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

构成大陆的大多数物质在构造事件中都被加热和变形，比如大陆碰撞形成山脉，而发生的大多数变化发生在地下深处。关于地球表面以前很深的岩石如何变得炎热的经典观点调用了与它们的高温转变(变质作用)无关的过程，例如变质作用很久之后可能发生的数百万年的侵蚀。最近，地球科学家发现了一些地方，在那里，以前很深的热岩被迅速运往地球表面，不是作为岩浆(快速运输的一种机制)，而是作为主要是固体的流动地壳。这项研究的中心假设是，深部岩石迅速上升到地球表面的情况是大规模的，而不仅仅是在不寻常的局部地点。大多数岩石没有记录它们更早的、深刻的历史，但有些岩石记录了，这些可以进行研究，以了解深部热岩向地球表面输送的条件、机制和时间，这对于理解大陆的热和构造演化，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实地培训，以及一个重要的科学、技术、工程和科学(STEM)学科的分析技能。该项目还代表了明尼苏达大学和蒙彼利埃大学(法国)的研究人员之间的合作，包括为自然历史博物馆的普通公众互动和创建教育材料。首席研究人员部分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(片麻岩穹隆)组成的穹隆结构是深部地壳流动系统在山脉系统中的常见表现，代表了大陆地壳中强大而有效的质量和热传递机制。穹隆中的物质来源可能很深，但大多数片麻岩穹顶记录的侵位都是在相对较浅的地壳水平上进行的，因为穹顶中占主导地位的石英岩不能很好地记录压力-温度路径。这项研究利用了保存在石英长石片麻岩中镁铁质岩石包裹体中较为完整的压力-温度-时间历史记录，重点研究了法国中央地块五个片麻岩穹顶中的造山榴辉岩和角闪岩。基于前人的研究表明，该地区同一穹隆中的榴辉岩和寄主岩石记录了相同的年龄，并且这一年龄代表了高压变质作用的时间，本研究验证了穹顶是区域深部地壳流动系统的冰山一角的观点。根据这一假设，穹顶岩可用于通过现场和分析研究来调查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(榴辉岩)和伴生的角闪岩的压力-温度-时间历史。这项研究中的关键数据包括综合热压计和激光烧蚀分离流感应等离子体质谱(LASS-ICPMS)铀铅(U-Pb)年代学和微量元素分析确定的锆石、金红石和钛铁矿的组成和年龄，以及锆石的原位氧同位素和显微结构分析，以评估流体在深部造山地壳高压高温变质过程中重结晶中的作用。这一裁决反映了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，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.