From Variscan to Neotethyan tectonic processes in the Central Srednogorie and Sakar-Strandja Zones in Bulgaria: Evidence from Geochronology and Geochemistry

The Apuseni-Banat-Timok-Srednogorie (ABTS) belt, located in east-central Europe, forms part of the Alpine-Mediterranean orogenic system and represents a continental margin magmatic arc. Its development is generally interpreted to be closely related to the northwestward subduction of the Neotethyan Vardar Ocean beneath the Serbo-Macedonian-Rhodope Massif. The Srednogorie Zone is composed of a Variscan basement overlain by a Permo–Mesozoic cover sequence and an Upper Cretaceous sedimentary basin. To the south, the Sakar-Strandja Zone is exposed and consists of a crystalline basement intruded by Permian to Triassic granites, which relate to Paleotethys subduction processes. However, detailed geochronological constraints and sedimentary provenance data for both tectonic zones remain scarce. Based on systematic field investigations in the Srednogorie and Sakar-Strandja zones, this study presents integrated petrological, geochemical, and geochronological analyses of basement gneisses, Upper Cretaceous sedimentary rocks, and granites. Geochemical analyses reveal that the granites in both zones are peraluminous, exhibiting similar rare earth element distribution patterns characterized by relative fractionation of light rare earth elements over heavy rare earth elements and distinct negative Eu anomalies (δ_Eu = 0.08–0.46). They are consistently enriched in Rb, Pb, and Th but depleted in Ba, Nd, and Eu. Geochronological results show that the basement gneisses in the central Srednogorie zone have crystallization ages of Ediacaran (612.6±2.2 Ma) and Ordovician (475.0–454.8 Ma), and record a distinct Variscan metamorphic age (351.4–327.7 Ma). Detrital zircon ages from Upper Cretaceous sandstones indicate that their provenance is the Srednogorie basement, with dominant ages of Ordovician and Carboniferous. Additionally, their Ediacaran and Late Cambrian age components constrain connections the link to the Cadomian-Avalonian belts. The Upper Cretaceous sheared granites in the southern Srednogorie tectonic belt have ages of 85 and 83 Ma, and their formation is related to the subduction of the Vardar Ocean, which also constrain a second stage of the ductile overprint at the boundary to Rhodopes in the south. In contrast, the Sakar granite yielded an Early Triassic age (248 Ma), indicative of magmatism associated with Paleotethys subduction.