Structural and geochronological constraints on the ...

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Structural and geochronological constraints on the role of partial melting during the formation of the Shuswap metamorphic core complex at the latitude of the Thor–Odin dome, British Columbia Olivier Vanderhaeghe, Christian Teyssier, and Richard Wysoczanski

Abstract: At the latitude of the Thor–Odin dome, the Shuswap metamorphic core complex exposes a -15 km thick structural section composed of an upper unit that preserved Mesozoic metamorphism, structures, and cooling ages, separated from the underlying high-grade rocks by low-angle detachment zones. Below the detachments, the core of the complex consists of an amphibolite-facies middle unit overlying a migmatitic lower unit exposed in the core of the Thor–Odin dome. Combined structural and super high resolution ion microprobe (SHRIMP) U–Pb geochronology studies indicate that the pervasive shallowly dipping foliation and east–west lineation developed in the presence of melt during Paleocene time. SHRIMP analyses of complexly zoned zircon grains suggest that the migmatites of the lower unit crystallized at -56 Ma, and a syntectonic leucogranite at -60 Ma. We suggest that leucogranite migrated upward from the migmatites through an array of dikes and sills that permeated the middle unit and ponded to form laccoliths spatially related to the detachment zones. The similarity in ages of inherited zircon cores in the two migmatite and the leucogranite samples suggests a genetic link consistent with the structural analysis. Following the crystallization of migmatites, the terrane cooled rapidly, as indicated by argon thermochronology. We propose that exhumation of the core of the Canadian Cordillera during the formation of the Shuswap metamorphic core complex occurred from -60 to 56 Ma at a time when the crust was significantly partially molten. These structural and temporal relationships suggest a genetic link between mechanical weakening of the crust by partial melting, late-orogenic collapse, and exhumation of high-grade rocks in the hinterland of a thermally mature orogenic belt. Vanderhaeghe et al. 943 Résumé : À la latitude du dôme du Thor-Odin, le complexe métamorphique du Shuswap expose une coupe structurale de -15 km au coeur de la Cordillère canadienne. Le complexe métamorphique du Shuswap comprend une unité supérieure formée de roches ayant enregistré une évolution tectonique et des âges de refroidisement Mésozoique et qui est séparée des roches métamorphiques exhumées au coeur du Tertiaire par des zones de détachements subhorizontales. En dessous des détachements, les roches métamorphiques comprenent une unité médiane à faciès des amphibolites et d'une unité migmatitique exposée au coeur du dôme du Thor-Odin. La combinaison des données structurales et des âges U–Pb obtenus par analyse avec la sonde ionique « SHRIMP », révèle que la foliation pénétrative à faible pendage et que la linéation est–ouest ont été développés en présence de magma durant le Paléocène. Les zircons issus des migmatites et d'un leucogranite syntectonique sont zonés et les analyses par la sonde ionique SHRIMP suggèrent un âge de cristallisation de -56 Ma pour les migmatites et de -60 Ma pour le leucogranite. Nous proposons que le leucogranite représente une partie de la fraction liquide issue des migmatites et ayant migré le long d'un réseau de dikes et de sills lardant l'unité médiane et étant accumulée au niveau des zones de détachments sous forme de laccolites. La similitude des âges U–Pb obtenus sur les coeurs de zircons hérités dans les échantillons des migmatites et du leucogranite suggèrent un lien génétique entre migmatites et leucogranite compatible avec l'analyse structurale. À la suite de la cristallisation des migmatites, les roches métamorphiques ont refoidi rapidement comme l'indiquent les données de thermochronologie argon. Nous proposons que l'exhumation du coeur métamorphique de la Cordillère canadienne durant la formation du complexe métamorphique du Shuswap a eu lieu entre 60 et 56 Ma, à un moment ou un volume important de la croûte orogénique était partiellement fondu. Ces relations structurales et temporelles suggèrent un lien génétique entre affaiblissement mécanique de la croute par fusion partielle, effondrement tardiorogénique et exhumation de roches métamorphiques dans la zone interne d'un orogène thermiquement évolué. Received May 28, 1998. Accepted February 23, 1999. O. Vanderhaeghe1 and C. Teyssier. Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, U.S.A. R. Wysoczanski. Research School of Earth Sciences (RSES), Australian National University, Canberra, ACT 0200, Australia. 1

Corresponding author. Present address: Department of Oceanography, Dalhousie University, Halifax, NS B3H 4J1, Canada (e-mail: [email protected]).

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Introduction The formation of the Canadian Cordillera is the result of Paleozoic to early Tertiary accretion and collision of magmatic arcs to the western edge of the North American craton (Monger et al. 1982; Gabrielse et al. 1991). Eastward thrusting of allochthonous terranes over the sedimentary sequences accumulated on the North American paleomargin (Brown et al. 1986; Price 1986) resulted in the formation of a 50–60 km thick crustal welt (Coney and Harms 1984) and the development of a flexural foreland basin at the front of the Rocky Mountain fold-and-thrust belt (Price and Mountjoy 1970). Crustal thickening and burial of the sedimentary sequences were associated with widespread hightemperature metamorphism and crustal anatexis of rocks forming the Omineca belt in the hinterland of the Cordillera (Reesor 1970; Brown and Read 1983; Brown and Journeay 1987; Sevigny et al. 1989, 1990; Carr 1992; Nyman et al. 1995). Final exhumation of the high-grade rocks corresponds to the formation of the Shuswap metamorphic core complex (hereafter called the Shuswap MCC) attributed to a period of early Tertiary regional extension (TempelmanKluit and Parkinson 1986; Brown and Journeay 1987; Parrish et al. 1988; Johnson and Brown 1996) with the activation of an array of normal faults linked by strike-slip faults at the scale of the southern Canadian Cordillera (Ewing 1981; Struik 1993) (Fig. 1). The Shuswap MCC is the largest of the Cordilleran metamorphic core complexes (MCC) (Crittenden et al. 1980) and straddles the suture between allochthonous terranes and the North American paleomargin (Wheeler and McFeely 1991) (Fig. 1). In contrast to Cordilleran MCC south of the 49th parallel, the Shuswap MCC has not been affected significantly by basinand-range type of extension and preserved high-grade fabrics. Although a genetic link between magmatism and exhumation of metamorphic core complexes has been suggested by several authors (Armstrong and Ward 1991; Lister and Baldwin 1993; Foster and Fanning 1997), the role of partial melting during metamorphic core complex formation remains to be demonstrated. The aim of this paper is to constrain the temporal and structural relationship between partial melting and deformation of the high-grade rocks exhumed in the Shuswap MCC. At the latitude of the Thor–Odin dome, the Shuswap MCC displays a -15 km thick structural section from migmatites to leucogranites, providing exposure to assess the role of partial melting during the orogenic evolution of the Canadian Cordillera. We present the results of field investigations between the Trans-Canada Highway 1 (TC1) and the Thor– Odin dome (Figs. 1, 2) which provide constraints on the history of fabric development at different levels of the crust and on the structural relationship between melting and formation of the Shuswap MCC. Following this structural framework, we present the results of U–Pb dating by Super High Resolution Ion Microprobe (SHRIMP) analysis on complexly zoned zircons from two migmatites sampled in the Thor– Odin dome, and from a leucogranite emplaced along a detachment zone. Based on these new constraints and a review of previous work, we propose that the formation of the Shuswap MCC corresponds to early Tertiary late-orogenic gravitational col-

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lapse of the Canadian Cordillera accommodated by normal faulting and extension of the upper crust, and by ductile thinning of the lower crust enhanced by the presence of melt. This model is consistent with thermal models indicating that crustal thickening is followed by thermal relaxation and rise of the isotherms after a characteristic time of 20–30 Ma (England and Thompson 1984), and with physical models showing that thermal weakening may affect the crustal strength profile (Sonder et al. 1987).

Geologic setting Previous work and models of formation of the Shuswap MCC The major stratigraphic and metamorphic units in the area between Shuswap Lake and the Columbia River Valley were identified by early mapping projects (Dawson 1898; Daly 1915; Brock 1934; Jones 1959). Mapping in the 1960s led to the publication of a Special Paper of the Geological Association of Canada (Wheeler 1970) which presented the first model taking into account the geologic constraints at the scale of the Cordillera and of the southern Omineca belt in particular. Detailed structural analysis in various parts of the “Shuswap metamorphic complex” resulted in the identification of several mantled gneiss domes aligned along the strike of the Cordillera (Fyles 1970; McMillan 1970; Reesor 1970; Reesor and Moore 1971). The gneiss domes were described as being cored by migmatites forming diapirs uprising into mantling gneisses of sedimentary origin (Fyles 1970; McMillan 1970; Reesor 1970; Reesor and Moore 1971). Since no unconformity had been observed between migmatitic Precambrian gneisses and Paleozoic gneisses, and migmatization was described as pervasive throughout the Shuswap metamorphic complex, these authors suggested that formation of the mantled gneiss domes was part of the tectonic evolution of the Cordillera during Mesozoic time. At the scale of the Cordillera, Price and Mountjoy (1970) proposed that buoyant upwelling and lateral spreading of the hot mobile infrastructure beneath a relatively passive suprastructure to the west were related to the northeasterly growth of the foreland fold and thrust belt. This elegant model was superseded in the 1980s by models emphasizing collision of allochthonous terranes with various geometries of crustal thickening involving décollement of a sedimentary cover over a Precambrian basement (Brown and Read 1983; Mattauer et al. 1983; Brown et al. 1986; Price 1986). These authors defined the Shuswap metamorphic complex as being composed of (1) an allochthonous unit, designated as the Selkirk allochthon, the Kootenay terrane, or the Shuswap terrane, depending on the authors; and (2) a Precambrian basement which appears in tectonic windows such as the “Monashee complex.” The presence of a Precambrian basement in the Monashee complex is based on U–Pb and Rb–Sr geochronology yielding Proterozoic ages (Wanless and Reesor 1975; Armstrong et al. 1991; Parkinson 1991; Crowley 1997a, 1997b) and on the inference of an unconformity between the migmatitic gneisses forming the core of the basement culminations and a basal quartzite considered to be Late Proterozoic or early Paleozoic in age (Brown and Read 1983; Scammell and Brown 1990; Crowley 1997a, 1997b). © 1999 NRC Canada

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Fig. 1. Geology of the southern Canadian Cordillera. (a) Geologic map modified after Wheeler and McFeely (1991) and Parrish et al. (1988). (b) Crustal-scale cross section of the Canadian Cordillera based on geologic information and interpretation of the Lithoprobe profile labelled “1b” in (a). th., thrust.

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Fig. 2. Lithotectonic map of the Shuswap metamorphic core complex. Compilation of data from Reesor and Moore (1971), Read (1980), Mathews (1981), Read and Brown (1981), Duncan (1984), Brown and Journeay (1987), Carr (1991a, 1991b, 1992), Parkinson (1992), Brown et al. (1993), McNicoll and Brown (1995), Johnson and Brown (1996), and Vanderhaeghe (1997). Numbers in white boxes correspond to the samples dated by U–Pb geochronology.

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In contrast to the metamorphic core complexes south of the 49th parallel (Crittenden et al. 1980), the importance of extension in the Shuswap MCC has been identified only recently. The contribution of crustal extension to the structural development of the Shuswap MCC was restricted to brittle normal faults (Read and Brown 1981; Lane 1984; Brown and Journeay 1987) until part of the mylonitic fabric characterizing the shear zones boarding the MCC was interpreted as being related to detachment of an upper plate from an exhumed metamorphic core (Tempelman-Kluit and Parkinson 1986; Carr et al. 1987; Parrish et al. 1988; Johnson and Brown 1996). Current problems and controversial issues Before presenting our own results we would like to point out several issues that remain debated about the formation and evolution of the Shuswap MCC in the area between the TC1 and the Thor–Odin dome. These points will be discussed at the end of the paper in the light of the data presented in this contribution. Timing of crustal anatexis Most constraints to date on the timing of metamorphism and crustal anatexis are provided by U–Pb ages obtained on zircons, monazites, and titanites. As stated above, migmatitic gneisses exposed in the Monashee complex comprise paragneisses yielding Rb–Sr and U–Pb zircon ages of -2.2 Ga intruded by orthogneisses yielding ages from 1.9 to 1.8 Ga (Wanless and Reesor 1975; Armstrong et al. 1991; Parkinson 1991; Crowley 1997a, 1997b). In contrast, leucogranites emplaced at various structural levels of the Shuswap MCC yield U–Pb ages ranging from 130 to 50 Ma and are attributed to crustal anatexis on the basis of their geochemical and zircon signature (Parrish et al. 1988; Sevigny et al. 1989; Carr 1991a, 1991b, 1992; Parkinson 1992; Scammell 1993). Precambrian ages at the lowest structural level exposed in the Shuswap MCC are interpreted to reflect a preCordilleran history in a basement not significantly affected by Cordilleran deformation and metamorphism (Parrish 1995; Crowley 1997a, 1997b). This implies the preservation of an inverted metamorphic gradient, and the discrepancy between the age of partial melting at the lowest structural level and the ages of granites emplaced at higher structural level raises the question of the source of the leucogranites. Significance of the ductile fabric The origin of the ductile fabric in the high-grade rocks exhumed in the footwall of the detachments is still debated. For most Cordilleran MCC the ductile fabric has been attributed to deformation during early Tertiary continental extension (see Crittenden et al. 1980, and references therein). In the Shuswap MCC, most authors attribute the ductile fabric to the Mesozoic crustal thickening event associated with thrusting along the Monashee décollement (Brown et al. 1986; Brown and Journeay 1987; Carr 1991a, 1991b), although several authors attribute part of it to the early Tertiary extensional event (Tempelman-Kluit and Parkinson 1986; Parrish et al. 1988; Carr 1992; Johnson and Brown 1996).

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Structural section Structural units and major tectonic contacts We propose to describe the Shuswap MCC in the area between the Trans Canada Highway 1 and the Thor–Odin dome as three superposed units (Figs. 2, 3). These units are identified on the basis of their contrasted structural evolution during the formation of the core complex and diachroneity of the cooling ages (Vanderhaeghe and Teyssier 1997). Lowangle detachment zones, defined as ductile shear zones cutting down structural section, separate remnants of a dismembered upper unit from the exhumed metamorphic core (Fig. 2). The upper unit preserved Mesozoic K–Ar, 40 Ar/39Ar, and Rb–Sr cooling ages, whereas the metamorphic core is characterized by Paleocene to Eocene cooling ages (Mathews 1981; Parrish et al. 1988; Vanderhaeghe 1997). Below the detachment zones, the middle unit is composed of a dominantly metasedimentary amphibolite-facies sequence. The lower unit consists of migmatites located in the core of dome-shaped structures. Above the detachment zones, tilted blocks are associated with pull-apart and extensional basins filled by Eocene sedimentary and volcanic sequences (Church 1981; Mathews 1981). The area is dissected by a number of brittle normal and strike-slip faults affecting all structural units. This framework serves as a basis for the presentation of our field investigations described in detail in the following sections. The migmatitic lower unit: southeast Thor–Odin dome The descriptions of the lower unit reported here are mainly based on observations made in the Thor–Odin dome. We present a detailed map of the southeastern edge of the Thor–Odin dome comprising Mount Odin (Fig. 4). Information collected in the area of the map (Fig. 4) was complemented by observations made at several localities around the dome, in particular to the west of Coursier Lake, to the west of Pingston Creek, and on the edges of Mount Burnham and Mount Thor. We also visited several localities at the southern edge of the Frenchman’s Cap dome in the Jordan River area mapped by Fyles (1970) along Kirkup and Hiren creeks. The southern limb of the Thor–Odin dome (Figs. 1, 2) has been the object of several detailed mapping projects (Reesor and Moore 1971; Duncan 1984). Following these authors, we identify a dominantly migmatitic core zone which we define as the lower unit and a metasedimentary mantling zone we assign to the middle unit. We follow the terminology defined by Brown (1973) and Burg and Vanderhaeghe (1993) adapted from Mehnert (1968) and distinguish two types of migmatites: (1) metatexites characterized by a continuous gneissic layering, encompassing “stromatites” and “lit-par-lit migmatites” and corresponding to the “layered gneisses” of Reesor and Moore (1971); and (2) diatexites composed of more or less heterogeneous granitoids encompassing anatectic granites and nebulitic migmatites and corresponding to the monzogranitic to granodioritic core gneisses of Reesor and Moore. The core of the Thor–Odin dome is dominated by diatexites, whereas its flanks are formed by metatexites. We describe the metatexite–diatexite transition in the detailed map of the southeast Thor–Odin dome (Figs. 4, 5). © 1999 NRC Canada


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Fig. 3. Geologic cross sections of the Shuswap metamorphic core complex. Locations are indicated in Fig. 2.

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Fig. 4. The southern limb of the Thor–Odin dome. (a) Detailed geologic map. (b) Geologic cross section (location indicated in a). Compilation of the structures measured in the area are plotted on equal-area, lower hemisphere stereonets. Coordinates on the map are given in the universal transverse Mercator (UTM) system.

The southern side of the map is formed by metatexites (Figs. 4, 5a) characterized by a well-developed compositional layering at various scales. Metatexites are dominated by quartz–feldspar–biotite gneisses containing more or less continuous layers of paragneisses with compositions ranging from pelitic to psammitic and layers of hornblendebearing orthogneiss with an intermediate composition (see Reesor and Moore 1971 for a detailed description of the litho-

logies). Metapelites contain leucosome (quartz, K-feldspar, plagioclase, biotite, ±garnet) surrounded by melanosome (biotite–sillimanite–garnet). In places, metapelitic gneisses contain large sillimanite pseudomorphs after kyanite. In thin section, metapelitic metatexite exhibits a coarse equigranular texture containing myrmekite and interfingered K-feldspar, quartz, and plagioclase. Melanosome–leucosome contacts show a concentration of myrmekite and, in places, larger bi© 1999 NRC Canada


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Fig. 5. (a, b) Metatexite of the Thor–Odin dome characterized by continuous synmigmatitic layering. Metre-scale compositional layering with relatively more mafic and more felsic layers probably represents a transposed bedding. Superimposed on this initial layering, decimetre-scale alternations of grey mesosomes with white leucosomes surrounded by dark melanosomes are interpreted in terms of melt segregation. Notice shear zones and boudin necks filled by granitic material. (c, d) Metatexite–diatexite transition. (c) With increasing amount of granite the continuity of the foliation is lost and boudins of metatexites are individualized in a granitic matrix. (d) Internal deformation of the metatexite in the transition zone displays complex folds affecting leucosomes but also crosscut by granitic veins. (e, f) Diatexite of Mount Odin. (e) Summit of Mount Odin showing a southwest-dipping synmigmatitic foliation delineated by the alignment of large amphibolite boudins. Approximate width of the picture is 60 m. (f) Detail of the diatexite showing the synmigmatitic foliation delineated by alignment of xenoliths and restitic mafic minerals Approximate width of the picture is 1 m. All pictures are taken looking to the west from the southern flank of Mount Odin.

otite grains. Hornblende intermediate gneisses are associated with tonalitic hornblende-bearing leucosome surrounded by hornblende–biotite melanosome. Metatexites also include boudins of various compositions aligned along the strike of the layering and with trails of quartzite and mafic boudins composed of hornblende, garnet, and pyroxene. The layering of these rocks is likely to be the result of a combination of processes. The metre- to decametre-scale compositional layering marked by the alternation of psammitic and pelitic layers (Fig. 5a) and boudinaged amphibolite layers probably corresponds to a transposed inherited bedding reflecting the sedimentary nature of the protolith. Superimposed on the transposed bedding, the alternation at the centimetre to decimetre scale of homogeneous mesosome with granitic leucosome surrounded by mafic melanosome (Fig. 5b) is interpreted to represent in situ segregation of a granitic melt fraction from a partially molten protolith. This layering is characteristic of metatexites and is defined as a synmigmatitic layering (Burg and Vanderhaeghe 1993; Brown et al. 1995). The synmigmatitic layering is roughly parallel to lithological contacts and concordant with the internal ductile fabric of resisters less affected by partial melting such as quartzite or amphibolite. In quartz–feldspar–biotite gneisses and hornblende intermediate gneisses, the synmigmatitic layering is boudinaged on a metre scale and displaced by shear zones. Toward the core of the dome, the development of a north–south-trending preferred orientation of prismatic sillimanite, fibrolite, and biotite associated with the synmigmatitic layering contrasts with the regional east–westtrending lineation. The mineral lineation is more difficult to detect in granitic material. Apparent offsets associated with the ductile shear zones are consistent with a dextral component on north–south-trending shear zones, and a sinistral component on east–west-trending zones. Boudin necks and shear zones are typically filled by granitic material. At the contact with intrusive granitic veins, mafic minerals are dragged and aligned into parallelism to the internal fabric of the crosscutting granitic veins delineated by the orientation of biotite and hornblende. These features are interpreted to represent flow of the melt phase into dilational jogs opened during heterogeneous deformation coeval with the development of the synmigmatitic layering. In contrast, massive amphibolite layers behave in a more brittle fashion and are affected by fractures filled by granitic material. The foliation of the metatexites is also affected by steep discrete sillimanite- or chlorite-bearing shear zones with a normal sense of shear. Some of the granitic leucosome is characterized by synmigmatitic way-up criteria (Burg and Vanderhaeghe 1993)

such as cauliflowers developed preferentially in metapelitic layers, interpreted as incipient diapiric intrusions, or asymmetric vein clusters attributed to trapping of the melt typically underneath more competent amphibolite layers. Assuming that the synmigmatitic foliation formed in a subhorizontal position and that the low-density and lowviscosity melt phase preferentially migrates upward toward the free surface, these structures are consistent with outward tilting of the metatexites with respect to the core of the dome. Similar way-up criteria and tilting of the synmigmatitic foliation have also been described to the east of the Thor–Odin dome (Vanderhaeghe and Teyssier 1997). Diatexites are composed of a heterogeneous mixture of granitoids ranging from medium- to coarse-grained quartz monzonite to biotite granodiorite (Reesor and Moore 1971) including a garnet–sillimanite–kyanite leucogranite. Xenoliths and enclaves (Fig. 5e), up to a few tens of metres long, representing the various lithologies recognized in the surrounding metatexites, and centimetre-scale schlieren of restitic minerals, represent a residual and restitic solid fraction in the magmatic rock. The orientation of these xenoliths and the alignment of mafic minerals in the granitic matrix delineate a dominantly magmatic fabric (Fig. 5f). This fabric is affected, in places, by steep ductile shear zones filled by granitic material and consistent with upward motion of the diatexite with respect to the metatexite. The microscopic texture is characterized by myrmekite and interfingered feldspar and quartz showing embayment of grain boundaries. This magmatic fabric is only slightly overprinted by solid-state deformation marked by undulose extinction of quartz and discrete steep chlorite-bearing shear zones with a normal sense of shear. Although contacts are typically intermingled, magmatic fabrics are concordant across the various types of granitoids. Despite the heterogeneity of the granitic facies and the large proportion of resisters and restites, the diatexite core of the dome appears as a structurally coherent magmatic body at the scale of the mapped area (Fig. 4). The transition from metatexites to diatexites (Figs. 5c, 5d) is characterized by the loss of continuity of the synmigmatitic layering. On the southern limb of the Thor–Odin dome, this transition occurs within -200 m. Going towards the core of the dome on a southwest–northeast section, the amount of leucosome and intrusive granitic material increases. Metatexites with a continuous gneissic framework contain up to 30–50% of granitic material at the outcrop scale. For migmatites containing more than approximately 50% granitic material, the synmigmatitic layering is disrupted, individualizing panels that exhibit complexly contorted folds (Figs. 5c, 5d). Across the transition, the fabric in the metatexite, defined by the gneissic layering in the solid © 1999 NRC Canada


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framework, is concordant with the fabric in the diatexite, defined by the preferred orientation of xenoliths and schlieren. We recognized a similar metatexite–diatexite transition at various localities around the Thor–Odin dome, to the northeast of Mount Thor and on the eastern flank of Mount Burnham, and also to the south of Frenchman’s Cap dome (Fig. 1), in Mount Copeland. For the area that we did not investigate ourselves, we extrapolated the metatexite–diatexite contact based on interpretation of existing maps (Reesor and Moore 1971; Duncan 1984; McNicoll and Brown 1995). Summary The Thor–Odin dome is cored by diatexites (heterogeneous granite-dominated migmatites) surrounded by metatexites (migmatites with a well-developed synmigmatitic layering). The major foliation of the diatexites and metatexites developed in the presence of melt. The synmigmatitic layering of the metatexite and the magmatic foliation of the diatexite are concordant with the attitude of the metatexite– diatexite transition which delineates the elliptical map shape of the domes cored by heterogeneous but coherent bodies of diatexites. Synmigmatitic way-up criteria suggest that the synmigmatitic layering was tilted outward with respect to the core of the dome. To the north of Frenchman’s Cap dome, a transposed but continuous stratigraphic sequence is described above a basal quartzite horizon considered to be Late Proterozoic or early Paleozoic (Scammell and Brown 1990; Crowley 1997a, 1997b). In contrast, in the Thor–Odin dome, migmatization affects fertile units above and below the quartzite marker horizon, which appears in large boudins in the metatexites and diatexites. Amphibolite-facies middle unit: TC1 and Mount Symonds Most of the present-day surface in the footwall of the detachments is represented by rocks of the middle unit. Our field investigations consisted in visiting all the outcrops exposed along roadcuts and trails in the area between the TC1 and the Thor–Odin dome. The main features of the middle unit are presented here based on a transect along the TC1 (Fig. 3) and detailed maps of Mount Symonds (Fig. 7) and Mount Hall (Fig. 9). Trans-Canada Highway 1 (TC1) The thickest structural section of the amphibolite-facies middle unit is exposed along the TC1 (Fig. 3). The lowest structural level is composed of paragneisses of pelitic and calc-silicate composition. This sequence is overlain by a thick quartzite horizon, identified as a regional-scale marker horizon (Brown 1980; Read 1980; Scammell and Brown 1990; Carr 1991a, 1991b). The quartzite horizon delineates a 10–15 km wide upright antiform with a horizontal northnorthwest–south-southeast axis. To the west, the quartzite dips westward and disappears, at Victor Lake, underneath a sequence of quartz–feldspar–biotite paragneisses which grade into a sequence dominated by calc-silicate, marble, metapelite, and metapsammite units containing large amphibolite boudins and a sheet of hornblende granodiorite a few hundred metres thick. To the east, facing the Columbia River Valley, the quartzite horizon forms cliffs at the top of the highest peaks of the area, such as Mount Begbie culminating


at 2956 m (Figs. 2, 3). Regional-scale repetition of the quartzite horizon has been interpreted to reflect isoclinal folding and, in particular, the quartzite layer forming the top of Mount Begbie has been described as an overturned limb of a kilometre-scale northward-verging fold (Read 1980). To the southeast of Mount Begbie, the thick quartzite gently dips towards the shore of Upper Arrow Lake where preserved cross-beds to the north of Mount Hall indicate that the sequence is right-way-up. This polarity is not consistent with the interpretation of Read (1980) that this part of the quartzite is overturned. To the east of Mount Begbie repetition of the quartzite horizon and other stratigraphic markers probably reflects tectonic accretion accommodated by imbrication of thrust slices during the early stages of crustal thickening. Directly to the east of Mount Begbie, large eastdipping panels of intensely sheared quartzite are found on the western shore of Upper Arrow Lake and are interpreted to be boudinaged equivalents of Mount Begbie quartzite. The entire structural section is characterized by widespread amphibolite-facies metamorphism, and typical mineral assemblages in rocks of pelitic composition comprise quartz–biotite–sillimanite–feldspar–garnet. The section is also permeated by a network of sills and dikes of leucogranite (Figs. 3, 6). Part of the granitic fraction was probably generated in situ, as suggested by petrologic and thermobarometric studies (Nyman et al. 1995), but a significant part is intrusive, as indicated by granitic veins crosscutting the compositional layering of the host rock. The geometric characteristics of the granitic network are controlled in part by the lithological composition and thus the physical properties of the host rock. In the metapelite units, the leucogranite forms relatively thin layers concordant with the foliation and is located in ductile shear zones or in the necks of boudins. In contrast, leucogranite in amphibolite or quartzite forms a network of veins with sharp intrusive contacts disrupting the host rock that may occur as angular blocks. The amphibolite-facies fabric of the rocks of the middle unit consists of a shallowly dipping, dominantly north– south-striking and west-dipping composite foliation carrying an east–west- to northeast–southwest-trending mineral lineation. The foliation is defined in paragneisses, marble, and quartzite by centimetre- to metre-scale layers with different mineral compositions. In addition, this foliation is delineated by the orientation of metamorphic minerals such as biotite in paragneisses, white mica in quartzite, and graphite in marble and calc-silicate rocks. The quartzite is typically pervasively recrystallized and contains large flakes of white mica, but in places deformed clast and cross-stratification structures delineated by fine layers of oxides are preserved. The foliation in the hornblende granodiorite is delineated by a compositional layering with the alternation of felsic and mafic layers. Accordingly, for most lithological units except the hornblende granodiorite and part of the amphibolites, the foliation represents transposition of bedding during amphibolite-facies deformation. Leucogranitic sills and dikes have either preserved a magmatic fabric concordant with the contacts of the veins or developed a fabric reflecting solid-state deformation concordant with the host rock. The lineation is defined by biotite and sillimanite in metapelite, hornblende © 1999 NRC Canada


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in amphibolite and hornblende granodiorite, and white mica and stretched quartz in quartzite and leucogranite. Heterogeneous deformation of this compositionally layered package of rocks is characterized by several generations of superimposed structures. At the outcrop scale three generations of folds are distinguished. Preserved hook-like interference patterns indicate two generations of synmetamorphic coaxial folds with shallow-dipping axial planes concordant with the amphibolite-facies foliation. Sheath folds are common in quartzite and marble layers. The third generation of folds is responsible for the formation of long-wavelength, regional, post-metamorphic open upright undulations with roughly north–south horizontal axes. The amphibolite-facies foliation is boudinaged and disrupted by shear zones showing a normal sense of shear and filled with granitic material. Granitic veins either preserved a magmatic fabric or are transposed in the amphibolite-facies foliation. However, a network of discordant and concordant granitic veins, with respect to the compositional layering of the host rock, is preserved throughout most of the TC1 section except at the top of the middle unit where the amphibolite-facies foliation is transposed under greenschist-facies conditions, and near Victor Lake where the Monashee décollement is exposed (Brown et al. 1993). At this level, the Monashee décollement corresponds to a -100 m thick amphibolite-facies shear zone, localized in metapelite and running parallel to the gently west-dipping lithological layering. Kinematic criteria such as C–S fabric, asymmetric pressure shadows, and crystallization tails around rotated porphyroclasts are consistent with a top to the east sense of shear. The continuity of the Monashee décollement to the south is uncertain. Although it has been identified on the western limb of the Thor–Odin dome (Carr 1991a, 1991b; McNicoll and Brown 1995), we did not recognize it in the area of Mount Symonds. The direction of maximum finite shortening, perpendicular to the generally flat-lying foliation, is close to vertical. The direction of maximum finite stretching, represented by the lineation, is horizontal and trending east–west to northeast–southwest. Boudinage of the foliation in several directions is consistent with a finite strain ellipsoid in the flattening field. Leucogranite bodies exhibit a more homogeneous internal fabric than the metasedimentary host rock, and record a simpler strain history. Leucogranite bodies along the TC1 typically exhibit a plano-linear fabric consistent with the east–west-elongated finite strain ellipsoid estimated from structural analysis of the host rocks. Mount Symonds To the south of the migmatitic dome, the lithological package exposed in the Mount Symonds area (Fig. 7) comprises, from bottom to top, a sequence of psammitic gneisses containing about 40% intrusive granitic sills and dikes overlain by a sequence of pelitic schists containing quartzite horizons and massive garnet amphibolite. This sequence ends with a thick quartzite marker horizon, repeated by isoclinal folding and thrusting on the scale of the studied area (Fig. 7). Above the quartzite, a white marble containing graphite and diopside constitutes another marker horizon. The metapelitic sequence enclosing the quartzite and the marble horizons grades upward into a sequence dominated by calcsilicate and quartz–feldspar–biotite paragneisses.

927 Fig. 6. Network of granitic sills and dikes in metapelites (a) and in amphibolites (b). Notice high-angle normal faults crosscutting foliation in (b). Both pictures are taken along the Trans-Canada Highway 1, to the west of Three Valley Gap.

Paragneisses with a pelitic composition show synkinematic mineral assemblages comprising sillimanite–biotite–garnet and quartz–feldspar leucosome. The area of Mount Symonds displays a superposition of structures and mineral assemblages similar to that described above for the TC1 area. In contrast, the major amphibolitefacies foliation strikes east–west and dips on average 40°S (Fig. 7). The mineral lineation is prominent and trends east– west. Following previous authors (Reesor and Moore 1971; Duncan 1984; Carr 1991a, 1991b; McNicoll and Brown 1995), we recognized the presence of three generations of folds (Fig. 7). The major foliation is axial planar to metre- to hectometre-scale noncylindrical sheath folds, with secondorder fold axes subparallel to the lineation. In early formed fold hinges, the folded surface is a transposed compositional layering. The presence of a first generation of folds is evi© 1999 NRC Canada

928



Fig. 7. Geologic map of Mount Symonds (a) and cross sections (b) located on the map by letters. The inserted circle shows a detail of fold interference pattern. Coordinates on the map are given in the UTM system.

denced at the metre scale by the presence of hook-like fold interference patterns indicating coaxial refolding of tight isoclinal folds. At the decametre to hectometre scale, the asymmetry of second-order, second-generation folds is not inverted on opposite limbs, suggesting that the larger fold hinges are related to the first generation. The relationship between the original compositional layering and the two subsequent foliation surfaces is only distinguished in fold hinges at the contact between contrasted lithological units. The last generation of folding affects all lithologies of the middle unit, including the leucogranite sheets, and is responsible for large-amplitude hectometric to kilometric open upright folds that wrap around the Thor–Odin dome. Granitic

bodies are deformed by and also crosscut all sets of structures. In particular, granite veins are localized in shear zones with an S-side-down normal component, and in the necks of boudins. The leucogranite bodies are folded by the second and third generation of folds but are also present in axial planes of the latter. These features indicate that most of the ductile structural development of the area occurred in the presence of melt. The foliation is also affected by steep discrete chlorite-bearing normal shear zones Leucogranite and metaconglomerate units in the area to the south of the Thor–Odin dome exhibit a strong east–west linear fabric (constrictional finite strain). In this area, the subvertical shortening is accompanied by a second direction © 1999 NRC Canada

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of finite shortening, subhorizontal and north–south trending. This shortening direction is consistent with the axial planes of folds and affects layers that were previously boudinaged. Summary The amphibolite-facies middle unit is exposed along a 10– 15 km thick structural section at the latitude of the TC1 and to the south of the Thor–Odin dome, at Mount Symonds. The lithological sequence is dominated by metasedimentary rocks comprising calc-silicate rocks, metapelite, quartzite, and marble, attributed to the North American paleomargin (Brown et al. 1986; Scammell and Brown 1990). Although detailed stratigraphic correlations are uncertain in this area affected by intense deformation and widespread amphibolite-facies metamorphism, the presence of a thick quartzite horizon associated with a more or less continuous marble layer constitutes a stratigraphic marker at the scale of the Shuswap MCC (Read 1980; Scammell and Brown 1990). This continuity of stratigraphic markers, despite the degree of transposition, contrasts with the loss of coherence of the stratigraphy observed in the lower migmatitic unit. On the regional scale, the major fabric of the middle unit corresponds to the transposition of an original compositional layering during amphibolite-facies metamorphism. The major fabric consists of a shallowly dipping foliation carrying an east–west-trending mineral lineation. In the vicinity of the Thor–Odin dome, the major foliation and enclosed granites, including the large laccolith of Ladybird leucogranite, and fold-axial planes (synmetamorphic as well as postmetamorphic) are warped and delineate the shape of the dome, suggesting that the doming event was coeval or late with respect to the development of this fabric. To the south of Mount Odin, the mineral lineation trends north–south in the migmatites, at 90° to that of the middle unit (Fig. 2). According to this structural analysis the footwall of the major detachments is characterized by penetrative, homogeneous, amphibolite-facies ductile deformation associated with the development of a plano-linear fabric in general and a linear fabric south of the dome. The maximum finite direction of shortening is close to the vertical for most of the area. The maximum direction of finite stretching is horizontal and east–west-trending. For the area to the west and east of the Thor–Odin dome, the resulting finite-strain ellipsoid is in the flattening field, whereas to the south of Thor–Odin, it is in the constriction field. Detachments and high-angle normal faults The boundaries of the Shuswap MCC are marked by complex tectonic contacts characterized by the superposition of several generations of structures (Read and Brown 1981; Lane 1984; Tempelman-Kluit and Parkinson 1986; Carr 1992; Johnson and Brown 1996). We propose to distinguish ductile shear zones, referred to as detachment zones, from brittle faults which either root into detachments or offset them. In the studied area we distinguish two detachment zones, each composed of several segments, on the basis of the kinematic criteria observed in the associated mylonitic zones. The western detachment displays kinematic criteria consistent with a top to the west sense of shear, whereas the eastern detachment is marked by kinematic criteria indicat-

929

ing a top to the east sense of shear. The western detachment is composed of the Beaven, Trinity Hills, Silver Hills, and Okanagan – Eagle River detachments, and the eastern detachment comprises the Columbia River detachment and the detachment at the base of three upper unit klippen south of the Thor–Odin dome (Fig. 2). Following Parrish et al. (1988), we consider that the portion of the Clachnacuadainn complex which displays Paleocene and younger K–Ar and 40 Ar/39Ar cooling ages is part of the Shuswap MCC. This interpretation implies that the trace of the Columbia River detachment departs from the Upper Arrow Lake south of Revelstoke and a shear zone in its continuity should then cross the TC1 east of Revelstoke (Fig. 2). In the studied area, all three structural units are also affected by an array of high-angle brittle normal faults dominantly north–south trending and connected by east–west-trending transfer zones (Fig. 2). The north–south-trending normal faults define a series of tilted blocks and comprise the east-dipping Columbia River fault, crosscutting the Columbia River detachment, the west-dipping Victor Lake and Three Valley Gap faults along the TC1, the west-dipping Enderby and Mabel Lake faults, and the east-dipping Cherryville and Sugar Lake faults at the latitude of the Thor–Odin dome (Fig. 2). Western detachment The detachment marking the western boundary of the Shuswap MCC was defined as the Okanagan detachment to the south of Vernon (Tempelman-Kluit and Parkinson 1986) and as the Eagle River detachment near Sicamous (Brown and Journeay 1987; Johnson and Brown 1996). The detachment zone is well exposed south of Sicamous, where it is marked by progressive overprinting of an amphibolite-facies mylonitic fabric by a chlorite-bearing shear zone, cataclasite and breccia going upsection. Between Sicamous and Vernon, the Okanagan – Eagle River detachment is exposed in places. North of Vernon, on Highway 97A, the detachment is represented by a mylonitized granite exhibiting rotated Kfeldspar porphyroclasts with asymmetric tails indicating a top to the west sense of shear (Fig. 2). At Enderby, the hills are capped by rocks of the upper unit comprising coarse clastic sediments overlain by volcanic rocks. The sediments grade downward into a breccia with angular clasts and, within a few tens of metres, into a fine-grained slaty chloritic cataclasite in which there are layers of mylonitic rhyolite with blue quartz porphyroclasts. To the east of the trace of the Okanagan – Eagle River detachment zone, the contact between the early Tertiary volcanics and sediments of the upper unit forming the top of Trinity Hills (Fig. 2), and the exhumed high-grade metamorphic rocks of the middle unit, is occupied by a syntectonic laccolith of Ladybird leucogranite, a few hundred metres thick, sampled for U–Pb dating (see below). The magmatic fabric of the granite is overprinted by a pervasive greenschist-facies C–S fabric (Berthé et al. 1979) indicating a top to the west sense of shear (Fig. 8). Similar fabrics are observed in pegmatites higher in the section. Above this syntectonic granite, the top of the detachment zone is marked by a zone of chloritic cataclasite in which the protolith is crushed and retrogressed. The Silver Hills, to the southeast of Mabel Lake, and the Cherry ridge are also © 1999 NRC Canada


930 Fig. 8. Microstructures in the leucogranite. (a) Hand sample from Mount Symonds showing a strong lineo-planar fabric (constrictional finite strain) with develoment of C–S fabric in the plane perpendicular to foliation parallel to lineation. (b) Photomicrograph under crossed polars of a mylonitic leucogranite sampled at Trinity Hills within the western detachment zone (sample 97046 dated by U–Pb geochronology). Notice quartz ribbons and small white micas delineating the mylonitic foliation and development of shear bands with grainsize reduction (width of picture is 6.25 mm). (c) Photomicrograph

Can. J. Earth Sci. Vol. 36, 1999 under crossed polars of a mylonitic leucogranite sampled at Mount Hall within the eastern detachment zone. White mica and K-feldspar porphyroclasts enclosed in a fine-grain matrix (width of picture is 3 mm).

capped by rocks of the upper unit lying on top of an amphibolite- to greenschist-facies mylonitic detachment defined as the Beaven detachment to the southeast of Sugar Lake (Carr 1991a, 1991b). Along the trace of the Okanagan – Eagle River detachment, and to the east of Trinity Hills, the amphibolite-facies and mylonitic fabrics dip eastward, giving them an apparent thrust motion. To explain this discrepancy, we infer that the current dip of this fabric is due to eastward tilting of blocks related to the activity of north–south-trending high-angle normal faults such as the Enderby fault, crosscutting all units. This tilting is also implied by the eastern dip of the strata of the Enderby and Trinity Hills basins (Figs. 2, 3). Eastern detachment The significance of the eastern contact of the Shuswap MCC is still debated, and mylonitic rocks in the “Columbia River fault zone” have been variously ascribed to normal displacement during early Tertiary extension or to thrusting in relation to the Monashee décollement (Read and Brown 1981; Lane 1984; Parrish et al. 1988; Carr 1992; Johnson and Brown 1996). We investigated the structural evolution of the eastern detachment from north of Revelstoke to Galena Bay, south of Mount Hall (Fig. 2), and present a detailed map of the Mount Hall region (Fig. 9). The major amphibolite-facies fabric is progressively overprinted by a retrograde greenschistfacies mylonitic fabric over a few hundred metres. Figure 10 illustrates the microstructural progression from amphiboliteto greenschist-facies fabric. The mylonitic fabric corresponds to the transposition of a preexisting fabric in anastomosing east-dipping shear surfaces associated with growth of chlorite. These surfaces are well developed in metapelite, whereas quartzite and marble units tend to act as decoupling layers that are attenuated and boudinaged. The transposed foliation dips to the east and the apparent repetition of the quartzite is attributed to broad undulations and back rotation of the quartzite when disrupted by a shear zone (cross section, Fig. 9). Reconstructed foliation patterns to the southeast of the Thor–Odin dome indicate that the detachment surface is corrugated with an east–west axis following the edge of the Thor–Odin dome (Fig. 2). The detachment zone is also associated with the emplacement of leucogranite and pegmatite bodies (Fig. 8). The greenschist-facies mylonitic fabric is defined by dynamically recrystallized quartz ribbons and kinked and boudinaged white mica grains (Fig. 8). Porphyroclasts of K-feldspar or garnet are preserved but intensely fractured. In general, the east–west-trending mineral lineation of the middle unit, defined by sillimanite and biotite, is colinear with the chlorite stretching lineation of the mylonites. Kinematic criteria such as asymmetric recrystallized tails around porphyroclasts, bookshelf sliding in K-feldspar along antithetic fractures, mica fish, and crystallographic preferred orientation of recrystallized quartz grains are ubiquitous in the detachment © 1999 NRC Canada

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Fig. 9. Geologic map of Mount Hall (a) and cross section (b), the location of which is shown in (a).

and indicate a significant component of noncoaxial deformation. These criteria indicate a top to the east sense of shear for the Columbia River detachment (Fig. 8). The greenschist-facies mylonitic fabric is truncated by zones of cataclasite and pseudotachylite. Pseudotachylite is identified, at the northwestern entrance of Revelstoke and at the dam to the north, by the presence of black material truncating the fabric of the host rock as small fractures and veins. However, in thin section the texture of pseudotachylite veins is microcrystalline rather than amorphous. All previously described fabrics are affected by cataclasis associated with extreme grain-size reduction along discrete zones. The

resulting fine-grained quartz–feldspar matrix contains a varied population of porphyroclasts comprising K-feldspar but also angular fragments of the host rocks. To the east of Upper Arrow Lake, south of Revelstoke, a shear zone with a mylonitic fabric similar to the one described for the Columbia River detachment is observed. However, it gently dips to the west (Fig. 3). On both sides of Upper Arrow Lake, the mylonitic fabric is affected by north–south-trending, dominantly east-dipping, high-angle normal faults with downdip striations (Lane 1984). Therefore, we suggest that the present west-dipping attitude of the mylonitic fabric east of the Upper Arrow Lake is related to © 1999 NRC Canada


932 Fig. 10. Microstructural progression from amphibolite-to greenschist-facies mylonites. (a) Photomicrograph of an amphibolite-facies mylonite of metapelitic gneiss from the TC1 at the location of the Monashee décollement at Victor Lake. Garnet porphyroclasts are enclosed in a biotite–sillimanite– quartz–feldspar matrix (natural light; width of picture is 6 mm). (b) Photomicrograph of a mylonitic granite from the eastern detachment on the eastern flank of Mount Hall. K-feldspar porphyroclasts with sigma tails in a fine-grained quartz – white mica – feldspar matrix (natural light; width of picture is 3 mm). (c) Photomicrograph of a cataclastic gneiss from the eastern detachment on the western shore of Upper Arrow Lake, at the latitude of Revelstoke dam. Polycrystalline clasts enclosed in a fine-grained matrix of crushed rocks (crossed polars; width of picture is 6 mm).


tilting of the hanging wall toward a major east-side-down high-angle normal fault, defined as the Columbia River fault (Lane 1984). Similar structural relationships are described about 100 km south of this area in the Valhalla complex where the Slocan River fault, which affects at least half of the crust based on the interpretation of the Lithoprobe seismic profile (Parrish et al. 1988; Cook et al. 1992), offsets the mylonitic Valkyr shear zone (Carr et al. 1987; Simony and Carr 1997). Summary Detachment zones, delimiting the Shuswap MCC to the east and to the west, display similar features. Development of a mylonitic amphibolite-facies fabric was spatially associated with the presence of laccoliths of leucogranites and pegmatites. Overprinting of the amphibolite-facies and magmatic fabric by greenschist-facies mylonitic fabric, cataclasite, and breccia suggests that the activity of the detachment zones was associated with a decrease in the metamorphic conditions during deformation. Kinematic criteria indicate outward motion of the upper unit with respect to the metamorphic core of the complex about a north–south axis which is roughly aligned along the domes cored by the migmatites of the lower unit. Tilting of the upper unit above the detachment results in the opening of basins filled by volcanic lavas and sedimentary strata. All units are affected by high-angle normal and strike-slip faults. Due to the shallow dip of the detachment zones and tilting related to the activity of late high-angle faults, it is not straightforward to ascribe a particular shear zone to regional extension or compression unless the breakaway and (or) the root zones can be observed.

U–Pb dating A U–Pb geochronology study was conducted to determine the age of crystallization of the leucogranite bodies and the leucosome in the migmatitic gneisses. To compare these two igneous components of the Shuswap MCC, we sampled one leucogranite emplaced along the detachment zone to the west of Mabel Lake (Fig. 2) and two leucosome fractions from the migmatites within the diatexite–metatexite transition exposed to the south of Mount Odin (Figs. 2, 4). The leucogranite (sample 97046) was sampled on the western shore of Mabel Lake along the western detachment (Fig. 2). The granite is a typical Ladybird leucogranite (Carr 1992), converted into a C–S orthogneiss composed of quartz, K-feldspar, muscovite, and minor amounts of sillimanite and garnet. Microscopic observations (Fig. 8) reveal undulatory extinction of large K-feldspar porphyroclasts with recrystallization of small grains in pressure shadows. The K-feldspar clasts are enclosed in a foliation defined by muscovite fish and quartz, either as single ribbon grains or ribbons of dynamically recrystallized grains. Rotation of Kfeldspar and asymmetric tails are consistent with a top to the west sense of shear. This C–S fabric is partially overprinted by discrete shear bands marked by grain-size reduction. The two migmatite samples dated by U–Pb geochronology represent two different rock types from the Thor–Odin dome (Figs. 2, 4). Sample 97010 is a quartz–feldspar–biotite granite representative of the dominant rock type in the diatexite core of the Thor–Odin dome, and sample 97013 is © 1999 NRC Canada

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from a 50 cm thick vein of coarser quartz–K-feldspar– sillimanite granite with minor amounts of garnet and kyanite, surrounded by a biotite melanosome representative of the metatexite. For both samples, the original magmatic texture of these leucosomes is mostly preserved except for some localized chlorite-bearing shear zones and brittle overprint. Analytical procedure Zircons were separated from rock samples by standard heavy-liquid and magnetic separation techniques at the Research School of Earth Sciences, Australian National University (ANU). Individual grains were then hand picked, mounted in epoxy with the reference standard AS3, and polished. Cathodoluminescent imaging was undertaken to observe the internal structure of selected grains (Fig. 11) and to avoid overlapping two distinct growth zones during analysis. The U–Th–Pb dating by SHRIMP analysis of zircon was undertaken at ANU, following operating techniques and data-reduction procedures similar to those of Williams and Claesson (1987) and Muir et al. (1996). The Pb/U ratios have been normalized relative to a value of 0.1859 for the 206 Pb/238U ratio of the AS3 reference zircon, equivalent to an age of 1099.1 Ma (Paces and Miller 1993). Ages reported here (Table 1) are 206Pb*/238U ages for grains younger than 800 Ma and 207Pb*/206Pb* ages for grains older than 800 Ma, as 207Pb*/206Pb* is more sensitive than 206Pb*/238U for older ages. Uncertainties given for individual analyses presented here (ratios and ages) are at the 1σ level, however all uncertainties in calculated weighted mean ages are reported as 95% confidence limits. Results Migmatite 97010 The zircon population in sample 97010 (Fig. 11a) consists mainly of euhedral, magmatic grains with large, highly altered centers, and a small proportion of rounded, pitted grains. The magmatic rims of euhedral grains display oscillatory zones, and may or may not include a high-U outer rim. A weighted mean 206Pb*/238U age of 56.4 ± 1.4 Ma (n = 17), mean square of weighted deviates (MSWD) = 1.06) (Fig. 12; Table 1) was obtained for the magmatic rims. Cathodoluminescent imaging indicates that the inherited centers of many grains were melted, particularly those rimmed by high-U overgrowths (Fig. 11a). Some centers are less altered and exhibit magmatic zoning sometimes overgrown on cores which appear homogeneous in cathodoluminescent imaging. An age range of 1.0–2.4 Ga has been obtained for the 13 centers analyzed (Fig. 12; Table 1), with unaltered centers giving concordant ages at ca. 1.8, 2.0, and 2.2 Ga. The magmatic inherited component yields ages of 1.8 Ga or less, with concordant grains at ca. 1.8 Ga. Migmatite 97013 The zircon population in sample 97013 (Fig. 10b) consists mainly of elongate, rounded, and pitted grains. In addition, a small proportion of the population consists of clear euhedral zircon grains that show magmatic oscillatory zoning, with some having high-U outer rims. The small number of analy-

933

ses on the clear magmatic rims yield a near-concordant 206 Pb*/238U weighted mean age of 55.9 ± 3.1 Ma (n = 4, MSWD = 1.18) (Fig. 12; Table 2). Where structurally distinct centers are present in these grains, they are invariably highly altered and this texture suggests melting. Rounded zircons in this sample include magmatic elongate grains, as well as equant grains with magmatic rims similar in structure to the elongate rounded grains. The age range of the rounded grains and inherited centers is ca. 1.6–2.4 Ga, with near-concordant zircon at ca. 1.8, 2.1, and 2.4 Ga. The range of the magmatic overgrowths, however, is ca. 1.6–1.8 Ga, with concordant zircon only at ca. 1.8 Ga (Fig. 12; Table 2), indicating a magmatic inherited component of this age. Leucogranite 97046 The zircon population of the Ladybird leucogranite analyzed in this study consists largely of clear, euhedral magmatic grains that show oscillatory zoning growth structures under cathodoluminescent imaging (Fig. 11c). The magmatic zircon population yielded a weighted mean 206 Pb*/238U age calculation of 59.8 ± 1.0 Ma (n = 16, MSWD = 1.66), interpreted to be the crystallization age of the leucogranite. High-U rims are apparent on some magmatic zircon grains; however, there is no significant difference in age between these rims and the centers of the magmatic grains. A small proportion of grains have inherited centers evident in cathodoluminescent imaging (Fig. 10c). Although these centers are generally clear and unaltered, they are nevertheless highly discordant, yielding ages from ca. 1.0 to 2.2 Ga for the 11 centers analyzed (Fig. 12; Table 3). Inherited centers showing distinct magmatic zoning under cathodoluminescent imaging invariably have ages 2.4 Ga for inherited cores of zircons from migmatitic leucosomes of the Thor–Odin dome, and - 60 Ma for magmatic rims and discordant ages for preserved inherited cores of zircons from a leucogranite emplaced within the detachment zone bordering the Shuswap MCC to the west. These results imply that (1) the migmatitic gneisses forming the core of the Thor–Odin dome were formed during an early Tertiary melting event and thus do not necessarily represent basement culminations or the top of a basement duplex; (2) leucogranites were generated during this melting stage at various levels of the terrane, suggesting that a plumbing system for melt existed from the migmatitic core to the detachment between at least -60 Ma and -56 Ma; (3) leucogranitic magma accumulated in the detachments and crystallized first at -60 Ma, and the majority of the granitic bodies had crystallized by -56 Ma; and (4) the rela-

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Acknowledgments This work was supported by National Science Foundation Grant NSF-EAR-9526915. We would like to thank Tim Benedict, Virginia Davies, Denis Cohen, Jerry Mullin, Britt Norlander, and Matevz Lorencak for sharing the joy of scenic British Columbia and the rock load, and John Mya and Shane Paxton for mineral separation at ANU in exchange of a few cakes at tea time. The constructive participation of Karen Kleinspehn and Jean-Pierre Burg in discussing interpretations in the field was much appreciated. The paper benefitted from the reviews of Richard Friedman and, in particular, Philip Simony who gave critical comments addressing the regional significance of the data presented in this paper. OV would like to express his gratitude to the Revelstoke community and more specifically to the Johnston-McKnight family, the McNutt’s, Dave Hickey, and Nan Campbell (what a party animal!) for their spontaneous and warm hospitality.

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