Description:

_Journal of Structural Geology 33 (2011) 399-421_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Deciphering cryptic PeT-det histories in the western Thor-Odin dome, Monashee Mountains, Canadian Cordillera: A key to unravelling pre-Cordilleran tectonic signatures M. Iole Spalla a,*, Davide Zanoni b, Paul F. Williams b, Guido Gosso a Dipartimento di Scienze della Terra “A. Desio”, Università di Milano and CNR-IDPA, Via Mangiagalli 34, 20133 Milano, Italy b Department of Geology, University of New Brunswick, 2 Bailey Dr., Fredericton, NB, Canada E3B 5A3 Article info Article history: Received 3 May 2010; Received in revised form 16 November 2010; Accepted 17 November 2010; Available online 2 December 2010 Keywords: PeT-det paths, HT exhumation, HT transposition, Tectonometamorphic memory Abstract A detailed structural and metamorphic study along a transect from the core of the Thor-Odin dome to its western margin (South-eastern Canadian Cordillera) has allowed the reconstruction of the PeT-det history estimating PT changes during successive fabric development. The dome comprises a recumbently-folded and transposed Proterozoic high-grade basement and Late Proterozoic-Palaeozoic metasedimentary cover, characterised by a regional foliation (ST). An inferred nearly-isothermal decompressional path from a maximum depth of ca. 40 km transgresses the “maximum relaxed geotherm”, consistent with exhumation during lithospheric extension. Contrasting pre-ST PT conditions indicate that different units followed different PT paths before ST development. Traces of these early and variable structural and metamorphic re-equilibration steps are preserved in meter-scale boudins and granular-scale domains poorly affected by transposition. The various paths represent events from Palaeoproterozoic to Proterozoic Rodinia break up, to Jurassic-Cretaceous convergence and final exhumation in the Eocene. Much of the deformational and metamorphic history may have been accumulated before onset of ST. However, the early structural history is overprinted, and mineral assemblages and their relative ages are mostly obscured by ST development. A glimpse of the earlier geologic history is possible only by combining micro-structural analysis and PT estimates. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction PeT-det (Pressure, Temperature and relative or absolute time) paths integrated with thermal and mechanical models (e.g., Thompson, 1981; Cloos, 1993; Stöckhert and Gerya, 2005; Spalla and Marotta, 2007; and refs. therein), produced to simulate the dynamics of convergent and divergent tectonic environments, have been widely used to elucidate the geodynamic evolution of orogenic domains (Spalla et al., 2010 and refs. therein). Reference to a previously defined structural history is essential for reconstruction of PeT-det paths (Spalla, 1993; Johnson and Vernon, 1995), due to the number of factors (rock type, P-T-?uid conditions, deformation history, diffusion etc.) influencing the accomplishment of metamorphic reactions during deformation (e.g., Williams, 1985; Vernon, 1989; Passchier and Trouw, 1996). * Corresponding author. Tel.: +39 (0)250315550; fax: +39 (0)250315494. E-mail addresses: iole.spalla@unimi.it (M.I. Spalla), dzanoni@unb.ca (D. Zanoni), pfw@unb.ca (P.F. Williams), guido.gosso@unimi.it (G. Gosso). 0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.11.014 Spalla et al., 2000; Brown, 2001; Hobbs et al., 2010). For this purpose, emphasis in the field has to be on establishing sequences of fabric overprinting and on distribution of fabric gradients (e.g., Williams, 1985). This involves sampling the widest range of thermo-mechanical stages and discrimination of micro-structural sites in which evidence of the metamorphic and deformational history is best preserved. Our sampling strategy therefore is based on the assumption that rocks, which preserve a relatively longer metamorphic memory, are those in which the superposed planar fabrics are less evolved locally. Consequently, the structural correlation in a heterogeneously deformed area has to be grounded on geometrical criteria and metamorphic compatibility of the mineral assemblages defining different fabrics. Our aim is to produce an accurate reconstruction of the PeT-det evolution of rocks from the Thor-Odin dome (Fig. 1) in order to shed light on debated tectonic interpretations (e.g., Brown and Carr, 1990; Brown et al., 1992; Johnston et al., 2000; Kruse and Williams, 2007). For this purpose new results are presented for metapelites and amphibolite boudins sampled along a 5 km long E-W transect from the core of the dome to its western margin. 400 M.I. Spalla et al. Journal of Structural Geology 33 (2011) 399-421 Mineral abbreviations are as in Kretz (1983) after Whitney and Evans (2010), except for white mica (Wm). 2. Geological setting Fig. 1. Geological sketch map of the Omineca belt at the latitude of the Thor-Odin dome, modified after Carr (1991). 1 ? Proterozoic North American basement rocks. 2 ? Late Proterozoic or younger North American cover sequence. 3 ? Late Proterozoic to Mesozoic metamorphic assemblages. 4 ? Palaeozoic to Lower Jurassic strata. 5 ? Jurassic plutonic rocks. 6 ? Cretaceous to Eocene plutonic rocks. 7 ? Fault and shear zones; teeth are on the hanging wall. 8 ? Monashee décollement; triangles on the hanging wall. Rectangle with thick outline locates Fig. 2. Black dots and bold numbers locate the age data shown in Table 1a. Monashee décollement has been drawn only north of Three Valley Gap as shown by Read and Brown (1981), Journeay (1986) and Brown et al. (1992). We have found no evidence of its southward prolongation. Inset map shows the five belts constituting the Canadian Cordillera according to Gabrielse et al. (1991); RMT ? Rocky Mountain Trench; FRF ? Fraser River Fault. (Greenbush Lake shear-band zone - GLSBZ; Figs. 1 and 2). The deformation history has been determined by means of detailed mapping (Kruse et al., 2004) and structural analysis (Johnston et al., 2000; Williams and Jiang, 2005; Kruse and Williams, 2007). The sequence of metamorphic reactions has been inferred taking into account the timing of mineral growth with respect to successive stages of fabric development. Subsequently PT conditions have been derived from chemical data acquired for minerals defining chronologically selected fabric elements. Using fabric evolution and the sequence of mineral assemblages supporting the fabrics in various rock groups, together with numerous radiometric data available in the literature (Table 1a and 1b and Fig. 2), we construct a relative chronology of tectonometamorphic stages. The Canadian Cordillera comprises five belts, parallel to the strike of the chain, that differ in lithostratigraphic associations and tectonic style viz. the Foreland, Omineca, Intermontane, Coastal and Insular belts (inset of Fig. 1). Major tectonic boundaries commonly separate these belts. The Omineca belt represents the hinterland of the Foreland belt and was exhumed as a result of the collision between the accreted terranes (Intermontane, Coastal and Insular belts) and the North American (Laurentian) margin (Monger et al., 1982; Gabrielse et al., 1991) between Middle-Late Jurassic and Cretaceous (Gibson et al., 2008). The Monashee complex is one of the structural culminations of the southern Omineca belt, which exposes the deepest part of the Canadian Cordillera. It is divided into two domes: Frenchman Cap and Thor-Odin (Fig. 1). The cores of these domes consist of Palaeoproterozoic basement with high-grade polymetamorphic migmatitic para- and ortho-gneisses and metabasics as well as Proterozoic intrusions (Reesor and Moore, 1971; Wanless and Reesor, 1975; Armstrong et al., 1991; Parkinson, 1991; Carr, 1995; Crowley, 1997). This basement is overlain unconformably (Ross and Parrish, 1991 and refs. therein) by a Proterozoic to Palaeozoic metasedimentary sequence, comprising quartzites, pelitic schists, semi-pelitic schists, marbles, calcsilicate gneisses, and various quartzo-feldspathic gneisses (e.g., Brown, 1980; Kruse et al., 2004). At all scales from regional to microscopic, basement to cover are repeated by transposition, and the ratio of cover and basement increases upwards. The Monashee complex is tectonically overlain by relatively lower grade (amphibolite and greenschist facies) metamorphic rocks, which comprise a unit known in the literature as the Selkirk Allochthon (Brown et al., 1992). The boundary between Monashee complex and the Selkirk Allochthon, in the Thor-Odin dome, like all boundaries in the complex, is strongly transposed by folding into a common ST (Johnston et al., 2000; Kruse et al., 2004; Kruse and Williams, 2007). The ubiquitous transposition fabric (ST) is the product of a penetrative non-coaxial flow. The sense of shear is consistently top-to-the-northeast in the Monashee complex and the lower part of the Selkirk Allochthon, and is top-to-the-southwest at higher structural levels (Journeay, 1986; Johnston et al., 2000; Williams and Jiang, 2005; Kuiper et al., 2006). The western boundary of the Thor-Odin dome is marked by a west dipping high strain zone with a top-to-the-west sense of shear, successively overprinted by north-south trending Eocene brittle strike-slip, and normal faults (Johnston et al., 2000; Kruse and Williams, 2007). Spatially, the normal shear zone coincides approximately with the southern prolongation of the Monashee décollement and the transition between the Monashee complex and the Selkirk Allochthon (Journeay and Brown, 1986; Brown et al., 1992; McNicoll and Brown, 1995). The Monashee décollement is interpreted as a to-the-northeast movement zone for which no evidence is found along the transect here discussed, other than the ubiquitous tra' Ключевые слова: grt-bearing amphibolite, contributions mineralogy, garnet, kruse, pl, fault, ? qz, altot content, petrology holland, geobarometer, foliation, calcic amphibole, pre, mineral chemistry, lavrent eva, thor-odin dome, adams, breton, mineralogist, canadian cordillera, shear, monashee complex, micro-structural site, powell, ha, fabric evolution, compositional trend, re-equilibration stage, condition, experimental investigation, earth, williams, ky-bearing, brown, reaction, syn, syn-st, altot, journal structural, sil-granulite facies, partial melting, amphibolite, chl, nal cooling, gosso, geological survey, thompson, age data, rim, qz, canada, journal, cordillera, granulites, ernst, holness, valley gap, cover, newton, spalla journal, journal petrology, high angle, ky-bearing spl-bearing, age, geological map, experimental data, pegmatite, kfs, metabasics, fabric, ampiii, contributions, channel, ti, structural evolution, grain, geological, bt, zrn, kuiper, constant table, ky, semi-pelitic gneiss, tectonics, thor, monashee dcollement, bearing, lithospheric thinning, pressure, late, radiometric age, micro-structural, xmg, country rock, post-st, pre-st, pre-st stage, carr, petrology, bulk chemistry, composition, metamorphic imprint, metamorphic reaction, amphibolite facies, early stage, transposition, spl-bearing metapelites, ky-bearing metapelites, british, sil, content, thesis university, american journal, red arrow, monashee, thor-odin, nal stage, immature fold, spear, ab, development, alvi, shear zone, monashee decollement, estimate, stage, sciences, re-equilibration, structural, dome, pt, pb pb, contributions mineralogy petrology, northern margin, deformation, thor odin, hornblende, ampii grain, geology, petrology bohlen, compositional variation, mineral, liou, british columbia, core, amp, spalla, ma, compositional range, assemblage, bulk composition, reaction rim, parkinson, garnet amphibolite, quarter-wave plate, grt, blanket glacier, structural geology, dt, zone, boudin neck, history, american mineralogist, columbia, mineral pair, basement, hbl, metamorphic, sample, gneiss, tectonic evolution, mineralogical, gpa, johnston, locally, experimental calibration, hinchey hinchey, relict, omineca belt, belt, dt ?, odin, metamorphic age, northern slope, metamorphic re-equilibration, rational boundary, duncan, metapelites, evolution, experimental study, american, ghent, fold, bohlen, prismatic sil, lms, thermocalc holland, crowley, relationship, massonne, blanket area, canadian, hinchey, independent thermometer, complex, eds, pli, st, review, university, plane, mineral phase, wm, porphyroclasts, southern, ampii, micro-structural feature, re-equilibration occurred, high, journal structural geology, selkirk allochthon, experimental, relict grt, internal strain, small grain, ky-spl-free metapelites, cpx, pb, generally, area, blanket, rock, table, progress report, absolute age