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

合作研究：限制美国西北部科迪勒拉元古代变质作用的时间和性质

• 批准号: 0855392
• 负责人: William McClelland
• 金额: $ 4.56万
• 依托单位: University of Iowa
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-07-01 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0855392&HistoricalAwards=false
• 关键词: Collaborative; Research; Constraining; Timing; Nature

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.

该项目的目标是检验以下假设：来自美国西北部科迪勒拉山脉的含石榴石角闪岩和泥质片岩保留了中元古代晚期高压变质作用的记录，该记录可归因于劳伦西亚西缘在超大陆罗迪尼亚的集合。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年代学和变质分析，以便：1）利用石榴石的 Lu-Hf 和 Sm-Nd 地质年代学以及独居石的 U-Th-Pb 地质年代学，以确定整个美国科迪勒拉山脉西北部的元古代变质作用； 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天文仪的结果，并使用详细的岩相学将这些信息与变质织物相结合； 4）将地质年代学和压力-温度信息与岩石中保存的渗透变形结构联系起来。中元古代晚期变质作用与这些岩石中大面积存在的高温高压组合之间的联系的建立将表明，在汇聚构造作用期间，地壳增厚的一个主要时期发生在劳伦西亚西缘。中元古代。如果美国西北部存在一条大约有十亿年历史的收缩带得到证实，则意味着科迪勒拉山脉的变质作用和变形历史比之前认为的更为复杂。这些结果将为晚中元古代构造重建提供重要的突破点。特别是，该项目的结果将为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约束，并最终影响罗迪尼亚的各种重建。格伦维尔造山运动是一个重要事件，极大地改变了大约十亿年前的北美大陆。这次事件产生的变质岩可以在从加拿大东部延伸到美国西南部和墨西哥北部的宽带中发现。初步地质年代学数据得出了意想不到的结果，即该年龄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山脉形成过程中发生的变质作用中通常产生的矿物的年龄。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。美国西北部十亿年前变质事件的证据也需要修改当前关于当时大陆构造的想法。