Geotraverses of the Japanese Islands

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Geotraverses of the Japanese IiandsThe Japanese Islands are divided into two principal geologic provinces, Northeast Japan and Southwest Japan. Both provinces are composed of many tectonostratigraphic terrenes. Without knowledge of these terrenes, the complex geological history of the Japanese Islands can not be properly understood.The purpose of this study is to examine intra- and inter-terrene relations as well as stratigraphic, sedimentological, geochronological and metamorphic characteristics of these terrenes through geotraverses in many regions in the Japanese Islands, from Hokkaido to Okinawa, and to overview the Paleozoic to Cenozoic geologic evolution.As a result of this study, the following lines of new evidence were obtained.(1). In the central part of the Hida terrene the so-called Precambrian Gray Granite occurs as veins and dikes cutting the Hida gneiss. The detailed geochronological studies on the basis of CHIME ages of monazites and zircons from the Gray, franite have revealed that it intruded into the Hida gneiss in early Triassic time (ca. 240 Ma). not in Precambrian time.(2). CHIME age data of monazite and zircon grains from the Carboniferous llikami granite in the Soutil Kitakami terrene have shown that the ca. 250 Ma thermal event is also recorded in tile rims of many zircon crystals in the granite. It seems probable that the latest Permian magnatism was much more extensive both in Northeast Japan and Southwest Japan than has been thought.(3). The oldest radiometric date is a 2050 Ma Rb-Sr whole-rock isochron age (originally 1985 Ma, it was recalculated using the currently-accepted decay constant) for quartzo-feldspathic gneisses in the Kamiaso conglomerate. CHIME ages of monazites in quartzo-feldspathic gneiss clasts and sandstones in the Mino terrene also give ca. 1700-1400 Ma, showing that the middle Precambrian rocks were widespread in the provenance of the classic rocks.