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Geology of the Pinchi Lake area, central British Columbia Paterson, Ian Arthur

Abstract

The area mapped, 75 square miles, lies astride the Pinchi Fault near Fort St. James in central British Columbia. Northeast of the fault system is the lower Mesozoic Takla Group composed of greywackes, conglomerates and minor limestones. Southwest of the fault system is the Pennsylvanian-Permian Cache Creek Group, made up of limestones, cherts, argillites and greenstones. Between these regions, a complex northwesterly-trending fault system involves a series of elongate fault-bounded blocks of contrasting lithology and/or metamorphic grade. Rock types making up individual blocks include: (a) lawsonite-glaucophane metasediments and meta- volcanics, (b) pumpellyite-aragonite greenstones, (c) serpentinized harzburgites and dunites and (d) a sequence of amphibolitized gabbro-diabase basalt. Boulders of lawsonite-glaucophane eclogite are also found in the area. Within the glaucophanitic rocks a metamorphic foliation (S₁) parallels the bedding and contains a mineral lineation (L₁). This foliation is deformed by F₂ folds and mullions which are accompanied by a prominent L₂ crenulate lineation. These structures are deformed by F₃ kink folds. Significant mineral assemblages within the glauco-phanitic rocks include: (i) quartz + lawsonite + sphene + phengite + glaucophane ± carbonaceous material (metacherts) (ii) aragonite + dolomite (limestones) (iii) jadeitic pyroxene + lawsonite + quartz + white mica + chlorite (metagreywacke) (iv) quartz + white mica + lawsonite ± glaucophane + pyrite + carbonaceous material (carbonaceous schists) (v) acmitic pyroxene + lawsonite + sphene + chlorite (metavolcanics) (vi) glaucophane + lawsonite + sphene + chlorite (metavolcanics). Comparison of these assemblages with experimentally determined phase equilibria favours the hypothesis that the glaucophanitic rocks formed at high lithostatic pressures and relatively low temperatures. Because there is no evidence for a metamorphic zonation with respect to the Pinchi Fault, and because metagreywackes and metavolcanics are commonly unsheared, tectonic overpressures are not considered to have contributed appreciably to the total pressure. In the late stages of metamorphism the fluid phase in carbonaceous schists became progressively more reducing and may have contained appreciable methane. This fluid reacted with acmitic metavolcanics (v) to give glaucophane bearing assemblages (vi). Thus high pressure mineral assemblages with high Fe³⁺/Fe²⁺ (i.e. v) existed within the metavolcanics prior to reaction with the reducing fluid. A three stage tectonic model is proposed. Firstly, during the Late Permian, a narrow wedge of Upper Paleozoic sediment was metamorphosed in the blueschist facies along an easterly dipping subduction zone approximately on the site of the Pinchi Fault. This event is considered to be contemporaneous with the formation of F₁ structures. At a higher crustal level, an ophiolite sequence of which unit (d) is a remnant was obducted over the Cache Creek Group sediments which had accumulated above the subduction zone. During the second stage, a change in the stress system converted the subduction zone into a right-lateral strike-slip or oblique-slip fault. This event marked the beginning of the F₂ deformation. Strike-slip movement resulted in formation of zones of low pressure where there were deflections or offsets in the original fault. At Pinchi, such zones were at once filled from below by diapirs of low average density consisting of subducted blueschist, serpentinized ultramafites and minor eclogite which rose in the crust to a level governed by isostasy. It is considered that the Middle or Upper Triassic K-Ar dates obtained from phengitic micas (211, 214, 216 and 218 ± 7 m yrs) record either the time of cooling of the uplifted blueschists or the end of the F₂ deformation. The third stage, during the Upper Triassic, involved erosion of the ophiolites exposed on a topographic high to the west of the Pinchi Fault and flysch deposition in the adjacent trough to the east. The F₃ deformation was contemporaneous with carbonatization of fault zones and possibly occurred during the Eocene.

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