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

Abstract

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.

Fluid-metapelite interaction in an ultramafic mélange: implications for mass transfer along the slab-mantle interface in subduction zones

“…The ultramafic mélanges contain tectonic blocks of metagabbro, mafic schist, pelitic schist, and albitite in a matrix of chlorite-actinolite schist, talc schist, and schistose serpentinite (Nishiyama 1989(Nishiyama , 1990. Small blocks (<5 m in diameter) of jadeitite, omphacitite, rodingite, and zoisitite are found locally (Nishiyama 1978;Shigeno et al 2005Shigeno et al , 2012aMori et al 2011).…”

Jadeitites and associated metasomatic rocks from serpentinite mélanges in the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan: a review

“…This indicates the origin of the jadeitite as metasomatic replacement (R-type as defined by Tsujimori and Harlow, 2012), whereas both R-type and P-type (fluid-precipitates) are recognized in jadeitites from the Renge belt (Kunugiza and Goto, 2010;Tsujimori and Harlow, 2012). Thus the Nishisonogi jadeitite has a unique feature among Japanese jadeitites, which has been well documented by Nishiyama (1978), Shigeno and Mori (2003), Shigeno et al (2005Shigeno et al ( , 2012a, and Mori et al (2008Mori et al ( , 2010Mori et al ( , 2011. This paper reviews the nature of the Nishisonogi jadeitite in terms of occurrence, petrography, P-T conditions, and geochronology to coincide with the occasion of the selection of jadeitite as the 'national rock' of Japan.…”

“…As well as the case of Mie, no outcrop was found. Instead, more than ten jadeitite boulders of 1-4 meters in size were found (Shigeno and Mori, 2003;Shigeno et al, 2005). One of the boulders shows a zonation with albitite and muscovite rock and was studied in detail by Shigeno et al (2012a).…”

Phase relation in the NaAlSiO₄–SiO₂–H₂O system for the hydrothermal precipitation of jadeite, albite, natrolite, and analcime in jadeitite of the Itoigawa–Omi area, Japan

“…In Japan, jadeite with quartz often occurs in high P/T metamorphic rocks, with or without peridotite and serpentinite, in subduction zone metamorphic belts (e.g., Maruyama et al, 1978;Hirajima, 1983;Takayama, 1986;Shigeno et al, 2005;Osada et al, 2007;Tsujimori, 2017;Tsujimori and Harlow, 2017;Hirajima, 2017). Jadeitite without quartz is however limited to occur in the serpentinite mélange of Japan (Fig.…”

“…The jadeite-quartz assemblage, an index mineral assemblage characteristic of high-pressure metamorphism, occurs in the Japanese Islands (e.g., Maruyama et al, 1978;Hirajima, 1983;Takayama, 1986;Shigeno et al, 2005;Osada et al, 2007). These rocks are stable in the jadeitequartz field of the P-T grid (Fig.…”