Petrogenesis of mafic–silicic lavas at Mt. Erciyes, central Anatolia, Turkey

Abstract

New compositional data are presented for a suite of basaltic andesite to rhyolite flows from the flanks of the Pliocene–Quaternary Mt. Erciyes stratovolcano in Central Anatolia, Turkey. The samples are all part of the most recent (<0.9Ma) New Erciyes eruptive phase. Temperatures estimated from FeTi-oxides vary from ~1000°C in the andesites to ~820°C in the rhyolites, while the dacites preserve a very broad temperature range (~620°C–1020°C), consistent with textural and mineral chemistry evidence for their hybrid nature. Oxygen fugacity is about NNO +1 for most samples. The whole rock compositional data, in conjunction with published data, reveal two distinct trends, one formed by the mafic samples (basalts to basaltic andesites) and one by the silicic samples (andesites–dacites–rhyolites). All Erciyes samples plot above the MORB-OIB array on a Nb/Yb–Th/Yb plot, consistent with involvement of subduction-modified mantle, and their low MREE/HREE ratios imply melting of garnet-free mantle. Ratios of incompatible trace elements demonstrate that the basaltic andesite flows are not related to the basalt flows by closed-system fractional crystallization, but instead are derived from a more enriched mantle source, given their higher La/Sm and lower Zr/Nb. The silicic lavas all have higher Th/Nb than the mafic samples, indicating more extensive crustal assimilation. The silicic lavas show linear compositional trends for major elements and most trace elements for SiO2 contents between 56wt.% and 72wt.% that are attributed to mixing processes, consistent with textural observations and mineral data particularly in the intermediate dacite flows (two distinct plagioclase phenocryst populations — clear An33–53 cores vs. sieve-textured cores of An59–80; wide compositional range for orthopyroxene phenocrysts — Mg# from 63 to 90). We infer that basaltic melts of a heterogeneous, shallow mantle source ponded in the mid- to lower-crust and underwent variable differentiation accompanied by crustal assimilation to form more evolved melt compositions with SiO2>~55wt.%. These melts ascended to shallower crustal levels (4–10km based on amphibole geobarometry) where mixing of diverse melts and entrained crystals gives rise to the linear mixing trends shown by the erupted andesite–dacite–rhyolite lavas.