PETROLOGY OF THE GREAT ABITIBI DYKE, SUPERIOR PROVINCE, CANADA

The Great Abitibi Dyke (GAD) which can be traced northeast, for > 700 km, across the Abitibi Belt in the southeastern Superior Province of the Canadian Shield, is composed of olivine gabbro to monzodiorite, weakly saturated to undersaturated in silica. All rocks of the GAD can be derived by mainly plagioclase and olivine fractionation from a parental magma corresponding in composition to chilled margin samples. Two units can be distinguished, a marginal unit (Unit 1) representing 0-50% crystallization and a central unit (Unit 2), found over about half of the dyke length, representing 50-70% crystallization. Modelling, using Pearce elemental ratio analysis, quantifies the fractionation history and allows mass-balance calculations over the present exposure level of the dyke. The approximate balance between the amounts of cumulate and fractionated rocks suggests that Unit 1 differentiated essentially in situ as a closed system. In contrast, Unit 2 rocks were formed by loss of substantial plagioclase and olivine from the parental magma. This fractionate must have either been lost to depth or left behind in an external chamber. Unit 2 rocks show depletion in plagioclase and enrichment in mafic minerals along strike towards the southwest (deeper exposure level?), a trend explained by density stratification. Regional variation in Unit 1 chemistry is interpreted in terms of lateral magma injection towards the northeast from the locus of Keweenawan rift volcanism. Feldspar, olivine, and augite compositions are linearly correlated with equilibrium temperature and extent of magma evolution. The parent magma had a trace element chemistry corresponding to a 'within-plate' setting and was probably derived from an incompatible-element enriched mantle similar to the source for ocean island basalts (OIBs).