Proterozoic polyphase metamorphism in the Chhotanagpur Gneissic Complex (India), and implication for trans-continental Gondwanaland correlation

Abstract

In recent attempts to reconstruct the Early Cambrian architecture of Gondwanaland, the Prydz Bay Pan African suture separating Archean and Proterozoic terrains in Antarctica is suspected to extend inwards into Eastern India, presumably through the Chhotanagpur Gneissic Complex (CGC). However, the lack of thermo-chronologic information in the CGC impedes the proposed Gondwanaland reconstruction. In the Bero-Saltora area, located in the eastern fringe of the CGC, three deformation events were identified (D 1, D 2 and D 3) in granulite facies gneisses that were intruded by anorthosites (post-D 1, pre-D 2) and expansive granites (post-D 2, pre-D 3). The prograde pre/syn-D 1 granulite facies metamorphism in the enclave suite (750–850 °C and 4–6 kbar) is suggested to be >1.4 Ga (EPMA monazite age) or even older. The D 2 and D 3 deformation events (1.1–1.4 Ga EPMA monazite age) occurred at amphibolite facies conditions, manifested by the stabilization of hornblende and/or biotite in almost all lithologies. The post-D 3 stage was marked by the stabilization of prograde garnet + quartz at upper amphibolite facies (650 ± 50 °C and 4.5 ± 0.5 kbar) conditions replacing syn-D 2–3 hornblende/biotite + plagioclase assemblages in anorthosites, foliated granites and granulite enclaves, and the re-stabilization of a two-pyroxene + plagioclase assemblage at the expense of hornblende in mafic granulites. EPMA dating of monazites in post-D 3 garnets in sheared granites yielded ∼0.9–1.0 Ga. The reconstructed thermo-chronology of events is the first record of Meso- and Neoproterozoic (Grenvillian) polyphase prograde metamorphism in the CGC. The lack of Early Paleozoic ages in the gneissic complex rules out the suspected continuation of the Early Cambrian Prydz Bay suture within the CGC. The suture arguably should lie to the east of the CGC.