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

大陆的形成、生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。 这些岩石在地球历史的早期形成，并为大陆的建立提供了基础。 然而，这些基底大陆岩石的年龄，它们是如何形成的，以及它们是如何被合并和修改以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被构造带到地表的孤立块体中--这些暴露提供了大陆生长、演化和修改之谜的关键部分。 克利尔沃特复合体就是这样一个基岩区，它是一个几乎连续的结晶基底块体，暴露在爱达荷州中北部115公里长的区域内。最近，研究所及其学生对克利尔沃特杂岩中的岩石进行的锆石U-Pb定年表明，地壳形成的离散时期为2.66和1.86 Ga，石榴石Lu-Hf和独居石U-Th-Pb年代学表明，变质历史为~ 1.3 - 1.0 Ga。这些新的数据以及该地区代表了怀俄明州以外美国西北部暴露的最大的新太古代和古元古代地壳带这一事实，使其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清水杂岩来研究三个重要的过渡（2.7-2.6 Ga，1.9-1.7 Ga和1.3-1.1 Ga）？从新太古代到中元古界。这项工作将通过两项主要任务来完成。首先，我们将研究保存在正片麻岩中的岩浆记录，以记录该地区2.6 - 1.8Ga的地壳形成性质的变化，这是保存在锆石U-Pb和Hf，以及全岩Hf-Nd同位素记录。其次，我们将研究保存在空间相关的变质沉积岩的变质记录，以记录的时间和地壳条件在随后的修改，这一地区在中元古代。我们这部分工作的工具将是P-T相平衡研究，结合独居石U-Th-Pb和石榴石Lu-Hf和Sm-Nd年代学。这些岩石上获得的详细年代学将通过石榴石内独居石包裹体的原位分析和随后的Lu-Hf和Sm-Nd方法对同一石榴石的定年来完成。这种综合的方法将允许洞察这些计时器的耦合在区域变质作用，将广泛使用的变质社会的性质。

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.