Mesozoic alteration of Permian volcanic rocks (Thüringer Wald, Germany): Ar, Sr and O isotope evidence

Abstract Besides older granites and gneisses, the Thuringer Wald in central Europe consists predominantly of altered Permian volcanic and pyroclastic rocks The alteration history of these volcanic rocks was investigated by K–Ar, 39 Ar K – 40 Ar, Rb–Sr, δ 18 O, δ D and X-ray studies on feldspar and biotite separates. Alteration is accompanied by increases in the triclinicity and δ 18 O values (+14‰ to +19‰) of alkali feldspars, an increase in the δ 18 O values of low-potassium biotites (>+10‰) and in sericitized plagioclases (+12‰ to +19‰) and also the generation of apparent Mesozoic isotopic ages. In contrast, volcanic biotites with primary magmatic δ 18 O signatures still yield Early Permian ages, and biotites of the granitic basement give Carboniferous ages. The observed isotope changes of the volcanic feldspars indicate episodic fluid exchange with meteoric and formation brines ( δ 18 O≤+5‰) at low temperatures (200 to 100 °C) during tectonic fracturing of the basement. As a result, sanidines were replaced by porous microclines which were subject to isotope exchange and Ar loss. Feldspars and biotites of the granitic wall rocks seem to be less affected by alteration processes, which may be due to their lower permeabilities. Plateau-shaped 40 Ar/ 39 Ar age spectra of volcanic alkali feldspar indicate complete resetting of the K–Ar system, whereas staircase spectra point to partial retention or repeated Ar loss. Because the feldspars represent mixtures of microcline and sanidine remnants, the apparent ages (260 to 160 Ma) represent maximum ages for the time of replacement. Rb–Sr feldspar model ages (180 Ma or 230 to 170 Ma, respectively) support the K–Ar data. The minimal 40 Ar/ 39 Ar spectra ages are concordant with both K–Ar and 40 Ar/ 39 Ar sericite model ages (160 to 150 Ma) of sericitized plagioclases. The model ages favour Jurassic episodic fluid reactions with alkali feldspars and plagioclases in the Thuringer Wald.