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

The Plio-Quaternary alkaline volcanic rocks at Gölcük, 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 Gölcük 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.