| Abstract |
Two kinds of igneous xenoliths of andesitic composition were found in the granodiorite porphyry in the Miocene Osuzuyama felsic igneous complex in the Outer Zone of Southwest Japan. Both black-coloure...d lens and crystal clot were found in the rhyolite to dacite welded tuff of the complex. Phenocrysts of the granodiorite porphyry are plagioclase, orthopyroxene and quartz. One of the xenoliths is the Oseuchi type (OsX) composed of plagioclase, orthopyroxene and (±） quartz, and the other is the Mimikawa type (MiX) composed of orthopyroxene, augite, subcalcic-augite, pigeonite, plagioclase and devitrified glass. Black lens consists of plagioclase phenocryst, pyroxene (?) and plagioclase microphenocrysts, cryptocrystalline matrix and crystal clot. Crystal clot in the welded tuff, as well as in the black lens, shows the texture of plagioclase-orthopyroxene aggregation. Pigeonite geothermometry gives about 1000°C for MiX, while equilibrium temperature of the host magma had been estimated at about 800°C. It may be suggested that MiX is a part of quenched andesite magma which was injected into the felsic magma chamber before the eruption, and that OsX is crystal cumulation transported from the bottom of the magma chamber during the agitation by the injected magma (MiX). Black lens may be a part of the andesite magma with some small pieces of OsX, crystal clot. The granodiorite porphyry of 14 m. y. ago had a 67^Sr/66^Sr ratio of 0. 7099 and belongs to the S-type (CHAPPELL & WHITE, 1974), while OsX and MiX with low 67^Sr/86^Sr ratios to the I-type. In explanation of the above petrographical and geochemical data for the host rock and xenolith, a possible manner of ascent of the felsic magma is partial zone melting. Xenoliths similar to OsX and MiX were also found in other felsic igneous complexes of the S-type in the Outer Zone, Southwest Japan.show more
|
Export Mendeley
1302_p163