Zircon SHRIMP U-Pb geochronology, geochemical and Nd isotope systematics of Neoarchean granitoids, Gadag Greenstone Belt, Dharwar Craton, southern India: Petrogenesis and tectonic significance

Coupling of the geological processes in the arc-magmatic and back-arc provinces of accretionary orogens in Neoarchean plate tectonic setting is a subject of current research all over the world. The Dharwar Craton of southern India is an example of such an accretionary orogen, with an arc-magmatic province in the east and a back-arc province in the west, referred as the Eastern Dharwar Craton and the Western Dharwar Craton, respectively. The boundary between the two provinces is considered to be marked by a 400-km long shear zone along the eastern margin of the Gadag-Chitradurga-Karighatta greenstone belt which is called as the Chitradurga Boundary Shear Zone. Potassic, metaluminous, I type, calc-alkalic to alkali-calcic, arc-magmatic granitoids are widespread in the EDC. But they are also found to occur along the western margin of the Chitradurga-Gadag greenstone belt. SHRIMP U-Pb zircon ages of the granitoids in the western back-arc province in the Gadag region occurring near Srimant Gudda, Mulgund and Chabbi have been determined as 2565 +/- 24 to 2591 +/- 64 Ma old. Within errors, the ages of these granitoids are the same as the Lakundi and Turchihal granitoids occurring to the east of the Gadag Greenstone Belt in the arc-magmatic province. Nd isotope systematics of the granitoids suggest that they were formed from magmatic melts that were produced by remelting of 3200-3500 Ma old heterogenous continental crust. Rare inherited zircons support this antiquity of the protoliths. Occurrence of granitoids of similar age and origin, in the western back-arc province and eastern arc-magmatic province in the Gadag area was attributed to thrust duplex structure in the Gadag region. However, elsewhere, along the western margin of the Chitradurga greenstone belt near Harpanhalli, Hosdurga-Nagamangala-Pandavapura sector, or away from it in the Arsikere-Banavara, where repetition by thrusting is not obvious, late potash arc-magmatic type granitoids of similar age as the Gadag arc-magmatic granitoids are observed. The arc-magmatic type granitoids appear to have overstepped the boundary shear into the back-arc province at several places. Arc-magmatic and back-arc boundary may be diffuse rather than sharp, as also suggested by some earlier workers.Research highlightsThe ages and petrogenesis of the granitoids around GGB of both the sides of EDC (i.e., eastern magmatic-arc province) and WDC (i.e., western back-arc provinces) are very much similar.The geochemistry and isotope systematics (i.e., Nd TDM2 ages and epsilon NdT values at 2.5-2.6 Ga) of the granitoids exposed one both the sides are also similar.This shows not only a similar antiquity of both the provinces, but also gives evidence for a possible diffusive nature of boundary between the EDC and the WDC around GGB.