Garnet reaction rims from the breakdown of Staurolite in polymetamorphic micashists from the Rappold complex, Austroalpine basement, Eastern Alps

Abstract

In polymetamorphic pelites of the Rappold complex in the Wolz crystalline basement (Eastern Alps) reaction rim garnets at staurolite-quartz interfaces (type I) and single grain garnets along previous staurolite-white mica interfaces (type II) were formed. The garnet reaction rims were formed during the Cretaceous amphibolite facies metamorphic overprint of the pre-existing mineral assemblages comprising garnet, staurolite, and kyanite from an amphibolite facies metamorphic event probably of Variscian age. The newly formed garnet may take the form of reaction rims along the margins of large pre-existing staurolite blasts. The initial growth increments of garnet have low grossular content, and reaction rim growth was controlled by the transfer of Fe, Mg and Mn components from the staurolite-garnet interface to the quartz-garnet interface. Later garnet growth increments have relatively high grossular content due to consumption of matrix plagioclase, which was destabilized by successive pressure increase. The grossular content of newly formed garnet shows systematic increase towards sites where plagioclase breaks down indicating that transport of calcium through the matrix was sluggish. On the basis of reaction microstructures it is demonstrated that the mineral assemblage garnet + kyanite + biotite + paragonite was formed at the conditions of eo-alpine amphibolite facies overprint while staurolite and plagioclase broke down successively with increasing pressure.