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

利用新生代火成岩的地球化学定义美国西南部大盆地的岩石圈边界

• 批准号: RGPIN-2019-06141
• 负责人: Cousens, Brian
• 金额: $ 3.13万
• 依托单位: Carleton University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2022
• 资助国家: 加拿大
• 起止时间: 2022-01-01 至 2023-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=751129
• 关键词: Defining; Lithospheric; Boundaries; Great; Basin

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.

在过去的20亿年中，北美大陆是通过板碰撞和板裂式事件组装而成的。已添加到北美的不同地壳块包括不同年龄和起源的岩石，确定这些地壳块是了解北美集会的关键。火成岩是地壳块的出色探针，因为它们起源于地球上地幔的部分融化，通常在融化到地球表面时被上覆的大陆地壳岩石污染。特别是，火成岩中的元素，近代，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物，这些岩石和地壳岩石的组合对于定义地壳块边界至关重要。但是，不同的同位素系统定义了不同的块边界，并且报告了西部北美一些火成岩的同位素数据通常落在所提出的同位素边界之外。数据与拟议边界之间的差异告诉我们这些边界的形状和结构？美国西南部的大盆地是大陆延伸的地区，其火成岩的年龄范围从约250年到<100万年。我们提出了对年轻（<4000万年历史）的火成岩系统的年代学，地球化学和多同位素系统研究，以定义中北部大盆地不同地壳块之间边界的位置和精细形状，以解决几个科学问题。首先，我们需要对跨块边界的地幔源组成有了深入的了解。其次，大盆地中地壳块边界的形状和垂直结构是什么？提出了同位素组合物中标记变化的同位素“线”，以描绘地壳块边界，但不同的同位素系统产生不同的边界线。此外，具有高度不同同位素组合物的火成岩已被并排放置在这些边界线附近。块之间的过渡区有多大？年龄较大的地幔和地壳岩石是否在过渡区交错，或者后置换后变形使一些火成岩从其原始结晶区域移开？如果后者是正确的，那么年龄较大的岩石应在真实边界周围显示更多的分散体，而不是年轻的岩石。第三，岩石岩石是否记录了与火山岩相同的地幔与地壳输入的记录，并且这会随着时间的变化而变化吗？两种火成岩类型以前已被用于测量地幔/地壳贡献并定义地下室边界，但是这两种火成岩类型中的记录是否相当？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，这将对我们星球其他地方的大陆集会的研究产生影响。