; Our understanding of the nature of crustal formation in the Eoarchean is; severely curbed by the scarcity and poor preservation of the oldest; rocks, and variable and imperfect preservation of protolith magmatic; signatures. These limitations hamper our ability to place quantitative; constraints on thermomechanical models for early crustal genesis and; hence on the operative geodynamical regimes at that time. Controls on; the liquid line of descent responsible for Eoarchean crust petrogenesis; could help us understand more, but these remain vague. Growth of Archean; crust may have occurred dominantly via processes akin to modern oceanic; crustal genesis, coupled to a vertical geodynamic regime. Equally,; convergent boundary processes, including subduction, are argued to be; important in the development of the crust before about 3.8 Ga. The; recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern; Québec) is mainly composed of serpentinized ultramafic rocks and; amphibolitized mafic schists. Inferred protoliths to the Ukaliq; serpentinites include dunites, pyroxenites, and hornblendites with; compositions similar to that of arc crust cumulates, whereas the mafic; rocks were probably basalts to basaltic andesites. The Ukaliq cumulates; record two liquid lines of descent: (i) a tholeiitic suite, partially; hydrated, resulting from the fractionation of a basaltic liquid; and; (ii) a boninitic suite documenting the evolution of an initially; primitive basaltic to andesitic melt at ~0.5 GPa and; containing >6 wt% H; 2; O. Together with the; presence of negative; μ; 142; Nd anomalies, this; information points to a deep fluid input via recycling of Hadean crust; in the Eoarchean via modern-style subduction.;
