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

Abstract

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 (XH2O≤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.