High-pressure/low-temperature metamorphism of the Kő-hegy gneiss, Sopron (W-Hungary); phengite barometry and fluid inclusions
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 917 - 926
published: Oct 30, 1996
manuscript accepted: Apr 16, 1996
manuscript received: Sep 22, 1995
ArtNo. ESP147050805004, Price: 29.00 €
Abstract A first record of Alpine high-pressure/low-temperature, blueschist-facies metamorphism is reported from a garnet-bearing orthogneiss in the vicinity of Sopron area, western Hungary, in the Grobgneiss series of the Eastern Alps. The high-pressure conditions are marked by phengite-3T with Si contents up to 3.49 atoms p.f.u., which is in close association with biotite, K-feldspar, quartz and grossular-rich garnet. Peak metamorphic pressure conditions were about 1.2 GPa at 450-500°C. Some of the white micas preserved low-Si (about 3.1 atoms p.f.u.) and high-Ti magmatic cores. Four types of aqueous fluid inclusions can be distinguished in the studied rock: 1. Primary, intermediate salinity (10.9-12.4 NaCl eq. wt%) NaCl-H2O inclusions in albite with Th = 178-217°C. This type may be the high-pressure fluid, reequilibrated at lower pressures. 2. Secondary, intermediate salinity (7.1-12.4 NaCl eq. wt%) NaCl-H2O inclusions in quartz veins, cross-cutting the S2 schistosity. Homogenisation temperatures range from 85 to 176°C. 3. Secondary, high salinity CaCh-H2O ± NaCl inclusions (24 and 28.4 CaCh eq. wt%) in quartz veins, crosscutting the S2 schistosity. Homogenisation temperatures range between 61 and 171°C. The type 2 and 3 fluids are connected with retrograde metamorphism. 4. Secondary, high salinity NaCl-CaCl2-H2O inclusions with salinity of about 20.4 and 22.2 NaCl eq. wt% and homogenisation temperature between 64°C and 99°C.