Proterozoic Granitoids of Rajasthan

Two granitoid bodies each from Palaeo-, Meso- and Neo-Proterozoic periods have been studied for their chemical, petrological and source characteristics. The Proterozoic granitoids are megacrystic as well non-Megacrystic with variable effect by tectonometamorphic events. The Palaeo-Proterozoic granitoids are peraluminous, corundum normative with high proportion of biotite in their modal composition. They contain either orthoclase/Sanidine or andesine as feldspars and iron rich biotite. The Meso-Proterozoic granitoids have high silica with meta-to peraluminous composition. The feldspar chemistry indicates albite/Oligoclase composition whereas they exhibit high iron rich biotite and some being lepidomelane in composition. The Neo-Proterozoic granitoids are invariably porphyntic having higher silica content (>72 wt%), as compared to other granitoids and exhibit strong peraluminous affinity. The feldspar geothermometry indicate 1050°C for Palaeo-, 900-950°C for Meso- and 750°C for Neo-Proterozoic granitoids. The Proterozoic granitoids are orogenic, the degree of deformation varies from pristine, non-Foliated to gneissic and augen gneissic, mostly 'S' type, indicating a sedimentary source material for their generation. The biotite discriminant diagrams also support 'S' type collisional tectontic setting. The garnet-Biotite geothermometery for Palaeo-Proterozoic granitoids where these minerals co-Exist suggest equilibrium temperature of -540°C at 1 or 2 kb pressures. There is appreciable replacement in magnetites of Palaeo-Proterozoic granitoids by K₂O, SiO₂, Al₂O₃ and negligible in Meso- and Neo-Proterozoic rocks. There is gradual increase in SiO₂ and DI from Paleo- to Neo-Proterozoic granitoids. The basement complex, after attaining rigidity and cratonisation by the end Archaean igneous activity was rifted periodically during the Proterozoic times. Extensional events led to the formation of mafic dyke swarms, plutonic and volcanic activity and sedimentary platform covers. However, the aborted rifts were further effected by compressive tectonics giving rise to syn and late tectonic deformational signatures to the intrusive phases and their host rocks.