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2.7 Ga shoshonites occur in the Upper Keewatin Assemblage of the Shoal Lake-Lake of the Woods area in the northwestern part of the late Archean Wabigoon Belt, Superior Province, Canada. They are enriched both in large-ion lithophile elements (K, Rb, Sr, Ba and Th) and compatible elements (Mg, Ni and Cr). Their incompatible element abundance patterns exhibit a negative Ta anomaly and high La/Yb and Sr/Y ratios, similar to adakitic volcanic rocks in the same area. We suggest that the shoshonite magmas were formed by partial melting of a mantle source enriched by contemporaneous adakitic magmas derived by melting of subducted oceanic crust. We further speculate on the origin of Timiskaming alkaline volcanic rocks, located 1100 km to the east in the Abitibi Greenstone Belt, Superior Province, the archetypal example of late Archean alkaline magmatism. It seems probable that the mantle sources of Timiskaming alkaline rocks also were metasomatized by adakitic magmas, but that this source enrichment occurred 20-30 m.y. prior to the Timiskaming magmatism.

Geochemistry of shoshonites from the Upper Keewatin Assemblage, western Wabigoon Belt, Superior Province, Canada.