Serbo-Macedonian revisited: A Silurian basement terrane from northern Gondwana in the Internal Hellenides, Greece

Abstract

New geochronological and geochemical data on basement orthogneisses from the Vertiskos Unit of the Serbo-Macedonian Massif (SMM), Internal Hellenides, northern Greece, are used in order to constrain the pre-Alpine tectonic history of the basement units in the metamorphic hinterland of the Hellenides. The prevailing rock types in the Vertiskos crystalline basement are coarse-grained biotite augengneisses with subordinate leucocratic muscovite gneisses and two-mica gneisses. Zircon Pb–Pb and U–Pb ages on 20 samples range from 425.9 ± 4.2 Ma to 443.4 ± 5.5 Ma with a mean of 432.2 ± 3.2 Ma and are interpreted as primary crystallisation ages of the basement granites on the basis of the magmatic internal structure of the zircon grains. Trace-element and isotope geochemistry of the gneisses show that they originated in a magmatic-arc setting, but also contain material from pre-existing continental crust. The rocks are slightly peraluminous and some leucocratic gneisses are strongly depleted in incompatible trace elements. The zircon ages document an early Silurian magmatic phase in the Internal Hellenides. The association of lithologies and ages is distinctly different from that of the adjacent basement massifs. This difference in basement provenance and the fact that the Vertiskos Unit is bordered by ophiolitic material both to the west and to the east leads to the conclusion that this part of the SMM is an exotic terrane of northern Gondwanan origin, which was finally accreted to its present position during the Alpine orogeny. The Vertiskos Terrane may have been part of the Hun Superterrane, which evolved at the northern active continental margin of Gondwana in the early Palaeozoic and rifted away from it during the opening of the Rheic Ocean in the Cambrian to Silurian. Parts of this superterrane such as the Vertiskos Terrane are present throughout the Variscan and Alpine orogens.