Stable and Sr-isotope evidence for fluid advection during thrusting of the glarus nappe (swiss alps)

Abstract

At the Glarus thrust in the Swiss Helvetic Alps, Permian Verrucano siltstones are allochthonously superimposed over Tertiary Flysch with an intermediate, about 1 metre thin layer of intensively deformed calcmylonite of probable Mesozoic provenance. The H−O−C- and Sr-isotope compositions of minerals from the calc-mylonite and strongly mylonitized Verrucano siltstones were determined in order to assess: (1) equilibrium-disequilibrium relationships; (2) isotopic composition of the fluid phase, its provenance and water/rock ratios; (3) sources of Sr in the calc-mylonite; (4) deformation temperatures. The isotopic composition of cale-mylonite micro-samples from five sites along the thrust varies from 22 to 12‰ and 2 to-10‰ for δ18O and δ13C respectively. All samples are 18O depleted by up to 14‰ relative to the presumed marine Helvetic carbonate protoliths (δ18O=25.4‰±2). A pronounced geographic trend of 18O depletion from 22‰ in the north to 12‰ in the south is observed. In calc-mylonites, 87Sr/86Sr ratios range from typical Mesozoic marine carbonate protolith signatures (0.708±0.005) to more radiogenic values as high as 0.722. A variable contribution of radiogenic 87Sr to the calc-mylonite is though to reflect interaction with fluids that aquired their Sr from the Hercynian granitic basement. Chlorites and muscovites from the calc-mylonite and Verrucano have uniform δ18O values but display δD values from-40 to-147%: the D-enriched values correspond to the primary metamorphic or formational fluids expelled during thrusting, whereas the D-depleted samples reflect selective H-isotope exchange with meteoric fluids during uplift of the Alpine belt. The isotopic composition of the calc-mylonites requires exchange with 18O—depleted, 87Sr—enriched fluids at very high water/rock ratios. Possible sources for these are dewatering of the underlying Flysch and/or metamorphic fluids, or formation brines expelled along the thrust from greater depth. These could be derived from compaction and dewatering of the Flysch in the northern part of the thrust; in the south, however, where Verrucano is thrust over 18O-rich Mesozoic carbonates, the extreme 18O depletion of the calc-mylonite has to be explained either by fluid advection within the Verrucano hanging wall and thrust zone or alternatively by exchange with metamorphic fluids from greater depth, expelled along the thrust. Microstructural evidence (abundant veins, stylolites, breccias) suggests that fluids played an important role in deformation and strain localization. Excepting albite all major components (quartz, chlorite, muscovite, calcite) are both dynamically recrystallized and crystallized as secondary minerals in pressure shadows and syn-mylonitic veins, indicating that these minerals were potentially open to oxygen isotopic exchange during alpine metamorphism and thrust deformation. Within the mylonitized Verrucano silstones, isolated quartz-chlorite and quartz-calcite fractionations yield temperatures of around 320°C close to values obtained from calcite-dolomite thermometry (355°C±30) and in agreement with the regional lower greenschist facies metamorphism. Quartz-calcite and quartz-albite fractionations indicate slightly lower temperatures around 250°C, owing to selective lower temperature re-equilibration of the calcite and albite during post peak metamorphism.