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Eoarchean subduction-like magmatism recorded in ca. 3750 Ma mafic-ultramafic rocks of the Ukaliq supracrus-tal belt (Québec)
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  • Thomas Grocolas,
  • Pierre Bouilhol,
  • Guillaume Caro,
  • Stephen J Mojzsis
Thomas Grocolas
University of Lausanne

Corresponding Author:thomas.grocolas@unil.ch

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Pierre Bouilhol
Université de Lorraine
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Guillaume Caro
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Stephen J Mojzsis
University of Colorado Boulder
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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% H2O. Together with the presence of negative μ142Nd anomalies, this information points to a deep fluid input via recycling of Hadean crust in the Eoarchean via modern-style subduction.