A Petrognetic Model for Evidence of Asthenospheric Input Into the Source of Central Anatolia Quaternary Basalts (Central Anatolia)

Abstract

In this study, we present for the first time a petrogenetic model of the Quaternary aged nepheline normative Karapınar basalts (Karapınar-Karacadağ Volcanic Complex-KKVC) to explain the asthenospheric material involvement into their source. In recent studies, it has been proved that nepheline normative basalts cannot only be derived from peridotite melts, and they can also be originated from pyroxenite melts. Investigated basalts from KKVC show orogenic pattern in MORB normalized diagram. Although having flat HREE patterns, which is interpreted as originating from garnet-free source in many studies, these basalts may be derived from a garnet bearing source which underwent higher degree partial melting. Their moderate to low Y (< 25 ppm) and low HREE concentrations (Yb < 2 ppm), along with fractionated HREE patterns (Dy/YbN > 1) indicate that garnet was likely to have been involved in the source of the rocks. Furthermore, the most primitive basalts (B1; MgO~10-12 wt%) contain Zn/Fe (*10000) ratio (~16) resembling those of pyroxenite derived melts. Here, we present partial melting and binary mixing models for the Karapınar basalts. The results for modal batch partial melting model of spinel lherzolite and garnet pyroxenite show that mixing between pyroxenitic and peridotitic melts can produce the observed trace element heterogeneities of the most primitive basalts. Although melting model shows that the basalts have plottod into the area between the pyroxenitic and peridotitic sources whose melting degrees are minimum 30% and 5% respectively, the binary mixing model indicates that the rocks were derived from a mixture source of 50% molten pyroxenite and 3% molten peridotite. Furthermore, the mixing model also suggest that the proportion of the pyroxenitic melts in the source should be minimum 45%. According to geochemical data and petrogenetic calculations, we propose a mixing model that nepheline normative basalts were derived from mixing of a peridotitic source with a pyroxenitic source which was brought from the deeper parts of the upper mantle by asthenospheric upwelling.