Alpine deformation of the Ograzhden basement, Serbo-Macedonian Massif

The Ograzhden Unit, a part of the Serbo-Macedonian Massif (SMM), comprises a suite of ortho- and paragneisses with lenses of amphibolites, intruded by the Triassic Igralishte granite (Peytcheva et al., 2009, Geol. Balcanica 38, 5-14). The orthometamorphic suite, representing the bulk of the basement, consists of different migmatized granites to granodiorites and unmigmatized granites with ages from 440 to 480 Ma. Amphibolites have slightly younger ages of 430-440 Ma (Peytcheva et al., 2015, Geol. Balcanica 44, 51-84). An eclogite lens locally preserved in the amphibolites was reported to be recording a Variscan ultra-high pressure metamorphic event (Trapp et al., 2021, Terra Nova 33, 174-183).

Although Alpine Cretaceous amphibolite-facies imprint was reported from the Vertiskos unit in the Greek part of the SMM (Kilias et al., 1999, Int. J. Earth Sci. 88, 513-531), the Ograzhden Unit is considered to be largely unaffected by it (Kounov & Gerdjikov, 2024, Geol. Balcanica 53, 29-85). However, new data suggest that also the Ograzhden Unit, including the Igralishte pluton, was deformed under upper greenschist- to lower amphibolite-facies conditions. We present new data for the age of the Nikudin pluton (445 Ma), previously assumed to be of Triassic age like the Igralishte pluton. Both plutons record similar deformation patterns, and both plutons as well as their host rocks record top-to-SE shearing. Thus, we suggest a ductile Alpine deformation with top-to-E thrusting (rotated by later extension to top-to-SE), probably associated with thrusting of the Circum-Rhodope Belt onto the European margin in the Late Jurassic to Early Cretaceous.