Analysis of Mn-bearing lawsonite occurring in meta-siliceous rocks in Hakoishi serpentinite mélange of Kurosegawa Belt, Central Kyushu, Japan

Abstract

Mn -bearing lawsonite was discovered in meta -siliceous rocks metamorphosed under the lawsonite -blueschist facies formed at less than 350 °C and 0.8 -1.0 GPa in the Hakoishi serpentinite mélange of the Kurosegawa Belt, Central Kyushu, Japan. The lawsonite deposits were accompanied by those of hematite, braunite (Mn 2+Mn 6 3+SiO 12 ), and quartz, indicating that the rock had metamorphosed under high Oxygen fugacity (f O 2 ) (~ -20 < log f O 2 < -5) at the abovementioned P -T conditions. The Oxygen fugacity led to the conversion of all the iron into ferric compounds and some of the manganese into its trivalent form. As a result, the lawsonite was found to contain a significant amount of (Mn 3+ + Fe

White mica K–Ar ages from lawsonite–blueschist facies Hakoishi sub–unit and from prehnite–pumpellyite facies Tobiishi sub–unit of the Kurosegawa belt, Kyushu, Japan

“…The original igneous textures of hyaloclastite and pillow breccia are well preserved, although a schistose structure is developed in some metabasites in the western part of the Hakoishi sub-unit (Fujimoto et al, 2010). The chert shows various colors in the field such as white, red to pinkish, pale-green, brown and black depending on metamorphic minerals (Ibuki et al, 2008(Ibuki et al, , 2010, and commonly shows banding structures with a few cm thick and complicated folding structures, probably representing a slump structure. Metagabbro containing jadeite but without quartz is also reported by Saito and Miyazaki (2006).…”