Structural and geochronological constraints on the magmatic and tectonic events in the pre-Alpine basement of the central parts of the Balkan fold–thrust belt (Central Stara Planina Mountains, Bulgaria)

Abstract

The north-vergent Balkan fold–thrust belt of the Balkanide orogen, extending from northeast Serbia to the Black Sea region, was formed during at least two Alpine compressional events. However, remnants of the Variscan orogen are still preserved in its western and central parts. In the central part (Central Stara Planina Mountains), two pre-Permian metamorphic complexes of contrasting metamorphic degrees, the low-grade Stara Planina and high-grade Sredna Gora complexes, are juxtaposed along a major Variscan tectonic zone and intruded by voluminous granitoids of previously unknown age. Here, we present an extensive structural and U–Pb (TIMS and LA-ICP-MS) geochronological analysis, constraining several magmatic and tectonic events in the pre-Alpine evolution of the area. For the first time, early Cambrian magmatism (531.7 ± 1.5 Ma/519.4 ± 2 Ma) is reported from a granite intruding the low-grade Stara Planina complex. The juxtaposition of the metamorphic complexes before 314 Ma was followed by a transpressional stage between 313 and 306 Ma. The last Variscan penetrative ductile deformation was associated with the final emplacement stages of the Ambaritsa sheet-like pluton at about 306 Ma under still ongoing compression. In addition, at least two post-Variscan magmatic pulses of granitoid magmatism (at ca. 306–304 Ma and 250 Ma) have been distinguished in the studied area. Later, Alpine ductile to brittle deformational events led to an additional reworking of the Variscan edifice.