Petrogenesis of Late Cretaceous Adakitic Magmatism in the İstanbul Zone (Çavuşbaşı Granodiorite, NW Turkey)

The Cavusbasi granodiorite intrudes Ordovician sedimentary rocks in the western part of the Istanbul Zone (NW Turkey). The intrusion is made up mainly of granodiorite, and subordinate tonalite and quartz diorite, and has a granular texture and some special mixing textures such as antirapakivi, blade-shaped biotite, acicular apatite, spongy cellular dissolution/melting plagioclase textures. The main mafic minerals are hornblende and biotite. U-Pb in-situ dating of zircons from two samples via SHRIMP yielded weighted age values of ~68 Ma, suggesting emplacement during the Late Cretaceous. Geochemically the Cavusbasi granodiorite resembles adakites with high Sr/Y and La/Yb ratios, low Y and HREE contents and no Eu anomaly. It contains 63.4 (wt%) >SiO2, and is I-type, metaluminous, middle-K calc-alkaline. These adakitic rocks have high values of MgO (0.77-2.56 wt%), Mg# (45.3-59.3) and LILE (e.g., Rb, K, Ba, Sr). Initial eNd and 87Sr/86Sr values are 3.2-3.7 and 0.7035-0.7036, respectively. Based on the continuing subduction along the Izmir-Ankara and Intra-Pontide Neo-Tethyan oceanic domains and depleted Sr-Nd isotopic signatures, we suggest that the adakitic magmas may be derived from the partial melting of an oceanic slab under amphibole-eclogite facies conditions.