Petrology and possible significance of barroisite–bearing metabasite from the Kebara Formation in NW Kii Peninsula

Abstract

We report the new finding of barroisite (Brs)-bearing metabasites within a metabasite layer from the Kebara Formation, a unit exposed between the Sanbagawa and the Chichibu belts in NW Kii Peninsula. The dominant lithotype of the metabasite layer shows pale-green colors and is mainly composed of sodium amphibole, actinolite, pumpellyite and epidote. It is in agreement with reported mineral assemblages in the Kebara Formation which document pumpellyite-actinolite (PA) or pumpellyite-blueschist (PBS) facies conditions (<340°C and 0.8 GPa), and with geothermometry based on the Raman analysis of carbonaceous material from metapelite samples which give peak metamorphic temperatures of 300-340°C. Brs grains are identified from metabasites with dark-green color in the layer, and are closely associated with epidote, chlorite, white mica, albite and quartz, but not with pumpellyite. Brs grains are replaced by sodium amphibole and/or winchite at the rim with a distinct compositional gap. Thermodynamic calculation suggests that the Brs + epidote + chlorite + albite + quartz assemblage is stable at P-T conditions higher than 450°C and 0.4 GPa. The abovementioned data suggest that the Brs-bearing metabasites suffered an early higher temperature (>450°C) metamorphism and then overprinted by PA or PBS facies metamorphism along with the main constituents of the Kebara Formation. In the Besshi area of the Sanbagawa belt, the earlier subducted higher grade rocks are considered to juxtapose to the newly subducted rocks and overprinted retrograde metamorphism during their exhumation stage. Our new finding suggests that the similar phenomenon was took place in the lower grade part of the Sanbagawa belt.

2.13 Ga Lawsonite/Barroisite-Bearing E-Morb Signature Metagabbro Associated with Spinel Metaperidotite from Itaguara (São Francisco Craton, Brazil): Oldest Blueschist-Facies Fragment of Oceanic Moho?

“…Several studies demonstrate that barroisite-bearing metabasites retrometamorphosed at epidote amphibolite-facies have been formed in association with tectonic processes of subduction followed by collision-related exhumation [10,11] . By using petrography, mineral chemistry, thermobarometry, geochemistry, and titanite U-Pb geochronology, the petrology of the recently discovered lawsonite and barroisite-bearing metagabbro associated with spinel metaperidotite, which occur alongside 2.…”

Amphibole in UHP eclogite from the Sulu region, eastern China

“…Its breakdown can trigger melt generation in a subduction zone (Tatsumi, 1989). On the other hand, natural amphiboles in mafic rocks of a subducting plate are generally sodic and sub-calcic, such as glaucophane/riebeckite and barroisite, at medium depths of a subduction zone, respectively (e.g., Otsuki and Banno, 1990;Vitale Brovarone and Agard, 2013;Hyppolito et al, 2016;Kabir and Takasu, 2016;Kato and Hirajima, 2017). Furthermore, some petrological observations of natural UHP metamorphic rocks suggest that Na-rich amphiboles can be stable even in the UHP conditions, such as glaucophane in the sodic whiteschist of the Dora-Maira Massif, western Italy (e.g., Kienast et al, 1991), and nyböite in eclogites from the Western Gneiss Region of Norway and the Sulu region of eastern China (e.g., Smith, 1988;Hirajima et al, 1992).…”