Reappraisal of the Blake River Group stratigraphy and its place in the Archean volcanic record (French and English text)

The Archean Blake River Group is located at the southern limit of the Southern Volcanic Zone of the Abitibi Sub-province of the Superior Province of Canada. It is interpreted to be an incipient back-arc basin formed during a circa 2700 Ma extension event recognized in the area.; The Blake River Group is subdivided into three sub-groups based on lithological and geochemical variations. The Noranda sub-group is composed of interlayered andesites and rhyolites. Andesites dominate the Misema sub-group; there are few rhyolites. The Rouyn-Pelletier sub-group is characterized by basalts with rare andesites and rhyolites.; The stratigraphic relationship between the three sub-groups is established using a key-bed, the highly spherulitic Fish-roe rhyolite. This unit is defined by its crystallization spherulites, and distinct geochemistry compared to the general Blake River Group rhyolite data. Statistical analysis of the geochemical data for the Blake River Group rhyolites supports the conclusion that the highly spherulitic rhyolites constitute a single population that is distinct from the general Blake River Group rhyolite population. This suggests that the Fish-roe spherulitic rhyolites are a single unit.; The presence of the Fish-roe rhyolite in all three of the sub-groups suggests that the sub-groups are, at least in part, contemporaneous, and that the Blake River Group is the product of complex interaction between magmatic systems rather than of the evolution of one system. This interpretation, that the lithological and geochemical variations are lateral within a single basin, indicates along-ridge variations with a system peripheral to the main ridge. The along-ridge systems are the Misema and Noranda sub-groups, the Rouyn-Pelletier sub-group being the peripheral system. The Misema is the more mature part of the basin where rhyolitic volcanism has subsided.; Petrogenetic modelling of the Noranda sub-group lavas shows that the transitional andesites may be produced by binary mixing of the tholeiitic and LREE-enriched magmas. Crystallization modelling shows that the transitional lava models, with and without rhyolite contamination, and the tholeiitic model with rhyolite contamination produce evolutionary trends that approximate the trends of the actual data.; Of the modern geotectonic analogues suggested by the lithological and geochemical variations in the Blake River Group, and the geographic distribution of the subgroups, the nascent back-arc basin environment best fits the observed characteristics. Such basins contain basalts, andesites and rhyodacites, the felsic rocks being produced at the propagating tip of the ridge. (Abstract shortened by UMI.)...