The Lac Guyer greenstone belt in the James Bay region of New Quebec was a zone of active volcanism in Archean times and constituted part of a system of "rifts" known as the La Grande superbelt. The rifting of continental crust initiated with the deposition of a sequence of volcanics dominated by pillowed basalts. An overlying volcanic sequence began with the deposition of rhyodacites and other felsic volcaniclastics, intercalated with mafic flows and tuffs. These, in turn, are overlain by peridotitic and pillowed pyroxenitic komatiite flows and then by pillowed basalts. The rocks in this Archean belt have experienced at least two periods of deformation and attained amphibolite facies. / The early voluminous basalts of the first sequence probably reflect the eruption of steady state liquids from periodically replenished, periodically tapped, magma chambers located near the interface of an early crust and Archean mantle. With time the mafic crust overlying these magma chambers, melted to produce tonalitic-granodioritic magmas. These magmas rose and experienced amphibole fractionation. Residual liquids produced by this fractionation erupted as rhyodacites contemporanously with later basalts. The eruption of komatiitic liquids indicates a major failure of the crustal screen enabling primitive magmas to reach the surface. The spectrum of primitive komatiitic compositions reflects variable degrees (15 to 40%) of partial melting of a garnet lherzolite in the Archean mantle. Pyroxenitic komatiite magmas represent the least degree of melting with garnet remaining in the mantle residue. Garnet was completely consumed, however, in the more extensive melting involved in the production of the peridotitic komatiite magmas. As the volcanic pile of the second volcanic sequence grew crustal magma reservoirs were re-established. Primitive komatiitic magmas held in these reservoirs underwent extensive crystal fractionation to yield residual steady state liquids which erupted as the upper basalts of the second volcanic sequence.

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