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

由于板块构造作用，地球各大洲之间的碰撞导致地壳变厚、弯曲、变热，从而导致持久的变化：变质矿物反应、岩浆的形成以及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°C。认识到地球过去岩石的这些变化是理解大陆如何随时间变化的基础。然而，为什么UHTM发生在某些大陆碰撞带，而不是其他大陆碰撞带，仍然是未知的。这个项目将通过调查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超高温度的悬而未决的问题。具体来说，这项工作将检验这样一个假设，即岩浆从地幔上升，并将马达加斯加南部的岩石加热到超过典型山带的温度，从而导致了UHTM。锆石的年代测年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代表性不足的少数民族学生参与地球科学。这个合作项目将测试一个假设，即大型岩体（其起源和年龄未知）是造成马达加斯加南部UHTM的主要热源，这是一个典型的UHTM地体，形成于冈瓦纳的新元古代晚期到早寒武纪。先前的研究表明，导电热源、机械热源和放射成因热源结合起来无法将中地壳温度提高到900℃，因此地幔岩浆可能是缺失的热源。锆石Hf、δ 18O和U-Pb年龄，结合全岩主元素和微量元素数据，将用于约束马达加斯加南部主要深部构造套中地幔与地壳熔体的比例，并估算岩浆通量。另外对寄主片麻岩中的锆石和独居石进行分析，将确定岩浆活动和超热成因是否发生在同时期。通过解析地幔热平流的时间尺度和程度，这些结果将检验岩浆活动与区域UHTM之间是否存在因果关系。因此，本研究将增强对马达加斯加南部区域热收支的认识，并提供一个全面的U-Pb锆石数据集。这些结果将对在斯里兰卡、印度和南极洲记录的同期UHTM事件有直接的影响，并将告诉我们对显生宙碰撞造山带的一般理解。此外，该项目将开发双多集热器激光烧蚀分流ICPMS技术，该技术将能够在多集热器仪器上同时测量Hf和U-Pb同位素。该奖项反映了美国国家科学基金会的法定使命，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.