Jadeitite exhibiting jadeite with quartz inclusions has been newly found in the Nishisonogi metamorphic rocks, Kyushu, Japan. The jadeitites occur in boulders from a riverbed, together with boulders of albitites, clinozoisite muscovite rocks, and serpentinites. The distribution of these boulders is confined to an area that is downstream from an outcrop of a serpentinite melange, suggesting that they were originally tectonic blocks in this serpentinite melange. The jadeitites consist mainly of jadeite with small amounts of muscovite, paragonite, phlogopite, albite, analcime, clinozoisite, and titanite. The jadeite consists of a core with abundant inclusions of quartz and omphacite, and a rim that is free from quartz inclusions. The quartz inclusions are in direct contact with the host jadeite, which has an almost pure NaAlSi 2 O 6 composition (Jd 100 Jd 95 ). The volume fraction of the quartz inclusions (V Qtz /(V Jd + V Qtz ) = 0.20 0.28) in the jadeite core is close to the value (V Qtz /(V Jd + V Qtz ) = 0.27) of quartz produced by the reaction albite = jadeite + quartz. These findings suggest that the jadeite core was produced by an isochemical breakdown of albite at high P/T conditions. In addition, the jadeite is locally replaced by albite and/or analcime at the rim and along microfractures. These microtextures provide information to deduce a retrograde P T path during the exhumation of the jadeitite.

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.

The slab-mantle interface in subduction zones is a site of tectonic mixing of crustal and mantle rocks. It is the interface for fluid flow of slab-derived components into the mantle wedge. To assess the fluid-rock interaction along the slab-mantle interface, we studied the bleaching of pelitic schist in an ultramafic mélange. The Nishisonogi metamorphic rocks in Kyushu, Japan, comprise ultramafic mélanges intercalated with epidote-blueschist facies schists. The ultramafic mélange consists of tectonic blocks of various lithologies and a matrix of chlorite-actinolite schist and serpentinite. Along the contact with the mélange matrix, pelitic schist blocks are bleached mainly due to the modal increase of albite and the consumption of carbonaceous material. The bleaching is probably attributed to infiltration of Na-rich external fluid from the mélange matrix. Mass balance analysis indicates losses of C, Rb, K 2 O, Ba, Pb, and SiO 2 from the bleached pelitic schist, although Al 2 O 3 , TiO 2 , Sc, Y, Zr, Nb, La, Ce, and Nd remain immobile. This suggests fractionation of large-ion lithophile elements (LILE) and Pb from the high-field-strength elements and rare earth elements during the bleaching. If this ultramafic mélange is analogous to the slab-mantle interface, similar infiltration metasomatism will promote liberation of C, Si, LILE, and Pb from subducting metapelites and enhance metasomatism of the mantle wedge.

Two types of omphacitites are distinguished in ultramafic mélanges that are intercalated with epidote-blueschist facies schists of the Nishisonogi metamorphic rocks in western Kyushu, Japan. One is an omphacitite layer in a metabasite block, and the other is an omphacitite lens in an albitite block. These omphacitites are principally composed of omphacite with minor amounts of winchite/magnesiohornblende, epidote, albite and titanite. Although the petrographic features of both types are similar, the omphacitite layer is depleted in light rare earth elements (LREE; La, Ce and Nd) compared with the omphacitite lens. In the case of jadeitites, those interpreted as being formed by direct precipitation from aqueous fluids are LREE-depleted, while those interpreted as being formed by metasomatic replacement from protoliths are LREE-enriched. Thus, by analogy, the omphacitite layer may have directly precipitated from aqueous fluids, and the omphacitite lens may be metasomatic in origin. The omphacitite layer has amphibole-rich selvages, which are likely formed by the reaction between the omphacitite-forming fluid and the host metabasite. Mass-balance analysis using the isocon method indicates that the formation of the selvages involves additions of large ion lithophile elements (LILE; Rb, K and Ba), Na and Si to the host metabasite. These components were likely introduced via fluid infiltration. Although the source of the omphacitite-forming fluid is not clear, the LILE-rich composition suggests participation of fluids derived from metapelitic rocks.