[BCG07-P01] Geology of the Eoarchean Nuvvuagittuq supracrustal belt, northeastern Canada: Reappraisal of Protoliths and ages of the sedimentary rocks
Keywords:Hadean, Early Earth, Early life, Surface environment
The belt is intruded by some precursors of orthogneiss, whose igneous ages of ca. 3.8 Ga and constrain the minimum age of the belt. O'Neil et al. (2008, 2012) proposed that the igneous age of the fine-grained amphibolite is ca. 4.38 Ga based on a putative 147Sm/144Nd-142Nd/144Nd isochron age. On the other hand, Cates et al. (2013) proposed that a felsic layer within the faux-amphibolite originates from a clastic felsic sedimentary rock, and the maximum age of the belt is ca. 3.78 Ga based on ages of the zircons. David et al. (2009) interpreted a felsic layer as a volcanosedimentary rock, and proposed that the supracrustal rock was formd at 3817 Ma based on the ages of zircons.
We made a quite detailed geological map using a drone, which enables easily to trace even tens centimeter layers and intrusions. Many tens centimeter thick intrusions with an intermediate composition are present in the faux-amphibolite, whereas no intrusions are apparently found in the green-amphibolite. The distribution of the two types of amphibolite corresponds to that of three geochemical types of greenstone proposed by O'Neil et al. (2011), suggesting that the geochemical signature is due to secondary alteration. In addition, the apparent isochron on the 147Sm/144Nd-142Nd/144Nd diagram possibly originates from mixing between the mafic/ultramafic component and secondary intermediate component. O'Neil et al. (2008) proposed that the Nd isochron age of gabbroic dikes is 4.3 Ga. But, the gabboric dikes are younger than 3.8 Ga, and the isochron is putative because our map clearly shows that some thin gabbroic dikes are intruded into ambient orthogneiss, whose age is ca. 3.8 Ga. We found many conglomerate layers over the belt, but the layers finally change felsic intrusions. The occurrence means that they are putative conglomerate. In addition, the carbonate rocks occur along the felsic intrusions, suggesting they also have metasomatic origin. The petrograhy and occurrence of the BIF in the northwestern area indicate that the BIF was formed from metamorphism of the ultramafic rock. The siliceous rock is intruded by many felsic intrusions and quartz veins, but the detailed map can differentiate between the siliceous layers and intrusions, and shows that the siliceous layers originate from siliceous sediment.