| Abstract |
The Shirasu, the great rhyolitic pyroclastic flow from the Aira volcano of south Kyushu, contains glass fragments, crystal grains, and rock fragments. The crystal grains are quartz, plagioclase, hyper...sthene, hornblende, augite, biotite, and magnetite, together with only trace of ilmenite, apatite, and zircon. The "corroded" quartz grain is bipyramidal crystal without prismatic face. Many plagioclase phenocrysts consist of two parts: the labradorite core and more sodic rim with oscillatory zoning. The boundary between the core and rim is somewhat irregular and embayed. The core, almost lacking in zonation, is more cloudy than the rim, with many inclusions of glassy substance, iron ores, apatite, hornblende, pyroxene, and indistinct dust. In the core, two plagioclases, labradorite-bytownite and andesine, into which a homogeneous plagioclase was separated on heating, make an irregular intergrowth. The rim, free from all inclusions, is clear, having exceedingly delicate and rhythmical oscillatory zoning. The albite twinning observed in the rim does not extend into the core. The abrupt transition from the core to oscillatory zoned rim may be explained that the core is a foreign crystal to the Shirasu magma.show more
|
Export Mendeley
0701_p011