Origin and evolution of the Tertiary Evros volcanic rocks, Thrace, Northeastern Greece: major, trace and Sr, Nd, Pb geochemistry

The Tertiary Evros volcanic rocks (EVR), which comprise one of the two widespread volcanic occurrences in northeastern Greece, crop out in Thrace, in close association with fault-controlled sedimentary basins, which developed synchronously with post-Cretaceous collision and subsequent Tertiary extension. Three volcanic areas, namely Loutros-Feres-Dadia, Kirki-Esimi and Mesti-Petrota, after the corresponding basins, are distinguished. Rock bulk chemistry and compositional variations show magmas with calc-alkaline to high-K calc-alkaline and, locally, shoshonitic features associated with magmatism at convergent margins. The EVR consists of pyroclastic flows interbedded with conglomerates at its base, consisting of volcanic layers locally associated with lahars. The upper part of the volcanic succession consists of lava flows, covering an ignimbrite sequence, domes and dykes. Their chemical composition ranges from basaltic andesite to rhyolite through andesite, trachyandesite, trachydacite and dacite. Basaltic andesites have two pyroxenes while andesites are either pyroxene andesites or biotite-hornblende andesites. Trachyandesites and trachydacites have pyroxenes and biotite. Dacites have mostly biotite and hornblende and locally pyroxene. Rhyolites have mainly biotite and rarely hornblende. All 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 clastic sediments indicate that the volcanic activity started earlier than Oligocene. Two main groups of rocks have been distinguished, the PxBt group comprising basaltic andesites, pyroxene andesites, trachyandesites and trachydacites, and the HblBt group comprising hornblende-biotite andesites, dacites and rhyolites. Two, parallel, sub-parallel or cross-cutting geochemical trends are distinguished, indicating different evolutionary histories for the two groups. The isotopic composition of Sr, Nd and Pb differs in the two groups, with Sr I.R. ranging between 0.7074 and 0.7077 in the PxBt group, and between 0.7054 and 0.7085 in the HblBt group. Nd I.R. is 0.512447-0.512630 and 0.512352-0.512378 and 206Pb/204Pb is 18.745-19.898 and 18.228-18.963 in PxBt and HblBt, respectively. The parental magmas of the PxBt group originate in an inhomogeneous and strongly metasomatized mantle through a slight modification of a primary basaltic melt. Isotope (Sr, Nd, Pb) correlation diagrams support a metasomatized (enriched) mantle as source region since the samples analyzed plot between EMI and EMII. 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, is responsible for the genesis of melts similar to the less evolved rhyolites. The PxBt group was evolved through an open system process (MFC) in which basaltic andesite and trachydacite represent the basic and the acid end-members respectively. Although AFC is not excluded, MFC 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 HblBt group.