Collaborative Research: The Building of a Continent: The Archean to Proterozoic Growth and Modification of Western Laurentia

合作研究：大陆的形成：太古代到元古代的生长和西劳伦大陆的改造

• 批准号: 1427473
• 负责人: Jeffrey Vervoort
• 金额: $ 20.1万
• 依托单位: Washington State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-09-01 至 2019-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1427473&HistoricalAwards=false
• 关键词: Collaborative; Research; Building; Continent; Archean

The formation, growth, and modification of the continents is a fundamental topic in the geosciences. The key to understanding these processes lies in the rocks that form the "basement" of the continents. These rocks formed earlier in the history of the Earth and provide the foundation on which the continents were built. The age of these basement continental rocks, how they formed, and how they have been amalgamated and modified to form the foundation of the present day continents, however, is far from clear. The main reason for this is that these basement rocks are often buried under younger rocks or sediments and consequently are not exposed at the surface of the Earth. Where basement exposures do exist--at the cores of many continents and in isolated blocks that have been tectonically brought to the surface--these exposures provide critical pieces of the puzzle of the growth, evolution, and modification of the continents. One such area of basement rocks is the Clearwater complex, a nearly continuous block of crystalline basement exposed across a 115-km-long region of north central Idaho. Recent U-Pb zircon dating of rocks in the Clearwater complex by the PIs and their students have identified discrete periods of crust formation at 2.66 and 1.86 Ga and garnet Lu-Hf and monazite U-Th-Pb geochronology have demonstrated a complex metamorphic history from ~ 1.3 to 1.0 Ga. These new data and the fact that this area represents the largest belt of Neoarchean and Paleoproterozoic crust exposed in the Northwest U.S. outside of the Wyoming Province, that has been largely unexplored, make it a compelling area for further research.The primary goal of this proposed research is to use the Clearwater complex to examine three important transitions (2.7-2.6 Ga, 1.9-1.7 Ga, and 1.3-1.1 Ga) in Earth?s history from the Neoarchean to the Mesoproterozoic. This work will be accomplished through two principal tasks. First, we will examine the magmatic record preserved in orthogneisses to document the change in the nature of crust formation in this region from 2.6 to 1.8 Ga, which is preserved in the zircon U-Pb and Hf, and whole-rock Hf-Nd isotopic records. Second, we will examine the metamorphic record preserved in spatially associated metasedimentary rocks in order to document the timing and crustal conditions during the subsequent modification of this region during the Mesoproterozoic. The tools for this part of our work will be P-T phase equilibria studies that are integrated with monazite U-Th-Pb and garnet Lu-Hf and Sm-Nd geochronology. The detailed chronology obtained on these rocks will be accomplished by analysis of monazite inclusions in situ within garnet and subsequent dating of the same garnet by Lu-Hf and Sm-Nd methods. This integrated approach will allow insight into the nature of the coupling of these chronometers during regional metamorphism that will be of broad use to the metamorphic community.

大陆的形成、增长和改造是地球科学的一个基本课题。理解这些过程的关键在于形成大陆“基底”的岩石。 这些岩石形成于地球历史的早期，为大陆的形成提供了基础。 然而，这些基底大陆岩石的年龄、它们是如何形成的，以及它们是如何合并和改变形成当今大陆的基础的，还远不清楚。 其主要原因是这些基底岩石通常埋藏在较年轻的岩石或沉积物之下，因此不会暴露在地球表面。 在确实存在基底暴露的地方——在许多大陆的核心和在构造上被带到地表的孤立块中——这些暴露提供了大陆生长、演化和改造之谜的关键部分。 克利尔沃特复合体就是这样的基底岩石区域之一，它是一块几乎连续的结晶基底，暴露在爱达荷州中北部 115 公里长的地区。最近，PI 及其学生对克利尔沃特杂岩中的岩石进行了 U-Pb 锆石测年，确定了 2.66 和 1.86 Ga 处的地壳形成的离散时期，石榴石 Lu-Hf 和独居石 U-Th-Pb 地质年代学证明了复杂的变质历史~ 1.3 至 1.0 Ga。这些新数据以及该区域代表了暴露在美国西北部怀俄明省以外的地区大部分尚未勘探，因此成为进一步研究的引人注目的地区。这项拟议研究的主要目标是利用克利尔沃特复合体来检查三个重要的转变（2.7-2.6 Ga、1.9-1.7 Ga，以及1.3-1.1 Ga）在从新太古代到中元古代的地球历史中。这项工作将通过两项主要任务来完成。首先，我们将检查正片麻岩中保存的岩浆记录，记录该地区地壳形成性质从 2.6 Ga 到 1.8 Ga 的变化，这些记录保存在锆石 U-Pb 和 Hf 以及全岩 Hf-Nd 同位素中记录。其次，我们将检查空间相关的变质沉积岩中保存的变质记录，以记录中元古代该地区随后变化的时间和地壳条件。我们这部分工作的工具将是与独居石 U-Th-Pb 和石榴石 Lu-Hf 和 Sm-Nd 地质年代学相结合的 P-T 相平衡研究。在这些岩石上获得的详细年代学将通过对石榴石内的独居石包裹体进行原位分析以及随后使用 Lu-Hf 和 Sm-Nd 方法对同一石榴石进行测年来完成。这种综合方法将有助于深入了解这些天文钟在区域变质作用过程中耦合的本质，这将对变质社区具有广泛的用途。