Isotopic evidence for a long-lived Hadean basaltic–andesitic precursor to the Acasta Gneiss Complex, Canada

Understanding the genesis and composition of Earth’s earliest crust is critical for reconstructing the planet’s early differentiation and geodynamic evolution. However, direct evidence from the Hadean (>4.0 Ga) remains elusive owing to the near-complete absence of preserved rocks from that eon. Paleoarchean layered gneisses from the Acasta Gneiss Complex (AGC, Northwest Territories, Canada) preserve isotope signatures inherited from a Hadean crustal precursor. To investigate the petrogenesis of this precursor, whole-rock 146,147 Sm‒ 142,143 Nd data were obtained from a suite of samples cut parallel to the compositional layering of the gneiss to obtain chemically and mineralogically diverse rock slices. Additional analyses, including whole-rock major and trace element compositions, 176 Lu‒ 176 Hf isotope systematics, and apatite ( 147 Sm‒ 143 Nd and 176 Lu‒ 176 Hf) and zircon (U‒Pb) geochronology, were conducted to evaluate the preservation of primary isotope systematics and reconstruct the evolutionary history of the AGC. Zircon U‒Pb data from felsic layers define major age clusters at 3.6 to 3.5 Ga, consistent with a whole-rock 147 Sm‒ 143 Nd isochron age of 3.53 ± 0.11 Ga, supporting their interpretation as crystallization age. In contrast, apatite Sm‒Nd and Lu‒Hf dates between 2.5 and 2.1 Ga record a mostly undocumented recrystallization event, likely linked to a major phase of cratonization and breakup of the Sclavia supercraton, constituting the Slave and neighbouring cratonic blocks. Two mafic samples yield Nd- and Hf-isotope compositions indicative of a primitive or enriched Archean source, whereas five intermediate-to-felsic samples with calc-alkaline affinity indicate derivation from a Hadean crustal source. This Hadean crust, represented by three felsic samples, had an average µ 142 Nd of −8.1 ± 0.7 and a corresponding ε 143 Nd 3.53Ga of −5.3 ± 0.4. From these values, a two-stage 142,143 Nd model age of 4.21 ± 0.01 Ga and a 147 Sm/ 144 Nd source value of ∼ 0.14 are estimated for the AGC precursor crust. Considering the observed correlations among Sm/Nd, Lu/Hf, and SiO 2 in global crustal rocks, the AGC protocrust is inferred to have had a SiO 2 content of ≥ 55 wt% and a 176 Lu/ 177 Hf source value of ∼0.014. Combined with an extraction age of 4.21 Ga, the modeled isotopic evolution of this reservoir aligns with ε 176 Hf i of zircon from the Acasta Gneiss Complex. Together, the mineral and whole-rock isotope data document the preservation of a long-lived (≥600 Myr) basaltic-andesitic Hadean protocrust that underwent episodic re-melting and differentiation into felsic continental crust, revealing a protracted history of crustal reworking and chemical differentiation in the Acasta Gneiss Complex.