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Metamorphism southwest of Yale, British Columbia Pigage, Lee Case

Abstract

Pelitic metasediments immediately southwest of Yale, British Columbia contain mineral assemblages characteristic of staurolite through sillimanite zones of the Barrovian facies series. Isograds are steeply dipping. Two phases of deformation are recognized with metamorphism being syn- to post-tectonic. Pseudomorphs after andalusite indicate that contact metamorphism preceded regional upgrading of the pelites. Microprobe analyses of major silicate phases in the pelites are combined with linear regression techniques in discussing possible kyanite- and sillimanite-forming reactions. A zone some 3 kilometers wide contains the assemblage staurolite-kyanite-garnet-biotite-muscovite-quartz which is univariant in AFM projection. Regression analysis of the staurolite-kyanite assemblage reveals the sensitivity of regression methods to error limits associated with the different minerals present. Precision of the analyses was not high enough for regression analysis to differentiate between stable, divariant equilibrium and a buffered, univariant reaction relation for this assemblage. Pelitic and calc-silicate assemblages from the metasediments restrict pressure-temperature conditions during regional metamorphism to 5½-8 kilobars and 550-700°C. It is suggested that deformation, emplacement of granitic intrusions, and regional metamorphism are all part of the Cretaceous orogeny which formed structures of the Cascades Mountains.

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