Genesis of the Singhbhum Craton, eastern India; implications for Archean crust-mantle evolution of the Earth

The Singhbhum Craton in eastern India, one of the four major Archean cratons of the Indian Shield, is underlain by Paleoarchean granitoids (trondhjemite-tonalite-granodiorite i.e. TTGs, and potassic granites) that are encircled by greenstone belts rich in banded iron formation known as Iron Ore Group. This study presents whole rock major-trace element geochemistry, Sr and Nd isotopes, K-feldspar common Pb isotopes, along with in-situ zircon U-Pb ages and Hf isotope data from the granitoids and the greenstone volcanic rocks to characterize their source(s) and to constrain the evolution of the Paleoarchean continental crust. Zircon U-Pb ages show that these granitoids were emplaced during two magmatic episodes at ca. 3.47-3.44 Ga and 3.36-3.28 Ga. The greenstone volcanic rocks in the eastern part of the craton are basalts and basaltic andesites with rare komatiites, whereas those in the western part are basaltic-andesites and andesites. Whole rock Sm-Nd isotope regression lines for volcanic rocks from the eastern (including komatiites) and western regions yield ages of 3746 +/- 340 Ma (MSWD=169) and 2961 +/- 420 Ma (MSWD=3.3), respectively. The major and trace element data show that the volcanic rocks of the eastern and western greenstone belts had a different petrogenesis. The granitoids are highly evolved with SiO2 contents up to 76 wt%. The major and trace element data indicate that the granitoids of the Singhbhum Craton were derived by partial melting of a mafic crust at variable depths (ca. 10-15 kbar). The isotopic compositions of Pb measured on leached K-feldspars from the granitoids show undepleted mantle-like characteristics and negate the possible involvement of much older evolved crust in the formation of these rocks. The whole rock initial Nd isotopic compositions (epsilon Nd-i = -0.3 to + 2.2) and zircon in-situ initial Hf isotopic compositions (epsilon Hf-i = -0.3 to + 2.0) of the granitoids calculated using zircon crystallization ages indicate that they were derived from a near-chondritic reservoir. These data suggest that only a limited volume of continental crust formed prior to ca. 3.5 Ga, and major crust extraction from the mantle started at ca. 3.5 Ga. Using this time as the start of major crustal growth and concomitant mantle depletion, Nd and Hf mantle depletion histories are proposed for the Singhbhum Craton that can be described by the equations epsilon Nd-t = -2.85 x (t(Ga))+ 10, and epsilon Hf-t = -4.68 x (t(Ga))+ 16.4. These curves do not change the model ages for young rocks, but they make a significant difference for the Archean rocks.