Crystallization condition and mineral chemistry of adakitic rocks in north of Dehgolan, east of Kurdistan

Dehgolan region is located in the northwest of Iran. In geological classification it is a part of the Sanandaj-Sirjan zone. In this region, several intermediate to acidic volcanic rocks with Upper Miocene-Pliocene age are outcropped such as Sheida- Akhikamal and Kaniderezh. The volcanic rocks from this region rocks are mainly andesite, trachy andesite and dacite. White to gray dacitic ash has been thrown around the outcrops have pozzolanic functon. Main phenocrysts are plagioclase, hornblende and biotite. Clinopyroxene with a very low frequency is also detectable in phenocrysts. In this research, mineral chemistry is used in order to investigate kinetic of the crystallization and genesis of rock forming minerals.

In this study, after field sampling and preparation of thin sections and petrographic of the samples, in order to study the chemical composition of the minerals of plagioclase, amphibole, clinopiroxen and biotite minerals in Dehgolan volcanic rocks, Eleven samples of the freshest rocks were selected for EMPA point analysis and about 80 points of four types of minerals were analyzed.

The results of the EPMA analysis of minerals indicate that plagioclases have a composition of andesine to oligoclase, Most have polysynthetic macles and also zoning is present. Their zoning is mostly normal, with the calcium content decreasing from core to rim. The amphiboles of the study area are calcic (calcium-rich and titanium-rich) and mainly of edenitic hornblende and magnesiohornblende types. Clinopyroxene are of augite type and based on clinopyroxene data, most of the samples in the study area are in the volcanic arcs (VAB) and (WPA). Biotites are Mg-biotite and are mostly of primary magmatic type and located between phlogopite and annite poles. The magma oxygen fugacity was high during the magma crystallization. Based on biotite composition, the Dehgolan volcanic samples belong to calc-alkaline magmatic assemblages of the orogenic environment. Clinopyroxenes in intermediate-acidic volcanic rocks in Dehgolan region show mainly crystallization at medium pressures and crystallize on average at 5 to 6 kb at temperatures of about 1000 to 1110 ° C, amphibole crystalized at 4 to 6 kb and 723 to 861 °C respectively. While the estimated temperature for plagioclase is between 550 to 750 °C and for biotite this is 715°C.

Volcanic rocks north of Dehgolan have a predominantly riodasite and sometimes andesitic composition with Calc-Alkaline nature, which erupts with large volumes of acid ash. The magma of the Adakitic rocks in northern Dehgolan was dehydrated during its formation, and only high-temperature, water-free minerals such as pyroxene and plagioclase were crystallized. In the following or on higher horizons collision with water in the crust iccurs and the crust is contaminated. The preponderance of xenolite confirms this, Oxygen fugitives are rised and biotite and amphibole minerals are crystallized. The biotite and clinopyroxenes geochemistry is indicative of the Calc-Alkaline Magmatic nature of the region, which erupted in the periphery of the active continental margin at the last phases of neotethys subduction in the northern Sanandaj-Sirjan region during the Mio Pliocene.