MAGMATIC PLUMBING DURING THE FINAL STAGES OF THE KEWEENAWAN LARGE IGNEOUS PROVINCE: EVIDENCE FROM THE MICHIPICOTEN ISLAND FORMATION

The volcanic products erupted during the terminal phases of large igneous provinces are rarely preserved, leaving ambiguity about the magmatic processes that operate during the shutdown of magmatism. Due to the failure and inversion of the North American Mid-Continent Rift, the 1.1 Ga Keweenawan large igneous province (LIP) is widely preserved and accessible, including its latest stages. Lavas exposed at Michipicoten Island in Lake Superior provide a record of such late-stage magmatic processes. Elsewhere in the Keweenawan LIP (e.g. Lake Shore Traps), volcanics from this terminal period show evidence for crustal contamination. At Michipicoten Island, by contrast, such contamination is largely absent. This unexpected difference points to heterogeneity in the magmatic differentiation processes operating during the late stages of the Keweenawan LIP. To further probe these magmatic processes, we present petrographic and laser ablation ICP-MS major and trace element data on mineral phases and cumulates from Michipicoten Island lavas. We report that Michipicoten Island volcanics show clinopyroxene-plagioclase-oxide cumulate glomerocrysts that are large and abundant in the lowest stratigraphic units, decrease in size and abundance towards younger units and are largely absent within the upper lava flows. We explore these petrographic results in the context of new in-situ mineral trace element data. These data will provide constraints as to the nature and number of magmas, and constraints as to where these magmas stalled within the crust. This contribution is more broadly applicable in aiding our understanding of magmatic processes that drive the terminal stages of LIPs.