A Low-Variance Mineral Assemblage with Talc and Phengite in an Eclogite from the Saxonian Erzgebirge, Central Europe, and its P-T Evolution

Abstract

A light-coloured, fine-grained eclogite sample from near the village of Hammerunterwiesenthal in the Erzgebirge (NW Bohemian Massif ) preserves the low-variance mineral assemblage of garnet, omphacite, phengite, talc, amphibole, clinozoisite, quartz, rutile, and accessory phases. Porphyroblasts of amphibole, clinozoisite, and phengite formed during a late stage (III) of metamorphism. Paragonite joined the assemblage late in this stage (IIIb). The chemical zonation of the minerals was carefully studied. Various geothermobarometric methods were applied, especially involving phengite and talc. The constrained P–T path for the eclogite starts at about 480 C and 25 kbar (stage Ib), followed by a significant temperature rise (stage II) at slightly increasing pressure. At the peak P–T conditions of 720 C and 27 kbar, blastesis of amphibole, clinozoisite, and phengite was caused by infiltrating hydrous fluids. The resulting density reduction may have allowed buoyant uplift of the eclogite. Subsequently, significant cooling occurred at high pressures. Stage IIIb is characterized by P–T conditions around 520 C and 18 kbar at reduced water activities. This unusual late P–T evolution might explain the freshness of the eclogite, including the preservation of chemical zonation on the micrometre scale.