Compositional zoning and inclusions of garnet in Sanbagawa metapelites from the Asemi-gawa route, central Shikoku, Japan

Abstract

Compositional and textural characteristics of the zonal structure and inclusions of garnet in metapelites from areas of high -grade greenschist facies (garnet zone) to areas of epidote -amphibolite facies (albite -biotite and oligoclase biotite zones) located along the Asemi -gawa route were examined using EPMA and Raman spectroscopy to investigate evolution of the P -T history of Sanbagawa metamorphism. Garnet grains exhibit two types of compositional zoning patterns: bell -shaped and composite types with spessartine variation. Bell -shape zoning is characterized by a monotonous decrease in the spessartine component from the crystal core towards the margin. Almandine and pyrope components exhibit the opposite trend. The grossular content reaches a maximum at an intermediate position between the core and the rim, and then decreases towards the outermost rim. The composite -zoned garnet is divided into core and mantle parts and it exhibits discontinuous compositional variations according to the associated boundary, evidencing resorption of the core and overgrowth of the mantle during crystallization; implying a two -stage growth of garnet during the Sanbagawa metamorphism. Composite -zoned garnet is characteristically observed in metapelites from a part of the albite -biotite zone, and quartz grains included in its core retain a higher residual pressure than those in garnet from other mineral zones. Isolated paragonite crystal occurs particularly as an inclusion in the core part of this composite -zoned garnet. These data suggest that part of the Sanbagawa metamorphic rocks in the Asemi -gawa region recrystallized under higherpressure conditions up to the eclogite facies, prior to regional metamorphism from the greenschist, to the epidote -amphibolite facies that formed the regional thermal structure of the Sanbagawa belt of central Shikoku.

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

“…Consequently, they assumed the ages of peak eclogites facies as~120-110 Ma by reference to the data of Okamoto et al (2004), though these age data were not obtained from the Asemi-gawa zircons, and interpreted that the young rim ages (88-81 Ma) represent zircon recrystallization during exhumation and retrograde stages. However, direct evidence for the eclogite facies equilibria are not confirmed from the basic and pelitic schists of the Asemi-gawa oligoclase-biotite zone, from which the sample studied by Aoki et al (2009) was collected (Taguchi and Enami, 2014a;Taguchi, T., personal communication, 2017). Additionally, zircon grains are a typical refractory phase, and thus it seems unlikely that recrystallization with decreasing temperature down to epidote-amphibolite facies conditions obliterated the ages for the eclogite facies stage of~120-110 Ma, as assumed by Aoki et al (2009).…”

New finding of paragonite–clinozoisite association in garnet from the type locality of Sanbagawa belt (Kanto Mountains, Japan)

“…Preserved paragonite and clinozoisite within garnet have also been reported in Sanbagawa schist in the Asemi-gawa area of central Shikoku (Taguchi and Enami, 2014). Therefore, prograde-zoned garnets can be potential rigid capsules retaining prograde mineral assemblages (Maruyama et al, 2010).…”

Coexistence of jadeite and quartz in garnet of the Sanbagawa metapelite from the Asemi–gawa region, central Shikoku, Japan

“…They suggested that a part of the epidote-amphibolite facies zone (albite-biotite zone) has probably experienced the eclogite facies metamorphism, although no index phases of the eclogite facies equilibrium have been identified. In this study, coexistence of jadeite + quartz was newly discovered as an inclusion in garnet of a metapelite from the western extension of the southern albite-biotite zone along the Asemi-gawa route that was discussed by Taguchi and Enami (2014). This is direct evidence that at least part of the Asemi-gawa region has experienced high-pressure blueschist or eclogite facies metamorphism, and this finding prompted us to reevaluate the P-T path of the Sanbagawa metapelite in the Asemi-gawa region.…”

“…The Asemi-gawa region is divided into the chlorite, garnet, southern (lower) albite-biotite, oligoclase-biotite, and northern (upper) albite-biotite zones from south to north. Taguchi and Enami (2014) reported that composite-zoned garnet containing inclusions of paragonite and quartz with high residual retaining pressure characteristically occurs in the metapelite (sample AST72) from the southern albite-biotite zone (Fig. 1b), and they suggested the possibility that part of the schists of the Asemi-gawa region experienced the eclogite facies equilibrium.…”

“…1a), epidote-amphibolite facies rocks are widely distributed. Taguchi and Enami (2014) reexamined the P-T history of metapelites in the Asemi-gawa region based on datasets derived from (I) the zonal structure of garnet, (II) the paragenesis of inclusions in garnet, and (III) the quartz Raman barometry along the Asemi-gawa route. They suggested that a part of the epidote-amphibolite facies zone (albite-biotite zone) has probably experienced the eclogite facies metamorphism, although no index phases of the eclogite facies equilibrium have been identified.…”