Petrology and geochemistry of augite trachytes and porphyritic trachytes from the Gölcük volcanic region, Isparta, SW Turkey: A case study

Abstract

Abstract The Plio-Quaternary alkaline volcanic rocks at Golcuk, Isparta, SW Turkey, include older porphyritic trachytes, augite trachytes and younger trachyandesitic and tephriphonolitic dikes. In order to better understand their pre-eruptive evolution the geochemical variations of porphyritic trachytes and augite trachytes have been modeled quantitatively using the MELTS algorithm ( Ghiorso and Sack, 1995 ). Their geochemical variations are also compared with those of the younger series. Results show that two distinct magma series (trachyte-trachyandesite and tephriphonolite) governed the petrogenesis of Golcuk volcanic rocks. Geochemistry of the trachytic-trachyandesitic series is controlled predominantly by clinopyroxene and plagioclase fractionation at a shallow depth under oxygen fugacity conditions, two units above the QFM buffer. MELTS calculations also show that the crystallisation of trachytic-trachyandesitic magma produces clinopyroxene, plagioclase, alkali feldspar, biotite, apatite and magnetite+quartz, consistent with the mineralogy of the augite trachyte, porphyritic trachyte and trachyandesitic dikes.