ALKALIC ROCKS OF THE NEMURO PENINSULA WITH SPECIAL REFERENCE TO THEIR PILLOW LAVAS

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Abstract
Mafic alkalic rocks occur as lava flows, sheets or laccoliths in the Upper Cretaceous rocks in the Nemuro Peninsula, Hokkaido, Japan. Crystallization differentiation owing to gravitational separation of olivine and pyroxene is observed in some sheets or laccoliths, while remarkable pillow structure is observed in some sheets or lava flows. Pillows are mostly 50 cm - 2 m in diameter and usually ellipsoidal in shape. Difference in crystallinity and mineral composition is observed in a pillow; i. e. its central portion is holocrystalline dolerite and its margin is tachylite with very high water concentration. Origin of the pillow lavas in this district is interpreted as follows. Very fluid doleritic lava was erupted on deep ocean bottom, sometimes intruded into thick wet sediments. Quenching by water or sediments and convection current resulted in the formation of ellipsoidal cracks in the lava flows or sheets. Under such conditions there was not much difference between lava flows and sheets. Because of its ...

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