The Tertiary Evros volcanic rocks (Thrace, northeastern Greece):: Petrology and K/Ar geochronology

The Tertiary Evros volcanic rocks (EVR) crop out in northeastern Greece (Thrace), in close association with fault-controlled sedimentary basins, formed in an extensional regime. Three volcanic areas, called the Loutros-Feres-Dadia, Kirki-Esimi and Mesti-Petrota after the corresponding basins, could be distinguished. The rock bulk chemistry shows features of calc-alkaline to high-K calc-alkaline and, locally, shoshonitic rock series. Compositional variations indicate magmatism of convergent margins. The EVR form lava flows, domes, dykes and abundant pyroclastics. Their chemical composition ranges from basaltic andesite to rhyolite through andesite, trachyandesite, trachydacite and dacite. The basaltic andesites are two-pyroxene rocks, while the andesites are either pyroxene andesites or biotite-hornblende andesites. The trachyandesites and trachydacites have pyroxenes and biotite. The dacites mostly have biotite and hornblende and locally pyroxene. Rhyolites have mainly biotite and rarely hornblende. All the rocks are porphyritic with glassy, holocrystalline or semicrystalline textures. The K/Ar ages range from 33.4 to 19.5 Ma, establishing an Oligocene (33.4-25.4 Ma) and an Early Miocene (22.0-19.5 Ma) volcanic activity. Intercalations, however, of pyroclastic materials with Priabonian elastic sediments indicate that the volcanic activity started earlier than the Oligocene. Two main groups of rocks have been distinguished, the PxBt group comprising basaltic andesite, pyroxene andesites, trachyandesites and trachydacites, and the HblBt group comprising hornblende-biotite andesite, dacite and rhyolite. On the basis of the rock chemistry two parallel, sub-parallel or cross-cutting geochemical trends are distinguished, indicating different evolutionary histories for them. The Sr isotopic composition differs in the two groups, with Sr I.R. ranging between 0.7057 and 0.7074 in the PxBt group, and between 0.7071 and 0.7080 in the HblBt group. The PxBt group was evolved through an open system process (MFC - mixing plus fractional crystallization) in which basaltic andesite and trachydacite represent the basic and the acid end-members respectively. Although assimilation plus fractional crystallization is not excluded, WC between a basic end-member, similar to the HblBt andesites, and an acid end-member, having rhyolitic composition, is suggested as a possible process for the evolution of the Hbfl3t group. The parental magmas for the evolution of the PxBt group originate in an inhomogeneous and strongly metasomatized mantle through a slight modification of a primary basaltic melt. The parental magma for the evolution of the HblBt group could be a hybrid magma of the PxBt group, which evolves, under different conditions, to give the HblBt group rocks. Although the involvement of a mantle component cannot be excluded for the origin of the rhyolitic melts, partial melting of crustal material (amphibolite, basalt, andesite, gneisses, pelites, greywackes), under various P-T conditions, are responsible for the genesis of melts similar to the less evolved rhyolites.