پترولوژی (Dec 2022)
nPetrography, geochemistry, and tectonomagmatic model of igneous masses in Gorgan Schist of the south of Behshahr, east of Mazandaran
Abstract
The Gorgan schist complex, composed of low-grade metamorphic rocks, are widespread in the Alborz highlands in the south of Mazandaran Sea and extended from Behshahr to Aliabad. The complex is characterized by alternating of phyllite, sericite, chlorite schist, and quartzite along with ophiolite interlayers at the base part of the interval (Tietze, 1877; Stahl, 1911). The shale layers of the studied area are discontinuously covered by non-metamorphosed sediments, in the lower part, these sediments include a layer of conglomerate in which the fragments of Gorgan schists occurred (Hubber, 1957).Materials and methods25 thin sections were prepared from the collected samples. and were studied in the microscopic laboratory of Kharazmi University in Tehran with a Zeiss Axioplan 2 polarizing microscope. 10 samples with minor alterations were analyzed by ICP-MS and XRF methods at the Zarazma laboratory in Tehran. The data obtained from the chemical analyses are given in Tables 1 and 2. Abbreviations used in the text, figures, and tables of the present study are based on Whitney and Evans (Whitney and Evans, 2010). GCDKit, Excel, and Adobe Illustrator softwares were used.to analyze the geochemical data and to draw the required diagrams.Regional GeologyThe studied area lies in the north of Iran, the east of Mazandaran province, and the middle-eastern Alborz zone. The northern part of this zone is Gorgan-Rasht zone, including the marginal areas of the Mazandaran Sea as well as the north of Alborz fault (Darvishzadeh, 2013). Some workers believe that the protoliths of these metamorphic complexes belonging to Paleozoic or Mesozoic rocks deformed by collisional tectonics in the Late Triassic (Khosrotehrani, 2009). The Behshahr and the Galogah regions are located in the geological divisions of Iran in the central Alborz zone and the west of eastern Alborz zone.PetrographyAccording to the petrography, the investigated masses in the south of Behshahr include metagabbro, metabasalt and orthogneiss. Metagabbros in the hand sample are often seen as grayish-green, fine-grained to medium-grained and granular texture. In the microscopic studies, plagioclase and pyroxene phenocrysts are place in the microlithic to crypto-crystalline matrix, which shows porphyry texture. Also, metamorphic amphiboles such as tremolite-actinolite time with pseudomorph texture have grown in the pyroxene forms. Plagioclases are illustrated with polysynthetic twinning. Some plagioclases have been sericitized, and pyroxenes altered to serpentine. Pyroxene and plagioclase are the major rock-forming crystals and make the major granular texture. Mineral deforming such as amphiboles bending can show the syn-tectonic phases with the peak of metamorphism. The petrographic evidence demonstrates high-grade metamorphism, in the border of the upper amphibolite facies and granulite facies. The presence of gneisses containing high-grade metamorphic minerals such as garnet and clinopyroxene proves the high-grade metamorphic series in the central part of the Gorgan metamorphic complex. The identification of dynamo-thermal metamorphic fabrics such as Bookshelves and Micafish indicates the influence of special tectonic processes on the margin of the subduction zone with exhumed metamorphic complexes during the closing subduction systems and upwelling the metamorphic complex into the upper crust levels.GeochemistryThe chemical analysis show, the amount of SiO2 varies between 35.94 and 49.26 and they are in the range of basic rocks. Also, the Al2O3 content in these samples is between 13.80 and 20.32 which mines our samples are in the range of per aluminum. Also, the results of the chemical analysis represented our samples belonging to the calc-alkaline rock series. Low titanium content (1.59 to 2.72 wt%) and low potassium content (0.06 to 1.87 wt%) are special characteristics of these rocks. The low amount of P2O5 and Ti2O in the samples, which are less than 0.5 and 2.5, respectively, are similar to subduction-related rocks (Defant et al., 1992). Based on the TAS diagram (Cox et al., 1979), SiO2 versus Na2O+K2O, all the studied samples except the BMG125 sample are placed in the gabbro range. These study rocks are often included in the group of intrusive rocks resulting from melts created in subduction zones. The study of rare elements shows that Behshahr's intrusive complex shows features close to volcanic arcs on the edge of the subduction zone and close to oceanic islands. In the analyzed samples, LREE elements are more enriched than HREE elements, and LILE elements are also more enriched than HFSE elements, which indicates magma contamination with the crust and partial melting.DiscussionAccording to the studies, all of the samples are in the calcalkaline range in the related diagrams. The calc-alkaline magmatic series is related to subduction setting. Therefore, it can be expected that the formation setting of the studied gabbroid mass is a subduction setting. Of course, due to the fact that some samples are in the intermediate range of calcalkaline and alkaline, the intraplate scenario is also possible to describe the tectonomagmatic setting of the region, and in this regard, we examine both subduction and intraplate theories. Considering that the age of the metamorphisms of Gorgan is related to the Paleozoic and Triassic periods (Zanchi et al., 2009), most probably the age of the studied intrusive masses and gabbros before the Paleozoic is related to the Cambrian. It is noteworthy that dating and providing a more accurate theory about the tectonomagmic setting of the studied area requires more sampling and geochronologic data. In this regard, it is suggested that in the continuation of this research, dating studies should be carried out to investigate and determine the tectonomagmic pattern and dating of intrusive masses in the Gorgan schists.
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