Implications for ancient tectonics in the Himalaya

The Paleoproterozoic (1900 ±200 Ma) granitic magmatism has been reported from all along the 2000km Lesser Himalayan belt staring from the Besham gneiss in the Lower Swat Valley, NW Himalaya to the Bomdila gneiss in the NE Himalaya (Sharma, 1998). The extensive studies of these granites (Sharma, 1983; Gupta et al., 1994; Prabha and Rawat, 1999; Sharma and Rashid, 2001) indicate that they have unequivocal similarities in terms of their geological setting, tectonic setting and geochemistry. All of these studies show that the granites are formed during syn-collisional tectonic environment, suggesting that collision of two continental blocks might have occurred during Proterozoic period along this linear belt, similar to the already established Paleozoic Lesser Himalayan granitoid belt (Le Fort et al., 1986) related to Pan-African orogeny. This further implies an episode of significant Proterozoic orogenic events in the Lesser Himalaya: and, possibly the later tectonics exposed these granites to weathering and provided detritus to the then formed large Proterozoic sedimentary basins in the Lesser Himalaya. This deduction is in strong agreement with the inference drawn by many researchers who, on the basis of textural and geochemical investigations of the Proterozoic sediments from the Lesser Himalaya (Rashid, 2002, 2005), suggested that the sediments were derived exclusively from felsic (leuco-, S-type-granitic) sources occurring in close proximity or adjacent to the basins. This argument rules out the hypothesis that the relatively mafic, I-type-granitic plutons from the Peninsular India, such as Bundelkhand granite and granites from Aravalli region, may have contributed sediments to the Lesser Himalayan sedimentary basins.