Alkaline magmatism along the Southeastern margin of the Indian shield: Implications for regional geodynamics and constraints on craton-Eastern Ghats Belt suturing

摘要

Deformed alkaline complexes at the contact between the Eastern Ghats Belt (EGB) and the Archaean cratons in southeastern India mark the location of a collisional suture superimposing an earlier Mesoproterozoic (similar to 1.5-1.3 Ga) rift. This article offers a plausible geodynamic model explaining the alkaline magmatism and its relationship to regional crustal evolution. The Mesoproterozoic rift can be correlated to the breakup of the supercontinent Columbia and may have opened an ocean between eastern India and east Antarctica where the sedimentary sequences of the Eastern Ghats Province (EGP) were deposited between 1.4 and 1.2 Ga. Inversion of the rift basin during the late Mesoproterozoic (similar to 1.0 Ga) led to the collision of eastern India with east Antarctica during Rodinia assembly. The collision formed the Grenvillian EGP-Rayner complex orogen where the EGP sediments and the rocks of the Rayner complex were deformed, metamorphosed and migmatized at granulite-facies condition. The Mesoproterozoic rifting and Grenvillian basin closure may thus represent a Wilson cycle related to the breakup of Columbia and the assembly of Rodinia. The craton-EGB suture was subsequently modified significantly during Pan-African tectonism (0.5-0.6 Ga) when the EGB granulites were thrust westward over the cratonic foreland along a number of amphibolite-facies shear zones at the contact. The shearing deformed and metamorphosed the cratonic margin and the alkaline complexes it hosts. The thrust stacking in many places has the craton boundary and the alkaline rocks in the footwall and the EGB granulites in the hanging wall. The continuity of the Pan-African shear fabric across the craton-granulite contact and the lack of Grenvillian ages from the alkaline and other craton margin rocks in such regions give the apparent impression that the two crustal segments amalgamated during Pan-African tectonism and not during the earlier Grenvillian collision. Geodynamic constraints however indicate that this was unlikely and the inconsistency arises because the present crustal geometry exposes only the eroded Pan-African thrust contact between the craton and the EGB and not the original Grenvillian suture, which may be lying unexposed under the granulite thrust sheets.