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20亿年中通过板块碰撞和板块断裂事件组装而成的。已经添加到北美的不同地壳块体包括不同年龄和起源的岩石，识别这些地壳块体是理解北美组装的关键。火成岩是地壳块体的极好探针，因为它们起源于地球上地幔的部分熔体，并且通常在熔体行进到地球表面时被覆盖的大陆地壳岩石污染。特别是，火成岩中的锶、钕、铅和氧等元素的同位素记录了地幔和地壳岩石成分的混合，这对确定地壳块体边界至关重要。然而，不同的同位素系统定义了不同的块体边界，并且报告的来自北美西部的一些火成岩的同位素数据通常落在提议的同位素边界之外。这些数据和建议的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区域，暴露出年龄从250年到<100万年的火成岩。我们建议对年轻（<4000万年）的火成岩系统进行地质年代学，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，我们需要更好地了解跨块体边界的地幔源的组成。第二，大盆地地壳块体边界的形状和垂直结构是什么？同位素“线”标志着同位素组成的变化，建议划定地壳块体边界，但不同的同位素系统产生不同的边界线。 此外，具有高度不同的同位素组成的火成岩已被并排放置在这些边界线附近。街区之间的过渡区有多宽？是年轻和年老的地幔和地壳岩石在过渡带中交错，还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显示出更多的分散。第三，深成岩是否记录了与火山岩相同的地幔与地壳输入，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/地壳的贡献和定义基底边界，但这两种火成岩类型的记录具有可比性吗？这些问题的答案将揭示火成岩如何定义地壳块体边界的形状和块体的年龄，这将对我们星球上其他地方的大陆组装研究产生影响。

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.