飛騨外縁帯,青海-蓮華メランジの変成岩類  [in Japanese] Metamorphic rocks in the Omi-Renge serpentinite melange, Hida Marginal Tectonic Belt, Central Japan  [in Japanese]

The Omi-Renge region, which is considered to be the northeastern part of the Hida Marginal Tectonic Belt (Hida-Gaien Belt), is composed of five tectonic units which generally have NNW-SSE trends with westward dips except for Unit 1 and Unit 2, and as a whole form a shuppen structure. The five units are 1) middle Permian sedimentary rocks and metaophiolite (Maizuru Terrane), 2) middle Permian chaotic sedimentary rocks (Akiyoshi Terrane), 3) Omi limestone (Akiyoshi-type) and adjacent olistostrome (Akiyoshi Terrane), 4) serpentinite melange including Carboniferous blueschists blocks and 5) chaotic terrigeneous sedimentary rocks including metaophiolite blocks (Shiroumadake melange). The last two units form the Hida-Gaien complex. Serpentinite is the dominant and most important constituent in the Omi-Renge region. It forms not only the serpentinite melange (Unit 4) but also serpentinite nappe that bounds Unit 2 and Unit 3. The serpentinite that forms the nappe include relatively small amount of metamorphic blocks, such as albitite, jadeite rock, meta-carbonate rock and meta-sedimentary rock. The serpentinite melange forms a wide zone between Units 3 and 5, but the zone is largely covered by Lower Jurassic Kuruma Group and younger sedimentary rocks and volcanics. Hence the serpentinite melange appears to be unevenly distributed in the area of Omi, Renge and Happo. Metamorphic rocks included in the serpentinite melange are classified into high-pressure type (Sanbagawa type) and low pressure type (ocean-floor type), but there are some other types of rocks such as garnet-brown hornblende amphibolite, high-Na pargasite metagabbro and metagranitic rock. They are different in origin from the two types of metamorphic rocks abovementioned. The high pressure type metamorphic rocks have no lawsonite, aragonite nor jadeite + quartz and constitute the facies series from the pumpellyite-actinolite facies through the epidote glaucophene schist facies to the epidote amphibolite facies. In the Happo region, blocks of coarse-grained garnet glaucophene schist, garnet epidote amphibolite, garnet mica schist and eclogite are included in serpentinite. The radiometric ages of the high-pressure type schists are 311 to 362 Ma, and those of the low-pressure type metagabbro-amphibolites are 336 to 370 Ma. The high-pressure schist ages are slightly older than those of the oldest schists (305 to 280 Ma) in the Sangun Terrane.