Quaternary bimodal volcanism in the Niğde 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 δ 18 O isotopes) and geochronological (U–Pb zircon and Ar–Ar amphibole and whole-rock ages) data for bimodal volcanic rocks of the Niğde 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 87 Sr/ 86 Sr = 0.7038, 143 Nd/ 144 Nd = 0.5128, 206 Pb/ 204 Pb = 18.80, 207 Pb/ 204 Pb = 15.60 and 208 Pb/ 204 Pb = 38.68, suggesting a moderately depleted signature of the mantle source. Felsic volcanic rocks define a narrow range of 143 Nd/ 144 Nd isotope ratios (0.5126–0.5128) and are homogeneous in Pb isotope composition ( 206 Pb/ 204 Pb = 18.84–18.87, 207 Pb/ 204 Pb = 15.64–15.67 and 208 Pb/ 204 Pb = 38.93–38.99). 87 Sr/ 86 Sr 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 δ 18 O 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 87 Sr/ 86 Sr and Th, suggesting simultaneous assimilation and fractional crystallization, whereas the felsic rocks are characterized by a flat or slightly negative variation. High 87 Sr/ 86 Sr gneisses are a potentia