Detrital and metamorphic record of the Lebnitsa metasediment, Ograzhden Unit, Serbo-Macedonian Massif, SW Bulgaria

The Serbo-Macedonian Massif consists of high-grade orthogneisses with lenses of amphibolites, eclogites, and paragneisses derived from Gondwana-affiliated protoliths. In the Ograzhden Unit, amphibolite-facies paragneisses represent a diverging sedimentary succession based on their detrital ages (Peytcheva et al., 2015, Geol. Balcanica 44, 51-84). Near the village of Lebnitsa, a metasediment succession occurs that is different from these paragneisses in composition and local preservation of cross-bedding. The metasediment comprises quartzite and mica-Al₂SiO₅-garnet-rich layers. These observations suggest a mature fluvial sediment protolith.

We analysed zircons to characterize the detrital age component and origin of the Lebnitsa metasediment. The zircon population indicates various age clusters, with major peaks at 530 Ma and 450 Ma. The Struma Diorite and Frolosh Formations could have been the source for the former peak (516-579 Ma, Zagorchev et al., 2011, Bulg. Geol. Soc. “Geosciences 2011”), whereas the 450 Ma cluster is representative of the post-Cadomian orthometamorphic basement (440-480 Ma) of the Ograzhden Unit (Peytcheva et al., 2015, Geol. Balcanica 44, 51-84). Unlike most of the Ograzhden paragneisses, no peaks younger than 425 Ma are present. The metamorphic assemblage and Quartz-in-Garnet Raman elastic barometry of the Lebnitsa metasediment suggest amphibolite-facies metamorphism. The tectonic foliation and lineation correspond to the ones observed in the surrounding basement, suggesting common tectonic history.

Based on the newly obtained data, we propose that the Lebnitsa metasediment represents a monocyclic cover deposited on the Ograzhden basement in post-Cadomian times and metamorphosed at the amphibolite facies conditions during the Alpine orogeny together with the surrounding basement.