Study of protolith, thermobarometry and formation condition of garnet cordierite hornfelses from south of the Serkan area (SW of the Hamedan Province)

The Alvand batholith contact aureole hornfelses in south of the Serkan area (NW Hamedan province) contain metamorphic assemblage cordierite + garnet + biotite + plagioclase + K-feldspar. Whole rock compositions of the hornfelses show that they formed by metamorphism of a pelitic protolith (with some CaCO3 impurities) that originated from weathering of felsic to intermediate igneous rocks. Electron microprobe analyses indicate that the cordierites and biotites are of Fe-rich associations, the garnets are rich in almandine and spessartine and the plagioclases are oligoclase. Thermobarometry calculations using the in-equilibrium minerals obtained 550 to 620 oC T and >3 kbar P for these rocks. Also, the calculated pseudosections for average composition of these rocks demonstrate that the cordierite + garnet assemblages occurred due to high Mn and Ca contents of the garnets, high Mg contents of the cordierite and probably during uplifts during intrusion of the igneous bodies. Moreover, investigation of variations in fluids during metamorphism show that stability of the cordierite + garnet assemblages needed some CO2 contribution.