Dharwar Craton - opengeology.in

The structure of the Dharwar Craton is interpreted in terms of a foreland in the west and an accretionary complex in the east.

The foreland comprising the Western Dharwar Craton consisting of Neoarchaean schist belts and older plutonic and supracrustal rocks.
The Eastern Dharwar Craton is made up of the accretionary complex that is dominated by calc-alkaline plutonic rocks and meta-sediments of intra-arc basins.
The Eeastern and Western blocks of the Dharwar Craton are separated by NNW-SSE trending shear zone. Shear zone is characterised by the presence of mylonite and a zone of “high strain”.
The Southern part of the Dharwar Craton is separated from the Granulite Zone of South India by a transition zone is known as Fermor Line.

The older sequence that shows a relatively higher degree of metamorphism and intrusive contact with the granitic and gneisses has been named as Sargur Group. The granitic gneisses that show intrusive contact with Sargur Group are formally grouped as Peninsular Gneiss. It formed the basement for the deposition of a younger volcano-sedimentary sequence comprising the Dharwar Supergroup.

The two volcano-sedimentary sequence, which are made up of predominently green coloured rocks, are known as Older and Younger Greenstone Belts. The youngest element of the Precambrian Basement of the Dharwar Province is a group of plutonic rocks of granitic composition known as Closepet Granite.

The gneisses named as Gourur Gneiss are well exposed on Gorur-Hasan road in Karnataka. They consist of trondhjemite gneisses, tonalite and granodiorite. These rocks were formed by Partial melting of basalts that formed the primordial crust of the earth.

Sargur Group is consists of Quartzite are often fuschsitic (chromite bearing) and are characteristically inte-bedded with sedimentary pyrite. Stratiform deposits of barite have been sometimes observed in association with fuchsitic quartzite. Pillow structure in komatites of the Sargur Group indicates a sub-marine environment of volcanic eruption.
- The rocks have undergone a high degree of metamorphism ranging from amphibolite to lower granulite facies. The schist contain abundant lenses of anorthosite and anorthosite gabbro.
- The schist belts of Kolar and Hutti, which are known to contain deposits of gold and other noble metals, belong to this ancient group of schistose rocks of Dharwar Craton.

The Peninsular Gneiss is typically gneiss is typically made up of migmatitic gneisses alternating with bands of amphibolite and tonalite.

Dharwar Supergroup :-

The sediments of the Dharwar Supergrou were laid down in elongate proto-geosynclines over a basement of the rocks belonging to Sargur Group and Peninsular Gneiss.

There is a two fold division of Dharwar sequence “Hornblendic Division” and “Chloritic Division” . There is a three fold classification in which hornblendic division is termed as Lower Dharwar and chloritic division as Middle and Upper Dharwars. Each division was said to be characterised by a horizon of conglomerate at its base.

The two fold classification formally known as Bababudan Group and Chitradurga Group –

Bababudan Group, basal part of the succession is often marked with the presence of quartzite and conglomerate that is known as Kartikere Conglomerate. The upper parts of the seauence has been further subdivided into four formations that were named as Kalaspura, Allampur, Santaveri and Mulaingiri Fromations. Presence of sedimentary structures shows the shallow water conditions of deposition.

Chitradurga Group, begins with a basal horizon of conglomerate. A major part of the Chitradurga Group, however rests directly over the Peninsular Gneiss. Chitradurga Group is further divided into two sub-units named as Vanivilas Subgroup and Ranibennur Subgroup. Interclations of metabasics in the form of flows and sills are common. The succession also contains local concentrations of Mn-ores.

The Dharwar Group has been assigned a stratigraphic age in the range of ~2900 Ma to 2600 Ma.

Closepet Granite :-

The Closepet granite constitutes a suit of post-tectonic granites that were emplaced at the culmination of the lastest phase of the orogeny of the Dharwar Craton. The NNW-SSE trending belt of the granite is tilted northward that leads to exposure of gradually deeper rocks in the southern part of the belt.

The belt has been divided into three parts –
- Root Zone rock forms the southern part of the belt depict the precesses of generation of magma and its interaction with the basement.
- Transfer Zone that occupies the middle part of the belt depicts the textures of magmatic processes of magma chamber and its upward movement.
- Intrusion Zone is the northern most part that shows the presence of feeder dikes.

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