Mafic volcanic rocks associated with Upper Proterozoic diamictites are widespread in western North America, and a continental rift setting has been proposed for their origin. We report trace-element and rare-earth-element geochemistry for these volcanics from northern Utah and southeastern Idaho. The analyzed samples are high-Ti within-plate basalts, and they vary from transitional tholelitic-alkaline in the south to alkaline in the north. The geochemistry, together with the geologic setting of the volcanics, indicates a rift setting, but the timing of this rifting and its relationship to final continental separation are still controversial.

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