TERMINAL STAGES OF A DYING RIFT

The terminal stages of continental rifts are rarely preserved due to subsequent erosion and sedimentation. Consequently, most models of rift development focus on modern rift environments such as East Africa. Such approaches do not incorporate the potentially rich record of rift evolution preserved within failed continental rifts, where late stage dissection and erosion have been minimized. Among the best-preserved failed rifts is the 1.1 Ga Midcontinent Rift (MCR) in North America. Intensive parameters of melt generation and magma-lithosphere interaction are recorded within the volcanic sequences in a rift, and thus provide a window into rifting processes. Here we examine the final stages of magmatic activity within the Midcontinent Rift (ca. 1083 Ma), which are preserved on Michipicoten Island in NE Lake Superior. We examine whether this activity relates to late stage continental rifting prior to ocean basin formation, or processes associated with the failure of the rift. We present a geochemical and isotopic study of a well-preserved section of volcanic stratigraphy from the Michipicoten Island Formation. The composition of this formation ranges from basaltic andesite to rhyolite - a near continuous evolution trend that is abnormal both for the typically bimodal MCR and most other continental rifts. Trace element patterns show an enrichment in the highly incompatible elements and LREE, a pattern that has been commonly attributed to crustal contamination. However, unlike other evolved magma suites in the MCR, the isotopic results of the Michipicoten Island Formation show that most samples have near depleted mantle to primitive ε_Nd values (-0.5 - +6.5) and ε_Hf of (-1.5 - +4.0). We interpret these data as reflecting melt generation from both a depleted mantle source and a more primitive source. These observations are inconsistent with a significant role for continental crust in the petrogenesis of these rocks. We explore the implications of these data in light of continuing geophysical studies of lithospheric structure in this region.