Abstract

Near Pinchi Lake, central British Columbia, a fault-bounded alpine peridotite occurs within the Cache Creek Group. It comprises massive harzburgite, dunite, and pyroxenite, all of which display tectonite fabrics. Near isoclinal folds (F1), outlined by dunite, pyroxenite, and spinel layers (compositional layering) constitute the earliest recognizable penetrative deformation. A second set of near-coaxial open folds (F2) refolds the earlier set; both are cut by a later fracture cleavage (F3). Flattened, elongate olivine and orthopyroxene grains within the harzburgite outline foliations and lineations parallel with axial surfaces and minor fold axes of both fold sets. Narrow zones of mylonitization within the isoclinally folded dunites transect the composition layering, are parallel with the axial surfaces of the F1 isoclines, and appear to be related to refolding of F1 by F2. Relic olivines are kinked along {OKL}:[100], have pronounced α-maxima perpendicular to F1 and γ-axes maxima parallel to fold axis L1. Orthopyroxene, kinked on (100):[001] shows weaker fabric, but is symmetric with earliest composition layering, some 15°–20° away from F1. Asymmetry of fabrics results from olivine recrystallizing under conditions where orthopyroxene only kinks.It is speculated that the peridotite is a fragment of Paleozoic oceanic lithosphere. F1 and F2 are believed to be mantle transport fabrics resulting from a northerly driven plate, whereas F3 fractures are high-level emplacement features produced during late Paleozoic time.

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