Mantle Sources and Tectono-Magmatic Evolution of the Keweenawan Mid-Continent Rift

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The volcanic rocks associated with the Keweenawan Midcontinent Rift of the Lake Superior region afford an excellent opportunity to study the mantle sources of picritic and basaltic primitive magmas. From previous work the PIs have acquired major and trace element data on 350 consecutive lava flows comprising a 5,350 meter continuous section of the Mamainse Point volcanics of Ontario. This section records almost the entire history of the aborted rifting episode from initial subsidence through nearly to the cessation of volcanism. The data reveal numerous stratigraphically-constrained, chemically distinct groups of lavas. There are also limited data on an additional 1200 meter volcanic section at Alona Bay. Trace element characteristics of the Mamainse Point groups indicate the presence of heterogeneity and variable enrichment in the mantle source of the basalts. Crustal contamination is indicated locally, but does not appear to be pervasive. Because of the amount of data, superb stratigraphic control, continuous section, and near completeness of record, these lavas should provide invaluable evidence about the nature, heterogeneity, and structure of the Middle Proterozoic mantle. In order to be conclusive, however, present data need to be augmented by radiogenic isotope studies and crucial trace elements. Selected critical samples representative of the groups will be analyzed for Sm-Nd, Rb-Sr, and U-Th-Pb isotopic systematics and important trace elements (e.g., Th, Ta, Hf). Specifically, the the isotopic systematics and trace elements will be used to test ideas about: 1) the presence and character of old, enriched lithosphere beneath the Archean craton, 2) the nature of the underlying asthenophere (N- MORB-like or enriched?), 3) the presence of OIB-like melts near the top of the section, 4) the extent and character of crustal contamination, 5) the formation and evolution of the Midcontinent Rift.