Sub-lithospheric origin of Na-alkaline and calc-alkaline magmas in a post-collisional tectonic regime: Sr-Nd-Pb isotopes in 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 MiocenetotheQuaternary.Calc-alkalinepyroclasticproductsandlavaerupteddiffuselyduring thelate Mio ceneandPliocene.Bothcalc-alkaline and Na-alkalinevolcanicrocks wereemplacedinclosespatialandtemporal proximity during the Plio-Pleistocene. Here we focus on the Plio-Pleistocene monogenetic activity, which pro ducedtwotypesofrocks:calc-alkalinevolcanicrocksrangingincompositionfrombasaltstorhyolitesandoccur ring aroundtheAcıgölcaldera,theGöllüDağdomeandtheHasanDağandErciyesDağcompositevolcanoes;Na alkaline volcanic rocks ranging from alkali basalts to mugearites and cropping out in monogenetic centres in Karapınar, along the WSW flank of Hasan Dağ,inAcıgöl and near the Kızılırmak River, northwest of Nevşehir. Thecoevaloccurrenceofcalc-alkaline and Na-alkalinevolcanism observed here isinstrikingcontrastwith activ ity in thesurroundingWesternandEasternAnatoliaregions, wherethecalc-alkalinevolcanicrocksaredistinctly 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 (87Sr/86Sr~0.70395–0.70474 and 143Nd/144Nd~0.51268–0.51281 for the calc-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 calc-alkaline magma is enough to change a SiO2-oversaturated magma into an ol-hy-normative and ne-normative one, respectively, whereas theadditionof15Êlc-alkalinemagmatoanOIB-typemagmaresultsinanegativeNb\\Taanomalyintheprim itive mantle-normalized spider diagrams. The various types of products of recent volcanism in Cappadocia reflect the complex tectonic setting of the Cen tral Anatolian Block: characterised by strike-slip tectonics and local extensional pull-apart basins, different magmatypes formed contemporaneously through decompression melting at different depths. In this scenario, the calc-alkaline magmas derivedfrom partialmelting of a mantle wedgemodifiedbya subduc tion component, whereas the within-plate (OIB) magmas derived from a deeper, unmodified sub-slab mantle source. Mixing between calc-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 calc-alkaline magmas in the late stages of Cappadocia volcanism.