Abstract

The Spi Group is a package of plagioclase porphyritic, amygdaloidal basalt flows and sandstone (Spi Lake Formation) conformably overlain by coarse conglomerate and sandstone (Old Boot Formation). These unconformably overlie the Kaminak Lake segment of the Central Hearne supracrustal belt, western Churchill Province. The Spi Group developed in a small, isolated basin (8 km2) and is older than the Paleoproterozoic Hurwitz Group. Our data demonstrate that the basalts of the Spi Lake Formation are genetically related to the ca. 2450 Ma Kaminak dyke swarm, a speculation previously promoted by some workers based on field characteristics alone. They are continental tholeiites with large ion lithophile and light rare-earth element enriched multielement profiles with prominent high field-strength element troughs. The 143Nd/144Nd compositions overlap the contemporaneous chondritic bulk-earth data. All of these are features we infer to have resulted from mixing of melts from dominantly depleted mid-ocean ridge basalt (MORB) mantle with low-degree partial melts of metasomatically enriched sub-continental lithospheric mantle. These composite magmas then underwent clinopyroxene + plagioclase ± olivine fractionation with minor assimilation of local tonalitic middle crust. The Spi Lake basin is <1 km wide, trends 020° parallel to Kaminak dykes but is discordant to the more common 045° to 070° trends of fold keels of the Hurwitz Group. The basin is a graben filled first with basalt and then molassoid sediments, representing the expression of crustal extension associated with Kaminak dyke emplacement. This indicates that the present erosional surface in the region is broadly at the same level as at the time of both Hurwitz and Spi Group deposition.

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