Magmatic and high-pressure metamorphic development of orthogneisses in the Sopron area, Eastern Alps (W-Hungary)
published: Mar 24, 1998
ArtNo. ESP154017301004, Price: 29.00 €
Orthogneisses with quartz-albite- K-feldspar-white mica ± biotite ± garnet ± clinozoisite ± epidote ± chlorite ± ilmenite ± rutile ± monazite ± apatite were studied from the Grob gneiss series (Lower Austroalpine Unit, Eastern Alps) near Sopron. Relict magmatic phases, such as zoned Fe-Mn garnet, muscovite, corroded biotite with exsolved rutile needles and perthitic K-feldspar are ubiquitous. On the basis of relict garnet composition the pressure of garnet formation was estimated to be below 0.4 GPa at temperatures below 700°C. Subsolidus breakdown of garnet and K-feldspar and formation of muscovite and albite is established from textural evidence. The high pressure metamorphic assemblage consists of Ca-Fe-Mn gamet, biotite, phengite, K-feldspar, albite and clinozoisite. Two-feldspar thermometry provides the minimum temperature estimate of about 450°C for the high pressure metamorphism overprinting the magmatic assemblages. Phengites with 7.03 Si atoms pfu. indicate 1.3 and 1.4 GPa peak pressure at 450 and 550 'C, respectively. Peak temperature conditions postdate the pressure peak, as indicated by the phengite inclusions in garnet. Peak temperature probably did not exceed 600°C. The high-pressure metamorphism occurred in a fluid-rich environment, as indicated by the breakdown of the K-feldspar to albite+phengite and by primary aqueous fluid inclusions trapped in the core of the albite, together with the phengites.