EXPERIMENTAL, GEOCHEMICAL, MINERALOGICAL AND O-ISOTOPE CONSTRAINTS ONTHE LATE-MAGMATIC HISTORY OF THE FICHTELGEBIRGE GRANITES (GERMANY)

Mineralogical, experimental, geochemical and stable-isotope data give evidence for an aqueous late-stage fluid which develops increasingly f rom the less to the more highly evolved granites of the Fichtelgebirge (G1-G4). In less-evolved granites (particularly G1) only minor chlori tization and sericitization, minor hydration and corrosion of zircons, low leachable fractions of Sr and Zr, the nearly chondritic ratios of Zr/Hf and Y/Ho, and rare-earth-element (REE) patterns lacking the tet rad effect provide evidence of a less-differentiated granitic melt sys tem, poor in complexing agents and water, with insignificant late magm atic fluid-rock interaction. The subsequent low-temperature alteration , which is suggested by oxygen-isotope composition of quartz and bioti te, did not affect the whole-rock chemical composition. In the highly evolved granites (particularly G4), albitization, sericitization and f luoritization. ubiquitous hydration and corrosion of zircons, high lea chable fractions of Sr and Zr, Zr/Hf and Y/Ho ratios very different fr om the chondritic ratios, and REE patterns showing a marked tetrad eff ect point to the presence of a highly specialized fluid system, rich i n water and complexing agents. such as fluorine. leading to a marked m agmatic-hydrothermal alteration. The oxygen isotopes also indicate alt eration by high-temperature fluids which were likely exsolved during t he final stage of crystallization.