This thesis concerns the tectonic evolution of the La Grande greenstone belt and the neighbouring gneissic-granitic terrane in the James Bay area of central Quebec.Two sequences of basalt and an overlying komatiitic sequence have been recognized in the study area. The geochemical variation of the basalts is compatible with a crystal-fractionation controlled magma evolution. Crystallization calculation suggest that the evolution of the basaltic magmas may have been somewhere between fractional cristallization and equilibrum cristallization, with most olivine and some pyroxene crystals removed while most plagioclase and some pyroxene crystals remained in the magma. The observed abnormal behavior of Rb and K is interpretated to result from contamination by continental crust when the magmas passed through it. Despite the intimate spatial relationship of the komatiites to the basalts, and their similar trace element ration, the basalts cannot be derived directly from the komatiites by fractionation. They may, however, have been derived from the same mantle diapir at different depths during rifting. A comparisom with the geochemistry of basalts in other sections in the La Grande greenstone belt leeds to a westward-propagating rifting model for the tectonic evolution of the whole belt. Migmatization is widespread in the gneissic-granitic terrane. Mass balance calculations show that the low grade migmatite may have formed in an approximately closed sustem, controlled dominantly by partial melting. However, the generation of diatexite and anatectic granite involved significant mass transfer, particularly the gain of K and loss of Mg, Ca, Al and Na by the system. This may be due to the introduction of a metamorphic fluid, which may have been produced during crustal thickening following the rifting event.