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 (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.
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