The intrusive rocks of the Nipigon Embayment comprise a series of four mafic to ultramafic intrusions and a number of laterally extensive diabase sills that are among the oldest expression of the ∼1.1 Ga Mesoproterozoic Mid-continent Rift. New geochemical data indicate that the sills can be subdivided into five distinct groups: three mafic sills (Nipigon, Inspiration, and McIntyre diabase sills), with the Nipigon sills forming the bulk of the outcrop, and two spatially restricted ultramafic to mafic sills (Jackfish and Shillabeer sills). The latter mafic sills are typically massive, medium-grained, intergranular textured gabbros ranging in thickness from a few metres to more than 250 m. Two of the ultramafic intrusions included in this study (Disraeli and Hele) consist of a core of pyroxene peridotite with olivine gabbro along the margins. The geochemical characteristics of the ultramafic intrusions and diabase sills are consistent with plume-derived melts that have undergone subsequent fractionation and been contaminated by continental crust, likely at depth, but a few samples from the Hele and Disraeli intrusions have the characteristics of primary, uncontaminated melts that have been rapidly transported through the lithosphere with little interaction with wall rocks. The field and geochemical characteristics of the intrusions and sills are consistent with the ultramafic intrusions having been emplaced before the diabase sills and indicate that the history of the Midcontinent Rift is more complex and protracted than previously recognized.

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