Bihar Mica Belt Plutons-an Example of Post-Orogenic Granite from Eastern Indian Shield

A group of small isolated domical to phacolithic granite plutons of 2 to 5 km in diameter have intruded the metasedimentary rocks of Bihar mica belt (BMB), representing north-eastern extremity of the Satpura orogenic belt of central and eastern India. The metasedimentary rocks of BMB rest on the Chotanagpur gneissic complex (CGC) with an erosional base and have recorded three phases of folding (F₁, F₂ and F₃), where the first two phases are the most dominant. A re-examination of structural, petrological and geochemical data, as well as study on new Rb-Sr whole rock isotopic ages of the BMB granite plutons, suggest that they are post-orogenic in nature in relation to the evolution of the basement (Chotanagpur gneissic complex). These plutons were emplaced between the second phase of compressive deformation (F₂,) of the Satpura orogeny, and before the establishment of post-compressional extension, when the regional stress pattern was likely to be transpressional or transextensional and/or mildly extensional type. The new Rb-Sr whole rock ages provided by Geological Survey of India suggest that the BMB plutons were formed between - 1.2 and 1.0 Ga. Though the parent magma of the plutons was earlier suggested to have been generated by partial melting of the tonalitcgranodiorite component of the Chotanagpur gneissic complex, a subordinate amount of BMB metasedimentary country rock is found to be involved in the genesis of the plutons. The pressure and temperature of emplacement as well as formation of a large number of BMB granite plutons is estimated around 5 kbar and slightly above 1000°C in a relatively anhydrous condition. The pegmatites intruding the BMB metasedimentary rocks are found to be emplaced either before F₁, folding or after F₂ folding of the host metasediments and are not strictly time correlative with the emplacement of the BMB granite plutons. It appears that the necessary fluids for the formation of pegmatites were generated by muscovite dehydration reaction(s) of the metasedimentary country rocks during their metamorphism.