Large-volume and swift magmatic response to Late Cenozoic segmentation of the subducted Neotethyan oceanic slab: evidence from the Galatian Volcanic Province, northwestern Turkey

The Miocene Galatian Volcanic Province (GVP) is one of the largest volcanic provinces in central-western Anatolia, with an extent of similar to 8,900 km(2). The volcanic activity is extended from 22.5 to 7.5 Ma. The volcanic compositions straddle the alkaline-subalkaline fields, from basic to acid compositions and mostly transitional to sodic affinity. Major oxides show good correlation with SiO2 indicating prolonged effects of fractional crystallization. Primitive mantle-normalized multi-element patterns indicate overall similarities among the different samples of the three geographic sectors, sharing strong negative anomalies in Nb-Ta-Ti, strong positive peaks at Cs and K, coupled with a common, albeit not always present, positive anomaly at Pb. Mineral-melt geothermobarometric estimates indicates similar to 1070-1235 degrees C and similar to 7-19 kbar for melting conditions of basaltic compositions and similar to 1000-1150 degrees C and similar to 3-12 kbar for andesitic-dacitic rocks. The absence of correlation between radiogenic isotopes and SiO2 and MgO is here interpreted as consequence of assimilation-fractional-crystalizationprocesses involving lower continental crust as contaminant. The GVP parental magmas are generated from similar to 2% to 10% partial melting of a lherzolitic mantle with high spinel/garnet ratio based on intra-REE fractionation constraints. The subduction-related metasomatism inferred for the GVP mantle sources based on their chemistry is interpreted to be linked to the northward subduction of the northern branch of the Neo-Tethys slab. Successive slab retreat resulted in extension for the critical stress distribution through the Cyprus slab, favouring magma propagation for the GVP volcanic region. The eventual break-off of the slab after the continent-continent collision of Arabian with Eurasian could have caused a toroidal mantle flow, favouring the widely distributed 15-16 Ma alkaline magmatism in the eastern GVP, associated with passive hot asthenospheric upwelling imaged by teleseismic P-wave tomography.