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[SMP24-P15] Allochthonous schist blocks at the Atokura nappe–Mikabu greenstone boundary in northern marginal area of the Kanto Mountains
Keywords:Sanbagawa metamorphic rocks, Mikabu greenstone, Atokura Nappe, lawsonite, actinolite rocks
Introduction
The Atokura nappe emplaces upon the Mikabu greenstone unit in the Kanto Mountains. Pelitic schists and mafic rocks occur between them in the Shimonita and Kamiyama area. The pelitic schists were subdivided into Type Ⅰ, II and Ⅲ by phengite K–Ar ages and metamorphic mineral assemblage. Type Ⅰ (78 Ma) belongs to the Mikabu unit, whereas Type Ⅱ and Ⅲ show a younger metamorphic age (72 Ma) and older metamorphic age (90–82 Ma), respectively (Kobayashi and Tanaka, 1994; Kobayashi, 1996). Type III is also older than the chlorite zone of the currently exposed Sanbagawa metamorphic rocks (84–82 Ma; Miyashita and Itaya, 2002). Based on the mineral assemblage, Type II corresponds to the garnet zone of the Sanbagawa belt and Type III to the chlorite zone. These overlie Type I (Mikabu unit) and are considered to be allochthonous blocks constituting as a member of the Atokura nappe (Kobayashi, 1996). We have conducted a detailed petrographic investigation of these allochthonous blocks and we will report the results and discuss their geological significance.
Petrography of schists
(a) Pelitic schists with a mineral assemblage corresponding to the biotite zone of the Sanbagawa Belt are newly found in the Type II area. The compositions of garnet grains show no difference from those in the biotite zone in the Ayukawa–Sanbagawa area (Miyashita, 1997). Biotite schists with coarse biotite grains of about 200 μm are also found in the Type II area.
(b) Lawsonite occurs in pelitic schist in the Type III area. The lawsonite-bearing pelitic schist clasts are known in the Miocene Aoiwa Conglomerate (Arai et al., 2009). The clast of the lawsonite-bearing pelitic schist yields phengite K–Ar age of 96−91 Ma and is correlative with Type III (Arai et al., 2009). Finding the lawsonite in the TypeⅢ schist supports this correlation.
(c) Actinolite rocks were found in the Type Ⅱ (Kobayashi, 1996). These have been reported in the matrix of the greenstone complex at the southern margin of Shimonita town (Arai et al., 2011), and from a member of amphibolite in Kiroko mélange in Ogawa town (Miyashita et al., 2016).
(d) In the northern foot of Kamiyama area, mafic and pelitic schists, and granitic mylonite were found between Atokura Formation and Mikabu greenstone. The mylonite indicates a top-to-the-SE reverse shear sense.
Discussion
In the Kanto Mountains, the Sanbagawa unit structurally underlie the Mikabu unit by south-dipping fault. Based on the metamorphic mineral assemblage, the pelitic schists upon the Mikabu unit are considered to belong to the underlying Sanbagawa unit. Its occurrence suggests that some of an upper unit of the Sanbagawa metamorphic rocks was moved to the south together with the Atokura nappe, and also the other part of an upper unit eroded and deposited as clasts in the Aoiwa Conglomerate at about 15 Ma (Arai et al., 2009). The mylonites with top-to-the-SE sense in the Kamiyama area suggest a relationship with an early-stage movement (to the south) of the Atokura Nappe (Arai et al., 2008).
References
Arai, H., Kobayashi, K. and Takagi, H., 2008, Gondwana Research, 13, 319–330.
Arai, H. and Takagi, H., 2009, Jour. Geol. Soc. Japan, 115, 233–241.
Arai, H., Miyashita, A., Tanabe, K. and Murata, M., 2011, Japan. Mag. Mineral. Petrol. Sci., 40, 177–194.
Kobayashi, K., 1996, Ph.D Thesis, Waseda Univ..
Kobayashi, K. and Tanaka, H., 1996, 101nd Ann. Meet. Geol. Soc. Japan, 250.
Miyashita, A., 1997, Bulletin of the National Science Museum, Series C, 23,1–25.
Miyashita, A. and Itaya, T., 2002, Gondwana Research, 5, 837–848.
Miyashita, A., Tsutsumi, Y. and Sano, T., 2016, Jour. Geol. Soc. Japan, 122, 511–522.
The Atokura nappe emplaces upon the Mikabu greenstone unit in the Kanto Mountains. Pelitic schists and mafic rocks occur between them in the Shimonita and Kamiyama area. The pelitic schists were subdivided into Type Ⅰ, II and Ⅲ by phengite K–Ar ages and metamorphic mineral assemblage. Type Ⅰ (78 Ma) belongs to the Mikabu unit, whereas Type Ⅱ and Ⅲ show a younger metamorphic age (72 Ma) and older metamorphic age (90–82 Ma), respectively (Kobayashi and Tanaka, 1994; Kobayashi, 1996). Type III is also older than the chlorite zone of the currently exposed Sanbagawa metamorphic rocks (84–82 Ma; Miyashita and Itaya, 2002). Based on the mineral assemblage, Type II corresponds to the garnet zone of the Sanbagawa belt and Type III to the chlorite zone. These overlie Type I (Mikabu unit) and are considered to be allochthonous blocks constituting as a member of the Atokura nappe (Kobayashi, 1996). We have conducted a detailed petrographic investigation of these allochthonous blocks and we will report the results and discuss their geological significance.
Petrography of schists
(a) Pelitic schists with a mineral assemblage corresponding to the biotite zone of the Sanbagawa Belt are newly found in the Type II area. The compositions of garnet grains show no difference from those in the biotite zone in the Ayukawa–Sanbagawa area (Miyashita, 1997). Biotite schists with coarse biotite grains of about 200 μm are also found in the Type II area.
(b) Lawsonite occurs in pelitic schist in the Type III area. The lawsonite-bearing pelitic schist clasts are known in the Miocene Aoiwa Conglomerate (Arai et al., 2009). The clast of the lawsonite-bearing pelitic schist yields phengite K–Ar age of 96−91 Ma and is correlative with Type III (Arai et al., 2009). Finding the lawsonite in the TypeⅢ schist supports this correlation.
(c) Actinolite rocks were found in the Type Ⅱ (Kobayashi, 1996). These have been reported in the matrix of the greenstone complex at the southern margin of Shimonita town (Arai et al., 2011), and from a member of amphibolite in Kiroko mélange in Ogawa town (Miyashita et al., 2016).
(d) In the northern foot of Kamiyama area, mafic and pelitic schists, and granitic mylonite were found between Atokura Formation and Mikabu greenstone. The mylonite indicates a top-to-the-SE reverse shear sense.
Discussion
In the Kanto Mountains, the Sanbagawa unit structurally underlie the Mikabu unit by south-dipping fault. Based on the metamorphic mineral assemblage, the pelitic schists upon the Mikabu unit are considered to belong to the underlying Sanbagawa unit. Its occurrence suggests that some of an upper unit of the Sanbagawa metamorphic rocks was moved to the south together with the Atokura nappe, and also the other part of an upper unit eroded and deposited as clasts in the Aoiwa Conglomerate at about 15 Ma (Arai et al., 2009). The mylonites with top-to-the-SE sense in the Kamiyama area suggest a relationship with an early-stage movement (to the south) of the Atokura Nappe (Arai et al., 2008).
References
Arai, H., Kobayashi, K. and Takagi, H., 2008, Gondwana Research, 13, 319–330.
Arai, H. and Takagi, H., 2009, Jour. Geol. Soc. Japan, 115, 233–241.
Arai, H., Miyashita, A., Tanabe, K. and Murata, M., 2011, Japan. Mag. Mineral. Petrol. Sci., 40, 177–194.
Kobayashi, K., 1996, Ph.D Thesis, Waseda Univ..
Kobayashi, K. and Tanaka, H., 1996, 101nd Ann. Meet. Geol. Soc. Japan, 250.
Miyashita, A., 1997, Bulletin of the National Science Museum, Series C, 23,1–25.
Miyashita, A. and Itaya, T., 2002, Gondwana Research, 5, 837–848.
Miyashita, A., Tsutsumi, Y. and Sano, T., 2016, Jour. Geol. Soc. Japan, 122, 511–522.