جستجوی مقالات مرتبط با کلیدواژه "گرگان" در نشریات گروه "زمین شناسی"

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.

The Shirinabad clay-bauxite deposit with more than 1 km long and about 8 m in thickness is located in 60 km south-east of Gorgan. The Shirinabad deposit has been developed as a stratiform horizon along the contact zone of Triassic dolomitic limestones and Jurassic shales and sandstones. The basal contact zone of the horizon is mainly undulatory¡ whereas the upper contact zone is concordant with the hanging-wall shales and sandstones. The rocks within the horizon show pelitomorphic¡ microgranular¡ oolitic and pisolitic textures. Textural analysis indicates both allochthonous and autochtonous origins for the Shirinabad deposit. Based on textural and mineralogical evidences the deposit can be divided into four distinct units. Kaolinite¡ anatase¡ routile¡ bohemite¡ hematite¡ goethite and berthierine are the principal constituents. From geochemical data¡ it is concluded that the Shirinabad deposit probably originated from basaltic volcanic rocks. Combination of mineralogical and geochemical data shows that the Shirinabad deposit formed in two stages. First¡ bauxite materials and clay minerals were developed as authigenic bauxitization processes of alkaline basaltic parent rock. Then¡ these materials were transported to karst depressions and formed the Shirinabad clay-bauxite deposit.

More than 80% of drinking water of highly populated and expanding city of Gorgan, as one of agricultural centers of Iran, is supplied by local groundwater resources (Ziarat, Garmabdasht, and Shast-kola aquifers). Chemical analysis of groundwater samples of Gorgan district indicates two regions of anomalously high concentration of nitrate (Ziarat aquifer) in the urban area. High chloride concentration in water from dug wells corresponds with the anomalous high values of nitrate. Low concentration of nitrate in Shast-kola as well as Garmabdasht aquifers, where main land use is agricultural activities respect to Ziarat aquifer, demonstrates low/no effects of fertilizers on the contamination of groundwater. Determination of NO3/Cl and K/Cl ratios depicts sewage effluents and urban runoff as the main sources of nitrate contamination of the groundwater. Moreover, rapid urbanization and the encroachment of residential development into surrounding forests release considerable nitrate to the underlying shallow aquifers.