Collaborative Research: Did Mantle Magma Cause Ultrahigh-Temperature Metamorphism (UHTM) in Southern Madagascar?

由于板块构造，地球大陆的碰撞导致地壳变形，弯曲，变得更热，导致持久的转变：变质矿物反应，岩浆的形成，以及整个地壳的元素重组。这些永久改变大陆的过程在超高温变质岩（UHTM）中最为极端，温度超过900 °C。认识到地球过去岩石的这些变化是理解大陆如何随着时间变化的基础。然而，为什么UHTM发生在一些大陆碰撞区，而不是其他人仍然未知。该项目将通过调查马达加斯加南部暴露的UHTM岩石来解决岩石如何以及为什么达到如此高的温度这一悬而未决的问题。具体来说，这项工作将测试岩浆从地幔上升并将马达加斯加南部的岩石加热到超过典型山脉带的温度，从而导致UHTM的假设。锆石的年龄测定和化学分析将分别确定岩浆侵入的时间和起源，以确定它们与UHTM的关系。该项目将支持伍兹霍尔海洋学研究所和密歇根大学的研究生教育，并将包括来自美国，瑞士和马达加斯加。本组织与一个地方组织合作，为马萨诸塞州的残疾人提供就业和康复机会。此外，一个实践教育项目将鼓励未被充分代表的少数民族学生参与地球科学。这个合作项目将检验一个假设，即相当大的岩体（其起源和年龄未知）是造成马达加斯加南部超高温岩浆的主要热源，这是一个在冈瓦纳古陆新元古代晚期至寒武纪早期组装期间形成的典型超高温岩浆。先前的研究表明，传导性、机械性和放射性热源结合在一起无法将中地壳温度提高到900 °C，因此地幔岩浆可能是缺失的热源。锆石Hf，三角洲18 O，和U-Pb年龄，结合全岩主要和微量元素的数据，将被用来约束地幔与地壳熔体在马达加斯加南部的主要深成岩套的比例，并估计岩浆通量。对寄主片麻岩中的锆石和独居石的进一步分析将确定岩浆作用和超高温热处理是否为同生。通过解决地幔热平流的时间尺度和程度，这些结果将测试是否有岩浆活动和区域UHTM之间的因果关系。在这样做的过程中，这项研究将提高区域热收支的理解，并提供一个全面的U-Pb锆石数据集马达加斯加南部。结果将有直接的影响，在斯里兰卡，印度和南极洲记录的同时代的UHTM事件，并将告知我们的了解一般的中生代碰撞造山带。此外，该项目还将开发双多接收器激光烧蚀分流ICPMS技术，使多接收器仪器能够同时测量Hf和U-Pb同位素。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。

The collision of Earth’s continents, as a result of plate tectonics, causes the crust to thicken, buckle, and become hotter, leading to lasting transformations: metamorphic mineral reactions, the formation of magma, and the reorganization of elements throughout the Earth’s crust. These processes that permanently alter the continents are most extreme in ultrahigh-temperature metamorphic (UHTM) rocks, where temperatures exceeded 900 °C. Recognizing these transformations in rocks from the Earth’s past is fundamental to understanding how the continents have changed through time. Yet, why UHTM occurs in some continental collision zones, but not others, remains unknown. This project will address the unresolved question of how and why rocks reach such ultrahigh temperatures by investigating the UHTM rocks exposed in southern Madagascar. Specifically, this work will test the hypothesis that magmas ascended from Earth’s mantle and heated the rocks of southern Madagascar to temperatures exceeding those of typical mountain belts, causing UHTM. Age dating and chemical analyses of the mineral zircon will establish the timing and origins of magma intrusions, respectively, to determine their relationship to UHTM. This project will support graduate student education at Woods Hole Oceanographic Institution and the University of Michigan, and will include cross-cultural collaboration among scientists from the U.S., Switzerland, and Madagascar. In partnership with a local organization, it provides employment and rehabilitation opportunities for disabled individuals in Massachusetts. In addition, a hands-on education project will encourage underrepresented minority students to participate in the geosciences.This collaborative project will test the hypothesis that sizable plutons (the origins and ages of which are unknown) were the principal heat source responsible for UHTM in southern Madagascar, an archetypal UHTM terrane formed during the Late Neoproterozoic to Early Cambrian assembly of Gondwana. Prior research suggests that conductive, mechanical, and radiogenic heat sources combined were incapable of raising mid-crustal temperatures to 900 °C, so mantle magmas may be the missing heat source. Zircon Hf, delta 18O, and U-Pb ages, combined with whole rock major- and trace-element data, will be used to constrain the proportion of mantle versus crustal melts in the major plutonic suites in southern Madagascar and estimate the magmatic flux. Additional zircon and monazite analyses in host gneisses will establish whether magmatism and UHTM were contemporaneous. By resolving the timescale and extent of mantle heat advection, these results will test whether there was a causal link between magmatism and regional UHTM. In doing so, this study will enhance understanding of the regional heat budget and provide a comprehensive U-Pb zircon dataset for southern Madagascar. The results will have direct implications for coeval UHTM events recorded in Sri Lanka, India, and Antarctica, and will inform our understanding of Phanerozoic collisional orogens in general. Additionally, this project will develop dual-multicollector laser ablation split stream ICPMS techniques that will enable simultaneous measurement of Hf and U-Pb isotopes on multicollector instruments.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.