The Dharwar or Karnataka Craton in South India is a piece of the earth's crust that dates back to the late Archean. As part of the Indian Shield, it has been a relatively stable geologic terrain for several billion years. The bedrock in this region formed between 3.6 and 2.5 billion years ago. The Dharwar Craton lies roughly between Chennai, Goa, Hyderabad, and Mangalore in the Karnataka and Andhra Pradesh states.
Geological history and significance
The Dharwar Craton presents a natural cross-section of late-Archaean continental nuclei lying between longitude 72°45´–80° and latitudes 11°–19°. It is an elliptical region comprising a number of subparallel supracrustal belts. The term Dharwar craton was introduced by the Geological Survey of India to avoid confusion with early lithologies. There are three main structural zones: a root zone of highly heterogeneous petrology (from monzonite to granite) and texture (phenocryst accumulation), a "channel zone" where evidences of large scale magma ascent can be observed, and a zone of superficial intrusions, consisting in independent homogeneous intrusive bodies. In the root zone, mantle-derived magma underwent fractional crystallisation which was followed by mingling between the residual liquids and melts generated by anatexis of the surrounding gneissic basement. It is divided into eastern Dharwar Craton and western Dharwar Craton owing to their differences in lithologies and ages.
The term Dharwar Supergroup is now used as synonymous with metamorphosed Precambrian sediments and including all the schistose series below the Eparchaean Unconformity. The Dharwarian rocks are mostly unfossiliferous except for the stromatolitic limestones.
Lithology of the Dharwars
The rocks of this age show extremely complex nature with clastic and chemically precipitated sediments, volcanic and plutonic rocks — all of which show varying degrees of metamorphism. The majority of the rocks are often phyllites, schists and slates. There are hornblende-, chlorite-, haematite-, and magnetite- schists, felspathic schists: quartzites and highly altered volcanic rocks, like rhyolites and andesites turned into hornblende-schists; abundant and widespread granitic intrusions; crystalline limestones and marbles; serpentinous marbles; steatite masses; beds of jaspers and massive beds of iron and manganese oxides.
- Moyen, J. F.; Jayananda, M.; Nedelec, A.; Martin, H.; Mahabaleswar, B.; Auvray, B. (2003). "From the Roots to the Roof of a Granite: the Closepet Granite of South India" (PDF). Journal of the Geological Society of India. 62 (6): 753–768. Retrieved 23 April 2016.
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