Petrogenesis and Provenance of the Triassic Metasedimentary Succession in the Sakar Unit, Bulgaria: Constraints from Petrology, Geochemistry and U-Pb Detrital Geochronology

This study investigates the Terrigenous-carbonate Sakar-type Triassic (TCSTT) and Sakar-type Triassic (STT) metasedimentary successions in the Sakar Unit, SE Bulgaria. Both share lithological similarities (alternation of carbonate-silicate schists, mica schists, marbles, and impure marbles) and are affected by post-Triassic metamorphism, but exhibit differences in metamorphic grade and provenance. The STT displays a higher metamorphic grade (low amphibolite facies) compared to the TCSTT (low greenschist facies). Petrographic observations and geochemical analyses indicate protoliths composed of arkosic sandstones, shales, and limestones, derived from a quartz-dominated source with minor contributions from intermediate magmatic sources. U-Pb geochronology of detrital zircons reveals a dominant Carboniferous age, supplemented by early Ordovician ages, consistent with the presence of Carboniferous-Permian magmatic rocks in the basement. The presence of Early Paleozoic and Cambrian-Neoproterozoic zircons in the detrital zircon populations suggests that older rocks in the basement of the Sakar Unit and Srednogorie zone are also sources of the sedimentary material. Based on immobile trace elements contents and discrimination diagrams, the siliciclastic component comes from rocks formed in a continental arc tectonic setting. REE patterns exhibit a negative Eu anomaly, inherited from the granitic source rocks.