Abstract
Ultramafic xenoliths in alkali basalt from Arato-yama, Okayama Prefecture, southwestern Japan, consist of various kinds of peridotites and pyroxenites derived from the upper mantle. As mantle peridotites, spinel harzburgite surpasses spinel lherzolite in volume. Cumulus mantle supplied the magma with dunite, wehrlite, clinopyroxene-rich lherzolite and pyroxenites.Pyroxenites are mainly composed of clinopyroxenite, olivine websterite and websterite, and can be divided into two groups, I and II, in terms of textures. Pyroxenites of Group I are more fine-grained than those of Group II, and have metamorphic textures.
Petrological model of the upper mantle underneath Arato-yama is as follows. Spinel harzburgite with subordinate amounts of spinel lherzolite represents the mantle peridotite. Cumulus mantle between the mantle peridotite and the Moho is composed of dunite with pockets of websterites, wehrlite, Group I pyroxenites, and clinopyroxene-rich lherzolite, upwards in this order. Pyroxenites of Group II make a gigantic pocket in piles of cumulus peridotites and Group I pyroxenites. Cumulus mantle, especially its upper part, is characterized by the common occurrence of pyrrhotite.
Equilibrium temperatures of the ma n tle peridotites and the cumulus mantle just below the Moho are about 1200°C and 1050°C, respectively. A relatively steep geotherm can be estimated also.
