Mafic inclusions in migmatite domes: exceptional pressure-temperature-time records of crustal flow

构成大陆的大多数材料在构造事件中被加热和变形，例如当大陆碰撞形成山脉时，大多数发生的变化都发生在地下深处。关于热的、以前很深的岩石如何暴露在地球表面的经典观点，调用了与它们的高温转化（变质作用）无关的过程，例如在变质作用之后很久可能发生的数百万年的侵蚀。最近，地球科学家们发现了一些地方，以前深，热的岩石被迅速运到地球表面，而不是作为岩浆（快速运输的一种机制），而是作为大多数固体，流动的地壳。这项研究的中心假设是，深层岩石快速上升到地球表面发生在大规模，而不仅仅是在不寻常的地方。大多数岩石没有记录它们早期的深层历史，但有些可以，这些可以被调查以了解深层热岩石向地球表面运输的条件，机制和时间，对了解大陆的热演化和结构演化，包括矿产资源的分布。该项目的社会效益涉及研究生和本科生在一个重要的科学，技术，工程和科学（STEM）学科领域和分析技能的培训。该项目还代表了明尼苏达大学和蒙彼利埃大学（法国）研究人员之间的合作，包括为自然历史博物馆的公众互动和创作教育材料。主要研究人员通过组织一个关于地壳流动系统的国际实地论坛来传播他们的研究成果，由高级变质岩（片麻岩穹丘）组成的穹丘结构是山区系统中地壳深部流动系统的可获得的表现，是大陆地壳中物质和热量转移的有力和有效的机制。穹丘中的物质可能来源较深，但大多数片麻岩穹丘记录的侵位位置相对较浅，因为穹丘中占主导地位的石英长石岩记录的压力-温度路径较差。这项研究利用了更完整的记录的压力-温度-时间的历史保存在石英长石质片麻岩中的镁铁质岩石包裹体，特别关注造山榴辉岩和角闪岩在法国中央地块的五个片麻岩圆顶。根据以前的工作表明，榴辉岩和寄主岩石在一个圆顶在这个地区记录相同的年龄，这个年龄代表了高压变质作用的时间，这项研究测试的想法，圆顶是“冰山一角”的区域深部地壳流动系统。根据这一假设，圆顶岩石可用于调查的条件，时间，并通过现场和分析研究，包括高压镁铁质岩（榴辉岩）和相关的斜长角闪岩在一个系统的空间相关的圆顶的压力-温度-时间的历史计算的横向和垂直流的后果。本研究的关键数据包括锆石、金红石和钛铁矿的成分和年龄，这些数据是通过综合热压法和激光烧蚀分流-电感耦合等离子体质谱法确定的（LASS-ICPMS）铀铅（U-Pb）地质年代学和微量元素分析，以及原位氧同位素和锆石显微结构分析，以评估流体在高压-高压重结晶过程中的作用。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。

Most of the materials that make up continents have been heated and deformed during tectonic events, such as when continents collide to create mountains, and most of the transformations that occur take place deep underground. The classical view of how hot, formerly-deep rocks become exposed at the Earth’s surface invokes processes unrelated to their high-temperature transformation (metamorphism), such as erosion that may occur over many millions of years, long after metamorphism. More recently, geoscientists have discovered some places where formerly-deep, hot rocks were rapidly transported toward the Earth’s surface, not as magma (one mechanism for rapid transport), but as mostly-solid, flowing crust. The central hypothesis of this research is that rapid ascent of deep rocks to the Earth’s surface occurs on a large-scale, not just in unusual, local sites. Most rocks do not record their earlier, deep history, but some do, and these can be investigated to understand the conditions, mechanisms, and timing of transport of deep, hot rocks toward the Earth’s surface, with implications for understanding the thermal and structural evolution of continents, including the distribution of mineral resources. Societal benefits of this project involve the training of graduate and undergraduate students in field and analytical skills in an important science, technology, engineering, and science (STEM) discipline. The project also represents a collaboration between researchers at the University of Minnesota and the University of Montpellier (France), including interaction and creation of educational materials for the general public for natural history museum. The principal investigators disseminate their research findings in part through the organization of an international field-forum on crustal flow systems.Domal structures comprised of high-grade metamorphic rocks (gneiss domes) are accessible manifestations of deep-crustal flow systems in mountain systems and represent powerful and efficient mechanisms of mass and heat transfer in continental crust. The material in domes may be deeply sourced, but most gneiss domes record emplacement at relatively shallow crustal levels, as the dominant quartzofeldspathic rocks in domes are poor recorders of pressure-temperature paths. This research takes advantage of the more complete record of pressure-temperature-time history preserved in mafic rock inclusions in quartzofeldspathic gneiss, with a specific focus on orogenic eclogite and amphibolite in five gneiss domes of the French Massif Central. Based on previous work showing that eclogite and host rocks in one dome in this region record the same age, and that this age represents the timing of high-pressure metamorphism, this research tests the idea that domes are the ‘tip of the iceberg’ of regional deep-crustal flow systems. According to this hypothesis, dome-rocks can be used to investigate the conditions, timing, and consequences of lateral and vertical flow via field and analytical studies that include calculation of pressure-temperature-time histories of high-pressure mafic rocks (eclogite) and associated amphibolite in a system of spatially-related domes. Key data in this research include the composition and age of zircon, rutile, and titanite as determined by integrated thermobarometry and Laser ablation split stream - inductively coupled plasma mass spectrometry (LASS-ICPMS) Uranium-Lead (U-Pb) geochronology and trace element analysis, as well as in situ oxygen isotope and microstructural analysis of zircon to evaluate the role of fluids in recrystallization during high-pressure - high-temperature metamorphism in the deep orogenic crust.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.