Characterisation and U–Pb–Hf isotope record of the 3.55 Ga felsic crust from the Bundelkhand Craton, northern India

Abstract

To date, ca. 3.3Ga tonalite-trondhjemite-granodiorite (TTG) gneisses from the Aravalli and Bundelkhand Cratons are the oldest rocks in northern India. However, the results of our combined U–Pb and Lu–Hf isotope study on zircon grains from a trondhjemite gneiss of the Bundelkhand Craton provide evidence for the formation of even older felsic crust of 3.55Ga age in this part of India. The petrological and geochemical data reveal that the trondhjemite gneiss from the Bundelkhand Craton is characterised by a highly fractionated REE pattern [(La/Yb)N=34.4] with low HREE contents, and positive Eu (Eu/Eu*=1.52) and Sr anomalies. These features are consistent with melt extraction from a garnet-bearing and plagioclase-free source. Slightly subchondritic Hf compositions (ɛHf3.55Ga of −0.8±0.3) and Hf model ages between 3.80Ga and 3.95Ga additionally indicate that the trondhjemite was formed by reworking of Eoarchaean mafic crust. This interpretation is in good agreement with age-Hf isotope data obtained from zircon grains worldwide. Furthermore, our new results indicate that the Bundelkhand and Bastar Cratons experienced an important granitoid emplacement event at 3.58–3.55Ga.