Fluid-driven resetting of titanite following ultrahigh-temperature metamorphism in southern Madagascar

LA-ICP-MS U-Pb dates and EPMA trace-element maps of titanite were collected from calc-silicate gneisses of the Ikalamavony (peak T: 750-800 degrees C) and Anosyen (peak T: 900-950 degrees C) domains in southern Madagascar to evaluate how titanite responds to high-temperature metamorphism, cooling, and retrogression. Fluid-mediated replacement of precursor titanite by titanite domains of different composition (interface-coupled dissolution-precipitation, ICDR) was the primary mechanism by which titanite was reset following high-grade metamorphism. Comparison of titanite U-Pb dates (530-490 Ma) with independent petrology and thermochronology indicates that the alteration occurred at temperatures as low as 300-500 degrees C. Apparent Zr temperatures (temperatures calculated assuming titanite-quartz-zircon equilibrium) in altered titanite are less than or equal to the metamorphic peak, but higher than the inferred alteration temperature, implying that Zr was removed, but that titanite-quartz-zircon equilibrium was not achieved during alteration. Although evidence for ICDR was observed in titanite samples separated by 10's to similar to 100 km, differences in U-Pb dates among samples, and even among titanite grains in the same thin section suggest that alteration at any given time was localized.