Adel Saki, Mirmohammad Miri, Roland Oberhänsli

• Precambrian meta-pelites of the Poshtuk area in northwest Iran contain the prograde mineral assemblage staurolite-garnet-chloritoid-muscovite-biotite that was replaced by the assemblage garnet-staurolite-chlorite-muscovite-biotite at peak metamorphic condition. Whole-rock compositions reveal that high Fe, Al and Mn contents of their protolith rendered them prone to form these assemblages. Pseudosections calculated in KFMASH, MnKFMASH, and MnNCKFMASHO systems were used to investigate the P-T evolution of the samples. They clearly show the significant effect of MnO on the stability of the chloritoid-bearing assemblages and the formation of garnet through consumption of chlorite and chloritoid. The pseudosection in a T- aH(2)O diagram shows that the studied assemblage could be stable only at a(H2O)>0.8. X-Mg isopleths for garnet and biotite point to peak P-T conditions of about 3.75 kbar and 575 degrees C. Chloritoid stability is overstepped with such conditions. This can be attributed to thermal perturbation due to plutonism. It isPrecambrian meta-pelites of the Poshtuk area in northwest Iran contain the prograde mineral assemblage staurolite-garnet-chloritoid-muscovite-biotite that was replaced by the assemblage garnet-staurolite-chlorite-muscovite-biotite at peak metamorphic condition. Whole-rock compositions reveal that high Fe, Al and Mn contents of their protolith rendered them prone to form these assemblages. Pseudosections calculated in KFMASH, MnKFMASH, and MnNCKFMASHO systems were used to investigate the P-T evolution of the samples. They clearly show the significant effect of MnO on the stability of the chloritoid-bearing assemblages and the formation of garnet through consumption of chlorite and chloritoid. The pseudosection in a T- aH(2)O diagram shows that the studied assemblage could be stable only at a(H2O)>0.8. X-Mg isopleths for garnet and biotite point to peak P-T conditions of about 3.75 kbar and 575 degrees C. Chloritoid stability is overstepped with such conditions. This can be attributed to thermal perturbation due to plutonism. It is concluded, metamorphism was primarily controlled by advective heat from magmatic intrusions in the Poshtuk area. The Precambrian basement complexes were extensively overprinted by the Pan-African Orogeny as well as younger magmatic and metamorphic activities associated to Alpine Orogeny during convergence of Arabian and Eurasian plate.…