Japanese Magazine of Mineralogical and Petrological Sciences
Online ISSN : 1349-7979
Print ISSN : 1345-630X
ISSN-L : 1345-630X
Volume 32, Issue 2
March
Displaying 1-4 of 4 articles from this issue
Original Articles
  • Kazuhito YAJIMA
    2003 Volume 32 Issue 2 Pages 51-67
    Published: 2003
    Released on J-STAGE: April 21, 2008
    JOURNAL FREE ACCESS
    Chichijima, located in Ogasawara (Bonin) arc, Japan has been known as the type locality of boninite. Meanwhile, extremely evolved volcanics (SiO2>70 wt%) also occur together with the high-magnesian andesites. The evolved rocks have been considered as the products of differentiation from the boninitic parental magma by fractional crystallization processes. The differentiation processes are considered to produce low-Ca boninite through bronzite-andesite, dacite, quartz-dacite and quartz-rhyolite. Some of the quartz-dacites, however, would not be necessarily regarded as differentiates from the dacite magma. Quartz-dacites are enriched in SiO2 for the fractional crystallization products from dacite. Some quartz-dacites show mineralogical evidences of magma mixing, while such features can not be observed in the dacites. Quartz phenocrysts in the silica rich quartz-rhyolite exhibit euhedral form, but quartz-dacites have corroded quartz phenocrysts. The compositions of mafic pyroxene phenocrysts in quartz-dacites are similar to those in bronzite-andesites. Groundmass pyroxenes in some quartz-dacites include orthopyroxene and augite. In contrast, pigeonite and augite are ubiquitous groundmass pyroxenes in the quartz-free dacites. These chemical and petrographical features imply that the quartz-dacites have calc-alkalic affinities, whereas the dacites have tholeiitic affinities. The tholeiitic trend of the dacites might be formed by fractional crystallization under low fO2 conditions, judging from the crystallization of Fe-rich pigeonite phenocrysts, and the quartz-rhyolite would be produced from the dacite magma by fractional crystallization in the tholeiite suite. Mixing calculations between bronzite-andesite and quartz-rhyolite reproduce the observed chemical variations of quartz-dacites. Therefore, calc-alkalic trend of the quartz-dacites might be formed by internal magma mixing between the bronzite-andesite magma and the quartz-rhyolite magma.
    Download PDF (1131K)
  • Daisuke NISHIO, Tetsuo MINAKAWA
    2003 Volume 32 Issue 2 Pages 68-79
    Published: 2003
    Released on J-STAGE: April 21, 2008
    JOURNAL FREE ACCESS
    The reinvestigation of so-called titanian fassaite from Myojin island, Seto Inland Sea of Japan, has revealed that the minerals are titanian-esseneitic-diopside by chemistry. The diopsides with similar chemical compositions were found in Mutsuki island.
    These diopsides occur as dark crystalline aggregates of nodule in saccharodial limestone. The dark fragments in this area are recrystallized skarns, which underwent Al-Ti-Fe concentration composed of titanian esseneitic diopside, potassic-magnesiosadanagaite, hercynite, ilmenite, and aluminian titanite, as accessory phlogopite, apatite, and perovskite. These minerals almost all are rich in Ca-Al-Ti-Fe.
    The electron microprobe analysis shows that the diopsides in this area have high Al2O3, TiO2 and Fe2O3 contents. Al ≥0.894, Ti ≥0.183, and Fe3+≥0.276 on the basis of O=6 and total cations=4. The diopsides have high molecule contents of esseneite, Ti-Tschermaks, and Ca-Tschermaks diopsides.
    It is suggested that occurrence of the titanian esseneitic diopsides rock from Myojin and Mutsuki islands are from the sediment such as laterite or terra rossa, which is rich in the Al, Ti, and Fe, through the metamorphism with high temperature which is 600°C or more.
    Download PDF (1997K)
  • Hikaru YOKOYAMA, Masahiro YAHATA, Satoshi OKAMURA, Hirotsugu NISHIDO
    2003 Volume 32 Issue 2 Pages 80-95
    Published: 2003
    Released on J-STAGE: April 21, 2008
    JOURNAL FREE ACCESS
    Late Pliocene-Pleistocene Akaigawa Caldera, 6-7 km in diameter, is located in southwest Hokkaido. It mainly consists of felsic caldera-forming welded tuffs, somma lavas, and lacustrine deposits in order of decreasing age. Caldera-forming pyroclastic flows of the Nagasawa Volcanics were erupted at the southwestern caldera rim and are characterized by high SiO2 content (76-79%). Contrary to this, the subsequent somma lavas were erupted from multiple vents along ring fractures and have variable SiO2 contents ranging from basaltic andesite to dacite (56.6-66.7%).
    The whole rock K-Ar age of the lower member of the welded tuffs is 2.06±0.57, whereas the age of the middle member 1.62±0.14 Ma. The Maruyama Lava and the Daikokuyama Lava from the somma lavas give ages of 1.65±0.07 Ma and 1.55±0.06 Ma, respectively. The age of the middle member of the welded tuffs coincides within measurement errors with ages determined for the somma lavas. The age and geological structures of the lower member of the welded tuffs suggest that the pre-Akaigawa, outer Yoichigawa Caldera formation began before 2 Ma.
    On the basis of petrography and geochemistry, the somma lavas are divided into three units; the eastern somma lavas, the western somma lavas, and the post-caldera central cone. The lithologies of eastern somma lavas range from olivine basaltic andesite to andesite, and the western somma lavas range from quartz andesite to dacite, indicating a similar geochemical trend to that of the eastern somma lavas. The post-caldera central cone is composed of andesite to dacite, and has a different geochemical signature from that of the somma lavas.
    The volcanic structure, topography, eruption volume, and petrography indicate that the Akaigawa Caldera-collapse took place after eruptions of both welded tuffs and somma lavas, and that the western side of the caldera is characterized by a higher eruption rate of felsic to intermediate magma and more extensive subsidence compared with the eastern side. This may indicate the existence of a heterogeneous magma chamber beneath the Akaigawa Caldera.
    Download PDF (1304K)
Short Review & Scientific Communication
feedback
Top