Geological outline
Al-rich metaclastic rocks in Carboniferous deposits occur in five areas in Japan (Fig. 1; Seki, 1955; 1957; Asami, 1979; Hiroi, 1983; Sakashima et al., 1999).
While their deposition took place during the Carboniferous, their metamorphic ages are of ca. 240 Ma or ca. 100 Ma (Ueda et al., 1969; Yamaguchi and Yanai, 1970; Sugiyama, 1972; Hiroi, 1978; Kawamura et al., 1985; Suzuki and Adachi 1994; Sakashima et al., 1999). The Tono metamorphic rocks also partly include Permian protolith (Yoshida et al., 1994).
Paleo-Mesozoic systems in the Hida Mountains consist of eight geological units from northwest to southeast (Fig. 1C; Tsukada et al., 2004): (1) Hida Metamorphic Rocks are a series of low-P polymetamorphic rocks. (2) Unazuki Metamorphic Rocks are a series of ca. 240 Ma medium-P metamorphic rocks, which include Al-rich metapelites. (3) Hida Marginal (Gaien) Belt is composed of Paleozoic shelf sediments. (4) Renge Belt is composed of Ordovician ophiolite and Carboniferous high-P metamorphic rocks. (5) Akiyoshi accretionary complex shows a limestone-dominated facies with Permian accrecionary age. (6) Maizuru Belt is considered as arc-back arc system. (7) Ultra-Tanba accretionary complex shows a deficient limestone facies with Permian accretionary age. (8) Mino accretionary complex is a series of Jurassic accretionary complex.
In the Hida Mountains, Carboniferous shelf deposits belong to the Hida Marginal Belt and their metamorphic equivalents belong to the Unazuki Metamorphic Rocks and possibly a part of Hida Metamorphic Rocks (Tsukada et al., 2003; Kamikubo and Takeuchi, in press). These consist of two types (Tsukada, 2003; Tsukada et al., 2004). One is the dominantly carbonatic rocks Fukuji-Type that includes the Shimozaisho Group, Ichinotani Formation, Fujikuradani Formation, Nagano Formation and Unazuki Metamorphic Rocks. The other is the volcaniclastic dominated rocks Moribu-Type including the Arakigawa and Konogidani Formations. The limestone dominated type rocks commonly include Al-rich horizons and mafic volcaniclastic layers (Kawai, 1956; Igo, 1956; 1961).
Amidst the Fukuji-type rocks, the Shimozaisho Group, studied herein, correlates with the low-grade part of the Unazuki metamorphic rocks, and forms part of the Unazuki Belt (Hiroi, 1981).
Figure 1. Index map of the Japanese islands showing occurrences of Paleozoic chloritoid-bearing metamorphic rocks.
(A) Location of Carboniferous-origin Al-rich clastic rocks in Japan (stars). (B) Distribution of Mesozoic and Paleozoic rocks in the Hida mountains region (after Niwa et al., 2002; Takeuchi et al., 2004; Tsukada et al., 2004; Maki, 2006). Cretaceous volcanic rocks are omitted. Gr. = group, Fm.= Formation.