Pb–Nd–Sr isotope and trace element geochemistry of Quaternary extension-related alkaline volcanism: a case study of Kula region (western Anatolia, Turkey)

Abstract

The Quaternary alkaline volcanism of Kula is located in a western Anatolian graben system that resulted from an Aegean extensional regime. The typically silica-undersaturated, alkaline lavas of Kula are distinguished in three different sequences, namely Burgaz, Elekçitepe and Divlittepe, with a range of basanite, tephrite and phonotephrite compositions. To evaluate the source characteristics of the Kula lavas, rare earth elements as well as Sr, Nd and Pb isotopic compositions were determined. The volcanic rocks show significant enrichment in high field strength elements (such as Nb and Ta) and large ion lithophile elements. Furthermore, they exhibit high Nb/Y (>3) ratios, which are typical characteristics of within-plate alkaline volcanic rocks. However, the Rb, Ba, Sr, Nb and Ta contents and Rb/Nb and K/Nb ratios of the Kula lavas are considerably higher than in typical ocean island basalts (OIB), implying a contribution of lithospheric mantle in their genesis. Isotopic compositions of 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb for the Kula lavas range from 0.703029 to 0.703490, from 0.512773 to 0.512941, from 18.689 to 19.064, from 15.606 to 15.683 and from 38.560 to 39.113, respectively. Assimilation and fractional crystallisation modelling indicates that crustal contamination does not play a significant role in the evolution of the Kula lavas but that they evolved via fractional crystallisation processes. Elemental variations are also related to fractional crystallisation. The geochemical characteristics of the Kula lavas suggest a contribution from two mantle sources: (1) mainly an OIB-like asthenospheric component and (2) a limited contribution from a lithospheric mantle component. The volcanism of Kula is linked to an extensional setting in western Anatolia that developed during Late Miocene–Pliocene. It is correlated to the thinning of the crust in a post-collision period and concomitant with the upwelling of the underlying upper mantle.