Collaborative Research: Constraining the Timing and Nature of Proterozoic Metamorphism in the Northwest U.S. Cordillera

该项目的目标是检验一种假设，即来自美国西北部科迪勒拉地区的含石榴石角闪岩和泥质片岩保存了中元古代晚期高压变质作用的记录，这可以归因于在罗迪尼亚超大陆组装之前或期间沿劳伦蒂亚西缘显着的地壳增厚。为此，来自华盛顿州立大学、西弗吉尼亚大学和爱达荷大学的一个研究小组正在对美国西北部科迪勒拉地区含石榴石的变质组合进行综合的年代学和变质分析，目的是：1)利用石榴石的Lu-Hf和Sm-ND年代学和独居石的U-Th-Pb年代学确定变质年龄，以确定整个美国西北部科迪勒拉地区元古代变质的程度；2)建立这些变质组合中矿物生长的压力-温度条件；3)比较石榴石和独居石的年代学结果，并利用详细的岩相学将这一信息与变质组构相结合；(4)将地质年代学和压力-温度信息与岩石中保存的侵入性变形组构联系起来。建立晚中元古代变质作用与相当大范围内这些岩石中存在的高温、高压组合之间的联系，将表明中元古代末劳伦蒂岛西缘发生了一个主要的会聚构造增厚时期。美国西北部存在一条大约10亿年的收缩带，如果得到证实，将意味着科迪勒拉的变质和变形历史比之前所认为的要复杂得多。这些结果将为中元古代晚期构造改造提供重要的切入点。特别是，该项目的结果将对西劳伦西亚与澳大利亚、西伯利亚、南极洲和中国等其他克拉通碎片的相关性提供额外的限制，并最终将影响罗迪尼的各种重建。格伦维尔造山运动是大约10亿年前大幅改变北美大陆的一次重要事件。该事件产生的变质岩分布在从加拿大东部到美国西南部和墨西哥北部的宽阔地带。初步的地质年代学数据得出了意想不到的结果，即这个时代的变质作用也发生在美国西北部。为了证实这些数据，该项目正在使用新的地质年代学方法来确定矿物的年龄，这些矿物通常是在山脉带形成期间发生的变质作用中产生的。如果初步结果出来，目前对格伦维尔事件性质的理解将需要修改。美国西北部发生的10亿年前的变质事件的证据也需要修改目前关于当时大陆构造的想法。

The goal of this project is to test the hypothesis that garnet-bearing amphibolites and pelitic schists from the northwest U.S. Cordillera preserve a record of late Mesoproterozoic high pressure metamorphism that can be attributed to significant crustal thickening along the western margin of Laurentia before or during the assembly of the supercontinent Rodinia. To this end a research team from Washington State University, West Virginia University, and University of Idaho are undertaking an integrated geochronologic and metamorphic analysis of garnet-bearing metamorphic assemblages in the northwest U.S. Cordillera in order to: 1) determine the ages of metamorphism using Lu-Hf and Sm-Nd geochronology of garnets and U-Th-Pb geochronology of monazites in order to determine the extent of Proterozoic metamorphism throughout the northwest U.S. Cordillera; 2) establish the pressure-temperature conditions of mineral growth in these metamorphic assemblages; 3) compare the results from the garnet and monazite chronometers and integrate this information with the metamorphic fabrics using detailed petrography; and 4) link both the geochronologic and pressure-temperature information to the penetrative deformational fabrics preserved in the rocks. Establishment of a linkage between late Mesoproterozoic metamorphism and the high temperature, high-pressure assemblages that exist in these rocks over a considerable area will show that a major period of crustal thickening during convergent tectonism occurred on the western margin of Laurentia at the end of the Mesoproterozoic. The presence of an approximately one billion year old contractional belt in the northwestern U.S., if confirmed, would imply that the history of metamorphism and deformation in the Cordillera is more complicated than previously appreciated. These results would provide important piercing points for late Mesoproterozoic tectonic reconstructions. In particular, results of this project will provide additional constraints on correlations of western Laurentia with other cratonal fragments such as Australia, Siberia, Antarctica, and China and will ultimately impact various reconstructions of Rodinia.The Grenville orogeny was an important event that substantially modified the North American continent approximately one billion years ago. The metamorphic rocks produced in this event can be found in wide belt that extends from eastern Canada to southwestern U.S. and northern Mexico. Preliminary geochronological data yielded the unexpected result that metamorphism of this age also occurred in the northwestern U.S. To confirm these data, this project is using new geochronological methods to determine the ages of minerals that are commonly produced during the metamorphism that occurs during mountain belt formation. If the preliminary results are born out, current understanding of the nature of the Grenville event will require revision. Evidence for a billion year old metamorphic event in the northwest U.S. would also require modification of current ideas about the configuration of continents at that time.