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

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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.