Abstract

The development of the North American Midcontinent rift can be understood in the context of a tectonic and magmatic event in which the far-field effects of continental drift influenced magmatism produced by a mantle plume. We report and interpret new data for samples collected from stratigraphically controlled sequences of basaltic rocks in the Osler Volcanic Group and the Mamainse Point Volcanic Group in Ontario, Canada. We confirm that the earliest phase of rift development involved primitive melts that produced basalts in the Lower Formation of the Osler Volcanic Group with Fo74–80 olivine (1,490–2,280 ppm Ni). The earliest basalts are primitive in both the Mamainse Point Volcanic Group and Osler Volcanic Group sections, with elevated MgO, Cr, and Ni; these rocks also have high TiO2 and high Gd/Yb, which are features that indicate low degrees of melting of a garnet-bearing lherzolite mantle. The Central Formation A and B basalts of the Osler Volcanic Group that overlie the Lower Formation lavas record an upward increase and then decrease in the degree of contamination of basaltic rocks with normal Ni and platinum group element (PGE) abundance levels that require no sulfide saturation event to accompany contamination. The Upper Formation of the Osler Volcanic Group is characterized by a limited number of PGE-depleted flows; although these basalts are highly fractionated and typically develop textures with stellate to laminated plagioclase feldspar, they are usually not significantly contaminated. The Lower and Upper formations of the Mamainse Point Volcanic Group show a diversity in geochemistry consistent with contributions from different mantle sources, but contamination is restricted to a small number of flows with normal Ni and PGE abundance levels. There is no indication of a regional metal depletion signature on the scale of the Nadezhdinsky Formation at Noril’sk, but there is evidence that local S saturation events were triggered in the unexposed conduits feeding some of the upper sequence flows in response to fractionation of the magma.

Massive to heavy disseminated Ni-Cu-PGE sulfide mineralization in the Midcontinent rift is hosted by small primitive intrusions that were formed early in the evolution of the rift and are typically associated with transtensional structures in the cratonic blocks adjacent to the main rift. Examples include the Eagle deposit in Michigan, the Tamarack mineralization in Minnesota, and the Current Lake Complex in Ontario. The American examples are associated with splays of the Great Lakes tectonic zone and the Canadian example is associated with the Quetico fault zone. The intrusions share the geochemical fingerprints of the more primitive lowermost volcanic rocks of the Midcontinent rift and they are broadly contemporaneous with the development of the lower stratigraphy of the Osler Volcanic Group and Mamainse Point Volcanic Group. Although the low-grade but large-tonnage Cu-Ni-PGE deposits of the Duluth were formed significantly later than the small primitive intrusions, their host rocks share the same lithophile trace element signature as these early intrusions. The parental magmas that produced all of these intrusions were derived from deeper mantle depths, where garnet was a stable phase, than the magmas that produced the Central Formation basalts and some of the Upper Formation basalts investigated in this study.

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