Petrology, geothermobarometry, and P-T path of spinel-bearing symplectite migmatites from the Simin area, Hamedan, Sanandaj-Sirjan Zone, Iran

Authors : Ali Akbar BAHARIFAR, Donna L WHITNEY, Kwannang PANG, Sunlin CHUNG, Yoshiyuki IIZUKA

Pages : 275-298

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Publication Date : 0000-00-00

Article Type : Research Paper

Abstract :Migmatitic rocks in the Hamedan area, Sanandaj-Sirjan Zone, Iran, contain symplectite that has partially replaced andalusite. In metatexite, symplectite typically consists of spinel + cordierite + Ca-rich plagioclase ± orthoamphibole, but in diatexite, symplectite consists of spinel + Ca-rich plagioclase; cordierite is typically absent from symplectite although it is present in the matrix of the rock. A calculated P-T pseudosection for an average, nonmigmatitic bulk-rock composition indicates that the whole-rock composition was not the effective bulk composition during the formation of symplectite. Therefore, effective bulk compositions insert ignore into journalissuearticles values(EBCs); were calculated using modal abundance and the average composition of minerals in contact with Al2SiO5 in metatexite and diatexite. The EBC pseudosections predict a stability field for spinel + plagioclase ± cordierite in both types of migmatite and estimate peak conditions at ~740 °C, 4.3 kbar, the same results for peak P-T conditions obtained by the average, nonmigmatitic rock composition. P-XH2O diagrams calculated assuming different H2O contents indicate that the effect of additional H2O on reaction history could resemble the effect of decompression. The Simin spinel-bearing symplectite associated with andalusite is similar to other examples reported in the literature in that it contains Ca-rich plagioclase, although the Simin rocks lack other Ca-rich phases such as garnet. The role of migmatization, including the introduction of external melt, is therefore considered as a possible agent in the generation of Ca-rich plagioclase. Based on calculated pseudosections, migmatite of the Simin area of the Hamedan metamorphic sequence exhibits an anticlockwise P-T path, and its formation could be linked to that of the granitoid intrusion during the main plutonic phases in the middle Jurassic under hightemperature - low-pressure metamorphic conditions.
Keywords : Andalusite, spinel, symplectite, Sanandaj Sirjan, migmatite, Iran