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 No rthern Volcanic Zone of the Abitibi greenstone belt (Quebec, Canada) i s composed of tholeiitic basalt, komatiitic basalt and komatiite. The mafic and ultramafic rocks are pillowed, brecciated, and massive colum nar-jointed flows. The SRG conformably overlies the 2730 Ma Hunter Min e Group, a volcanic complex dominated by calc-alkaline felsic rocks. T he 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 ratio s (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 A l-undepleted Munro-type komatiites. The Al-depleted komatiitic basalts occur at the base of the SRG, whereas the Al-undepleted komatiites ar e prevalent higher up in the stratigraphy. The association of calc-alk aline rhyolites with rifted arc-related basalts passing upward into MO RB-like basalts, which in turn are capped by komatiitic rocks, reflect s an evolution in magma genesis from crustal melting (rhyolites) and a rc rifting to melting of a mantle plume. The preferred petrogenetic mo del for the SRG involves a rising mantle plume below an arc. The Al-de pleted komatiitic basalts were generated by mantle melting with garnet in the residue at the periphery of the plume whereas the Al-undeplete d komatiites were formed by a higher degree of melting in the plume ax is. The MORB-like basalts were produced from the cooler plume head at a shallower depth.