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769 ARTICLE Barrovian metamorphism in the central Kootenay Arc, British Columbia: petrology and isograd geometry Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. D.P. Moynihan and D.R.M. Pattison Abstract: A narrow, partly fault-bounded belt of Barrovian amphibolite facies rocks transects the central Kootenay Arc in the internal zone of the southeastern Canadian Cordillera. The following zones of increasing metamorphic grade are recognised in metapelites: chlorite/biotite, garnet, staurolite, kyanite, sillimanite, and sillimanite + K-feldspar. The garnet and higher-grade zones outline two joined domains: a N- to NNE-trending curved belt, >100 km long and 5–20 km wide, that is approximately parallel to the strike of stratigraphic units; and a SSE-trending belt, >70 km long and 10–15 km wide, that transects strike. Isograds in the N–NNE-trending belt outline an elongate bull's-eye pattern with highest-grade rocks in the centre, coincident with the position of a structural culmination. Isogradic surfaces have an elongate domal form centred around this culmination. Rocks in the kyanite and sillimanite zones were metamorphosed at ~25 km (⬃7 kbar (1 kbar = 100 MPa)) and >650 °C during Early Cretaceous crustal thickening; rocks in the garnet zone experienced peak temperature conditions (⬃500 °C) at lower pressure, suggesting the piezothermic array for the belt has a positive slope. The western flank of the N- to NNE-trending belt is cut by the west-dipping Gallagher fault zone (GFZ), whereas the eastern boundary of the SSE-trending fork is marked by the Purcell Trench fault (PTF). These Palaeocene–Eocene normal faults truncate Early Cretaceous isogradic surfaces and juxtapose regions with contrasting structural and metamorphic histories. Low-grade rocks in the hanging wall of the GFZ underwent peak regional metamorphism during the Early–Middle Jurassic, prior to intrusion of the 159–173 Ma Nelson batholith at a depth of ⬃12–14 km. With the exception of a zone along the southern “tail” of the Nelson batholith, rocks in the hanging wall of the GFZ were not affected by Cretaceous metamorphism or penetrative deformation. Rocks in the amphibolite-facies belt yield Palaeocene– Eocene K–Ar and 40Ar/39Ar mica cooling ages, whereas K–Ar biotite ages in the hanging wall of the GFZ record Jurassic – Early Cretaceous cooling, and those in the hanging wall of the PTF are mid-Cretaceous. Although the GFZ and PTF accommodated differential exhumation during the Palaeogene, significant relief on isogradic surfaces was established prior to normal faulting, during Early Cretaceous metamorphism and deformation. Résumé : Une ceinture étroite de roches barroviennes au faciès des amphibolites, en partie limitée par des failles, recoupe l'arc central Kootenay dans la zone interne de la Cordillère canadienne sud-est. Les zones suivantes, à degré croissant de métamorphisme, sont reconnues dans les métapélites : chlorite/biotite, grenat, staurotide, kyanite, sillimanite et sillimanite + feldspath-K. Les zones à grenat et les zones à métamorphisme plus élevé définissent deux domaines conjoints : (1) une ceinture courbée, de direction N à NNE d'une longueur de plus de 100 km et d'une largeur de 5 à 20 km, qui est presque parallèle à la direction des unités stratigraphiques et (2) une ceinture de direction SSE, d'une longueur de plus de 70 km et d'une largeur de 10 à 15 km qui recoupe la direction. Les isogrades dans la ceinture à direction N–NNE définissent un patron de cercles concentriques allongés dans lequel les roches à degré de métamorphisme plus élevé sont situées au centre, ce qui concorde avec une position de culmination structurale. Les surfaces des isogrades ont une forme de dôme allongé centré autour de cette culmination structurale. Les roches dans les zones de kyanite et de sillimanite ont été métamorphisées à une profondeur de 25 km (⬃7 kbar (1 kbar = 100 MPa)) et à une température >650 °C durant l'épaississement de la croûte au Crétacé précoce; les roches dans la zone à grenat ont subi les conditions de température de crête (⬃500 °C) à des pressions moindres, suggérant une pente positive pour le groupement piézothermique de la ceinture. Le flanc ouest de la ceinture à tendance N à NNE est recoupé par la zone de faille Gallagher (GFZ) à pendage ouest, alors que la limite est de la fourche à tendance SSE est marquée par la faille Purcell Trench (PTF). Ces failles normales (Paléocène–Éocène) recoupent les surfaces isogrades du Crétacé précoce et juxtaposent des régions ayant des historiques structuraux et métamorphiques différents. Les roches à faible métamorphisme dans l'éponte supérieure de la GFZ ont subi un métamorphisme de crête au cours du Jurassique précoce à moyen, avant l'intrusion du batholite de Nelson (159–173 Ma) à une profondeur d'environ 12–14 km. À l'exception d'une zone le long de l'extrémité sud du batholite de Nelson, les roches dans l'éponte supérieure de la GFZ n'ont pas été touchées par le métamorphisme au Crétacé ni par une déformation de pénétration. Les roches dans la ceinture au faciès des amphibolites ont donné des âges de refroidissement Paléocène–Éocène, déterminés par K–Ar et 40Ar/39Ar sur du mica, alors que les âges déterminés par K–Ar sur biotite dans l'éponte supérieure de la GFZ documentent un refroidissement au Jurassique–Crétacé précoce et que ceux dans l'éponte supérieure de la PTF datent du Crétacé moyen. Bien que la zone de faille Gallagher et la faille Purcell Trench soient le reflet d'une exhumation différentielle durant le Paléogène, des reliefs importants sur les surfaces des isogrades ont été établis avant les failles normales, durant le métamorphisme et la déformation au Crétacé précoce. [Traduit par la Rédaction] Received 10 April 2012. Accepted 17 February 2013. Paper handled by Associate Editor Marc St-Onge. D.P. Moynihan* and D.R.M. Pattison. Department of Geoscience, University of Calgary, Calgary, AB T2N 1N4, Canada. Corresponding author: D.P. Moynihan (e-mail: [email protected]). *Present address: Yukon Geological Survey, P.O. Box 2703 (K-14), Whitehorse, YT Y1A 2C6, Canada. Can. J. Earth Sci. 50: 769–794 (2013) dx.doi.org/10.1139/cjes-2012-0083 Published at www.nrcresearchpress.com/cjes on 3 April 2013. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. 770 Can. J. Earth Sci. Vol. 50, 2013 Introduction Geology of the central Kootenay Arc The southeastern Canadian Cordillera comprises a complex series of structural and metamorphic domains that record a prolonged period of orogenesis lasting from the Early Jurassic to the Eocene (Parrish 1995; Evenchick et al. 2007). Reconstruction of the development of the orogen is complicated by a heterogeneous pre-Cordilleran basin architecture, multiple superposed episodes of deformation and associated medium–high grade metamorphism, and late orogenic extension. Given this complexity, characterization of its varied structural and metamorphic domains is a prerequisite for understanding the tectonic processes that led to its development. The focus of this paper is a narrow, elongate belt of Barrovian amphibolite-facies rocks that transects the Kootenay Arc in the internal zone of the orogen (Fig. 1; Archibald et al. 1983; Read et al. 1991). We document the regional metamorphism of this metapelites in this belt, the relationships between metamorphism and deformation, the geometry of isograds and isogradic surfaces, and interpret the processes that gave rise to its formation. The Kootenay Arc (Figs. 1–3) is an arcuate zone in the hinterland of the orogen characterised by a distinctive stratigraphy and intense polyphase deformation. The structural trend is SW–NE in the southernmost part of the Arc and changes gradually to NW–SE in the northern part, where it is approximately parallel to the overall trend of the orogen. The Kootenay Arc includes the zone of overlap between Proterozoic – Early Palaeozoic rocks that formed on the western margin of ancestral North America or precursor rift basins, and mostly Late Palaeozoic – Mesozoic rocks that formed in island arc or back arc settings that developed after late Proterozoic rifting (Quesnel and Slide mountain terranes; Klepacki 1985). Geological background The southeastern Canadian Cordillera includes a metamorphic– plutonic internal zone that lies to the west of a NE-vergent fold and thrust belt and associated foreland basin (Fig. 1). This part of the orogen developed in response to convergence between cratonic North America and westward terranes in the Early Jurassic, and continued convergence led to further shortening, thickening, and eastward propogation (relative to North America) until the Eocene (Parrish 1995; Evenchick et al. 2007; Simony and Carr 2011). Three major episodes of deformation and metamorphism are recognised in the southeastern Canadian Cordillera: the first in the Early–Middle Jurassic, the second in the Early Cretaceous and a third during the Late Cretaceous – Eocene (Simony and Carr 2011 and references therein). Convergent orogenesis was followed in parts of the internal zone by a period of extension during the Eocene. Upper amphibolite- to granulite-facies rocks are exposed in a series of gneissic domal culminations that are exposed in the western part of the internal zone; individual domal culminations include the Monashee, Valhalla, Grand Forks, and Priest River complexes (Fig. 1; Doughty and Price 1999; Johnston et al. 2000; Carr and Simony 2006; Hinchey et al. 2006; Cubley et al. 2012). These culminations experienced high-grade metamorphism during the Late Cretaceous – Eocene and expose a number of thick top-to-the-east shear zones that were active during this time (Simony and Carr 2011). The area in which the gneiss domes are exposed experienced widespread Eocene extension (Parrish et al. 1988), and domal culminations are typically bounded by normal faults of this age. Lower-grade areas surrounding the gneissic domes typically record an older (Jurassic – Early Cretaceous) deformational and metamorphic history, as does much of the area that lies between the gneiss complexes and the Late Cretaceous – Eocene foreland fold and thrust belt (Currie 1988; Hoy and van der Heyden 1988; Scammell 1993; Parrish 1995; Colpron et al. 1996; Digel et al. 1998; Crowley et al. 2000; Reid 2003; Larson et al. 2007; Gibson et al. 2008). Early Cretaceous structures are dominant in a broad zone that stretches from the central Kootenay Arc across the Purcell Anticlinorium, as far east as the Porcupine Creek anticlinorium (Simony and Carr 2011; Fig. 1), whereas Middle Jurassic structures are more widely preserved in low-grade domains (i.e., at high structural levels) to the west and north of this region (Colpron et al. 1996; Acton et al. 2002; Carr and Simony 2006; Cubley et al. 2013). The area under consideration in this study — the central Kootenay Arc — includes the interface between these regions, and also lies within the area that was affected by lateorogenic extension. Stratigraphy of the central Kootenay Arc The Kootenay arc lies on the west flank of the Purcell Anticlinorium, a large north-plunging structure cored by Mesoproterozoic rocks of the Belt-Purcell Supergroup (Lydon 2007) and partially mantled by the unconformably overlying Neoproterozoic Windermere Supergroup (Fig. 1; Warren 1997 and references therein). These supergroups comprise thick sequences of clastic and carbonate sedimentary rocks with subordinate mafic volcanic rocks that were deposited in partially overlapping intracontinental rift zones. Rifting that led to formation of the western margin of ancestral North America took place in the late Neoproterozoic (Devlin and Bond 1988; Colpron et al. 2002) and is recorded by Neoproterozoic – Lower Cambrian rocks of the correlative Gog and Hamill groups (Fig. 2). The Hamill/Gog Group is dominated by coarse quartz-rich sedimentary rocks, with minor (meta-) pelite, carbonate rocks, and mafic volcanic rocks. There is a regional unconformity in the Hamill Group (Devlin and Bond 1988; Likorish and Simony 1995; Warren 1997), separating units that were deposited in faultbounded basins during rifting from an upper part distinguished by laterally continuous units deposited in a shallow-marine setting. The unconformity within the Hamill Group is interpreted as a “break-up” unconformity, representing continental separation and the change from active continental rifting to thermal subsidence on the newly formed western margin of Laurentia (Devlin and Bond 1988; Warren 1997). In the Kootenay Arc and western Purcell anticlinorium, the Hamill Group is overlain by the Mohican formation, a mixture of siliciclastic and carbonate rocks, followed by the Badshot Formation — a distinctive, Lower Cambrian pale-coloured marble. The Badshot marble is conformably overlain by rocks of the Lardeau Group, which comprises Lower Palaeozoic pelitic and semi-peltic schist, mafic volcanic rocks, carbonate and calcsilicate rocks, coarse siliciclastic rocks and conglomerate (Fyles and Eastwood 1962, Fyles 1964, 1967; Hoy 1980; Colpron and Price 1995 and references therein). The lowest part of the Lardeau Group is a fine-grained black metapelite that records deposition under tectonically quiescent, deep-water conditions; however, a return to active rifting is recorded by metavolcanic rocks and coarse grits of upper parts of the Lardeau (Logan and Colpron 2006). The Lardeau Group is unconformably overlain by the Mississippian– Permian Milford Group, which comprises conglomerate, phyllite, basalt, limestone, sandstone, tuff, and siliceous argillite. This is followed in conformable succession by the Kaslo group, which is dominated by basalt and mafic volcaniclastic rocks, but also includes serpentinite, siliceous sediments, greywacke, and minor conglomerate. The Milford and Kalso groups are interpreted to have been deposited in a rift setting, probably in a back-arc basin (the Slide Mountain or Kaslo basin) to the west of cratonic North America (Klepacki 1985; Nelson 1993; Roback et al. 1994). The Kaslo and parts of the Milford Group represent the Slide Mountain terrane in this part of the Cordillera. Published by NRC Research Press Moynihan and Pattison 771 Fig. 1. Geological map of the southeastern Canadian Cordillera, adapted from Wheeler and McFeely (1991). The Kootenay Arc forms an eastward-convex arcuate belt that includes the overlap between rocks deposited on the Laurentian margin and those of pericratonic (Quesnel and Slide Mountain) terranes to the west. A cross section from the Kootenay Arc to the foreland (Price 1981) is also shown. The subdivisions in the cross section do not correspond exactly with those shown on the map. In the cross section, the Neoproterozoic – Lower Cambrian Hamill–Gog groups are combined with Palaeozoic rocks, and Triassic–Palaeocene sedimentary rocks of the foreland belt are grouped together. 118 W 52 N FO 116 W 52 N RE °W 135 °W 130 115° T ef rm Val ha lla AS CO AR Und ed fo t and el rel CANADA U.S.A. b Gran d Fo rk s ED RM NE UL Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. FO DE UN TA ON INS 49°N o st Cenozoic volcanic & sedimentary rocks Creston 60°N CAN USA 1 ru Quaternary N th A c Prie st R iver ld & tain fo t e n a y r 48 N 118 W ium inor ticl An ll rce Pu o Nelson 50° un o Kelowna O ka n a g a n Calgary Mo K °N M ER y Revelstoke INT ck 55 120°W CA ND INE LA OM Ro ee sh na Mo Golden 48 N 114 W 48 N 116 W Late Cretaceous-Palaeogene foreland basin sedimentary rocks Triassic - Jurassic Mesozoic intrusions Palaeozoic (shelf/slope) Palaeogene intrusions Palaeozoic (basinal) Quesnel & Slide Mountain terranes Undifferentiated gneiss in metamorphic core complexes Palaeoproterozoic basement (Laurentian) W KOOTENAY ARC PURCELL ANTICLINORIUM Neoproterozoic - Lower Cambrian (Hamill & Gog groups) Neoproterozoic (Windermere Supergroup) Mesoproterozoic (Purcell Supergroup) Porcupine Creek Anticlinorium E Triangle Zone km 0 -5 -10 Published by NRC Research Press 772 Can. J. Earth Sci. Vol. 50, 2013 Fig. 2. Geological map of the central Kootenay Arc. From Fyles (1964, 1967), Reesor (1973), Hoy (1980), Klepacki (1985), Leclair (1988), Reesor (1996), and Warren (1997). The area covered is shown in Fig. 1. Intrusive Rocks A’ 50 15’ Late Cretaceous/Palaeocene A 117 00’ Middle Jurassic Stratified Rocks Purcell Supergroup Sandstone, siltstone, pelite, carbonate, mafic sills, Lakeshore fault lt ard fau West B ern Gallagher fault C’ Riondel Ainsworth Nelson batholith Crawford Bay t Arm Wes Ko ot en ay La ke D 49 30’ E Midge Creek stock Crawford stock ? D’ Seeman Creek fault Mesoproterozoic 49 45’ Proctor plut on Windermere Supergroup Sandstone, conglomerate, pelite, carbonate C ke Fault y Lreanch teunrcaell T Koo P Neoproterozoic Shoreline stock Kaslo plu ton NeoproterozoicLower Cambrian Hamill Group Grey and brown micaceous quartzite, mica schist, white quartzite, amphibolite, marble, calc-silicate B’B’ B B dy Lower Cambrian Badshot and Mohican Formations Grey and white calcite and dolomite marble, mica schist, amphibolite 50 00’ Bal Lardeau Group Calc-schist, calc-silicate gneiss, pelitic schist, marble, quartzite, amphibolite, greenschist, quartzofeldspathic gneiss lt CambrianDevonian? Fry Creek batholith au MississippianPermian Milford Group Siliceous argillite, schist, grey marble, chert, quartzite rf Kaslo Group Greenstone, amphibolite, chert de Permian Slocan Group Grey argillite and phyllite, light grey to black limestone oe hr Triassic Sc Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. mid-Cretaceous Bayonne batholith E’ 2 km Published by NRC Research Press Moynihan and Pattison 773 Fig. 3. Geological cross sections of the central Kootenay Arc. All are vertical sections with equal horizontal and vertical scales. Section lines A–E are shown in Fig. 2; section F–F= is marked on Fig. 5d. (a) A–A= is from Fyles (1967), (b) B–B= from Klepacki (1985), (c) C–C= from Fyles in Brown et al. (1981), (d–f) D–D=, E–E=, and F–F= are from Leclair (1988). a A A’ 1 km A‘ S2 S1 Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Anticline Creek dow Mea b B B‘ 1 km ult r fa e ed hro c S re/ l ho s ke Ka slo sil La c C lt 1 km r he lt ult au Fa e F ore n i sh ph se Lake Jo u Fa lag l Nelson S2 S1 E D‘ lt au kf Cr ee on fa ul t row s Cr eek fau F2 ils W Bald y pluto n Proc te pluto r n Nelson batholi th M id ge Cr ee k 1 km e e pp Na el d on Ri S2 lt D S2 Nar d Ga S1 C‘ E‘ Midge Creek fault Seeman Creek fault Purcell Trench fault 1 km f F Midge Creek fault Wurttemberg anticline F‘ Purcell Trench fault 2500 m 2000 m 1500 m 1000 m 500 m Published by NRC Research Press 774 Can. J. Earth Sci. Vol. 50, 2013 Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Fig. 4. (a) Refolded tight-isoclinal F1 fold in the Slocan Group above Fletcher Lake; hanging wall of the Gallagher fault. Limestone is folded around recessively weathering shale. The hillside relief is ⬃500 m. (b) Quartz-filled strain shadows on pyrite grains define a (L1) stretching lineation in phyllite of the Slocan Group; DM-07-124. (c) Polydeformed “felsite sills” in Grt–Ctd schist of the Milford Group. The “sills” contain a margin-parallel foliation that can be traced around hinges of the tight (D2) folds. (d) Tight-isoclinal F2 fold outlined by the buff-coloured Badshot Formation on a mountainside east of Riondel. Visible relief on the slope facing the viewer is ⬃300 m. (e) L2 stretching lineation defined by sillimanite nodules in Hamill Group schist on the shoreline of Kootenay Lake at Riondel. (f) Shear bands in metapelite from north of the Fry Creek batholith. S2 is deflected into a sigmoidal geometry between shear bands (C=). Length of thin section is 7.5 cm. View is to the SW, down the plunge of the intersection of shear bands with S2. (g) Cataclasite–mylonite from the Gallagher fault. The white material in the upper part of the photograph is a deformed quartz vein. Length of rock slab is 7.5 cm. Cut plane is approximately vertical, oriented E–W. (h) S–C= fabric in deformed quartz monzonite from the NW margin of the Fry Creek batholith. Field of view is 7.5 cm long. Cut plane is approximately vertical, oriented E–W. The S–C fabric indicates west-side down shearing. Thrust duplication of the Kalso Group accompanied closure of the Slide Mountain basin in the Late Permian – Middle Triassic (Klepacki 1985). The Kalso Group was subaerially exposed, eroded, and its detritus deposited in the Marten conglomerate, a Late Permian – Middle Triassic greenstone conglomerate intercalated with limestone (Klepacki 1985). The Marten conglomerate is unconformably overlain by the Slocan Group, which comprises Carnian–Norian dark grey slate and phyllite, locally interbedded with limestone, limestone arenite, quartzite, and intermediatemafic volcanic rocks. It was deposited in a regionally extensive back-arc basin that developed east of the Late Triassic Nicola/ Quesnel magmatic arc, which formed above the Cache Creek accretionary complex to the west (Mortimer 1987). The Slocan Group overlies a varied basement and provides part of the abundant stratigraphic and geochemical evidence that links the Quesnel Arc to ancestral North America in the Triassic (Dostal et al. 2001; Unterschutz et al. 2002; Petersen et al. 2004; Simony et al. 2006; Thompson et al. 2006). Though not exposed in the area covered by Fig. 2, Early Jurassic volcanic and sedimentary rocks of the Rossland Group lie on top of the Slocan Group (Hoy and Dunne 1997). The upper unit of the Rossland Group — the clastic Pliensbachian–Toarcian Hall Formation — is the youngest unit that was deposited prior to Cordilleran deformation in this part of the Cordillera. Middle Jurassic D1 deformation, M1 metamorphism, and magmatism The central Kootenay Arc is dominated by two regionallydeveloped fold generations (Fyles 1964, 1967; Hoy 1976; Leclair 1988; Figs. 2, 3) and records two episodes of regional metamorphism (M1, M2). Phase 1 folds are a series of high-amplitude isoclines with gently-plunging axes (Figs. 2–4a). The most clearly defined F1 folds in the study area are westward-closing recumbent anticlines cored by the Hamill Group; the largest of these, the Riondel nappe (Hoy 1976; equivalent to the Meadow Creek anticline in the Duncan Lake area) has a 20 km long overturned lower limb (Fig. 3). S1 schistosity is axial planar to F1 folds, and contains a gently-plunging stretching lineation defined by aligned minerals and quartz strain shadows (L1; Fig. 4b). D1 was accompanied by low-grade regional metamorphism that was locally overprinted by contact metamorphism around Middle Jurassic intrusions (Pattison and Vogl 2005). The high-amplitude, isoclinal D1 folds are similar to Middle Jurassic structures (e.g., the west-vergent Scrip/Carnes nappe) that are developed approximately along strike in the Selkirk and Cariboo mountains (Raeside and Simony 1983; Murphy 1987; Brown and Lane 1988; Colpron et al. 1996). Although there is evidence for older (Paleozoic) deformation northwest of the study area (Read 1973; Read and Wheeler 1976; Klepacki 1985), F1 folds in the central Kootenay Arc probably also formed during the Middle Jurassic (see discussion in Warren 1997). Early–Middle Jurassic deformation and regional metamorphism was followed by intrusion of Middle Jurassic plutons and associated minor bodies. In the central Kootenay Arc, this suite is represented by the calc-alkaline Nelson Batholith and associated minor bodies (Fig. 2). The Nelson Batholith is an 1800 km2 composite body that was intruded during the interval 159–173 Ma (Ghosh 1995). It ranges in composition from diorite to granite, but is dominated by porphyritic hornblende granodiorite. The Nelson batholith is truncated on its western side by the east-dipping Eocene Slocan Lake (normal) fault (Carr et al. 1987), and is also fault-bounded on part of its eastern side, as described later in the text. Most of the Nelson Batholith is unaffected by ductile deformation; however, a subhorizontal stretching lineation and subvertical to west-dipping foliation is developed on its eastern margin along its southern “tail” and to the north of the West Arm of Kootenay Lake (Crosby 1968; Vogl 1992). In addition to plutons, the region contains numerous felsic dikes and sills. On the west side of the northern part of Kootenay Lake, and to the north of the lake, sills of blocky, rusty-weathering “felsite” (Fyles 1964) intrude all units. These sills are commonly foliated and lineated and some are folded (Fig. 4c), with gently north-plunging axes parallel to the lineation. The sills range in size from tens of centimetres to large map-scale bodies. Smith et al. (1991) reported a U–Pb zircon age of 173 ± 5 Ma for the largest body of this type — the “Kaslo sill”. Early Cretaceous D2 deformation and M2 metamorphism A second period of deformation and regional metamorphism (D2/M2) took place in the central Kootenay Arc, after intrusion of the Nelson batholith suite. This is the period of metamorphism that gave rise to the narrow Barrovian belt that is the focus of this paper. F2 folds are approximately coaxial with F1 (Figs. 2, 3) but are overturned to the east (Fig. 4d). F2 axial planes and their axial– planar schistosity (S2) dip gently to moderately-steeply to the west, but steepen towards higher structural levels in the Purcell Anticlinorium (Fyles 1964; Hoy 1980; Fig. 3). L2 stretching lineations (Fig. 4e), which are defined by aligned minerals (sillimanite, micas, amphiboles, tourmaline), strain shadows on porphyroblasts, sillimanite-rich nodules, and quartzofeldspathic aggregates plunge gently, parallel to F2 fold axes. North of the West Arm of Kootenay Lake, D2 lineations and folds plunge gently north, exposing deeper structural levels towards the south. Approximately south of the West Arm, lineations are subhorizontal to gently SSW-plunging (Fyles 1967; Leclair 1988; Vogl 1992). In the region adjacent to the SSW-trending tail of the Nelson batholith, gently-plunging stretching lineations are accompanied by abundant shear bands and C–S fabrics, indicating dextral shearing during D2. Foliation within the tail of the batholith displays a transition towards parallelism with this SW-trending zone (Vogl 1992), as does the foliation in screens within the tail. There is considerable along- and cross-strike variation in the intensity of D2. In the eastern part of the Kootenay Arc and the western part of the Purcell anticlinorium, D2 structures generally form the dominant fabrics, whereas they are weakly-developed or absent from lower-grade areas to the north and west (Figs. 3, 4a). A southward increase in D2 strain is manifested in tightening of F2 folds (Figs. 3, 4d) and more pervasive development of D2 fabrics. Around Duncan Lake and the northern end of Kootenay Lake, D2 folds are open to close (Fig. 3a), S1 is commonly preserved (Fyles 1964) and F2 folds have curvilinear hinges. In the Riondel area (Hoy 1977) and further south (Leclair 1988), however, F2 folds are Published by NRC Research Press UP W Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Moynihan and Pattison g) 775 a) b) F1 L1 c) d) e) N UP W F2 S1 f) NW SE C‘ (S3) S2 L2 h) UP E E S C Published by NRC Research Press 776 Can. J. Earth Sci. Vol. 50, 2013 Fig. 5. Metamorphic maps of the central Kootenay Arc. (a) A compilation showing the distribution of regional metapelitic metamorphic zones in the central Kootenay Arc. (b–d) Mineral assemblage maps, each covering part of the area. The key for mineral assemblages is contained in (d). Assemblages observed in rocks collected for this study are labelled in black, whereas those reported in previous studies have red outlines (in the Web version). Samples discussed in the text or illustrated in figures are labelled. In b–d, grid lines are spaced at 5 km intervals. Data are from this study, Fyles (1964, 1967), Reesor (1973), Crosby (1968), Hoy (1974), Archibald et al. (1983), Klepacki (1985), Leclair (1988), and Warren (1997). The section lines of metamorphic cross sections presented in Fig. 11 are marked on (c) and (d). And, andalusite; Bt, biotite; Chl, chlorite; Crd, cordierite; Ctd, chloritoid; Grt, garnet; Kfs, K-spar, K-feldspar; Ky, kyanite; Qtz, quartz; Sil, sillimanite; St, staurolite. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. tight-isoclinal (Fig. 3d), rectilinear F2 fold hinges are invariably parallel to the L2 stretching lineation and D1 fabrics are typically not discernible (Hoy 1980; Leclair 1988). Mid-Cretaceous magmatism A second major plutonic suite was intruded during the interval 117–95 Ma. This suite includes hornblende and biotite granodiorite, biotite granite, and two-mica granite, which are interpreted to have been derived from crustal anatexis (Brandon and Lambert 1993). The largest bodies of this age in the area are the Fry Creek batholith and the Heather Creek, Drewry Point, Steeple Mountain, and Mount Skelly phases of the Bayonne batholith. These largely undeformed intrusions crosscut the major structures in the area (Fig. 2) and are younger than D2/M2 penetrative deformation and regional metamorphism. The oldest member of this suite — the Baldy pluton — is an elongate, sheet-like granodiorite body that was deformed during D2 (Fig. 2). It has yielded a combined titanite and allanite U–Pb date of 117 +4/–1 Ma, which was interpreted to represent the age of crystallization (Leclair et al. 1993). The nearby, largely undeformed Midge Creek stock yielded a U–Pb allanite age of 114 +1/–3 Ma. This suggests that, contrary to earlier interpretations, D2/M2 took place during the Early Cretaceous (Leclair et al. 1993). White leucogranitic pegmatite, locally containing biotite or muscovite, forms abundant sills, dikes, pods, and small plutons in western part of the Riondel area (Hoy 1980), on much of the Crawford peninsula, and on the west side of Kootenay Lake south of the West Arm. Pegmatite exhibits a variety of relationships with respect to the (D2) dominant structures. Much of the pegmatite is deformed — it is foliated, boudinaged, and locally folded. Large bodies of pegmatite are commonly deformed on their margins and more massive in their interior. Crosscutting, undeformed pegmatite and aplite/leucogranite dikes are also present in the Crawford Bay – Riondel area (Crosby 1968; Livingstone 1968; Hoy 1980). D3, D4, D5 Over a large area adjacent to the northern, north-trending part of Kootenay Lake, S2 in micaceous rocks is overprinted by shear band cleavage (Platt and Vissers 1980; White et al. 1980; Williams and Price 1990; Passchier and Trouw 2005; Fig. 4f). Shear bands dip NW, and lines of intersection of shear bands with S2 and associated sigmoidal deflections of the foliation plunge SW. Although most of these “extensional crenulations” (Platt and Vissers 1980) are centimetre-scale, some metre-scale and larger folds of this nature are also developed. These SW-plunging warps were noted by Fyles (1967), and referred to as F3 folds by Livingstone (1968) and Hoy (1980). The area affected by this S2 reactivation is >60 km long, and ⬃15 km wide. On the west side of the lake, shear bands are developed east of the Gallagher fault between the West Arm and Kalso, and are found east of the Lakeshore fault (Fyles 1967) north of Kaslo. On the east side of the Lake, they are restricted to the region close to the shore of Kootenay Lake, extending north from the highway west of Crawford Bay, past Riondel (Livingstone 1968; this work). They are also developed in the area north of the Fry Creek batholith (Fig. 2). The age of these structures is poorly constrained; they are crosscut by earliest Palaeocene pegmatites (Moynihan 2012) but could be as old as Early Cretaceous (the age of D2). Two generations of folds not observed elsewhere are developed on and around the Crawford peninsula. Fourth-generation folds are buckles with axes that plunge gently SW or NE and axial planes with low to moderate dips. Unlike the F3 folds described earlier in the text, they formed by shortening rather than extension of S2. These folds post-date intrusion of the Palaeocene Crawford Bay stock (see later in the text) and are overprinted by NNW-trending folds (F5) that are probably related to motion on the Eocene Purcell Trench fault (PTF). Faults Hoy (1977) and Leclair (1988) identified a number of regionallysignificant, approximately strike-parallel west-dipping faults in the central Kootenay Arc. The West Bernard fault (WBF; Fig. 2) separates the Riondel nappe from its root zone and probably had reversesense displacement (Hoy 1977). A possible along-strike continuation to the south is the Seaman Creek fault (Leclair 1988; Figs. 2, 3), which separates domains with contrasting Neoproterozoic – Lower Palaeozoic stratigraphy. Leclair (1988) interpreted this fault, the Wilson Creek fault and the Midge Creek fault (Fig. 3) as major east-vergent contractional structures that stacked panels of rock from the continental margin and accommodated eastward translation of rocks belonging to the Quesnel and Slide mountain terranes. Transcurrent motion on the Midge Creek fault (see Fig. 5) was suggested by Vogl (1992), who noted that the most highly deformed rocks in the tail zone of the Nelson batholith occur where the fault marks the margin of the batholith. Fyles (1967) mapped three west-dipping faults in the Ainsworth area along the west side of Kootenay Lake. From west to east these are the Gallagher, Josephine, and Lakeshore faults (Fyles 1967; Figs. 2, 3). These faults are marked by zones of sheared, brecciated rock up to ⬃30 m thick, and in places by zones of localised folding (Fyles 1967). The Gallagher fault is particularly significant as it marks the location of an abrupt westward decrease in the intensity of D2 deformation. S2 forms the dominant west-dipping foliation in the footwall whereas in the hanging wall penetrative D2 deformation is absent and the fault cuts S1 at a high angle (Fig. 3). The Gallagher fault has been observed in two locations close to the eastern margin of the Nelson batholith during this study. The fault rock, which occupies a relatively narrow zone (less than ⬃30 m thick), comprises a mylonitised carbonate or phyllonite matrix with clasts and lenses of deformed quartz-rich, muscoviterich, and chlorite-rich rock (Fig. 4g). The origin of these fragments is uncertain but they are probably derived from adjacent metasedimentary rocks, quartz veins, and fragments of the Nelson batholith. Shear bands in this zone dip to the west, more steeply than foliation. The Nelson batholith on the west side of the fault zone is affected by an anastomosing network of dark deformation seams and contains abundant secondary muscovite. Fyles (1967) mapped the Lakeshore fault north of Kaslo, and showed it extending northwards parallel to Kootenay Lake. In contrast, Klepacki (1985) correlated the Lakeshore fault with the Schroeder fault, a large west-side-down structure that extends along strike to the NW where it is plugged by a ca. 180 Ma intrusion. For reasons outlined later in the text, we consider the former interpretation more likely. West-dipping normal faults were also mapped by Fyles (1964) in the Duncan Lake area, one of which projects into the NW margin of the Fry Creek batholith (Fig. 2). Published by NRC Research Press Moynihan and Pattison 777 50 30’ a) b) Chlorite/Biotite zone Garnet zone Staurolite zone 5590000 Kyanite zone Dun Sillimanite zone can Sil+Kfs zone 117 00’ r ie k k ac ee oc Gl Cr st 50 15’ Middle Jurassic 5570000 DM-07-133 DM-07-131 50 00’ Fry Creek batholith Kaslo DM-07-142 Gallagher fault see (b) Kootenay Lake 49 45’ DM-06-157 Nelson batholith Riondel 5550000 Crawford Bay Sc hr oe de see (c) 49 30’ Mid ge Cre ek fau lt ault nch f ll Tre Drewry Creek intr. 15 5530000 Steeple Mtn. Shaw Creek 30 km 510000 Wall stock Bayonne batholith 500000 117 00’ k ree tC Los lt fau 0 Fry Creek pluton lt DM-07-124 see (d) 49 15’ rf au DM-07-166 e Purc Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Lake ice Eocene Late Cretaceous/ Palaeocene mid-Cretaceous Published by NRC Research Press 778 Can. J. Earth Sci. Vol. 50, 2013 Fig. 5 (continued). Shore c) fault * DM-05-80 yC ree Lakeshore 5520000 k Gallagher fault DM-05-98 DM-05-88 DM-07-86 DM-07-58 5510000 * Co ee C ree k DM-07-99 y Lake G DM-05-47 DM-05-147 Koo tena Nelson batholith DM-05-04 G’ 5500000 DM-07-212 DM-05-53 DM-05-55 DM-07-178 DM-05-30 DM-05-44 DM-07-31 Crawford Bay stock DM-09-CBAG DM-09-CBA 520000 DM-07-192 510000 tor Procpluton Arm West 500000 Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. DM-06-163 Woo dbu r tock line s DM-06-164 Fry Creek batholith Published by NRC Research Press Moynihan and Pattison 779 Fig. 5 (concluded). d) DM-05-04 5500000 DM-07-212 DM-05-55 DM-05-53 DM-07-178 st Arm We DM-07-192 DM-07-31 Pro 5490000 DM-07-233 plu ldy Ba E pluton to n 92-MC-6.1 Creek Nelson batholith ult h fa renc ell T cto Purc r p luton DM-09-CBAG DM-09-CBA Heather Intrusions Late Cretaceous/Palaeocene E’ mid-Cretaceous (deformed/undeformed) Middle Jurassic (deformed/undeformed) Metamorphic Zones 5480000 e C re ek fa ul t Midge Creek stock Drewry Point intrusion dg Garnet zone Staurolite zone Kyanite zone Sillimanite zone Sil+K-spar zone Mineral Assemblages Chl St+Grt+Bt St+Bt St+And+Grt+Bt St+And+Bt Kfs+Sil+Grt+Bt Kfs+Grt+Bt Kfs+Sil+Bt Sil+Ky+St+Grt+Bt Sil+St+Grt+Bt Sil+St+Bt Sil+Ky+Grt+Bt Sil+Ky+Bt Sil+Grt+Bt Sil+Bt Crd+And+Grt+Bt Crd+And+Bt Crd+Sil+And+Bt Grt+Bt Grt+Ctd+Chl Grt+Bt+Chl & Wurtenberg Lake Ky+St+Grt+Bt Ky+Grt+Bt Ky+Bt Ky+And+St+Grt+Bt zones F’ (Leclair, 1988) 510000 Ctd 500000 F Chlorite/Biotite zone Observed contact aureole Mi Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Crawford Bay stock DM-05-44 And+Bt Bt Bt+Chl Ctd+Chl Chl And+Grt+Bt symbols with black outlines - this study symbols with red outlines - previous work And+Sil+Bt * Qtz+Ky vein Published by NRC Research Press Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. 780 Can. J. Earth Sci. Vol. 50, 2013 Fig. 6. Photomicrographs and photographs of metapelitic rocks from the central Kootenay Arc. (a) F2 crenulations in chlorite-bearing phyllite of the Slocan Group. Field of view is 6 mm; DM-06-124. (b) Chloritoid porphyroblast in Chl + Ms (muscovite) + Qtz-bearing phyllite of the Windermere Supergroup. Field of view is 1.5 mm; DM-07-GC1. (c) Garnet–biotite–chlorite-bearing metapelite from the garnet zone near the northern end of Kootenay Lake. Sigmoidal inclusion trails in garnet are interpreted to reflect rotation during syn-D2 growth; field of view is 6 mm; DM-07-131. (d) Garnet–chloritoid assemblage in aluminous metapelite of the Milford Group; field of view is 6 mm; DM-06-123. (e) F2 crenulations locally preserved in Grt–St schist of the Lardeau Group. Field of view is 3 mm; DM-07-86. (f) Large staurolite porphyroblasts on a weathered surface adjacent to a quartz vein. Although they retain their idioblastic shape, the porphyroblasts are largely pseudomorphed by chlorite and sericite. Quartz veins at this locality contain boudinaged kyanite crystals (inset); the inset photograph covers an area ⬃4 cm wide. Milford Group; DM-05-80. (g) Partially pseudomorphed staurolite crystal in metapelite of the Milford Group. Garnet forms small crystals that occur in the matrix and as inclusions in staurolite. Field of view is 12 mm. (h) Staurolite crystal partially replaced by kyanite; field of view is 2.5 mm; DM-05-88. (i) Kyanite and garnet porphyroblasts visible on etched surface of Sil–Ky–St–Grt schist; shoreline of Crawford Bay; DM-0504. (j) Photomicrograph of the sample shown in (i). Staurolite has been partly replaced by kyanite; sillimanite largely grew by replacement of biotite. Field of view is 6 mm; DM-05-04. (k) Sil–Ky–Grt schist of the Lardeau Group from the west shoreline of Kootenay Lake. Most of the rock is a kyanite–garnet–biotite schist; however, sillimanite is present in thin Sil-rich seams and Qtz–Sil–Plag (plagioclase) veins. Rutile (Rut) forms inclusions in some kyanite crystals. Field of view is 6 mm; DM-05-47. (l) Sillimanite crystals included in leucosome in K-feldspar + sillimanitebearing gneiss; Highway 3, west of Crawford Bay. Field of view is 6 mm; DM-05-147. (m) Garnet crystal in garnet-zone metapelite of the Lardeau Group. The matrix fabric, which is a crenulation cleavage (S2), wraps around the crystal, indicating post-garnet tightening of S2. Inclusion trails in the rim of the crystal also wrap around the inclusion-free core. These relationships are interpreted to result from growth of the garnet rim during progressive tightening of S2; field of view is 6 mm; DM-07-133. (n) Another garnet crystal from the same samples as e. F2 crenulations are included in the garnet rim, indicating growth of garnet rims after the onset of D2. Straight inclusion trails in the cores of other crystals in this sample suggest garnet started to grow before D2 fabrics developed. Field of view is 1.5 mm; DM-07-133. (o) Kyanite crystal that overgrew F2 crenulation; northernmost part of the kyanite zone; field of view is 3 mm; DM-07-142. (p) Contact metamorphic sillimanite overprinted by tight (F2) crenulations; Nelson batholith contact aureole south of the West Arm. Field of view is 6 mm; 92-MC-6.1; Slocan Group. Although most of the batholith is undeformed, this part contains a west-dipping foliation, a down-dip lineation and well-developed C–S fabrics indicating normal-sense motion (Reesor 1973; Warren 1997; Figs. 4h, 5b). As discussed later, the Gallagher fault, the Midge Creek fault, the Lakeshore fault and possibly also the Josephine fault, are interpreted to form partially overlapping strands of a west-dipping, Palaeocene–Eocene normal fault zone that marks an important metamorphic and geochronological discontinuity. This fault zone, which has a strike length of >80 km is herein referred to as the Gallagher fault zone (GFZ). The Gallagher fault is the bestcharacterised and centrally-located strand in this fault zone. The GFZ and PTF (a NE-side-down normal fault; Figs. 2, 3) are manifestations of a period of late orogenic extension that affected a broad zone in the internal part of the orogen. This has been attributed to transtension in the transfer zone between major dextral strike-slip faults (Price and Carmichael 1986; Harms and Price 1992). Regional metamorphism of the central Kootenay Arc The isograd pattern In map view the Barrovian garnet and higher-grade zones outline two joined, obliquely trending belts (Fig. 5). A N- to NNEtrending curved belt extends >100 km, from adjacent to the tail of the Nelson batholith to the area east of Duncan Lake (Fig. 5a). The axis of this belt is approximately parallel to the strike of stratigraphic units. Isograds in this part of the belt outline an elongate bull's-eye pattern with highest-grade rocks in the centre. The highest-grade region approximately coincides with the crest of the D2 structural culmination (described earlier in preceding text), which is marked by a reversal in the plunge of fold axes and lineations. The west side of the N–NNE-trending part of the amphibolite facies belt is cut by the GFZ, which marks a metamorphic discontinuity. To the east of the axis of the metamorphic high, grade decreases gradationally towards the centre of the Purcell anticlinorium. A SSE-trending fork of the amphibolite-facies belt is 10–15 km wide and extends >70 km, continuing across the Canada–USA border (Archibald et al. 1983). This belt is approximately parallel to the PTF, which marks its eastern boundary. Close to the Crawford peninsula, the PTF obliquely cuts strike-parallel isograds belonging to the N–NNE-trending belt. The two amphibolite-facies belts are separated in their region of overlap by a southward widening region of low metamorphic grade. Regional metamorphic zones The regional chlorite/biotite, garnet, staurolite, kyanite, sillimanite, and sillimanite + K-feldspar zones are described in the following text; in addition to major phases, all rocks contain accessory phases including tourmaline and (or) graphite. Ilmenite is the dominant oxide mineral. Rutile is present in some samples but is typically only preserved as inclusions in garnet or kyanite, suggesting it was not stable at peak temperature (T) conditions. Hematite is present in heavily altered rocks. Low-grade zones (chlorite and biotite zones) Compositional variability renders meaningful distinction between the chlorite and biotite zones difficult as many of the rocks are high-Al pelites that do not contain biotite at low grade. Metapelitic rocks in these zones are well-foliated phyllites (Fig. 6a), and typically contain the assemblage Chl + Ms + Qtz ± Bt ± Pl (Chl, chlorite; Ms, muscovite; Qtz, quartz; Bt, biotite; Pl, plagioclase). Chloritoid (Ctd) forms small porphyroblasts as part of the assemblage Ctd + Chl + Ms + Qtz + Pl at some localities in the Windermere Supergroup (Purcell anticlinorium; Fig. 6b), and in the Milford Group on the west side of Kootenay Lake. Leclair (1988) also reported, but did not document the location of, additional occurrences of chloritoid in the low-grade area south of the Midge Creek stock. Garnet zone Metapelitic rocks in the garnet zone are generally fine-grained schists. The diagnostic garnet (Grt)-zone assemblage Grt + Bt + Chl + Ms + Qtz ± Pl (Fig. 6c) occurs in a few localities in the north of the area (Fig. 5b). Hoy (1974) reported the assemblage Grt + Bt + Chl + Ms from a number of localities east of Riondel; however, we interpret chlorite in this area as a retrograde phase. Garnet in this and higher-grade zones is commonly partly replaced by chlorite, biotite, and other phases. The assemblage Grt + Ctd + Chl + Ms + Qtz occurs in Milford Group schists close to where the Gallagher fault intersects the Nelson Batholith (Fig. 5c), and east of the Gallagher fault near where the staurolite isograd diverges from the fault trace (SW of Kaslo; Fig. 6d). Some garnet crystals from the garnet and higher-grade zones (up to the sillimanite zone) display welldeveloped textural sector zoning (Moynihan and Pattison 2013). Published by NRC Research Press Moynihan and Pattison 781 a) b) Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Ctd F2 c) d) Ctd Grt pseudomorph e) f) Grt S2 St g) h) Grt Ky St St Published by NRC Research Press 782 Can. J. Earth Sci. Vol. 50, 2013 Fig. 6 (concluded). i) j) Ky Sil Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. St Grt k) l) Sil Sil Ky Rut m) n) S2 S2 o) p) F2 Sil Ky Grt Published by NRC Research Press Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Moynihan and Pattison Staurolite zone The staurolite (St)-zone assemblage St + Grt + Bt + Ms + Pl + Qtz is mostly developed in rocks of the Lardeau Group and Milford Group. Whereas the mineralogy of the rocks is similar, there are textural contrasts between metapelites from the two units. Staurolite-zone metapelites of the Lardeau Group (Fig. 6e) are rusty-weathering silvery schists containing garnet crystals approximately 0.5–1 cm in diameter in a mica-rich, well-foliated matrix. Staurolite occurs as porphyroblasts up to 1 cm long, or as small (⬃1 mm) blocky crystals commonly aligned in trains parallel to the foliation. In contrast, rocks of the Milford Group are what Fyles (1967) termed “knotted schists”. They are medium–dark grey, poorly foliated schists with large (up to 2.5 cm) staurolite crystals and small “pinhead” garnet crystals (approx. 1 mm) in a fine–medium-grained, commonly graphitic matrix (Figs. 6f, 6g). Milford Group schists on the west side of Kootenay Lake are extensively retrogressed. Most staurolite has been pseudomorphed, typically by chlorite and muscovite, but locally by chloritoid. In addition, much of the biotite has been replaced by secondary chlorite, which commonly contains rutile inclusions. Pseudomorphs of staurolite are idiomorphic, suggesting little deformation after their retrogression. Kyanite zone Textural contrasts between metapelitic schists in the Lardeau and Milford groups described earlier in the text also pertain to the kyanite (Ky) zone, which is defined by occurrences of the diagnostic assemblages Ky ± St + Grt + Bt + Ms + Qtz. Kyanite typically forms blocky to elongate crystals ranging from 1 mm to 1.5 cm long; however, as much of the kyanite has been partially replaced by muscovite, it is commonly inconspicuous in the field. Locally, kyanite has partially replaced staurolite crystals directly (Fig. 6h). A feature of the kyanite zone is the abundance of non-kyanitebearing (St + Grt) assemblages within its bounds, for example, on the west side of Kootenay Lake, and in the area north of the Fry Creek batholith. Quartz veins containing kyanite were observed at two locations in the Milford Group on the west side of Kootenay Lake (Figs. 5b, 6f). As the host schists contain staurolite but not kyanite, these were not used to constrain the position of the kyanite isograd. The Milford Group schist is extensively retrogressed, however, and it is possible that kyanite was once distributed more widely in these rocks. Sillimanite zone Metapelities in the sillimanite (Sil) zone contain the assemblage Sil ± Grt ± Ky ± St + Bt + Ms + Qtz + Pl (Figs. 6i–6k). Sillimanite typically forms aggregates of fine-grained crystals within biotite crystals, and (or) as idioblastic, elongate matrix columns; in most samples there is a gradation between these two modes of occurrence. Segregations of quartz and coarse-grained plagioclase are conspicuous in many sillimanite-zone metapelites. Sillimanite does not occur in contact with kyanite in rocks that contain both phases. Locally, sillimanite is restricted to thin, S2-parallel Sil-rich seams, Qtz–Pl–Sil veins, and their immediate margins, whereas the body of rock contains scattered idiomorphic kyanite crystals (Fig. 6k). Several occurrences of the assemblage Sil + Ky + St + Bt + Ms + Qtz + Pl (Figs. 5d, 6i, 6j) are interpreted to reflect metastable persistence of staurolite and kyanite into the stability field of sillimanite rather than an unusually low-variance equilibrium assemblage. Sillimanite + K-feldspar zone Hoy (1974) reported a single occurrence of coexisting K-feldspar, sillimanite (altered to white mica), muscovite, and quartz in the southwestern part of the Riondel peninsula. To the south, Crosby (1968) reported two other gneiss localities with K-feldspar west of Crawford Bay — one with sillimanite and biotite, the other with 783 garnet and biotite; muscovite was not reported from these locations. On the north side of Highway 3 (DM-05-147) west of Crawford Bay, outcrops of schist/gneiss contain abundant deformed pegmatite bodies and granitic leucosomes on a variety of scales. The host rocks contains sillimanite, biotite, and small garnet crystals (Fig. 6l). Muscovite is present, but much or all of it is retrograde. Granitic leucosome contains coarse-grained quartz, plagioclase, K-feldspar, and in places, sillimanite. K-feldspar is most common in coarse-grained leucosome but is also present in small crystals scattered through the rock matrix. K-feldspar (Kfs) is variably replaced by myrmekite and skeletal mica–quartz symplectic intergrowths (cf. Ashworth 1972), while sillimanite is partly replaced by muscovite. Formation and partial replacement of the peak metamorphic assemblage (Sil + Kfs) is interpreted to have resulted from prograde progress and subsequent reversal on cooling of the muscovite dehydration melting reaction: [1] Ms + Qtz + Pl = Sil + Kfs + melt The extent and significance of the sillimanite + K-feldspar zone is difficult to assess. The paucity of localities with this assemblage most likely reflects a combination of a small area that was metamorphosed at sufficiently high temperatures, and a low proportion of rocks with prograde muscovite. Hoy (1980) interpreted the abundant granitic (Kfs + Pl + Qtz ± Bt ± Ms ± Grt) pegmatite dikes, sills, and small plutons in the highest-grade parts of the amphibolite-facies belt to have formed by partial melting of surrounding metasedimentary rocks. Although these bodies were intruded during D2/M2, this is unlikely, as only a small proportion of rocks in the sillimanite + K-feldspar zone underwent muscovite dehydration melting. Except for unusually muscovite-rich rocks, this reaction produces small amounts of melt (⬃5%; Clemens and Vielzeuf 1987; Patino Douce et al. 1990) and cannot account for the voluminous granitic intrusions. It is more likely the melt was transported from deeper levels where other more significant melt-producing reactions (i.e., biotite dehydration) may have operated. Microstructures Microstructures allow assessment of the relative timing of metamorphism and structural development. The variation in D2 strain that is evident in major structures is also manifested in variable microstructures. In the northern part of the amphibolitefacies belt, and within the staurolite and kyanite zones on the west side of Kootenay Lake, S2 is commonly a crenulation cleavage (Figs. 6e, 6n, 6o), with S1 preserved in microlithons. This contrasts with the parts of the belt in the vicinity of Riondel–Crawford Bay and south of the West Arm, where D2 strain and metamorphic grade is highest. Here, S2 schistosity and L2 stretching lineations are generally the only structures preserved in the rocks. Informative garnet microstructures are preserved in the garnet–kyanite zones in the northern part of the area (north of the Fry Creek batholith). Here, interpretation of microstructures is aided by the fact that continuity is locally preserved between the foliation in the matrix and that preserved as inclusion trails in garnet porphyroblasts. F2 crenulations are included in some, but not all garnet crystals. Generally, F2 crenulations in garnet are more open than corresponding folds in the matrix. In some crystals, crenulations are included in the rim (Figs. 6m, 6n), whereas inclusions in the core are straight. These observations indicate a temporal overlap between garnet growth and D2 deformation. Further examples of microstructures in rocks from this area are illustrated in Moynihan and Pattison (2013). In the southern part of the amphibolite-facies belt, garnet porphyroblasts are wrapped by S2, with no continuity of included fabrics with the matrix Published by NRC Research Press 784 Can. J. Earth Sci. Vol. 50, 2013 Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Fig. 7. (a) Representative X-ray maps of garnet crystals from each of the metamorphic zones showing relative concentrations of the elements Fe, Mg, Mn, and Ca. Warmer colours (in the Web version) indicate higher concentrations. Lowest concentrations are indicated by shades of blue, intermediate concentrations by red–pink, and highest concentrations by yellow. Scale bars are 1 mm in all cases. (b–e) Compositional profiles for crystals shown in a; the position of the transects are shown in a. The staurolite zone example (DM-06-157) is from a rock containing the assemblage St + Grt + Bt + Ms + Pl + Qtz + Ilm. Although the sample is located in the kyanite zone, its garnet zoning characteristics are that of the staurolite zone, as processes related to kyanite formation did not affect this rock. Bs, backscatter image; Ksp, K-feldspar; M/(M+F), Mg/(Mg + Fe). fabric. Inclusion trails are typically straight or wavy, and commonly lie at a high angle to matrix S2. In an area of low D2 strain in the north of the area, staurolite and kyanite crystals overprint D2 crenulations (Fig. 6o), with little post-porphyroblast tightening of the fabric. Elsewhere, however, these minerals are bent around large garnet crystals, aligned in the plane of S2, host strain shadows, and locally form mineral lineations. Inclusion trails in staurolite range from straight to curved. Regional-metamorphic sillimanite invariably defines a shallow-plunging mineral lineation and lies in the plane of the S2 foliation. These observations indicate that peak Barrovian (M2) amphibolite-facies metamorphism overlapped with D2 deformation and that D2 generally outlasted peak metamorphism. Mineral compositions Mineral compositions were determined using a JEOL JXA-8200 Superprobe at the University of Calgary Laboratory for Electron Microbeam Analysis. Carbon-coated polished sections were analysed at 15 kV using a 20 nA beam and a range of natural and synthetic standards. Garnet, plagioclase, and staurolite were analysed using a 5 m beam diameter, with a 10 m spot size used for micas. Element-distribution maps of garnet grains were generated using WDS stage scans for the elements Fe, Mg, Ca, Mn, and Y. The mapping was conducted with a focussed beam using step sizes of 2–10 m, beam currents of 50–200 nA and dwell times of 30–50 ms. Representative compositional data for garnet and other minerals is included in Appendix A (see supplementary data).1 The most complete dataset from rocks across the full range of metamorphic grades is that of garnet compositions. Compositional zoning in garnet Compositional zoning in garnet was assessed by carrying out X-ray mapping on 26 crystals from 22 different rock samples, combined with spot analyses and quantitative compositional transects. Many garnet crystals are partially resorbed, and display Mn-enrichment at their rims (Kohn and Spear 2000); unresorbed examples are illustrated (Fig. 7) and discussed in the following text. Garnet crystals in rocks containing Grt + Bt or St + Grt + Bt assemblages share similar, mostly smoothly varying composition trends (e.g., Figs. 7a, 7b). The same compositional patterns are seen in rocks from the Grt zone, the St zone and rocks from the kyanite zone in which kyanite did not develop. Xsps (sps, spessartine) typically decreases outwards, from ⬃0.15–0.3 towards zero at the rim. Xalm (alm, almandine) typically falls in the range 0.5–0.8; it rises outwards from the core and flattens off towards the rim. Xprp (prp, pyrope) increases gradually outwards from core (<0.05) to rim (>0.1). Xgrs (grs, grossular) is relatively high in the core (typically 0.15–0.2); it is generally constant or decreases gradually through the core, and falls more steeply close to the rim. Mg/(Mg + Fe) increases outward from core to rim, generally with a steeper gradient towards the rim. Partially resorbed crystals display a sharp increase in Xsps at the rim, and commonly display further compositional modification around cracks. Moynihan and Pattison (2013) used thermodynamic forward modeling to replicate garnet zoning and calculate a pressure–temperature (P–T) path for a sample from the kyanite zone north of the Fry Creek 1Supplementary batholith. The P–T path has a clockwise form with an initial steep dP/dT, followed by an interval dominated by an increase in temperature (Fig. 10c). Compositional zoning in garnet from two samples with the assemblage Ky + St + Grt + Bt is broadly similar to that described earlier in the text, but each displays additional zoning features not seen in Ky-free samples. In DM-07-142, a metapelite containing Ky + St + Grt + Bt + Ms + Pl + Qtz + Ilm (ilmenite) (Fig. 7c), there is a low-Xgrs, high-Xalm band close to the rim (zone “B” in Fig. 7c). The outer margin of this band (the boundary between zones B and C) is marked by a discontinuity in all end-members; there is a significant rise in Xgrs, a drop in Xalm, a small rise in Xsps, and a small drop in Xprp. This step is interpreted to mark the location of a hiatus in garnet growth, with minor garnet dissolution. A possible explanation for this feature is that garnet stopped growing during formation of staurolite by a whole-rock equivalent of the KFMASH reaction: [2] Grt + Chl + Ms = St + Bt + H2O followed by renewed growth when kyanite entered the assemblage according to the reaction: [3] St + Ms = Ky + Grt + Bt + H2O If so, a separate explanation is required for the origin of the inner step (between zones A and B). This discontinuity, which is only evident in alm and grs end-members, may reflect the consumption of a minor phase, possibly an epidote group mineral (e.g., Menard and Spear 1993). A second sample with the assemblage Ky + St + Grt + Bt + Pl + Qtz + Ilm displays only a single step, similar to that developed at the boundary between zones B and C in DM-07-142. Garnet crystals from the sillimanite zone (Fig. 7d) do not show any discontinuities or sharp gradients. Instead, they are similar to those seen at lower grades, with relatively flat Xgrs in their cores, bell-shaped Xsps profiles, gradually increasing Xprp in the core with steeper increases near the rim, and outward increasing Xalm that flattens towards the rim. In addition, however, wellpreserved crystals feature a decrease in Xprp and Mg/(Mg + Fe) close to the rim. This is interpreted to reflect retrograde diffusional exchange with the rock matrix. Crystals from the Sil + Kfs zone show little zoning, apart from a small rimward increase in Xalm and decrease in Xprp and Mg/(Mg + Fe) (Fig. 7e). The flat zoning and decrease in Mg/(Mg + Fe) is similarly interpreted to result from diffusional homogenisation and retrograde exchange with the rock matrix. Bulk composition of metapelites and phase-diagram topology The bulk compositions of 46 metapelite samples, covering all stratigraphic units and metamorphic zones, were determined by X-ray fluorescence (XRF) using a Philips PW2440 4 kW automated spectrometer system at the McGill University geochemical labora- data are available with the article through the journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/cjes-2012-0083. Published by NRC Research Press Moynihan and Pattison (a) Fe 785 Grt zone St zone Fe Fe Ky zone Sil zone Fe Sil+Ksp zone Fe A .78 .79 A A B A B Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. B DM-07-133 DM-06-157 Mn Mn B .76 .76 .79 .77 DM-07-142 DM-07-192 Mn DM-05-147 Mn Bs, Xsps .06 .06 .06 .06 .06 .06 Mg Mg Mg Mg Mg .10 .09 .12 .12 .09 .11 Ca Ca Ca Ca Ca .06 .06 .06 .06 .06 .06 (b) 0.8 A B (c) A 0.75 B DM-07-142 DM-07-133 0.75 0.7 0.7 Xalm 0.65 0.65 Xprp Xsps Xgrs M/(M+F) 0.2 0.15 A B C 0.2 0.15 0.1 0.1 0.05 0.05 0.1 0.2 0.3 0.4 0.5 0.6 0.7 (d) A (e) 0.8 DM-06-157 1 2 3 4 5 6 B DM-07-192 0.8 0.75 0.7 0.65 0.60 0.2 0.15 Xalm 0.75 Xprp Xsps Xgrs M/(M+F) 0.14 0.7 0.12 0.1 0.1 0.08 0.05 0.06 0.04 0 1 2 3 4 5 mm 6 8 9 7 10 0 1 2 3 mm 4 5 Published by NRC Research Press 786 Can. J. Earth Sci. Vol. 50, 2013 Fig. 8. Modified AFM projection of metapelitic rocks analysed by XRF. The composition was simplified to the AFM system by projection from Ab, An, Ap, Ilm, Qtz, Ms, and where applicable, Po. The rocks have a limited range in Mg/(Mg + Fe), but span a wide range in A=. Metapelitic rocks of the Slocan and Milford groups generally plot above the garnet–chlorite tie line, whereas most samples from other units are less aluminous. Mineral composition fields from Spear (1993). ab, albite; an, anorthite; Ctd, chloritoid; Grt, garnet; Ky, kyanite; Sil, sillimanite; St, staurolite; Prl, pyrophyllite. A’ Slocan Gp. Milford Gp. Lardeau Gp. Hamill Gp./Mohican Fm. Windermere Sgp. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Ky, Sil, and, Prl + muscovite + quartz + H2O + apatite + ab, an + ilmenite + pyrrhotite St Ctd Crd Grt chlorite F’ M’ A’ = Al2 O3 - 3K2 O-Na2 O-{CaO-[(10/3)P2O5]} F’ = FeO - TiO2 - SO3 M’ = MgO tories, and abundances of S and C established by combustion infrared spectroscopy in the same laboratory (see supplementary data, Appendix B1). Compositional variation is illustrated using an aluminium–iron–magnesium (AFM) diagram (Fig. 8). The rocks span a considerable range in A= values (range, –19.8 to 51.3; mean, 24.74; standard deviation, 17.08), but most fall within a limited M=/(M= + F=) range (range, 0.31–0.63; mean, 0.40; standard deviation, 0.06). Metapelites of the Milford Group and most of those in the Slocan Group are aluminous and therefore plot above the Grt–Chl tie line, whereas the Lardeau Group and Windermere Supergroup span a large range in A=. To illustrate the effects of differences in bulk composition on predicted phase relations, phase diagrams were constructed in the chemical system MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3−SiO2− H2O–TiO2 (Fig. 9). Diagrams were produced using compositions representing average Milford Group (A= = 43; Mg/(Mg + Fe) = 0.37 and average Lardeau Group (A= = 29; Mg/(Mg + Fe) = 0.35 in MnNCKFMASHT) metapelites. The phase diagrams (Fig. 9) were calculated using the Gibbs free energy minimization software THERIAK/DOMINO (de Capitani and Brown 1987; de Capitani and Petrakakis 2010) in conjunction with version ds5.5 of the Holland and Powell (1998) database. Excess H2O was assumed, and a melt phase was not considered. Solution models are those described in Moynihan and Pattison (2013). Whereas there are large differences at lower temperatures, the two diagrams are very similar above the point at which biotite enters the assemblage (Fig. 9). The diagram representing the average Lardeau metapelite predicts biotite above ⬃450 °C, and a large field with the assemblage Grt–Bt–Chl. In the diagram representing the high-Al metapelite, biotite is only pres- ent above 550–600 °C, at a higher temperature than both the chloritoid field and the low-T limit of staurolite. The similarity in the position of the Ky ± St + Grt + Bt + Ms + Pl + Qtz + Ilm fields in both diagrams is of particular significance, as these assemblages help constrain the peak P–T conditions reached in higher-grade portions of the belt. P–T estimates from equilibrium assemblage diagrams Estimates of peak P–T conditions are determined by comparing observed mineral assemblages with those predicted in phase diagrams constructed for individual samples (Figs. 10a, 10b). These P–T estimates do not account for potential fractionation of the effective bulk composition due to garnet growth; however, this has a limited effect on the position of other major phase boundaries (e.g., Evans 2004; Zuluaga et al. 2005). DM-07-133 is a metapelitic schist from the garnet zone in the northern part of the area containing the assemblage Grt + Bt + Chl + Ms + Pl + Qtz + Ep + Ilm + Tur. As this assemblage (with zo/cz in place of epidote (Ep); zo, zoisite; cz, clinozoisite) is not predicted to be stable under any P–T conditions in the model system MnNCKFMASHT, modeling was carried out in the MnNCKFMASHTO system. An unknown variable is the Fe2+/Fe3+ ratio at the time of peak metamorphism; however, the observed assemblage is only predicted to be stable in cases where Fe3+ is less than ⬃6% of total Fe (magnetite appears at higher values of Fe3+), and there is little variation in P–T position within the range Fe3+/FeTotal = 0%–6% (Fig. 10a). The observed assemblage is predicted in this model system over a narrow P–T interval at ⬃500 °C, 5.5–6 kbar (1 kbar = 100 MPa) (Fig. 10a). Assemblages from higher-grade zones are adequately reproduced in MnNCKFMASHT. The main P–T constraints for Ky or Sil-bearing samples are provided by the low-T limit of Al2SiO5 minerals, the high-T limit of staurolite, the low-P limit of rutile and the Ky–Sil phase boundary (Fig. 10b). The high-P/low-T stability limit of plagioclase also provides an additional constraint for some of the samples. The upper pressure limit implied by stable ilmenite is ⬃7500–8000 bar, whereas the low-P limit for kyanite in these samples is typically ⬃6500 bar. P–T conditions for the assemblage Ky + St + Grt + Bt + Ms + Pl + Qtz + Ilm are best constrained, falling in the narrow range of 6500–7500 bar, 640–660 °C. The presence of Fe3+ would result in a displacement towards higher pressure of the ilmenite–rutile boundaries, but the positions of other boundaries are predicted to be insensitive to small variations in oxidation state. Peak P–T conditions of samples with both kyanite and sillimanite are more difficult to assess. In Fig. 10b, Sil + Ky-bearing assemblages are plotted along the Ky–Sil boundary, unless there is petrographic evidence that Ky is a relict phase. In these rocks, there is no textural evidence that sillimanite formed significantly later than other minerals as it invariably defines the gentlyplunging L2 lineation that is developed throughout the area; equally, there is no textural evidence that kyanite was out of equilibrium with other phases. It is likely that there was metastable persistence of kyanite in these rocks, but peak conditions may not have been far from the equilibrium boundary. The presence of the metamorphic K-feldspar + sillimanite in DM-05-147 implies P–T conditions above the muscovite dehydration reaction (Ms + Qtz + Pl = Sil + Kfs + melt). For a pressure range of 6.5–8 kbar, this suggests temperatures greater than approximately 690–710 °C (Spear et al. 1999). These P–T estimates help constrain the piezothermic array (Thompson and England 1984). The suggested array is approximately linear, with a positive slope (Fig. 10c), indicating greater amounts of post-peak-metamorphic exhumation in higher-grade parts of the belt. Published by NRC Research Press Moynihan and Pattison 787 Fig. 9. Simplified equilibrium assemblage diagram (EAD) for bulk compositions representing (a) average Lardeau Group (medium-Al) and (b) average Milford Group (high-Al) metapelites. The EADs were constructed in the 10-component system MnO–Na2O–CaO–K2O–FeO–MgO– Al2O3–SiO2–H2O–TiO2 (MnNCKFMASHT) system, using THERIAK-DOMINO software in conjunction with version db5.3 of the Holland and Powell (1998) database. A melt phase was not considered. Minor and (or) ubiquitous phases are omitted for clarity. 1 bar = 100 kPa. a) 9000 +Qtz+H20 MnNCKFMASHT Grt Bt Rut Grt Chl Rut Chlorite - in/out 8000 Biotite - in/out Chl Rut 6000 St Grt Bt Ilm Grt Bt Chl Ilm Garnet - in/out Sil Grt Bt Ilm 5000 Chl Ilm +M +K s fs Pressure (bar) 7000 4000 Bt Chl Ilm 3000 Staurolite - in/out And Grt Bt Ilm Average Lardeau Group A‘ = 29; Mg/(Mg+Fe) = 0.35 Crd Bt Ilm 2000 9000 Ky Grt Bt Rut Grt Chl Rut Ilm Grt Chl Rut 8000 Aluminosilicate - in/out MnNCKFMASHT +Qtz+H20 Ky Grt Bt Ilm Grt Chl Ilm 7000 St Grt Bt Ilm St Grt Chl Ilm Chl Rut 6000 Cordierite - in/out Chloritoid - in/out Sil Grt Bt Ilm 5000 Ctd Chl Ilm Sil Grt Bt Ilm + -P l Pl 4000 3000 Chl Ilm Average Milford Group A‘ = 43; Mg/(Mg+Fe) = 0.37 +M +K s fs b) Pressure (bar) Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Ky Grt Bt Ilm l -P l +P Grt Chl Ilm Ilmenite - in/out And Bt Ilm Crd And Bt Ilm Rutile - in/out 2000 400 500 600 700 Temperature (°C) Orientation of isogradic surfaces and thickness of metamorphic zones Vertical cross sections across the amphibolite-facies belt illustrate the orientation of isograds and their relationship to structures (Fig. 11). In the central and northern part of the belt (Fig. 11a), isograds are domed over Kootenay Lake. The core of the isogradic antiform is marked by rocks of the Sil + Kfs zone in the Riondel– Crawford Bay areas. To the north of this core, the axis of this isogradic antiform plunges gently to the north, whereas southwest of this culmination it plunges gently southwest. Isograds Published by NRC Research Press 788 Can. J. Earth Sci. Vol. 50, 2013 Fig. 10. (a) Equilibrium assemblage diagram for garnet zone sample DM-07-133 in the system MnNCKFMASHTO. The field containing the observed assemblage is shaded for different values of 100Fe3+/FeTotal. The other assemblage fields shown are for the case where Fe3+ represents 2% of total Fe. (b) P–T diagram showing the boundaries of peak metamorphic assemblages in aluminosilicate-bearing metapelitic rocks. The boundaries are taken from EADs constructed for each sample in the system MnNCKFMASHT. The locations of samples that are plotted are shown in Fig. 5. (c) Summary P–T diagram showing the different pressure ranges of Barrovian M2 metamorphism and contact metamorphism discussed in the text. The P–T path from Moynihan and Pattison (2013) is also shown, as is an approximate piezothermic array. (d) Schematic illustration of the contrasts in structural and metamorphic geology across the Gallagher fault. b) +Qtz+H2O Observed assemblage Grt Chl Bt Ms Pl Ep Ilm 1% Fe 3+ DM-07-99 (Ky-St-GrtBt-Ms-Pl-Ilm) DM-05-55 (Ky-Grt-BtMs-Pl-Ilm) 6% Fe 3+ 7.5 3+ 5% Fe St Grt Bt Ms Pl Ilm Grt Chl Ms Ilm Ep Pl Grt Chl Bt Ms Pl Ilm DM-05-44 (Ky-St-GrtBt-Ms-Pl-Ilm) 6.5 Grt Bt And Ms Pl Ilm DM-07-133 DM-05-53 (Sil-St-Grt-Bt-Ms-Pl-Ilm) 6 550 625 600 650 d) 9 675 Contact metamorphism St-And (3.5-4 kbar) @ 165 Ma 8 Jurassic-Early Cretaceous Bt cooling ages Peak M2 Barrovian regional metamorphism 7 othe piez rox. app 6 rmic 700 725 750 Temperature (°C) t 500 fa ul 450 DM-04-17 (St-Grt-Bt Ms-Pl-Ilm) 7 Grt Chl Ms Ilm Pl DM-05-04 DM-05-47 DM-07-166 DM-07-178 (Sil-Ky-GrtBt-Ms-Pl-Ilm) Gentle-Open F2 folds F2 er 5 3 DM-07-142 (Ky-St-GrtBt-Ms-Pl-Ilm) 8 2% Fe 3+ 4 c) 8.5 Grt Chl Bt Ms Ilm MnNCKFMASHT Tight F2 folds gh Pressure (kbar) Grt Chl Ms Ilm Ep Pressure (kbar) Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. 7 6 +Qtz+H2O MnNCKFMASHTO lla 8 Ga a) D3 shear bands F2 y arra Nelson batholith 5 mid-K plutons 4 Nelson batholith aureole Crawford Bay (65 Ma) Crd & And 3 Regional metamorphism at 6.5-7 kbar (23-25 km) Crawford Bay stock Mineral Zones: Grt, St, Ky, Sil, Sil + Kfs 2 400 Temperature (°C) 600 St, And Palaeocene-Eocene mica cooling ages 700 crosscut S2 at a high angle in the centre and on the east flank of the metamorphic antiform (Fig. 11a). Given the constraint that isograds dip to the east on the east flank of the isogradic dome, a minimum value for the thickness of the St + Ky zones around the centre of the isogradic culmination can be estimated by measuring the elevation difference between the St and Sil isograds along the line of section G–G=; this value (1.25 km) would be the thickness if the isograds were subhorizontal. A maximum value for the thickness of the two zones is given by the distance between the St and Sil isograds measured parallel to the land surface. This value is ⬃3.4 km. Tighter constraints rely on various assumptions. One endmember possibility is that isotherms were subhorizontal during meta- morphism, peak metamorphism was synchronous across the area, and the domal form of isogradic surfaces is due to post-peakmetamorphic differential exhumation. If it is assumed that there has been negligible post-metamorphic structural thickening/thinning of mineral zones, the difference in pressure between the intersections of the St-in and Sil-in curves with the piezothermic array (Fig. 10c) provides an estimate of the thickness of the combined St and Ky zones (~0.6 km = ~2.25 km). This value is in the centre of the range suggested above and is used in the cross section (Fig. 11a). The other end-member possibility — that all of the relief on isogradic surfaces was caused by domal isothermic surfaces at the time of peak metamorphism — is ruled out because the piezothermic array has a positive slope. It is possible, howPublished by NRC Research Press Moynihan and Pattison 789 Fig. 11. Cross sections showing the orientation of isograds and their relationships to structures. All cross sections are vertical, with equal vertical and horizontal scales. The metamorphic zones are colour-coded according to the scheme used in Fig. 5. (a) Cross section G–G= (Fig. 5c) across the crest of the isogradic antiform, showing truncation of the western flank of the isogradic antiform by the Gallagher fault. Gently west-dipping isobars in the Nelson batholith are also plotted (Pattison and Vogl 2005). The Riondel Nappe (Hoy 1976) and adjacent structures are projected onto the line of section. Isograds cut structures at moderate to high angles. (b) Metamorphic cross section along the line E–E= (Figs. 2, 3, 5d). Isograds define an open synform. The crest of the isogradic antiform to the west is cut by the Midge Creek fault. (c) Metamorphic section along line F–F= (Figs. 2, 3, 5d). A large low-grade region sits in the core of an isogradic synform. A sillimanite + K-feldspar zone is not illustrated in b and c, as it has not been mapped in this area, but presumably exists at depth. a) Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. G r) 3.5 kba r) 3.5 kba And (~ r) a b 3.5-4 k nd (<~ Crd+A St+And lt fau r he ag l l Ga G’ 5 km 4 km 3 km 2 km 1 km 0 km Mt. Hooker (~ th batholi Nelson Purcell Trench b) E Midge Creek fault fault Purcell Trench fault E’ 2500 m 2000 m 1500 m 1000 m 500 m c) F Midge Creek fault Purcell Trench fault F’ 2500 m 2000 m 1500 m 1000 m 500 m ever, that isothermic surfaces were domal to some degree at the time of metamorphic quenching, and postmetamorphic processes enhanced the domal form of isogradic surfaces. South of the West Arm of Kootenay Lake, a gently southplunging isogradic synform separates the southern part of the N–NNE trending belt from SSE-trending part of the amphibolitefacies belt. (Figs. 11b, 11c). As is the case further north, isograds transect fold axial planes and associated foliations. It is likely that isogradic surfaces in the SSE-trending amphibolite-facies belt also have an antiformal geometry; there is no evidence for an eastward increase in metamorphic grade in the hanging wall of the PTF, as would be expected if the fault cut westward-dipping isograds. In much of the belt the garnet, staurolite, kyanite, and sillimanite zones have similar thicknesses to one another. Using the phase diagrams (Figs. 9, 10c) and piezothermic array (Fig. 10c) as a guide, broadly similar thicknesses for the staurolite, kyanite and possibly the sillimanite zones would be expected, with a significantly thicker garnet zone. North of the Fry Creek batholith, on the unfaulted eastern flank of the amphibolite-facies belt, the garnet zone is thicker than other metamorphic zones, but elsewhere is has a comparable thickness. The anomalously thin nature of the garnet zone in much of the area may reflect bulk compositional control of the position of the garnet isograd. Metamorphic zones that outline the isogradic synform region south of the Midge Creek stock (Fig. 5d) are poorly defined, but appear thinner than those in the N–NNW-trending belt. It is likely that the apparently condensed nature of metamorphic zones rePublished by NRC Research Press 790 flects, at least in part, a paucity of observations from appropriate metapelitic lithologies. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Contact metamorphism Nelson batholith aureole A contact aureole is developed around unfaulted margins of the Nelson batholith in metapelitic rocks of the Slocan Group and correlative units (Pattison and Vogl 2005). The batholith was emplaced at a pressure of 3–4 kbar, equivalent to a depth of 11–14.5 km (assuming an average crustal density of 2800 kg/m3) and overprints D1 (Early-Middle Jurassic) structures. Based on the distribution of pressure-sensitive mineral assemblages in various parts of the aureole, Pattison and Vogl (2005) inferred post-emplacement tilting of the batholith ⬃10° to the west. A wide (approx. 1.75 km) contact aureole, characterised by St + And ± Grt assemblages at medium grade is developed in the hanging wall of the Gallagher fault in the region approximately north of Woodbury Creek (Fig. 5c). St + And-bearing assemblage, which are typical of the eastern parts of the Nelson aureole (Pattison and Vogl 2005) indicate emplacement at 3.5–4 kbar, equivalent to a depth of 12.5–14.5 km (Fig. 9). Further evidence that primary intrusive relationships are preserved in this area is provided by a localised zone of folding related to emplacement of the batholith (Fyles 1967). To the south, along the eastern side of the deformed “tail” of the Nelson batholith, a contact aureole is narrow or absent. At Coffee Creek (Fig. 5c), the eastern margin of the batholith is faultbounded, and is in direct contact with rocks of the regional (M2) kyanite zone. South of here, there is a narrow panel of rock separating the tail of the batholith from Barrovian amphibolite-facies rocks in the footwalls of the Gallagher and Midge Creek faults. The regional metamorphic grade in this panel is low, and though a narrow contact aureole is locally present, it appears to be absent elsewhere (Vogl 1992; this work). Contact metamorphic assemblages were studied at two locations within this panel (Fig. 5d) and in each location contact-metamorphic sillimanite is overprinted by tight F2 crenulations (Fig. 6p). The limited evidence for contact metamorphism along the eastern part of the Nelson batholith tail is interpreted to reflect deformation along this margin after (D2), and possibly also during (Vogl 1992), its emplacement. Mid-Cretaceous contact aureoles Mid-Cretaceous plutons crosscut Barrovian isograds, and their andalusite-bearing contact aureoles record exhumation of rocks in the area to upper crustal levels after amphibolite-facies (M2) regional metamorphism. The Fry Creek batholith intruded across the northern part of the east flank of the domal metamorphic culmination (Fig. 5a). Reesor (1973) reported assemblages containing combinations of andalusite, cordierite, garnet, staurolite and sillimanite from the contact aureole of the batholith. Whereas a contact metamorphic aureole is well-developed in its eastern part, an aureole was not identified in the western part, where it intruded rocks of higher regional metamorphic grade (Reesor 1973). An andalusite-bearing contact aureole surrounds the 114 +1/–3 Ma Midge Creek stock and occupies a broad region to its south, between it and an extension of the Drewry Point intrusion (Fig. 5d). Assemblages in this region reported by Leclair (1988) include And + Bt ± Grt, And + St + Grt + Bt and Sil + And + Bt. And-bearing contact metamorphic assemblages are superimposed on kyanite-bearing regional assemblages on the northern margin of the Midge Creek stock. And + Bt ± Grt and And + St + Grt assemblages are developed around a small intrusion near Wurtenberg Lake (Fig. 5d), whereas contact metamorphism around the Drewry Point intrusion produced And + St + Grt, Sil + Grt + Bt, and Crd + Bt-bearing assemblages (Leclair 1988). The reported occurrence of St + Can. J. Earth Sci. Vol. 50, 2013 And- and Crd-bearing assemblages from the contact aureoles of mid-Cretaceous intrusions suggests variation in the depth of emplacement within and between intrusions (c.f. Pattison and Vogl 2005), and warrants further investigation. Crawford Bay stock aureole The Crawford Bay stock is a small, equigranular to porphyritic biotite granodiorite (Crosby 1968) that is exposed on either side of Crawford Bay, on the east side of Kootenay Lake. The stock was intruded during the latest Cretaceous-Palaeocene (Moynihan 2012) and crosscuts D2 folds, fabrics and regional metamorphic isograds (Fig. 2.5c); it has undergone minor ductile deformation but is not foliated or lineated. On the eastern side of the stock, a contact aureole is developed in low-grade rocks of the Windermere Supergroup. Metapelitic rocks contain the assemblages Crd + And + Bt + Ms + Qtz ± Grt and And + Sil + Crd + Bt + Ms + Qtz (Crosby 1968; Fig. 5d), indicating emplacement at low pressure (approximately 6–10 km depth; Moynihan 2012). The western part of the stock intruded rocks of the regional staurolite zone (Fig. 5d). Andalusite is present in some metapelitic rocks near this contact, where it forms large, generally poikiloblastic crystals that are typically elongate in the plane of the foliation. Folding of partly sericitised contactmetamorphic andalusite crystals around F4 crenulations demonstrates a latest Cretaceous-Palaeocene or younger age for D4. Discussion Formation and geometry of Early Cretaceous isogradic surfaces Barrovian amphibolite-facies metamorphism in the central Kootenay Arc took place in the Early Cretaceous during D2. The metamorphism is interpreted as a consequence of burial and radiogenic heating. D2 structures are compatible with crustal thickening and the P–T path calculated by Moynihan and Pattison (2013) suggests burial of ⬃2 kbar (~7 km) during garnet growth in the kyanite zone. The elongate domal geometry that characterises isogradic surfaces in the N–NNE-trending part of the Barrovian belt is interpreted to have formed during D2/M2, prior to the intrusion of mid-Cretaceous intrusions that crosscut dipping isogradic surfaces. Isograds transect F2 axial planes at a high angle, suggesting they were established after F2 folds formed and much of the related shortening had taken place; however, deformation of peak metamorphic minerals indicates that, in general, D2 outlasted peak metamorphism, and it is likely that there was some further tightening of these folds after isograd formation. As S2 is at a moderate to high angle to isograds, late (post peak metamorphic) D2 shortening may have resulted in some thickening of mineral zones and modification of the isogradic geometry. The highest-grade rocks in the central Kootenay Arc (Sil + Kfs zone) are located close to the crest of the structural culmination, where there is a change in the plunge direction of F2 folds and L2 lineations (Fyles 1967). To the north of this culmination, F2/L2 plunge gently north, whereas to its south, L2/F2 plunge gently to the south-southwest. The angle of plunge of minor folds to the north of the culmination crest is approximately 5°–20° (Fyles 1964; Hoy 1980). Using the map and cross sections as a guide, a comparison can be made between these plunge values and the plunge of isogradic surfaces. The combined thickness of the staurolite and kyanite zones is ⬃2.25 km (range of 1.25–3.4 km). Combining this thickness estimate with the distance between the staurolite and sillimanite isograds along the axis of the isogradic antiform (Fig. 5) yields an average plunge of 4°–10° (7° using the preferred thickness value). As the isogradic antiform has a subhorizontal axis in its crestal region (Sil + Kfs zone), this is an overestimate of the average plunge from the crest of the isogradic culmination to the northern tip of the staurolite zone. The estiPublished by NRC Research Press Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Moynihan and Pattison mated plunge of isogradic surfaces is at the low end of the range of minor fold plunges, most of which suggest greater amounts of structural relief. This difference could be explained by a discrepancy between the orientation of minor folds and large-scale plunge, or the possibility that the isograds were “frozen in” to the rock after growth of the culmination had commenced. The piezothermic array (Fig. 11b) suggests that higher-grade rocks have been exhumed from deeper levels. Assuming the piezothermic array is representative of the entire belt, this implies that there was Early Cretaceous differential exhumation, concentrated around the crest of the culmination. Rather than simply recording the presence of a thermal dome, the isogradic geometry was, at least in part, acquired as a result of progressive growth of the structural culmination during the later stages of D2. The westward tilt of the Nelson batholith may have been acquired during formation of this metamorphic and structural culmination. S3 shear bands, which dip NW and formed by reactivation of S2, are developed in the NW quadrant of the culmination. Their age is poorly constrained but it is possible that they are a manifestation of Early Cretaceous, post-peak-metamorphic doming. Large relief on isogradic surfaces has been reported in other parts of the southeastern Cordillera that record Early Cretaceous Barrovian metamorphism (Ghent et al. 1980; Digel et al. 1998; Ghent and Simony 2005). In these areas, curviplanar isogradic surfaces were interpreted to reflect the thermal-baric structure at peak metamorphic conditions rather than being a consequence of post-metamorphic differential exhumation. Isograds that define the SSE-trending part of the amphibolitefacies belt are poorly constrained, but their geometry also suggests that relief on isogradic surfaces developed during the Early Cretaceous. The Midge Creek stock crosscuts the hinge zone of the south-plunging isogradic synform and west-dipping isogradic surfaces of the SSE-trending belt are cut by the Heather Creek pluton and Drewry Creek intrusion (Fig. 5d). There is no indication from the orientation of structures documented by Leclair (1988) that the attitudes of Early Cretaceous structures or isogradic surfaces were significantly modified by post-metamorphic doming or by motion on the PTF (Fig. 11). Following Barrovian metamorphism, kyanite zone rocks in the vicinity of the Midge Creek stock were exhumed to <4.5 kbar (<14.5 km depth; Fig. 10c) by ca. 114 Ma, as evidenced by the presence of andalusite in its contact aureole (Leclair 1988; Leclair et al. 1993). Similarly, andalusite developed in the contact aureoles of the Fry Creek Stock (Reesor 1973), Drewry Point intrusion and other mid-Cretaceous bodies that crosscut Early Cretaceous Barrovian isograds. Relationship to Late Cretaceous metamorphism and deformation in the Priest River complex The SSE-trending part of the amphibolite-facies belt extends beyond the area covered in Fig. 4, across the USA–Canada border into the Priest River complex (Doughty et al. 1997; Doughty and Price 1999; Fig. 1). Peak metamorphism and deformation in the Priest River complex took place in the Late Cretaceous – Eocene (Doughty and Price 1999; Doughty and Chamberlain 2004). Late Cretaceous (ca. 82 Ma) metamorphism and deformation also affected the SSE-trending amphibolite-facies belt in the area west of Creston (Fig. 4), around the southern end of Kootenay Lake (Brown et al. 1999). Late Cretaceous metamorphism and deformation did not affect the northern part of this belt, however, as the 95 Ma Steeple Mountain pluton (Brown et al. 1999; see Fig. 5) is locally foliated adjacent to the PTF but not otherwise penetratively deformed (Leclair 1988). The Drewry Creek intrusion and Heather Creek pluton, intrusions of presumed (but unconfirmed) midCretaceous age (Leclair 1988) are undeformed and crosscut regional isograds (Fig. 5a). While forming a continuous region with an apparently simple isograd pattern, this belt evidently has a polymetamorphic origin. 791 Palaeogene normal faults and their role in exhumation of the belt The amphibolite facies belt that transects the central Kootenay Arc is partially bounded by two normal faults or fault zones — the PTF and the GFZ. The PTF has long been recognised as a significant Eocene structure, but the significance of the GFZ, which forms the western margin of the Barrovian belt, has hitherto not been fully appreciated. Palaeocene–Eocene normal faulting augmented the isogradic structure that was established during the Early Cretaceous, and juxtaposed regions with contrasting structural and metamorphic histories along fault-bounded margins of the amphibolite-facies belt. The GFZ The western flank of the Barrovian metamorphic antiform is truncated by the GFZ. From the West Arm of Kootenay Lake northwards to near Kaslo, the Gallagher fault separates M1 chlorite/ biotite zone hanging wall rocks from M2 amphibolite facies (staurolite–kyanite zone) rocks in the footwall. Low-grade hanging wall rocks record a simple Middle Jurassic metamorphic and deformation history, whereas amphibolite-facies footwall rocks underwent further metamorphism and deformation during the Early Cretaceous (Fig. 11d). The Gallagher fault marks the western boundary of penetrative D2 deformation, the western boundary of amphibolite-facies metamorphism that was concurrent with D2, and the western boundary of young 40Ar/39Ar mica cooling ages (Figs. 11d, 12). It also marks the western boundary of D3 structures. Palaeocene–Eocene K–Ar cooling ages in the immediate footwall (Mathews 1983; Archibald et al. 1984; Fig. 12) suggest that it, like the PTF, is a Palaeogene extensional fault. To the east, K–Ar biotite cooling ages increase gradually towards the centre of the Purcell anticlinorium (Archibald et al. 1984). South of the West Arm of Kootenay Lake, geological relationships are similar to those further north: a west-dipping fault (in this case the Midge Creek fault) separates Barrovian M2 kyanite or sillimanite-zone rocks from low-grade phyllites of the Milford Group, which lie east of a narrow, locally developed contact aureole adjacent to the Nelson batholith. The Midge Creek fault has previously been interpreted as a thrust or a strike-slip fault (Leclair 1988; Vogl 1992), whereas the relationships described earlier in the text suggest normal faulting. One possibility is that a thrust-sense/strike-slip fault was reactivated during Palaeocene– Eocene normal faulting. Alternatively, the Gallagher fault may extend south of the West Arm and cut the older Midge Creek fault at a low angle. Although the Gallagher fault marks the western limit of D2 penetrative strain north of where it forms the eastern boundary of the Nelson batholith, deformation of contactmetamorphic sillimanite suggests some D2 strain affected rocks in the hanging wall of the Midge Creek fault. The Gallagher fault dies out around Kaslo, and north of here the western boundary of amphibolite-facies rocks is marked by the Lakeshore fault. The garnet isograd leaves the Gallagher fault around its termination and crosses a relay zone between these two structures, which are interpreted as individual strands of a west-dipping normal fault zone. Interpretation of the Lakeshore fault as a Palaeogene normal fault conflicts with Klepacki's (1985) suggestion that the Lakeshore fault is part of a long, strike-parallel pre-Middle-Jurassic fault – the NW-trending Schroeder fault. We favour Fyles' (1967) interpretation (his Fig. 2) that the Lakeshore fault continues north, approximately parallel to Kootenay Lake. Given their relative ages, it is likely that the Lakeshore fault cuts the Schroeder fault, rather than being part of it. Around the northernmost part of Kootenay Lake and Duncan Lake, the western boundary of the amphibolite-facies belt does not coincide with known faults. Normal faults were mapped in the area north of the Fry Creek batholith by Fyles (1964) and a normal-sense shear zone overprints the NW margin of the mid-Cretaceous Fry Creek batholith. The zone of normal faulting may step eastwards in this Published by NRC Research Press 792 Can. J. Earth Sci. Vol. 50, 2013 Fig. 12. Map showing K–Ar and 40Ar/39Ar dates from the area. Data from Archibald et al. (1984) and references therein. 50 30’ 120 117 00’ 50 15’ 45,(63) (56) 50 00’ 53 [161] 50,(51) Kaslo 120 146 135 51 130 (59) 48,(51) {53} {131} 50,(55) 45 54 52,(60) {67} 63 {75} {67} Ainsworth 53, [126] 56,[160] A West 54 54 rm 60 [104] 62 48 [83] 49 30’ PTF 47,(50) 51 51,(52) [161] 111 0 ult k fa ree xt C Ne [157] 49 15’ 107 142 81 103 79 50 83 92 43,(51) 89 82 90 51,(54) 45,(50) 41,(49) 72 54 53,(50) 51 52 44,(50) 71 49 39 15 Thanks to Travis Murphy, Hannah Mills, Gennyne McCune, and Chris Coueslan for assistance in the field, to Rob Marr for electron microprobe support, and to Doug Tinkham for providing solution models used in thermodynamic modeling. The paper benefitted from criticisms of an early draft by Philip Simony and Sharon Carr and from recommendations provided by associate editor Marc St-Onge and anonymous journal reviewers. Funding was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) discovery grants to D.R.M. Pattison and P. Simony and by the Geological Survey of Canada through the Targeted Geoscience Initiative (TGI-3). References 49 [131] [99] The PTF The east side of the SSE-trending fork of the amphibolite-facies belt is bounded by the Eocene Purcell Trench normal fault. This fault forms the eastern boundary of the Priest River metamorphic complex (Doughty and Price 1999) to the south of the central Kootenay Arc (Fig. 1) and coincides with the location of an antecedent east-side-down normal fault that was active during the Neoproterozoic (Leclair 1988). The fault extends northwards across the international border and occupies the southern part of Kootenay Lake before dying out around the latitude of the West Arm. Along the southern, NW-trending portion of Kootenay Lake, the fault separates sillimanite zone rocks with Palaeocene– Eocene 40Ar/39Ar mica cooling ages in the footwall from biotite zone rocks with mostly mid- to Late Cretaceous cooling ages (Fig. 12). North of its termination, the sillimanite zone extends across the Purcell Trench (occupied by Kootenay Lake), and there is little, if any, difference in 40Ar/39Ar cooling ages (Fig. 12). Contrasts in cooling ages demonstrate differential exhumation across the fault during the Eocene; however, as discussed earlier in the text, isogradic relief in the footwall of the fault existed prior to its formation. As is the case with the GFZ, the PTF modified an isogradic geometry that was established during Early Cretaceous metamorphism and deformation. Acknowledgements (70) 49 45’ GFZ Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. biotite (muscovite) [hornblende] {whole rock} region and continue northwards, in a diminished fashion, as suggested by Warren (1997). Restoration of apparent stratigraphic cut-offs in the cross section across the Kootenay Arc by Fyles in Brown et al. (1981) suggests offset of ⬃7.5 km (throw of ⬃5 km) on the Gallagher fault in the Ainsworth–Kalso area (Fig. 3c). This estimate is similar to that suggested by juxtaposition of rocks of the kyanite and sillimanite zones against low-grade phyllites (Figs. 6a, 11a). The Josephine and Lakeshore faults also separate lithological units for considerable distances along strike, suggesting they are significant structures. The faults are approximately parallel to lithological boundaries, however, and some of the structural complexity in this zone may arise from earlier compressional structures, as is the case along strike (Klepacki 1985; Leclair 1988). 30 km Acton, S.L., Simony, P.S., and Heaman, L.M. 2002. Nature of the basement to Quesnel Terrane near Christina Lake, southeastern British Columbia. Canadian Journal of Earth Sciences, 39: 65–78. doi:10.1139/e01-056. Archibald, D.A., Glover, J.K., Price, R.A., Farrar, E., and Carmichael, D.M. 1983. Geochronology and tectonic implications of magmatism and metamorphism, southern Kootenay Arc and neighbouring regions, southeastern British Columbia. Part I: Jurassic to mid-Cretaceous. Canadian Journal of Earth Sciences, 20: 1891–1913. doi:10.1139/e83-178. Archibald, D.A., Krogh, T.E., Armstrong, R.L., and Farrar, E. 1984. Geochronology and tectonic implications of magmatism and metamorphism, southern Kootenay Arc and neighbouring regions, southeastern British Columbia. Part II: Mid-Cretaceous to Eocene. Canadian Journal of Earth Sciences, 21: 567–583. doi:10.1139/e84-062. Ashworth, J.R. 1972. Myrmekites of exsolution and replacement origins. Geological Magazine, 109: 45–62. doi:10.1017/S0016756800042266. Brandon, A.D., and Lambert, R. STJ. 1993. Geochemical characterisation of midCretaceous granitoids of the Kootenay Arc in the southern Canadian Cordillera. Canadian Journal of Earth Sciences, 30: 1076–1090. doi:10.1139/e93-091. Published by NRC Research Press Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. Moynihan and Pattison Brown, D., Doughty, T., Glover, K., Archibald, D., and Pattison, D.R.M. 1999. Field trip and road log: Purcell Anticlinorium to the Kootenay Arc, Southeastern British Columbia: Highway 3 - Creston to Summit Pass, and Northern Priest River Complex, West of Creston. B. C. Ministry of Energy, Mines and Petroleum Resources, Information Circular 1999-2. Brown, R.L., and Lane, L.S. 1988. Tectonic interpretation of west-verging folds in the Selkirk Allochthon of the southern Canadian Cordillera. Canadian Journal of Earth Sciences, 25: 292–300. doi:10.1139/e88-031. Brown, R.L., Fyles, J.T., Glover, J.K., Hoy, T., Okulitch, A.V., Preto, V.A., and Read, P.B. 1981. Southern Cordillera Cross-section - Cranbrook to Kamloops. In Geological Association of Canada, Field Guide to Geology and Mineral Deposits. Edited by R.I. Thompson and D.G. Cook. Calgary ‘81 Annual Meeting, pp. 335–343. Carr, S.D., and Simony, P.S. 2006. Ductile thrusting versus channel flow in the southeastern Canadian Cordillera: evolution of a coherent crystalline thrust sheet; Geological Society, London, Special Publications 268. pp. 561–587. Carr, S.D., Parrish, R.R., and Brown, R.L. 1987. Eocene Structural development of the Valhalla Complex, southeastern British Columbia. Tectonics, 6: 175–196. doi:10.1029/TC006i002p00175. Clemens, J.D., and Vielzeuf, D. 1987. Constraints on melting and magma production in the crust. Earth and Planetary Science Letters, 86: 287–306. doi:10. 1016/0012-821X(87)90227-5. Colpron, M., and Price, R.A. 1995. Tectonic significance of the Kootenay terrane, southeastern British Columbia: An alternative model. Geology, 23: 25–28. doi:10.1130/0091-7613(1995)023<0025:TSOTKT>2.3.CO;2. Colpron, M., Price, R.A., Archibald, D.A., and Carmichael, D.M. 1996. Middle Jurassic exhumation along the western flank of the Selkirk fan structure: Thermobarometric and thermochronometric constraints from the Illecillewaet synclinorium. Geological Society of America Bulletin, 108: 1372–1392. doi:10.1130/0016-7606(1996)108<1372:MJEATW>2.3.CO;2. Colpron, M., Logan, J.M., and Mortensen, J.K. 2002. U-Pb zircon age constraint for late Neoproterozoic rifting and initiation of the Lower Palaeozoic passive margin of western Laurentia. Canadian Journal of Earth Sciences, 39: 133– 143. doi:10.1139/e01-069. Crosby, P. 1968. Tectonic, plutonic, and metamorphic history of the Central Kootenay Arc, British Columbia, Canada. Geological Society of America, Special Paper 99, 94 p. Crowley, J.L., Ghent, E.D., Carr, S.D., Simony, P.S., and Hamilton, M.A. 2000. Multiple thermotectonic events in a continuous metamorphic sequence, Mica Creek area, southeastern Canadian Cordillera. Geological Materials Research, 2: 1–45. doi:10.1007/978-3-642-56949-4_1. Cubley, J.F., Pattison, D.R.M., Tinkham, D.K., and Fanning, C.M. 2012. U-Pb geochronological constraints on the timing of metamorphism and high-T exhumation in the Grand Forks complex, British Columbia. Lithos, 156-159: 241–267. doi:10.1016/j.lithos.2012.10.015. Cubley, J.F., Pattison, D.R.M., Archibald, D.A., Hamilton, B.M., and Jolivet, M. 2013. Thermochronological constraints on the Eocene exhumation of the Grand Forks complex, British Columbia, based on 40Ar/39Ar and apatite fission track geochronology. Canadian Journal of Earth Sciences, 50(5): 576– 598. doi:10.1139/cjes-2012-0058. Currie, L. 1988. Geology of the Allen Creek area, Cariboo Mountains, British Columbia. Unpublished Ph.D. thesis, University of Calgary, Calgary, Alberta, 152 p. de Capitani, C., and Brown, T.H. 1987. The computation of chemical equilibrium in complex systems containing non-ideal solutions. Geochimica et Cosmochimica Acta, 51: 2639–2652. doi:10.1016/0016-7037(87)90145-1. de Capitani, C., and Petrakakis, K. 2010. The computation of equilibrium assemblage diagrams with Theriak/Domino software. American Mineralogist, 95: 1006–1016. doi:10.2138/am.2010.3354. Devlin, W.J., and Bond, G.C. 1988. The initiation of the early Paleozoic Cordilleran miogeocline: Evidence from uppermost Proterozoic-Lower Cambrian Hamill Group of southeastern British Columbia. Canadian Journal of Earth Sciences, 25: 1–19. doi:10.1139/e88-001. Digel, S.G., Ghent, D.G., Carr, S.D., and Simony, P.S. 1998. Early Cretaceous kyanite-sillimanite metamorphism and Palaeocene sillimanite overprint near Mount Cheadle, southeastern British Columbia: geometry, geochronology, and metamorphic implications. Canadian Journal of Earth Sciences, 35: 1070–1087. doi:10.1139/e98-052. Dostal, J., Church, B.N., and Hoy, T. 2001. Geological and geochemical evidence for variable magmatism and tectonics in the southern Canadian Cordillera: Paleozoic to Jurassic suites, Greenwood, southern British Columbia. Canadian Journal of Earth Sciences, 38: 75–90. doi:10.1139/e00-078. Doughty, P.T., and Chamberlain, K.R. 2004. New U-Pb shrimp evidence for multiple metamorphic events in the northern U.S. Cordillera: Eocene, Cretaceous, and late Precambrian (Grenville) events. Geological Society of America Abstracts with Program, volume 36: No. 5, p. 271. Doughty, P.T., and Price, R.A. 1999. Tectonic evolution of the Priest River complex, northern Idaho and Washington: A reappraisal of the Newport fault with new insights on metamorphic core complex formation. Tectonics, 18: 375–393. doi:10.1029/1998TC900029. Doughty, P.T., Brown, D.A., and Archibald, D.A. 1997. Metamorphism of the Creston map area, southeastern British Columbia; Ministry of Employment and Investment, Open File 1997-5. Evans, T.P. 2004. A method for calculating effective bulk composition modifica- 793 tion due to crystal fractionation in garnet-bearing schist: implications for isopleth thermobarometry. Journal of Metamorphic Geology, 22: 547–557. doi:10.1111/j.1525-1314.2004.00532.x. Evenchick, C.A., McMechan, M.E., McNicoll, V.L., and Carr, S.D. 2007. A synthesis of the Jurassic-Cretaceous tectonic evolution of the central and southeastern Canadian Cordillera: exploring links across the orogen. Geological Society of America Special Paper 433, pp. 117–145. Fyles, J.T. 1964. Geology of the Duncan lake Area, Lardeau district, British Columbia. British Columbia Ministry of Mines, Energy and Resources, Bulletin 49, 87 p. Fyles, J.T. 1967. Geology of the Ainsworth-Kaslo area, British Columbia. British Columbia Department of Mines, Energy and Resources, Bulletin 53, 125 p. Fyles, J.T., and Eastwood, G.E. 1962. Geology of the Ferguson area, Lardeau district, British Columbia. British Columbia Ministry of Mines, Energy and Resources, Bulletin 45, 92 p. Ghent, E.D., and Simony, P.S. 2005. Geometry of isogradic, isothermal and isobaric surfaces: interpretation and application. Canadian Mineralogist, 43: 295–310. doi:10.2113/gscanmin.43.1.295. Ghent, E.D., Simony, P.S., and Knitter, C.C. 1980. Geometry and pressuretemperature significance of the kyanite-sillimanite isograd in the Mica Creek area, British Columbia. Contributions to Mineralogy and Petrology, 74: 67– 73. doi:10.1007/BF00375490. Ghosh, D.K. 1995. U-Pb geochronology of Jurassic to early Tertiary granitic intrusives from the Nelson-Castlegar area, southeastern British Columbia, Canada. Canadian Journal of Earth Sciences, 32: 1668–1680. doi:10.1139/e95-132. Gibson, H.D., Brown, R.L., and Carr, S.L. 2008. Tectonic evolution of the Selkirk fan, southeastern Canadian Cordillera: a composite Middle-Jurassic-Cretaceous orogenic structure. Tectonics, 27: TC6007. doi:10.1029/2007TC002160. Harms, T.A., and Price, R.A. 1992. The Newport fault: Eocene listric normal faulting, mylonitisation, and crustal extension in northeast Washington and northwest Idaho. Geological Society of America Bulletin, 104: 745–761. Hinchey, A.M., Carr, S.D., McNeill, P.D., and Rayner, N. 2006. Palaeocene-Eocene high-grade metamorphism, anatexis and deformation in Thor-Odin dome, Monashee complex, southeastern British Columbia. Canadian Journal of Earth Sciences, 43: 1341–1365. doi:10.1139/e06-028. Holland, T.J.B., and Powell, R. 1998. An internally consistent thermodynamic data set for phases of petrological interest. Journal of Metamorphic Geology, 16: 309–343. doi:10.1111/j.1525-1314.1998.00140.x. Hoy, T. 1974. Structure and metamorphism of Kootenay Arc rocks around Riondel, B.C. Unpublished Ph.D. thesis, Queen University, Kingston, Ontario, 201 p. Hoy, T. 1976. Calc-silicate isograds in the Riondel area, southeastern British Columbia. Canadian Journal of Earth Sciences, 13: 1093–1104. doi:10.1139/e76112. Hoy, T. 1977. Stratigraphy and structure of the Kootenay arc in the Riondel area, southeastern British Columbia. Canadian Journal of Earth Sciences, 14: 2301– 2315. doi:10.1139/e77-198. Hoy, T. 1980 Geology of the Riondel area, central Kootenay Arc, southeastern British Columbia: British Columbia Ministry of Mines, Energy and Resources, Bulletin 73, 89 p. Hoy, T., and van der Heyden, P. 1988. Geochemistry, geochronology and tectonic implications of the quartz monzonite intrusions, Purcell Mountains, southeastern British Columbia. Canadian Journal of Earth Sciences, 25: 106–115. doi:10.1139/e88-011. Hoy, T., and Dunne, K.P.E. 1997. Early Jurassic Rossland Group, Southern British Columbia. British Columbia Department of Mines, Bulletin 102, 124 p. Johnston, D.H., Williams, P.F., Brown, R.L., Crowley, J.L., and Carr, D. 2000. Northeastward extrusion and extensional exhumation of crystalline rocks of the Monashee complex, southeastern Canadian Cordillera. Journal of Structural Geology, 22: 603–625. doi:10.1016/S0191-8141(99)00185-6. Klepacki, D.W. 1985. Stratigraphy and structural geology of the Goat Range area, Southeastern British Columbia. Unpublished Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. 268 p. Kohn, M.J., and Spear, F. 2000. Retrograde net transfer reaction insurance for pressure-temperature estimates. Geology, 28: 1127–1130. doi:10.1130/00917613(2000)28<1127:RNTRIF>2.0.CO;2. Larson, K.P., Price, R.A., and Archibald, D.A. 2007. Tectonic implications of 40Ar/ 39Ar muscovite dates from the Mt. Haley stock and Lussier River stock, near Fort Steele, British Columbia. Canadian Journal of Earth Sciences, 43: 1673– 1684. doi:10.1139/e06-048. Leclair, A.D. 1988. Polyphase structural and metamorphic histories of the Midge Creek area, southeast British Columbia: implications for tectonic processes in the central Kootenay Arc. Unpublished Ph.D. thesis, Queens University, Ontario, Canada. 264 p. Leclair, A.D., Parrish, R.R., and Archibald, D.A. 1993. Evidence for Cretaceous deformation in the Kootenay Arc on U-Pb and 40Ar/39Ar dating, southeastern British Columbia. Current Research, Part A, Geological Survey of Canada, Paper 93-1A, pp. 207–220. Likorish, H.W., and Simony, P.S. 1995. Evidence for late rifting of the Cordilleran margin outlined by stratigraphic division of the Lower Cambrian Gog Group, Rocky Mountain Main Ranges, British Columbia and Alberta. Canadian Journal of Earth Sciences, 32: 860–874. doi:10.1139/e95-072. Livingstone, K.W. 1968 Geology of the Crawford Bay map-area: Unpublished M. Sc. thesis, University of British Columbia, Vancouver, Canada, 87 p. Published by NRC Research Press Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13 For personal use only. 794 Logan, J.M., and Colpron, M. 2006. Stratigraphy, geochemistry, syngenetic sulphide occurrences and tectonic setting of the lower Paleozoic Lardeau Group, northern Selkirk Mountains, British Columbia, in M. Colpron and Nelson, J.L., eds., Paleozoic Evolution and Metallogeny of Pericratonic Terranes at the Ancient Pacific Margin of North America, Canadian and Alaskan Cordillera: Geological Association of Canada, Special Paper 45, p. 361–382. Lydon, J.W. 2007. Geology and metallogeny of the Belt-Purcell Basin. In Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods. Edited by W.D. Goodfellow. Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, pp. 581–607. Mathews, W.H. 1983. Early Tertiary resetting of potassium-argon dates in the Kootenay Arc, southeastern British Columbia. Canadian Journal of Earth Sciences, 20: 867–872. doi:10.1139/e83-076. Menard, T., and Spear, F.S. 1993. Metamorphism of calcic pelitic schists, Strafford Dome, Vermont: compositional zoning and reaction history. Journal of Petrology, 34: 977–1005. doi:10.1093/petrology/34.5.977. Mortimer, N. 1987. The Nicola Group: Late Triassic and early Jurassic subductionrelated volcanism in British Columbia. Canadian Journal of Earth Sciences, 24: 2521–2536. doi:10.1139/e87-236. Moynihan, D.P. 2012. Metamorphism and deformation in the central Kootenay Arc, southeastern British Columbia. Unpublished Ph.D. thesis, University of Calgary, Calgary, Alberta, Canada. Moynihan, D.P., and Pattison, D.R.M. 2013. An automated method for the calculation of P-T paths from garnet zoning, with application to metapelitic schist from the Kootenay Arc, British Columbia, Canada. Journal of Metamorphic Geology, 31: 525–548. Murphy, D.C. 1987. Suprastructure/infrastructure transition, east-central Cariboo Mountains, British Columbia: geometry, kinematics and tectonic implications. Journal of Structural Geology, 9: 13–29. doi:10.1016/0191-8141(87)90040-X. Nelson, J.L. 1993. The Sylvester Allochthon: upper Palaeozoic marginal-basin and island-arc terranes in northern British Columbia. Canadian Journal of Earth Sciences, 30: 631–643. doi:10.1139/e93-048. Parrish, R.R. 1995. Thermal evolution of the southeastern Canadian Cordillera. Canadian Journal of Earth Sciences, 32: 1618–1642. doi:10.1139/e95-130. Parrish, R.R., Carr, S.D., and Parkinson, D.L. 1988. Eocene extensional tectonics and geochronology of the southern Omineca belt, British Columbia and Washington. Tectonics, 7: 181–212. doi:10.1029/TC007i002p00181. Passchier, C.W., and Trouw, R.A.J. 2005. Microtectonics, 2nd ed. Springer, New York. 366 p. Patino Douce, A.E., Humphreys, E.D., and Johnston, A.D. 1990. Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America. Earth and Planetary Science Letters, 97: 290–315. doi:10.1016/0012-821X(90)90048-3. Pattison, D.R.M., and Vogl, J.J. 2005. Contrasting sequences of metapelitic mineral-assemblages in the aureole of the tilted Nelson Batholith, British Columbia: implications for phase equilibria and pressure determinations in andalusite-sillimanite-type settings. Canadian Mineralogist, 43: 51–88. doi: 10.2113/gscanmin.43.1.51. Petersen, N.T., Smith, P.L., Mortensen, J.K., Creaser, R.A., and Tipper, H.W. 2004. Provenance of Jurassic sedimentary rocks of south-central Quesnellia, British Columbia: implications for paleogeography. Canadian Journal of Earth Sciences, 41: 103–125. doi:10.1139/e03-073. Platt, J.P., and Vissers, R.L.M. 1980. Extensional structures in anisotropic rocks. Journal of Structural Geology, 2: 397–410. doi:10.1016/0191-8141(80)90002-4. Price, R.A. 1981. The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky Mountains. In Thrust and Nappe Tectonics. Edited by K.R. McClay and N.J. Price. Geological Society of London Special Publication 9, pp. 427–448. Raeside, R.P., and Simony, P.S. 1983. Stratigraphy and deformational history of the Scrip nappe, Monashee Mountains, British Columbia. Canadian Journal of Earth Sciences, 20: 639–650. doi:10.1139/e83-059. Read, P.B. 1973. Petrology and structure of the Poplar Creek map area, British Columbia. Geological Survey of Canada, Bulletin 193, 144 p. Read, P.B., and Wheeler, J.O. 1976. Geology, Lardeau west-half, British Columbia. Geological Survey of Canada, Open File Map 432. Can. J. Earth Sci. Vol. 50, 2013 Read, P.B., Woodsworth, G.J., Greenwood, H.J., Ghent, E.D., and Evenchick, C.A. 1991. Metamorphic Map of the Canadian Cordillera. Geological Survey of Canada Map 1714A, scale 1:2,000,000. Reesor, J.E. 1973. Geology of the Lardeau Map-Area, East Half, British Columbia; Geological Survey of Canada, Memoir 369, 129 p. Reesor, J.E. 1996. Geology, Kootenay Lake, British Columbia; Geological Survey of Canada, Map 1864A; scale 1:100,000. Reid, L.F. 2003. Stratigraphy, structure, geochronology and geochemistry of the Hobson Lake area (Cariboo Mountains, British Columbia), in relation to the tectonic evolution of the southern Canadian Cordillera. Unpublished Ph.D. thesis, University of Calgary, Calgary, Alberta, 221 p. Roback, R.C., Sevigny, J.H., and Walker, N.W. 1994. Tectonic setting of the Slide Mountain terrane, southern British Columbia. Tectonics, 13: 1242–1258. Scammell, R.J. 1993. Mid-Cretaceous to Tertiary thermotectonic history of former mid-crustal rocks, southern Omineca belt, Canadian Cordillera. Unpublished Ph.D. thesis, Queens University, Kingston, Ontario, 576 p. Simony, P.S., and Carr, S.D. 2011. Cretaceous to Eocene evolution of the southeastern Canadian Cordillera: Continuity of Rocky Mountain thrust systems with zones of “in-sequence” mid-crustal flow. Journal of Structural Geology, 33: 1417–1434. doi:10.1016/j.jsg.2011.06.001. Simony, P.S., Sevigny, J.H., Mortensen, J.K., and Roback, R.C. 2006. Age and origin of the Trail Gneiss Complex: Basement to Quesnel terrane near Trail, southeastern British Columbia, in Colpron, M. and Nelson, J.L., eds., Paleozoic Evolution and Metallogeny of Pericratonic Terranes at the Ancient Pacific Margin of North America, Canadian and Alaskan Cordillera: Geological Association of Canada, Special Paper 45: 505–515. Smith, M.T., Gehrels, G.E., and Klepacki, D.W. 1991. 173 Ma U-Pb age of felsite sills (Kalso River intrusives) west of Kootenay Lake, southeastern British Columbia. Canadian Journal of Earth Sciences, 29: 531–534. doi:10.1139/e92-046. Spear, F.S. 1993. Metamorphic phase equilibria and pressure-temperature-time paths. Mineralogical Society of America Monograph, Washington, D.C., 799 p. Spear, F.S., Kohn, M.J., and Cheney, J.T. 1999. P-T paths from anatectic pelites. Contributions to Mineralogy and Petrology, 134: 17–32. doi:10.1007/ s004100050466. Thompson, A.B., and England, P.C. 1984. Pressure-temperature-time paths of regional metamorphism II. Their inference and interpretation using mineral assemblages in metamorphic rocks. Journal of Petrology, 25: 929–955. doi: 10.1093/petrology/25.4.929. Thompson, R.I., Glombick, P., Erdmer, P., Heaman, L.M., Lemieux, Y., and Daughtry, K.L. 2006. Evolution of the ancestral Pacific margin, southern Canadian Cordillera: Insights from new geologic maps, in Paleozoic Evolution and Metallogeny of Pericratonic Terranes at the Ancient Pacific Margin of North America, Canadian and Alaskan Cordillera, Colpron, M. and Nelson, J.L., (eds.), Geological Association of Canada, Special Paper 45: 433–482. Unterschutz, J.L.E., Creaser, R.A., Erdmer, P., Thompson, R.I., and Daughtry, K.L. 2002. North American margin origin of Quesnel terrane strata in the southern Canadian Cordillera: Inferences from geochemical and 150Nd isotopic characteristics of Triassic metasedimentary rocks. Geological Society of America Bulletin, 114: 462–475. Vogl., J.J. 1992. The southern tail of the Nelson batholith, southeast British Columbia: emplacement and deformational history. Unpublished M.Sc. thesis, University of Calgary, Calgary, Alberta, 102 p. Warren, M.J. 1997. Crustal extension and subsequent crustal thickening along the Cordilleran rifted margin of ancestral North America, western Purcell Mountains, southeastern British Columbia: Unpublished Ph.D. thesis, Queens University, Ontario, Canada, 361 p. Wheeler, J.O., and McFeely, P. 1991. Tectonic assemblage map of the Canadian Cordillera and adjacent parts of the United States of America. Geological Survey of Canada Map 1712A, scale 1:2,000,000. White, S.E., Burrows, S.E., Carreras, J., Shaw, N.D., and Humphreys, F.J. 1980. On mylonites in ductile shear zones. Journal of Structural Geology, 2: 175–187. doi:10.1016/0191-8141(80)90048-6. Williams, P.F., and Price, G.P. 1990. Origin of kinkband and shear-band cleavage in shear zones: an experimental study. Journal of Structural Geology, 12: 145–164. doi:10.1016/0191-8141(90)90001-F. Zuluaga, C.A., Stowell, H.H., and Tinkham, D.K. 2005. The effect of zoned garnet on metapelite pseudosection topology and calculated metamorphic P-T paths. American Mineralogist, 90: 1619–1628. doi:10.2138/am.2005.1741. Published by NRC Research Press