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

与基韦纳万中大陆有关的火山岩 苏必利尔湖上级地区的裂谷提供了一个极好的机会 研究苦橄质和玄武质原始岩石的地幔来源 岩浆 从以前的工作中，PI已经获得了主要和痕量 350个连续熔岩流的元素数据， Mamainse Point火山岩的连续部分， 安大略。 这一部分几乎记录了整个历史 从初始沉降到近 火山活动的停止 数据显示， 受地层限制，化学性质不同的 熔岩 另外1200米的数据也很有限 阿罗纳湾的火山区 微量元素特征 Mamainse点群表明存在异质性 地幔源区玄武岩的富集程度不同。 地壳污染局部显示，但没有出现 to be pervasive普遍. 由于数据量大， 地层控制、连续剖面、接近完整 这些熔岩应该提供了宝贵的证据 中部地区的性质、异质性和结构 元古宙地幔 然而，为了得出结论，目前 数据需要通过放射性同位素研究来补充， 关键的微量元素 选定的关键样品 将分析各组代表的Sm-Nd，Rb-Sr， 和U-Th-Pb同位素系统学及重要微量元素 (e.g., Th、Ta、Hf）。 特别是同位素系统学 和微量元素将被用来测试的想法：1） 存在和性质的老，丰富的岩石圈下， 太古宙克拉通; 2）下伏软陶层（N- MORB样或富集的？），3)OIB类熔体的存在， 剖面顶部; 4）地壳活动的范围和性质 污染，5）的形成和演变 中大陆裂谷。

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.