Kapadokya İgnimbiritlerinde Bulunan Minerallerin ve Kapanımların Mikro Ölçekte Petrolojik İncelemesi: Silisik Magma Rezervuarlarının Zamansal Değişimi

Abstract

The melt inclusions representing the samples of juvenile melt trapped in the host mineral phase during crystallization, have an important role in controlling the trace element abundances in magmatic systems. The melt inclusions isolated and separated by crystallization from the main magma, reveal the petrogenetic development of the crystal phase and the magmatic system. Thus, trace element partition values have been calculated from trace element concentrations obtained by Secondary Mass Spectrometer (SIMS) analytical method of zircon containing melt inclusion of ignimbrites in the Cappadocian Volcanic Province (from older to younger; Kavak, Zelve, Sarımadentepe, Sofular, Cemilköy, Tahar, Gördeles, Kızılkaya and Valibabatepe). The trace element partition values of the zircon/melt inclusion pairs exhibit compatible behavior due to the increase in the atomic number of the related trace element. Furthermore, correlation of melt inclusion with whole rock and matrix glass trace element compositions has been shown that the dominant process in the petrogenetic development of pyroclastic flow deposits, is fractional crystallization. The Cappadocia Volcanic Province (CVP) is defined as a region that has hosted SiO2-rich rhyolitic magma development and consequently the caldera-forming eruptions. Most of the studies conducted in this region on the geochemical and petrological basis of whole rock composition of pyroclastic flow units, show little information about extraction and storage processes of large volume of silicic magmas prior to eruption, existence of processes that control caldera formation and which are thought to have an effect on melt extraction process. Here, we present the extraction and deposition model of the magma source of common pyroclastic flow units in Central Anatolia. The ignimbrites in the Cappadocia region present almost identical pumice composition and similar phenocrystal contents (except for Tahar and Valibabatepe). In order to create the model of the magmatic system before the eruption and reveal the potential triggering effect of each eruption processes, matrix glass, melt inclusions in the zircon, whole rock trace element contents and mineral associations belonging to the units were used. In order to support these studies, trace element compositions obtained from zircon crystals and Hf isotope data were also used. Accordingly, it was concluded that the magma chambers with almost similar compositions developed from the same source and that these magma chambers were stored at the upper crustal levels representing the shallow depths isolated from each other and the melt continued to evolve until the time of the eruption. It has been shown that fractional crystallization represents the most dominant process in the generation of the units stored in the upper crust and in some units (Kavak, Zelve, Cemilköy, Tahar and Gördeles), as well as fractional crystallization, crustal contamination has been observed as the effective process. The effect of the mafic magma recharge before the eruption observed as zircon crystal CL zoning pattern variation and increasing in U and Th content. However, Hf isotope values measured from zircon crystals indicate that crustal effect not only is gradually decreasing towards the older to the young as proposed by the oxygen isotope studies, but also there is a periodic effect of the crustal contribution during the formation of the units.