Metamorphism of the Canadian Shield, Ontario, Canada. I. the Superior Province

This paper is an outgrowth of the compilation of metamorphic information for the Canadian Shield in Ontario, and complements the recently compiled Metamorphic Map of the Canadian Shield. The paper contains a summary of the Archean metamorphic history of the Superior Province in Ontario on a subprovince basis, with an emphasis on the limitations of the existing data and the reasoning applied in extrapolating metamorphic boundaries. Little of the early metamorphic history of the Superior Province (pre-2715 Ma) is preserved, but there is local evidence for events at 2870–2850, 2810, and 2730 Ma within some of the older blocks of crust. The present distribution of metamorphic grade and age of metamorphism largely reflects pan-Superior events in the interval 2710–2640 Ma that occurred subsequent to coalescence of a system of island arcs, back-arcs, oceanic plateaus and microcontinents between 2720 and 2690 Ma. The distribution of metamorphic events and facies is the result of three interrelated patterns. 1) There is a relationship between subprovince type, metamorphic grade and age, with granite–greenstone subprovinces generally preserving older greenschistto lower-amphibolite-facies events, metasedimentary subprovinces preserving younger middle-amphiboliteto granulite-facies events, and high-grade gneiss subprovinces preserving the youngest events. 2) Discrete metamorphic episodes between 2720 and 2640 Ma are associated with periods of major plutonism. 3) There is a pattern of increasing complexity of metamorphic history with increasing metamorphic grade. The present distribution of metamorphic facies in the Superior Province also was influenced by uplift, tilting, and erosion during the Paleoproterozoic. The timing of lode-gold and rare-element-pegmatite mineralization within the Superior Province corresponds closely with metamorphic evolution; it is consistent with models whereby gold-bearing fluids and pegmatite-forming melts develop, in part, as a result of granulite-facies metamorphism of the lower crust.