Komatiite flooding of a rifted Archean rhyolitic arc complex: Geochemical signature and tectonic significance of the Stoughton-Roquemaure Group, Abitibi Greenstone Belt, Canada

The 0.2-2 km thick, Archean Stoughton-Roquemaure Group (SRG) in the Northern Volcanic Zone of the Abitibi greenstone belt (Quebec, Canada) is composed of tholeiitic basalt, komatiitic basalt and komatiite. The mafic and ultramafic rocks are pillowed, brecciated, and massive columnar-jointed flows. The SRG conformably overlies the 2730 Ma Hunter Mine Group, a volcanic complex dominated by calc-alkaline felsic rocks. The tholeiitic basalts of the SRG resemble MORB. The komatiitic basalts and komatiites have positive epsilon(Nd) values, overlapping those of the tholeiitic basalts. Komatiitic basalts, with low Al2O3/TiO2 ratios (similar to 10) and fractionated heavy REE patterns, are similar to Al-depleted komatiites. In contrast, the komatiites have high Al(2)O3/TiO2 (similar to 20), unfractionated heavy REE patterns and resemble Al-undepleted Munro-type komatiites. The Al-depleted komatiitic basalts occur at the base of the SRG, whereas the Al-undepleted komatiites are prevalent higher up in the stratigraphy. The association of calc-alkaline rhyolites with rifted arc-related basalts passing upward into MORB-like basalts, which in turn are capped by komatiitic rocks, reflects an evolution in magma genesis from crustal melting (rhyolites) and arc rifting to melting of a mantle plume. The preferred petrogenetic model for the SRG involves a rising mantle plume below an arc. The Al-depleted komatiitic basalts were generated by mantle melting with garnet in the residue at the periphery of the plume whereas the Al-undepleted komatiites were formed by a higher degree of melting in the plume axis. The MORB-like basalts were produced from the cooler plume head at a shallower depth.