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 (εHf = +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 εHf 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 ∼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.
Bibliographical noteFunding Information:
We thank Steve Mozjsis group and The John Templeton Foundation – FfAME Origins program in the support of CRiO for help with zircon separation, Nancy Cherim and Mark Townley for help with CL image acquisition, Julia G. Bryce and Maria F. Fahnestock for technical help at UNH. Jean-Louis Paquette and Abdel-Mouhcine Gannoun are thanked for assistance with analyses at the Laboratoire Magmas et Volcans. We are grateful to Claire Fonquernie and Mhammed Benbakkar for rock crushing and major-oxide analyses at Laboratoire Magmas et Volcans. Discussions with Hervé Martin helped improve the present manuscript. Martin Guitreau acknowledges financial support from Matthias Willbold through NERC grant R117299 and the Région Auvergne through the Auvergne Fellowship program. This is Laboratory of Excellence ClerVolc contribution 266. The University of New Hampshire supported this research project through start-up funds granted to Dr. Samuel B. Mukasa. Last but not least, we would like to thank Randall Parrish for efficient editorial handling as well as Tsuyoshi Iizuka, Sukanta Dey, and an anonymous reviewer for insightful comments that helped improve this manuscript.
© 2017 Elsevier B.V.
- Lu-Hf isotope system
- Western Dharwar Craton
- Zircon U-Pb geochronology