The Aravalli craton is located in the northwestern part of peninsular India, bounded on the north by the Himalaya Mountain chain, the Cambay graben in the southwest, and the Narmada-Son lineament on the south and southeast. Young sediments cover the western boundary and may extend farther into Pakistan.
Rocks of the Aravalli craton are quite different from the Dharwar and other cratons of the Indian shield. They consist mostly of Proterozoic phyl-lites, graywackes, quartzites, and carbonates, with minor mafic and ultramafic schists. Stromatolites are common in the carbonates and are associated with phosphorite deposits. Banded-iron formations are almost totally absent from the Aravalli craton but common in other cratonic blocks of the Indian shield.
Major structures of the Aravalli craton include the Great Boundary fault on the eastern edge of the Aravalli-Delhi belt, the Delhi-Haridwar ridge, and the Faizabad ridge, which is an extension of the Bun-delkhand massif under the Indo-Gangetic Plain. The Aravalli-Delhi belt contains a large number of gra nitic rocks emplaced over a wide range of time from
3.5 billion to 750 million years ago.
The oldest rocks of the Aravalli craton are found as metasedimentary/metavolcanic inclusions, named the Bhilwara suite, in the Banded Gneiss complex. The gneisses include metasedimentary units, migma-tites, granitic gneisses and pegmatites, and metabasic layers. The Bhilwara suite is preserved in several elongate belts within the Banded Gneiss complex, especially between Karera and the Great Boundary fault. Rock types include shales, slates, quartzites, dolostones, marbles, cherts, graywackes, and hornblende and mica-schists. Sedimentary structures in these rocks suggest that they were deposited in a shallow shelf platformal environment.
The Aravalli supergroup unconformably overlies this Banded Gneiss complex in some places but is intruded by the gneissic rocks in others. These conflicting relationships show that the Banded Gneiss complex consists of several different units that still need additional work to separate them from one another, as confirmed by a range of isotopic ages on the gneisses that extends from 3.5 to 2.0 billion years. Rock types in the Aravalli Supergroup, including quartzites, greywacke, and carbonates, have experienced greenschist facies metamorphism and are intruded by pre-, syn-, and posttectonic granitoids and mafic-ultramafic suites. Ages of the Aravalli Supergroup range between 2.5 and 2.0 billion years. The Aravalli Supergroup is complexly folded by as many as four generations, including early reclined, typically rootless folds, followed by later generations of folds distorting the outcrop belts into hook-shaped synclines, anticlines, and more complex shapes.
The Delhi Supergroup apparently overlies the Aravalli Supergroup, and parts of it are at least
1.6 billion years old, though other sections may be significantly younger. The Delhi Supergroup is the main rock suite outcropping in the Aravalli Mountains, over a distance of more than 350 miles (700 km) from Gujarat to Delhi. The Delhi Supergroup includes quartzites, conglomerates, arkoses, phyllites, slates, limestones, marbles, mafic volcanic rocks, and amphibolites. The rocks are complexly deformed and metamorphosed between greenschist and granulite grades. Two main deformation events are recognized from structures in the Delhi Supergroup. The first deformation event produced isoclinal folds with axial planar foliation parallel to bedding surfaces, whereas the second produced open to tight asymmetrical folds that plunge north to north-northwest.
Exposures of rock are few and far between in the Bundelkhand area, but the rocks there include granites and gneisses deposited over a considerable time spanning much of the Precambrian. The northeastern part of the craton is overlain by generally flat-lying rocks of the Late Proterozoic Vindhyan Supergroup, deformed only near the Great Boundary fault and in the south near the Satpura orogen. Vind-hyan sedimentary rocks include basal conglomerates and quartzite, and grades up into quartz arenite, and higher in the section into limestones, shales, and other shallow-water sedimentary deposits. The upper part of the Vindhyan Supergroup includes sandstones and conglomerates, and some have diamonds probably eroded from nearby kimberlite pipes.
The Arvalli craton hosts a few alkaline igneous intrusions and kimberlite pipes. The alkaline rocks were emplaced around 1.5 billion years ago, and the kimberlites have yielded a variety of isotopic ages ranging from 1.63 to 1.12 billion years, all with large uncertainties.
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