U-Pb Zircon Age Constraints on the Paleozoic Sedimentation, Magmatism and Metamorphism of the Sredogriv Metamorphics, Western Balkan Zone, NW Bulgaria

The Sredogriv greenschist facies rocks belong to the Western Balkan Zone in northwestern Bulgaria. The low-grade rocks consist of clastic-tuffaceous precursors and presumably olistostromic magmatic bodies. We present U-Pb LA-ICP-MS zircon age constraints for the Sredogriv metaconglomerate, intruding metaalbitophyre and a breccia-conglomerate of the sedimentary cover. Detrital zircons in the Sredogriv metaconglomerate yielded a maximum depositional age of 523 Ma, with a prominent Neoproterozoic-Early Cambrian detrital zircon age clusters derived from igneous sources. The metaalbitophyre crystallized at 308 Ma and contains the same age clusters of inherited zircons. A maximum age of deposition is defined at 263 Ma for a breccia-conglomerate of the Smolyanovtsi Formation from the sedimentary cover that recycled material from the Sredogriv metamorphics and Carboniferous-Permian magmatic rocks. Proximity to Cadomian island arc sources and provenance from the northern periphery of Gondwana outline the depositional setting of the Sredogriv sedimentary succession. The timing of the Variscan greenschist facies metamorphism of the Sredogriv metamorphics is bracketed between 308 Ma and the depositional age of 272 Ma of another adjacent clastic formation. The results obtained allow us to identify the timing of the Cadomian sedimentary history and the Variscan magmatic and tectono-metamorphic evolution in part of the Western Balkan Zone.