Geology of the Eoarchean, > 3.95 Ga, Nulliak supracrustal rocks in the Saglek Block, northern Labrador, Canada: The oldest geological evidence for plate tectonics

Abstract

The Earth is a unique planet, which has been highly evolved, diversified and complicated through geologic time, and underwent many key events, including giant impact, magma ocean, core formation, large-scale mantle differentiation and late heavy bombardment, especially in its dawn. But, our knowledge of early Earth is limited due to the lack of the Hadean supracrustal rocks. The supracrustal rocks with the Eoarchean ages provide key evidence for the Earth's early evolution, but few supracrustal rocks have been comprehensively investigated. Therefore, we mapped in seven areas of the Saglek Block, northern Labrador, where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses. Early studies suggested that some of them have the Mesoarchean ages because of the lack of the Mesoarchean Saglek dyke, but we found the Saglek dykes in the areas to recognize the Eoarchean Nulliak supracrustal rocks and Uivak Gneiss in all the areas. Recent reassessment of U-Pb dating and cathodoluminescence observation of zircons from the oldest suites of the Uivak Gneiss showed that the Uivak Gneiss has the Eoarchean age, N 3.95 Ga, and forms the Iqaluk-Uivak Gneiss series. Because our geological survey clearly showed that the Iqaluk-Uivak Gneisses were intruded into the Nulliak supracrustal belts, the Nulliak supracrustal rocks are the oldest supracrustal rock in the world. The supracrustal belts consist of piles of fault-bounded blocks, which are composed of the ultramafic rocks, mafic rocks and sedimentary rocks in ascending order, similar to modern ocean plate stratigraphy (OPS). In addition, small-scale duplex structures are found over the areas. The presence of duplex structure and OPS indicates that the N 3.95 Ga Nulliak supracrustal belts originate from an accretionary complex. The presence of the accretionary complex, ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early Earth.

Peak to post-peak thermal history of the Saglek Block of Labrador: A multiphase and multi-instrumental approach to geochronology

“…The dominant component of the Saglek Block is the Uivak gneiss, which is subdivided into two types: Eoarchean (> 3.6 Ga) Uivak I gneisses of tonalite-trondhjemite-granodiorite (TTG) composition, and slightly younger (~3.6 Ga) Uivak II augen gneisses, which include Fe-rich porphyritic granodiorites and diorites (Bridgwater and Schiøtte, 1991;Collerson and Bridgwater, 1979). The Uivak I gneiss is a composite of different magmatic protoliths, and contains enclaves of Fe-rich monzodiorites (Collerson et al, 1992) and tonalities (Komiya et al, 2015). These authors have suggested the enclaves were derived from > 3.9 Ga protoliths, the 'Nanok' and 'Iqaluk' gneisses, respectively.…”

Plate-tectonic evolution of the Earth: bottom-up and top-down mantle circulation

“…Based on Nd-Sm bulk-rock analyses, O'Neil et al (2008O'Neil et al ( , 2011 reported the existence of yet older ϳ4.28 Ga rocks in the Nuvvuagittuq greenstone belt, northeastern Canada, but Cates et al (2013) indicated a possible maximum age of ϳ3.78 Ga for these supercrustals. In any case, water-laid metasedimentary rocks in western Greenland requires that tracts of the Earth's surface had cooled below the boiling point of H 2 O by ϳ3.8 Ga at the very latest (Moorbath et al 1972;Moorbath 1984;Nutman et al 1997;Komiya et al 2002Komiya et al , 2015 but possibly up to at least 300 Myr earlier (Wilde et al 2001;Mojzsis et al 2001, Valley et al 2002Harrison 2009;Sleep et al 2014). Apparently, segments of solid crust and mantle were present in the early Earth (Davies 2006(Davies , 2007Hynes 2008).…”

“…Bulk-rock isotopic data suggest that thorough reincorporation of sialic materials into the mantle continued until ϳ3.8 Ga, that the average age of continental crust separated from the mantle is ϳ2.3 ± 0.5 Ga, and that much of this material (70% or more) was generated by Late Archean or Early Proterozoic time (Veizer 1976;Jacobsen and Wasserburg 1981;DePaolo 1981;Nelson and DePaolo 1985;Taylor and McLennan 1985;Turcotte and Kellogg 1986;DePaolo et al 1991;Condie 1998;Hawkesworth et al 2013). Calcalkaline igneous arcs also were produced by Hadean subduction, but long-term preservation of continental crust did not begin until the Early Archean (Moorbath et al 1972;Komiya et al 2015). The existence of igneous zircons as old as ϳ4.3-4.2 Ga (Compston and Pidgeon 1986;Wilde et al 2001;Harrison 2009;Hopkins et al 2008Hopkins et al , 2010 in mid-Archean sedimentary rocks reflects at least short-term near-surface retention of some ancient arc material.…”

“…The occurrence of ductile microplates also may account for the small scale of the surviving Archean granite-greenstone belts, assuming that the mafic rocks formed as oceanic crust, then were sutured against primitive sialic island arcs (e.g., Percival et al 1994;Mueller et al 2002;Wyman et al 2002;Garde 2007;Szilas et al 2012;Komiya et al 2015). These oceanic arcs include abundant pillow lavas, so they were covered by shallow seas, reflecting early Earth condensation of H 2 O from the cooling atmosphere.…”

Asteroid bombardment and the core of Theia as possible sources for the Earth's late veneer component

“…Furthermore Hadean mantle geochemical domains and maybe even Hadean crust exist on the Earth [e.g., O'Neil et al ., ]. For reference, the oldest undisputed sedimentary rocks on the Earth are 3.95 Ga [ Komiya et al ., ].…”