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Geology and metamorphism of the Yale Creek area, B.C. Bartholomew, Paul Richard

Abstract

The Yale Creek area is underlain by the pelitic Settler Schist, Spuzzum hornblende quartz diorites, and minor gneiss and granodiorite. Mineral textures in the schists define three phases of folding. The structural geology is dominated by the second phase, f₂, characterized by northwest plunging isoclinal folds. Regional metamorphism reached its culmination just after f₂. The structurally concordant and broadly syn-tectonic Spuzzum intrusions were emplaced over a period of time encompassing the period of folding and regional metamorphism and served as a heat source for the regional metamorphism and early contact metamorphism. Late faulting has juxtaposed Settler Schist and Custer Gneiss across the Hope Fault, and Settler Schists of contrasting metamorphic grades across the Cogburn Creek Fault. A Triassic Rb-Sr date obtained for the Settler Schist is interpreted as the date of deposition of original—eugeosyclinal sedimentsHornblende K-Ar ages of 75.5±2.6 and 92.1±3.2 Ma obtained for Spuzzum quartz diorites conform to a regional east-younging trend in Spuzzum K-Ar ages which reflects the cooling history of the area rather than age of intrusive emplacement. The date of emplacement of the structurally discordant and post-tectonic body of granodiorite was defined as 32±2Ma by concordant Rb-Sr and K-Ar dates. The distribution of aluminosilicate assemblages in the Settler Schist defines three regional metamorphic zones increasing in grade to the north. The kyanite zone and two sillimanite zones are separated by two isograds which mark equilibrium and overstepped kyanite-sillimanite transitions. The metamorphic conditions near the first sillimanite iso- grad are estimated to be 705±45°C and 7.6±0.5kb from the mutual intersection of equilibria calculated from thermodynamic data and microprobe analyses. Anatectic relations established to date for pelitic systems appear to be inconsistent with, this thermodynamic P-T estimate. Mineral assemblages in scattered ultramafic pods are consistent with the metamorphic P-T estimates for the pelitic assemblages.

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