Detailed study of the basal 500 m of the Keweenawan Mamainse Point volcanics in Ontario, Canada, has revealed that the section is dominated by basaltic to picritic lavas with up to 23 wt% MgO. Two stratigraphic suites with distinct chemical characteristics are separated by four flows of high-TiO2 ferrobasalt containing large and abundant plagioclase spherulites. The focus of this study is on the lower suite that contains the most MgO-rich compositions and that generally has lower TiO2 content. Although most of the flows are olivine-phyric, the olivine phenocrysts are skeletal, suggesting in situ crystallization from magmas with high normative-olivine content. Furthermore, the compositional variation of the lavas does not fall along an olivine-only control line. Taken together, the data indicate that the compositional variation of the lavas represents fractionation of high-MgO liquids rather than accumulation of olivine in basaltic liquids. Most of the fractionation apparently involved crystallization of olivine, clinopyroxene, and Cr-spinel, but later fractionation involved olivine, clinopyroxene, and plagioclase. The occurrence of picritic lavas low in the overall volcanic section is similar to relations in other flood-basalt provinces, and should eventually yield important tectonic and magmatic constraints for the development of the Keweenawan midcontinent rift of North America.

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