Abstract
To understand the character of deep-seated crustal components, we have investigated the crustal xenoliths, in Pleistocene andesites from the Kinbo volcano, northwest Kyushu, Japan. Various types of rocks, such as gneiss, Px granulite, Ho-Pl amphibolite, hornblendite, Ho±Cpx gabbro and granitoids, are found as crustal xenolith in the andesites. The average chemical compositions of the crustal xenoliths in Araoyama and Ishigamiyama are similar to those of tholeiitic basalt and basaltic andesite, respectively.
There is no interaction zone between the xenoliths and host andesites in their appearance indicating that the thermal difference between them was small at entrainment. However, complex disequilibrium textures, including breakdown of hydrous minerals, plagioclase with sieve texture and quenched anatectic melt, occur in thin section of almost all the samples. These textures indicate that pyrometamorphism of the xenoliths resulted from the heating of basaltic magma erupted at early stage of the volcano. The parent rocks, eliminating the influence of pyrometamorphism from the xenoliths, presumably equilibrated under amphibolite facies condition, and were heterogeneous basic gneiss as a whole. Based on the lithologic and chemical character of the basic gneiss combined with reported geophysical data in this region, it is expected that the basic gneiss may widely distribute as the crustal components under the Conrad discontinuity in the northwest Kyushu.
