Paragenesis of staurolite in pelitic schists of Kishangarh, District Ajmer, India

Summary The staurolite-bearing schists of the area were metamorphosed in the staurolite-kyanite zone of the amphibolite facies. The various assemblages of the pelitic schists are represented in the AKF and Thompson's AFM diagrams. Within the PT conditions of stability of staurolite, this mineral develops in pelitic schists that have a high Al2O3 (excess)/(K2O+FeO+MgO) ratio so that the plots of analyses lie above the muscovite-garnet join in the AKF diagram. The three-phase field of staurolite-kyanite-biotite in the AFM diagram migrates towards the lower FeO/MgO side with increasing oxidation ratio and thus staurolite-bearing rocks are not necessarily restricted to pelitic schists with high FeO/MgO ratio as is commonly believed. On the other hand, the three-phase field shifts towards the higher FeO/MgO side with increasing grade of metamorphism assuming oxidation ratio and pressure to be constant, thus reducing the composition field of staurolite schists in the AFM diagram.