. Standard heavy mineral separation procedures include the Wilfley wet shaking table, Franz magnetic separator, and heavy liquids. After initial zircon purification, various size fractions were collected using nylon sieves, and aliquots for analysis prepared by handpicking under a binocular microscope. Analytical aliquots contained 0.1-1.0 mg of zircon. Fractions coarser than 44 /xm were air abraded with pyrite for 1.5 to 10 hours. Dissolution and column chemistry used a procedure modified from Krogh (1973). Purified Pb and U were loaded together on Re filaments with silica gel. U and Pb concentrations were determined using a mixed 205Pb/235u spike (Parrish and Krogh, 1987). Mass spectrometric analysis was carried out on a VG Isomass 54R solid source mass spectrometer with single Faraday and Daly collectors. Pb/U and Pb/Pb errors were obtained by propagating all calibration and analytical uncertainties through the entire date calculation program, and summing the individual contributions to the total variance. Analytical results are reported in Table 2.1. 2.2 K-AR ANALYTICAL TECHNIQUES About 0.5-2 kg samples were pulverised and 50-80 mesh size fractions obtained for analysis. Biotite and hornblende were concentrated with heavy liquids, dry shaker table and Franz magnetic separator. K was determined in duplicate by atomic absorption using a Techtron AA4 spectrophotometer and Ar by isotope dilution using an AEI MS-10 mass spectrometer, high purity 38 Ar spike, and conventional gas extraction and purification procedures. The constants used are: 4 0Ke=0.581xl0- 1 0y- 1 , 4 0 K =4.962xl0' 1 0y- 1, 4 0K/K=0.01167 atom%. K was analysed by D. Runkle and Ar by J. Harakal at the Department of Geologcal Sciences, University of British Columbia. Analytical results are reported in Table 2.2. Table 2.1. U-Pb zircon isotopic data. 1 3 4 Sample Weight U Pb Pb isotopic abundance (mg) (ppm) (ppm) 2 0 6 P b = 100 2 0 6 P b / 2 0 4 P b Common Pb 2 0 7 P b / 2 0 6 P b 2 0 6 P b / 2 3 8 U 2 0 7 P b / 2 3 5 U Blank 207 JP9) 208 204 Observed 5 Date^ Date' (Ka) (Ha) Date (Ma) Date (Ma) Late to post-kinematic dykes west of SRF, in SRMZ and lower plate TH89131: N2A/1°, 0.7 177.5 3.09 8.830 21.407 0.2656 2006 + 149|Ltm, 300 abr (C) 85±100 159±31 94.4±0.3 96.9±1.3 N2A/1 , 0.2 1625 15.55 5.044 11.549 0.0162 838.6 85±100 102116 61.1±0.4 62.110.6 -149 + 134/xm, 200 N1.5A/30, 0.2 3522 30.2 6.584 17.160 0.1251 743.5 85+100 70±24 50.4±0.2 50.810.6 -74+44/im, 30 abr (C) M1.5A/30, 0.1 4329 31.4 5.261 13.228 0.0510 1292 -44Min (C) 50 851100 -51126 45.3+0.4 43.610.5 TH8 9253e: N2A/1°, 0.1 425.2 10.49 12.825 30.058 0.5346 116.1 +149am, 200 abr (C) N2A/1°, 0.2 997.3 10.34 8.133 17.020 0.2318 255.4 -149 + 134/im, abr (C) 200 65+100 179+54 -56 651100 56+22 -23 114.910.5 117.9+2.8 59.110.2 59.010.6 Continued Table 2.1 (continued) Sample Weight U (mg) (ppm) (ppm) Blank 207 .(pg) Pb3 Pb isotopic abundance4 2 0 6Pb/ 2 0 4Pb Common Pb 2 0 7Pb/ 2 0 6Pb 2 0 6Pb/ 2 3 8U 2 0 7Pb/ 2 3 5U 206 Pb = 100 208 204 Observed- Date' (Ma) Date' (Ma) Laie-kinematic granitoid east of SRF: TH89179: N 2 A / 1 ° , 0.1 519.5 5.92 5.833 16.784 0.0757 756.1 + 1 3 4 / i m , 20 abr (C) 6 5 ± 1 0 0 58132 Date (Ma) Date (Ma) 68.310.6 68.010.8 N2A/1 , 0.4 283.8 5.94 21.582 55.568 1.1321 81.8 -134+74Ltm, abr (C) 1 0 0 651100 162+38 - 3 9 68.910.3 71.611.2 N1.5A/3 , 0.7 297.4 3.40 5.313 17.348 0.0375 562.9 -74+44/im, abr 100 651100 79+23 -24 69.010.4 69.3+0.7 Continued Table 2.1 (continued) Sample 1 Weight U Pb3. Pb isotopic abundance4 2 0 6 P b / 2 0 4 P b Common Pb 2 0 7 P b / 2 0 6 P b 2 0 6 P b / 2 3 8 U 2 0 7 P b / 2 3 5 U (mg) (ppm) (ppm) 2 0 6 P b = 100 Blank 207 208 204 5 S ' 7 Observed Date Date (Ha) (Ha) Date Date (Ha) (Ha) Late-kinematic granitoids west of SRF, in upper plate of SRMZ: THB9207C N l A / 5 ° , 0 . 8 260 .6 3 . 1 1 6 .532 19 .157 0 .1200 6 4 3 . 4 6 5 ± 1 0 0 8 1 ± 8 6 9 . 2 1 0 . 2 6 9 . 5 1 0 . 3 + 134/im, 50 a b r (C) N 2 A / 0 . 5 0 , 2 . 3 274 .7 3 . 0 9 5 .542 16 .248 0 .0536 1286 -134+74/im, 100 a b r (C) 651100 7615 6 8 . 2 1 0 . 2 6 8 . 4 1 0 . 3 N 1 . 5 A / 1 0 , 1 .0 2 5 5 . 9 2 .94 4 . 9 1 1 1 9 . 2 0 7 0 .0149 1361 -74+44/xm, 100 a b r (C) 651100 45111 6 8 . 8 1 0 . 2 6 8 . 1 1 0 . 4 TH89404d: N 2 . 1 A / . 5 0 1 .0 597 .7 9 . 2 8 13 .723 3 5 . 3 1 6 0 .6053 1 4 1 . 0 651100 104+30 6 8 . 6 1 0 . 3 6 9 . 6 1 1 . 0 -134 200 -31 +74um, a b r (m) N 1 . 5 A / 3 , 1 .0 668 .1 8 .24 8 . 0 2 0 19 .392 0 .2220 3 7 6 . 7 651100 73118 + 134/im, 200 a b r (m) 6 9 . 0 1 0 . 3 6 9 . 1 1 0 . 6 Pre-kinematic granodiorite orthogneiss in SRMZ: THS9257c: N 2 A / 1 ° 0 . 1 303 .1 8 . 8 3 5 .222 9 .3157 0 .0075 5 4 6 . 0 1801100 246121 -149 + 134/im 100 a b r (m). 1 8 7 . 6 1 0 . 5 1 9 2 . 0 1 1 . 7 N 1 . 5 A / 3 0 , 0 .4 650 .9 1 8 . 7 2 5 . 1 3 0 8 . 2 2 5 0 0 .0088 5134 -74+44/ilti 50 a b r (m) 1801100 19515 1 8 7 . 2 1 4 . 8 1 8 7 . 8 1 4 . 5 C o n t i n u e d Table 2.1 (continued) Sample 1 Weight U Pb 3 Pb iso top ic abundance 4 2 0 6 P b / 2 0 4 P b Common Pb 2 0 7 P b / 2 0 6 P b 2 0 6 P b / 2 3 8 U 2 0 7 P b / 2 3 5 U (mg) (ppm) (ppm) 2 0 6 P b = 100 Observed 5 Date 6 Date 7 Date Date Blank 207 208 204 (Ma) (Ma) (Ma) (Ma) (P9) Pre-kinematic quartz diorite east of SRF: TH89265: N2A/1° 0.3 87.3 4.28 5.405 15.122 0.0108 719.1 280±100 306114 297.010.7 298.011.7 +134/Jm 100 -13.8 abr (m) N1.5A/30, 0.4 95.9 4.97 5.565 15.620 0.0200 2697 2801100 31712 311.410.7 312.110.7 -134+74/iiti 100 -2.5 abr (ro) Sample locat ions plotted in Plate 3. Locations of samples are: TH89257c-54°24 '30"N, 128°53'48"W; TH89265-54°25 '45"N, 128°51 '00"W; TH89179-54°29 '12"N, 128°49'42"W; TH89404d-54°26 '45"N, 128°57'30"W; TH89131-54°23 '20"N, 12 9 o06'20"W; TH89253e-54°24 '33"N, 128°53*46"W; 2 abr = a i r abraded with p y r i t e ; M = magnetic, N = non-magnetic; C = c l e a r , c o l o u r l e s s , euhedral c r y s t a l s ; m = metamict.. 3 Radiogenic and common lead. 4 Radiogenic and common Pb, corrected for 0.0043 per a.m.u. f rac t ionat ion and for 20 ± lOpg to 300 ± 100 pg blank Pb with composition 208:207:206:204 = 3 7 . 3 0 ± 0 . 2 9 : 1 5 . 5 0 ± 0 . 1 7 : 1 7 . 7 5 ± 0 . 1 9 : 1 . 0 0 . 5 Not corrected for f rac t iona t ion . 6 Common Pb assumed to be Stacey and Kramers (1975) model Pb with 6/4=11.152, 7/4=12.998, 8/4=31.23 at 3.7Ga. 7 A l l dates with 1 sigma e r r o r s . Decay constants used: 2 3 8 U = 1.55125 x 1 0 _ 1 0 y r - 1 ; 2 3 5 U = 9.88485 x 1 0 - 1 0 y r - 1 ; 2 3 8 u / 2 3 5 u = 137.88. 126 ... (NTS /03 2/7 T ^ r r t t c c U_ n u Q Mineral analysis "~ r D 0Concordia interpretation • Mineral or rock isochron Sample Number(s) and Reference(s) Upper Intercept 2a error Lab No: TH8<113\ Computed^} Assumed • Z / y ± T 7 Ma Ref: T- H&euh Lower Intercept 2a error Computed 0" Assumed • ^ ± Ma Record No: 238Tt 206T Suite No: U- Pb date SioiiU.- (^JLrrud ftmrdk. dykt, 2 3 5 u- 2 0 7Pb date decay constant 207 206 • old: 0.1537/0.9722/0.0499/137.8 P b / P b d a t e • not reported Number of Points: n» Ma Ma Ma CTnew: 0.155125/0.98485/0.049475/137.88 , „ . „ Th- Pb date •.other: + M Latitude: Longitude: (X° Y' Z" or X° Y.Y') ( tTtj.0 2a" N, f2J? ° 04 ' ZO " W (± ); Elevation: 4"&> UMI Zone 3 Z-flO E (* UffO N; Province: 8-C.  Sec. , T. ,R. ; Co., State (NTS /t>3 I ) Terrace. Map Area (1:250,000) Location: &" -Uy*j+«tAj r&*-d /? £vs/g*^ fi-t-irt* n*rH *£-Z*SLO &urtsr Source Type: , , ' j Rock Types: ftlooljZsv+eU, gLzJirrr^J (njrP>l< - a*~r\sf L> Jjyfr Geologic Unit: Dyfca > u {/ u / Geologic Setting:  Material Analysed: -^trr** . CLL**T) r^n^Ae^a A, Mfkt vinlc . OjtfAeJraJ. UieiJ JesmjnautcJ u ; Comment on Analyses: /flccLervd-C jo y&oA . ^ Interpretation: H 8 ~ 3 Mm UA&d -un dLooculM Collected by: Dated by: T S.T. H<Ldk r-s.r. H<!*Jk Date of l i s t i n g : / 2/z/?/ Sample Name or Number: Tn/ff'<? /3 / Sheet Split-Mineral ppm U ppm Pb 206 207 208 20«l " - • IS Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe *t>r ' 117-5 3-o1 loo- oo 2)to7 o • us(, » 950 SSdtav 0-70 tn<\ 206 Pb ... . 2 W T T r a t l ° ± 207 Pb ... , 2«u r 3 t l°i 207 Pb . . A IbTTb r a t , ° i 206 Pb j . 238 U d a t e * 207 Pb . , ^ 2 „ u date + 207 Pb j . 206-PF d a t e t 0-01475 + o-otooS O-IDOIZ to-ovi$j o-oVf23 ±o-m44 '59 ' t~3t Spli t-Mineral ppm U ppm Pb 206 207 208 201) Meas. 20T Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe | 00i oo 5- o f f II- f f ? (.Zoo, %5t tot o-i o vnoi 20b Pb .., . I W T - r a t l o + 207 Pb . 215 U r a t l ° i 207 Pb . 206-pb r a t i ° t 206 Pb . , A 2WTT d a t e t 207 Pb . , A 2 « 0" d a t e - 207 Pb j . 206-Tb d 3 t e t R J ( i - l - o-i lot i / f c o^> bpli t-Mineral ppm U ppm Pb 206 207 208 20^  Meas. 1°* Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe 3«-t . e • 1 (3.4yf g S ± i b 0 0-2 0 206 f'b „. 2 W T r r a t , ° t 207 Pb . . . 215 U r a " ° i 207 Pb . . . 206-p¥ r a t ' ° i 206 Pb j . 2 W i r d a t e t 207 Pb . . ^ 235 U d a t C ± 207 Pb j . 206" Pb d a t e - K 5~0-$ t 0 • 2- 70 t** o-tt bpli t-Mineral ppm U ppm Pb 206 207 208 20l( Mole * Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe * HUM** ice•oo 5-261 13- ZZ8 0 • 051 0 1211-71 V1!* 0-1&8 }tro o\ 206 Pb ... ^ 2 W T T r a t , ° t 207 Pb . . . 235 U r a t ' ° * 2f>7 Pb „. ^ 206-pb r a t ' ° t 206 Pb . . ^ 23fl u d a t e i 207 Pb , . . 235 U d a t e -207 Pb j . 2oTTb d a t e i 3 45-3 - o - f H1-t t 0-5 - 5i " - i6 o-fi Spli t-Mineral ppm U ppm Pb 206 207 208 IK Mole * Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe 206 Pb . 2 3 F T T r a t i o t 207 Pb . 235 U r a t ' ° ^ 207 Pb . 206-pb r a t i ° t 206 Pb j . 2 W T T d a t e t 207 Pb . . ^ 235TT d a t e i 207 Pb J . . 206"~Pb d a t e t J + + + + + + Statement of U n c e r t a i n t i e s : I • / Q* I so top ic composi t ion of b lank : Q S-K Modern Pb ( 6 / ^ : 1 8 . 7 , 7/*»: 15.63. 8/11:38.63) or 0 Other ( 6 / ' i : l T 7 S 7 / ' i : l^--5^8/ '* : 57"§0 ) I so top ic composi t ion of common Pb based on S-K growth curve : 6/'t= 11.152, 7/^=12.998, 875=31.23 at 3.7Ga w i t h 238lJ/20't r" - - 9 - 7 ' l , 232Th/20'4pb=37. 1 9 ; decay constants 0. 15512 r 0.98 '(35, 137-88; or Q Other (6/*): 7/h: S/h : ) U_ Q U • Mineral analysis " U £jConcordia interpretation O Mineral or rock isochron (NTS /0 3 J T&rm Sample Number(s) and Reference(s)  Lab No: THSI "ZS39. Ref: T- UeaJn ~ Upper Intercept Computed 0 As sumed • Record No: Lower Intercept Computed 0 As sumed Q Suite No: 238,, 206.,, , _ U- Pb. date Sample Name: decay constant • old: 0.1537/0.9722/0.0499/137.8 0new: 0.155125/0.98485/0.049475/137.88 • other: • not reported 235TT 207,., , _ U- Pb date + 1-2. 2 0 7Pb/ 2 0 6Pb date a m i * * rt-* t * S 2 3 2Th- 2 0 8Pb date Number of Points: n= 128 2a error ZHHt 2<r Ma_ 2a error (>0 ± 6 Ma Ma Ma Ma Ma Latitude: Longitude: ( o"f ° 24 ' 3J" N, / Z J ° 5 r w ( (X° Y' Z" or X° Y.Y') ); Elevation: /t* UMT Zone SOC 5~OQ E joyg ?Qo N; Province: &' ^  '  Sec. , T. ,R. ; Co., State (NTS / g 3 l ) _Map Area (1:250,000) Location: Source T y p e : " source lype: -Rock Types: UdLtt, H> ,O»TI fcinj»A*frc k>,' - g - T T j u o J T J Geologic Unit: Duke v Geologic Setting: Material Analysed: "Zlnon . EuJx  &U>\JLTLU» . cAtjur . Coast*. C) Comment on Analyses: f<ro~r rruo Interpretation: Cry a fatttzaJ^** «^ S f - o i I-o ^La poil cLaJ*4 "ylonih* djJrtn-ry^aJ-c^^ AAJ chatty "f>rt-"tL*J-e* LrftrU - *l t',rl*U. <L&,l+ctimJU*J f & i U w kr- Ar fa; ohk t.o*lir+ aJte. J-*,„ -rt,^ Mftc*. Uo 4A-2M*. Collected by: Dated by. •rs.r. H"J* Date of listing: 7<& /¥ /?/ Sample Name or Number: "T//$^ 2-5"2 Sheet Q Split-Mineral ppm U ppm Pb 206 207 208 201) Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe H»m/i} -Mailt*' IOO-60 o • • " * • < > o - i s / g -206 Pb ^ 2 i 8 - u r r a t i 0 i 207 Pb . 235 U r a t ! ° ± 207 Pb . IbTTb r a t I ° ± 2 3 0 - d a t C * 207 Pb . . ^ 235 U d a t e t 207 Pb j . IbTTb i T 3 ooj«J79 ± ° o o © o i Sb -2.1 bp I i t-Mlneral ppm U ppm Pb 206 207 208 20*1 Mole * Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe 42S--2- t OO • B D 3©- oj» 6 52 V4 1/6-/1 OJT Cjt it* 0 1 vnc\ 206 Pb „, ^ 2 i r r r a t i o i 207 Pb „. , 207 Pb /..-J IbTTb r a t i ° t 206 Pb j . 238 U d a t e -207 Pb . . ^ 235 U d a t e t 207 Pb j . 2061 Pb d a t e t R J 0 0 m £ tooooej o;ijit> to-6»jpj oo«f946 to-ooii? II4--5 - 0 177 ± J J oft Spl i t-Mineral ppm U ppm Pb 206 207 208 204 Mole % Blank Pb Rad. Pb Rad+ComPb Common' Pb Aqe 206 Pb ... . 2 W T T r a t i 0 t 207 Pb . 2 „ u ratio + 207 Pb ^ 2 0 6 - P b r a t i o t 206 Pb j . . ; 238 U d a t e -207 Pb . . 235 U d a t e t 207 Pb j . IbTTb d a t e t R j + + + + + + Split-Mineral ppm U ppm Pb 206 207 ; 208 ' - 20*1 Meas. |06 20M Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe m« 206 Pb ^ 2 l O - r a t l 0 t 207 Pb , 235 u r a t ! ° i 207 Pb , . '.• • IbTTb r a t , ° i : 206 Pb . „ ^ JWW d a c e i 207 Pb , 235 U d a t e i • 207 Pb j . IbTTb" t J + + + + + + Split-Mineral ppm U ppm Pb 206 207 :?o8 204 Mole % Blank Pb Rad. Pb Rad+ComPb Common . Pb Aqe j- •"»* 206 Pb , 2 3 F T T r a t l ° t 207 Pb „, . 235 U r a M ° t 207 Pb ... , J IbTTb r a t , ° I*- f - ; 2 ! F T r d a t e i 207 Pb . . ^ 235 U d a t e t 207 Pb j . 206 Pb d a t e i J + + + + 1 i — • + + - >-Statement of U n c e r t a i n t i e s : I s o t o p i c composition of blank: \~\ S-K Modern Pb (6/4 : 18 .7 , 7A:15.63, 8/4:38.63) or 0 Other (6/4 :lTtt' 1/h: IS'TO 8/4 :37-^ > ) Isotopic composition of common Pb based on S-K growth curve: 6/4=11.152, 7/4=12.998, 87M=31.23 at 3.7Ga with 238u/204Pb=9.74, 232Th/204Pb=37-19; decay constants 0.155125. 0.98485. 137.88: or r~l Other f6/4: 7//.- fi/k • ^ 130 (NTS /OI T/7 TcrfVCz. ) U_ D U • Mineral analysis *0 0"Concordia interpretation • Mineral or rock isochron Sample Number(s) and Reference(s) Upper Intercept 2a error Lab No: Tttftl f7<f Computed • Assumed Lj ± Ma Ref: Lower Intercept Computed• Assumed •  2a error Record No: Suite No: 238lT 206„, , U- Pb date Sample Name: 61 ± 0-3 235TI 207_ . U- Pb date decay constant • old: 0.1537/0.9722/0.0499/137.8 *7 ± / 207.,, ,206^ , Pb/ Pb date »0fiew: 0.155125/0.98485/0.049475/137.88 ._. . J Th- U 8Pb date • o ther: • not reported 6 ? ± u Number of Points: n= -3 Ma Ma Ma Ma Ma Latitude: Longitude: (x£c Y* Z" o r ^ C Y.Y*) ( 5¥ ° 7<\ ' 12. " N, fZt° 4f ' ^ 1 "£w (± ); Elevation: SJ tSO UMT Zone E N; Province: & • C • Sec. , T. Co., State (NTS IbiZ ) TCrrxuz. Map Area (1:250,000) Location: On knob rtorfLv^ah cf) StCZ/U* PurCr b VT. Urn <~Q£kO/ J) SAg* Source Type: ^ ~ ^ Rock Types: Uor-nigLt-r^JjL o^-wurL+aL} trritc. i/1-hnA} lr* ptiajasji . Geologic Unit: PondjLr fiJjvn Geologic Setting:  Material Analysed: -Zirvon . C U J L T ; C^Cow6t^» A> MfM ^ A t , PIJACLZ^/ o*;H\ p*t>cl rcrvtiMfiena - . °  Comment on Analyses:  Interpretation: Collected by: TS . 7* • Afex/l Dated by: Date of l i s t i n g : WaLr. 3/f/ S a m p l e Name o r N u m b e r : 77/°?/7y S h e e t S p l i t -M l n e r a l p p m U p p m P b 2 0 6 207 208 2 0 4 M e a s . 2 0 6 20T M o l e % B l a n k P b R a d . P b R a d + C o m P t C o m m o n P b A q e 5t<fS I OV.0D ) o-9SS I5t loo 01 2 0 6 P b „ , ^ 2 i8 -T r r a t , °1 207 P b „ . , 2 3 5 U r a t ' ° * 207 P b t , , 206-pb r a t i 0 t 2 0 6 Pb , ^ ^ 21FTT d a t e i 2 0 7 Pb j , 2 3 5 U d a t e - 207 P b J , , 206-Tb d a t e t ^ • 3 - °-*> 68o t OS S p l i t -M i n e r a l p p m U p p m P b 2 0 6 207 208 20*1 M e a s . 2 0 6 M o l e % B l a n k P b R a d . P b R a d + C o m P t Common P b A q e 2.83 8 190-00 /•/Ja o+ 2 0 6 P b , . ^ 2 i c n r r a t ( ° ± 207 P b 2 3 5 u r a t l ° i 2 0 7 Pb „ . ^ 2 0 6 - ? b r a t i o t 2 0 6 P b j . I W i r d a t e ± 207 Pb j , 2 3 5 U d a t e -207 P b j , 2 0 6 P b d a t e -J 0 OIOTS toocoof 7/6 - / ^ , b i " If Spl i t -M i n e r a l p p m U p p m P b 206 207 208 2 0 4 M e a s . 2 0 6 20T M o l e % B l a n k P b R a d . P b R a d + C o m P b Common-P b A q e •ftftf MM, air 2«?7- u 1 oO- ro 5--3/J » -0 37r 7-7/^ * m 651 too O'l 2 0 6 P b „ . 2 l O - r a t l 0 + 207 P b 2 „ n r a t i o + 2 0 7 P b „ , ^ 206-Tb r a t , ° i 2 0 6 Pb j . , 2WTT d a t e *-207 Pb j , 2 3 5 U d a t e t 207 P b j , 206" P b d a t e i R fy.tHoTJ toomi e>-o<f7T<f -"-anit-j 6?o to-* *S S p l i t - " M i n e r a 1 p p m U p p m P b 2 0 6 2 0 7 208 2 0 4 M e a s . 2 0 6 20T M o l e % B l a n k P b R a d . P b R a d + C o m P t Common P b A q e 266 P b „ . ^ 2 T c n r r a t l ° ± 207 P b , 2 „ U r a t i ° ^ 207 Pb , . ^ 2 0 6 - p b r a t ' ° t 2 0 6 Pb j , 23f i U ' d a t e -2 0 7 P b , , , 2 3 5 U d a t e t 2 0 7 P b j , 206 1 P b d a t e - R j + + + + + + b p I i t -M i n e r a l p p m U p p m P b 2 0 6 207 208 2 0 4 M e a s . 2 0 6 20T M o l e % B l a n k Pb R a d . P b R a d + C o m P t C o m m o n P b A q e I M 2 0 6 P b „ , ^ 2!8-TT r a t i 0 + 207 P b . 2 3 5 U r a t i ° t 2 0 7 P b „ . ^ loTlb r a t i 0 t 2 0 6 P b , h L 2 3 i n r d a t e t 207 P b j fc , 2 3 5 U d a t e t 207 Pb j , 206- P b d a t e - ft J + + + + + + S t a t e m e n t o f U n c e r t a i n t i e s : \ (T I s o t o p i c c o m p o s i t i o n o f b lank : Q S - K M o d e r n Pb ( 6 / 4 : 1 8 . 7 , 7 / 4 : 1 5 . 6 3 , 8 / 4 : 3 8 . 6 3 ) o r R/j O t h e r ( 6 /4 : P'75'7/4: i s ' ' j 5 8 / 4 : J 7 - J o ) I s o t o p ' c o m p o s i t i o n o f c o m m o n Pb b a s e d o n S - K g r o w t h c u r 6 / 4=1 1 . 1 5 2 , 7 / 4 = 1 2 . 9 9 8 , 874~=31.23 a t 3 . 7Ga w i t h 2 3 8 U/2L J = 9 . 7 4 , 2 3 2 T h / 2 0 4 P b = 3 7 - 1 9 ; d e c a y c o n s t a n t s 0.1551 - J . 0 . 9 8 4 8 5 . 1 3 7 . 8 8 : o r I I O t h e r (h/U- l/U- fi.fl- ^ 1 3 2 U— D K n M i n e r a l a n a l y s i s ~"i U N 0 Concordia i n t e r p r e t a t i o n • M i n e r a l or r o c k i s o c h r o n (NTS /OS 1:1 ' ~fermce. Sample Number(s) and R e f e r e n c e ( s )  Lab No: - TAf?9 2^>7c Ref: ~ ~ ~ Record No: S u i t e No: Sample Name: decay c o n s t a n t • o l d : 0.1537/0.9722/0.0499/137.8 ^ h e w : 0.155125/0.98A85/0.049475/137. • ot h e r : • not r e p o r t e d Upper I n t e r c e p t Computed 0 2a e r r o r Assumed • i Ma Lower I n t e r c e p t Computed • Assumed • + 2a e r r o r Ma 238.. 2 0 6 D U . U- Pb date 68-7± o-S Ma 235,. 207.,, , U- Pb date 0*1+ Ma 2 0 7 P b / 2 0 6 P b date n ± Zo Ma 2 3 2 T h - 2 0 8 P b date Ma Number of P o i n t s : n= 3 L a t i t u d e : L o n g i t u d e : (X° Y' Z" or X° Y.Y') ( ° ' " N, P - S ° V? ' i f " W (± ); E l e v a t i o n : ^Z.SV fcaf UMT Zone 6~°0'?~CO E feC33i?^N; P r o v i n c e : • Sec. _,R. Co., S t a t e (NTS /o3 ^ •/?) T^rraat. _Map Area (1:250,000) L o c a t i o n : fa*L&e. &ao{- J] txr S^em &rzr /u>nt% J] £hvy /£ of,Ske^ftA. &ui*-r Source TvpeT" ^ ® ' ^ Rock Types: Li/^jzje^ hb ffraruksLnsn A*. /7&LC/. QrzUn* J , sfrrZoU' • Geologic U n i t : PvnJL&s- Plu.hr: G e o l o g i c S e t t i n g :  M a t e r i a l A n a l y s e d : 2"/rz^ >n • E<jJ\j^Ju~aA ct^ioJ. rz^hzca 3 ' / /c 2 / . £tthi-J^~*J. Comment on A n a l y s e s : I n t e r p r e t a t i o n : C o l l e c t e d by:_ Dated by: "T- H<^1\  Date of l i s t i n g : S a m p l e Name o r N u m b e r : /H S'i 2-o~7o S h e e t [~~) S p l i t -M i n e r a l ppm U p p m P b 2 0 6 2 0 7 2 0 8 2 0 4 M e a s . loT M o l e % B l a n k P b R a d . P b Rad+ComPt Common P b A q e -UltfAlh 3 1/ 6 - 1 3 2 / ? • / / 7 O • iz c r o A* ^ 0' ?/( 0-gO 266 P b ^ 2 W T T r a t , ° i 207 P b , 235 U r a t ' ° * 207 Pb „ . ^ IbTTb r a t ' ° i 206 P b . . ^ 238 u d a t e -207 P b j t , 235 U d a t e t 207 P b j . IbTTb d a t e t j oojoSl to-Wf 0-CU76H- to-own 6?-r + 0 . 3 £7 i f on b p l i t-M i n e r a l p p m U p p m P b 2 0 6 2 0 7 2 0 8 20«t M e a s . IbT M o l e % B l a n k P b R a d . P b R a d + C o m P t C o m m o n P b A q e 274-7 I oy / O S ' 00 7.30 rvift 206 P b . . ^ 2 l B - l T r a t l 0 t 207 P b . . . 235 U r a t ' ° ± 2 0 7 Pb . IbTTb r a t i o i 206 P b . . ^ i 3 i n r d a t e t 207 P b j ^ . 235 U d a t e ± 207 P b j . IbTTb d a t e t R • j - o-o) oQ - O'0000! 0-9/ b p l 1 t " M i n e r a 1 p p m U p p m P b 2 0 6 2 0 7 2 0 8 2 0 4 M e a s . 206 IbT M o l e % B l a n k P b R a d . P b R a d + C o m P I C o m m o n P b A q e A/I r r t / V M - 255. '? 2'7^ /ov- CD 4-1/1 0 cr/<rf 3 - 2 | • 0" 206 P b 2 l F T T r a t ' ° ± 2 0 7 P b . . . 235 U r a t ' 0 t 2 0 7 Pb „ . t IbTPb r a t ' ° i 2 0 6 P b , „ ^ 2 i i n r d a t e -207 P b , , , 235 U d a t e t 2 ° 7 P b . . ^ IbTTb d a t e ± R 0•ojo~i3 t 0 csmt 0s-* - o-Z H5 - // 085 S p l i t -M i n e r a l p p m U p p m P b 2 0 6 2 0 7 2 0 8 2 0 4 M e a s . ™T IbT M o l e $ B l a n k P b R a d . P b R a d + C o m P t C o m m o n P b A q e '206 p b • 238" U r a t , ° t 2 0 7 P b . . . 235 U r a t ' ° ± 2 0 7 Pb j . loTTb r a t ' ° i 2 0 6 P b , . ^ 2 3 i n r d a t e i 207 P b , . . 235 U < l a t e -2 0 7 P b . „ . IbTTb" i P< S p l i t -M i n e r a 1 p p m U p p m P b 2 0 6 2 0 7 2 0 8 2 0 4 M e a s . 206 IbT M o l e % B l a n k P b R a d . P b R a d + C o m P l C o m m o n P b A q e 206 P b , 2 3 F T r r a t , ° t 2 0 7 P b . . . 235 U r a t ' ° i 2 0 7 P b . . J_ IbTTb r a t , ° t 206 P b , „ ^ i 3 i n r d a t e i 207 P b . „ ^ 235 U d a t e t 2 ° 7 P b . , ^ IbTTb d a t e * J + + + + + + S t a t e m e n t o f U n c e r t a i n t i e s : ! •* ]S I s o t o p i c c o m p o s i t i o n o f b l a n k : [ ] s - K M o d e r n P b ( 6 / 4 : 1 8 . 7 , 7 / 4 : 1 5 . 6 3 , 8 / 4 : 3 8 . 6 3 ) o r 0 O t h e r ( 6 / 4 r l7/) '7/4 : /J-fo 8 / 4 : 3 7- 5* ) I s o t o p i c c o m p o s i t i o n o f c o m m o n P b b a s e d o n S - K g r o w t h c u r v e : 6 / 4 = 1 1 . 1 5 2 , 7 / 4 = 1 2 . 9 9 8 , 8 7 4 = 3 1 . 2 3 a t 3 • 7 G a w i t h 2 3 8 U / 2 0 4 P K = 9 . 7 4 , 2 . 3 2 T h / 2 0 4 P b = 3 7 . 1 9 ; d e c a y c o n s t a n t s 0 . 1 5 5 1 2 5 0 . 9 8 4 3 5 , 1 3 7 - 8 8 ; o r [~~] O t h e r ( 6 / 4 : 7 / 4 : 8 / 4 : ) 134 U_ D U Q M i n e r a l a n a l y s i s r U g Concordia i n t e r p r e t a t i o n D M i n e r a l o r rock i s o c h r o n (NTS JQ3 i/? Terrace Sample Number(s) and R e f e r e n c e ( s )  Lab No: TH ?<t 4<> Vol Ref! Upper I n t e r c e p t Computed Q Assumed • 2a e r r o r Record No: S u i t e No: Sample Name: decay c o n s t a n t • o l d : 0.1537/0.9722/0.0499/137.8 gjnew: 0.155125/0.98485/0.049475/1 • o t h e r : • not r e p o r t e d Ma Lower I n t e r c e p t Computed Q Assumed • + 2a e r r o r Ma 238 T T 206„ , „ U- Pb date 6f-8± Ma 235.. 207„, , ^ U- Pb date o-l Ma 207 D, ,206„ . _ Pb/ Pb dat e 3o Ma 232_. 208,,, , Th- Pb dat e + Ma Number of P o i n t s : n= Z L a t i t u d e : L o n g i t u d e : (X V Z" o r X Y.Y 1) ( 54 0 21, * kS " N, /29 ° 5?' 3d " W. (± ); E l e v a t i o n : UMT Zone 501350 E (>0ZSt>b N; P r o v i n c e : 8 • C • Sec. Co., S t a t e ( N T S / 0 3 2 . ) Terrace Map Area (1:250,000) L o c a t i o n : W• ° i fVuiy id , ft. a f Shames ft<rer (2 km) Source Type: Rock Types: ff\v/oniHse_c( } n-i-rueU^f h Is <trnn oJ> artAe G e o l o g i c U n i t : ' !)ukjL •> u G e o l o g i c S e t t i n g : 1 M a t e r i a l A n a l y s e d : ) r Con &u.hexLrcuL & aar>e)t y^er/n/na/-/ a n c ; /orismahc . elefousiess 4o / / - a/rilc ; L •• <AJ r*+1o i / 3: ' = 2oV. : 2-'/ ^crt>V. IntCvO** Comment on A n a l y s e s : I n t e r p r e t a t i o n : QynaorAzrd crqsh^^1'Z&h*>r) qgg prr -<Jjjjt^ cLuc^rfe my Iv/life c f e ^ g n n t / t j ^ c/7 / rin>t>aJrLe T<zrf%cuij cute. C o l l e c t e d by: Dated by: ~T 5- T. Heak I C.J. (green  ~T- 5 T. H*a-l> D a t e o f l i s t i n g : Tan. 2-S/<?t> Sample Name or Number: f fj 6 t ¥<> ? d Sheet | \ SpT i f -H i nera1 ppm U ppm Pb 206 207 208 204 Meas. 126-IbT Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe H» iif-l b°-2t IOO-0 19- 392 0-2220 l7i-7Sl-l o-P72 l-o 20b Pb „ . ! f T r a t l ° i 207 Pb , 2 „ u r a t i o J 207 Pb „, , IbTTb r a t i o ± 206 Pb j . IWIT d a t e i 207 Pb j . 235 U d a t e t 207 Pb j . IbTTb d a t e t R j o-ol°7C - oeoooi o-o</-7<f7 to-dooji 6f.o  +- °-3 (?•/ t <>•£, 73  +- n Oft S p l i t -Minera l ppm U ppm Pb 206 207 208 204 Meas. 206 IbT Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe A/2-IA/oi'fibt £111 1ZS too* /3- 72 3 35- 3/1 o • 6os-3 ] 0-7D1 20b PIT , 2^1T r a t l 0 t 207 Pb , 235 U r a t t o + -207 Pb . IbTTb r a t i o t 206 Pb j . 238 I T d a t e -207 Pb j . 235 U d a t e t 207 Pb j . 206" Pb d a t e - K O- o\o~fo- OtMoS o-oifgo^ to-oooH 6f-6 - ° "*• \\' 0-?} S p l i t -Mi nera 1 ppm U ppm Pb 206 207 208 204 Meas. 206 IbT Mole % Blank Pb Rad. Pb Rad+ComPt Common-Pb Aqe 206 Pb „. nanr r a t l° ± 207 Pb . , 235 u r a t!° i 207 Pb . . . IbTTb r a t l ° t 206 Pb , 238 U d a t e t 207 Pb A , 235 U d a t e -207 Pb J . . IbTTb d a t e i + + + + + + S p l i t-Mine ra l ppm U ppm Pb 206 207 208 204 Meas. 206 IbT Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe 206 Pb „ . 2WTT r a t , ° t 207 Pb . . . 2V, U r a t ! ° -207 Pb IbTTb r a t , ° t 206 Pb . . ^ i 3 i n r d a t e i 207 Pb , . , 235 U d a t e i 207 Pb j , IbTTb d a t e i j + + + + + + b p l1 t -M i nera1 ppm U ppm Pb 206 207 208 204 Meas. 206 IbT Mole % Blank Pb Rad. Pb Rad+ComPb Common' Pb Aqe rt\<\ 206 Pb „. 2 l F T r r a t i o ± 207 Pb . . . 2 „ u r a t i o + 207 Pb , IbTTb" ± 206 Pb j . 2 W l T d a t e t 207 Pb j . 235 U d a t e -207 Pb j . IbTTb i J + + + + + + - >-S t a t e m e n t o f U n c e r t a i n t i e s : I s o t o p i c c o m p o s i t i o n o f b l a n k : Q s - K M o d e r n P b ( 6 / 4 : 1 8 . 7 , 7 / 4 : 1 5 . 6 3 , 8 / 4 : 3 8 . 6 3 ) o r O t h e r ( 6 / 4 : fi-lSl/U -. R ^ 8 / 4 : £ > 3 P ) I s o t o p i i m p o s i t i o n o f c o m m o n Pb b a s e d o n S - K g r o w t h c u r v e / 4 = 1 1 . 1 5 2 , 7 / 4 = 1 2 . 9 9 8 , 8/4= 3 1 . 2 3 a t 3 . 7 G a w i t h 2 3 8 U/iJs . , = 9 . 7 4 , 2 3 2 T h / 2 0 4 P b = 3 7 - 1 9 ; d e c a y c o n s t a n t s 0 . 1 5 5 1 2 5 . 0 . 9 8 4 8 5 . 1 3 7 . 8 8 : o r I I O t h p r ((./h- ->//•• on... \ 136 U_ Q L . • M i n e r a l analysis ' *J •Q'Concordia interpretation • Mineral or rock isochron Sample Number(s) and Reference(s) Upper Intercept Lab No: T M g 9 Z J " f C Computed • (NTS /ol r / 7 Terra ce. 2a error Ref: Record No: Suite No: Sample Name: decay constant • old: 0.1537/0.9722/0.0499/137.8 • other:_ • not reported Assumed • + Ma Lower Intercept 2a error Computed Q Assumed O + Ma 238TI 206„, , „ U- Pb date Ma 2 3 5 U - 2 0 7 P b date \% ± 3 Ma 2 0 7 P b / 2 0 6 P b date \1S± 1 0 Ma 232„, 208p, Th- Pb date ± Ma Number of Points: : n= . 2. (X° Y' Z" or X° Y.Y') Latitude: Longitude: ( Sj-° Zlf. * 3o " N, /Z$ ° 53 • 4tf " w (± ); Elevation: / ^6 UMT Zone 6 S~DD E &OZA10QlA; Province: ^ C ' Sec. Co., State_ (NTS 101 I ) ^LrTClVL Map Area (1:250,000) Location: Source Type: ' " Rock Types: (fUi(s*rjf?S*J ki - ka <ftrtucv <Jtv-ri 1* Geologic Unit: Shjxsr^Oo £*tsy-ls~ r*Jf 7J>A/7*. ZZ>nj CSHniib^ Geologic Setting: Material Analysed: Zin^n .t^*fi****^rei<J-i**l4 ^uS^edr*/ f> suj,ej^uanJ . L: u) r*Ag Comment on Analyses: rfleJjutJ*cbi ej^cJi . Weak U- Vj^ruxJ n> Hon - msLBui^uraJo^ . Interpretation: Dale Jut flr& ~ rtU/haiM qUJorrrxaA^Qri . CLAJ. rrxAM hefrc+z*/-Collected by: T-S- 7. ffeqti Dated by: 7 5 - 7 f~Ujxk Date of l i s t i n g : Sample Name or Number: &<?Z-S'7- c Sheet S p l I t -Mineral p p m U p p m Pb 206 207 208 204 * - s Mole % Blank Pb Rad. Pb Rad+ComPh Common Pb Aqe -MUM,ah ? o 3 - l i l l 100 o 5 J 2 Z 0 • oo ? f <>•••) l o - 13 o-\ 206 pb . . . 2 l8~TT r a t , ° t 207 Pb . • 235 U r a t ' ° i 207 Pb „. A 206-Tb r a t ' ° i 206 Pb j . 23(5 U d a t e -207 Pb , . ^ 235 U d 3 t e i 207 Pb . . 206-Tb d a t e t R j o .o2J)Sl-<>0<i<*t o lotM to-ecioq. o o r n z - toooow I 8 !•(. t o-S |<U-o t I T - a-81 S p l i C-Mlneral p p m U p p m Pb 206 207 208 204 Mole * Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe NM l-ir>/»* |8-72 | CV-0 130 €•H5o 0 • <5 08cf ' 0 ? H 20b Pb . , , 207 Pb . . . 215 U r a t ' 0 i 207 Pb . , . 20S-Pb r a t , ° t 206 Pb . . ^ 2 3 F T T D A T E i 207 Pb , . ^ 235 U d a t e i 207 Pb . . , 206-Tb d a t e t R J be 2.1 «ffc± o-o»»77 <>ZOi|( to-ooS32 O • 05080 i O'NOK \tlt t H-g- l ^ r + 5 p-fi S p l i t-Mi nera1 p p m U p p m Pb 206 207 208 204 Meas. 106 20¥ Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe 20b Pb . . . 2 i c n r r a t , ° -207 Pb 235 U r a t ' ° i 207 Pb . . . 206-pb r a t ' ° i 206 Pb j , 2 i t n r d a t e i 207 Pb A . . 235 U d a t e t 207 Pb j . 2birPb d a t e i R + + + + + + Split- • Hi nera1 p p m U p p m Pb 206 207 208 204 Meas. 206 20¥ Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe 20b Pb f . _ r _ r a t l o + 207 Pb . . , 235 U r a t ' ° ± 207 Pb , . ^ 206-Tb r a t , ° t 206 Pb . . J_ 2 3 i n r d a t e +-207 Pb , . . 235 U d a t e t 207 Pb , , . 206-Tb d a t e +- ft - j - — + + + + + + S p l i l-M i nera1 p p m U p p m Pb 206 207 208 204 Meas. 2p6. 20T Mole % Blank Pb Rad. Pb Rad+ComPt Common Pb Aqe 20b Pb . , , 2 3 n r r a t , ° t 20? Pb . . . 235 U r a U o i 207 Pb . . . 206-pb r a t ' ° t 206 Pb . . x 2 3 i n r d a t e i 207 Pb . . ^ 235 U d a t e t 207 Pb . . ^ 206-Tb d a t e t ft 1 J + + + + + + Statement of U n c e r t a i n t i e s : 1 t£* Isotopic composition of blank: j ^ J s -K Modern Pb (6/4:18.7, 7/4:15.63, 8/4:38.63) or 0 Other (6/4: 11 ^'7/4: |rt"0&8/4: *7-3i>) Isotopic composition of common Pb based on S - K growth curve: 6/4=1 1.152, 7/4=12.998, 8/4=31 - 23 at 3.70a with 238U7204Pb=9.74 , 232Th/204Pb=37 • 19; decay constants 0.155125, 0.984R5, 1 37-88; or Other (6 /4 : 7/4: 8/4: ) 138 D K • Mineral analysis r D 0 Concordia interpretation • Mineral or rock isochron ( N T S IQJ 1/7 To-rec* Sample Number(s) and Reference(s)  Lab No: TH89Z6S Upper Intercept Computed 0 Assumed D 2a error 6 2 Ref: Record No: Suite No: Sample Name: decay constant • old: 0.1537/0.9722/0.0499/137.8 0new: 0.155125/0.98485/0.049475/1 • other: • not reported Ma Lower Intercept Computed 0 Assumed • 22-5" ± 2a error Ma 238TT 206^ , U- Pb date + Ma 235IT 207^ , U- Pb date + Ma 207^.206^ . Pb/ .Pb date , 3(7± Ma -frs****l\ 6. o 2 3 2 T h - 2 0 8 P b date + Ma Number of Points: n= Latitude: Longitude: ( ° 25 ' 4-5" N , 1-22 ° 51 UMT Zone 509 ?SD E 603/2JPN; Sec. , T. ,R. ; (NTS /OS Z ) (x° . y Z" or f Y.Y') OO " W (+ ); Elevation: A e £ Province: *3 «C •  Co., State _Map Area (1:250,000) Location: A<£mtf /fWy • lb, <gt»o/ /7 Sh*Jneo &<\rt,r Source Type  Rock  Geolog Geologic Setting c i. y L ie .  Types: &ri4He.ly +htu«LeJ . cMorlf-KJ^ bio/if^- qKArh. cLoorite  ic Unit: SHkl**. AsCOAtM+fiX. ' Material Analysed: / T ^ m Comment on Analyses: Collected by: Dated by: rs. r. f^j,  T.s-r. M*JJ> Date of l ist ing: 19/2/9/ S a m p l e Name o r N u m b e r : TH?9ZCS QI r S h e e t S p l i t -M l n e r a l p p m U ppm P b 2 0 6 2 0 7 1 T~" 1 20& 2 0 4 M e a s . 2 ? 6 IbT M o l e % B l a n k P b R a d . P b R a d + C o m P t Common P b A q e Mt*\ lot>o 5 V o 5 0 • 0 1 <jy 7/9-/itj ((00 T2o ' 0-993 2 0 6 P b ... ^ Twirratl0 i 207 P b , 2 „ „ ratio + 207 P b , IbTTb r a t i o t 2 0 6 P b j , i 3 F i r d a t e t 207 P b . . ^ 2 3 5 U d a t e +-207 P b j . IbTTb d a t e i w 2-970 - 0-7 1 1 8 - 0 - 1-7 •a at, t /if. S p l i t -M i n e r a l p p m U ppm P b 206 207 208 2 0 4 M e a s . 2 p 6 IbT M o l e % B l a n k P b R a d . P b R a d + C o m P t Common P b A q e NM2fl/| '-M* <?S'f (OO - * isizo e) • 0 2. 00 2&>±tcb oy vno, 2 0 6 P b , . ^ 2 T n r r a t i 0 * 2orpb . 2 3 5 U r a t l 0 + -207 Pb „ . • IbTTb r a t , ° i 2 0 6 P b , „ 23TXT d a t e ± 207 P b . . J 2 3 5 U d a t e ± 207 Pb j , 206 - P b d a t e - K J o • 04914<?-<>• Wt>|/ 3II-* - <W 3(2/ i o-7 3/7 + - 2 yff b p l 1 t -M i n e r a l p p m U p p m P b 206 207 208 2 0 4 M e a s . 2 0 6 IbT M o l e % B l a n k P b R a d . P b R a d + C o m P b Common' P b A q e 2 0 6 P b ... 2 T O - r a t , ° ± 207 P b . 2 3 5 U r a * ! ° ± 207 Pb , IbTTb r a t , ° i . 2 0 6 P b . . ^ 2 T t n r d a t e t 207 P b . . 2 3 5 U d a t e t 207 P b j . IbTTb d a t e t + + + + + + b p l 11-M i n e r a l p p m U ppm P b 206 207 208 2 0 4 M e a s . 2 0 6 IbT M o l e % B l a n k P b R a d . P b R a d + C o m P t Common P b A q e m a 2 0 6 P b „ . 2 T o r r a t l ° t 207 P b . . . 2 3 5 U r a U 0 +-2 0 7 Pb , . ; IbTTb r a t , ° t • 2 0 6 P b , ,. ^ • I3TTT d a t e ± 2 0 7 P b . . ^ 2 3 5 U d a t e t 207 Pb j . 206 P b d a t e -j + + + + + + S p l i t -M i n e r a l p p m U ppm P b 2 0 6 207 .708 2 0 4 M e a s . 20?-IbT M o l e B l a n k P b R a d . Pb R a d + C o m P b Common' P b A q e 2 0 6 P b , 2 T T i r r a t l 0 i 207 P b , 2 3 5 U r a t ' . ° i 2 0 7 Pb „ . *• IbTTb r a t l ° i 2 0 6 P b . . , M T d a t e t 2 0 7 P b . . ^ 2 3 5 U d a t e i 2 0 7 Pb j , 206 P b d a t e -J + + + + + + S t a t e m e n t o f U n c e r t a i n t i e s : | <5~ I s o t o p i c c o m p o s i t i o n o f b l a n k : [ ] s - K M o d e r n Pb (6/4:18.7, 7 / 4 : 1 5 . 6 3 , 8/4:38.63) o r - / O t h e r (6 /4: IT757/4: (ST-5b8/4 : 37Jo) I s o t o p i c c o m p o s i t i o n o f c o m m o n Pb b a s e d o n S - K g r o w t h c u r v e : 6 / 4 = 1 1 . 1 5 2 , 7/4=12.998, 8/4=31.23 a t 3-7Ga w i t h 238u/204Pb=9.74, 232Th/204Pb=37.19; d e c a y c o n s t a n t s 0 . 1 5 5 1 2 5 . 0.98485, 1 37.88: o r HH O t h e r (6/4 • 7 / t - ft/i. • > Table 2.2. K-Ar analytical data and results. Sample Rock Type Material %K Radiogenic 4 0 A r % Radiogenic x 10"6 cc/gm 4 0 A r Date (Ma± la) TH89207c TH89253e TH 89257c TH89286b TH89363b TH89404d Lineated hornblende granodiorite Biotite granite Mylonitic biotite hornblende granodiorite Metavolcanic Biotite granodiorite Mylonitic biotite granite Hornblende 1.63 ± 0.03 4.723 Biotite Biotite 5.61 ±0.10 10.201 7.41 ±0.04 13.576 Whole rock 1.58±0.01 3.260 Biotite 6.86±0.01 12.958 Biotite 5.54±0.09 11.162 87.6 96.1 90.3 88.0 92.4 93.9 73.1 ±2 .6 46 .2±1 .6 46 .5±1 .6 52 .3±1 .8 48 .0±1 .6 51.1 ±1 .8 Sample locations are: TH89207c: 54°26'45"N, 128°59'58"W; TH89253e: 54°24'33"N, 128°53'46"W; TH89257c: 54°24'33"N, 128°53'46"W; TH89286b: 54°28'20"N, 128o46'00"W; TH89363b: 54°31'02"N, 128058'14"W; TH89404d: 54°26'45"N, 128°57'30"W. 141 (NTs /oil Temce) K-Ar Sample Number (s ) and Reference (s) m a t e r i a l Date l a e r r o r Lab No: 7 7 / 94 2n7 (L d e c a y c o n s t a n t s : O 4. 7 2 / . 5 8 4 / 1 . 1 9 R e f : 77 rV**A. -O 4 . 7 2 / . 5 8 4 / 1 . 1 8 -m 4 . 9 6 / . 5 8 1 / 1 . 1 6 7 Record No: ( ML ) 73../ + 3.^ Ma ( ) + Ma ( ) + Ma ( ) + Ma Suite No: o n o t r e p o r t e d Sample Name: L.jnjLaJ<-J k^mbk^vdte. ^-nxAodionJ^ Longitude: (X Y' Z" or X° Y . Y ' ) -Latitude: (Sif> 2t' L£ N , J2?° 5"9* St" W (± ) ; E l e v . 5. 2 S~0 fee.tr UTM Zone 5"Q O 2 0 O E 6o3g05QN; P r o v i n c e £ . S e c . , T . , R . ; C o . , S t a t e ( N T S lojZll) Terence. Map A r e a , S c a l e I:SO, one* Location: fcelge Ujeaf A S hame* &\rt.r, nprfj, <fl frtoy. l(>  Source Type: 0 / Rock:  Geologic Uni t : , — » — . Geologic Age: Lg/-*f r Cf4.tB,C&oxJo ~ Materia l Analyzed: A n a l y t i c a l D a t a : ( l i s t duplicate analyses or indicate n = 2, n = 3, etc.) K = X = /. £5 t 0.03 %. , a r . 4 0 * _ r . / <* J x i o - c c / g m ) * K 2 0 = ri = 2_ % ' l A r - ' ' =v,_40, H . % ; ( A r 4 0 * = x l 0 " ° c c / g m ) ; ( . K 2 0 = • ! » < A r 4 0 * - X " - W C C ^ i ' ( % E A r 4 0 ) K = % , a 4 0 * - Aj.u VJW / yiu ; K 2 0 = % ' l A r ^ n - 1 0 _ w _ , ; ( % E A r 4 0 ) Comment on Analyses: ¥ 74 3 i " 6  AJ07 x l O ~ 10 . . , ; mol /gm) x l O " c c / g m ) . x l O " •10 ' mol /gm) x l 0 ~ ^ c c / g m ) x l O " mol /gm) x l O " ^ c c / g m ) x l O ~ i n m o l / g m ) ' Interpretat ion: CZyoCcr^ cia/e C o l l e c t e d by; ~J. HeaA  D a t e d by: ^ //ar^/^ J). Listed by: T- H&xM ( U 8 C ^  Date: / / . r)7. <?Q ^ n a m e - i n g C i t u 1 . i o n ) " w l v 142 (NTS (°SX T<TKlCg ) K-Ar m a t e r i a l D a t e l c f e r r o r Lab No: THMZLttE  Ref: T. He.a.L. Record No: d e c a y c o n s t a n t s : ( Bio. ) ± /.A Ma 0 4 . 7 2 / . 5 8 4 / 1 . 1 9 ( ) ± Ma -D 4 . 7 2 / . 5 8 4 / 1 . 1 8 ( ) ± Ma - • 4 . 9 6 / . 5 8 1 / 1 . 1 6 7 ( ) ± Ma Suite No: o n o t r e p o r t e d Sample Name: T~* Longitude: (X° Y* Z" or X" Y . Y ' ) i I I Lat i tude: i II   ) ; E l e v . IjOfeaJt (5<f 2.£f 50 N ,|2£ S3 ^ ^LJ.v UTM Zone 50 6 £ o o _ E dO2SSoo N? P r o v i n c e /3.<Z . S e c . , T . , R. ; C o . , S t a t e (NTS l o 3 l / ? ) Terra.cx- Map A r e a , S c a l e /-' 3D, Do a Locat ion: dooj Crut~CSVf3 lktr> J) S~hffJW*o fZ+X/V <xQnj> Hunt / L Source Type: \ Q / \J J" Rock: Geologic Unit: Geologic Age: Mater ia l Analyzed: A n a l y t i c a l D a t a : ( l i s t dupl icate analyses or indicate n = 2, n = 3, e tc . ) K 2 0 = K = K 2 0 = K = K 2 0 = K 2 0 = ; (Ar 40" ; (Ar 40* ; (Ar 40*= /o.aci x l O - ^ c c / g m ) ¥• SSI. x i o " 10 . . . ; mol/gm) x lO~ c c / g m ) x l O " • i o , , . ; mol/gm) x l 0 ~ ^ c c / g m ) x l O " mol/gm) x l O " ^ c c / g m ) x l O " 10 . . . ; mol/gm) ( U.I % Z A r 4 0 ) ( % 2 A r 4 0 ) % E A r 4 0 ) % r A r 4 0 ) Comment on Analyses: Inte rpreta t ion: CtroU^j rJn±je C o l l e c t e d b y : 7"- ff&AJ>  D a t e d b y : J". MarakJ 4 J). fomlsle Lis ted by: (.name, i n s t i t u t i o n ) Date: //. 23.4o 143 (NTS loll T-emtce) K-Ar Sample Number(s) and Reference(s) material Date la e r r o r Lab No; TH <7 £57C, decay constants: ( Bin. ) 46. S ± / t> Ma 0 4.72/.584/1.19 , \ ) ± Ma Ref: T.fUnh, -a 4.72/.584/1.18 ( } -• 4. 96/. 581/1.167 , Ma ) + Ma Record No: Suite No: o not reported Sample Name: . . . Latitude: ' Longitude: (X Y' Z" or X""1 T.Y') " (?tf 2-tf' 30 N , J 2 ? ° 5-3 ^f" W (± ); Elev. /?t> Tee/-UTM Zone _ £ 0 6 5*°° E 6o2^goON; Province B.C. Sec. , T. , R. ; Co., State (NTS 1*031/7 ) TtmautA Map Area, Scale t'^o, o»o Location: RJOOLJ trujxrvf) /£rv« LKxCoh JI J f W u o /ZoC-ty- , * . £ > V Source Type: / Rock: Geologic Unit: Geologic Age:  Material Analyzed: Blr^J-.Tfi (-^Q-r^oJ A n a l y t i c a l Data: (l i s t duplicate analyses or indicate n = 2, n = 3, etc.) K = x = 7 . y / ^ . 0 y K 2 O = n-2.. % 40* % ; ( A r = (,.05% xlO ^ cc/gm ) xlO umol/gm)' ( 90. 3 %£Ar 4 0) K = K20= % ; ( A r 4 0 * = % xlO 6 cc/gm ) - i n ' xlO ^Vol/gm) ( %£Ar 4 0) K = K 20= xlO ^ cc/gm ) -10 ' xlO mol/gm) ( %EA r 4 0 ) K = K20= % S(Ar4°*= xlO ^ cc/gm ) xlO umol/gm)' ( % I A r 4 0 ) Comment on Analyses: Interpretation: Ce-etCr^ C/LAJUL C o l l e c t e d by: 7T ffZOL^ Dated by: //a^l/^J^ ~b. Listed by: 7T //e^uA C *<gC 1  (.name, institution) Date: /I.3L/.9O 1 4 4 (NTS fQlX T*rrztcf) Sample Number(s) and R e f e r e n c e ( s ) K-Ar m a t e r i a l D a t e Iff e r r o r Ref: 71 U**A ~D 4. 7 2 / . 5 8 4 / 1 . 19 -a 4 . 7 2 / . 5 8 4 / 1 . 1 8 4 . 9 6 / . 5 8 1 / 1 . 1 6 7 Record No: ( W.rQy.) 5"2.3 ± / . ? Ma ( ) ± Ma ( ) + Ma ( ) ± Ma S u i t e No: Sample Name: n o t r e p o r t e d L a t i t u d e : • II L o n g i t u d e : (X Y' Z" or X Y.Y') I l l ) ; E l e v . SW fe&t ( 5 ^w 1? W N , 111 H-  0 0 W (± UTM Zone i r / 5 / 5 ~ c E i^Hoso N ; P r o v i n c e £> .C, . S e c . , T . , R. _ ; C o . , S t a t e (NTS fojl-lf ) Terra OL. _Map A r e a , S c a l e / :Sb, <Tvt> L o c a t i o n : Atonf tfvoy (i (AJCO)L A T^rfZLCZ Source Type: J / Rock:  G e o l o g i c U n i t :  G e o l o g i c Age:  M a t e r i a l A n a l y z e d : WAr>/s MrU- (- 7r>) A n a l y t i c a l D a t a : ( l i s t d u p l i c a t e a n a l y s e s o r i n d i c a t e n = 2, n = 3, e t c . ) K = X - /• 5*8 ~t D. O/ %. , , 4 0 * / > = a . % ' ( = K 2 0 = K = K 2 0 = K = K 2 0 = K = K 2 0 = % ; < A r 4 ° * = % % ; (Ar 40' ; (Ar 40*= x l O " 6 c c / g m ) x l O " 1 0 m o l / g m ) ; x l O " 6 c c / g m ) , x l O " •10 ' mol /gm) x l O " ^ c c / g m ) # x l O " •10 ' mol /gm) x l O " ® c c / g m ) x l 0 ~ 1 0 m o l / g m ) ? 40, ( % l A r 4 0 ) % I A r 4 0 ) %£Ar 4 0) Comment on A n a l y s e s : I n t e r p r e t a t i o n : CooU^ rtaJb. C o l l e c t e d b y : _ D a t e d b y : Markka]4 3). QunUe.. L i s t e d by: ~T- HeaJrs ( US C)  (name, i n s t i t u t i o n ) Date: //.OT^O 145 (NTS /Oil 7-erfad K-Ar Sample Number(s) and Reference(s) m a t e r i a l D a t e t e r r o r Lab No: 7 7 / ff? 362 8 d e c a y c o n s t a n t : O 4. 7 2 / . 5 8 4 / 1 . 19 4 . 9 6 / . 5 8 1 / 1 . 1 6 7 ( Sic ) M.o Ma ( ) + Ma ( ) + Ma ( ) + Ma Record No: Suite No: a n o t r e p o r t e d Sample Name. grn ^ U^. - bfohi-t ^u^oai^crnAc ffhana t^J cf&rrtaf Lat i tude: Longitude: (X° Y' Z" or X° Y . Y ' ) (5f-° 3/'oz N , IZH° IH- " W (± ); E l e v . ^ <ra? UTM Zone _5£_ /J_oo___E H5Q N; Province /g. g . Sec. , T. , R. ; Co., State (NTS ) Terrace. Map Area, Scale /; Sd,em> Location: &i<fy*. ruyrH ^7 S.Jwruu> fZ; ^ cr  Source Type:  Rock:  Geologic Unit:  Geologic Age:  Material Analyzed: Z=f;/) j-l ?e (- Vn-r Kn) A n a l y t i c a l D a t a : ( l i s t dupl icate analyses or indicate n = 2, n = 3, etc . ) * ; ( A r 4 0 * = M S I % 5". 7 8 2. x l O 6 c c / g m ) %ZAr u ) K 2O= n - £. x l O ^ m o l / g m ) ' ( K = K 2 0 = % ; ( A r 4 0 * = % x l O c c / g m ) . -10 ' x l O mol/gm) ( % l A r 4 0 ) K = K 2 0 = ;,».»*- x l O ^ c c / g m )_ -10 ' x l O mol/gm) ( % E A r 4 0 ) K = K 2 0 = x l O - 6 c c / g m ) — i n x l O • L U mol/gm) ' ( % £ A r 4 0 ) Comment on Analyses: Interpretation: CoffCjut^ dtJbg, C o l l e c t e d b y : 7T M^O-ln D a t e d b y : ZT. Hard/^J<t 3>. ft„„jJt>_ Listed by: 77 //&g/> U g C ) Date: //. 2$.4o (name, institution) 146 (NTS tOjl 7**r*cjt) Sample Number(s) and Reference(s) Lab No: X9*rO'S 35~ K-Ar material Date Ifferror Ref: 7~. #4a.A n4.72/.584/1.19 -O 4 . 7 2 / . 5 8 4 / 1 . 1 8 - * 4 . 9 6 / . 5 8 1 / 1 . 1 6 7 ( ) + • Ma ( ) ± Ma ( ) + Ma Record No: Suite No: a not reported Sample Name Latitude: Longitude: i (5T 26 oo N 3o w (± (X Y* Z" or X Y.Y') ) ; Elev. 5,56>Q Peat UTM Zone jTD 3 Q o O E 6 OiSOO N; Province J3. (L . Sec. , T. , R. ; Co., State (NTS l°i J ) Te.rna.ta- Map Area, Scale /•' SO, OoO Location: rZlcLqjz UHtOr 4 ShosyiO* 04.\r€JT OOrfL J) Hwy • id Source Type: » 0 / Rock:  Geologic Unit:  Geologic Age:  Material Analyzed: B\r>-f; T& ( - 4 W g f l ) A n a l y t i c a l D a t a : ( l i s t dupl icate analyses or indicate n = 2, n = 3, etc, .) K = X = 5.59 + D.Q*} K 2 O = n - z . % 40* % ; ( A r x l O - 6 c c / g m ) - i n x l O u m o l / g m ) ' ( 93-9 % l A r4 0 ) K = K 20= % ; < A r 4 0 * % x l O - 6 c c / g m ) . -10 ' x l O mol /gm) ( % i : A r 4 0 ) K = K 20= x l O 6 c c / g m ) -10 ' x l O mol /gm) ( % i : A r 4 0 ) K = K 2 0 = x l O - ^ c c / g m .) x l 0 ~ 1 0 m o l / g m ) '" ( % I A r4 0 ) Comment on Analyses: Interpretation: Cool/./^ d.«.fe. C o l l e c t e d by: Dated by: 3~- r(ara.kaJ <t J). Qunkh. Listed by: T-H^Ak CfABC) Date: 0 9.17^0 (.name, institution) 147 APPENDIX 3. ELECTRON MICROPROBE ANALYTICAL TECHNIQUES, DATA REDUCTION PROCEDURES AND THERMOBAROMETRIC RESULTS 3.1. ANALYTICAL TECHNIQUES Tea polished, carbon-coated thin-sections were analysed with a Cameca SX-50 automated electron microprobe in order to obtain quantitative thermobarometric data. Geothermometers employed in metamorphosed pelitic and volcanic rocks included garnet-biotite (Ferry and Spear, 1978), and garnet-amphibole (Graham and Powell, 1984). The geobarometer used was the GASP barometer of Ghent et al. (1979). Emplacement pressures in plutonic rocks were estimated using A l j in hornblende (Hollister et al., 1987; Hammarstrom and Zen, 1986; Johnson and Rutherford, 1989 and Rutter and Wyllie, 1989). The calibration of Johnson and Rutherford (1989) was used throughout. Most analyses were performed on rims of grains in mutual contact. Core compositions were also obtained to determine peak conditions of metamorphism. An effort was made to analyse only fresh, unaltered grains that attained or approached equilibrium conditions, although these conditions are very difficult to prove (Spear, 1989). Within each thin section, at least six domains, each containing one or more minerals or mineral assemblages of interest, were anlaysed. At least three rim and two core compositions were determined for each mineral in a domain. The following operating conditions were used: - Specimen current: 20 nA - Accelerating potential: 15 kV - Beam diameter: 10 fim - hornblende and biotite 5 /xm - plagioclase 3 jum - garnet - Counting times: 20-30 seconds Standards were analysed after every 15 to 20 unknown analyses, and at the beginning and end of every automated run. Table 3.1 lists the standards used. Garnet, plagioclase and biotite standards were calibrated by previous workers at the University of British Columbia. The amphibole standards were calibrated during the course of the present study by J. Knight (UBC) and the author. 148 Analytical results are shown below in Table 3.2, and are arranged by mineral species. 3.2 DATA REDUCTION PROCEDURES The following procedure was followed in order to reduce the data obtained from the electron microprobe: 1. Convert all files with extension .COR in PDP computer to DOS readable format, following instructions in microprobe mount preparation room. 2. All .COR files now contain the raw data, either in element weight % (amphibole and biotite) or oxide weight % (garnet and plagioclase). 3. Load and run program TRANSFORM to convert .COR files to a format usable in LOTUS 123 and FORMULA-I, which enables you to calculate structural formulas. 4. Within program TRANSFORM: a. If LOTUS 123 will be used to manipulate the data, answer "yes" when asked if LOTUS will be used. Also choose "quotes" option around titles, so that titles will be displaed correctly in LOTUS 123. b. If program FORMULA will be used, answer "no" to LOTUS and quotation marks options. 5. To calculate structural formulas: Program FORMULA-I a. Make sure the files containing the weight % oxides or elements obtained in 4. above is in same directory/drive as the program FORMULA-I. b. Load FORMULA-I and follow the directions. A caveat for calculating amphibole structural formulas is discussed in 7. below. 6. Hints on using FORMULA-I: a. Written in Fortran, spaces of labels, data, etc. is critical. The 1988 version of FORMULA-I requires that each element/oxide title occupies exactly 5 spaces, while the 1989 version requires 6 spaces. Check the instructions and test data for the version you are using. An error message will appear if this caution is not heeded. b. You must remove "TOTAL" and "RATIO" from the second line of every file, otherwise the program will bomb. There is no need to remove the actual numbers. 149 c. Make sure there is only one paragraph mark at the end of the document when you view it in MS WORD. d. When making changes to the files, make sure it is saved as ASCII. e. The program accepts a maximum of 100 analyses at any one time, so divide your data up accordingly using any text processor. f. Results are listed in TABLE.OUT and ECHO.OUT. These files are overwritten each time the program is run, so do rename them with a different extension if you need these output files for future reference. 7. If 23 oxygen amphibole formula is desired for use in hornblende geobarometry: a. Remove F and CI analyses from every line in the data file obtained using program TRANSFORM. For this purpose, use LOTUS 123, delete appropriate columns, and save as a print (ASCii) file, with top, bottom and side margins set to 0 inches. Verify you have saved it as an ASCII file by viewing it with MS WORD or the "view" option in XTREE. b. The correct 23-oxygen formula will be given by choosing option 2 in the AMPHIBOLE menu in program FORMULA-I. 8. To obtain P, T information, a number of different programs are available. The present study used the program PTMETER written by J. Percival. Also available is program THERMOBAROMETRY, written by F. Spear and S. Peacock. The latter program has been updated for Macintosh computer users, and has calibrations and for most geothermobarometers available to 1989/1990. Table 3.1. Microprobe standards used. Garnet: Elements Standard Source Si, AI,Ca 007-grossularite V Lualenyi, Kenya Ti 013-rutile Synthetic Mn 015-spessartine Gerass, Brazil Cr 222-chromite Tiebaghi mine, New Caledonia Na 229-hornblende Kakanui, New Zealand Mg 235-pyrope Kakanui, New Zealand Fe 237-fayalite Rockport, Maine Hornblende: Elements Standard Source Ti 13-rutile Synthetic Mn 15-spessartine Gerass, Brazil K 24-fluorophlogopite Synthetic, USBM, Norris, Tennessee F 92-richterite-NaF Synthetic Cr 222-chromite Tiebaghi mine, New Caledonia Na 229-hornblende Kakanui, New Zealand Fe 246-aegirine unknown CI 286-sylvite unknown Si, Mg 379-diopside Wakefield, N.J. Feldspar: Elements Standard Source Ba 016-barite Rock Candy Mine, B.C. Si, K 028-orthoclase Unknown Al, Ca 101-anorthite Synthetic Na 105-albite Amelia, Co., Va. Fe 246-aegirine Unknown Sr 274-strontianite Unknown Si, Mg 379-diopside Wakefield, N.J. Micas: Elements Standard Source Ti 13-rutile Synthetic Mn 15-spessartine Gerass, Brazil K 24-fluorophlogopite Synthetic, USBM, Norris, Tennessee Fe 82-biotite Wells Gray Park, B.C. Si, Al 164-muscovite Wildrose Canyon, Cal. Cr 222-chromite Tiebaghi mine, New Caledonia Na 229-hornblende Kakanui, New Zealand CI 286-sylvite unknown 151 Table 3.2. Representative microprobe hornblende analyses. Cations reported on the basis of 23 O; total Fe as FeO. c = core analysis, r = rim analysis. 4 2 4 1 c l 4 2 1 1 r l 4 2 1 1 r 2 1 7 9 1 1 r l 1 7 9 1 1 r 2 4 1 3 5 1 r 2 4 1 3 2 1 r 4 4 1 3 6 2 c 2 S i 0 2 4 4 . 3 4 4 4 . 3 4 4 5 . 0 0 4 5 . 71 4 5 . 9 5 4 4 . 3 3 4 4 . 4 7 4 4 . 9 9 A 1 2 0 3 8 . 3 9 9 . 5 2 8 . 7 1 7 . 32 7 . 4 2 8 . 6 9 9 . 1 4 8 . 0 4 T i 0 2 1 . 3 0 0 . 7 1 0 . 5 3 0 . 81 0 . 8 3 0 . 9 1 0 . 9 6 1 . 3 0 MgO 1 1 . 7 8 1 1 . 5 5 1 2 . 0 0 1 2 . 93 1 2 . 9 4 1 1 . 6 0 1 1 . 6 9 1 2 . 0 5 F e O 1 5 . 8 2 1 6 . 0 2 1 5 . 8 7 1 4 . 15 1 4 . 5 3 1 6 . 4 4 1 6 . 7 2 1 5 . 9 6 MnO 0 . 5 2 0 . 4 7 0 . 4 7 0 . 53 0 . 4 6 0 . 5 7 0 . 4 6 0 . 4 5 C a O 1 1 . 5 9 1 2 . 0 8 1 1 . 6 9 1 1 . 75 1 1 . 7 5 1 1 . 8 7 1 2 . 0 9 1 1 . 6 3 N a 2 0 1 . 3 9 1 . 0 7 1 . 3 6 1 . 36 1 . 2 3 1 . 2 4 1 . 3 4 1 . 3 0 K 2 0 0 . 9 3 0 . 9 2 0 . 9 0 0 . 74 0 . 7 6 0 . 9 3 0 . 8 1 0 . 9 3 C r 2 0 3 0 . 0 6 0 . 0 0 0 . 0 5 0 . 02 0 . 0 0 0 . 0 5 0 . 0 0 0 . 0 7 F 0 . 1 2 0 . 0 4 0 . 1 2 0 . 12 0 . 0 4 0 . 0 0 0 . 0 0 0 . 0 2 C I 0 . 0 6 0 . 0 6 0 . 0 6 0 . 04 0 . 0 6 0 . 0 2 0 . 0 9 0 . 0 0 T o t a l 9 6 . 3 0 9 6 . 7 8 9 6 . 7 6 9 5 . 48 9 5 . 9 7 9 6 . 6 5 9 7 . 7 4 9 6 . 7 4 C a t i o n s o n t h e b a s i s o f 23 O . T o t a l F e a s F e O . S i 6 . 7 5 9 5 6 . 7 0 6 5 6 . 8 0 9 1 6 . 9 4 6 7 6 . 9 4 2 5 6 . 7 3 6 5 6 . 6 8 4 6 . 8 0 5 6 A 1 I V 1 . 2 4 0 5 1 . 2 9 3 5 1 . 1 9 0 9 1 . 0 5 3 3 1 . 0 5 7 5 1 . 2 6 3 5 1 . 3 1 6 1 . 1 9 4 4 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 8 . 0 0 0 0 A 1 V I 0 . 2 6 7 5 0 . 4 0 4 2 0 . 3 6 3 2 0 . 2 5 8 5 0 . 2 6 3 3 0 . 2 9 3 5 0 . 3 0 0 3 0 . 2 3 8 4 T i 0 . 1 4 8 8 0 . 0 8 1 2 0 . 0 6 0 4 0 . 0 9 2 5 0 . 0 9 3 8 0 . 1 0 3 5 0 . 1 0 8 4 0 . 1 4 8 4 Mg 2 . 6 7 7 4 2 . 6 0 4 5 2 . 7 0 7 5 2 . 9 2 9 9 2 . 9 1 3 8 2 . 6 2 7 8 2 . 6 1 8 9 2 . 7 1 5 7 F e 2 . 0 1 7 0 2 . 0 2 6 9 2 . 0 0 8 6 1 . 7 9 8 9 1 . 8 3 5 8 2 . 0 8 9 3 2 . 1022 2 . 0 1 9 2 Mn 0 . 0 6 6 9 0 . 0 5 9 9 0 . 0 5 9 9 0 . 0 6 8 3 0 . 0 5 8 3 0 . 0 7 3 5 0 . 0 5 8 2 0 . 0 5 7 4 5 . 1 7 7 6 5 . 0 9 5 5 5 . 1 9 9 6 5 . 1 4 8 1 5 . 1 6 5 0 5 . 1 8 7 6 5 . 1 8 8 0 5 . 1 7 9 1 C a 1 . 8 9 3 5 1 . 9 5 7 4 1 . 8 9 5 1 1 . 9 1 3 9 1 . 9 0 2 7 1 . 9 3 2 9 1 . 9 4 6 7 1 . 8 8 5 2 N a ( A ) 0 . 4 0 9 6 0 . 3 1 5 1 0 . 3 9 8 6 0 . 4 0 1 2 0 . 3 6 0 1 0 . 3 6 6 6 0 . 3 9 0 9 0 . 3 8 1 1 K 0 . 1 8 0 2 0 . 1 7 7 1 0 . 1 7 4 2 0 . 1 4 6 4 0 . 1 4 6 7 0 . 1 8 0 5 0 . 1 5 4 5 0 . 1 7 8 7 Sum A 0 . 5 8 9 8 0 . 4 9 2 2 0 . 5 7 2 8 0 . 5 4 7 6 0 . 5 0 6 8 0 . 5 4 7 1 0 . 5 4 5 4 0 . 5 5 9 8 2 0 7 5 1 r 5 2 0 7 1 1 r 2 2 0 7 5 1 c 3 2 5 7 2 1 r 5 2 5 7 5 1 r l 2 5 7 5 1 c l 1 9 0 6 1 r l S i 0 2 4 6 . 4 0 4 5 . 4 6 4 5 . 7 7 4 1 . 3 7 4 2 . 8 7 4 0 . 4 8 4 0 . 7 5 A 1 2 0 3 7 . 5 6 7 . 9 1 9 . 2 9 1 1 . 50 1 0 . 4 9 1 2 . 9 6 1 3 . 1 0 T i 0 2 0 . 8 0 0 . 8 0 1 . 2 5 0 . 9 7 0 . 4 9 0 . 5 6 0 . 5 0 MgO 1 3 . 1 5 1 2 . 3 2 1 1 . 5 4 9 . 6 0 9 . 9 2 9 . 1 0 7 . 5 1 F e O 1 4 . 7 5 1 5 . 4 1 1 6 . 4 2 1 7 . 7 7 1 7 . 6 6 1 8 . 2 6 2 0 . 2 5 MnO 0 . 1 3 0 . 3 3 0 . 3 4 0 . 6 3 0 . 6 5 0 . 6 7 1 . 3 4 C a O 1 2 . 1 6 1 1 . 9 1 1 1 . 7 2 1 1 . 9 0 1 2 . 0 1 1 1 . 9 0 1 1 . 2 3 N a 2 0 0 . 9 9 1 . 1 2 1 . 5 1 1 . 1 1 0 . 9 4 1 . 3 7 1 . 2 0 K 2 0 0 . 5 5 0 . 7 2 0 . 9 2 1 . 2 3 0 . 8 6 0 . 8 8 0 . 9 8 C r 2 0 3 0 . 0 0 0 . 0 4 0 . 0 0 0 . 0 0 0 . 0 2 0 . 0 2 0 . 0 0 F 0 . 0 0 0 . 0 1 0 . 0 0 0 . 1 4 0 . 0 0 0 . 0 2 0 . 0 0 C I 0 . 0 2 0 . 0 4 0 . 1 5 0 . 0 2 0 . 0 3 0 . 0 3 0 . 1 9 T o t a l 9 6 . 4 9 9 6 . 0 7 9 8 . 9 1 9 6 . 2 4 9 5 . 9 4 9 6 . 2 5 9 7 . 0 5 S i 6 . 9 3 8 6 6 . 8 8 2 5 6 . 7 6 7 6 6 . 3 9 4 5 6 . 5 9 6 7 6 . 2 6 1 2 6 . 2 5 8 0 A 1 I V 1 . 0 6 1 4 1 . 1 1 7 5 1 . 2 3 2 4 1 . 6 0 5 5 1 . 4 0 3 3 1 . 7 3 8 8 1 . 7 4 2 0 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 0 8 . 0 0 0 0 A 1 V I 0 . 2 7 0 3 0 . 2 9 3 1 0 . 3 8 7 0 0 . 4 9 0 3 0 . 4 9 9 3 0 . 6 2 3 3 0 . 6 0 3 0 T i 0 . 0 9 6 2 0 . 0 9 1 0 0 . 1 3 9 5 0 . 1 1 2 6 0 . 0 5 7 1 0 . 0 6 5 0 0 . 0 5 8 0 Mg 2 . 9 2 9 9 2 . 7 8 0 6 2 . 5 4 2 7 2 . 2 1 1 7 2 . 2 7 5 9 2 . 0 9 7 0 1 . 7 1 9 0 F e 1 . 8 4 5 1 1 . 9 5 1 4 2 . 0 3 0 6 2 . 2 9 6 9 2 . 2 7 3 4 2 . 3 6 1 4 2 . 6 0 0 0 Mn 0 . 0 1 6 8 0 . 0 4 2 7 0 . 0 4 2 7 0 . 0 8 2 7 0 . 0 8 5 3 0 . 0 8 7 5 0 . 1 7 4 0 5 . 1 5 8 3 5 . 1 5 8 8 5 . 1 4 2 5 5 . 1 9 4 2 5 . 1 9 1 0 5 . 2 3 4 2 5 . 1 5 4 0 C a 1 . 9 4 8 5 1 . 9 3 1 1 1 . 8 5 7 1 1 . 9 7 0 4 1 . 9 8 0 8 1 . 9 7 1 8 1 . 8 4 7 0 N a ( A ) 0 . 2 8 6 5 0 . 3 2 9 2 0 . 4 3 3 2 0 . 3 3 1 6 0 . 2 7 9 9 0 . 4 0 9 5 0 . 3 5 7 0 K 0 . 1 0 5 7 0 . 1 3 9 1 0 . 1 7 3 1 0 . 2 4 3 2 0 . 1 6 9 1 0 . 1 7 3 3 0 . 1 9 2 0 Sum A 0 . 3 9 2 2 0 . 4 6 8 3 0 . 6 0 6 3 0 . 5 7 4 8 0 . 4 4 9 0 0 . 5 8 2 7 0 . 5 4 9 0 153 Table 3.3. Representative microprobe garnet rim analyses. Cations reported on the basis of 12 O; total Fe as FeO. 4 4 6 2 r l 1 9 0 2 1 r 2 8 2 3 2 r l 8 2 4 3 r 3 S i 0 2 A 1 2 0 3 T i 0 2 C r 2 0 3 F e O MnO MgO C a O N a 2 0 V 2 0 5 3 5 . 3 9 2 1 . 4 5 0 . 0 4 0 . 0 2 3 5 . 3 5 4 . 6 2 1 . 9 0 1 . 0 3 0 . 0 2 0 . 0 6 3 6 . 4 7 2 0 . 9 5 0 . 0 8 0 . 0 0 2 8 . 3 6 4 . 3 9 2 . 0 7 6 . 7 2 0 . 0 1 0 . 0 0 3 5 . 0 9 2 1 . 1 1 0 . 0 4 0 . 0 0 3 6 . 2 4 4 . 2 7 1 . 0 4 1 . 1 7 0 . 0 1 0 . 0 3 3 5 . 3 0 2 1 . 3 2 0 . 0 3 0 . 0 0 3 8 . 1 7 2 . 5 2 1 . 0 8 1 . 4 2 0 . 0 0 0 . 0 1 T o t a l 9 9 . 8 8 9 9 . 0 5 9 9 . 0 0 9 9 . 8 5 S i 2 . 8 2 0 2 . 9 1 9 2 . 8 4 5 2 . 8 3 2 A l 2 . 0 1 4 1 . 9 7 6 2 . 0 1 7 2 . 0 1 6 T i 0 . 0 0 2 0 . 0 0 5 0 . 0 2 0 . 0 0 2 F e 2 . 3 5 5 1 . 8 8 1 2 . 4 5 6 2 . 5 6 0 Mn 0 . 3 1 2 0 . 2 9 7 0 . 2 9 3 0 . 1 7 1 Mg 0 . 2 2 6 0 . 2 4 7 0 . 1 2 6 0 . 1 2 9 C a 0 . 0 8 8 0 . 5 7 6 0 . 1 0 2 0 . 1 2 2 N a 0 . 0 0 3 0 . 0 0 2 0 . 0 0 3 0 . 0 0 0 T o t a l : 7 . 9 1 9 7 . 8 4 5 7 . 8 2 0 7 . 8 3 2 x A l m 0 . 7 9 0 2 0 . 6 2 6 8 0 . 8 2 5 0 0 . 8 5 8 4 x P r p 0 . 0 7 5 7 0 . 0 8 2 2 0 . 0 4 2 4 0 . 0 4 3 3 x G r s 0 . 0 2 8 9 0 . 1 8 3 8 0 . 0 3 4 2 0 . 0 4 0 9 x S p s x U v 0 . 1 0 4 6 0 . 0 9 9 0 0 . 0 9 8 4 0 . 0 5 7 4 0 . 0 0 0 6 0 . 0 0 0 0 0 . 0 0 0 0 0 . 0 0 0 0 x A n d 0 . 0 0 0 0 0 . 0 0 8 0 0 . 0 0 0 0 0 . 0 0 0 0 154 Table 3.4. Representative microprobe biotite rim analyses. Cations occupy 8 sites; total Fe as FeO. 4 4 3 1 r l 1 9 0 2 1 r 2 8 2 3 2 2 r 2 8 2 4 1 1 r 2 S i 0 2 3 4 . 6 9 3 4 . 5 3 3 3 . 0 0 3 2 . 97 A 1 2 0 3 1 9 . 2 8 1 5 . 5 6 1 8 . 8 8 1 9 . 41 T 1 0 2 2 . 2 9 2 . 4 0 3 . 1 2 3 . 19 C r 2 0 3 0 . 0 8 0 . 0 0 0 . 0 5 0 . 00 F e O 2 3 . 2 1 2 2 . 4 7 2 6 . 6 6 2 6 . 53 MnO 0 . 1 2 0 . 6 7 0 . 1 1 0 . 09 MgO 6 . 8 3 8 . 9 4 3 . 8 6 3 . 41 C a O 0 . 0 0 0 . 0 2 0 . 0 0 0 . 01 N a 2 0 0 . 3 0 0 . 0 8 0 . 1 8 0 . 23 K 2 0 9 . 1 1 9 . 1 4 8 . 7 3 8 . 90 B a O 0 . 3 1 0 . 4 1 0 . 2 4 0 . 20 F 0 . 1 1 0 . 1 9 0 . 2 2 0 . 14 C I 0 . 1 1 0 . 1 3 0 . 0 2 0 . 02 T o t a l 9 6 . 4 4 9 4 . 5 4 9 5 . 0 7 9 5 . 10 S i A 1 I V A 1 V I 2 . 7 4 6 2 . 7 8 5 2 . 715 3 . 071 0 . 2 5 4 0 . 2 1 5 0 . 285 0 . 0 0 0 1 . 5 4 4 1 . 2 6 4 1 . 545 2 . 130 T i 0 . 1 3 6 0 . 1 4 6 0 . 193 0 . 223 C r F e 2 + 0 . 0 0 5 0 . 0 0 0 0 . 003 0 . 0 0 0 1 . 5 3 6 1 . 5 1 5 1 . 834 2 . 0 6 6 Mn 0 . 0 0 8 0 . 0 4 6 0 . 0 0 8 0 . 007 Mg 0 . 8 0 6 1 . 0 7 5 0 . 4 7 3 0 . 4 7 3 C a 0 . 0 0 0 0 . 0 0 2 0 . 0 0 0 0 . 0 0 0 N a 0 . 0 4 6 0 . 0 1 3 0 . 0 2 9 0 . 002 K 0 . 9 2 0 0 . 9 4 0 0 . 9 1 6 0 . 027 T o t a l : 8 . 0 0 0 8 . 0 0 0 8 . 0 0 0 8 . 0 0 0 155 Table 3.5. Representative microprobe plagioclase rim analyses. Cations based on 8 oxygens; total Fe as FeO. 4 4 6 1 r l 8 2 3 2 r 2 8 2 4 3 2 r l S i 0 2 6 0 . 8 5 6 1 . 6 6 6 1 . 9 8 A l 2 ° 3 2 4 . 6 7 2 3 . 9 5 2 4 . 0 4 F e O 0 . 2 5 0 . 3 9 0 . 3 7 MgO 0 . 0 0 0 . 0 1 0 . 0 0 C a O 5 . 5 6 5 . 4 7 4 . 9 8 N a 2 0 8 . 2 8 8 . 0 5 8 . 1 7 K 2 0 0 . 1 6 0 . 1 2 0 . 2 3 B a O 0 . 0 1 0 . 0 2 0 . 0 3 T o t a l 9 9 . 7 8 9 9 . 6 7 9 9 . 8 0 S i 2 . 7 3 1 2 . 7 5 8 2 . 7 6 5 A l 1 . 3 0 5 1 . 2 6 2 1 . 2 6 4 F e 2 + 0 . 0 0 9 0 . 0 1 5 0 . 0 1 4 Mg 0 . 0 0 0 0 . 0 0 1 0 . 0 0 0 C a 0 . 2 8 4 0 . 2 6 0 0 . 2 3 8 N a 0 . 6 6 3 0 . 6 9 8 0 . 7 0 7 K 0 . 0 0 7 0 . 0 0 7 0 . 0 1 3 S u m : 5 . 0 0 0 5 . 0 0 0 5 . 0 0 0 

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