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

Abstract

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°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.

Deep Tectonics in the Eastern Hellenides Uncovered: The Record of Variscan Continental Amalgamation, Permo‐Triassic Rifting, and Early Alpine Collision in Pre‐Variscan Continental Crust in the W‐Rhodope (Vertiscos‐Ograzden Complex, N‐Greece)

“…Hence, it was not possible to infer better accuracy for the P‐T conditions of the UHP stage as the measured garnet composition has been reequilibrated during decompression and the subsequent MHP metamorphic cycle. Similar P‐T conditions (temperatures ~700–750°C and pressures at 3–3.5 GPa) are also reported from kyanite eclogites in the adjacent Bulgarian VOC (Janak et al, 2011; Trapp et al, 2020). Additionally, Kostopoulos et al (2000) described graphite pseudomorphs after diamond in an amphibolite xenolith within the Triassic Arnea granite intruding the Vertiskos Unit.…”

“…Recent Lu‐Hf dating of two kyanite eclogites from southern Rila Mountains (Belica area) and Ograzden Mountains (Gega area) of southwestern Bulgaria of the Ograzhden complex (Ograzhden refers to the Bulgarian part and Vertiskos to the Greek part of the same complex), yielded ages of 334.1 ± 1.8 and 334.0 ± 2.2 Ma, respectively (Trapp et al, 2020). The kyanite eclogites record UHP conditions of 3.0–3.5 GPa and 700–750°C.…”

Sm-Nd age determination of eclogites from Sredna Gora Mountains – HP metamorphic event or final retrogression

“…The age of HP peak of metamorphism is also debatable. Gaggero et al (2009) propose an age of 403±4 Ma by analogy with the age of zircon rims in eclogites from Sardinia (Cortesogno et al, 2004), whereas Trapp et al (2021) obtained Carboniferous ages (334.1±2.2 Ma) for eclogites from the Serbo-Macedonian Massif.…”