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The internal fabric of an alpine peridotite near Pinchi Lake, central British Columbia

1977; NRC Research Press; Volume: 14; Issue: 1 Linguagem: Inglês

10.1139/e77-004

1480-3313

John V. Ross,

High-pressure geophysics and materials

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 (F 1 ), outlined by dunite, pyroxenite, and spinel layers (compositional layering) constitute the earliest recognizable penetrative deformation. A second set of near-coaxial open folds (F 2 ) refolds the earlier set; both are cut by a later fracture cleavage (F 3 ). 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 F 1 isoclines, and appear to be related to refolding of F 1 by F 2 . Relic olivines are kinked along {OKL}:[100], have pronounced α-maxima perpendicular to F 1 and γ-axes maxima parallel to fold axis L 1 . Orthopyroxene, kinked on (100):[001] shows weaker fabric, but is symmetric with earliest composition layering, some 15°–20° away from F 1 . 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. F 1 and F 2 are believed to be mantle transport fabrics resulting from a northerly driven plate, whereas F 3 fractures are high-level emplacement features produced during late Paleozoic time.