Quaternary bimodal volcanism in the Nigde Volcanic Complex (Cappadocia, central Anatolia, Turkey): age, petrogenesis and geodynamic implications

The late Neogene to Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia is one of the most impressive volcanic fields of Turkey because of its extent and spectacular erosionally sculptured landscape. The late Neogene evolution of the CVP started with the eruption of extensive andesitic-dacitic lavas and ignimbrites with minor basaltic lavas. This stage was followed by Quaternary bimodal volcanism. Here, we present geochemical, isotopic (Sr-Nd-Pb and delta O-18 isotopes) and geochronological (U-Pb zircon and Ar-Ar amphibole and whole-rock ages) data for bimodal volcanic rocks of the Ni g de Volcanic Complex (NVC) in the western part of the CVP to determine mantle melting dynamics and magmatic processes within the overlying continental crust during the Quaternary. Geochronological data suggest that the bimodal volcanic activity in the study area occurred between ca. 1.1 and ca. 0.2 Ma (Pleistocene) and comprises (1) mafic lavas consisting of basalts, trachybasalts, basaltic andesites and scoria lapilli fallout deposits with mainly basaltic composition, (2) felsic lavas consisting of mostly rhyolites and pumice lapilli fall-out and surge deposits with dacitic to rhyolitic composition. The most mafic sample is basalt from a monogenetic cone, which is characterized by Sr-87/Sr-86 = 0.7038, Nd-143/Nd-144 = 0.5128, Pb-206/Pb-204 = 18.80, Pb-207/Pb-204 = 15.60 and Pb-208/Pb-204 = 38.68, suggesting a moderately depleted signature of the mantle source. Felsic volcanic rocks define a narrow range of Nd-143/Nd-144 isotope ratios (0.5126-0.5128) and are homogeneous in Pb isotope composition (Pb-206/Pb-204 = 18.84-18.87, Pb-207/Pb-204 = 15.64-15.67 and Pb-208/Pb-204 = 38.93-38.99). Sr-87/Sr-86 isotopic compositions of mafic (0.7038-0.7053) and felsic (0.7040-0.7052) samples are similar, reflecting a common mantle source. The felsic rocks have relatively low zircon delta O-18 values (5.6 +/- 0.6 %) overlapping mantle values (5.3 +/- 0.3 %), consistent with an origin by fractional crystallization from a mafic melt with very minor continental crustal contamination. The geochronological and geochemical data suggest that mafic and felsic volcanic rocks of the NVC are genetically closely related to each other. Mafic rocks show a positive trend between Sr-87/Sr-86 and Th, suggesting simultaneous assimilation and fractional crystallization, whereas the felsic rocks are characterized by a flat or slightly negative variation. High Sr-87/Sr-86 gneisses are a potential crustal contaminant of the mafic magmas, but the comparatively low and invariant Sr-87/Sr-86 in the felsic volcanics suggests that these evolved dominantly by fractional crystallization. Mantle-derived basaltic melts, which experienced low degree of crustal assimilation, are proposed to be the parent melt of the felsic volcanics. Geochronological and geochemical results combined with regional geological and geophysical data suggest that bimodal volcanism of the NVC and the CVP, in general, developed in a post-collisional extensional tectonic regime that is caused by ascending asthenosphere, which played a key role during magma genesis.