Conditions and timing of high-pressure Variscan metamorphism in the South Carpathians, Romania

High-pressure (HP) metamorphic rocks, including garnet peridotite, eclogite, HP granulite, and HP amphibolite, are important constituents of several tectonostratigraphic units in the pre-Alpine nappe stack of the Getic-Supragetic (GS) basement in the South Carpathians. A Variscan age for HP metamorphism is firmly established by Sm-Nd mineral-whole-rock isochrons for garnet amphibolite, 358±10 Ma, two samples of eclogite, 341±8 and 344±7 Ma, and garnet peridotite, 316±4 Ma.A prograde history for many HP metamorphic rocks is documented by the presence of lower pressure mineral inclusions and compositional zoning in garnet. Application of commonly accepted thermobarometers to eclogite (grt+cpx±ky±phn±pg±zo) yields a range in "peak" pressures and temperatures of 10.8-22.3 kbar and 545-745 °C, depending on tectonostratigraphic unit and locality. Zoisite equilibria indicate that activity of H₂O in some samples was substantially reduced, ca. 0.1-0.4. HP granulite (grt+cpx+hb+pl) and HP amphibolite (grt+hbl+pl) may have formed by retrogression of eclogites during high-temperature decompression. Two types of garnet peridotite have been recognized, one forming from spinel peridotite at ca. 1150-1300 °C, 25.8-29.0 kbar, and another from plagioclase peridotite at 560 °C, 16.1 kbar.The Variscan evolution of the pre-Mesozoic basement in the South Carpathians is similar to that in other segments of the European Variscides, including widespread HP metamorphism, in which P - T - t characteristics are specific to individual tectonostratigraphic units, the presence of diverse types of garnet peridotite, diachronous subduction and accretion, nappe assembly in pre-Westphalian time due to collision of Laurussia, Gondwana, and amalgamated terranes, and finally, rapid exhumation, cooling, and deposition of eroded debris in Westphalian to Permian sedimentary basins.