Recognition of the ‘early’ Sambagawa metamorphism and a schematic cross-section of the Late-Cretaceous Sambagawa subduction zone

Abstract

Recent petrological studies on the Sambagawa metamorphic belt in Shikoku have recognized that the coarse-grained eclogite-bearing lithologies (so-called tectonic blocks in earlier studies) in the Besshi area exclusively preserve evidence for the early Sambagawa metamorphism, which can be related to onset of the Sambagawa subduction system during Early Cretaceous (c. Ma). Geological mapping and associated multidisciplinary studies on the regional (spatially widespread) Sambagawa metamorphism (both the eclogite-facies and main metamorphisms) have revealed the tectonic framework of the Late-Cretaceous Sambagawa subduction zone as follows: (i) a spreading ridge was approaching close to the trench; (ii) the subducting slab was coupled with the convective mantle at depth of > km; (iii) thickness of the hanging-wall continental crust waskm; and (iv) the forearc mantle wedge ( -km depth) was largely serpentinized. These features allow us to draw a semi-quantitative cross-section of the Sambagawa subduction zone at around -Ma, implying that boundary conditions for thermo-mechanical modeling aiming to simulate exhumation of high-P/T metamorphic rocks are now well constrained. It has also become clear that ultramafic blocks dispersed in the higher-grade part of the Sambagawa belt were derived from the mantle wedge, i.e. the corresponding part of the belt has been re-evaluated as a fossil subduction boundary of a relatively warm subduction zone. Field-based petrological studies in the Sambagawa belt can, therefore, have potential to provide invaluable information on material behaviors at the slab-mantle wedge interface including domains of episodic tremor and slip (ETS) in presentday warm subduction zones.

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

“…In Japan, eclogites are restricted to the following locations: the Sanbagawa metamorphic belt (e.g. Aoya, 2001; Aoya & Endo, 2017; Aoya et al., 2013; Ota, Terabayashi, & Katayama, 2004; Wallis & Aoya, 2000; Wallis & Okudaira, 2016; Weller, Wallis, Aoya, & Nagaya, 2015); the Chugoku Mountains (Nakamizu, Okada, Yamazaki, & Komatsu, 1989; Tsujimori, 1998, 2010; Tsujimori & Itaya, 1999; Tsujimori & Liou, 2004); and the HCB (Tsujimori, 2002; Tsujimori et al., 2000a, 2000b). The eclogites of the HCB are particularly significant because they are the remnants of Palaeozoic subduction along the margin of the South China craton, which were ultimately incorporated into the Japanese archipelago (Isozaki, Maruyama, Nakama, Yamamoto, & Yanai, 2010; Tsujimori, 2010).…”

A new occurrence of retrogressed eclogite from the Sanbagawa belt of southwest Japan and its significance

“…The P–T data for the non‐eclogite unit (Chlorite, Garnet, Albite–biotite and Oligoclase–biotite zones) are from Enami et al (). The source of the P–T data regarding eclogite unit is the compilation by Aoya and Endo (). The reaction curve: albite = jadeite + quartz was calculated using the THERMOCALC software ver.…”

“…Two different types of eclogites (coarse‐ and fine‐grained types) exist within the Sanbagawa eclogite unit, with comparable peak eclogite facies P–T conditions (see review by Aoya & Endo, ). Fine‐grained schistose eclogites (Seba type) in the Seba and Kotsu areas have a Lu–Hf age of ca 89 Ma (Late Cretaceous), and the ages are interpreted as the peak eclogite facies metamorphism (Wallis et al, ).…”

“…Aoya, ). Wallis and Aoya () proposed the presence of two units in high‐grade area along this belt: an independent large‐scale eclogite facies tectonic unit (eclogite unit) and the underlying non‐eclogite unit, which is referred to as the Shirataki unit (Aoya & Endo, ). Moreover, petrological studies have reported omphacite inclusions within the garnet grains of the pelitic schists from the Besshi region (Kouketsu & Enami, ; Kouketsu, Enami, & Mizukami, ; Sakurai & Takasu, ).…”

Origin of the Tonaru body in the Sanbagawa metamorphic belt, SW Japan

“…Ota et al (), Terabayashi et al () and Utsunomiya et al () pointed out that the early metamorphism may have been caused by crustal thickening associated with the formation of oceanic crust prior to subductions. On the other hand, Endo et al () and Aoya and Endo () estimated that the early‐stage metamorphism was caused by hot‐slab subduction. With regards to the prograde stage metamorphism, the metamorphic grade varies from epidote‐blueschist facies (T = 300–450 °C and P = 0.7–1.1 GPa) to eclogite facies (T = 700–730 °C and P ≥ 1.5 GPa) (e.g.…”