Review of the Isua supracrustal belt area (Greenland) Eoarchean geology from integrated 1:20,000 scale maps, field observations and laboratory data: Constraints on early geodynamics
In the early 1970s, Vic McGregor and Stephen Moorbath demonstrated that the Isua supracrustal belt (Greenland) and its surrounding orthogneisses formed in the Eoarchean (Moorbath et al., 1972, 1973). Fifty years later, these rocks are a key resource globally for understanding the Eoarchean Earth. This is because there is an extensive area (∼200 km2) of abundant outcrops where superimposed Neoarchean strain is modest, and Neoarchean metamorphism was only at lowest amphibolite facies; these are low tectonothermal conditions compared with other Eoarchean crustal remnants globally. Therefore, this region can provide the clearest window onto early Earth geodynamics. Within the Isua supracrustal belt there are rare areas where Eoarchean strain is also low, and these areas provide unambiguous recognition of diverse lithologies including sedimentary carbonates with the oldest proposed but contested stromatolites, pillow lavas, volcano-sedimentary lithologies and ultramafic rocks derived from layered intrusions (accepted) and slivers of mantle-derived meta-serpentinite that underwent Eoarchean ultra-high-pressure metamorphism (contested). As explored in previous papers (e.g., Polat et al., 2002), the geochemical signatures of the mafic igneous rocks, which include a boninite-like suite, indicate their magmas were derived by fluid fluxing of peridotites. Radiogenic isotopic signatures define discrete pulses of Eoarchean juvenile crustal growth, until a tectonic event at ∼3660 Ma with high pressure metamorphism that was followed by a switch to crustal recycling with the production of granites sensu stricto. Our appraisal of the integrated evidence points to complex superimposed Eoarchean events (∼3820–3600 Ma) driven by lateral lithosphere movements in convergent boundary settings, rather than a single crustal overturn such as within a model heat-pipe scenario.
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Nationale Geologiske Undersøgelser for Danmark og Grønland