Origin of zircon in jadeitite from the Nishisonogi metamorphic rocks, Kyushu, Japan

On the basis of internal structures, laser ablation U-Pb ages and trace element compositions, the origin of zircon in jadeitite in the Nishisonogi metamorphic rocks was examined. The zircon comprises euhedral zoned cores overgrown by euhedral rims. The cores contain inclusions of muscovite, quartz, albite and possibly K-feldspar, yield (238)U-(206)Pb ages of 126 +/- 6 Ma (+/- 2 SD, n = 45, MSWD = 1.0), and have Th/U ratios of 0.48-1.64. The rims contain inclusions of jadeite, yield (238)U-(206)Pb ages of 84 +/- 6 Ma (+/- 2 SD, n = 14, MSWD = 1.1), and have Th/U ratios of < 0.06. The cores are richer in Y, Th, Ti and rare earth elements (REEs), but the rims are richer in Hf and U. Chondrite-normalized REE patterns of the cores indicate higher Sm(N)/La(N) ratios, lower Yb(N)/Gd(N) ratios and larger positive Ce anomalies compared with those of the rims. Thus, the cores and rims have different (238)U-(206)Pb ages and trace element compositions, suggesting two stages of zircon growth. Although the (238)U-(206)Pb ages of the rims are consistent with the reported (40)Ar/(39)Ar spot-fusion ages of matrix muscovite in the jadeitite, the (238)U-(206)Pb ages of the cores are older. The mineral inclusions and high Th/U ratios in the cores are best explained by crystallization from felsic magma. Therefore, the cores are considered relicts from igneous precursor rocks. The rims surrounding the inherited cores possibly precipitated from aqueous fluids during jadeitite formation. The elevated U concentrations in the rims suggest that infiltration of external fluids was responsible for the precipitation. This study provides an example of jadeitite formation by metasomatic replacement of a protolith.