U-Pb and Hf isotope study of detrital zircon and Cr-spinel in the Banavara quartzite and implications for the evolution of the Dharwar Craton, south India

Abstract

The Sargur Group has been considered to be the oldest group (>3.0 Ga) in the Archaean sequence of the Dharwar Craton in south India, whereas the rocks of the Dharwar Supergroup are younger (between 3.0 and 2.55 Ga). The supracrustal units of the Sargur Group were deposited during the Archaean period. The Banavara quartzite forms part of the supracrustal Sargur Group and contains significant amounts of chromian spinel (Cr-spinel). Here, U-Pb and Hf isotopes of detrital zircons are integrated with compositional data and X-ray refinement parameters for Cr-spinels to decipher the provenance of the metasediments. Zircons show an age spectrum from 3.15 to 2.50 Ga, and juvenile Hf isotopic compositions (IHf = +0.8 to +6.4) with model ages between 3.3 and 3.0 Ga. Major-and trace-element contents of the Cr-spinels do not resemble those in the Sargur ultramafic rocks, but resemble well-characterized Archaean anorthosite-hosted chromites. Cr-spinel trace-element signatures indicate that they have undergone secondary alteration or metamorphism. X-ray refinement parameters for the Cr-spinels also resemble the anorthosite-hosted chromites. We conclude that the detrital minerals were probably derived from gneissic and anorthositic rocks of the Western Dharwar Craton, and that the Sargur Group sequences have experienced a younger (2.5 Ga) metamorphic overprint. Show more