The Nishisonogi unit of the Nagasaki Metamorphic Rocks represents a part of a Late Cretaceous subduction complex exposed in western Kyushu, Japan. We estimate peak metamorphic temperatures using a Raman carbonaceous material (CM) geothermometer on 60 pelitic schists. No systematic regional changes were observed in the mineral assemblage of samples collected over a large area (about 30 × 15 km), which include chlorite ± garnet + white micas + albite + quartz + titanite + CM. However, the estimated peak metamorphic temperature increases structurally upward from 440 to 524 °C, suggesting an inverted thermal gradient.
An albite-omphacite rock, containing the three sodic pyroxenes of the jadeite-omphacite-diopside/ hedenbergite ternary system, has been investigated in detail to better characterize the phase relationships in this system. The rock is from a serpentinite mélange in the Kurosegawa Zone of Central Kyushu, Japan, and exhibits an omphacite overgrowth over diopside/hedenbergite and the further overgrowth of jadeite over omphacite, indicative of the successive replacement of pyroxenes during metamorphism. Partial replacement of omphacite by diopside/hedenbergite at the margins of K-feldspar veins suggests that all three pyroxenes were stable locally at the same stage of metamorphism. The peak temperature and pressure conditions are roughly estimated to be 350 °C and 5-10.8 kbar. Partitioning of Fe2+-Mg between pyroxene pairs is consistent with a recent thermodynamic model, indicating the attainment of local equilibrium for pyroxene pairs. The observed compositional gap is also consistent with the miscibility gap reported in many previous studies. The compositional field of omphacite in the ternary jadeite-aegirine-diopside/hedenbergite diagram extends from the jadeite-diopside/hedenbergite line toward the aegirine-rich field, maintaining approximately constant the jadeite content, resulting in an asymmetric phase diagram.
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.
Phase relation in the NaAlSiO 4 -SiO 2 -H 2 O system for the hydrothermal precipitation of jadeite, albite, natrolite, and analcime in jadeitite of the Itoigawa-Omi area, Japan ･････････････････････････････････････････Atsushi GOTO, Keitaro KUNUGIZA, Hiroshi MIYAJIMA ･･････ 271 Chemical composition of fluid inclusions in the Yorii jadeite-quartz rocks from the Kanto Mountains, Japan ･･････････････････････Mayuko FUKUYAMA, Tatsuhiko KAWAMOTO, Masatsugu OGASAWARA ･･････ 281 Cathodoluminescence petrography of P-type jadeitites from the New Idria serpentinite body, California
