Early Triassic Granitic Magmatism - Arnea and Kerkini Granitic Complexes - in the Vertiskos Unit (Serbo-Macedoniam Massif, Northeastern Greece) and its Significance in the Geodynamic Evolution of the Area

The Hellenides are an accretionary Alpine orogen, which is the result of the closure of Tethys and the subsequent accretion of terranes along the Eurasian margin. They are conventionally distinguished, from west to east, into External Hellenides and Internal Hellenides. The Serbo-Macedonian Massif belongs to the latter and is subdivided into two units: the Kerdyllia and Veriskos Unit in the eastern and central and northwest Chalkidiki Peninsula, respectively. The Vertiskos Unit mostly comprises various types of gneisses, associated with amphibolites and metasediments, and is intruded by mainly Mesozoic leucocratic granites and dykes. The largest magmatic bodies are the Arnea and Kerkini granitic complexes, which comprise two-mica syenogranites and alkali-feldspar granites containing quartz, K-feldspar, plagioclase, and biotite, often replaced by chlorite, and allanite, titanite, zircon, and fluorite, as typical accessory minerals. They are variably peraluminous with molar Al2O3/CaO+Na2O+K2O values of 0.96 - 1.34, and straddle the high-K calc-alkaline and shoshonite fields in the K2O vs. SiO2 diagram. The Arnea and Kerkini rocks are studied using new U-Pb geochronological zircon data, as well as whole rock geochemical data, aiming at investigating the origin and evolution of the two complexes, and providing constraints on the relative geodynamic environment. Ages of Arnea and Kerkini complexes, obtained from zircon oscillatory rims and with excellent concordance, are at the boundary between Permian and Triassic, with Arnea complex showing differences in the rocks cropping out at South and North of Volvi Lake, 254 Ma and 244 Ma, respectively, and Kerkini having an age of 247 Ma. Both complexes evolve mainly by Fractional Crystallization, separating mineral assemblages consisting of feldspars and biotites, and allanite and zircon. Parental magmas are crustal melts derived by partial melting of TTG sources. Employing a statistical approach, using all the main features able to discriminate A-type granite igneous suites, all the rocks from Arnea and Kerkini complexes show variable behaviour: some parameters indicate a clear A-type affinity for both the complexes, but conversely other parameters indicate a clear differentiated I-type affinity. In addition all the rocks are A2 subtype, indicating that, more than connected to an intra-plate rifting, the magmatism is likely to be connected with a post-collisional environment. At the boundary between Permian and Triassic the area was in an incipient rift environment, and as such oceanization had not still occurred, and the continental crust, accreted during a previous continental collision, is present at the rift floor. Such continental crust could melt partially to produce batches of magma similar to the studied magmatism. A submarine rhyolitic magmatism, outcropping on the floor of epicontinental sea and having similar age, exists and is geochemically similar to the granitic magmatism, indicating that the compositionally granitic melts produced by partial melting of the crust either remained at depth, generating the granitic complexes, or extruded as rhyolitic lavas. We suggest that magmatism intruding Vertiskos unit emplaced in a post-orogenic environment following the subduction of Godwana under Eurasia during the first period of the extensional rifting leading to the opening of the Vardar-Meliata ocean.