YEŞİM YÜCEL ÖZTÜRK, CAHİT HELVACI & MUHARREM SATIR

Beypazarı Granitoyidinin (KB Ankara, Batı-Orta Anadolu, Türkiye) Petrojenezi Üzerine Jeokimyasal ve İzotopik Sınırlamalar

Ankara (Türkiye) batısında yer alan Geç Kretase yaşlı Beypazarı granitoyidi, petrografi ve jeokimyasal bileşimine dayanarak, granodiyorit ve diyorit olmak üzere iki farklı birime ayrılmıştır. Granitoyid subalkalin özellikte ve yüksek-K’lu seriye aittir. Granitoyidin bileşimi granitten diyorite değişim sunmaktadır. Bu kayaçlar göreceli olarak düşük 87Sr/86Sr (0.7053–0.7070) oranına sahiptir. Mineralojik ve dokusal veriler, ve ana ve iz element jeokimyası ile birlikte, tüm bu karakteristik özellikler, Beypazarı granitoyidiinin üst kabuk etkileşimi ile kirlenmiş manto ve kabuk bileşenlerine sahip, hibrid bir kaynaktan, magmatik bir yay ortam içinde oluştuğuna işaret etmektedir. Bu kayaçlar –5.5’den –2.0’a değişen aralıkta εNd(75Ma) değerlerine sahiptir. Bu karakteristikler aynı zamanda, kabuk bileşeninin Beypazarı granitoyidinin petrojenezinde önemli bir rol oynadığına işaret etmektedir. Beypazarı (Ankara) yakınında yüzlek veren, orta derecede evrim geçirmiş granitoyid stoğunun, toplam kayaç, kuvars ve silikat minerallerinin oksijen ve hidrojen izotop jeokimyası çalışılmıştır. Beypazarı granitoyidinin δ18O değerleri normal I-tipi granitler için tanımlanan değerlerden daha yüksektir. Bu durum, Beypazarı granitoyidinin önemli bir kabuk bileşenine sahip olduğuna işaret eden arazi gözlemleri, petrografik ve tüm-kayaç jeokimyasal veriler ile uyum içindedir. Bununla birlikte, mineraller arasındaki δ18O ilişkileri yarı-katı koşullarda herhangi bir hidrotermal proses girişine işaret etmemektedir. Beypazarı granitoyid örneklerine ait oksijen izotop sistematikleri, düşük-δ18O akışkanlarının (meteorik su) belirgin bir girişi olmaksızın, yüksek δ18O değerlerine sahip akışkanların (magmatik su) aktivitesini sonuçlamaktadır. Analizi yapılan dört kuvars-feldispat çift i 0.5–2.0 arasında Δqtz-feld değerlerine sahiptir, bu da kapalı sistem koşulları altında denge kavramı ile uyumludur. Sonuçta, granitoyidlerin hidrotermal akışkanlarla yaygın etkileşimini gösteren herhangi bir duraylı izotop verisi bulunmamaktadır ve bu sonuç bölgede büyük ölçekli baz metal mineralizasyonunun olmaması ile uyumludur.

Anahtar Kelimeler:

Beypazarı granitoyidi, Üst Kretase, oksijen ve hidrojen izotopları, kabuk kirlenmesi, batı-orta Anadolu, Türkiye

Geochemical and Isotopic Constraints on Petrogenesis of the Beypazarı Granitoid, NW Ankara, Western Central Anatolia, Turkey

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 eNd(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. d18O 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 d18O 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-d18O fluids (magmatic water), with no major involvement of low-d18O fluids (meteoric water) evident. The analysed four quartz-feldspar pairs have values of Dqtz-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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