A billion years of crustal evolution recorded in the Nuvvuagittuq greenstone belt: Pb-Hf evidence for Eoarchean TTGs produced from melting of Hadean mafic crust

Investigation of Earth's primitive crust is limited by the scarcity of Eoarchean/Hadean terrains. Most of these terrains are dominated by felsic Tonalite-Trondhjemite-Granodiorite (TTG) rocks. These felsic rocks however, cannot be directly produced from melting of the mantle but must instead have been derived from the melting of an older mafic precursor. Obtaining accurate ages on old terrestrial mafic rocks is challenging. The geochronology constraints on Archean mafic rocks commonly come from long-lived radiogenic isotopic systems that can be affected by younger metamorphic/metasomatic events. The short-lived ¹⁴⁶Sm-¹⁴²Nd isotopic system is less susceptible to partial resetting because ¹⁴⁶Sm became extinct prior to ~4 Ga. The mafic rocks from the Nuvvuagittuq greenstone belt (NGB), called the Ujaraaluk unit, have ¹⁴⁶Sm-¹⁴²Nd systematics consistent with them being formed in the Hadean, between 4.3 and 4.4 Ga. This age has been challenged because the oldest U-Pb ages on zircons obtained in the NGB are ~3.8 Ga. The ~3.8 Ga zircons, however, are from trondhjemetic bands intruding the NGB and thus provide only a minimum age for the mafic rocks. The NGB is surrounded by 3.65 Ga tonalites having deficits in ¹⁴²Nd suggesting derivation from an Hadean precursor such as the Ujaraaluk unit. We have now identified two additional tonalite generations dated at 3.75 Ga and 3.4-3.5 Ga suggesting a more complex thermal history for the NGB. In order to better constrain the geologic relationship between the mafic and the felsic rocks and the evolution of the NGB through time, we present whole-rock Lu-Hf data for the Nuvvuagittuq rocks as well as combined Pb-Hf analyses in zircons from a series of surrounding TTGs dated at 3.35 Ga, 3.4-3.5 Ga, 3.65 Ga and 3.75 Ga. The Lu-Hf isotopic compositions of the NGB mafic rocks have been partially reset by a Neoarchean metamorphic/metasomatic event, consistent with what is observed for the long-lived ¹⁴⁷Sm-¹⁴³Nd system. Zircons from the 3.35 to 3.65 TTGs have strongly subchondritic initial ɛHf values and display an ɛHf vs. age array consistent with their derivation from a 4.3-4.4 Ga mafic precursor. The 3.75 Ga TTGs have initial ɛHf values consistent either with derivation from the ~4.2 Ga NGB gabbros or with incorporation of more juvenile mantle-derived material. The Hf-zircon and ¹⁴²Nd data for the Eoarchean NGB TTGs is consistent with their formation from the melting of the Hadean Ujaraaluk unit. The time integrated ɛHf-zircon data also is consistent with the Hf isotopic compositions of the Jack Hills zircons suggesting a similar primitive mafic precursor for the Eoarchean/Hadean TTGs forming Earth's early crust.