While most continental rifts progress toward rupture and eventual oceanic spreading, in certain circumstances, the rift may fail. Failed rifts provide a window into the transition from continental rifting to the formation of a passive margin, which in successful rifts is occluded by thick post rift sedimentary packages. Among the best-preserved failed rifts is the 1.1 Ga Midcontinent Rift (MCR) in North America. Within the MCR, the final stage of magmatism is preserved on Michipicoten Island. Here I present a geochemical and isotopic study of the Michipicoten Island Formation to probe conditions in the crust and mantle during the final stage. My results show that the volcanic units on Michipicoten Island have undergone magma mixing between rhyolitic and basaltic magma, dominating magmatic processes within the crust. During previous eruptive periods in the MCR, during which this observation has been made, the rhyolitic endmember has been interpreted to have experienced significant contribution from the Achaean crust based on profoundly unradiogenic Nd values. In contrast, the Michipicoten Island Formation exhibits some of the most radiogenic Ndi values in the MCR, indicating that the source of evolved melts in the MCR changes from melting of existing Archean crust to juvenile material. The isotopic data from Michipicoten Island also shows that the depleted mantle is the single largest contributor of any geochemical reservoir. In the absence of a strong thermal plume component, melting of the depleted mantle requires decompression. The Michipicoten Island Formation was erupted during the geophysically-defined post-rift phase. My results require plate thinning to have continued during this late stage, with the implication that plate deformation persisted even though the crustal structure may not have recorded this deformation.
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