New constraints on the early formation of the Western Dharwar Craton (India) from igneous zircon U-Pb and Lu-Hf isotopes

The Western Dharwar Craton (WDC) is an Archean crustal segment for which the earliest stages of development have remained poorly constrained because the oldest identified lithologies are chronologically indistinguishable despite vastly different compositions and origins (i.e., 3352 +/- 110 Ma Sargur-group komatiites and 3342 +/- 6 Ma Hassan-Gorur TTG gneiss). Indication for older crust come from ancient detrital zircons (3450-3610 Ma), although their genetic link to the WDC is purely conjectural. In order to bring new understanding to early development of the WDC, we studied orthogneisses around the Holenarsipur Schist Belt (HSB) for their petrography, major-oxide concentrations, zircon U-Pb geochronology, and Lu-Hf isotope systematics. Our results reveal that the WDC igneous record contains crust older than 3350 Ma in the form of a 3410.8 +/- 3.6 Ma granitic gneiss and inherited zircons with ages ranging from 3295 +/- 18 to 3607 +/- 16 Ma that were found within a 3178 +/- 10 Ma trondhjemitic gneiss and a biotite-rich enclave found within it. The presence of muscovite and the peraluminous signature of the granitic gneiss, in spite of mildly-depleted Hf isotopic signature (epsilon(Hf) = + 2.2 +/- 0.6 at 3410.8 Ma), suggest that this sample formed by reworking of a felsic precursor with short crustal residence time, possibly marking the beginning of WDC formation. The oldest inherited zircons display variable epsilon(Hf) ranging from + 10.4 at 3414 Ma to -2.3 at 3607 Ma that did not seem to have influenced the Hf isotopic composition of granitoids of the WDC that formed between 3200 and 3410 Ma, except perhaps in the Sargur area. We suggest that the WDC formed remote from continental crust until a crustal block containing > 3410 Ma zircons was accreted to it similar to 3200 My ago. This event resulted in the stabilization of the WDC which is marked by diapiric granitoids to which the 3178 Ma trondhjemitic gneiss belongs. After 3200 Ma, the crustal block together with granitoids formed between 3410 and 3200 Ma buffered the Hf isotopic signature of newly formed granitoids, hence, indicating that, by then, the WDC already was a stable continental segment.