Blueschists associated with serpentinites are found in the Mikabu Greenstones of eastern Kii Peninsula. The serpentinites are classified into two types: massive serpentinites and serpentinite conglomerates. Blueschists occur as thin layers (6-250cm wide), and are intercalated with the serpentinite conglomerates. Talc-carbonate rocks are developed along the boundary between blueschists and serpentinite conglomerates. The blueschists, talc-carbonate rocks and surrounding Sanbagawa schists are folded with E-W trending fold axis. Both the blueschists and Sanbag-awa schists have similar mineral lineations plunging gently to the east or to the west.
Major metamorphic minerals of the blueschists are magnesioriebeckite, aegirine-aegirine-augite, muscovite, calcite, quartz and hematite. The blueschists, talc-carbonate rocks and serpentinite conglomerates characteristically contain detrital minerals: brown amphibole, colorless amphibole and chromian spinel.
These petrographic data suggest that (1) the serpentinite conglomerates and protoliths of blueschists and talc-carbonate rocks were sedimentary mixture of siliceous, calcareous and mafic materials, and (2) they were recrystallized during the Sanbagawa metamorphism. The blueschists and serpentinite conglomerates represent siliceous and mafic parts of the original sedimentary sequence, respectively.

抄録全体を表示

Biotite separates from granodiorites of the Tanigawadake Pliocene plutonic body, North Fossa Magna, central Japan were dated by K-Ar method. The ages range from 3.91±0.27 Ma to 2.9±2.4 Ma.
Petrological and mineralogical features suggest that the youngest granodiorite is an intrusion into the Tanigawadake Pliocene plutonic body.
Relationship between altitude and ages of rocks inferred that uplift rate of the Tanigawadake Pliocene plutonic body was about 1, 400 m/Ma during 3.91-3.40 Ma. This value suggests that the North Fossa Magna region experienced at least 5, 500m of uplift which is in accordance with the previous conclusion expressed by other workers.

抄録全体を表示

三宅島火山噴出物中には,灰長石巨晶 (1mm-3cm) と大型のかんらん石斑晶 (1-3mm) を伴った玄武岩溶岩およびスコリアが各地域で観察される。灰長石巨晶の⁸⁷Sr/⁸⁹Sr比は0.70340-0.70352であり,一方大型のかんらん石斑晶と灰長石巨晶中に包有されているかんらん石は共に0.70357-0.70374であった。また,これらの斑晶を除いた玄武岩溶岩およびスコリアの値は,両者の中間の値を示した。これらの結果は灰長石巨晶とかんらん石斑晶が噴出時にマグマとSr同位体的に非平衡であったことを示している。野外における産状を考慮すると,これらの結晶はゼノクリストであった可能性がある。灰長石巨晶とその結晶中に包有されるかんらん石のSr同位体比の不一致は,これらの結晶が2種類の起源の異なったマグマの混合によって形成されたことを暗示している。また,玄武岩溶岩,スコリアのSr同位体比が両結晶の値の中間にバラつく点は,三宅島火山のマグマの成因を考える上で重要な意味をもっていると考えられる。

抄録全体を表示

On-yama is one of the Cenozoic small monogenetic volcanoes which are clustered in and around the Tsuyama Basin, southwest Japan. Both alkali basaltic breccia and lavas of On-yama have abundant ultramafic xenoliths, which are small in size (<5cm across). Lherzolite is predominant among the xenoliths. Small amounts of other xenoliths of both Group I (wehrlite and pyroxenites) and Group II (clinopyroxenite) and megacrysts (clinopyroxene, spinel and phlogopite) are associated with lherzolite. Absence of dunite is characteristic of the On-yama xenolith suite.
The On-yama lherzolite has a rock-by-rock uniformity in mineralogy, having olivine of Fo_89-90 and chromian spinel of Cr # (=Cr/(Cr+Al) atomic ratio)=0.10-0.15. It is always enriched in clinopyroxene; the cpx/(opx+cpx) volume ratio is higher than 0.1. The low refractoriness of the On-yama lherzolite is correlated with the low frequency of associated dunite and other cumulative ultramafic xenoliths. The On-yama lherzolite may be representative of the least refractory part of the upper mantle beneath the Japan arcs.

抄録全体を表示