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Hafnium isotopes in zircons track the changing sources for the Late Cretaceous magmatism in Eastern Srednogorie, Bulgaria

We present new Hf isotopic data of magmatic zircons from the Eastern Srednogorie zone. The data outline two clear temporal trends: rising initial εHf from the initiation of the magmatism at ~95 Ma to 81 Ma, followed by a rapid decline in the initial εHf in the 81–78 Ma time period. The first trend highlights the increasing participation of mantle melts in the formation of magmatic products in the East Balkan and Strandzha regions, which is likely dictated by the southward retreat of the subducting slab. This trend is also evident in published Hf isotopic data on zircons from Central Srednogorie zone. The second trend of rapidly decreasing initial εHf of zircons is interpreted to reflect increased proportion of lower crustal melts in an intra-arc rift extensional environment (the Yambol-Burgas region) between 81 and 78 Ma; this trend is not observed in the Central Srednogorie zone.