Application of clinopyroxene as a petrogenic mineral in determining the nature of Ordovician intrusive rocks in Ziarat (Southern Gorgan, Golestan Province)

The study area is located in the Eastern Alborz zone (Gansser, 1951) which is a part of the northern margin of Gondwana during the Paleozoic (Stöcklin, 1974; Salehi Rad, 1979; Berberian and King, 1981; Şengor, 1990; Alavi, 1991, 1996; Stampfli and Borel, 2002; Allen et al., 2003; Horton et al., 2008; Sinha, 2012, 2013). According to studies conducted by Gansser (1951) and Hubber (1957), the gabbro masses of South Gorgan (Nahar Khuran Valley) belong to the ophiolite collection. Meanwhile, 6km South-East of Galougah city, Gorgan schists have been covered by underlying Jurassic sandstones with s steep discontinuity. The parts resulting from the erosion of Gorgan schists were observed in the sequence conglomerate of the underlying Jurassic. At the beginning of the Ziarat Valley, about 800 meters above the Nahar Khuran, two small tectonic masses (the thickness of about 11 meters) are placed on the layers. These masses are mostly metamorphic. The metamorphic rocks (Gorgan schists), as one of the important geological units in Iran, are mainly formed from low-metamorphic rocks such as slate, phyllite, chlorite schist, greenschist and micaschist along with volcanic rocks and gabbrodiorite masses infiltrating them.

To prepare a geological map of the study area, field sampling and fieldwork were first done from the various units in the region. Over 100 samples were collected from the area. Approximately 70 thin cross-sectional samples of the tectonic rocks of the area were selected from them and investigated using a polarizing microscope. In this paper, 11 points of clinopyroxene were selected for microprobe analysis. The point analysis done on these minerals was conducted using the EPMA method using a microprobe analysis set in the Center of Mineral Processing of Iran (Karaj). The structural formula is calculated using the Excel (Spreadsheet) and dividing them was done by the MinPet 2.02 software package. Given that microprobe analysis is unable to distinguish Fe2+ and Fe3+ and given that it reports total iron as FeO*, it is necessary to separate these two from each other in order to calculate the minerals’ structural formula.

Petrographic studies identified rocks including gabbro, olivine gabbro, monzonite, gabbro to monzogabbro altered, metagabbro, and catalactic porphyry diorite. Mineralogically, these rocks consist of phenocrysts of plagioclase with labradorite composition, clinopyroxene and olivine with accessory minerals of apatite, sphene, biotite, and metal minerals. Secondary minerals are chlorite, sericite, clay mineral and epidote. The dominant textures in these rocks are granular and ophitic. The results of electron microprobe analysis of these clinopyroxenes show that they have augite compositions. In addition, gabbros also mostly show alkaline composition.

To precisely investigate the rocks in the research area, the results of the chemical analyses of minerals were used in determining their petrogenesis. Felsic minerals mainly include altered plagioclase, the main combination of which is labradorite and which is converted to the secondary albite due to hydrothermal alteration processes. The combination of clinopyroxenes available in the volcanic rocks is in Quad range and has a composition of augite. The temperature calculated for the clinopyroxenes in gabbros is 1277.7 to 1353.1°C and the pressure is more than 10 Kbars.  (On the baseline of 3.65kbar per 1km in depth), the formation of clinopyroxenes in the parent magma was over 37km. The primary water content of the gabbro magmas is estimated to have been between 0.5 and 5 wt.%. Distribution of aluminum in tetra and octa positions of clinopyroxenes depends on pressure and the amount of water available in the crystallization environment. Accordingly, the amount of AlIV decreases as the amount of water increases. Tectonomagmatic diagrams suggest that the host rocks are alkaline and are related to volcanic arc setting.