The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 119, Issue 12
Displaying 1-4 of 4 articles from this issue
Articles
  • Ayumi Kosugi, Mitsuhiro Nakagawa, Tomoko Seino
    2013 Volume 119 Issue 12 Pages 743-758
    Published: December 15, 2013
    Released on J-STAGE: June 05, 2014
    JOURNAL FREE ACCESS
    Supplementary material
    The volcanic island of Oshima-Kojima, which as an area of ~1.5 km2, is located 30 km west of the Oshima Peninsula in southwestern Hokkaido. We surveyed a part of the island to reveal the geology of the volcano and collected volcanic rocks to determine K–Ar ages and whole-rock chemistry. The volcano started the activity under the shallow subaqueous condition and continued to effuse lava flows in the subaerial condition with the shift of vent position. The newly obtained K–Ar ages of two samples from subaqueous and subaerial lavas are 0.11 ± 0.02 and 0.16 ± 0.01 Ma, respectively. Thus, the volcanic edifice developed over a relatively short period during the MiddleLate Pleistocene. The rocks of the island range from basalt to dacite and contain plagioclase, olivine, clinopyroxene, orthopyroxene, amphibole, biotite, quartz, apatite, and opaque minerals as phenocrysts. The rocks are classified as calc-alkaline series on a FeO*/MgO–SiO2 diagram, and as transitional between high-K and medium-K on a SiO2–K2O diagram. They have a similar geochemical signature to rocks from volcanoes on the back-arc side of the northeastern Japan arc with respect to incompatible elements and Sr–Nd isotopes. The petrological features and whole-rock chemistry indicate that mixing between basaltic magmas and dacitic magma was the main magmatic process. The variations in Sr and Nd isotopic compositions and the existence of several different trends in some of the oxide–SiO2 diagrams indicate that mafic end-member magmas are composed of several distinct magma types.
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  • Kazuma Seike, Hideki Iwano, Tohru Danhara, Hiromichi Hirano
    2013 Volume 119 Issue 12 Pages 759-775
    Published: December 15, 2013
    Released on J-STAGE: June 05, 2014
    JOURNAL FREE ACCESS
    The Late Cretaceous to early Paleogene evolution of the Ryoke-Izumi belt is examined using apatite and zircon fission-track (FT) dating of the Sennan Rhyolites, the Takijiri Adamellite, and the Izumi Group in the Izumi Mountains, southwestern Japan, together with pre-existing geochronological data and interpretation of geologic structures. The results reveal that the geology of the area can be divided into three blocks on the basis of different cooling histories and east-west-trending boundary faults; namely, blocks A, B, and C from south to north. Block A consists mainly of the Izumi Group, which accumulated over a part of the Sennan Rhyolites and the Takijiri Adamellite at 70.9 ± 3.4 Ma (zircon FT weighted mean age of pyroclastic rocks of the Izumi Group). Previously determined K-feldspar K-Ar ages of the Takijiri Adamellite of Block A (around 66 Ma) record the age of uplift of this block. Block B includes most of the Sennan Rhyolites, which erupted from 85.0 ± 5.2 Ma to 80.0 ± 11.2 Ma (zircon and apatite FT ages of a tuffaceous sample of the Sennan Rhyolites). The Sennan Rhyolites were also partly thermally metamorphosed by the Takijiri Adamellite and cooled before 73.9 ± 8.2 Ma (zircon FT age of a nontuffaceous sample of the Sennan Rhyolites). Block C includes the Takijiri Adamellite, which was exposed on the northern side of the faults after 50.0 ± 6.6 Ma (apatite FT age). The Sennan Rhyolites in Block B were not thermally affected by subsidence of the Izumi sedimentary basin (ISB), indicating that the ISB was formed at 71 Ma as a transtensional pull-apart basin by the subsidence of Block A. The subsequent rapid uplift of the Izumi Group and of the Takijiri Adamellite of Block A at 66 Ma was probably caused by reverse faulting under transpression.
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  • Hiromichi Kano, Hideo Takagi
    2013 Volume 119 Issue 12 Pages 776-793
    Published: December 15, 2013
    Released on J-STAGE: June 05, 2014
    JOURNAL FREE ACCESS
    Dextral ductile shear zones are found within the NW-SE-striking granitic porphyry dikes (older dikes) that intruded the older Ryoke granitoids on Awaji Island. The occurrence of these small-scale shear zones in the dikes is significant when considering the mechanism of strain localization, the history of stress, and the tectonics of the Ryoke Belt. This paper aims to clarify the geotectonic history of mylonite zones in the older dikes on the basis of petrological and structural analyses together with geochronological data. The mylonitized granitic porphyry yields K-Ar hornblende ages of 79.8 and 81.0 Ma. The mylonitic foliation of granite porphyry dikes is subparallel to the intrusion surfaces (NW-SE) and not parallel to the foliation of weakly to moderately mylonitized older Ryoke granitic rocks, which strikes E-W to ENE-WSW. Quartz porphyroclasts in granitic porphyry are commonly highly elongate and recrystallized, and the axial ratio of the porphyroclasts ranges from 3:1 to 17:1. Most of the estimated strain ellipsoids lie in the uniaxial shortening field. In the northern Shio area, granite porphyry is strongly mylonitized and contains recrystallized quartz with a mean grain size of ~100 μm. Quartz c-axis fabrics of the granite porphyry mylonites are midtemperature type (400-550°C), whereas those of the older Ryoke granitoids are low-temperature to transition (300-450°C) types. On the basis of the meso- and microstructural evidence, together with K-Ar ages, the generation process of mylonite prior to the Miocene clockwise rotation of southwestern Japan is inferred to be as follows: (1) mylonitization in Granitic Rocks I occurred at 90-85 Ma, (2) EW-striking intrusion of granite porphyry dikes along extension cracks took place at 85-80 Ma, and (3) dextral mylonitization along the E-W-striking dikes occurred at 80-70 Ma in a NW-SE compressional regime.
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