Origin of omphacitites from the Nishisonogi metamorphic rocks, western Kyushu, Japan: comparison with jadeitites

Abstract

Two types of omphacitites are distinguished in ultramafic melanges 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.