초록

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Abstract ▼ AI-Helper

The Dongmakgol Tuff, which is the important ash-flow ignimbrites in the Cheolwon basin, is composed of multiple-flow widespread ash-flow units. The ignimbrites display systematic chemical zonations with zigzaggy type in the vertical sections. All chemical analyses fall into the range of 67.1~79.4%...

The Dongmakgol Tuff, which is the important ash-flow ignimbrites in the Cheolwon basin, is composed of multiple-flow widespread ash-flow units. The ignimbrites display systematic chemical zonations with zigzaggy type in the vertical sections. All chemical analyses fall into the range of 67.1~79.4% in SiO2 contents, which gradually change from basal high-silica rhyolite to upper rhyodacite within many intervals of the sheet. These zigzaggy compositional variations appear to reflect vertical variations and discharge rate changes in the magma chamber from which the ignimbrites were erupted. The Rb-Sr isotopic data yield an isochron of 103.1±3.2 Ma with an initial 87Sr/86Sr ratio of 0.71426±0.00088. The systematic sequence of compositional variations suggests that the ignimbrites were formed by successive eruptions to the lower part from the top of a zoned magma in which relatively high-silica rhyolitic magma overlay rhyodacitic magma. This suggests that the zoned magma resulted from magmatic differentiations by fractional crystallization and/or crystal settling.

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