Defining Lithospheric Boundaries in the Great Basin, Southwest USA, Utilizing the Geochemistry of Cenozoic Igneous Rocks

北美大陆是在过去至少20亿年的板块碰撞和板块裂谷事件中形成的。不同的地壳块体添加到北美洲，包括不同年龄和起源的岩石，识别这些地壳块体是了解北美洲组合的关键。火成岩是探测地壳块体的极好工具，因为它们起源于地球上地幔的部分熔体，当熔体到达地球表面时，通常会受到上覆大陆地壳岩石的污染。特别是，火成岩中锶、钕、铅和氧等元素的同位素记录了地幔和地壳岩石组成的混合，这对确定地壳块体边界至关重要。然而，不同的同位素体系定义了不同的块体边界，北美西部一些火成岩的同位素数据通常落在建议的同位素边界之外。数据和建议边界之间的差异告诉我们这些边界的形状和结构是什么？美国西南部的大盆地是一个大陆伸展的地区，暴露出的火成岩年龄从250万年到100万年不等。我们提出对年轻（< 4000万年）火成岩系统进行年代学、地球化学和多同位素系统研究，以确定大盆地中北部不同地壳块体之间边界的位置和精细尺度形状，以解决几个科学问题。首先，我们需要提高对跨块界地幔源组成的认识。二是大盆地地壳块体边界的形态和垂向结构是怎样的？人们提出用同位素组成变化的同位素“线”来划分地壳块体边界，但不同的同位素系统产生不同的界线。此外，同位素组成差异很大的火成岩在这些界线附近被并排放置。街区之间的过渡区有多宽？在过渡带中是否有较年轻和较老的地幔和地壳岩石相互交错，或者是侵位后的变形使一些火成岩离开了它们原来的结晶区？如果后者是正确的，那么较老的岩石应该比较年轻的岩石在真实边界周围表现出更多的分散。第三，深成岩是否记录了与火山岩相同的地幔和地壳输入，这是否随时间而变化？这两种火成岩类型以前都被用来测量地幔/地壳贡献和确定基底边界，但是这两种火成岩类型的记录是否具有可比性？这些问题的答案将揭示火成岩如何定义地壳块边界的形状和块的年龄，这将对我们星球上其他地方大陆组合的研究产生影响。

The continent of North America was assembled through plate collision and plate rifting events over at least the past 2 billion years. Different crustal blocks that have been added to North America include rocks of different ages and origins, and identifying these crustal blocks is key to understanding the assembly of North America. Igneous rocks are excellent probes of crustal blocks, in that they originate as partial melts of the Earth's upper mantle and are usually contaminated by overlying continental crustal rocks as the melts travel to the Earth's surface. In particular, isotopes of elements such as strontium, neodymium, lead and oxygen in igneous rocks record the mix of mantle and crustal rock compositions, knowledge of which is crucial to defining crustal block boundaries. However, different isotope systems define different block boundaries, and the isotopic data reported for some igneous rocks from western North America commonly fall outside of the proposed isotopic boundaries. What do the discrepancies between the data and the proposed boundaries tell us about the shape and structure of these boundaries? The Great Basin of the southwestern United States, a region of continental extension, exposes igneous rocks ranging in age from ~250 to <1 million years old. We propose a geochronological, geochemical, and multi-isotope system study of young (< 40 million years old) igneous systems to define the location and fine-scale shape of boundaries between different crustal blocks of the north-central Great Basin, to address several scientific questions. First, we require an improved understanding of the composition of mantle sources across block boundaries. Second, what is the shape and vertical structure of crustal block boundaries in the Great Basin? Isotopic "lines" marking changes in isotopic composition are proposed to delineate crustal block boundaries, yet different isotope systems produce different boundary lines. Further, igneous rocks with highly different isotopic compositions have been emplaced side-by-side close to these boundary lines. How wide is the transitional zone between blocks? Are younger and older mantle and crustal rocks interleaved in the transition zone, or has post-emplacement deformation moved some igneous rocks away from their original crystallization regions? If the latter is correct, then older rocks should show more dispersion around the true boundaries than should younger rocks. Third, do plutonic rocks record the same mantle vs. crustal inputs as do volcanic rocks, and does this vary as a function of time? Both igneous rock types have been being used previously to measure mantle/crust contributions and define basement boundaries, but are the records in these two igneous rock types comparable? Answers to these questions will reveal how igneous rocks can define the shape of crustal block boundaries and the age of the blocks, which will have an impact on the study of continental assembly elsewhere on our planet.