Retrograde Andalusite and Staurolite Coronas around Spinel in Garnet-Cordierite-Sillimanite-Biotite Gneisses of the Dzirula Massif (Georgia)

摘要

The Dzirula crystalline massif is window into the Pre-Jurassic basement within the Black Sea-Central Transcaucasus terrain which is a part of the Alpine-Himalayan collision zone [1]. The massif is composed of a gneiss-migmatite complex, metabasites, and quartz-diorite orthogneisses intruded by Late Hercyn-ian granites, as well as a metavolcanic sedimentary complex with rhyolites and closely associated ophi-olitic fragments. Rocks comprising the gneiss-migmatite complex experienced regional metamorphism of a low-pressure And-Sil type~1 which, according to I.P. Gamkrelidze and D.M. Shengelia [1,2], was manifested twice: first, in the Cadomian cycle at a temperature transient from sillimanite-biotite-K-feldspar and garnet-cordierite-K-feldspar facies [3] and, second, during the Variscan cycle under medium biotite-andalusite-muscovite subfacies conditions [2-4]. There also exists some evidence for Late Hercynian greenschist retrograde metamorphism.