Abstract
Our understanding of the nature of crustal formation in the Eoarchean is limited 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 geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 °C. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative μ142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred: (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction.
Highlights
Plate tectonics has long governed the mode of crust formation and cooling on our planet, including the long-term operation of the geochemical cycles and, the evolution of the atmosphere, hydrosphere, and biosphere in what has been termed “biogeodynamics” (e.g., Stern 2002; Von Huene and Scholl 1991; Zerkle 2018)
The petrogenetic process associated with the crustal growth trajectory of the Archean Earth remains widely debated because of the poor preservation of crustal material from these early times that could provide direct information on the processes involved in crustal genesis
Age and origin of the inherited Hadean component of the Innuksuac complex We demonstrated that magmatic rocks of the Ukaliq supracrustal belt (USB) formed following two liquid lines of descent, one from a damp basaltic primitive melt and the other from a wet boninitic primitive melt that are ubiquitous and best produced during low-pressure mantle melting in subduction systems
Summary
Plate tectonics has long governed the mode of crust formation and cooling on our planet, including the long-term operation of the geochemical cycles and, the evolution of the atmosphere, hydrosphere, and biosphere in what has been termed “biogeodynamics” (e.g., Stern 2002; Von Huene and Scholl 1991; Zerkle 2018). The thermal and mechanical properties of such thick crust inhibited subduction processes to instead favor emplacement of a long-lived lithosphere susceptible to reworking via what may have been catastrophic vertical transfer events (e.g., Bédard 2006, 2018; Fischer and Gerya 2016a; O’Neill and Debaille 2014; Van Thienen et al 2004) At odds with these interpretations are recent numerical models showing that subduction can proceed even under the thermal boundary conditions of very thick and buoyant crust (Maunder et al 2016; Weller et al 2019), other models demonstrate that subduction is inhibited even with a mantle 30 °C hotter in the Proterozoic (e.g., Davies 1992). The petrogenetic process associated with the crustal growth trajectory of the Archean Earth remains widely debated because of the poor preservation of crustal material from these early times that could provide direct information on the processes involved in crustal genesis Resolving these conflicting conclusions about Earth’s history of plate tectonics requires analysis of the oldest rocks in addition to numerical modeling. This approach is only effective if the oldest preserved rocks can be shown to contain petrological and geochemical characteristics that allow for distinguishing between the competing Archean petrogenetic models cited above
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