O, Sr and Nd isotopic constraints on Cenozoic granitoids of Northwestern Anatolia, Turkey: Enrichment by subduction zone fluids

• Isotope geochemistry of Tertiary granitoids from northwestern Anatolia.
• Investigation of crust/mantle interactions by using O–Sr isotopic systematics.
• Hydrothermal mineralisations related to the Tertiary magmatism in NW Anatolia.

The oxygen and strontium isotope compositions of Cenozoic granitoids cropping out in the İzmir-Ankara-Erzincan suture zone help constrain the petrological evolution of magmatism in northwest Anatolia. The magmatism was mostly widespread between late Eocene (∼37 Ma) and the middle Miocene (∼14–15 Ma), and is represented by volcanic and plutonic rocks of orogenic affinity, of which Ezine, Eğrigöz, Çataldağ 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 87Sr/86Sr ratios of 0.7072–0.7094, and εNd(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, Eğrigöz, Çataldağ and Kozak granitoids, have δ18O values that are consistent with those of normal I-type granites (6–10‰), but the δ18O relationships among minerals of samples collected from the intrusive contacts which are closest to mineralized zones, 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-δ18O fluids (magmatic water), with major involvement of low-δ18O fluids (meteoric water) evident, near the edge zone of these plutons. This is most evident in δ18O 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.