5:15 PM - 6:30 PM
[SMP25-P06] Pseudotachylyte in a marble from the Takine Group (Abukuma Terrane) and its similarity to pseudotachylyte from the Higo Mertamorphic Rocks
Keywords:pseudotachylyte, marble, Abukuma Terrane, Takine Group, Higo Metamorphic Rocks
We have reported a pseudotachylyte in a marble from the Higo Metamorphic Rocks in central Kyushu (Nishiyama, et al., 2019 and 2020, JpGU Meeting). This time we investigated marbles in the Takine Group (Abukuma Terrane or Abukuma Belt), the Tohoku Region, and found pseudotachylytes which have characteristics very similar to those from the Higo Metamorphic Rocks. In this report, we present petrography of some pseudotachylytes in a marble at the Shin-Takine Mine (Asahi Koumatsu Co. Ltd.), Tamura City, Fukushima Prefecture, and discuss the possible genesis in comparison with pseudotachylytes from the Higo Metamorphic Rocks.
A large body (1×7 km2) of marble associated with metasediments, metabasites, and metaperidotites occurs as an isolated mass (roof pendant) in a granitic pluton in Tamura City, about 20 km apart from the main body of the Abukuma Metamorphic Terrane. Ehiro et al. (1989) named them the Takine Group and gave detailed petrography for the metamorphic rocks, revealing that the Takine Group has undergone the contact metamorphism by Cretaceous granites. It is unknown whether it had undergone the regional metamorphism of the Abukuma Metamorphic Rocks prior to the contact metamorphism. It is also unknown to which geological unit the Takine Group belongs (Ehiro et al., 1989). The protolith age is not known, either.
In the Shin-Takine Mine, several metabasite dykes up to 2 meters thick occur in a massive marble. The metabasite dykes are strongly weathered and the mineral assemblage cannot be identified. The marble is a massive monomineralic rock consisting of coarse-grained calcite. However, a layered structure can be seen in part of the marble, in which grey layers several millimeters thick recurrently occur in the marble with a frequency of several centimeters. Quartz, diopside, tremolite and dolomite occur in the grey layers, but no wollastonite occurs. Apatite also occurs as an accessory mineral. These layers show intersecting relations in part, implying the origin not of original bedding but of fluid pathway (i.e., veins).
A quartz (1st-stage quartz) vein develops along the contact between a metabasite dyke and a marble, and black veins 1 to 2 centimeters thick occur in this quartz vein and also in the marble, showing branching and layering. Observations with a SEM-EDS reveal following characteristics.
(a) The black vein consists mainly of dolomite, quartz (1st- and 2nd-stage), and kaolinite-group clay mineral (hereafter abbreviated as Al-Si clay) with a small amount of calcite. 1st-stage quartz is anhedral and polycrystalline, partially dissolved by dolomite. Some quartz grains show fracturing. Quartz (2nd-stage) also occurs as inclusions in dolomite, which have very irregular shape.
(b) Dolomite shows compositional heterogeneity with respect to Fe-Mg exchange. Some dolomite crystals show growth zoning.
(c) Al-Si clay rarely includes K-feldspar, indicating that the Al-Si clay is an alteration product of K-feldspar.
(d) Al-Si clay is frequently associated with quartz (2nd-stage). In a rare case, an idiomorphic quartz (2nd-stage) crystal occurs within Al-Si clay.
(e) Pyrite occurs in dolomite, showing dissolution texture.
(f) Calcite occurs as inclusions with irregular shape in dolomite.
These characteristics indicate that this black vein is not a simple fault gouge. Because the country rock is a marble (CaCO3), the occurrence of dolomite, Al-Si clay (originally K-feldspar) and 2nd-stage quartz implies mass transfer of Mg, Fe, Al, K, and Si from the exterior. Fracturing is only observed in 1st-stage quartz. Growth zoning in dolomite and an idiomorphic quartz (2nd-stage quartz) crystal in Al-Si clay suggest crystallization of these minerals in fluid or melt. Dissolution texture of pyrite prefers reaction with dolomite melt rather than fluid. These lines of evidence indicate that this black vein represents a recrystallization product of dolomite and silicate melts formed by frictional melting associated with faulting, namely a kind of pseudotachylyte.
Reference
Ehiro, M., Kanisawa, S., and Teketani, Y. (1989) Pre-Tertiary Takine Group in the central Abukuma Massif. Bull. Fukushima Museum, No.3, 21-37.
A large body (1×7 km2) of marble associated with metasediments, metabasites, and metaperidotites occurs as an isolated mass (roof pendant) in a granitic pluton in Tamura City, about 20 km apart from the main body of the Abukuma Metamorphic Terrane. Ehiro et al. (1989) named them the Takine Group and gave detailed petrography for the metamorphic rocks, revealing that the Takine Group has undergone the contact metamorphism by Cretaceous granites. It is unknown whether it had undergone the regional metamorphism of the Abukuma Metamorphic Rocks prior to the contact metamorphism. It is also unknown to which geological unit the Takine Group belongs (Ehiro et al., 1989). The protolith age is not known, either.
In the Shin-Takine Mine, several metabasite dykes up to 2 meters thick occur in a massive marble. The metabasite dykes are strongly weathered and the mineral assemblage cannot be identified. The marble is a massive monomineralic rock consisting of coarse-grained calcite. However, a layered structure can be seen in part of the marble, in which grey layers several millimeters thick recurrently occur in the marble with a frequency of several centimeters. Quartz, diopside, tremolite and dolomite occur in the grey layers, but no wollastonite occurs. Apatite also occurs as an accessory mineral. These layers show intersecting relations in part, implying the origin not of original bedding but of fluid pathway (i.e., veins).
A quartz (1st-stage quartz) vein develops along the contact between a metabasite dyke and a marble, and black veins 1 to 2 centimeters thick occur in this quartz vein and also in the marble, showing branching and layering. Observations with a SEM-EDS reveal following characteristics.
(a) The black vein consists mainly of dolomite, quartz (1st- and 2nd-stage), and kaolinite-group clay mineral (hereafter abbreviated as Al-Si clay) with a small amount of calcite. 1st-stage quartz is anhedral and polycrystalline, partially dissolved by dolomite. Some quartz grains show fracturing. Quartz (2nd-stage) also occurs as inclusions in dolomite, which have very irregular shape.
(b) Dolomite shows compositional heterogeneity with respect to Fe-Mg exchange. Some dolomite crystals show growth zoning.
(c) Al-Si clay rarely includes K-feldspar, indicating that the Al-Si clay is an alteration product of K-feldspar.
(d) Al-Si clay is frequently associated with quartz (2nd-stage). In a rare case, an idiomorphic quartz (2nd-stage) crystal occurs within Al-Si clay.
(e) Pyrite occurs in dolomite, showing dissolution texture.
(f) Calcite occurs as inclusions with irregular shape in dolomite.
These characteristics indicate that this black vein is not a simple fault gouge. Because the country rock is a marble (CaCO3), the occurrence of dolomite, Al-Si clay (originally K-feldspar) and 2nd-stage quartz implies mass transfer of Mg, Fe, Al, K, and Si from the exterior. Fracturing is only observed in 1st-stage quartz. Growth zoning in dolomite and an idiomorphic quartz (2nd-stage quartz) crystal in Al-Si clay suggest crystallization of these minerals in fluid or melt. Dissolution texture of pyrite prefers reaction with dolomite melt rather than fluid. These lines of evidence indicate that this black vein represents a recrystallization product of dolomite and silicate melts formed by frictional melting associated with faulting, namely a kind of pseudotachylyte.
Reference
Ehiro, M., Kanisawa, S., and Teketani, Y. (1989) Pre-Tertiary Takine Group in the central Abukuma Massif. Bull. Fukushima Museum, No.3, 21-37.