Chemical Petrology of a Suite of Calc-Alkaline Lavas from Mount Ararat, Turkey

Abstract

A suite of lavas from Great Ararat is described. Analyses are reported for Si, Al, Fe, Mg, Ca, Na, K, Ti, Mn, S, P, Ba, Nb, Zr, Y, Sr, Rb, Zn, Cu, and Ni. Two distinct series can be recognized, each consisting of andesite, dacite, and rhyodacite (designated Low-Y and high-Y series), they are distinguished by significantly different K. Mn, P, Nb, Zr, Y, Rb, Cu, and normative feldspar contents-The high-Y series corresponds closely to a normal calc-alkali series, but the low-Y series is relatively deficient in K, Mn, P, Nb, Zr, Y, and Rb. from both series is . Petrographi-cally, both series are characterized by three generations of feldspar, all rock types containing labradorite together with more sodic phenocrysts and groundmass plagioclase: orthopyroxene, clinopyroxene, magnetite and ilmenite are present. Quartz and K-feldspar phenocrysts are absent. Suggestions of the former presence of garnet and brown hornblende, together with relevant experimental and geo-chemical data, lead to the suggestion that a multi-stage evolution of primitive upper mantle material has occurred, involving equilibration of a liquid containing about 2-5% with garnet and amphi-bole, with subsequent crystal fractionation. Differentiation of the "parental" andesitic magma has led to the more silicic types, the low-Y series being generated under more hydrous conditions than prevailed during the evolution of the later high-Y series. Discussion of the geotectonic setting suggests that subduction zone theory cannot be applied to the Ararat situation without modification: a lithospheric shear-zone theory at a complex plate junction is preferred.