Detrital zircon age constraints on the deposition of the Sredogriv low-grade metasedimentry rocks, West Balkan Zone, Bulgaria

The Sredogriv low-grade metamorphic rocks belongs to the Montana Unit of the Western Balkan Zone in Northwestern Bulgaria. The low-grade rocks predominantly consist of metamorphosed in greenschist facies clastic and clastic-marly precursors and minor basic to acidic extrusive and intrusive presumably olistostromic rock bodies. Here, we present detrital zircon age constraint for clastic rock of the Sredogriv metamorphics of unknown till now depositional timing, together with an age of the clastic rock unconformably overlying these metamorphics. Detrital zircons in the Sredogriv metaconglomerate yielded a maximum depositional age of 523 Ma, additionally maximum age of deposition is defined at 263 Ma for a conglomerate of the Smolyanovtsi Formation from the first unconformable sedimentary cover. Mostly continental detrital material from magmatic source built the protolith of the Sredogriv metamorphics, which are recycled in the clastic cover rocks. Hence, the greenschist facies metamorphism of the Sredogriv metamorphics took place between 523 Ma and 263 Ma.