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

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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.