5:15 PM - 6:30 PM
[SGL23-P03] Itayama Nappe, meta-chert and gabbro in the Ryoke Belt of the Takato-Itayama area, central Japan
Keywords:Ryoke belt, Takato-Itayama, Median Tectonic Line, Kashio mylonite, Hase unit
Itayama Nappe, meta-chert and gabbro are distributed near the Median Tectonic Line (MTL) of the Itayama area, Takato Town. Here, we introduce these geological bodies. Small Nappe occurs in the Itayama area [1]. Here, this is called Itayama Nappe. The Itayama Nappe is composed of various rocks derived from the Ryoke and Sanbagawa Belts. However, low-grade metamorphic rocks and sedimentary rocks in the Itayama Nappe are not exposed in the nearby Ryoke and Sanbagawa Belts. The Itayama Nappe is sandwiched between two faults, MTL-E and MTL-W (Figure A). However, there is a problem with the geology of the northern part of the Itayama Nappe. In this area an outcrop of MTL exists near the Miyadonosawa dam [2]. That is, the blue fault in Figure A is the MTL. The two MTLs do not exist in this area. Hence, the Itayama Nappe is a narrow geological body extending from north to south that has entered the Kashio mylonite zone (Figures B and C).
Narrow and long meta-chert is distributed in the vicinity of MTL [3]. To the west of the long meta-chert, mylonitized pelitic and siliceous rocks are exposed as xenoliths of Hiji tonalite. For example, various siliceous rocks are exposed almost continuously over about 120 m along a small valley to the east of the Kajimura village (Figure C). These mylonites are derived from the Hase Unit of the accretionary complex [4]. Sizes of quartz grains are about 100 µm for the quartz-rich portion of a siliceous mylonite (Figures D and E) near the red dotted line in Figure C. The red dotted line is the western margin of the Kashio mylonite zone for granitic rocks [3]. The psammitic hornfels in Figure C are not metamorphic rocks derived from the Hase Unit.
Small hornblende gabrro is exposed in two places along the forest road leading to Mt.Gatsuzozan (Figure F). The gabbro intrudes into banded tonalite (Figure G). The Hiji tonalite in the Itayama area has a relatively uniform lithology, but tonalites near the gabbro exhibit remarkably diverse lithologies (Figures H and I). The geological relationship between the Hiji tonalite and Katsuma quartz diorite is unknown, but in the Shioku to the north of Itayama, an outcrop was found in which these granitoids are almost vertically in contact with each other.
The Itayama Nappe is a geological body that has entered the Kashio mylonite zone. The existence of allochthonous geological body in the Kashio mylonite zone was first reported from the Kashio area. Watanabe & Kinugasa [5] reported the occurrence of metamorphic rocks which are thought to be the Sanbagawa metamorphic rocks. Next, microbreccia is found near the Bungui Pass of the Hase area [6]. Subsequently, Awasawa metamorphic rocks were reported which belong to neither Sanbagawa metamorphic rocks, Kashio mylonite nor microbreccia [7]. Microbreccia is a fractured Awasawa metamorphic rock. The Awasawa metamorphic rocks are mainly composed of garnet-biotite-muscovite schist. This pelitic schist is finer than the Sanbagawa schist containing garnet and biotite. Moreover, white mica is not phengite.The (FeO+MgO) content of muscovite is not high as compared with that of the Sanbagawa schist.
[1] Hara et al., 2004, Struc. Geol, No 48, 1. [2]Ono, 2013, GSJ Meeting, R5-P-13. [3]Makimoto et al, 1996, Geology of the Takato district, Geol.Surv.Jpn. [4] Ono, 2018, GSJ Meeting, R15-P-13.[5] Watanabe & Kinugasa, 1970, GSJ Meeting, p.286. [6] Takagi, 1984, Geol. Soc. Jpn, 81. [7] Ono, 2002, Geol. Soc. Jpn, 733.
Narrow and long meta-chert is distributed in the vicinity of MTL [3]. To the west of the long meta-chert, mylonitized pelitic and siliceous rocks are exposed as xenoliths of Hiji tonalite. For example, various siliceous rocks are exposed almost continuously over about 120 m along a small valley to the east of the Kajimura village (Figure C). These mylonites are derived from the Hase Unit of the accretionary complex [4]. Sizes of quartz grains are about 100 µm for the quartz-rich portion of a siliceous mylonite (Figures D and E) near the red dotted line in Figure C. The red dotted line is the western margin of the Kashio mylonite zone for granitic rocks [3]. The psammitic hornfels in Figure C are not metamorphic rocks derived from the Hase Unit.
Small hornblende gabrro is exposed in two places along the forest road leading to Mt.Gatsuzozan (Figure F). The gabbro intrudes into banded tonalite (Figure G). The Hiji tonalite in the Itayama area has a relatively uniform lithology, but tonalites near the gabbro exhibit remarkably diverse lithologies (Figures H and I). The geological relationship between the Hiji tonalite and Katsuma quartz diorite is unknown, but in the Shioku to the north of Itayama, an outcrop was found in which these granitoids are almost vertically in contact with each other.
The Itayama Nappe is a geological body that has entered the Kashio mylonite zone. The existence of allochthonous geological body in the Kashio mylonite zone was first reported from the Kashio area. Watanabe & Kinugasa [5] reported the occurrence of metamorphic rocks which are thought to be the Sanbagawa metamorphic rocks. Next, microbreccia is found near the Bungui Pass of the Hase area [6]. Subsequently, Awasawa metamorphic rocks were reported which belong to neither Sanbagawa metamorphic rocks, Kashio mylonite nor microbreccia [7]. Microbreccia is a fractured Awasawa metamorphic rock. The Awasawa metamorphic rocks are mainly composed of garnet-biotite-muscovite schist. This pelitic schist is finer than the Sanbagawa schist containing garnet and biotite. Moreover, white mica is not phengite.The (FeO+MgO) content of muscovite is not high as compared with that of the Sanbagawa schist.
[1] Hara et al., 2004, Struc. Geol, No 48, 1. [2]Ono, 2013, GSJ Meeting, R5-P-13. [3]Makimoto et al, 1996, Geology of the Takato district, Geol.Surv.Jpn. [4] Ono, 2018, GSJ Meeting, R15-P-13.[5] Watanabe & Kinugasa, 1970, GSJ Meeting, p.286. [6] Takagi, 1984, Geol. Soc. Jpn, 81. [7] Ono, 2002, Geol. Soc. Jpn, 733.