Multiple fluid migration events in the Sopron Gneisses during the Alpine high-pressure metamorphism, as recorded by bulk-rock and mineral chemistry and fluid inclusions
published: Dec 14, 2001
Three fluid migration events were distinguished during the Alpine high-pressure and retrograde metamorphism of the orthogneisses and associated leucophyllites in the Sopron area on the basis of mineral chemistry, bulk rock chemistry and fluid inclusions. 1. Fluids close to the peak of the high pressure Alpine metamorphism (P = 1200-1400MPa, T = about 600 °C) are represented by medium salinity (8.7-12.4 NaCl eq. wt.%) NaCl-dominated primary fluid inclusions in the core of the albite grains which coexist with high Si (up to 7.03 a.p.f.u.) phengite. Similar primary fluid inclusions are present in quartz grains enclosed in K-feldspar (salinity between 10.2 and 15.5 NaCl eq. wt.%), and in matrix quartz (salinity between 10.6 and 14.5 NaCl eq.wt.%). Thin garnet-phengite-biotite-bearing quartz veinlets were also found in the orthogneiss and were attributed to small-scale fluid migration close to the peak metamorphic pressure conditions. 2. Fluids causing Mg-metasomatism along shear zones. The orthogneiss has gradually altered with the disappearance of garnet, Fe-rich biotite, K-feldspar and albite to form leucophyllite with or without kyanite. Increase in the Mg content is coupled with decrease of Na and Fe contents from the gneiss to the leucophyllite. Minor elements and REE do not show appreciable variations except of Sr, Zr and Ba which show depletion from metagranite to the leucophyllite. Microstructural observations show that the Mg-metasomatism occurred after the peak of the Alpine high-pressure metamorphism. Fluid inclusion studies indicate that the metasomatism may be attributed to a medium to high salinity (13-29 wt.% total salt content) NaCl- CaCl2-(MgCl2)-H2O fluid. Medium to high salinity NaCl-H2O fluids were also present during the retrogression. 3. Formation of minor phosphate mineralisation in some leucophyllites is attributed to the final stage of the high-salinity NaCl-CaCl2-H2O fluids which carried Mg, Al, Sr, Pb, and P. The effect of this fluid was restricted because the bulk rock chemistry of the leucophyllites does not show any enrichment in Ca, Sr, Pb, Al and P. Sequential crystallisation of different phosphates (Mg-rich lazulite, Pb-bearing goyazite and apatite) shows the change in composition of the latest fluids.