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

作为板块构造的结果，地球大陆的碰撞导致地壳增厚、弯曲并变得更热，导致持久的变化：变质矿物反应、岩浆形成和整个地壳元素的重组。这些永久改变大陆的过程在超高温变质岩(UHTM)中最为极端，那里的温度超过900摄氏度。识别地球过去岩石中的这些变化对于了解大陆如何随着时间的变化至关重要。然而，为什么UHTM发生在一些大陆碰撞带，而不是其他大陆碰撞带，仍然是未知的。这个项目将通过研究暴露在马达加斯加南部的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。具体地说，这项工作将检验一种假设，即岩浆从地球地幔上升，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。矿物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来源，以确定它们与UHTM的关系。该项目将支持伍兹霍尔海洋研究所和密歇根大学的研究生教育，并将包括来自美国、瑞士和马达加斯加的科学家之间的跨文化合作。它与当地组织合作，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。这一合作项目将检验这样一种假设，即相当大的深成岩(其来源和年龄未知)是马达加斯加南部UHTM的主要热源，UHTM地体是在新元古代晚期至冈瓦纳早寒武世组装期间形成的典型UHTM地体。先前的研究表明，传导热源、机械热源和辐射热源加在一起，无法将地壳中部的温度提高到900℃，因此地幔岩浆可能是缺失的热源。锆石Hf、18O和U-Pb年龄，结合全岩主量元素和微量元素数据，将被用来约束马达加斯加南部主要深成岩套中地幔熔体与地壳熔体的比例，并估计岩浆流量。对寄主片麻岩中的锆石和独居石的额外分析将确定岩浆作用和UHTM是否是同时代的。通过解析地幔热平流的时间尺度和范围，这些结果将检验岩浆活动与区域UHTM之间是否存在因果联系。通过这样做，这项研究将加强对区域热收支的了解，并为马达加斯加南部提供一个全面的锆石U-Pb数据集。这一结果将对斯里兰卡、印度和南极洲记录的同时代UHTM事件产生直接影响，并将有助于我们对显生界碰撞造山带的总体理解。此外，该项目将开发双多收集器激光消融分流ICPMS技术，使在多收集器仪器上同时测量Hf和U-Pb同位素成为可能。这一奖项反映了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，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.