Evolution of the Continental Crust in the Proterozoic Eastern Ghats Belt, India and new constraints for Rodinia reconstruction: implications from Sm-Nd, Rb-Sr and Pb-Pb isotopes

For this study Nd, Sr and Pb isotope compositions were analyzed for ortho- and paragneisses from the Eastern Ghats Belt of India in order to determine its crust formation and crustal evolution. This belt represents a Proteroz oic orogen that extends along the east coast of Peninsular India and forms part of the mobile belts in East Gondwana and Rodinia. The Eastern Ghats Be lt was affected by Mesoproterozoic granulite facies metamorphism in the wes tern segment (Western Charnockite Zone) and a Grenvillian regional-scale hi gh-grade event in the central and eastern segments (Western Khondalite Zone , Charnockite Migmatite Zone and Eastern Khondalite Zone) as well as a loca l Pan-African overprint. The results of the isotope studies are used for th e large-scale reconstruction of the indo-antarctic part of the Rodinia supe rcontinent. Based on Nd model ages and Pb isotope ratios from leached felds pars four crustal domains can be distinguished in the Eastern Ghats Belt. T hese domains can in part be correlated with the lithological division of th e belt: (1) The Western Charnockite Zone south of the Godavari Graben is ch aracterized by Nd model ages between 2.3 and 2.5 Ga for orthogneisses and 2 .6 and 2.8 Ga for metasediments (Domain 1). The Pb isotopes are primitive i ndicating reworking of dominantly Archean and mixing with minor Proterozoic material; (2) North of Godavari Graben Nd model ages for orthogneisses are significantly higher with values ranging from 3.2 to 3.9 Ga. The Pb isotop es are strongly retarded; (3) The north-eastern parts of the Charnockite Mi gmatite Zone and Western Khondalite Zone form a distinct and almost homogen eous crustal domain (Domain 3) with Nd model ages between 1.8 and 2.2 Ga; ( 4) Between the isotopically homogeneous terranes stretches a broad transiti on zone (Domain 2) enclosing parts of the Western Khondalite Zone, Charnock ite Migmatite Zone and Eastern Khondalite Zone. The Nd model ages for metas ediments (2.1-2.5 Ga) are younger than paragneiss ages of the adjoining Wes tern Charnockite Zone. The Nd model ages for orthogneisses (1.8-3.2 Ga) dis play a large spread, which is consistent with the Pb isotope signatures tha t indicate mixing of Archean with Proterozoic material. The border zone bet ween Domain 3 and the Archean Eastern Indian Craton forms a second transiti on zone (Domain 4) characterized by metasediments with Nd model ages betwee n 2.2 and 2.8 Ga and orthogneisses with model ages around 3.2 Ga. Reworking of Archean crustal material is most intense along the border zones of the belt and 'juvenile' material is more dominant away from the orogenic front. This scenario is indicative of an active continental margin setting for th e two Proterozoic episodes of orogenesis in the Eastern Ghats Belt. A corre lation of Domain 3 with the Rayner Complex and the Prydz Bay region, Antarc tica, for the early crustal evolution is supported by the similarity of the isotope signatures. The Napier Complex is very different to the Eastern Gh ats Belt and an early joint evolution of these terranes is ruled out on the basis of the Pb-Pb and Sm-Nd systematics. (C) 2001 Elsevier Science B.V. A ll rights reserved.