The Lac Guyer greenstone belt was one of a series of volcanic troughs active during the Archean in the James Bay territory of the Superior Province of Quebec. The belt consists of a succession of isoclinally folded volcanic rocks comprising a lower sequence of basalts overlain by felsic tuffs and rhyodacites that are in turn succeeded by an upper sequence of basalt and komatiite. Plutons of granodioritic composition syntectonically intrude the volcanic succession. The development of this volcanic succession can be interpreted in terms of a model involving an intimate interaction between a differentiated crust and Mg-rich magmas rising from the mantle. Although some of these magmas reached the surface to erupt as komatiites, the majority were trapped at the base of the crust and fractionated towards basaltic compositions. This process caused partial melting of the base of the crust, which was probably mafic in composition, and produced granodioritic magmas whose derivative liquids erupted as rhyodacites.