Timing and geochemistry of 1.88 Ga Molson Igneous Events, Manitoba: Insights into the formation of a craton-scale magmatic and metallogenic province

Abstract

The timing and origin of Paleoproterozoic mafic and ultramafic magmatism in the NW Superior craton is important for constraining the tectonic evolution of this margin during formation and closure of the 2.1–1.8 Ga Manikewan Ocean. An important component of this magmatism is the 1.88 Ga Molson Igneous Events, which comprise the Molson dyke swarm, various ultramafic sills and mafic volcanic rocks located in both the Fox River and Thompson Nickel belts, northern Manitoba. Many Molson dykes have relatively flat primitive mantle-normalized patterns, overlapping the compositions of coeval mafic/ultramafic volcanic rocks and sills in northern Manitoba, supporting a common origin. Six new U–Pb zircon ages were obtained for mafic and ultramafic units and one monzogranite from both within and external to the TNB. Within the TNB the 1876.7 ± 5.1 Ma Paint Lake pyroxenite sill provides further evidence that at least some ultramafic magmatism associated with nickel mineralization is linked to Molson Igneous Events. A deformed monzogranite unit encountered in a drill hole located at Mystery Lake East has a U–Pb zircon age of 1874.6 ± 3.4 Ma and provides further evidence for the generation of contemporaneous felsic and ultramafic magmas along the Superior margin. External to the TNB, magmatism linked to Molson Igneous Events has been confirmed within the TNB transition zone (1885.2 ± 2.1 Ma Bear Island dyke, Sipiwesk Lake) and up to 100 km east of the TNB, for example the 1900.0 ± 5.8 Ma Wakehao Lake and 1884.5 ± 3.8 Ma Molson Lake dykes. Molson Igneous Events span a period of approximately 25 m.y. but the majority of magmatism is short-lived; emplaced within the narrow time span of about 8 m.y. between 1885 and 1877 Ma. The tectonic setting of Molson Igneous Events is intriguing as it occurs at a convergent margin during initiation of oceanic arc formation and closure of the Manikewan Ocean. This magmatism is relatively short-lived (<20 m.y.), has a dominantly MORB-like chemical and neodymium isotopic composition, and can be traced into the highly deformed Thompson Nickel Belt where it forms sediment–sill complexes associated with Ni–Cu mineralization. The majority of the northern Superior craton margin, including the Thompson Nickel Belt, Fox River Belt, Cape Smith Belt and the New Québec Orogen, share a strikingly similar Paleoproterozoic evolution with two periods of mafic/ultramafic magmatism at ∼2.2–2.1 and 1.90–1.87 Ga, similarity in basalt geochemistry, and the development of ∼1.88 Ga fine-grained sediment–sill complexes that are host to significant nickel mineralization. The similarity in the timing and geochemistry of the 1.88 Ga mafic/ultramafic magmatism and its location along the western, northern and eastern Superior Province margin, extending for more than 3000 km, is interpreted to indicate Molson Igneous Events are linked to a unique and widespread Paleoproterozoic depleted mantle melting phenomena. Possible tectonic settings include development of a series of contemporaneous continental back-arcs during convergence and subduction of Manikewan oceanic crust or more likely passive upwelling of asthenosphere focused along the thinned margin of the Superior craton.