Eoarchean to Neoarchean crustal evolution of the Western Dharwar Craton, southern India: Clues from U-Pb-Hf isotope composition of detrital zircon.

In this study, we use the U-Pb-Hf isotope composition of detrital zircon in metasedimentary rocks from the Western Dharwar Craton to identify older crusts, their nature, source, timing of extraction, and reworking. The samples come from the Holenarsipur region in the central and Sargur region in the southern part of the craton and belong to both the Sargur and the Bababudan Groups. Both detrital and metamorphic domains were identified in the zircon. The metamorphic overgrowths correspond to two age populations at 2536–2516 Ma, and 2478–2460 Ma. Rutile grains provide either concordant 2453 Ma, or discordant younger ages suggestive of thorough re-equilibration during metamorphism. Detrital zircon grains define notable clusters at 3685–3638 Ma, 3573–3524 Ma, 3471–3411 Ma, 3373–3341 Ma, 3277–3249 Ma, 3172–3148 Ma, 3084–2996 Ma, and 2659–2647 Ma which correspond to major episodes of granitoid crust formation in the craton. Based on detrital and metamorphic zircon populations, deposition of both the older Sargur Group and the basal unit of the younger Bababudan Group in the Holenarsipur region were broadly contemporaneous after c. 3160 Ma, while the sediments of the purported Sargur Group in the Sargur region were deposited after c. 2650 Ma. The new age data requires revision of the stratigraphic positions of the Sargur-type and Dharwar-type greenstone belt successions. The detrital zircon suites have both radiogenic and crust-like Hf isotopic compositions. However, the majority have radiogenic compositions that were acquired during major events of juvenile crust extraction between c. 3850 Ma and 3250 Ma. Strongly unradiogenic ɛHf (t) of some Paleoarchean detrital zircon require protoliths extracted from the mantle in the Hadean, but which persisted for a long period and contributed to granitoid formation in the Paleoarchean and the Mesoarchean. Mixing of juvenile magmas with preexisting crustal components also played a significant role in the petrogenesis of the granitoids. A distinct shift in the ɛHf (t) of the detrital zircon from chondritic to positive value at c. 3600 Ma is noted, reflecting a marked increase in the contribution of strongly depleted mantle reservoir to the source of the post-c. 3600 Ma granitoids. [ABSTRACT FROM AUTHOR]