Assimilation and fractional crystallization of foid-bearing alkaline rocks: Buzlukdağ intrusives, Central Anatolia, Turkey

Authors: KIYMET DENİZ, YUSUF KAĞAN KADIOĞLU

Abstract: Felsic intrusive rocks within the Central Anatolian Crystalline Complex provide a window into the geodynamic processes in operation during the final closure of the Neotethys Ocean. Previous studies were largely restricted to the calc-alkaline granitoids, and the structural and petrogenetic relations of syenitoids are poorly studied. The Buzlukdağ Intrusive Complex is a silica-undersaturated alkaline syenite that is differentiated into three concentric subgroups according to texture and grain size. Mineral compositions do not vary between the subgroups but differentiation has resulted in different mineral proportions. Mafic microgranular enclaves are present throughout the suite, indicating mingling and mixing between the coeval felsic and mafic magmas. Major element concentrations are consistent with fractional crystallization of nepheline + K feldspar ± Na rich plagioclase + Na amphibole + pyroxene ± melanite ± cancrinite. Mineral chemistry reveals that the syenites are crystallized under a wide range of pressures (1.5-3.7 kbar), at varying temperatures (732-808 °C), and are likely emplaced at depths of 6-14 km. Large-ion lithophile element and light rare earth element enrichments with respect to high field-strength elements and heavy rare earth elements are consistent with their derivation from an incompatible element-enriched magma source. Incompatible trace element concentrations (e.g., Sr, Ba, Th, Ta, Pb, La, Ce, and Yb) revealed that the magma has a subduction fluid component, which can be distinguished from crustal assimilation. The Buzlukdağ alkaline intrusive rocks are likely to be derived from decompressional melting of the lithospheric mantle above asthenospheric upwelling as a result of crustal thinning of Central Anatolia during the Late Mesozoic-Early Cenozoic.

Keywords: Buzlukdağ syenite, alkaline rocks, assimilation and fractional crystallization, subduction zone metasomatism, lithospheric mantle, enclave

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