Fifty years of the Eoarchean and the case for evolving uniformitarianism
It is 50 years since the landmark paper where Black et al. (1971) presented whole-rock Pb-Pb and Rb-Sr isotopic evidence for some rocks in Greenland surviving from Earth's first billion years; the ≥ 3700 Ma Amîtsoq gneisses. This overturned ideas prevalent at that time that the young Earth was far too violent for such ancient rocks to survive. In the following few years it emerged how ‘normal’ this early Earth appeared to be, with a retained hydrosphere (oceans) by 3700 Ma and ‘continental’ crust dominated by rather normal granitic sensu lato rocks. By several decades ago this led to a vision of Eoarchean lithosphere development being controlled by a geodynamic regime with definite similarities to that in the Phanerozoic, particularly with evidence for lateral lithosphere movement (mobile lid) such as exemplified by arc-like magmatism and Barrovian metamorphism (the latter requiring tectonic crustal thickening). However, the literature in recent years is increasingly emphasising model-driven non-uniformitarian stagnant lid geodynamic scenarios to explain the Eoarchean geological record. This paper reviews the broad range of field and laboratory evidence extracted from the Eoarchean geological record in Greenland. We argue that the non-uniformitarian sagduction within a stagnant lid regime interpretation does not fit Eoarchean facts.
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Nationale Geologiske Undersøgelser for Danmark og Grønland