Cathodoluminescence,fluid inclusion and stable C–O isotope study of tectonic breccias from thrusting plane of a thin-skinned calcareous nappe

Basal hydraulic breccias of alpine thin-skinned Muráň nappe were investigated by means of cathodoluminescence petrography, stable isotope geochemistry and fluid inclusions analysis. Our study reveals an unusual dynamic fluid regime along basal thrust plane during final episode of the nappe emplacement over its metamorphic substratum. Basal thrusting fluids enriched in ¹⁸O, silica, alumina, alkalies and phosphates were generated in the underlying metamorphosed basement at epizonal conditions corresponding to the temperatures of 400–450°C. The fluids fluxed the tectonized nappe base, leached evaporite-bearing formations in hangingwall, whereby becoming oversaturated with sulphates and chlorides. The fluids further modified their composition by dedolomitization and isotopic exchange with the host carbonatic cataclasites. Newly formed mineral assemblage of quartz, phlogopite, albite, potassium feldspar, apatite, dravite tourmaline and anhydrite precipitated from these fluids on cooling down to 180–200°C. Finally, the cataclastic mush was cemented by calcite at ambient anchizonal conditions. Recurrent fluid injections as described above probably enhanced the final motion of the Muráň nappe.     

Dehydration melting and devolatilization during exhumation of high-grade metapelites: the Tatra Mountains, Western Carpathians

Partial melting and retrogression related to Variscan tectonic exhumation have been recognized in the high-grade metapelites of the Tatra Mountains, Western Carpathians. Staurolite and kyanite relics document an early stage of the prograde metamorphism at c. 600 °C and 9–10 kbar. An increase in temperature to >730 °C at 11–12 kbar resulted in partial melting and incipient migmatization in the stability field of kyanite. Further heating at decreasing pressure during the earliest stage of exhumation led to the dehydration-melting of muscovite and biotite at >750–800 °C and 6–10 kbar, producing garnet-bearing granite as leucosomes in migmatite. Subsequent cooling is documented by garnet resorption by biotite and sillimanite (a reversal of the prograde biotite dehydration-melting reaction). This was followed by nearly isothermal decompression to c. 4–5 kbar producing cordierite and some melt due to biotite decomposition. Later nearly isobaric cooling led to cordierite pinitization and formation of orthoamphibole, chlorite and carbonates. Densities of primary, monophase CO₂–N₂ inclusions (0.69–1.06 g cm^?3) from the migmatite leucosome are consistent with the near-peak and retrograde conditions. Highly varying N₂ contents (5–30 mol%) are thought to result from the nitrogen uptake in retrograde K-bearing minerals, or dilution by CO₂ liberated during interaction of melt-derived water with metapelite graphite. The relatively high nitrogen content, not observed until now in migmatites, could have been inherited from the high-pressure metamorphism stage. It is assumed that the water-absent composition of fluid inclusions is not representative of the bulk water content (X_H2O≤0.7), which was masked by mechanical separation of the CO₂- and H₂O-dominated immiscible phases, and/or by post-entrapment modifications of the fluid inclusions. Decompression and the final stage of exhumation were accomplished by top-to-the-south thrusting as well as west–east (orogen-parallel) extension. They were most probably related to regional uplift and gravitational collapse of thermally weakened Variscan crust.