Experimental, geochemical, mineralogical and O-isotope constraints on the late-magmatic history of the Fichtelgebirge granites (Germany)

Abstract

Mineralogical, experimental, geochemical and stable-isotope data give evidence for an aqueous late-stage fluid which develops increasingly from the less to the more highly evolved granites of the Fichtelgebirge (G1–G4). In less-evolved granites (particularly G1) only minor chloritization 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 tetrad effect provide evidence of a less-differentiated granitic melt system, poor in complexing agents and water, with insignificant late magmatic fluid–rock interaction. The subsequent low-temperature alteration, which is suggested by oxygen-isotope composition of quartz and biotite, did not affect the whole-rock chemical composition. In the highly evolved granites (particularly G4), albitization, sericitization and fluoritization, ubiquitous hydration and corrosion of zircons, high leachable fractions of Sr and Zr, Zr/Hf and Y/Ho ratios very different from the chondritic ratios, and REE patterns showing a marked tetrad effect point to the presence of a highly specialized fluid system, rich in water and complexing agents, such as fluorine, leading to a marked magmatic–hydrothermal alteration. The oxygen isotopes also indicate alteration by high-temperature fluids which were likely exsolved during the final stage of crystallization.