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中的一个基本问题。理解这些过程的关键在于构成大陆“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，这些基底大陆岩的年龄、它们是如何形成的，以及它们是如何合并和变质形成今天大陆的基础的，还远远不清楚。其主要原因是这些基岩往往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从构造上带到地表的孤立区块--这些暴露提供了大陆增长、演化和改造之谜的关键碎片。一个这样的基岩区域是Clearwater复合体，这是一个几乎连续的结晶基底块，暴露在爱达荷州中北部115公里长的地区。Pis和他们的学生最近对Clearwater杂岩中的岩石进行了锆石U-Pb定年，确定了2.66和1.86Ga的离散地壳形成时期，石榴石Lu-Hf和独居石U-Th-Pb年代学证明了~1.3~1.0Ga的复杂变质历史。这些新数据以及该地区是美国西北部怀俄明省以外最大的新太古代和古元古代地壳暴露带的事实，使其成为一个引人注目的进一步研究区域。这项拟议研究的主要目标是利用Clearwater复合体来研究地球历史上从新太古代到中元古代的三个重要转变(2.7-2.6Ga，1.9-1.7Ga和1.3-1.1Ga)。这项工作将通过两项主要任务完成。首先，我们将研究保存在正片麻岩中的岩浆记录，以记录该地区地壳形成性质的变化，从2.6到1.8Ga，保存在锆石U-Pb和Hf，以及全岩Hf-ND同位素记录中。其次，我们将研究保存在空间伴生的变质沉积岩中的变质记录，以记录该地区在中元古代后续改造过程中的时间和地壳条件。我们这部分工作的工具将是P-T相平衡研究，它与独居石U-Th-Pb和石榴石Lu-Hf和Sm-ND年代学相结合。这些岩石的详细年代学将通过对石榴石中独居石包裹体的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。这种综合的方法将使人们能够深入了解区域变质作用期间这些计时器耦合的性质，这将对变质岩社区有广泛的用途。

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.