The Oshima metamorphic complex, which is located on Oshima Island, western Ehime, Shikoku Island, is a rare high-temperature metamorphic complex in the outer belt of Southwest Japan. The complex and accompanying granitic mylonites were subjected to zircon U-Pb age dating by LA-ICP-MS and monazite U-Th-Pb chemical age dating by FE-EPMA. The complex yields detrital zircon ages of ca. 2000-140 Ma and might have undergone a main high-temperature metamorphism at ca. 120 Ma, intrusion by gabbroic to tonalitic magma at ca. 110 Ma, and brittle deformation associated with pseudotachylite formation at ca. 100 Ma. In contrast, the granitic mylonites were possibly derived from the felsic magmatism at ca. 170 Ma, indicating an origin independent from that of the Oshima metamorphic complex. The geology and geochronology of the complex are similar to those of the Higo metamorphic complex and possibly comparable with the Atokura nappes in the Kanto mountains, and the Takanuki and Nishidohira metamorphic rocks of the Abukuma plateau. The results indicate the presence of a large-scale Cretaceous plutono-metamorphic complex extending from Kyushu to the south Tohoku area.
Fission-track (FT) and U-Pb thermochronometries were applied to rock samples collected from the South Kyushu shear zone for constraining when the zone was active its timing of activity. Two samples were collected from thermally undisturbed portions of the Shibi-san Granodiorite, and one sample was collected along an aplite dike which was cut by a fault zone. The samples yield zircon U-Pb ages of 14.0-13.5 Ma, zircon FT ages of 14.7-13.7 Ma, and apatite FT ages of 12.6-10.0 Ma. were obtained from the three samples. The ages and thermal inversion results show that the Shibi-san Granodiorite intruded into a relatively shallow depth at 14.0-13.5 Ma and cooled below the partial annealing zone of the apatite FT system (60-130℃) within a few million years. The Hirabae and Kusubae rock types of the Shibi-san Granodiorite have similar formation ages and cooling histories. Based on meso-microscopic observations of various directions of sense of shear in the fault zone, the aplite dike was displaced multiple times during and immediately after its intrusion, at temperatures of more than ~200℃. Apatite FT data show that dike intrusion and the series of fault movements occurred at 10.0±0.8 Ma. The fault movements seem to be related to multiple tectonic movements at ca. 10 Ma, rather than to fracturing during the later stage of intrusion and cooling of the Shibi-san Granodiorite body at 14.0-13.5 Ma. Applying a similar approach to the study of further minor faults may provide more detailed understanding of the tectonics of the shear zone over geologic time.
The stratigraphic positions of the Upper Cretaceous Haraokayama and Tenjōgatake andesite members of the Abu Group, located in the Nagato-Hōhoku area of Yamaguchi Prefecture in Japan, were reexamined through field survey, petrography, whole-rock geochemistry, and plagioclase K-Ar dating. We found that: 1) the Haraokayama and Tenjōgatake andesite members unconformably overlie a volcanic basin covered by the Abu Group; 2) the andesite members yielded ages of 72.8±1.6, 72.6±1.6, 71.4±1.6, 70.5±1.6, and 69.9±1.6 Ma, which are significantly younger than that of the Abu Group (90-80 Ma); and 3) the Haraokayama andesites have slightly different geochemical characteristics to the intercalated andesites within the Abu Group. Andesites with K-Ar ages of 73-70 Ma should therefore be separated from the Abu Group, and redefined as the Upper Cretaceous Haraokayama Andesite, which unconformably overlies the Abu Group from Nagato City to Hōhoku-Cho of Shimonoseki City. These units were designated previously as the Haraokayama Andesite Member of the Nagato Formation and the Tenjōgatake Andesite Member of the Shiratakiyama Formation. We suggest that the transition from ignimbrite flare-up in the Abu Group to steady-state arc volcanism, constrained by the Haraokayama Andesite, dates back to at least 70 Ma.
We conducted U-Pb dating of detrital zircons from three clastic rock samples, which have been classified into the Chichibu Belt in the Koshibu-gawa area, central Akaishi Mountains. U-Pb ages of the samples fall into two main age groups at 280-100 Ma and 2600-1700 Ma. The youngest single grain ages are 105.5±1.3 Ma, 109.6±1.2 Ma, and 111.1±1.9 Ma, and the youngest cluster ages are 115.9±1.0 Ma, 115.1±0.4 Ma, and 113.1±1.3 Ma. These Aptian to Albian (latest Early Cretaceous) ages are considerably younger than previously reported youngest ages from the Jurassic accretionary complex of the Chichibu Belt in the southern part of the Koshibu-gawa area (Okuchausu-yama area). Our results indicate that the geological bodies, including clastic rocks studied in the Koshibu-gawa area, should be assigned to the Cretaceous accretionary complex of the Shimanto Belt, and not to the Jurassic accretionary complex of the Chichibu Belt. Reexamination of the structural evolution and geological architecture in the Koshibu-gawa area is thus warranted.
The Shibukawa region of the Sanbagawa belt, central Japan, is one of the historic jadeite localities in Japan. Although its petrological and mineralogical characteristics of the locality were briefly reported over 35 years ago, no further studies have been conducted since the disappearance of the jadeite-bearing outcrop. Recently, jadeite was discovered within a veinlet in a host dunite boulder, a member of an ultrabasic body that is surrounded by the Sanbagawa crystalline schists in the Shibukawa region. The veinlet is composed mainly of diopside, OH-bearing grossular, jadeite, chlorite, pargasite and serpentine. Relict olivine and spinel, which were probably members of the host dunite, rarely occur in the veinlet. The jadeite grains vary from <1 to 50 µm in size, and occur as patchy aggregates with amoeba-shaped grossular grains. The jadeite grains have Na2O and Al2O3 contents of 14.7-15.4 wt.% and 24.3-25.5 wt.%, respectively, and are 91-98% jadeite contents [(Na-Fe3+)×100].