| Abstract |
Nodules (with the maximum diameter of 20 cm) have been found abundantly within a vent of the Neogene two-pyroxene andesite flow on the river side at Matsubara, south f Hita City, Oita Prefecture. The ...nodules and their host rocks are of the same kind of, two-pyroxene andesite, though the groundmass of the former is richer in magnetite, hematite, brown glass and clay minerals than the latter. Various kinds of xenoliths are found, without exception, enclosed at the centers of all the nodules examined, forty in number. The xenolith is granitic rock, schistose twopyroxene rock, hornblende diabasic rock, two-pyroxene doleritic rock or quartzose rock, all of which are decomposed and slightly remelted. The nodules are often deformed to lenses and rods, and finally to sheets alternating with the host rocks to form a banded structure, which disappears beyond a distance of about 30 meters from the margin of the vent. It is considered that volatile components mainly of water released from the xenoliths by the heat effect of magma migrate into the magma around the xenolith, where the viscosity of the magma has been lowered, and Na and P ions in the magma selectively diffuse to less viscous part and are concentrated there, that oxydation also occurs in and around the xenoliths. Combined effects of the above mentioned phenomena with the lack of differential movement of the lava which prevails at the vent are favourable for the formation of the nodules.show more
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