Jadeite–bearing metaigneous rocks from the Northern Chichibu belt, SW Japan: implications for the lowest–grade Sanbagawa metamorphism

Abstract

Multiple generations of Na-Ca and Na pyroxenes (aegirine-augite, aegirine and jadeite) were found in a metamorphosed monzonitic dike from a coherent mafic layer of the Northern Chichibu belt in central Shikoku, Japan. The mafic layer was derived from alkaline basalt magma and its derivatives, and belongs to the Kamiyoshida unit (Middle Jurassic accretionary complex). The earliest-stage sodic pyroxene (jadeite-free Ti-rich aegirine-augite to aegirine) in the dike probably crystallized during a post-magmatic hydrothermal stage. Igneous Ti-rich augite phenocrysts were almost completely pseudomorphed by chlorite + phengite + Al-OH-rich titanite + sodic pyroxene (aegirine-augite with jadeite component) during early stages of subduction metamorphism. The earliest-stage sodic pyroxene and pseudomorphed Ti-rich augite were further overgrown by jadeite/ aegirine fringes during high-P/T metamorphism. Jadeite/aegirine pyroxene also occurs as small (~50 µm long) neoblasts within pseudomorphs (albite + phengite + pumpellyite ± Ba-rich K-feldspar) after igneous plagioclase. Individual grains of the fringe/neoblastic pyroxene are zoned with a jadeite core (up to 98% jadeite content) and an aegirine rim (down to 25% jadeite content). Jadeite-rich pyroxene (up to 92% jadeite content) was also found in basaltic rock from the mafic layer. The absence of quartz and the alkaline affinity of the protoliths suggest that the jadeite formation can be explained by the reaction analcime = jadeite + H 2 O. If H 2 O fluid was present, the decrease in jadeite content in the fringe/neoblastic pyroxene implies decompression from 0.6-0.7 GPa at 300°C to~0.4 GPa at 210-260°C. The formation of the jadeite-to-aegirine pyroxene during exhumation probably resulted from the lowest-grade Sanbagawa metamorphism.

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge, SW Japan

“…In contrast, Murata et al. () and Endo () showed widespread evidence for high‐ P metamorphism in the B unit, and suggested that there is no large gap in metamorphic conditions across the A–B unit boundary. Moreover, some authors treat the A and B units as a single high‐ P unit, and instead proposed the presence of a significant pressure gap between the A–B and C units (Murata et al., ; Sakakibara et al., ).…”

“…In some quartz-free samples, alkali pyroxene crystals exhibit concentric compositional zoning. One type of zoning showing a rimward increase in jadeite content occurs in the Mikabu unit, and the other type with a rimward decrease in jadeite content along the Jd-Aeg join has been reported from the B unit (Endo, 2015) (Figure 8a).…”

“…There are no age data for the metamorphic veins in the C unit, but decompression‐stage epidote‐ or Fe‐rich prehnite‐bearing veins may have formed during the Cretaceous metamorphism. Prograde to peak‐ P metamorphic records in the Northern Chichibu belt are limited to some single‐phase decomposition reactions such as calcite → aragonite in the A unit (Suzuki & Ishizuka, ), analcime → jadeite+H 2 O in the B unit (Endo, ) and laumontite → lawsonite+2quartz+2H 2 O in the C unit. Under low‐ T conditions (<300°C), multivariant reactions in a basaltic system may have only partially developed during burial stages due to the short timescales required for burial up to the maximum depths compared with post‐accretion, long‐term histories in the wedge.…”

“…Dallmeyer, Takasu, & Yamaguchi, 1995;Sakakibara et al, 1998). In addition, Suzuki and Ishizuka (1998) and Endo (2015) reported metamorphic aragonite from the Mikabu-A unit and near end-member jadeite in quartz-free metaigneous rocks from the B unit respectively. In contrast, the C and D units are composed mainly of mudstonematrix m elange units with no high-P index minerals, and metamorphism of these units has been considered as low-P (<0.3 GPa) prehnite-pumpellyite or zeolite facies (Murata, Yamazaki, & Maekawa, 2009;Sakakibara et al, 1998;Suzuki & Ishizuka, 1998).…”

Thermal structure of the Kebara Formation and its proximal areas in the western Kii Peninsula, SW Japan: Application of the carbonaceous material Raman geothermometry

“…On the basis of the Na-pyroxene composition and RSCM geothermometry, Endo (2015) estimated metamorphic conditions of the Kamiyoshida unit of the Northern Chichibu belt, central Shikoku, as~300°C and 0.6-0.7 GPa, suggesting the southwards gradual decrease of metamorphic temperature from the Sanbagawa metamorphic belt located to the north. The occurrences of metamorphic aragonite and sodic-amphibole + pumpellyite assemblage have been reported from the Mikabu belt and the northern part of the Northern Chichibu belt in central Shikoku (Suzuki, 1995;Suzuki and Ishizuka, 1998), which have not been identified in the low-grade part of the Sanbagawa metamorphic belt (sensu stricto).…”