P. V. Sunder Raju

Chapter 15 Evolution of c. 3.5–2.5 Ga basins of the Dharwar Craton

Abstract The geology and basin evolutionary history of the Dharwar Craton is discussed. The Dharwar Craton comprises western (WDC) and eastern (EDC) subdivisions (possibly separated by the Closepet granite), predicated on lithological contrasts and inferred metamorphic and magmatic evolution. A postulated genesis of the WDC comprises early, c. 3.5 Ga protocrust, which possibly formed as basement to the c. 3.35–3.2 Ga Sargur Group greenstone belts. The latter are thought to have formed through accretion of plume-related ocean plateaux. The approximately coeval Peninsular Gneiss Complex possibly originated from beneath plateau remnants, leading to metamorphism of Sargur Group belts at c. 3.13–2.96 Ga. At c. 2.9–2.6 Ga, the Dharwar Supergroup, comprising lower Bababudan (mainly braided fluvial, glaciomarine and subaerial volcanic strata) and upper Chitradurga (marine clastic, chemical sedimentary and subaqueous volcanic rocks) groups developed. This supergroup formed younger greenstone belts characterized by two distinct magmatic events, at 2.7–2.6 and 2.58–2.54 Ga; the latter was approximately coeval with c. 2.6–2.5 Ga granitic magmatism, which marked final cratonization of the WDC. The EDC consists of 2.7–2.55 Ga tonalite–trondhjemite–granodiorite gneisses and migmatites, essentially coeval greenstone belts (mainly volcanic lithologies), with minor inferred remnants of an older, c. 3.38–3.0 Ga crust, and voluminous 2.56–2.5 Ga granitoids (including the Closepet). An east–west accretion of EDC island arcs (or possibly of an assembled arc-granitic terrane) on to the WDC is postulated, and the Closepet granite perhaps accreted earlier on to the WDC to form a ‘central Dharwar’ terrane. A final voluminous granitic cratonization event affected the assembled Dharwar Craton at c. 2.5 Ga.

Textural and compositional relationships of rutile and chromite in Sittampundi anorthosite complex, Tamil Nadu

The anorthosite complexes and related rock types of the Indian Precambrian shield are primarily associated with either cratons or mobile belts. They were metamorphosed under amphibolites to granulite facies conditions. The major rock types are chromite-bearing meta-anorthosites, amphibolites, basic granulites, pink granites and gneisses. This study was carried out on chromitite bearing samples from Sittampundi layered anorthosite complex, Tamil Nadu, to evaluate the textural and compositional relationships of rutile and chromite. The pristine composition of the chromites is still preserved inspite of intense metamorphic and tectonic process. The rutiles are differentiated into isolated, clustered and exsolved rutile grains.