カナダラブラドル地方、ネーン岩体の地質：最古の海洋地殻の発見に向け

The Hadean is the most mysterious period because no rocks and geologic bodies are preserved except for only the zircons in\nWestern Australia, Canada, China and Greenland (Froude et al., 1983, Nature; Nelson et al., 2000, EPSL; Mojzsis & Harrison,\n2002 EPSL; Iizuka et al., 2006, Geology;Wang et al., 2007, CSB). But, it is the most important period because the early evolution\npossibly clinched the history of the earth. We try to find the earliest supracrustal rocks in the world to investigate the Hadean\ntectonics and decode surface environments. As far, the oldest supracrustal rocks are found in Akilia association in West Greenland,\nNuvvuagittuq in Quebec, and Nain Complex in Labrador (Nutman et al., 1996, Precamb. Res.; O’Neil et al., 2008, Science;\nSchiotte et al., 1989, Can Jour Earth Sci.). Because the Akilia association suffers from severe metamorphism and alteration, the\nprecursors are highly in debate (e.g. Fedo & Whitehouse, 2002, Science). Recent geological works in the Nuvvuagittuq, Quebec\nshowed the sequence contains amphibolite with a pyroclastic rocks affinity, ultramafic sills, felsic sediment, BIF and conglomerate.\nAlthough a pseudoisochron age of 147Sm/144Nd-142Nd/144Nd implies the Hadean age (O’Neil et al., 2008, Science), the\nsupracrustal belt possesses 3811 Ma by conventional U-Pb zircon ages (David et al., 2009, GSAB).\nWe made geological survey in the Nain Complex, and reinvestigated the occurrence of the supracrustal rocks and their relationship\nwith the ambient orthogneisses. Previous works focused on distribution of the supracrustal belts within the orthogneisses\n(e.g. Bridgwater et al., 1974 Geol Surv Canada, Paper), but the detailed field occurrence of the supracrustal rocks within the belts\nis still ambiguous. Therefore, we focus on their internal structures.\nThe supracrustal belts are repeatedly intruded by granitic intrusions with some ages and their original structures are obscured,\nbut their lithostratigraphies are relatively well preserved in Nulliak, Big and Shuldham islands and St Jones Harbor.\nThe supracrustal belts in Nulliak and Big islands comprise ultramafic rocks, mafic rocks and mafic sediments intercalated with\nfeldspathic sediments and banded iron formations in ascending order. In the St Jones Harbor, it is composed of ultramafic rocks,\nmafic rocks, banded iron formation, and clastic sediments, intercalated with chert in the middle and with bedded carbonate rocks\nin the upper part, respectively, in ascending order. In the Shuldham Island, it consists of ultramafic rocks, layered gabbro with\nprecursors of plagioclase and pyroxene accumulation layers, mafic rocks and terrigenous sediments in ascending order. The\nlithostratigraphies are very similar to oceanic plate stratigraphy. The fact that some supracrustal belts are intruded by Uivak I\northogneisses, and presence of >3.86 Ga zircons in the supracrustal rocks suggest that the supracrustal belts have early Archean\nages. In addition, despite of the still ambiguous relationship between Nanok Gneiss and supracrustal rocks, presence of Nanok\nGneiss (3.85 to 3.91 Ga) in this area (Collerson, 1983 in Abstracts for Early Crustal Genesis Field Workshop, LPI, Technical\nReport 83-03; Shimojo et al., 2012, Goldschmidt Conf.) implies that the supracrustal belts date back to the earliest Archean.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.