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

合作研究：将变沉积岩纳入大陆弧深层：来自北部喀斯喀特的见解

• 批准号: 1419787
• 负责人: Robert Miller
• 金额: $ 8万
• 依托单位: San Jose State University Foundation
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-08-01 至 2019-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1419787&HistoricalAwards=false
• 关键词: Collaborative; Research; Incorporation; Metasedimentary; Rocks

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.

该项目所包含的研究涉及破译在火山演化和融入地壳过程中，在形成极深火山根部的岩浆系统中添加了哪些化学成分。 在一些火山系统中，来自地幔的岩浆包含深埋的沉积岩，这对了解火山系统的化学成分及其喷发的潜在危害具有重要意义。这项研究的目的是评估沉积物岩石如何以及何时转移到火山系统下面的深层，以及它们如何影响地壳的化学演化。华盛顿的北瀑布代表了一个古老的岩浆系统，该系统经历了崩溃和严重的侵蚀，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同位素技术，结合实地研究，主要研究人员将确定位于古代火山系统深层的沉积岩的来源特征。 这种物质的起源可能包括沉积物，这些沉积物积聚在一个下行的海洋板块前面，然后随着板块俯冲而被掩埋和熔化。 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重要证据，并将使项目研究员能够确定为火山系统提供原料并可能导致大规模火山爆发的物质的指纹，这具有重要的社会影响。 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透彻的理解，这对新大陆地壳的形成具有影响。除了这项研究的科学目标，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。外联工作包括编制地质小册子，旨在帮助教育游客了解北喀斯喀特国家公园的地质演变，以及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的实地考察，作为将于2017年秋季在西雅图举行的国际会议的一部分。只有少数基于实地的研究已经研究了变质沉积岩如何融入大陆岩浆弧的中深部地壳，即使它们的存在对弧系统具有重要的机械和地球化学后果。变质沉积岩及其熔体，如果这些岩石穿过固相线，是弱的，因此强烈影响系统的整体流变性。沉积物通过底侵作用进入弧也可以控制岩浆活动的时间尺度，从而通过驱动多个古弧系统中记录的高岩浆流量事件（例如，内华达州山脉，海岸山脉-北喀斯喀特山脉）。主要研究人员正在评估沉积物如何以及何时转移到弧的中深层，方法是调查一系列过程的重要性，包括1）俯冲地壳物质的就位，这些物质从下行板片上垂直上升弧前或弧后沉积物的逆冲/叠瓦状埋藏，弧岩浆作用期间沉积物和火山岩的逐渐埋藏;和/或4）在已经由大量变质沉积岩组成的加厚地壳上建造弧。主要调查人员将对北喀斯喀特山脉晚白垩世-始新世结晶核进行综合实地和实验室研究。主要调查人员将通过实地测绘、大块岩石Nd分析、碎屑锆石的U-Pb和Hf同位素研究以及热压测量来确定变质沉积岩的起源，以确定这些岩石的构造背景、达到的最大压力和来源特征。还将对来自弧前和弧后单元的碎屑锆石进行分析研究，这些锆石代表斯卡吉特和斯瓦坎变质沉积岩的潜在原岩。这项研究的结果可以外推到主动电弧系统（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中就位的，以及它们的存在对弧的演化和潜在的安山质大陆地壳的整体生产有什么后果。另一个重要成果将是更好地了解其他古老的、可能类似的弧系，如峡湾、新西兰和内华达州山脉。