Hisarlikaya (Ankara Güneybatisi) Yöresi Volkanik Kayaçlarinin Petrolojik ve Jeokimyasal Özelliklerinin İncelenmesi

Abstract

This thesis has been carried out on the volcanic rocks in Hisarlıkaya, which covers an area of approximately 20 km2, located in Ankara-Temelli; and on xenolites of different shapes, sizes, colors and origins in these rocks. The aim of this study is to determine volcanic rock’s mineralogical, petrographic and geochemical properties.

With thin section studies it was determined that, the Hisarlıkaya volcanic rocks (HVR) contain feldspar, amphibole, biotite and rarely pyroxene minerals.

With electron microprobe analyzes, the character of minerals have been determined; amphibole minerals in host rock are magnesiohasdingsite, plagioclase minerals are andesine-labrador; the pyroxene minerals in xenolites are diopside-hedenberjit, plagioclase minerals are andesine-labrador-oligoclase in composition. Garnets are defined as andradite-grossular solid solution products.

Through the mineral associations in the host rock and xenoliths, temperature-pressure calculations were performed. According to this calculations, pressure values of amphibole minerals in host rock were 7,5-8,5 kbar and 6-7 kbar; formation temperature range was revealed to be 852-911oC. The formation temperature of garnet crystals in xenoliths was estimated as 410-470oC.

According to major element geochemistry results of HVK, %SiO2 values vary between 60,99-64,18%. This rocks have sub-alkaline trachyte and trachyandesite character and show a calc-alkaline feature. According to Harker diagrams, fractional crystallization is an effective process in the formation of the HVK; also, different processes such as magma mixing can be effective, too. According to multiple element diagrams which created from REE and trace element data, it was seen HVK enriched by LILE and LREE, depleted by Nb (16,7-23 ppm), Ti (2675-3782 ppm) and HFS elements. Considering the geochemical properties, HVK may have been derived from the melts formed as a result of the low-grade (3-6%) batch melting of the spinel peridotite source in relatively shallow depths, which was metasomatized with the effect of subduction. HVK also exhibit adacitic feature. The compiled information suggests that the melt formed from the metasomatized lithospheric source is hybridized with a melt formed with lower crust melting, and this hybridized melt is not exposed to effective crustal contamination during the ascension of the melt.

According to the macroscopic features of the xenolites in the host rocks, they can be divided into 5 basic categories: purple-crystallized, green-clayed, black-red massive, with graphite band and with garnet. Xenoliths contain feldspar, quartz, biotite, amphibole, graphite, pyroxene, garnet, epidote and large amount of iron oxides; they show different mineral associations. The origin of xenolits seems to be related with shallow pluton with unknown shape and dimensions, located beneath the region, metamorphosed the surrounding rocks and a contact zone formed at its contact. Metamorphic formations such as quartz-schist, graphite-schist, gneiss, quartzite, hornfels have occurred apart from the formation of skarn with the underlying metamorphism of different types of basic rocks in the environment. All these xenoliths that have different characters, are carried with the volcanic activity that will later form the HVR.