An Archaean tectonic model of the Dharwar craton, southern India: the origin of the Holenarasipur greenstone belt (Hussan district, Karnataka) and reinterpretation of the Sargur-Dharwar relationship

The occurrence of the Holenarasipur greenstone belt is explained by accretionary process at a trench. A sequence of amphibolite (MORB), chert, and banded iron formation associated with komatiitic amphibolite (oceanic island material) is an Archaean analogue to the modern oceanic crust grown through the migration from an active ridge to a trench. At a trench, slices of such a sequence mixed with or became covered by turbidite from land mass (quartzite and conglomerate) to form an accretionary complex. Available SHRIMP data suggest the synchronous formation of nearby tonalite-trondhjemite-granodiorite (TTG) with the greenstone belt by melting of subducting oceanic plate. The Dharwar craton has been built up stepwise: (1) Sargur stage—ocean-ocean collision to form oceanic island arc with TTG and an accretionary complex (older greenstone belt), followed by the collision of such arcs to form mini-continents at the early to middle Archaean (unit 3.0 Ga); (2) Dharwar stage—the amalgamation of mini-continents with newer accretionary complexes (younger greenstone belt) under the same subduction polarity caused by the change from two layered to whole mantle convection with the progressive cooling of the earth (3.0−2.5 Ga). The cessation of TTG activity was also due to the cooling of the earth. Tectospheric peridotite keel formed with TTG has acted as a thermal insulator to stabilize the Dharwar craton after the Archaean.