Reaction microtextures in jadeitites from the Nishisonogi metamorphic rocks, Kyushu, Japan

概要

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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 NaAlSi2O6 composition (Jd100-Jd95). The volume fraction of the quartz inclusions (VQtz/(VJd + VQtz) = 0.20-0.28) in the jadeite core is close to the value (VQtz/(VJd + VQtz) = 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.
日本鉱物科学会の論文
2005-12-01