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

与苏必利尔湖地区Keweenawan大陆中裂谷有关的火山岩为研究苦荞质和玄武质原始岩浆的地幔来源提供了很好的机会。从以前的工作中，pi已经获得了350个连续熔岩流的主要和微量元素数据，这些熔岩流包括安大略省Mamainse Point火山的5350米连续剖面。这一节几乎记录了中止裂谷期的整个历史，从最初的下沉一直到火山活动的停止。这些数据揭示了许多受地层限制的、化学性质不同的熔岩群。在阿罗纳湾另外1200米的火山剖面上也有有限的数据。马门塞点群微量元素特征表明玄武岩幔源存在非均质性和可变富集。地壳污染是局部的，但似乎不是普遍的。由于大量的资料、极好的地层控制、连续的剖面和近乎完整的记录，这些熔岩应该为中元古代地幔的性质、非均质性和结构提供宝贵的证据。然而，为了得出结论，目前的数据需要通过放射性成因同位素研究和关键微量元素加以补充。将对具有代表性的关键样品进行Sm-Nd， Rb-Sr和U-Th-Pb同位素系统和重要微量元素（如Th， Ta, Hf）的分析。具体而言，同位素系统和微量元素将用于测试以下想法：1)太古宙克拉通下古老、富集的岩石圈的存在和特征，2)下伏软流圈的性质（N- morb类或富集？），3)剖面顶部附近的obb类熔体的存在，4)地壳污染的程度和特征，5)中大陆裂谷的形成和演化。

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.