Retrograde pumpellyite in the Yunotani garnet blueschist of the Omi area, Japan: An update on the cooling path

Abstract

Retrograde pumpellyite was newly found in garnet blueschist that is Mg-rich equivalent of late Paleozoic retrograde eclogite of the Yunotani Valley in the Omi area, Hida-Gaien Belt. The pumpellyite with high Al/ (Al + Mg + Fe) occurs in pressure shadows around garnets; it is associated with secondary glaucophane, epidote, chlorite, titanite, phengite, albite, and quartz, which all characterize a retrograde blueschist-facies mineral assemblage after peak eclogite-facies mineral assemblage. This feature is comparable with retrograde pumpellyite in late Paleozoic garnet blueschist (with relict eclogite-facies mineral assemblage) in the Osayama area of the Chugoku Mountains. Equilibrium phase calculation confirmed that the pumpellyite is stable at a low temperature and pressure portion of the lawsonite-blueschist-facies. T-bulk-composition (Mg) pseudosection suggests that pumpellyite appears preferentially in high Mg/(Mg + Fe) bulk composition. The limited occurrence of retrograde pumpellyite in the Yunotani garnet blueschist and retrograde eclogite would be explained by Mg-rich bulk compositions. Also, the limited occurrence in pressure shadows around garnets suggests that the fluid trapped in the pressure shadows might have enhanced growth (or precipitation) of pumpellyite. This finding provides a strong evidence that the deeply subducted (eclogite-facies) metabasaltic rocks both in the Hida-Gaien Belt and the Chugoku Mountains were subjected to a very similar blueschist-facies overprinting locally reached the pumpellyite stability field. The 'Franciscan-type' cooling path suggests a 'steady-state' underflow of the paleo-Pacific oceanic plate in late Paleozoic at a convergent margin of the South China Craton.

Early Carboniferous HP metamorphism in the Hida Gaien Belt, Japan: Implications for the Palaeozoic tectonic history of proto‐Japan

“…Consequently, we provide a brief summary of the results of Tsujimori (2002), Tsujimori and Matsumoto (2006), Tsujimori et al. (2000b), and Shinji and Tsujimori (2019).…”

“…Some of the glaucophane grains in the Shinadani eclogite are surrounded by actinolite, indicating greenschist facies retrograde metamorphism (Tsujimori, 2002). In addition, retrograde pumpellyite occurs in garnet–glaucophane schist in the Yunotani area (Shinji & Tsujimori, 2019).…”

“…In the Omi area, a boulder of eclogitic glaucophane schist contain a layer ~1.2 m thick with omphacite and garnet (Tsujimori, Ishiwatari, & Banno, 2000a). In contrast to the Happo‐one area, a hairpin‐shaped P – T path that reached peak P – T conditions of 550–600°C and at least 1.8 GPa was proposed for the eclogitic glaucophane schist (Shinji & Tsujimori, 2019; Tsujimori, 2002; Tsujimori & Matsumoto, 2006). Tsujimori (2002) further divided the schists in the Omi area into an eclogite unit (EC unit) and a non‐eclogite unit (non‐EC unit).…”

Boron isotope compositions of antigorite–grade serpentinites in the Itoigawa–Omi area of the Hida–Gaien Belt, Japan

“…The high-pressure schists of the Hida-Gaien Belt have been named the 'Renge Schists'; they mainly record greenschist-to amphibolitefacies metamorphism and locally preserve blueschist-to eclogite-facies metamorphism, indicating subduction zone metamorphism (Nakamizu et al, 1989;Tsujimori et al, 2000). In the Omi area, the Renge Schists are divided into two distinct groups: an 'eclogitic unit' and a 'non-eclogitic unit' (Tsujimori, 2002;Tsujimori and Matsumoto, 2006;Shinji and Tsujimori, 2019) (Fig. 1b).…”

Two groups of fluid inclusions in the Yunotani eclogite from the Hida–Gaien Belt: Implications for changes of fluid salinity during exhumation

“…However, the matrix omphacite is commonly replaced by chlorite, and initially coarse-grained glaucophane is reduced in grain-size. A pumpellyite-bearing retrograde blueschist-facies mineral assemblage is found in the garnet blueschist, an Mg-rich equivalent of the eclogite (Shinji and Tsujimori, 2019).…”