Genesis of collision-related volcanism on the Erzurum-kars plateau, North eastern turkey

Abstract

The Eastern Anatolia Region exhibits one of the world's best exposed and most complete transects across a volcanic province related to continent-continent collision. Within this region, the Erzurum-Kars Plateau is of particular importance since it contains the full record of post-collision volcanism from 11 to 1.5 Ma. The volcanics of the Erzurum-Kars Plateau cover a broad compositional range from basalts to rhyolites displaying a calc-alkaline character. They show a distinctive subduction signature represented by selective enrichment of the large ion lithophile and light rare earth elements. Among trace elements, Y behaves in a quite different way forming two distinct trends against silica named as the low- and high-Y series. Lavas of the high-Y series are characterised by a distinct bimodal volcanism (from basalt to rhyolite), in contrast to the low- Y series which comprises an unimodal andesitic volcanism. Trace element systematics together with modelling of theoretical Rayleigh fractionation vectors suggest that the low-Y series underwent a hydrous crystallisation dominated by amphibole as the mafic phase, whereas the high-Y series was dominated by anhydrous (POAM) crystallisation. Al-in- amphibole geobarometer calculations on the plateau volcanics reveal that the low-Y series evolved in magma chambers located between 20 and 28 km. In contrast, magma chambers of the high-Y series were much shallower, around 14-22 km. The high-Y series dominates early and late stages of the volcanic activity, whereas the low-Y series dominates the middle (between 7.5 and 5 Ma) stage, probably coinciding with the most intensive stage of crustal thickening. Sr, Pb, Nd and δ(^18)O isotopic systematics also show significant differences between the high- and low-Y series. Lavas of the high-Y series are always more radiogenic with respect to the lavas of low-Y series. Results of assimilation combined with crystallisation (AFC) modelling suggests that the low-Y series assimilated a lower crustal material which is compositionally similar to the granulitic xenoliths from the Pannonian Basin in Hungary and from Central Europe. In contrast, the high-Y series assimilated two different upper crustal materials. In both the low- and high-Y series, the maximum assimilation rate was around 40% of the fractional crystallisation rate. Trace element and isotopic differences between the low- and high-Y series are not significantly dependent upon variations in the source. These differences appear to have been extensively controlled by the AFC processes.