Variscan ultra-high-pressure eclogite in the Upper Allochthon of the Rhodope Metamorphic Complex (Bulgaria)

The Rhodope Metamorphic Complex (RMC) in Bulgaria has been established as a Mesozoic ultra-high-pressure metamorphic province by findings of microdiamond in gneisses. Additionally, Variscan ultra-high-pressure metamorphism has been proposed for the Ograzhden/Vertiskos Unit in the Upper Allochthon of the RMC, based on findings of coesite, graphite pseudomorphs after diamond and indirect age constraints. We confirm ultra-high-pressure metamorphism of eclogites in this unit using thermobarometry, phase-equilibrium modelling and the Variscan age of metamorphism using Lu-Hf garnet-whole-rock dating. In Belica (southern Rila Mountains), kyanite- and phengite-bearing eclogite enclosed in high-grade gneisses records P-T conditions of 3.0-3.5 GPa and 700-750 degrees C. Lu-Hf dating of eclogite samples from Belica and Gega (Ograzhden Mountain), where coesite was found, yielded ages of 334.1 +/- 1.8 and 334.0 +/- 2.2 Ma, respectively, interpreted as the age of garnet growth during post-collisional subduction of continental crust after closure of the Rheic Ocean