Introduction
Shillong plateau in northeast India is a small but conspicuous gneissic complex. The Plateau has been described as a detached part of the Indian shield (Evans, 1964) or an extension of the Chtanagpur Gneissic Complex (CGC) (Desikachar, 1974) or the Eastern Ghat Granulite Belt (EGB) (Crawford, 1974). The available chronological data from the high grade rocks of CGC (Ray Barman et al., 1994; Harris, 1993) and the granulites of the EGB (Shaw et al., 1997; Dobmeier and Raith, 2003) suggest that the domains are largely Proterozoic in age with relict or reworked Archean components (Rickers et al., 2001; Ramachandra and Roy, 2001) and interspersed Grenvillian domains (Chattterjee et al., 2007).
Neoproterozoic felsic magmatism in Shillong plateau is represented by voluminous granitoid plutons around the plateau. Rb-Sr whole-rock isochron ages of the granite plutons range from 881 to 479 Ma (Ghosh et al., 1994) and contrast with the 1714-1150 Ma ages of the basement gneisses (Ghosh et al., 1994; Selvan et al., 1995, Chatterjee et al., 2007). Smilar granite plutons and felsic volcanic rocks of Neoproterozoic ages are widely recorded in other parts of India e.g. the granite plutons of Kerala (740-550 Ma, Santosh and Drury, 1988); Tamil Nadu (637 to 395 Ma, Santosh et al., 2005) and acid volcanic activity in Rajasthan (780 to 680 Ma, Rathore et al., 1999) and these are believed to be related to Pan-African Orogeny. Ghosh et al., (2005) have suggested that the granitoids belonging to Shillong plateau were formed due to tensional stress and thermal upwell related to the collisional and extensional episodes of the Gondwana supercontinent. Others working on the geochronology of the granitoids and the position of Shillong plateau in a reconstructed assembly of landmasses during Neoproterozoic period (see e.g. Rogers and Santosh, 2002; Collins, 2003; Chatterjee et al., 2007) suggested that the felsic magmatism within Shillong plateau was associated with the disintegration and dispersal of the supercontinent Rodinia that might have begun with the rising of a mantle plume.
Formation of the Rodinia supercontinent during the 1300-1000 Ma Grenvillian event is believed to have been followed by disintegration and dispersal of the crustal fragments around 750 Ma ago (Kröner and Cordani, 2003). Most of these dispersed terranes reassembled around 550 Ma ago in the Gondwana supercontinent (Meert and Van der Voo, 1997). These events left signatures in the rocks of the dispersed continental blocks (Windley, 1995). In this context the isotopic ages of basement gneisses and high-grade metamorphic rocks of Meghalaya suggest involvement of the Meghalaya region in an accretion event. Various attempts to reconstruct the Precambrian crustal blocks in the Neoproterozoic period place southwestern Australia with northeast India in the eastern Gondwana assembly (Fitzsimons, 2000; Rogers and Santosh, 2002; Ghosh et al., 2005 and references therein). Chattterjee et al., (2007) propose that the Pan-African suture passing through Prydz Bay in Antarctica possibly continued northward into India through the Shillong plateau.
The granitoids dealt in this study belong to the Mikir Hill Massif which is the eastern extension of the Shillong plateau. Petrological and geochemical characteristics of these granitoids are used here to constrain the crustal evolution of Shillong plateau during Neoproterozoic and their significance for dispersal of Rodinia and subsequent amalgamation of India in the supercontinent Gondwana.