TECTONIC EVOLUTION OF THE ARAVALLI OROGEN (NW INDIA) - AN INVERTED PROTEROZOIC RIFT BASIN

The Aravalli mountain range (AMR) in the northwestern part of the Indi an Peninsula consists of two main Proterozoic metasedimentary and meta igneous sequences, the Aravalli and Delhi Supergroups, respectively, w hich rest over the Archaean gneissic basement. A synthesis and reinter pretation of the available geological, geochronological and geophysica l data, including results of own field work and geophysical interpreta tions pertaining to the AMR, indicate its origin as an inverted basin: rifting into granitoid basement began ca. 2.5 Ga ago with Aravalli pa ssive rifting (ca. 2.5-2.0 Ga) and Delhi active rifting (ca. 1.9-1.6 G a). Associated mafic igneous rocks show both continental and oceanic t holeiitic geochemistry and are comparable with Phanerozoic, rift-relat ed magmatic products. Available data showed no 'conclusive evidence fo r oceanic lithoshere and island-arc/active margin magmatic activity in the AMR. Subsequent inversion and orogeny (Delhi orogeny, ca. 1.5-1.4 Ga) lead to complex deformation and metamorphism. Only in the western and central zones has the basement been involved in this mid-Proteroz oic (Delhi) deformation, whereas it is unaffected in the eastern part, except for local shear zones mainly along the basement/cover interfac e. The grade of metamorphism increases from the greenschist facies in the east to the amphibolite facies in the west with local HP assemblag es. These latter are explained by rapid burial and exhumation of thin and cool continental lithosphere. Subsequently, during a final, mild p hase of inversion, the Vindhyan basins consisting mainly of sandstones , limestones and shales, flanking the AMR formed which are comparable to foreland basins. The tectonic evolution of the AMR is therefore int erpreted as an example of a major inverted continental rift and of a P roterozoic intra-continental orogen.