The youngest blueschist belt in SW Japan: implication for the exhumation of the Cretaceous Sanbagawa high‐P/T metamorphic belt

Abstract

The tectonic evolution of the Northern Shimanto belt, central Shikoku, Japan, was examined based on petrological and geochronological studies in the Oboke area, where mafic schists of the Kawaguchi Formation contain sodic amphibole (magnesioriebeckite). The peak P-T conditions of metamorphism are estimated as 4-4.5 kbar (15-17 km depth), and 240-270°C based on available phase equilibria and sodic amphibole compositions. These metamorphic conditions are transitional between blueschist, greenschist and pumpellyite-actinolite facies. Phengite K-Ar ages of 64.8 ± 1.4 and 64.4 ± 1.4 Ma were determined for the mafic schists, and 65.0 ± 1.4, 61.4 ± 1.3 and 63.6 ± 1.4 Ma for the pelitic schists. The metamorphic temperatures in the Oboke area are below the closure temperature of the K-Ar phengite system, so the K-Ar ages date the metamorphic peak in the Northern Shimanto belt. In the broad sense of the definition of blueschist facies, the highest-grade part of the Northern Shimanto belt belongs to the blueschist facies. Our study and those of others identify the following constraints on the possible mechanism that led to the exhumation of the overlying Sanbagawa belt: (i) the Sanbagawa belt is a thin tectonic slice with a structural thickness of 3-4 km; (ii) within the belt, metamorphic conditions varied from 5 to 25 kbar, and 300 to 800°C, with the grade of metamorphism decreasing symmetrically upward and downward from a structurally intermediate position; and (iii) the Sanbagawa metamorphic rocks were exhumed from 60 km depth and emplaced onto the Northern Shimanto metamorphic rocks at 15-17 km depth and 240-270°C. Integration of these results with those of previous geological studies for the Sanbagawa belt suggests that the most probable exhumation mechanism is wedge extrusion.

Recognition of the Shimanto HP metamorphic belt within the traditional Sanbagawa HP metamorphic belt: New perspectives of the Cretaceous–Paleogene tectonics in Japan

“…Nano-SIMS zircon analyses, in situ REE analyses and inclusion mineralogy in zircons show that oligoclase-biotite zone metamorphic rocks, the highest-grade zone in the belt (Enami, 1982) after Aoki et al, 2007. (b) K-Ar phengite ages plotted along A-B cross-section (Itaya and Takasugi, 1988;Isozaki and Maruyama, 1991;Kawato et al, 1991;Aoki et al, 2008). (See Fig.…”

Complex microseismic activity and depth-dependent stress field changes in Wakayama, southwestern Japan

“…The Sambagawa belt consists of a variety of crystalline schists (e.g., Enami et al 1994) that formed as an accretionary prism at ~ 140-130 Ma and were metamorphosed at ~ 120-110 Ma under high-pressure, low-temperature conditions (Aoki et al 2008). Most of schistosity strikes N110°W to N80°W and dips 40°-90° to the north (Makimoto et al 2004).…”

Ca-Al hydrous silicates in the chlorite-grade pelitic schists in Sanbagawa metamorphic belt and a petrogenetic analysis in the model mixed-volatile system

“…1 and 2). Part of the chlorite zone in the lowest apparent structural horizon exposed in Oboke area, where younger radiometric ages than those of other Sanbagawa schists were reported long ago (Takasu and Dallmeyer, 1990;Shinjoe and Tagami, 1994), was proposed to be the outcrop of the blueschist part of the Northern Shimanto belt as a fenster (Aoki et al, 2008). Since pumpellyite + actinolite + chlorite + epidote is stable in hematite -free basic schists (Nakajima, 1982), our sampling areas belong to the pumpellyite -actinolite facies.…”