Mineralogy and thermobarometry of marbles unravel retrogression and hydrothermal imprint of a polymetamorphic terrane (Făgăraș Unit, Southern Carpathians, Romania)

This thermobarometry study employs calcite-dolomite solvus thermometry and thermodynamic pseudosections applied to marbles in the Făgăraș Unit, the uppermost unit of the Variscan metamorphic basement of the Alpine Supragetic units, Central Southern Carpathians. The marbles exhibit extensive relict granoblastic dolomitic bands and quartz inclusions, indicating formation through the metasomatic replacement of dolomite by calcite, aided by a hydrothermal F-rich fluid. The data reveal a thermal history ranging from amphibolite facies (572°–617°C) to greenschist and sub-greenschist facies (~ 435°C) during polyphasic metamorphism, followed by late-stage hydrothermal activity (105°–195°C). Mineral assemblages in the marbles suggest equilibration at 435 ± 40°C and 0.42 ± 0.12 GPa, with the CO₂/H₂O ratio influencing reactions. Talc formation indicates fluid interactions, while tectonic processes controlled hydrothermal fluid flow and pathways. This study quantitatively confirms and refines earlier P-T estimates regarding the metamorphic and hydrothermal evolution of the Făgăraș Unit. The results also reveal a distinct P-T history compared to other metamorphic units within the Alpine nappe stacking in the Făgăraș Mountains and support the interpretation of a complex nappe-stacking system composed of pre-Alpine units juxtaposed during the Variscan orogeny. These units were subsequently thrust during the Alpine orogeny over the Lerești-Călușu Unit, a low-grade metamorphic unit with no marbles and no evidence of polymetamorphism. The observed mineralogical composition, microtextures, and distinct P-T signatures of the dolomitic and calcitic marbles highlight their potential to preserve diagnostic geochemical and isotopic signals, offering valuable insights into the geologic evolution of the host petrologic assemblage.