Low Pressure Metamorphism, Deformation and Syntectonic Granite Emplacement in the Palaeoproterozoic Mahakoshal Supracrustal Belt, Central India

Porphyroblast assemblage of andalusite, garnet, staurolite and cordierite in the metasedimentary rocks of the Palaeoproterozoic Mahakoshal supracrustal belt suggests low pressure/medium temperature metamorphism. These porphyroblasts exhibit both dynamic (syn-kinematic) and static (post-kinematic) growth. Field and petrographic studies indicate that the growth of andalusite and garnet porphyroblasts initiated during the early stages of D2 (S2 cleavage) deformation and outlasted it. The staurolite porphyroblasts appeared during the late stages of D2 deformation and continued after it. Restricted occurrence of these syn-kinematic porphyroblasts to the vicinity of the shear zone suggest that intense deformation and repetitive cleavage development, apart from the elevated temperature conditions, enhanced the growth of the porphyroblasts. The cordierite porphyroblasts, whose growth is controlled essentially by the lithological composition, exhibit static growth, post-dating D1 (S1 cleavage) deformation. The mineralogical assemblages in the metasedimentary rocks suggest a peak P and T around 3.5 kb and 550–600°C respectively, which was attained during the D2 deformation. The D2 deformation was coeval with the development of reverse-oblique slip ductile shear zone and emplacement of linear, syn-kinematic granites along the southern margin of the belt. This indicates a contractional tectonic regime, wherein rising granitic plutons caused advective heat transfer to the middle and upper crust and created relatively higher temperature conditions at lower pressure.