15:00 〜 15:15
[SMP25-11] Sapphirine formation and metasomatic processes
キーワード：Metasomatism, Sapphirine, India
Sapphirine is a mineral usually formed during high to ultrahigh-temperature metamorphism. The association of sapphirine with quartz is the robust mineral association representing ultrahigh-temperature (UHT) metamorphism. These rocks are generally restite in nature with Mg-Al-rich bulk chemical composition. Such rocks are prominent in high-temperature orthopyroxene (high-Al) and Mg-rich garnet-bearing mineralogy. Many of these UHT samples are silica undersaturated with minor relicts of sapphirine-quartz or orthopyroxene-sillimanite-quartz assemblages. Sapphirine associations in meta-troctolite and ultramafic rocks are rare but reported from a few localities worldwide. Such sapphirine-bearing rocks are distinct and usually associated with clinopyroxene.
Sapphirine-bearing rocks rich in hydrous minerals such as gedrite, staurolite, and cordierite with or without garnet are another type of rock reported from various localities worldwide. These rocks are also Mg-Al rich in their bulk chemical composition. Hence such rocks are often interpreted as UHT occurrences. This study says petrologic investigations of hydrous sapphirine rocks from the Palghat-Cauvery Shear Zone, India. The garnet-bearing aluminosilicate rock preserves inclusions of gedrite, spinel and quartz within the garnet. In some domains, inclusions of sapphirine with spinel occur within the garnet, marking the prograde evolution of the garnet. Garnet-rim formed around gedrite with gedrite matrix further supports the garnet developing reaction after gedrite. The garnet-corundum-staurolite-kyanite/sillimanite assemblage formed as porphyroblasts. Most of the staurolite in the studied samples shows moderate to high Mg content. However, the staurolite composition varies in each microdomain. Various garnet breakdown symplectite textures in these rocks involved gedrite, cordierite sapphirine and spinel. Most textures are reported as evidence of very high pressure and temperature in the previous publication. However, present results demonstrated that the region’s geology and association of marble and amphibolite in the same locality indicate the possibly chemical-potential gradient and Ca-Si mobility-induced metasomatic process. This process can form sapphirine rock with high Mg-Al-OH-bearing mineral assemblages at relatively lower P-T conditions.
Sapphirine-bearing rocks rich in hydrous minerals such as gedrite, staurolite, and cordierite with or without garnet are another type of rock reported from various localities worldwide. These rocks are also Mg-Al rich in their bulk chemical composition. Hence such rocks are often interpreted as UHT occurrences. This study says petrologic investigations of hydrous sapphirine rocks from the Palghat-Cauvery Shear Zone, India. The garnet-bearing aluminosilicate rock preserves inclusions of gedrite, spinel and quartz within the garnet. In some domains, inclusions of sapphirine with spinel occur within the garnet, marking the prograde evolution of the garnet. Garnet-rim formed around gedrite with gedrite matrix further supports the garnet developing reaction after gedrite. The garnet-corundum-staurolite-kyanite/sillimanite assemblage formed as porphyroblasts. Most of the staurolite in the studied samples shows moderate to high Mg content. However, the staurolite composition varies in each microdomain. Various garnet breakdown symplectite textures in these rocks involved gedrite, cordierite sapphirine and spinel. Most textures are reported as evidence of very high pressure and temperature in the previous publication. However, present results demonstrated that the region’s geology and association of marble and amphibolite in the same locality indicate the possibly chemical-potential gradient and Ca-Si mobility-induced metasomatic process. This process can form sapphirine rock with high Mg-Al-OH-bearing mineral assemblages at relatively lower P-T conditions.