The lower crust of the Dharwar Craton, southern India: patchwork of Archean granulitic domains
Abstract
U-Pb geochronological, major and trace element and isotope geochemical data identify crustal domains of contrasted petrological origins and histories in the lower crust of the Dharwar Craton, all affected by granulite facies metamorphism and deformation in the Neoarchean. Four lower crustal domains have been identified and investigated across the Neoarchean amphibolite to granulite facies transition zone. (1) In the southern part of the Western Dharwar Craton (WDC), high-grade metamorphism affects rocks formed before 3.0 Ga. High-grade metamorphism is recorded at ca 2.51 Ga, followed by possible complex cooling and/resetting until 2.2 Ga. (2) The BR Hills-MM Hills domain represents the lower crust of the Central Dharwar Province (CDP) which is here defined as a transitional crust between the old (>3 Ga) WDC and the Eastern Dharwar Province (EDP), not older than 2.7 Ga. This granulitic domain contains the oldest rocks of the Dharwar Craton with TTG suites formed at ca 3.4 Ga and ca 2.6 Ga suites of probable crustal recycling origin. Both suites are involved in 2513±5Ma granulite facies metamorphism. (3) The Nilgiri granulitic domain is tectonically juxtaposed with the deepest level of the WDC as a consequence of Neoarchean dextral slip along the Moyar shear zone. The massif consists of rocks of orogenic affinities formed between 2.70 and 2.56 Ga and overprinted by granulite facies metamorphism at 2518±10 Ma. (4) The Krishnagiri domain exposes the deepest levels of the Eastern Dharwar Craton. It is composed of TTG and calc-alkaline igneous protoliths formed between 2.55 and 2.53 Ga. Migmatization in the upper part of the domain took place at 2507±6Ma and granulite facies overprint in the deepest parts of the domain is constrained at 2516±5 Ma. The Krishnagiri domain was magmatically accreted to the lower crust of both the CDP and EDP by interaction with mafic magmatism, inducing reworking of the 2.7-2.6 Ga or older overlying crust. The Nilgiri granulites share several temporal and petrological properties with the Madras granulites that make the easternmost outcrop of the Dharwar Craton. We tentatively suggest that the Nilgiri and Madras granulites evolved as parts of a common Madras domain in the lower crust of the EDP before activation of the Moyar shear zone. In this sense, the Nilgiri massif may be considered as an allochtonous unit in a context of Neoarchean ultra hot orogeny in South India.