Journal of Economic Geology (May 2020)
Study of Migmatization and Leucocrate granite formation processes in the Tuyserkan area, Hamedan
Abstract
Introduction Partial melting is an appropriate correlation process between metamorphism and magmatism which plays a key role in the development of migmatites, granulites and S-type granites during crust evolution (Kriegsman, 2001; Alvarez-Valero and Kriegsman, 2008; Sawyer, 2010). In this study, we tried to address the correlation between partial melting process and metapelites migmatization and the formation of adjacent granites through microscopic and field evidence and geochemical data. Materials and methods Petrography and field studies were carried out and in order to identify minerals’ composition and determine temperature and pressure. A few spots of different minerals were analyzed by microprobe electron method with CAMECA device model SX100 at the Geosciences Research Institute of China University. Also, in order to evaluate the geochemical and the correlation between migmatites, leucocratic granite and metapelites, several samples of the mentioned rocks were selected. Their major and minor elements were respectively analyzed by the XRF and ICP-MS methods at Beijing University of China. Results While the pattern of rare earth elements (REE) in migmatite leucosome and adjacent granites shows that leucosome and leucocratic granite do not have the same origin, the leucocratic granite influence has occurred after the migmatization event, geothermobarometric calculations of migmatites and intrusive bodies as well as age measurement of Alvand Plutonic mass and migmatite rocks confirm that anatexis and partial melting do not come from granitic body heat but also heat of older mafic bodies is the cause of partial melting and migmatization in the region. Therefore, migmatites have emerged because of contact metamorphism which itself is the result of injection of the same age mafic bodies with migmatites. Discussion Migmatites of the study area are composed of quartz, plagioclase, potassium feldspar, biotite, andalusite, cordierite, spinel, and sillimanite minerals. Temperature and pressure for metamorphism peak are approximately 700 ° C and 4 kbar, respectively. Based on these data, the formation depth of these rocks is about 11 km. Therefore, their geothermal gradient is 54 °C/km which is located in the contact metamorphism zone and the Buchan type metamorphism series and it is in accordance with high temperature-low pressure metamorphisms. Migmatites are located near the leucocratic granite in some parts of Tuyserkan. However, they do not have any contact with granites in other parts but they have outcrops with hornfels rocks instead. The pattern of rare earth elements (REE) has been used to find out the migmatites protolith in the Hamadan area. Since, the pattern of rare earth elements (REE) of migmatites and metapelites has a similar process, this lithology has been used as a probable protolith. In order to identify the distributed elements inside the molten or in the residual (restite), the average chemical composition of probable protolith (cordierite hornfels) was used as a normalization standard for restite geochemistry in multi-element diagrams. According to spider diagrams pattern (mesosome, leucosome) normalized to the average metapelites based on mass balance, it can be concluded that migmatites have been formed by evolution of cordierite hornfels. In order to investigate the origin and possible relations between leucosome and adjacent granites (leucocratic granite), the chemical composition of these rocks was compared. Leucocratic granite located in the migmatites immediate contact and leucosome which is a few centimeters thick are considered in this comparison. The pattern of rare earth elements (REE) shows a significant difference in the migmatite leucosome and adjacent granites. The most important results of REE patterns is the difference in HFSE value in granites and leucosome. Thermometry has been conducted on intrusive masses (gabbro) through various methods and by Sepahi et al. (2012). The approximate temperatures of 950 ° C for gabbro and 1300 ° C for olivine gabbro are estimated. Also, due to contact metamorphism reactions, the maximum contact temperature of porphyry granites (Alvand intrusive mass) is estimated to be about 530 to 550 ° C (Sepahi and Moein Vaziri, 2001). Such a temperature is not sufficient for migmatization in the region. Shahbazi et al. (2010) have acquired the age of Alvand plutonic rocks to be 166.5 ± 1.8 Ma for gabbro, 163.0 ± 9.9 and 161.7 ± 0.6 Ma for granites and 154.4 ± 1.3 and 153.3 ± 2.7 Ma for leucocratic granite. Jafari (2018) has acquired the age of Hamadan's Migmatites to be about 160 to 180 Ma and an average of 170 million years which is almost equal to the age of Alvand Plutonic body.
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