Authors: YEŞİM YÜCEL ÖZTÜRK, CAHİT HELVACI, MUHARREM SATIR
Abstract: The Upper Cretaceous Beypazarı granitoid of the western Ankara, Turkey, is composed of two different units, on the basis of petrography and geochemical composition; these are granodiorite and diorite. The granitoid is subalkaline, belonging to the high-K calc-alkaline I-type granite series, which have relatively low initial 87Sr/86Sr ratios (0.7053-0.7070). All these characteristics, combined with major, trace element geochemical data as well as mineralogical and textural evidence, reveal that the Beypazarı granitoid formed in a volcanic arc setting and was derived from a subduction-modified and metasomatized mantle-sourced magma, with its crustal and mantle components contaminated by interaction with the upper crust. The rocks have \epsilonNd(75Ma) values ranging from -5.5 to -2.0. These characteristics also indicate that a crustal component played a very important role in their petrogenesis. The moderately evolved granitoid stock cropping out near Beypazarı, Ankara, was studied using the oxygen and hydrogen isotope geochemistry of whole rock, quartz and silicate minerals. \delta^1^8O values of the Beypazarı granitoid are consistently higher than those of normal I-type granites. This is consistent with field observations, petrographic and whole-rock geochemical data, which indicate that the Beypazarı granitoid has significant crustal components. However, the \delta^1^8O relationships among minerals indicate a very minor influence of hydrothermal processes in sub-solidus conditions. The oxygen isotope systematics of the Beypazarı granitoid samples results from the activity of high-\delta^1^8O fluids (magmatic water), with no major involvement of low-\delta^1^8O fluids (meteoric water) evident. The analysed four quartz-feldspar pairs have values of \Deltaqtz-fsp between 0.5-2.0, which are consistent with equilibrium under close-system conditions. No stable isotope evidence was found to suggest that extensive interaction of granitoids with hydrothermal fluids occurred and this is consistent with the lack of large-scale base-metal mineralization.
Keywords: Beypazarı granitoid, Upper Cretaceous, oxygen and hydrogen isotopes, crustal contamination, western-central Anatolia, Turkey
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