What Causes UHT Metamorphism: Lengthscales and Timescales

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Crustal rocks that have undergone regional metamorphism at temperatures greater than 900°C are thought provoking; explaining how such high temperatures are attained in the crust is challenging. Central to developing a cogent explanation is quantifying the spatial and temporal scales over which such high temperature prevailed: short spatial and temporal scales imply localized plutonism or fluid flow, whereas long scales indicate larger scale processes. Southern Madagascar represents a key place for studying ultrahigh-temperature metamorphism. There are, however, two key pieces of information missing from our understanding of this otherwise excellent study area: a quantification of how peak temperature varied with depth and with position at a specific depth, and a quantification of how temperature evolved with time. Three endmember hypotheses for the cause of ultrahigh-temperature metamorphism--subduction beneath an arc, collisional thickening and plutonism, and extreme collisional thickening--will be tested using relatively new techniques: titanium-in-zircon, titanium-in-quartz, and zirconium-in-rutile thermometry and pseudosection modeling, in conjunction with laser-ablation split-stream uranium/thorium-lead dates and trace elements of monazite and zircon. Because Southern Madagascar is an archetypal example of exceptionally hot crust, ideas developed there can be applied globally. This proposal is targeted toward understanding how exceptionally hot crust develops, but it will also impact the following questions: What processes are active today beneath hot collision orogens such as Tibet? How do the rheology, topographic, density, and composition of Earth?s crust evolve during collisions? What role did Madagascar play in the evolution of the East African-Antarctica Orogen? These topics are of cross-disciplinary interest in tectonics, petrology, geochronology, geodynamics, geodesy and geophysics. This project brings together scientists from the U.S., Madagascar, Australia, and France. The PIs will interact closely and serve as mentors to each other's students. This provides cross-cultural experiences that ensure that the collaborative impact of this project will be long lasting. This project will provide training and research opportunities for an NSF Graduate Fellow, several undergraduates, and enable us to will continue visiting local elementary schools to expose potential future scientists to Earth science. Continued development of our laser-ablation split-stream technique will benefit the broad range of visitors to our laboratory, the development of laser-ablation systems, and the development of inductively coupled plasma mass spectrometers.