Abstract

Garnet–pyroxene–wollastonite zoned skarns in the Hirao limestone, Fukuoka, Japan, were formed during metamorphic activity associated with the emplacement of the Hirao granodiorite. In this area, rocks along the contact between a marble unit and a metamorphosed basic dike (MB) were replaced by clinopyroxene, garnet, wollastonite, and vesuvianite during infiltration metasomatism involving H2O-rich fluids. Whole-rock analysis of skarn-, marble-, and MB-hosted mineral zone samples indicates that the garnet zone skarn rocks are enriched in rare earth elements (REE). Marble trace element systematics indicate that Ce, alkali elements, Th, Pr, Zr, and Nb were all added during metamorphic activity. The majority of these elements were derived from metamorphic fluids, although some originated from the MB. Micro-scale mineral analysis was performed using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), revealing that vesuvianite preferentially hosts the REE within the garnet zone rocks, even though it is only present in small amounts (3% modal abundance). The vesuvianite commonly co-exists with a Ca-rich garnet and both minerals can contain significant concentrations of REE. The results suggest that the distribution of the REE in garnet and vesuvianite may be a function of fluid composition, with REE-enriched vesuvianite forming during interaction with light rare earth element (LREE)-enriched but heavy rare earth element (HREE)-unenriched fluids. The correlation of REE concentrations and Eu anomalies in vesuvianite indicates that the fluid was REE-enriched but Eu-depleted..

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