Evolutionary Model
The geochemical data-derived interpretations in combination with the overall lithological make-up of the Sonakhan granite-greenstone belt can be used to suggest a model for the tectonomagmatic evolution of the Bastar craton of the Indian shield. It is proposed that during late Archaean, a rising mantle plume generated a thickened, hot, buoyant and so unsubductable oceanic plateau, that is now represented by the lower unit basalts of the Baghmara Formation. The accretion of this plateau against an old continent margin could have triggered subduction. The thick oceanic plateau served as a base for further subduction of the oceanic crust producing subduction-related basaltic and felsic volcanic rocks, consisting of upper unit basalts, in an island arc setting (Fig. 11). The magmas might have been derived from the fluid-metasomatized mantle wedge producing the upper unit basalts of the island arc affinity. A similar model has been proposed for the evolution of the Western Dharwar Craton by Naqvi et al. (2006); Manikyamba and Khanna (2007) and Jayananda et al. (2008). In response to subduction slab roll-back, lithospheric extension in the over-riding plate may have occurred causing rifting of the arc. This extension may have eventually led to the formation of an intra-arc basin in which clastics of the Arjuni Formation were deposited. Petrographical and geochemical characteristics of these clastic rocks also corroborate this contention (our preliminary unpublished data). The plume-arc interaction model proposed here for the Sonakhan greenstone belt may be applicable for the Bastar craton as a whole during the Neoarchaean to Paleoproterozoic. Since such model has also been proposed for the Dharwar craton, it may further point to the dominance of such process for the evolution of the Indian shield during the Neoarchaean to Paleoproterozoic.