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都被加热和变形，例如当大陆碰撞形成山脉时，并且大多数发生的转变发生在地下深处。关于以前深层岩石如何暴露在地球表面的经典观点引发了与其高温转变（变质作用）无关的过程，例如可能在变质作用发生很久之后的数百万年里发生的侵蚀。最近，地球科学家发现，在一些地方，以前深部的炽热岩石被迅速输送到地球表面，不是以岩浆（快速输送的一种机制）的形式，而是以大多为固体的流动地壳的形式输送。这项研究的中心假设是，深层岩石快速上升到地球表面的现象是大规模发生的，而不仅仅是在不寻常的局部地点。大多数岩石不会记录其早期的深层历史，但有些岩石会记录这些历史，可以通过研究这些来了解深层热岩石向地球表面迁移的条件、机制和时间，这对于了解大陆的热和结构演化（包括矿产资源的分布）具有重要意义。该项目的社会效益包括对研究生和本科生进行重要科学、技术、工程和科学 (STEM) 学科的现场和分析技能培训。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制作教育材料。主要研究人员通过组织地壳流系统国际现场论坛来传播他们的研究成果。由高级变质岩（片麻岩穹顶）组成的穹顶结构是山地系统中深部地壳流系统的易见表现形式，代表了大陆地壳中强大而有效的传质和传热机制。圆顶中的物质可能来源很深，但大多数片麻岩圆顶记录了相对较浅的地壳水平的侵位，因为圆顶中占主导地位的石英长石岩石不能很好地记录压力-温度路径。这项研究利用了石英长石片麻岩中镁铁质岩石包裹体中保存的更完整的压力-温度-时间历史记录，特别关注法国中央高原五个片麻岩穹丘中的造山榴辉岩和角闪岩。基于之前的研究表明，该地区一个穹丘中的榴辉岩和围岩记录了相同的年龄，并且该年龄代表了高压变质作用的时间，本研究检验了穹丘是区域深部地壳流系统的“冰山一角”的观点。根据这一假设，穹顶岩石可用于通过现场和分析研究来研究横向和垂直流动的条件、时间和后果，包括计算空间相关穹顶系统中高压镁铁质岩石（榴辉岩）和相关角闪岩的压力-温度-时间历史。这项研究的关键数据包括通过集成温压测定法和激光烧蚀分流-电感耦合等离子体质谱法（LASS-ICPMS）、铀-铅（U-Pb）地质年代学和微量元素分析确定的锆石、金红石和钛矿的成分和年龄，以及锆石的原位氧同位素和微观结构分析，以评估流体在高压高温变质作用期间再结晶中的作用。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力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.