組成累帯クロムスピネルからみた青海蛇紋岩メランジュを構成する蛇紋岩の起源 [Origin of serpentinites in the Omi serpentinite melange (Hida Mountains, Japan) deduced from zoned chromian spinel]

Abstract

Ultramafic rocks of the Omi serpentinite melange are extensively serpentinized. However, newly discovered Cr-spinels in chromitite and massive serpentinite provide a key to decipher the origin and metamorphism of serpentinites. Cr-spinels show distinct chemical zoning: the cores have high Cr# [=Cr/(Cr+Al) atomic ratio] (0.70-0.77); Mg# [=Mg/(Mg+Fe2+) atomic ratio] decreases remarkably towards the rim through a transition zone between core and ferritchromite rim. Moreover, the cores of zoned Cr-spinel in chromitite contain abundant tiny inclusions of mainly pargasitic amphibole (up to 3.8 wt.% Na2O) and rare diopside. These inclusions were completely replaced by secondary tremolitic amphibole and chlorite at the rim, and by dolomite at the Ti-enriched metasomatized rim. Although the Mg# of the core (0.20-0.43) is significantly lower than those of Cr-spinel in both Alpine and abyssal peridotites, the observed mineral inclusions and the analysed Cr# at the core are interpreted to be relics of igneous stage. Compositional characteristics and the occurrence of hydrous mineral inclusions of the zoned Cr-spinel exhibit critical features that suggest a supra-subduction origin. Serpentinites of the Omi serpentinite melange may have derived from the mantle wedge above a subduction zone and subsequently metamorphosed at low- to mid-temperature condition of probably eclogite or amphibolite facies. [飛騨山地, 青海蛇紋岩メランジュを構成する超苦鉄質岩は著しく蛇紋岩化し, その起源や変成作用を明らかにするための情報に乏しかった. しかし, 塊状のアンチゴライト蛇紋岩と, それに伴うクロミタイト脈から, 初生的なCr/(Cr+Al) 原子比 (0.70-0.77) をコアに保持した組成累帯クロムスピネルと, 包有物としての初生的なパーガス閃石 (~3.8wt.% Na2O) を初めて見出した. 蛇紋岩の源岩は変成作用を被っており, クロムスピネルのコアは低Mg# (0.20-0.43) で特徴づけられる. さらに, リムにおいて初生鉱物包有物のドロマイト化と Ti の付加が部分的に認められる. これらの特徴から, 青海蛇紋岩メランジュを構成する蛇紋岩は, 沈み込み帯のマントルかんらん岩を起源とした低~中程度の温度の変成かんらん岩が, より低温で蛇紋岩化したものと推測される.]

Publication
地質学雑誌 (Journal of the Geological Society of Japan), v. 110, no. 10, p. 591-597, doi:10.5575/geosoc.110.591