Petrogenesis of Tarq-Mazdeh volcanic rocks based on clinopyroxene chemistry (South of Natanz, Urumieh Dokhtar volcanic belt)

Clinopyroxene is one of the most common of the rock forming minerals. Its long formation period (from the earliest crystallization of magma in the core of phenocrysts to the final microcrystalline crystallization in the rock background) can show the history of the host magma crystallization. The composition of clinopyroxene, especially those phenocrysts, in volcanic rocks could well establish the magmatic nature of the host lava.The clinopyroxene composition can point out the magmatic series, the tectonic environment and the source rock (Kushiro, 1960; Nisbet and Pearce, 1977, Leterrier et al., 1982). In addition, it is possible to estimate the temperature and pressure of rock formation by studying the chemistry of clinopyroxenes (Nimis and Taylor, 2000; Putirka, 2008).The study area is located in the middle part of the Urumieh- Dokhtar magmatic belt. It exactly lies in the area between Tarq and Mazdeh, its longitude is 51° 43' to 52° 00' E and its latitude is 33° 15' to 33° 30' N. The Eocene magmatic rocks vary from rather basic to acidic in composition, but they are mainly intermediate and they are rather basic rocks (Ghadirpour, 2017). Previously, various studies on using the chemical composition of the major elements of clinopyroxene were conducted to discover the conditions for the formation of igneous rocks in different parts of Iran (Sayari and Sharifi, 2016; Falahaty et. al., 2016; and Mohammadi et. al., 2017).So far, in all of these studies that have been conducted on volcanic rocks, the mineral chemistry of clinopyroxene has not been used to evaluate the magma's features such as temperature, pressure, and oxygen fugacity. In this article, we are going to study the mentioned features of magma by using chinopyroxene chemistry. To determine the geotectonic setting and the physicochemical conditions of volcanic rocks, thirty thin sections have been prepared. Their minerals and texture have been studied by using polarizing binocular microscope (Olympus BH-2). After detailed mineralogy and selection of suitable samples, microprobe analysis is done by EPMA (JEOL- JXA) in the Naruto University, Japan. The mineral analysis is performed at 15 nA intensity of current and accelerate voltage of 15 Kev. The study area is situated in the South of Natanz, between Tarq and Mazdeh villages. The volcanic rocks vary from acidic to rather basic (basaltic andesite to andesite and rarely rhyolite). Microlitic porphyric, glomeroporphyric and vesicular are some textures which are observed in the volcanics. Plagioclase and euhedral clinopyroxene occasionally with simple twinning are characteristic minerals of rocks.According to Wo-En-Fs diagram (Morimoto, 1989), clinopyroxene shows mainly the composition of augite.It is concluded that the magmatic series of rocks is calc-alkaline which is in relation to the subduction of the Neotethys oceanic crust under the central Iranian plate.There are several diagrams that are used for this purpose which are as follows.The Al2O3-Ti2O diagram (Le Bas, 1962): In this diagram, the studied clinopyroxene shows the nature of calc-alkaline. One of the diagrams used to determine the tectonic setting according to clinopyroxene composition is the F1- F2 diagram (Nisbet and Pearce, 1977). Based on this diagram, the Tarq- Mazdeh volcanic rocks belong to the magmatic arc environment.The Clinopyroxene temperatures are calculated by using a variety of methods which indicate that most of clinopyroxene in temperature range of 1150 to 1200°C has been crystallized (Soesoo, 1997). The temperature indicates changes in crystallization of clinopyroxene. The calculated temperatures of clinopyroxenes by using various methods show that they are crystalized in the temperature range of 1150 to 1200˚c. It mainly means that there is a change in temperature during clinopyroxene crystallization. By considering the barometric diagram, the pressure of clinopyroxene formation has been determined below 10 kb, in the depth range of 2 to 5 km.