On the petrogenesis of Paleoarchean continental crust: U-Pb-Hf isotope and major-trace element constraints from the Bastar Craton, India

Combined geochronological and (isotope) geochemical investigations of tonalite-trondhjemite-granodiorite (TTG) suites, which dominate Archean cratons, provide constraints on the sources and petrogenetic processes that gave rise to the Earth's early continental crust. In situ U-Pb and bulk Lu-Hf analyses of single zircon grains from ITGs of the Paleoarchean Bastar Craton in central India date the emplacement of the igneous rock suite and trace the chemical evolution of its source rocks. Complementary whole rock Lu-Hf and major and trace element data constrain the petrogenesis of the TTGs. The rocks are variably enriched in fluid mobile elements, particularly K and Pb, and are characteristically depleted in heavy rare earth elements. Calculated initial Hf isotopic compositions of zircon and whole rocks cluster around the chondritic value at ca. 3.55 to 3.45 Ga. The high precision Hf-isotope data reveal a trend of increasing radiogenic Hf-176/Hf-177 with age, the slope of which implies a Lu-176/Hf-177 = 0.02, and thus derivation from a mafic protolith. Geochemical modeling indicates that the granitoids can be directly produced by melting of a hydrous basalt in the garnet stability field, followed by minor fractional crystallization. Variations in the modal abundance of garnet suggest the source rocks melted at different depths. Rocks with moderately to strongly depleted HREE patterns require >10% garnet in the residue, consistent with derivation from a garnet-amphibolite or garnet-pyroxenite source. Modeling of the Lu-Hf systematics further highlights the important role of melting as opposed to fractional crystallization for reproducing the primary chemical characteristics of TTGs. The global Hf-isotope record of Archean mantle-derived rocks is considerably more radiogenic than that of the bulk silicate Earth, providing unambiguous evidence that parts of the mantle were significantly depleted in incompatible elements since at least the early Archean. The differences between Hf isotopic compositions of the mafic (greenstone belts) and felsic (gneissic terranes) rocks can be accounted for by crustal residence times of several 100 million years for the protoliths of the felsic magmatic suites. Consequently, TTGs integrate the geologic history of their precursor(s), complicating their isotopic record. Therefore, the near-chondritic Hf isotopic composition of TTGs is not direct evidence for their derivation from a primitive mantle, but rather a consequence of their specific petrogenesis involving an aged, mafic precursor. This relationship introduces uncertainties into models of crustal growth and geodynamics that are based on the felsic record, especially where the whole rock context is absent. In contrast, high-precision Hf-isotope analyses of single zircon grains, combined with whole rock isotope and elemental constraints, can be used to reliably identify the sources and processes involved in the generation of Earth's oldest continental crust.