Alpine deformation and pre-Alpine evolution of the Ograzhden Unit, Serbo-Macedonian Massif

Repeated deformation, metamorphism, remelting and crustal recycling are the processes that shape orogens. The Serbo-Macedonian Massif (SMM) stretches from the Pannonian Basin in the north to the Aegean Sea in the south, bound by the Vardar Zone and the Circum-Rhodope Belt in the west and neighboured by the Rhodope Metamorphic Core Complex in the east. Its present-day shape and structure are conditioned by the Alpine and pre-Alpine orogenies. In particular, the Ograzhden Unit in SW Bulgaria has been proven to possess Proterozoic and Paleozoic magmatic and metamorphic ages (Peytcheva et al., 2015, Geol. Balc. 44), incl. evidence for UHP metamorphism at 334 Ma (Trapp et al., 2020, Terra Nova), in its high-grade orthometamorphic basement. Furthermore, shortly before the onset of the Alpine orogeny, Triassic magmatism imprinted the basement, recorded in the Igralishte granitic body (Peytcheva et al., 2009, Geol. Balc. 38).

In this study, we investigate the evolution of the Ograzhden Unit as a part of the SMM. We report a new 444.8±1.5 Ma U-Th-Pb zircon age of the Nikudin granite. The granite body shows geochemical characteristics suggestive of a S-type leucogranite formed in a convergent setting. The high-grade metamorphism in a metasediment from Lebnitsa is characterized by a peak mineral assemblage Q-Ms-Ky-Grt, and MnNCKFMASHT equilibrium thermodynamic modelling suggests ≥ 1.1 GPa at 620°C, suggesting that lower eclogite facies conditions were possibly reached in the subducted continental crust during the Variscan orogeny. Triassic magmatism recorded in the Igralishte granite (Peytcheva et al., 2009, Geol. Balc. 38) shows trace and rare earth element patterns indicative of a highly evolved, fractionated material, associated with a high degree of crystallization. The basement was then structured by the Alpine orogeny, starting with ductile deformation in lower amphibolite facies in the Cretaceous and characterized by top-to-SE kinematics. Thus, the Ograzhden Unit comprises heterogeneous material formed and deformed during several orogenic cycles, and finally shaped into its current structure by Alpine deformation.