Mode of occurrence of alkali basaltic volcanic products and their inclusions of Noyamadake, Shimane Prefecture, southwestern Japan

Abstract

Geology and petrography of successive deposits of pyroclastics and lava flow of alkali basaltic composition with numerous xenoliths of Noyamadake, Shimane Prefecture, are examined to make clear the volcanological aspect of the Cenozoic alkali basalts of southwestern Japan. The pyroclastics, which occupy the lower part of the Noyamadake volcanic products, consist of several beds of pyroclastic fall deposits and pyroclastic flow deposits. From the mode of occurrence of these volcanic products, the eruptive history of Noyamadake volcano is roughly inferred as following; At first a large explosion occurred to make a small crater, and for a while, intermittent eruptions continued to produce pyroclastic fall and flow deposits. Later, eruptions became continuous, and produced pyroclastic flow deposits mainly, and at last lava erupted.
The relative abundance of xenoliths derived from the basement rocks (diorite and schist) drastically increases downwards in the Noyamadake volcanic products, and vice versa for the xenoliths of mantle peridotites (lherzolite and harzburgite). Xenoliths of the cumulus mantle origin (dunite, wehrlite and pyroxenites) also decrease in the relative abundance downwards, though less markedly than those derived from the mantle peridotites. To explain such a systematic change of frequency of xenoliths, a relatively simple model which concerns with the process of magma ascent and wall rock incorporation is proposed. Xenoliths are almost only incorporated in the front of the ascending magma column, in which xenoliths sink against the ascending magma due to the density difference. Therefore, the systematic change of the frequency of the rock species of xenoliths, depending on the duration and relative velocity of sinking of xenolith, is expected in the ascending magma column, which, consequently, determines the frequency of the xenoliths in the successive volcanic products derived from a magma column. We have to carefully examine the relative abundance of xenoliths when we estimate the petrological construction of the crust and upper mantle.