1.83 GA ULTRAPOTASSIC AND MAFIC MAGMATISM IN THE CHURCHILL PROVINCE: A TALE OF TWO SOURCES

The Christopher Island Formation (CIF), emplaced at ca. 1.83 Ga, is the world’s largest known suite of ultrapotassic rocks¹. The CIF occurs as minette dykes and flows in the eastern and north-central Churchill Province, extending east from Hudson Bay into central Nunavut over an area of 240,000 km². A coeval (1.83 to 1.82 Ga) series of mafic dyke swarms are present in the Churchill Province’s western margin, extending from Lake Athabasca, Saskatchewan (SK) to Great Slave Lake, Northwest Territories (NT)^2-4. This study examines the geochemistry of these dykes and compares their source to that inferred to have produced the CIF.

The mafic dykes on the western margin can be divided into two main groups, those from the NT and those sampled further east from near Uranium City, SK. The NT dykes are distinct from the CIF in terms of their higher TiO₂, and FeO contents, lower Mg#’s, K₂O and Th contents, and generally flatter REE profiles. The NT dykes also have less strongly negative εNd values (-2 to -6 vs. -6 to -10)¹. Interestingly, the SK dykes have elemental concentrations (e.g., Al₂O₃, TiO₂, K₂O, Mg#, Th) that generally fall between those of the NT dykes and the CIF. εNd values of the SK dykes (-4 to -6) are also transitional between these two magma suites.

Here, we argue that the source of the SK dykes, intermediate in geochemistry to the CIF and NT dykes, corresponds to the transition between a strongly metasomatized mantle in the eastern and central Churchill Province (CIF source), and a less metasomatized mantle in the west (source of the NT dykes with flat REE pattern). The enrichment of the CIF mantle source is thought to be due to Neoarchean metasomatism, from the dehydration of subducted oceanic crust¹. Thus, the SK dykes may mark the western edge of this subducted slab. The craton-wide thermal event that affected the Churchill Province at ca. 1.83 Ga tapped these contrasting mantle sources.

1) Cousens et al., 2004, The Precambrian Earth: 183—201; 2) Bostock & van Breemen, 1992, GSC Paper 92-2: 49–55; 3)Ewanchuk, 2006, U. of Alberta BSc thesis; 4) Morelli et al., 2009, Pre.Res. 175: 1–15; 5) Symons & Harris, 2005, CJES 42: 732—740.