Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
REVIEWS
Jadeitites and associated metasomatic rocks from serpentinite mélanges in the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan: a review
Tadao NISHIYAMAYasushi MORIMiki SHIGENO
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2017 Volume 112 Issue 5 Pages 197-216

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Abstract

The Nishisonogi jadeitite is a peculiar example among Japanese jadeitites in terms of the age of formation and its genesis as metasomatic replacement (R–type). The Nishisonogi jadeitite is characterized by jadeite with a quartz–inclusion–rich core and quartz–free rim. The pressure–temperature (P–T) conditions of the Nishisonogi jadeitite is constrained by the assemblage of jadeite + quartz and of clinozoisite + paragonite + quartz. The jadeitite occurs as tectonic blocks in a sepentinite mélange together with metasomatic rocks such as omphacitite, albitite and zoisite rock. The protolith of the jadeitite is likely plagiogranite or adakitic granite as suggested by the rare earth element (REE) pattern of zircon. The zircon has a euhedral core with a U–Pb age of 126 ± 6 Ma with exhibiting sharp and flat boundary with metamorphic rim, which has a U–Pb age of 84 ± 6 Ma. The rim age is almost consistent with the 95–90 Ma age of the peak metamorphism of the coherent schists, as a host of the serpentinite mélange, as determined by 40Ar/39Ar method applied to glaucophane and phengite. Some jadeitite blocks show lithological zonations with albitite and other rocks in scale of several tens of centimeters. These zonations likely formed in the stability field of albite during exhumation of the serpentinite mélange. The isocon analysis of whole rock compositions and the singular value decomposition analysis of mineral reactions show that SiO2, Na2O, and Fe2O3 evolved from the jadeitite towards serpentinite according to the retrograde reactions.

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© 2017 Japan Association of Mineralogical Sciences
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