A rhythmically banded basaltic andesite intrusion in the Shiretoko Peninsula, Hokkaido, Japan.

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  • GOTO Yoshihiko
    Experimental Forests, Faculty of Agriculture, Hokkaido University

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  • 北海道知床半島の縞状構造をもつ玄武岩質安山岩貫入岩体
  • rhythmically banded basaltic andesite i

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Abstract

The Oshinkoshin Intrusion is a mushroom-shaped, basaltic andesite intrusion of Pliocene age that occurs in the Shiretoko Peninsula, Hokkaido, Japan. The intrusion is 1000 m long and 400 m wide and consists of three concentric zones: inner massive zone, polyhedral jointed zone, and marginal banded zone. The inner massive zone occupies the central part of the intrusion, and is characterized by columnar joints 100 to 250 cm across. The polyhedral jointed zone encircles the inner massive zone, and is characterized by columnar joints 70 to 150 cm across and small polyhedral joints. The small joints outline 10 to 25 cm polyhedra on both longitudinal and transverse surfaces of the columns. The marginal banded zone encircles the polyhedral jointed zone, and shows rhythmic banding parallel to the margins and columnar joints perpendicular to the margins. The rhythmic banding is composed of alternating protruding, more weathering-resistant and recessive, less resistant bands, both of which are a few centimetres thick. Each protruding band characteristically encloses a central vein. Chemically, protruding bands have higher SiO2 and K2O and lower MgO and total Fe contents than recessive bands. Veins in the protruding band have much higher SiO2, TiO2, Na2O, K2O, P2O5, and lower Al2O3, MgO, CaO than the protruding band. The thicknesses of the protruding and recessive bands gradually decrease toward the margin of the intrusion. The concentric zonal structure in the Oshinkoshin Intrusion formed by cooling of a magma which was emplaced in a single intrusive phase into wet, poorly consolidated sediment. The rhythmic banding in the marginal banded zone formed by concentration of residual melt in numerous parallel cooling fractures along the margin of the intrusion, as the rheological behavior of magma changed from ductile to brittle.

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