Al-in-hornblende thermobarometry and Sr-Nd-O-Pb isotopic compositions of the early miocene Alaçam granite in NW Anatolia (Turkey)

During and aft er the closure of the Neo-Tethyan Ocean and progressive collision of the Tauride-Anatolide Platform with the Sakarya Continent, widespread magmatism occurred in NW Anatolia. This magmatism is manifested in a NW-trending belt along the northern border of the Menderes Massif. Due to the complex geodynamic setting of this region, the exact emplacement depth of the granitoids is still a matter of debate. Here we present Al-in-hornblende barometrical data and Sr-Nd-Pb isotope compositions of the Early Miocene Alaçam granite. The results imply a shallow emplacement depth of this granite (4.7±1.6 km) in contrast to previous studies which suggested emplacement along the brittle-ductile boundary of the crust. Furthermore, an evaluation of literature data let us reconsider the general emplacement mechanism of the Alaçam and other Early Miocene granitoids in the region. Initial isotopic signatures of the Alaçam granite are ⁸⁷Sr/⁸⁶Sr(I)= 0.70865-0.70915, e_Nd(I)= -5.8 to -6.4, δ¹⁸O= 9.5-10.5, ²⁰⁶Pb/²⁰⁴Pb isotope ratios vary between 18.87 and 18.90. These features indicate an assimilation-dominated crustal crystallization and melt derivation from an older middle crustal protolith.