Genesis of Na-alkaline and calc-alkaline magmas in a post-collisional transtensive tectonic regimes: the case study of the recent monogenetic volcanism of Cappadocia, Central Turkey

Intense Cenozoic magmatism accompanied the convergence and collision of the Eurasian, African and Arabian plates. In the Cappadocia Region of Central Anatolia, widespread and abundant volcanism developed from the late Miocene to the Quaternary. Calc-alkaline pyroclastic products and lava erupted diffusely during the late Miocene and Pliocene. Both talc-alkaline and Na-alkaline volcanic rocks were emplaced in close spatial and temporal proximity during the Plio-Pleistocene. Here we focus on the Plio-Pleistocene monogenetic activity, which produced two types of rocks: calc-alkaline volcanic rocks ranging in composition from basalts to rhyolites and occurring around the Aced caldera, the Gollu Dag dome and the Hasan Dag and Erciyes Dag composite volcanoes; Na-alkaline volcanic rocks ranging from alkali basalts to mugearites and cropping out in monogenetic centres in Karapinar, along the WSW flank of Hasan Dag, in Acigol and near the Kizilirmak River, northwest of Nevsehir. The coeval occurrence of talc-alkaline and Na-alkaline volcanism observed here is in striking contrast with activity in the surrounding Western and Eastern Anatolia regions, where the talc-alkaline volcanic rocks are distinctly older than the Na-alkaline ones. The Sr and Nd isotopic compositions of the mantle sources of both rock types show a narrow, overlapping range of values (Sr-87/Sr-86 similar to 0.70395-0.70474 and Nd-143/Nd-144 similar to 0.51268-0.51281 for the talc-alkaline products, 0.70334-0.70524 and 0.51268-0.51293 for the Na-alkaline ones). In addition, most of the Na-alkaline products, like the calc-alkaline rocks, show an arc-type distribution of incompatible trace elements, with marked enrichments in LILE (Large-Ion Lithophile Elements) and negative HFSE (High Field Strength Elements) anomalies. True intraplate magmas are absent in Cappadocia, where the genesis of Na-alkaline basalts is ascribed to mixing between different percentages of within-plate (OIB) and calc-alkaline magmas. Modelling reveals that the addition of 13% and 30% OIB-type melt to a talc-alkaline magma is enough to change a SiO2 -oversaturated magma into an ol-hy-normative and ne-normative one, respectively, whereas the addition of 15% calc-alkaline magma to an OIB-type magma results in a negative Nb-Ta anomaly in the primitive mantle-normalized spider diagrams. The various types of products of recent volcanism in Cappadocia reflect the complex tectonic setting of the Central Anatolian Block: characterised by strike-slip tectonics and local extensional pull-apart basins, different magma types formed contemporaneously through decompression melting at different depths. In this scenario, the talc-alkaline magmas derived from partial melting of a mantle wedge modified by a subduction component, whereas the within-plate (OIB) magmas derived from a deeper, unmodified sub-slab mantle source. Mixing between talc-alkaline and within-plate magmas during their rise to the surface produced the Na-alkaline Cappadocia basalts. The very low rate of extension allowed within-plate magmas (OIB) to reach the surface after having interacted with the overlying talc-alkaline magmas in the late stages of Cappadocia volcanism.