Age and origin of the Sept Iles anorthosite complex, Quebec

Rb-Sr studies of the Sept Iles anorthosite complex show that it was emplaced during the Cambrian and that it is unrelated to the Grenville province. The age of this complex is very similar to that of other igneous rocks associated with the St. Lawrence graben system. Both the acidic and basic components of the complex have similar ages and originated from magmas with similar, low('87) Sr/('86) Sr initial ratios. There is no evidence that crustal contamination played an important role in the generation of the acidic magmas. The earliest rocks of the complex exposed at Sept Iles are now observed on the mainland. They consist of cumulates rich in oxide minerals, mafic minerals and plagioclase. These rocks are thought to have been formed at the base of the complex. Further south, across the fault that defines the St. Lawrence graben in this area, the top of the complex is exposed on six islands and Presque-Ile Marconi. This part of the complex is thought to have formed by multiple injection of the magma differentiating at depth in the main chamber into the already solidified roof of the complex. Plagioclase cumulates, the earliest rocks exposed here, are cut by later, more differentiated gabbros. Syenite commonly overlies the basic rocks and is in turn overlain by granite. Geochemical and petrographic evidence indicate that the syenite was intruded as a magma and was not formed by accumulation of perthite precipitated from the granite. Geochemical studies show that all the rocks exposed at Sept Iles could have been formed by differentiation of a single gabbroic magma