Origin and tectonic evolution of the central Serbo-Macedonian Massif

The Serbo-Macedonian Massif (SMM) is a composite crystalline belt within the Eastern European Alpine orogen, outcropping from the Aegean Sea in the south, to the Pannonian basin in the north. Farther north, it is correlated with the Supragetic nappe sequences of the South Carpathians. The central part of the massif, located in southeastern Serbia, southwestern Bulgaria and eastern Macedonia, consists of the medium- to high-grade metamorphic Lower Complex, and the low-grade metamorphic Vlasina Unit. Due to sparsity of detailed structural studies, geo- and thermochronological constraints, the geodynamic history of this area was considered enigmatic. In this study, U-Pb LA-ICP-MS analyses of zircons, geochemical analyses of major and trace elements of whole-rock samples, outcrop- and micro-scale investigation of ductile structures as well as 40Ar/39Ar and fission-track thermochronology were applied in order to define the provenance and tectonic evolution of the central SMM.
The new results of U–Pb LA-ICP-MS analyses, coupled with Hf isotopic analyses of magmatic and detrital zircons, and major and trace element concentrations in whole-rock samples suggest that the central SMM and the basement of the adjacent units (i.e. Eastern Veles series and Struma Unit) originated in the central parts of the northern margin of Gondwana. These data provided a basis for a revised tectonic model of the evolution of the SMM from the late Ediacaran to the Early Triassic.
The earliest magmatism in the Lower Complex, Vlasina Unit and the basement of Struma Unit is related to the activity along the late Cadomian magmatic arc (562-522 Ma). A subsequent stage of early Palaeozoic igneous activity is associated with (i) the reactivation of subduction below the Lower Complex and the Eastern Veles series during the Early Ordovician (490-478 Ma), (ii) emplacement of mafic dykes in the Lower Complex due to aborted rifting in the Middle Ordovician (472-456 Ma), and (iii) felsic within-plate magmatism in the early Silurian (439±2 Ma). The third magmatic stage is represented by Carboniferous late- to post-collisional granites (328-304 Ma). These granites intruded the gneisses of the Lower Complex, in which the youngest deformed igneous rocks are of early Silurian age, thus constraining the high-strain deformation and peak metamorphism to the Variscan orogeny. The fourth, Permian-Triassic (255-253 Ma) stage is late- to post-collisional and it is documented by within-plate felsic magmatism, related to the opening of the Mesozoic Tethys.
Three major stages of ductile deformation were revealed based on the investigation of outcrop- and micro-scale ductile structures in the central SMM. The earliest stage of deformation D1 is related to isoclinal folding, commonly preserved as quartz-feldspar rootless fold hinges up to decimetre-scale. The second deformational stage D2 is associated with general southeastward tectonic transport and refolding of older structures into recumbent metre- to kilometre-scale tight to isoclinal folds. The deformational stages D1 and D2 could not be temporally separated, and probably occurred in close sequence. The age of these two ductile deformational stages was constrained to the Variscan orogeny based on indirect geological evidence (i.e. ca. 408-ca. 328 Ma). During this time the SMM was involved in a transpressional amalgamation of the western and eastern parts of the Galatian super-terrane and subsequent collision with Laurussia. Outcrop-scale evidence of the final stage of ductile deformation D3 is limited to the spaced and crenulation cleavage, which are probably related to the large-scale folding reported by earlier studies.
Results of the 40Ar/39Ar thermochronology revealed three major stages of cooling in the central SMM from the early Carboniferous until the Early Jurassic. Cooling below greenschist facies conditions in the western part of the Vlasina Unit took place in a post-orogenic setting (extensional or transtensional) in the early Permian (ca. 284 Ma). The age of activity along the top-to-the-west shear zone formed within the orthogneiss in the Božica area of the Vlasina Unit was constrained to the Middle Triassic (ca. 246 Ma). This age coincides with widespread extension and magmatism related to the opening of the Mesozoic Tethys. The greenschist facies retrogression in the Lower Complex probably occurred in the Early Jurassic (ca. 195 Ma) and is probably related to thermal processes in the overriding plate above the subducting slab of the Mesozoic Tethys Ocean.
Fission-track analyses on apatites and zircons revealed the existence of three major cooling stages from the late Early Cretaceous to the Oligocene. The first stage represents rapid cooling through zircon and apatite closure temperatures (300–60 °C) during the late Early to early Late Cretaceous (ca. 110–ca. 90 Ma). It is related to a post-orogenic extension following the regional nappe-stacking event in the Early Cretaceous. Middle to late Eocene (ca. 48–ca. 39 Ma) cooling is related to the formation of the Crnook-Osogovo-Lisets extensional dome and its exhumation along low-angle normal faults. The third event is related to the regional cooling following the late Eocene magmatic pulse. During this pulse the area surrounding the Surdulica granodiorite (36±1 Ma) and the slightly younger volcanic bodies (ca. 35 Ma) reached temperatures higher than the apatite closure temperature (110±10 °C) but lower than ca. 250 °C. The geochemistry of the igneous rocks reveals a late- to post-orogenic tectonic setting during magma generation.