Dharwar Craton

The Dharwar Craton is an Archean continental crust craton formed between 3.6 and 2.5 billion years ago, which is located in southern India and considered the oldest part of the Indian peninsula.
Studies in the 2010s suggest that the craton can be separated into three crustal blocks since they show different accretionary history. The craton includes the western, central and eastern blocks and the three blocks are divided by several shear zones.
The lithologies of the Dharwar Craton are mainly TTG gneisses, volcanic-sedimentary greenstone sequences and calc-alkaline granitoids. The western Dharwar Craton contains the oldest basement rocks, with greenstone sequences between 3.0 and 3.4 Ga, whereas the central block of the craton mainly contains migmatitic TTG gneisses, and the eastern block contains 2.7 Ga greenstone belts and calc-alkaline plutons.
The formation of the basement rock of the Dharwar Craton was created by intraplate hotspots, the melting of subducted oceanic crust and the melting of thickened oceanic arc crust. The continuous melting of oceanic arc crust and mantle upwelling generated the TTG and sanukitoid plutons over the Dharwar Craton.

Overview of the regional geology

As the Dharwar Craton is located in southern India, it is geographically surrounded by the Arabian Sea, the Deccan Trap, the Eastern Ghats Mobile Belt and the Southern Granulite Belt.
Traditionally, the Dharwar Craton includes the western block and eastern block. The mylonite zone at the eastern boundary of the Chitradurga greenstone belt is the margin between the western block and the eastern block. The Chitradurga greenstone belt is an elongated linear supracrustal belt which is 400 km long from North to South.
Cratonisation is an important process to form a craton with sufficient and stable continental masses. In terms of the ages of the blocks, the western blocks is older with a cratonisation age around 3.0 Ga while the eastern block is younger with the cratonisation age around 2.5 Ga.

Simplified stratigraphic column of the Dharwar Craton

Sargur group

The Sargur group mainly consist of ultramafic-mafic rocks like komatiite and basalt, as well as some quartzite and chert.
It is dominated in the western block from Paleoarchean to Mesoarchean.

Dharwar supergroup

The Dharwar Supergroup can be divided into two groups as well, including the Bababudan group with older age and the Chitradurga group with a younger age.

It is dominated in the western block from Paleoarchean to Mesoarchean.

Kolar group

Since the eastern block cratonised later than the western block, the eastern block is dominated by Kolar greenstone belts.

The Kolar-type greenstone mainly contains some metabasalts and felsic volcanic rocks.

Granitic plutons

The granitoids include high-magnesium sanukitoids and high-potassium granites.

Lithologies

TTG gneisses

TTG rocks are intrusive rocks with a granitic composition of quartz and feldspar but contain less potassium feldspar. In Archean craton, TTG rocks are usually present in batholiths formed by plate subduction and melting. Two kinds of gneisses can be found on the Dharwar Craton, which includes the typical TTG-type gneisses and the dark grey TTG banded gneisses :

Blocks	Associated group	Main TTG type	Characteristics
western block	Sargur Group	typical TTG gneisses	some of the TTG are with minor granitic intrusions
central block	Kolar Group	transitional TTG gneisses	the TTG shows foliation the abundance of the weakly foliated TTG gneisses decreases gradually from the west to the east the abundance of the dark grey banded gneisses with younger age increases gradually from the west to the east
eastern block	Kolar Group	banded gneisses	it contains less TTG than those of the western and central blocks

Volcanic-sedimentary greenstone sequences

Greenstone is metamorphosed mafic to ultramafic volcanic rock that formed in volcanic eruptions in the early stage of Earth formation. The volcanic-sedimentary greenstone sequence occupies the majority of the Archean crustal record, which is about 30%. The western block comprises the greenstone sequences with adequate sediments, while the central block and the eastern block comprise the greenstone sequences with adequate volcanic rocks but minor sediments.

Blocks	Associated group	Composition of the volcanic greenstone	Characteristics
Western block	Sargur group and Dharwar Supragroup	ultramafic komatiite with interlayered sediments	rocks were formed in calm and shallow water environments basaltic flows, conglomerate and some felsic volcanics can be found in the greenstone of the Dharwar Supragroup
Central block	Kolar group	basalts with minor ultramafic komatiite	the volcanic rocks comprised with minor sediment and some felsic rocks
Eastern block	Kolar group	basalts with minor ultramafic komatiite	basalts are with high magnesium the greenstone contains interlayered sediments like carbonate

Sanukitoids (Calc-alkaline granitoids)

Sanukitoids are granitoids with high-magnesium composition that are commonly formed by plate collision events in Archean. In the Dharwar Craton, there is no sanukitoid record in the western block. However, there are a lot of granitoid intrusions in the central block, which become less in the eastern block.

Blocks	Rock units intruded by granitoids	Main composition	Characteristics
Central block	TTG gneisses and volcanic greenstone	monzogranite and monzodiorite	they are comprised with pink phenocrysts. granitoid intrusions form plutons over the block that are north–south trending the largest pluton in the central block is the Closepet Batholith.
Eastern block	TTG gneisses and volcanic greenstone	monzogranite and monzodiorite	The granitoids are associated with the diatexites, indicating there was intense metamorphism which causes recrystallization of minerals

Anatectic granites

Anatectic granite is a kind of rock formed by the partial melting of the pre-existing crustal rock, which is relatively younger than the TTG and greenstone in the Dharwar Craton. The granites usually cut across the older rocks.

Blocks	Rock units intruded by granites	Main composition	Characteristics
Western block	TTG gneisses and volcanic greenstone	granite with high-potassium content	they occupy the ductile shear zone over the TTG gneisses, forming cross-cutting dykes and veins
Central block	TTG gneisses and volcanic greenstone	granite with high-potassium content	they occupy the ductile shear zone over the TTG gneisses, forming cross-cutting dykes and veins
Eastern block	TTG gneisses and volcanic greenstone	granite with high-potassium content	they occupy a large area in the eastern block in the southern part of the block, many veins and dykes cut across the gneisses some mafic to ultramafic xenoliths can be found

Metamorphic record

When the rocks were under subductions, they experienced high temperature and pressure leading to the chemical changes and textural changes of rocks. The mineral assemblages of the metamorphic rocks can tell us how high the temperature and pressure are when they are under the peak metamorphism. The metamorphic rocks in the Dharwar Craton usually recorded the mineral assemblages from amphibolite facies to granulite facies:

Blocks	Pressure-Temperature conditions	Metamorphic facies	Records
Western block	Progressive increase from the N to the S	From the greenschist facies to the hornblende-granulite facies	Holenarsipur greenstone belt mineral assemblages: kyanite-garnet pressure: 6–8 Kb temperature range: 500–675 °C Gundlupet region mineral assemblages: garnet-hornblende-clinopyroxene temperature range: 650-750 °C
Central block	Progressive increase from the N to the S	From the greenschist facies to the granulite facies	Pavagada region, the central part of the central block mineral assemblages: sillimanite-spinel-quartz temperature condition: ultrahigh B.R Hills region mineral assemblages: amphibolite-granulite pressure: 5–9 Kb temperature range: 600–775 °C
Eastern block	Poorly understood	Poorly understood	Hutti greenstone belt mineral assemblages: amphibolite Krishnagiri-Dharmapuri region, the southern part of the eastern block mineral assemblages: amphibolite-granulite temperature range: 650 to 800 °C

Archean crust accretions

Accretions mean the collisions between plates leading to the plate subduction. Crust accretions are important in the Dharwar Craton since the continuous volcanic eruptions caused by accretions led to the formation of Archean felsic continent crust.
For finding when the Archean crust accretions happened, the parent-daughter isotopes dating, like uranium-lead decay could be used to find out the ages of the events.
According to the zircon U-Pb ages of the TTG gneisses from the Dharwar Craton, there were 5 major accretion events leading to the formation of the Archean felsic continental crust. The events occurred with the ranges of age 3450–3300, 3230–3200, 3150–3000, 2700–2600 and 2560–2520 million years ago.
The western block records the two earliest crust accretion events, that happened in 3450 Ma and 3230 Ma. The rates of the continental growth of the two events are fast since the events led to the widespread of greenstone volcanism.
The central block records 4 major accretion events, that occurred in 3375 Ma, 3150 Ma, 2700 Ma and 2560 Ma. The isotopic data suggests that the scale of the continental growth due to felsic crust accretion was large during 2700–2600 Ma and 2560–2520 Ma, leading to the large-scale greenstone volcanism at that time.
The eastern block records the 2 latest major accretion events occurring in 2700 Ma and 2560 Ma with massive continental growth.