EMPLACEMENT AND DIFFERENTIATION OF THE BLACK STURGEON SILL, NIPIGON, ONTARIO

The Black Sturgeon sill, part of the 1.1 Ga Midcontinent Rift, is a 250 m thick olivine diabase intrusion located southwest of Lake Nipigon. It consists of four zones: Zone I (0-55 m) contains the lower chilled margin and ophitic olivine diabase; Zone II (55-119.5 m) contains coarse-grained, intergranular, olivine-free diabase; Zone III (119.5-201.2) contains subophitic olivine-rich diabase; Zone IV contains diabase with abundant lenses and patches of granophyric micropegmatite. In this study, the processes responsible for the heterogeneity of the Black Sturgeon sill are investigated using principal component analysis.

Over half of the compositional variation encountered in the sill can be explained by the first principal component, which is interpreted as a balance between primitive compositions with abundant plagioclase and olivine, and enriched compositions consistent with MELTS simulations of fractionation of a liquid with a chilled margin composition. Roughly one quarter of the compositional variation in the sill can be explained by the second principal component, which is interpreted as a balance between gabbroic compositions with abundant augite and plagioclase, and iron-rich compositions. A final 15% of the compositional variation can be explained by the third principal component, which is interpreted as a balance between felsic and mafic minerals. Based on the variations in the first three principle components, Zones I–III are interpreted as separate magma pulses, with differing crystal cargos, while Zone IV is attributed to localized redistribution of evolved interstitial liquids.

The presence of three distinct magma types in the Black Sturgeon sill can be used to interpret source conditions during Midcontinent Rift magmatism. The results of this study suggest that the Black Sturgeon sill, and by extension much if not all Midcontinent Rift magmatism, was tapping a mush column. Such a source would allow for the successive extraction and emplacement of compositionally distinct magmas into a single chamber.