The higher grade metamorphic zonation of the Sambagawa metamorphic belt in central Shikoku, Japan

Abstract

The higher grade metamorphic zonation of the Sambagawa ( = Sanbagawa) belt is established for the first time for the whole area of central Shikoku. As discontinuous reactions to define the isograd are absent, the metamorphic grade is primarily determined by the Mg-Fe partitioning between garnet and chlorite along representative traverses. However, for regional mapping, mineralogical features of the pelitic schists, such as using mineral assemblages of more than divariant equilibrium, the modal garnet to chlorite ratio, and the optical properties of chlorite, are employed as auxiliary criteria.The presence of the highest grade mineral zone in the middle of the structural level is confirmed, but its spatial distribution is far more complex than hitherto accepted. Thermal axes are now confirmed at three different structural levels. A model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress. Thermal readjustment brought a continuous metamorphic temperature gradient acros and within the thrust sheets. Tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re-equilibrated. Local anomalies of metamorphic grade, represented by mixing of schists of different metamorphic grade, exist, but they are due to a later stage event.

Structural and petrological constraints on the tectonic evolution of the garnet‐lherzolite facies Higashi‐akaishi peridotite body, Sanbagawa belt, SW Japan

“…In the Besshi unit, plagioclase porphyroblasts grew around the peak of metamorphic temperature [Otsuki and Banno, 1990;Wallis et al, 1992] and the appearance of associated minerals define the metamorphic zonation of the Besshi unit [Enami, 1982;Higashino, 1990]. If the phase of heating in Figure 7.…”

“…The peak metamorphic conditions are generally in the greenschist to epidote amphibolite facies and the unit is divided into chlorite, garnet, albite-biotite and oligoclase-biotite zones based on the changes of mineral paragenesis in metapelite [Banno et al, 1978;Enami, 1982;Higashino, 1990] (Figure 1a). These metamorphic zones show an overall inverted thermal structure with a general increase in grade up structural section.…”

“…The Higashi-akaishi body is composed mainly of dunite with intercalated layers of wehrlite, clinopyroxenite, chromitite, and minor amounts of garnet-bearing rocks, such as garnet clinopyroxenite, Figure 1. (a) Geological map of the Besshi area modified after Aoya [2001], Enami [1982], Higashino [1990] [Horikoshi, 1937;Yoshino, 1961Yoshino, , 1964Mori and Banno, 1973;Enami et al, 2004]. The ultramafic rocks show varying degrees of serpentinization and antigorite is the dominant polymorph of serpentine in the body [Kunugiza, 1984].…”

Rocks and minerals of the Sekigawa River

“…3 300 360°C 0.5 0.7 GPa 420 500°C 0.7 1.0 GPa -490 580 °C 0.8 1.1 GPa -580 640°C 0.9 1.1 GPa Enami et al, 1994 800°C 2.5 GPa Ota et al, 2004 Higashino, 1990). Higashino, 1990 Fig.…”

“…, 2009 1647.3 m Kunugiza et al, 1986Takasu et al, 1994, Aoya, 2001, 2002Endo et al, 2009-, Banno and Sakai, 1989Higashino, 1990 Higashino 1990 -- Fig. 3 300 360°C 0.5 0.7 GPa 420 500°C 0.7 1.0 GPa -490 580 °C 0.8 1.1 GPa -580 640°C 0.9 1.1 GPa Enami et al, 1994 800°C 2.5 GPa Ota et al, 2004 Higashino, 1990).…”