The oxygen and strontium isotope compositions of Cenozoic granitoids cropping out in the Izmir-Ankara-Erzincan suture zone help constrain the petrological evolution of magmatism in northwest Anatolia. The magmatism was mostly widespread between late Eocene (similar to 37 Ma) and the middle Miocene (similar to 14-15 Ma), and is represented by volcanic and plutonic rocks of orogenic affinity, of which Ezine, Egrigbz, cataldag and Kozak are the largest Tertiary granitic plutons exposed in northwest Anatolia. They vary from granite to granodiorite, and are subalkaline, belonging to the high-K calc-alkaline I-type granite series. All these characteristics, combined with major, trace element geochemical data as well as mineralogical and textural evidence, reveal that the Oligocene-Miocene granitoids of NW Anatolia are comparable with volcanic arc granites, formed in a transitional oceanic to continental collisional tectonic setting, from a hybrid source, having crustal and mantle components that underwent further interaction with the upper crust. These plutons have initial Sr-87/Sr-86 ratios of 0.7072-0.7094, and epsilon Nd(t) values ranging from -3.48 to -1.20. These characteristics also indicate that a crustal component played an important role in the petrogenesis of NW Anatolian Oligocene-Miocene granitoids. The moderately evolved Ezine, Egrigoz, cataldag and Kozak granitoids, have delta O-18 values that are consistent with those of normal I-type granites (6-10 parts per thousand), but the delta O-18 relationships among minerals of samples collected from the intrusive contacts which are closest to mineralized zone, indicate a major influence of hydrothermal processes under subsolidus conditions. The oxygen isotope systematics of the samples from these plutons result from the activity of high-delta O-18 fluids (magmatic water), with major involvement of low-delta O-18 fluids (meteoric water) evident, near the edge zone of these plutons. This is most evident in delta O-18 quartz feldspar pairs from these granitoids, which commonly have values characteristic of open-system hydrothermal conditions, and is consistent with the presence of large scale base-metal mineralization around the NW Anatolian granitoids. (C) 2016 Elsevier Ltd. All rights reserved.