Volcanostratigraphy and petrogenesis of the Nemrut stratovolcano (East Anatolian High Plateau): The most recent post-collisional volcanism in Turkey

Abstract

The Nemrut stratovolcano, with its well defined summit caldera, is the most recent volcanic eruption center (1441 A.D.) in the East Anatolian High Plateau, one of the best examples of an active colllision zone. Widespread volcanism has been active in this region for the last ∼ 10 Ma, producing large volumes of lavas and pyroclastics covering a broad belt, across the Bitlis Suture Zone, from the Anatolian plate in the north to the Arabian plate in the south. Three major evolutionary stages have been identified in the evolution of the Nemrut stratovolcano: pre-caldera, post-caldera and late stages. The pre-caldera stage is further differentiated into effusive, extrusive and explosive phases, the latter being responsible for the caldera formation. The products of this stage cover a broad compositional range from basalts to rhyolites, and consist of lava flows, domes, large volumes of ignimbrites and associated pyroclastics. The post-caldera stage consists of three phases comprising phreatomagmatic eruptions, vitrophyric rhyolitic lava flows and dykes. The late stage consists of the explosive and the following effusive phases with vitrophyric rhyolitic and basaltic lava flows, respectively. The post-caldera and late stages show a compositional gap between the (younger) mafic and the (older) felsic members. The products of all three stages are silica-saturated and display a gradual transition in nature from sodic in the mafic to potassic in the felsic members. The overlaps in the compositional spectra and the geochemical trends on bivariate plots suggest the derivation of the products of all three stages from a single parental magma (or similar parental magmas), yet the stratigraphic position of the mafic and felsic members points to a periodically replenished magma chamber. The major and trace element systematics, along with petrographic evidences, suggest clinopyroxene + plagioclase ± olivine ± amphibole as the major fractionating phases governing the liquid evolutionary paths, with a progressive increase in the proportions of plagioclase and amphibole (joined by K-feldspar, biotite and apatite) in the fractionating assemblages during the course of magmatic differentiation. The volcanics of all three stages exhibit enrichment in light rare earth elements (LREE) over heavy rare earth elements (HREE), and large ion lithophile elements (LILE) over high field strength elements (HFSE). The multi-element patterns and low Th/Y and Nb/Y ratios imply a within-plate type mantle asthenospheric source for this volcanics. Trace element modelling suggests derivation of parental magma(s) from 10% to 30% partial melting of a spinel–lherzolite source. Assimilation-fractional crystallization (AFC) modelling reveals significant but variable degrees of crustal contamination in the evolution of the volcanics with a range of r values (assimilation rate/crystallization rate) from 0.2 to 0.8. These interpretations are in conformity with the recent findings from seismic studies showing that lithospheric mantle is absent in eastern Anatolia. The crust in the region is about 45 km thick and directly underlain by the asthenosphere, possibly due to slab breakoff following the collision of the Arabian and the Anatolian plates along the Bitlis Suture Zone.