Stability of Al- and F-rich titanite in metacarbonate: petrologic and isotopic constraints from a polymetamorphic eclogitic marble of the internal Sesia Zone (Western Alps)

Abstract

Al- and F-rich titanite (3.86< Al2O3 < 9.33 wt%, 0.93< F < 2.53 wt%) from a polymetamorphic marble of the Sesia Zone (Western Alps) has been investigated in order to determine the behaviour of titanite during high-pressure metamorphism. Meso-structural to micro-structural relationships, mineral assemblages and petrological data indicate a pre-Alpine (low-pressure, high-temperature) to early-Alpine (high-pressure, medium-temperature) pressure-temperature-time evolution. Backscattered electron images, X-ray qualitative elemental maps and electron microprobe analysis show that most Al- and F-rich titanite that occurs as isolated crystals is rather homogeneous in composition. In contrast, titanite associated with, or rimmed by, typical early-Alpine, high-pressure minerals is characterized by variable Al1(F, OH)1 TlO1 substitution within single crystals, particularly at grain boundaries with omphacite and/or phengite. In-situ ion microprobe U-Pb analysis of titanite domains that have various Al and F contents yielded apparent 206Pb/238U ages scattering between 283 and 153 Ma. Chemical and petrological data are indispensable to interpret this complex age distribution, and the good correlation between 206Pb/238U ratios and Al content indicates that the Al- and F-rich titanite was formed during pre-Alpine metamorphism (≥281 ± 11 Ma). Progressively younger ages are obtained in domains with decreasing Al and F content, suggesting that partial chemical re-equilibration was responsible for the incomplete isotopic resetting during Alpine metamorphism. Petrological and U-Pb data show that Al- and F-rich titanite should be used with caution to infer high-pressure conditions in polymetamorphic carbonate systems.