Peak conditions of kyanite-bearing quartz eclogites in the Sanbagawa metamorphic belt, central Shikoku, Japan

Abstract

Kyanite bearing eclogitic assemblages occur in the highest grade zone of the Sanbagawa metamorphic belt, central Shikoku, Japan. The eclogites consist mainly of garnet, omphacite, phengite, kyanite, epidote, quartz and rutile. Compositionally variable amphibole (glaucophane/barroisite/pargasite), phengite and paragonite occur as inclusions in garnet and other eclogite facies phases. Careful examination of garnet zoning in kyanite eclogites suggests that (i) most garnet grains show complex zoning consisting of relatively Ca rich/Mg poor inner and Ca poor/Mg rich outer segments, (ii) the inner segment of the zoned garnet formed at the eclogite facies stage, and (iii) the Mg rich outermost rim of garnet does not always represents a composition at peak eclogite stage but could form at lower pressure conditions of subsequent epidote amphibolite facies. The assemblage of inner segment of garnet, omphacite, phengite, kyanite and quartz points to equilibrium conditions of 2.3 2.4 GPa/675 740 °C. The metamorphic P T conditions of the eclogite facies stage reported in literature have been estimated assuming that the outermost rim of garnet with Mg rich composition was in equilibrium with other eclogite facies phases. Therefore, P T estimations of the eclogite facies stage in the Sanbagawa metamorphic belt should be re examined carefully on the basis of textural and compositional heterogeneities of constituent minerals.

Late Cretaceous CHIME monazite ages of Sanbagawa metamorphic rocks from Nushima, Southwest Japan

“…Similar metamorphic process was also proposed for the Sanbagawa metamorphic rocks in the Besshi region by Ota et al (2004). On the other hand, Aoya (2001) and other researchers (e.g., Zaw Win Ko et al, 2005;Miyamoto et al, 2007;Kouketsu et al, 2014;Enami, 2014a, 2014b;Enami et al, 2017) have suggested that the samples in the eclogite unit, including quartz eclogite, experienced two distinct stages of prograde metamorphism associated with the eclogite facies and subsequent epidote-amphibolite facies (Fig. 6a).…”

“…The ages for the Seba-dani and Kotsu eclogites are younger than those estimated for quartz eclogites from the Gongen area (~120-110 Ma: Okamoto et al, 2004). Distinct differences in P-T conditions at peak eclogite facies stages were reported between the Gongen quartz eclogites (2.3-2.4 GPa/675-740°C: Miyamoto et al, 2007) and the Seba-dani (1.8-1.9 GPa/ 510-550°C: Zaw Win Ko et al, 2005;Kouketsu et al, 2010) and Kotsu-Bizan (1.8-2.0 GPa/580-600°C: Matsumoto et al, 2003;Tsuchiya and Hirajima, 2013;Kabir and Takasu, 2016) eclogites and related rocks. Therefore, the two groups of metamorphic ages reported by Okamoto et al (2004) and Wallis et al (2009) might suggest that eclogite facies rocks that formed at different depths and periods of the Sanbagawa subduction, which continued for more than 20-30 million years, occur in the Sanbagawa metamorphic belt (Fig.…”

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

“…Other 3 results of ridge-approach model by Iwamori (2000), which incorporates mantle convection at depths >30 km and uses v = 6.3 cm/year, are shown by dashed bold lines labeled by the corresponding ages of the slab at trench, 2.5, 1.0 and 0 Ma. Katayama et al 2001 540 Ma Okamoto et al 2004-Miyamoto et al, 2007 ; Engebretson 89 Ma Okudaira and Yoshitake, 2004Nakajima et al, 1990;, 1991;Otsuki, 1992;, 1993;Sakaguchi, 1996;Maruyama et al, 1997;Iwamori, 2000;Aoya et al, 2003Wallis et al, 2009Nakajima et al, 2004aNakajima et al, 1990;, 199385-80 Ma Wallis et al, 2009Taira et al, 2016 89 Ma 86 Ma…”

Experimental study of Fe3+ solubility in cristobalite and its application to a metamorphosed quartz-magnetite rock from Mt. Riiser-Larsen area, Napier Complex, East Antarctica

“…These rocks suffered the progressive metamorphism to the eclogite facies. The peak metamorphic condition was estimated as around 25 kbar and 750 °C by several authors (Kawasaki, 1987;Takasu, 1989;Aoya, 2001;Kugimiya and Takasu, 2002;Ota et al, 2004;Miyamoto et al, 2007;Kawasaki and Osanai, 2008). Data for the metamorphosed quartz of the eclogite plots above the experimental result at 1 bar in Figure 6.…”