Prof. M. K. Pandit Department of Geology, University of Rajasthan
The NW Indian block comprises of an eastern, Aravalli – Banded Gneiss Complex (Arcahen – Proterozoic) craton in the east and a younger Neoproterozoic – Phanerozoic (Marwar) terrane in the west, both sutured along the western flank of NE – SW trending Aravalli – Delhi Fold Belt. The region documents tectono-magmatic events between 1 and 0.75 Ga that define the time bracket between amalgamation and break-up of the Neoproterozoic supercontinent Rodinia. The 967 Ma, calc-alkaline, Sendra – Ambaj granites represent the early Neoproterozoic magmatic activity and occur as a series of linear exposures. The next episode is emplacement of typically S-type granites (870 – 820 Ma) related to the crustal convergence during Delhi Orogeny. The terminal magmatism is marked by initial outpouring of bimodal felsic (mainly) and mafic lava flows, followed by granite and felsic and mafic dyke emplacement, covering an area of more than 50,000 km2 . Known as the Malani Igneous Suite, it is regarded as the third largest felsic igneous province in the world. The rocks can be chemically subdivided into peraluminous (volumetrically abundant) and peralkaline types that also define temporal preferences. The magmatic event has been dated between 770 and 750 Ma and the rocks have also provided robust Neoproterozoic paleomagnetic poles that allow a much northerly paleoposition for the NW Indian block then envisaged in conventional paleogeographic reconstructions.