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

壮观的火山喷发是岩浆活动在塑造地球大陆过程中所起的根本作用的可见证据。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，岩浆从地幔中升起，并在地壳底部附近聚集，这一过程被称为“底板”。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。地球上很少有地方的热区被抬升并暴露在地表以供直接观测。这个项目探索了一个古老的地下热区岩浆的发展和演化，现在在萨斯喀彻温省北部的地表暴露出来。这个团队假设这个区域曾经存在于靠近大陆地壳底部的一个主要的下镀带之上。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北美各地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岩浆形成的过程，削弱或加强深层地壳的机制，以及最终促进地球大陆地壳形成和稳定的过程。岩浆底沉积是一个一级岩石学和构造过程，是许多大陆地壳发育和演化模式的核心。然而，通过地球物理或捕虏体对深部地壳进行的远程研究通常缺乏明确限制底板影响的分辨率。本项目探讨了“内镀”假设，即幔源岩浆进入底板上方的下大陆地壳，对岩浆过程（同化、混合、杂交）、变形（减弱和地壳流动）以及随后的下大陆地壳致密化、沉降或剥离至关重要。对阿萨巴斯卡麻粒岩地块（1.1-1.6 GPa）最深处的20,000平方公里的直接现场观测将收集到关键的见解。据推断，这些暴露在挖掘前经历了650米的下地壳居住。它们代表了一个难得的机会来探索地壳-地幔相互作用和下大陆地壳的长期演化，这是由于三个不同的构造环境中的内镀、变质、熔融和致密作用。交付成果包括：(i)限制岩浆内镀和变质作用在每种情况下的时间和持续时间，（ii）确定和量化致密化机制，（iii）限制变质沉积片麻岩对内镀下大陆地壳组成、密度和地震速度结构的影响，以及（iv）利用基于实地的观测和数据开发更可靠的下镀下大陆地壳地球物理模型。关键的地球物理目标包括：(1)评估在地震调查中区分底板和底板内背景的程度，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可以为北美部分地区探测到的地震快速下地壳模式提供更可靠的解释。更广泛的影响包括：(i) 2名博士和2-4名学士将进行研究项目；将为地球科学家和研究生举办一个虚拟国际讲习班，重点讨论下大陆地壳的起源和演化；pi和学生将召开下地壳场论坛，与会者将参观和探索阿萨巴斯卡麻粒岩地并讨论对下大陆地壳的一般影响；（iv） pi和学生将在萨斯喀彻温省石溪急流附近进行地球科学外展，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块边缘的文化、人民和景观的见解。该项目由三个EAR项目（岩石与地球化学、地球物理学和构造学）以及促进竞争性研究的既定项目（EPSCoR）共同资助。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.