Mineralogical, KAr, stable and Sr isotope systematics of K-white micas during very low-grade metamorphism of limestones (Helvetic nappes, western Switzerland)

Abstract

Mineralogical, KAr, RbSr and stable isotope analyses have been carried out on K-white micas from Helvetic Malm limestones in order to examine their evolution during very low- to low-grade Alpine metamorphism, associated with intense ductile deformation. Metamorphic temperatures were estimated at ∼300°C from stable isotopes (quartz-calcite thermometry), occurrence of chloritoid, and “epizonal” illite crystallinity index. K-white micas consist of variable mixtures of 2M 1 phengite and muscovite, as revealed by detailed X-ray diffraction analyses using peak decomposition of the (060, 331) spectra. KAr apparent ages display a strong grain-size dependence in which mainly fine-grained size fractions (<2 μm) record Alpine ages (37-15 Ma). However, these ages provide a relative rather than an absolute chronology of the diachronous Alpine metamorphic evolution of the Helvetic nappes. The resetting of the KAr isotopic system of K-white micas to Alpine metamorphic conditions reflects an apparent combination of crystallization/recrystallization and radiogenic 40Ar diffusion loss. The oxygen isotope compositions of micas (+15 to + 22‰) are intermediate between detrital and 18O-enriched values expected for micas neoformed within an abundant marine carbonate matrix. No isotopic equilibrium has been reached between calcite and micas. The variable depletion of hydrogen isotope compositions (−126 to −82‰) is influenced by the interaction with organic matter under closed-system conditions. Organic matter, if not removed, may also represent a serious source of error in KAr age determination, by introducing radiogenic 40Ar contamination. 87 Sr 86 Sr isotope ratios of micas range from 0.70879 to 0.70902 with one outlier at 0.71794. The low values reflect Sr exchange with calcite occurring during crystallization/recrystallization of micas under closed-system conditions.