Collaborative Research: Incorporation of Metasedimentary Rocks into the Deep Levels of Continental Arcs: Insights from the North Cascades

该项目的研究内容包括破译岩浆系统中添加了哪些化学成分，这些化学成分在火山演化和融入地壳的过程中形成了火山的根。在一些火山系统中，源自地幔的岩浆融合了深埋的沉积岩，这对了解火山系统的化学成分及其喷发的潜在危险具有重要意义。这项研究的目的是评估沉积岩如何以及何时转移到火山系统下的深层，以及它们如何影响地壳的化学演化。华盛顿的北喀斯喀特山脉代表了一个古老的岩浆系统，它经历了崩塌和严重的侵蚀，这使得该系统的深层暴露在今天的地球表面。利用各种不同的放射性成因同位素技术，结合实地研究，主要研究人员将对位于古火山系统深层的沉积岩的来源特征进行指纹识别。这种物质的来源可能包括沉积在一个下沉的海洋板块前面，然后随着板块的俯冲而被掩埋和融化。这些信息将为测试沉积地壳与岩浆结合的不同结构和化学模型提供关键证据，并将使pi能够识别火山系统的物质，并可能导致大规模的火山爆发，这对社会有重要影响。最终，这项研究提供了对控制现代和古代大陆火山弧岩浆活动性质和时间尺度的过程的更彻底的理解，这对新大陆地壳的形成具有重要意义。除了本研究的科学目标之外，该项目还支持STEM学科的研究生和本科生的培训，支持早期职业科学家的研究工作，并为两个合作机构的研究基础设施做出贡献。外联工作包括开发地质手册，旨在帮助游客了解北喀斯喀特国家公园的地质演变，并建立一个总结研究成果的网站。主要研究人员还将带领专业地球科学家和学生进行实地考察，作为将于2017年秋季在西雅图举行的国际会议的一部分。只有少数实地研究考察了变质沉积岩是如何融入大陆岩浆弧的中至深部地壳的，尽管它们的存在对弧系统具有重要的力学和地球化学影响。变质的沉积岩和它们的熔体，如果这些岩石穿过固相岩，是脆弱的，因此强烈地影响整个系统的流变学。沉积物通过底板进入弧内也可能控制岩浆活动的时间尺度，从而通过驱动在多个古弧系统（如内华达山脉、海岸山脉-北喀斯喀特山脉）中记录的高岩浆通量事件来控制弧的结构。主要研究人员正在通过调查一系列过程的重要性来评估沉积物是如何以及何时转移到弧的中深水平的，这些过程包括：1)俯冲地壳物质的就位，这些物质从下沉的板块浮力上升（再分层）；2)弧前或弧后沉积物的逆冲/叠瓦作用埋藏；3)弧岩浆作用期间沉积物和火山碎屑岩逐渐埋藏；和/或4)在已经由大量变质沉积岩组成的加厚地壳上建造一个弧。主要研究人员将对北喀斯喀特的晚白垩纪-始新世结晶岩心进行综合野外和实验室研究。主要研究人员将通过野外测绘、块状岩石Nd分析、碎屑锆石U-Pb和hf同位素研究，以及热气压测定法来确定这些岩石的构造背景、最大压力和来源特征，来确定变质沉积岩的起源。对弧前和弧后单元的碎屑锆石进行分析研究，这些碎屑锆石代表了Skagit和Swakane变质沉积岩的潜在原岩。这项研究的结果可以外推到活动的弧系统（例如安第斯山脉），以更好地了解变质沉积岩的背景：它们是如何在弧系统内安置的，它们的存在对弧的演化和潜在的安山岩大陆地壳的整体生产有什么影响。另一个重要的成果将是更好地了解其他古老的、可能类似的弧形系统，如峡湾、新西兰和内华达山脉。

The research contained in this project involves deciphering what chemical components are added into magmatic systems that form the roots of volcanoes at great depths during their evolution and incorporation into the Earth's crust. In some volcanic systems, magma derived from the Earth's mantle incorporates deeply buried sedimentary rocks, which has important implications for understanding the chemical composition of volcanic systems and the potential hazards associated with their eruption. The goal of this study is to evaluate how and when sediments rocks are transferred to deep levels beneath volcanic systems and how they affect the chemical evolution of the crust. The North Cascades of Washington represents an ancient magmatic system that has undergone collapse and significant erosion, which has allowed deep levels of the system to be exposed at the present-day Earth's surface. Using a variety of different radiogenic isotopic techniques, combined with field studies, the principal investigators will fingerprint the characteristics of source of the sedimentary rocks that were located in the deep levels of the ancient volcanic system. Possibilities for the origin of this material include sediments that were accumulated in front of a down going oceanic plate and then buried and melted as the plate was subducted. This information will provide crucial evidence for testing different structural and chemical models for the incorporation of sedimentary crust into magmas and will allow the PIs to fingerprint the material that feeds volcanic systems and possibly leads to large-scale volcanic eruptions, which has important societal impacts. Ultimately, this study is providing a more thorough understanding of the processes that control the nature and timescales of magmatism in modern and ancient continental volcanic arcs, which has implications for how new continental crust is formed. In addition to the scientific goals of this research, this project is supporting the training of graduate and undergraduate students in an STEM discipline, is supporting the research efforts of an early career scientist, and is contributing to research infrastructure at both collaborating institutions. Outreach efforts include the development of geologic brochures aimed at helping to educate visitors regarding the geologic evolution of North Cascades National Park and a website that summarizes research results. The principal investigators will also lead a field trip for professional geoscientists and students as part of an international meeting that will occur in Seattle in Fall 2017.Only a few field-based studies have examined how metasedimentary rocks become incorporated into the mid- to deep crust of continental magmatic arcs, even though their presence has significant mechanical and geochemical consequences for the arc system. Metamorphosed sedimentary rocks and their melts, produced if these rocks cross the solidus, are weak and thus strongly affect the overall rheology of the system. Emplacement of sediment into an arc via underplating may also control the timescales of magmatism and thus the architecture of the arc by driving the high-magma flux episodes noted in multiple ancient arc systems (e.g., Sierra Nevada, Coast Mountains-North Cascades). The principal investigators are evaluating how and when sediment is transferred into the mid to deep levels of arcs by investigating the importance of a range of processes, including 1) emplacement of subducted crustal material that rises buoyantly off of the downgoing slab (relamination); 2) burial by underthrusting/imbrication of forearc or backarc sediments; 3) gradual burial of sediments and volcanoclastic rocks during arc magmatism; and/or 4) construction of an arc on thickened crust that is already composed of voluminous metasedimentary rocks. The principal investigators will carry out an integrated field and laboratory study of the Late Cretaceous-Eocene crystalline core of the North Cascades. The principal investigators will determine the origin of the metasedimentary rocks via field mapping, bulk-rock Nd analyses, U-Pb and Hf-isotope study of detrital zircons, and thermobarometry to determine the tectonic context of these rocks, maximum-achieved pressures and source signatures. Analytical studies will also be conducted on detrital zircons from units in the forearc and backarc, which represent potential protoliths of the Skagit and Swakane metasedimentary rocks. The results from this study can be extrapolated to active arc systems (e.g., the Andes) to better understand the context of metasedimentary rocks: how they are emplaced within the arc system, and what the consequences are of their presence for the evolution of the arc and potentially for the overall production of andesitic continental crust. Another important outcome will be a better understanding of other ancient, potentially analogous arc systems, such as Fiordland, New Zealand and the Sierra Nevada.