MAGMATIC AND HIGH-PRESSURE METAMORPHIC DEVELOPMENT OF ORTHOGNEISSES IN THE SOPRON AREA, EASTERN ALPS (W-HUNGARY)

Orthogneisses with quartz-albite-K-feldspar-white mica +/- biotite +/- garnet +/- clinozoisite +/- epidote +/- chlorite +/- ilmenite +/- rut ile +/- monazite +/- apatite were studied from the Grob gneiss series (Lower Austroalpine Unit, Eastern Alps) near Sopron. Relict magmatic p hases, such as zoned Fe-Mn garnet, muscovite, corroded biotite with ex solved rutile needles and perthitic K-feldspar are ubiquitous. On the basis of relict garnet composition the pressure of garnet formation wa s estimated to be below 0.4 GPa at temperatures below 700 degrees C. S ubsolidus breakdown of garnet and K-feldspar and formation of muscovit e and albite is established from textural evidence. The high pressure metamorphic assemblage consists of Ca-Fe-Mn garnet, biotite, phengite, K-feldspar, albite and clinozoisite. Two-feldspar thermometry provide s the minimum temperature estimate of about 450 degrees C for the high pressure metamorphism overprinting the magmatic assemblages. Phengite s with 7.03 Si atoms pfu. indicate 1.3 and 1.4 GPa peak pressure at 45 0 and 550 degrees C, respectively. Peak temperature conditions postdat e the pressure peak, as indicated by the phengite inclusions in garnet . Peak temperature probably did not exceed 600 degrees C. The high-pre ssure metamorphism occurred in a fluid-rich environment, as indicated by the breakdown of the K-feldspar to albite+phengite and by primary a queous fluid inclusions trapped in the core of the albite, together wi th the phengites.