UC Santa Barbara

This dissertation is an attempt to gain a better understanding of the tectonic significance of a particular subset of metamorphic rocks—ultrahigh-temperature metamorphic rocks—that are prevalent throughout Earth’s history, but for which there is no consensus as to how they form. The dissertation consists of three chapters: (1) a case study of an ultrahigh-temperature terrane exposed in southern Madagascar, (2) the development of a new technique for calculating the maximum temperatures reached by a metamorphic rock—one of the most fundamental data types upon which tectonic interpretations are made—based on the oxygen-isotope composition of minerals within the rock, and (3) an empirical evaluation of how quickly elements diffuse in the mineral titanite to gain a better understanding of the geological significance of U–Pb titanite dates obtained from high-temperature rocks. Ultrahigh-temperature metamorphism in southern Madagascar is found to have been the result of combined magmatic advection, high rates of radiogenic heat production, and a thin mantle lithosphere. Peak temperatures of metamorphism can be recovered from the oxygen isotope compositions of minerals so long as one of the minerals has a sufficiently high closure temperature (such as garnet or sillimanite). Elemental diffusion in titanite is sufficiently slow that U–Pb dates likely reflect the time and conditions of titanite (re)crystallization, not cooling ages, in all but the highest grade rocks. Data tables referenced in the main text are available as spreadsheets in the supplementary materials accompanying this dissertation.