Petrology of Spinel-Gedrite-Cordierite Symplectites Replacing Andalusite in Migmatites from the Sarabi Area, Hamedan, Sanandaj–Sirjan Zone, Iran

Abstract

Metapelitic migmatites in the Hamedan area of Sanandaj–Sirjan Zone, Iran, contain spinel + gedrite + cordierite (± biotite, ± staurolite, ± ilmenite) symplectite, replacing andalusite in metatexite and diatexite. P-T conditions of metamorphism for cordierite-hornfels, andalusite-cordierite-hornfels and migmatites were estimated as 600°C, 2.2 kbar; 645°C, 2.9 kbar and 760°C, 3.7 kbar respectively, using calculated pseudosections for an average composition in the NCKFMASH system. Calculated pseudosection is in accordance with field and petrographic observation, except that symplectite minerals does not appear in the model, suggesting that formation of symplectite was affected by effective bulk compositions (EBC), rather than whole-rock composition. So EBC model was calculated using modal abundance and average composition of symplectite minerals. In pseudosection calculated for EBC, spinel + gedrite + cordierite is stable over wide range of P and T. Peak metamorphic condition for symplectites are estimated as 750°C at 4.1 kbar. Staurolite-bearing symplectites appeared during retrograde isobaric path at 600°C and 4.1 kbar. Based on calculated pseudosections and estimated P and T, an isobaric retrograde anticlockwise P-T path is proposed for the Sarabi migmatites. Metamorphism and migmatization in the area is linked to the major plutonism phases of the Sanandaj–Sirjan Zone in Middle-Jurassic under high T-low P conditions, during the evolution of Neo-Tethys.