Dating Oligocene metasomatism of skarns in Mogok metamorphic belt using in situ vesuvianite geochronology

Fluid metasomatism in marbles (skarnization) is a crucial mechanism for gemstone formation and metal deposits in the Mogok metamorphic belt (MMB, Myanmar). However, the precise time for the metasomatism is unknown due to a lack of suitable mineral chronometers. In this study, we use vesuvianite U–Pb chronology and mineral chemistry to constrain the timing of fluid metasomatism in marbles and the chemical characteristics of the reactive fluids. Two representative calc-silicate rocks from the MMB, which are the metasomatized products of calcite marbles by infiltrating fluids, were chosen. The occurrence, nanoscale structure, U–Pb ages, and compositions of vesuvianite in both samples were investigated. The MMB vesuvianite occurs as both porphyroblastic and matrix minerals in the calc-silicate rocks and is texturally equilibrium with garnet, clinopyroxene, plagioclase, and other silicates, indicating the growth of vesuvianite during metasomatism. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis reveals no U-rich nanoscale particles in the vesuvianite. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis yielded the well-defined lower intercept ages of 29.8 ± 1.8 Ma (2σ, MSWD = 2, n = 20) and 29.6 ± 2.0 Ma (2σ, MSWD = 1.5 Ma, n = 22) in the two samples. The ages are consistent with those of syenites in the MMB and thus reveal that the reactive fluids are most likely related to the syenite intrusions. The high concentrations of Th (80–2912 μg/g), U (34–1603 μg/g), and LREEs (216–735 μg/g for La) in the vesuvianite and high abundance of this mineral demonstrates that the reactive fluids transfer significant amounts of these elements into the marbles. This study indicates that the metasomatic formation of Ca-skarns in the MMB occurred in Oligocene during the India-Asia convergence. We highlight that vesuvianite is a powerful mineral that could be used for accurately determining the timing of skarn-type ores and gemstones, fluid-carbonate interaction, and orogenic CO₂ release.