Petrological evolution of Karlıova-Varto volcanism (Eastern Turkey). Magma genesis in a transtensional triple-junction tectonic setting

A diffuse and voluminous (>1400 km3) Miocene-Quaternary volcanic activity developed around the Karlıova Triple Junction in East Anatolia as a consequence of collisional tectonics among Anatolia, Arabia and Eurasia continental plates. The volcanic rocks of this region are grouped into three phases of activity: 1) Early Phase (Solhan volcanism; ~7.3–4.4 Ma), with emplacement of alkali basalt to trachyte lava flows and pyroclastic successions; 2) Middle Phase (Turnadağ and Varto volcanism; ~3.6–2.6 Ma), mostly with products with the same compositional range plus minor dacites and rhyolites, and 3) Late Phase (Özenç volcanism; ~2.6–0.5 Ma), with emplacement of alkali basaltic, hawaiitic and mugearitic lavas and dykes. Primitive Mantle-normalized patterns of the three rock groups share an enriched LILE and depleted HFSE contents, with overall positive spikes of Pb and mildly fractionated LREE/HREE trends showing more similar affinity to global subducting sediments rather than to magmas emplaced in mid-plate settings (i.e., OIB). Initial Sr isotopic ratios of the least evolved compositions range from values lower than BSE (87Sr/86Sri = 0.7041) to radiogenic compositions (87Sr/86Sri = 0.7050). They reflect either FC-like processes, with 87Sr/86Sri up to 0.7064, or closed system fractional crystallization, with 87Sr/86Sri = 0.7046–0.7049. Initial Nd are higher than ChUR estimate for the most and the least evolved compositions (143Nd/144Ndi = 0.51267–0.51280), indicating provenance from isotopically depleted sources. Lead isotopic ratios are characterized by a remarkable homogeneous 206Pb/204Pb (18.95–19.04), with 207Pb/204Pb (15.65–15.72) and 208Pb/204Pb (38.87–39.21) slightly above the Northern Hemisphere Reference Line, pointing towards the EMII end-member. Geochemical modelling for the least evolved volcanic units indicate the likely generation from an amphibole-bearing spinel-lherzolitic source. P-T calculations for partial melting calculations gave lithospheric pressures for initial magma generation (0.8–1.3 GPa). Possible cause of melting might be related to passive upwelling of asthenosphere as a response to the local extension linked to the development of North Anatolian and East Anatolian Fault Zones. Anyhow, volcanic units from the KTJ display only limited geochemical signatures of garnet-bearing sources, or any HIMU-OIB like characteristics, as instead observed in the other portions of the Eastern Anatolia. The long-lasting complex tectonic evolution of the Eastern Anatolia is responsible for the large geochemical variability of the magmatic products. However, the general characteristics of KTJ volcanic rocks are mainly dominated by subduction-related signatures, with most of the primary magma characteristics having been heavily masked by fractionation and crustal assimilation processes.