| چکیده انگلیسی مقاله |
Introduction Granitoids represent the primary components of orogenic belts with a wide range of compositional variations (Kaygusuz et al., 2008). Therefore, analyzing the composition of granitoids contributes significantly to the understanding of continental crust formation and evolution (Barbarin, 1999). Mineral compositions provide insights into the pressure-temperature conditions and the nature of the magma during granite emplacement, and estimating these parameters for a magmatic body is crucial for interpreting petrogenesis and regional tectonic settings (Abdel-Rahman, 1994; Moazzen and Droop, 2005; Gomes and Neiva, 2005; Zhang et al., 2006; Mazhari et al., 2008; Shabani et al., 2010; Sahin et al., 2010). Mineral composition reflects the pressure, temperature, and magmatic characteristics during granite emplacement, and determining these physical and chemical conditions is essential for accurately interpreting petrogenetic processes and understanding the broader tectonic framework of a region (Mazhari et al., 2008; Shabani et al., 2010; Sahin et al., 2010). The mineral assemblage and its chemical composition within igneous rocks are significantly influenced by the original magma composition and the prevailing physicochemical conditions during the crystallization process. The minerals analyzed in this study—namely plagioclase, biotite, and amphibole—exhibit geochemical signatures that offer valuable insights into magmatic crystallization dynamics and melt evolution. The purpose of this study is to investigate the chemistry of plagioclase, biotite, and amphibole minerals and to determine the magmatic origin on the base of chemical composition of aforementioned minerals. Geological Setting The Bagh pain granitoids are located between the geographical longitudes of 46'35°6'E and 46'33°46'E and the geographical latitudes of 36'25°36'N and 36'26°36'N, 50 km south of Shahin dej County and 50 km north of Takab City in the Sanandaj-Sirjan Zone. Based on geological maps and uranium-lead dating studies, the granitoid rocks of the region belonging to Late Cretaceous, were probably generated under the influence of the Late Laramide and Cimmerian orogenic phases. The rocks under study are classified as volcanic arc type and I-type granite. Materials and Methods In this study, a total of 120 samples were collected from all intrusive units during a field visit to the area, and the most intact specimens were selected based on minimal weathering. Subsequently, 40 thin sections were prepared at Bu-Ali Sina University in Hamedan, where petrological investigations were carried out using a polarizing microscope. In addition, 9 representative samples, carefully selected from over one hundred analyzed points, were sent to the University of Vienna, Austria, for detailed microprobe analysis. These samples were subsequently examined using high-resolution electron microscopy and backscattered electron (BSE) imaging techniques. After carbon coating, they underwent rapid semi-quantitative elemental analysis using a CAMECA SX Five Electron Microprobe equipped with a field emission cathode and an energy-dispersive X-ray (EDX) system, operating at an accelerating voltage of 20 keV, a probe current of 25 nanoamperes, and a beam diameter of 60 μm, in the Lithosphere Research Group laboratory at the University of Vienna. Discussion Abdel-Rahman (1994) through the analysis of MgO, Al₂O₃, and FeO oxides in biotite minerals, proposed several geochemical classification diagrams to categorize granitoid rocks into three distinct magma series, which correspond to three tectonic zones. These classifications are derived from the type and relative concentrations of iron, magnesium, and aluminum present within the mineral structure. Zone A corresponds to alkaline, non-orogenic igneous rocks. The C range encompasses calc-alkaline magmas typically generated in subduction-related orogenic settings and is representative of I-type granites, whereas the P range includes peraluminous magmas formed in collisional orogenic environments, which are characteristic of S-type granites. Based on the geochemical discrimination diagrams utilized in this study, all analyzed samples plot within the C range, thereby indicating that the biotites in the investigated area are genetically linked to subduction-related calc-alkaline magmatic systems. Considering the relative concentrations of Na₂O and Al₂O₃ compared to TiO₂, it is inferred that the amphiboles exhibit alkaline and pseudo-alkaline characteristics. Pseudo-alkaline amphiboles typically contain lower levels of Ti, Na, and Al than alkaline types. Accordingly, the amphiboles present in the regional granitoid rocks exhibit a pseudo-alkaline character, which is consistent with the whole-rock geochemical data. Moreover, the TiO₂ versus Al₂O₃ discrimination diagram clearly demonstrates the involvement of both mantle-derived magmatic input and crustal contributions in the genesis and subsequent evolution of these granitoid formations. Conclusion The Bagh-Pain granitoid body comprises granite, granodiorite, diorite, and aplite units, containing quartz, plagioclase, alkali feldspar, amphibole, and biotite as major minerals, along with zircon and apatite as accessory mineral phases. Detailed chemical analyses of biotite and plagioclase indicate that the plagioclases are oligoclase and andesine, while the biotites are magnesium-rich and re-equilibrated. This mineralogical composition corresponds with the green coloration of biotites in the region, their weak pleochroism, and the association of their source rocks with subduction-related tectonic settings. Additionally, the amphiboles are identified as calcium-rich edenite and pargasite types. Based on mineral chemistry data, these rocks are closely linked to calc-alkaline magmatism and are interpreted to have originated from mantle-derived magma that was subsequently modified by crustal contamination. |