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Interpretation for Variations in Mineral Contents in Volcanic Rocks Related to the Yangsan Caldera  

Hwang, Sang-Koo (Department of Earth and Environmental Science, Andong National University)
Kim, Se-Hyeon (Department of Earth and Environmental Science, Andong National University)
Jeong, Seong-Wook (Department of Earth and Environmental Science, Andong National University)
Publication Information
The Journal of the Petrological Society of Korea / v.17, no.3, 2008 , pp. 166-178 More about this Journal
Abstract
The modal analyses on the phenocryst phases and the normative mineralogies from the bulk chemical analyses record that the volcanic rocks related to the Yangsan caldera might been derived from compositionally zoned magma. The volcanic rocks show linearly continuous mineralogical gradients, not only totally in the relations between $SiO_2$ and proportion of phenocryst content, but also within each rock unit in the relations between total phenocryst content and the proportion, Q-Ab-Or and Q-An(Ab-Or) diagrams. The roughly gradational modal variations of the phenocryst phases are shown upward within each rock unit. However, the contents and proportion of the phenocrysts in the Yangsan Tuff and the Hwajeri Formation represent the zigzaggedly undulatory variations. The continuous mineralogical gradients without large gaps define a large zoned magma system in the pre-eruptive, later precollapse and postcollapse magma chambers respectively. The zigzagged variations reflect the intermittent eruptive pulses representing any time gaps.
Keywords
Yangsan magmatic system; Phenocryst phase; Normative mineral; Compositional gradient; Compositionally zoned magma;
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