講演情報
[R8P-06]下部地殻地震の化石：東南極ナピア岩体のザクロ石を含むシュードタキライトとザクロ石脈の鉱物学的・微細構造的な証拠
*村山 燎1、Sreehari Lakshmanan1、豊島 剛志2、Sajeev Krishnan3 (1. 島根大、2. 新潟大、3. インド理科大学院)
キーワード：
シュードタキライト、ザクロ石
Pseudotachylyte, formed by the rapid quenching of melt generated through frictional melting during fault activity, is known to form even in the lower crust (Austrheim and Boundy, 1994). However, because their mineral assemblages develop under non-equilibrium conditions involving supercooling (Spray, 2010), estimating the formation conditions of pseudotachylyte based on these assemblages is difficult. Studies by Papa et al. (2023) and Toffol et al. (2025, 2026) have demonstrated that melt-derived garnets in pseudotachylyte vein can also form in the upper crust.
Napier Complex in East Antarctica is an exposed lower crust characterized by the presence of ultra-high temperature metamorphic rocks that underwent metamorphism approximately 2500 Ma (Hokada and Harley, 2025); the occurrence of pseudotachylyte has been reported at several locations within the complex (McIntyre Island: Motoyoshi, 1996; Tonagh Island: Toyoshima et al., 1999; Mt. Riiser-Larsen: Ishikawa et al., 2000, Murayama et al., 2026; Mt. Sones: Toyoshima, JARE60). Microstructural observation of pseudotachylyte from Mt. Sones revealed the presence of melt-derived garnet, as well as garnet-rich veins that crosscut the foliation of the host gneiss. This study aims to characterize the melt-derived garnet within pseudotachylyte and garnet-rich veins associated with it and to clarify the depth and factors of pseudotachylyte formation in lower crustal depths.
Filed relation reveal that the garnet-rich vein developed by crosscutting the foliation of the host gneiss. This vein consists of the mineral assemblage Grt-Ky-Kfs-Pl-Spl-Rt-Qz; it is rich in garnet (~40%) and Kyanite (~30%) but extremely poor in quartz. The garnets range from euhedral to subhedral, and fine intracrystalline fractures are visible.
Pseudotachylyte formed at the boundary between the garnet-rich vein and the host gneiss, with injection veins crosscutting the garnet-rich vein. Evidence of ductile deformation in the pseudotachylyte is observed, such as the elongation and alignment of clasts derived from the host gneiss. Also, quartz and plagioclase in the host gneiss adjacent to the pseudotachylyte have undergone grain-size reduction, forming ultramylonite. Garnet within garnet-rich vein- specifically those adjacent to the pseudotachylyte or ultramylonite- shows evidence of being dragged in the direction of shear.Spherical garnet microlites are commonly observed within the injection veins. In contrast, pseudotachylyte unaffected by ductile deformation also occurs within the host gneiss; this type commonly contains cauliflower-like garnet microlites. The surrounding host rock shows no signs of plastic deformation, and both of the host gneiss and the pseudotachylyte remain in a relatively intact state.
Pseudotachylyte and garnet-rich vein are observed in this region; their structural relationships suggest that multiple events occurred at the same location. One of the mechanisms for embrittlement in the lower crust is fluid over pressure (e.g., Altenberger et al., 2011). Garnet-rich veins were possible to form during decompression, and pseudotachylyte may have utilized the boundaries of the vein and the host gneiss, which acted as localized fluid pathways. In particular, the fact that garnets adjacent to the damage zone—which contains both pseudotachylyte and ultramylonite—show signs of dragging suggests a relatively mid/lower crustal depth for pseudotachylyte. Conversely, pseudotachylyte not associated with such boundaries appear to have escaped repeated deformation, thereby avoiding ductile deformation and preserving an intact state.
Future research should aim to constrain the formation of the pseudotachylyte by estimating the formation conditions of the garnet-rich vein and analyzing the crystal deformation of garnet within both the veins and the host gneiss.
Napier Complex in East Antarctica is an exposed lower crust characterized by the presence of ultra-high temperature metamorphic rocks that underwent metamorphism approximately 2500 Ma (Hokada and Harley, 2025); the occurrence of pseudotachylyte has been reported at several locations within the complex (McIntyre Island: Motoyoshi, 1996; Tonagh Island: Toyoshima et al., 1999; Mt. Riiser-Larsen: Ishikawa et al., 2000, Murayama et al., 2026; Mt. Sones: Toyoshima, JARE60). Microstructural observation of pseudotachylyte from Mt. Sones revealed the presence of melt-derived garnet, as well as garnet-rich veins that crosscut the foliation of the host gneiss. This study aims to characterize the melt-derived garnet within pseudotachylyte and garnet-rich veins associated with it and to clarify the depth and factors of pseudotachylyte formation in lower crustal depths.
Filed relation reveal that the garnet-rich vein developed by crosscutting the foliation of the host gneiss. This vein consists of the mineral assemblage Grt-Ky-Kfs-Pl-Spl-Rt-Qz; it is rich in garnet (~40%) and Kyanite (~30%) but extremely poor in quartz. The garnets range from euhedral to subhedral, and fine intracrystalline fractures are visible.
Pseudotachylyte formed at the boundary between the garnet-rich vein and the host gneiss, with injection veins crosscutting the garnet-rich vein. Evidence of ductile deformation in the pseudotachylyte is observed, such as the elongation and alignment of clasts derived from the host gneiss. Also, quartz and plagioclase in the host gneiss adjacent to the pseudotachylyte have undergone grain-size reduction, forming ultramylonite. Garnet within garnet-rich vein- specifically those adjacent to the pseudotachylyte or ultramylonite- shows evidence of being dragged in the direction of shear.Spherical garnet microlites are commonly observed within the injection veins. In contrast, pseudotachylyte unaffected by ductile deformation also occurs within the host gneiss; this type commonly contains cauliflower-like garnet microlites. The surrounding host rock shows no signs of plastic deformation, and both of the host gneiss and the pseudotachylyte remain in a relatively intact state.
Pseudotachylyte and garnet-rich vein are observed in this region; their structural relationships suggest that multiple events occurred at the same location. One of the mechanisms for embrittlement in the lower crust is fluid over pressure (e.g., Altenberger et al., 2011). Garnet-rich veins were possible to form during decompression, and pseudotachylyte may have utilized the boundaries of the vein and the host gneiss, which acted as localized fluid pathways. In particular, the fact that garnets adjacent to the damage zone—which contains both pseudotachylyte and ultramylonite—show signs of dragging suggests a relatively mid/lower crustal depth for pseudotachylyte. Conversely, pseudotachylyte not associated with such boundaries appear to have escaped repeated deformation, thereby avoiding ductile deformation and preserving an intact state.
Future research should aim to constrain the formation of the pseudotachylyte by estimating the formation conditions of the garnet-rich vein and analyzing the crystal deformation of garnet within both the veins and the host gneiss.