The geology of Nuvvuagittuq greenstone belt and its implications for the early earth's evolution.

摘要

The Nuvvuagittuq greenstone belt is an Eoarchean/Hadean succession preserved along the eastern coast of Hudson Bay in the Northeastern Superior Province, Quebec, Canada. It is essentially composed of three major lithological units: (1) mafic cummingtonite-amphibolite, known as the "faux-amphibolite", which is the dominant lithology of the belt, (2) ultramafic and mafic sills that intrude the faux-amphibolite, and (3) chemical sedimentary rocks comprising a banded iron formation (BIF) and a silica-formation. The faux-amphibolite has a basaltic to andesitic composition. It is composed of variable proportions of cummingtonite + biotite + quartz, +/- plagioclase +/- garnet +/- anthophyllite +/- cordierite and ranges from cummingtonite amphibolite to garnet-biotite schist. Some rocks from the Nuvvuagittuq greenstone belt have lower 142Nd/144Nd ratios than the terrestrial standard (&egr142Nd = -0.07 to -0.15). The 142Nd/ 144Nd ratios for the faux-amphibolite correlate positively with their Sm/Nd ratios and produce a 146Sm-142Nd isochron with an age of 4280+53--81 million years. The 4.28 Ga faux-amphibolite is intruded by highly deformed gabbro sills yielding a 147Sm-143Nd isochron with a slope corresponding to an age of 4023 +/- 110 Ma. The Nuvvuagittuq greenstone belt also comprises 3.8 Ga felsic bands and is surrounded by 3.66 Ga tonalite interpreted to have been produced by the melting of the older mafic faux-amphibolite. The Nuvvuagittuq greenstone belt thus represents an Eoarchean/Hadean crustal section comprising the oldest rocks preserved on Earth. The 4.28 Ga faux-amphibolite is divided into a high-Ti group and a low-Ti group with co-genetic ultramafic sills and separated by a BIF/silica-formation. At the base of the sequence, the high-Ti faux-amphibolite is basaltic in composition and has "tholeitic" affinities. The low-Ti faux-amphibolite above the BIF exhibits a wider range of composition from basaltic to andesitic and shares geochemical characteristics with calc-alkaline volcanic suites. The high-Ti faux-amphibolite is consistent with crystal fractionation at low pressures, under dry conditions. In contrast, the low-Ti have high Al and low Ti for the andesitic range of compositions suggesting the early crystallization of Fe-Ti oxides and late appearance of plagioclase consistent with fractionation at elevated water pressures. The faux-amphibolite also includes cordierite-anthophyllite rocks having low Ca contents with low Na, low Sr and negative Eu anomalies. These cordierite-anthophyllite faux-amphibolites are interpreted to reflect intense weathering of their protolith perhaps by seawater in the Hadean. This is consistent with the interpretation of the faux-amphibolite being Hadean hydrothermally altered basalts transitioning from volcanics having tholeitic affinities to calc-alkaline affinities.