Collaborative Research: Impact of Magmatic Underplating on the Evolution of Lower Continental Crust

壮观的火山喷发是岩浆作用塑造地球大陆的根本作用的明显证据。但是岩浆作用是如何在看不见的深处创造和改变大陆的呢？地球科学家长期以来一直推断，镁铁质岩浆是从地壳底部附近的地幔和池塘中升起的，这一过程被称为“底侵”。这一过程在大陆地壳深处产生“热区”，导致熔融、岩浆生成和岩浆混合。地球上很少有热区被抬升并暴露在地表上进行直接观测的地方。这个项目探索了一个古老的底侵热区的岩浆发展和演化，现在暴露在萨斯喀彻温省北部的地表。这个研究小组假设，该地区曾经存在于大陆地壳底部附近的一个主要底侵带上方。学生和科学家将收集数据，以了解岩浆作用、变质作用和地壳深部致密化的时间和持续时间。他们将开发模型，以更好地理解地球物理学家在整个北美探测到的下大陆地壳的复杂图像。该项目将深入了解导致不同成分的岩浆形成的过程、削弱或加强地壳深层的机制，以及最终促进地球上陆壳形成和稳定的过程。岩浆底侵作用是一种一级岩石学和构造过程，对许多陆壳发展和演化模式至关重要。然而，通过地球物理或捕捉体对地壳深部的远程研究通常缺乏明确限制底侵影响的分辨率。该项目探讨了“内侵”假设，即地幔岩浆侵位到下陆壳底板上方，对岩浆作用(同化、混合、杂化)、变形(减弱和地壳流动)以及随后的致密化+/-下陆壳沉陷或拆沉至关重要。关键的见解将从对20,000平方公里阿萨巴斯卡麻粒岩地体(1.1-1.6 Gpa)最深出土部分的直接野外观察中获得。据推测，这些暴露经历了650英里。在掘出遗骸之前属于下地壳居住地。它们是探索壳幔相互作用和下大陆地壳在三个不同构造背景下的内侵、变质、熔融和致密作用的长期演化的难得机会。成果包括：(1)限制岩浆在每种背景下的侵入和变质作用的时间和持续时间；(2)识别和量化致密化机制；(3)限制变质片麻岩对板内下大陆地壳的成分、密度和地震速度结构的影响；以及(4)利用野外观测和数据，建立更可靠的下陆下地壳的地球物理模型。关键的地球物理目标包括：(1)评估在地震调查中能够区分底侵和内侵环境的程度，以及(2)评估每个构造域的综合地球物理性质在多大程度上能够更有力地解释在北美部分地区探测到的地震快速下地壳的模式。更广泛的影响包括：(I)两名博士生和2--4名理科学士学生将开展研究项目；(Ii)将为地球科学家和研究生举办一个以下大陆地壳起源和演化为重点的虚拟国际讲习班；(Iii)个人投资者和学生将召开下地壳实地论坛，与会者将参观和探索阿萨巴斯卡麻粒岩地体，并讨论对下大陆地壳的一般影响；(Iv)个人投资者和学生将在萨斯喀彻温省石急流附近进行地球科学宣传，作为实地工作的一部分。我们的目标是让感兴趣的教师和学生参与我们的虚拟研讨会和现场论坛，让他们有机会分享他们对阿萨巴斯卡麻粒岩地体边缘的文化、人和景观的见解。该项目由三个EAR项目(岩石学与地球化学、地球物理和构造学)以及既定的激励竞争研究计划(EPSCoR)共同资助。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Spectacular volcanic eruptions are visible evidence for the fundamental role of magmatism in shaping Earth’s continents. But how does magmatism create and alter the continents at great invisible depths? Earth scientists have long inferred that mafic magmas rise from the mantle and pond near the bottom of the crust in a process called “underplating”. This process produces “hot zones” in the deep continental crust which lead to melting, magma generation, and magma mixing. Few localities exist on Earth where hot zones have been uplifted and exposed at the surface for direct observation. This project explores the development and evolution of magma in an ancient hot zone of underplating, now exposed at the surface in northern Saskatchewan. This team hypothesizes that the region once existed above a major zone of underplating near the base of the continental crust. Students and scientists will collect data to understand the timing and duration of magmatism, metamorphism, and densification of the deep crust. They will develop models to better understand complex images of the lower continental crust detected by geophysicists throughout North America. The project will provide insight into the processes that lead to the formation of magmas of varying composition, the mechanisms that weaken or strengthen the deep crust, and ultimately, the processes that facilitate formation and stabilization of continental crust on Earth.Magmatic underplating is a first order petrologic and tectonic process that is central to many models for the development and evolution of continental crust. However, remote studies of the deep crust via geophysics or xenoliths commonly lack the resolution to unequivocally constrain the impacts of underplating. This project explores the hypothesis that ”intraplating”, i.e., the emplacement of mantle-derived magma into the lower continental crust above the underplate, is critical for magmatic processes (assimilation, mingling, hybridization), deformation (weakening and crustal flow), and subsequent densification +/- foundering or delamination of lower continental crust. Key insights will be gleaned from direct field observations of the most deeply exhumed section of the 20,000 km2 Athabasca granulite terrane (1.1-1.6 GPa). These exposures are inferred to have experienced 650 m.y. of lower crustal residence prior to exhumation. They represent a rare opportunity to explore crust-mantle interaction and long-term evolution of lower continental crust due to intraplating, metamorphism, melting, and densification in three distinct tectonic settings. Deliverables include: (i) constraining the timing and duration of magmatic intraplating and metamorphism in each setting, (ii) identifying and quantifying densification mechanisms, (iii) constraining the influence of metasedimentary gneisses on the composition, density, and seismic velocity structure of intraplated lower continental crust, and (iv) using field-based observations and data to develop more robust geophysical models of underplated lower continental crust. Critical geophysical goals include: (1) evaluating the degree to which the settings of underplating and intraplating can be distinguished in seismic surveys, and (2) evaluating the degree to which the integrated geophysical properties in each tectonic domain can inform more robust interpretations of the pattern of seismically fast lower crust detected beneath parts of North America. Broader impacts include: (i) Two Ph.D. and 2-4 B.S. students will carry out research projects; (ii) A virtual international workshop for Earth scientists and graduate students focused on the origin and evolution of lower continental crust will be hosted; (iii) PIs and students will convene a Lower Crustal Field Forum where participants will visit and explore the Athabasca granulite terrane and discuss general implications for lower continental crust; (iv) PIs and students will engage in Earth science outreach as part of field work in the vicinity of Stony Rapids, Saskatchewan. The goal is to involve interested teachers and students in our virtual workshop and field forum, giving them opportunities to share their own insights on the culture, people, and landscape along the edge of the Athabasca granulite terrane.This project is jointly funded by three EAR programs (Petrology & Geochemistry, Geophysics, and Tectonics) as well as the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.