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

壮观的火山爆发是岩浆活动在塑造地球大陆方面发挥重要作用的明显证据。但是岩浆活动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，镁铁质岩浆从地幔上升，并在地壳底部附近形成一个称为“底侵”的过程。这一过程在大陆地壳深部产生“热区”，导致熔融、岩浆生成和岩浆混合。地球上很少有地方存在热区已经隆起并暴露在地表以供直接观察。该项目探讨了一个古老的底侵热区岩浆的发展和演化，该热区现在暴露在萨斯喀彻温省北方的地表。该团队假设，该地区曾经存在于大陆地壳底部附近的一个主要底侵区之上。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，以更好地理解整个北美的地球物理学家探测到的下大陆地壳的复杂图像。该项目将深入了解导致形成不同成分岩浆的过程、削弱或加强深部地壳的机制以及最终促进地球上大陆地壳形成和稳定的过程，岩浆底侵作用是一个一级岩石学和构造过程，是大陆地壳发展和演化的许多模型的核心。然而，通过地球物理学或捕虏体对地壳深部的远程研究通常缺乏明确限制底侵作用影响的分辨率。这个项目探讨了“内板”的假设，即，幔源岩浆侵位进入下板之上的下陆壳，对于岩浆过程（同化、混合、混杂）、变形（弱化和地壳流动）以及随后的下陆壳致密化+/-沉降或分层至关重要。将从对20，000平方公里的阿萨巴斯卡麻粒岩碎屑（1.1-1.6 GPa）最深挖掘部分的直接实地观察中收集关键见解。这些暴露被推断为经历了6.5亿年。下地壳的居住地它们代表了一个难得的机会，探讨壳幔相互作用和长期演化的下陆壳由于内板作用，变质作用，熔融和致密化在三个不同的构造环境。可供选择的产品包括：（i）限制每种环境中岩浆内侵和变质作用的时间和持续时间，（ii）确定和量化致密化机制，（iii）限制变质沉积片麻岩对板内下陆壳成分、密度和地震速度结构的影响，（iv）利用野外观测和数据，建立更可靠的板下下陆壳地球物理模型。关键的地球物理目标包括：（1）评估在地震勘探中可以区分底侵和内侵背景的程度，以及（2）评估每个构造域的综合地球物理特性可以为更可靠地解释在北美部分地区下方检测到的地震快速下地壳模式提供信息的程度。更广泛的影响包括：（一）两个博士学位。2-4 B.学生将开展研究项目; ㈡将为地球科学家和研究生举办一个虚拟国际讲习班，重点是下大陆地壳的起源和演变; ㈢专业研究员和学生将召开一个下地壳实地论坛，与会者将参观和探索阿萨巴斯卡麻粒岩，并讨论下大陆地壳的一般影响;（iv）私人研究员和学生将在萨斯喀彻温省斯托尼拉皮兹附近参与地球科学外展工作，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，让他们有机会分享自己对阿萨巴斯卡麻粒岩边缘沿着文化、人民和景观的见解。该项目由三个项目（岩石学&地球化学、地球物理学、以及刺激竞争研究的既定计划（EPSCoR）该奖项反映了NSF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.