U-Pb ages and Hf-isotope systematics of detrital zircons from the Gadag Greenstone Belt: implications for the Archaean crustal growth processes in the western Dharwar Craton, India

The Western Dharwar Craton in peninsular India comprises a typical Meso- to Neo-Archean granite-greenstone terrain. Detrital zircons from two metamorphosed lithic sandstones in a late basin from the Gadag Greenstone Belt preserve at least eight age populations ranging in age from ca 3.34 to 2.55 Ga, and individual grains as old as ca 3.54 Ga. The zircon provenances for the two samples appear to be the same up to ca 3.25 Ga, with relatively juvenile Hf values largely between zero and depleted mantle values. Zircons younger than 3.25 Ga from one sample have similar Hf values to the older grains whereas the other sample gives only negative values indicative of Hf-evolution in a crustal environment. Source regions younger than ca 3.25 Ga for the two samples were distinctly different. Modelling the Hf isotopic evolution of the detrital zircons suggests two major crust forming events at ca. 3.6 and 3.36 Ga, and some indication of juvenile addition to the crust at ca 2.6 Ga. The maximum sedimentation age of the lithic sandstones is constrained by the youngest detrital zircon population at 2547+ or -5 Ma. Gold mineralization in the belt is dated at 2522+ or -6 Ma and constrains greywacke sedimentation, deformation and metamorphism to a ca 25 m.y. interval.