Do the deformed alkaline rocks always serve as a marker of continental suture zone? A case study from parts of the Chotanagpur Granite Gneissic complex, India

In Santuri- Kankarkiari area of Purulia, West Bengal, India, a nepheline syenite body cuts across the regional Grenvillian (similar to 1000-950 Ma) fabric (S-2) of the host Chotanagpur Granite Gneissic Complex (CGGC). Subsequently, the nepheline syenite of the studied area and its basement are metamorphosed and develop a pervasive regional fabrics (S-3). Isoclinal folding of S-2 fabric of the host gneissic rocks resulted in axial planar S-3 fabric. Field relations, 'frozen-in' textural feature, petrology and geochemistry of the studied rock and the published geological and geochronological information from the adjoining areas are consistent with (a) derivation of its magmatic protolith from the then sub-continental lithospheric mantle and emplacement of the melt in a rift basin (b) the rifting of the Grenvillian basement seems to have occurred during (similar to 950-900 Ma) and (c) burial, deformation and metamorphism (peaked at 9-10 kbar, 690-770 degrees C) of the studied alkaline rocks and its Grenvillian basement seem to have occurred in a continent-continent collisional tectonic setting during similar to 900-780Ma. The commonly perceived notion that the deformed alkaline rocks (DARs) are invariably linked to continental suture along which a former ocean basin floor was consumed is not supported in this study. Integrating all the information, we propose that the nepheline syenite of the studied area and also from parts of the Granulite Terrane of South India represent metamorphosed and deformed aulacogen rock ensemble that should not be associated with opening and closure of an oceanic crust. Combining the information of this study and the published works, it is demonstrated that the deformed alkaline rocks (DARs) can also form in a coherent crustal block without any connection with a continental suture zone. A plausible tectonic model for the evolution of the CGGC has also been presented.