Relationship between chemical composition of Japanese Island-Arc volcanic rocks and gravimetric data |
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| Authors: | Tadahide Ui Shigeo Aramaki |
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| Institution: | 1. Department of Earth Sciences, Faculty of Science, Kobe University, Nada, Kobe 657 Japan;2. Earthquake Research Institute, University of Tokyo, Hongo, Tokyo 113 Japan |
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| Abstract: | 1670 published chemical analyses from 137 Quaternary eruption centers of Kurile, Japan and Izu—Mariana arcs are collected and K55 and K60 values of individual centers are calculated. Along with the well-established trend of increase of K55 and K60 values across the arc towards the continent, longitudinal change is also recognizable. Interrelation between K55 or K60 value pattern and a map of Bouguer anomaly distribution along the volcanic front is characteristic.To develop one possible simplified model to explain the facts, we assume first that the asthenosphere has a homogeneous thermal state beneath the volcanic zone, second that the thickness of the crust is quite variable areally, and third that the mean density of the crust is constant throughout the area. Thus, the composition of the magma generated along a zone corresponding to equal depths along the Benioff zone should be identical throughout the volcanic arc, and lower values of the Bouguer anomaly should be observed where the crust is thicker. We then assume that magma generated in the asthenosphere ascends into the crust, then cools as fractional crystallization proceeds. Depths of fractional crystallization would be deeper where the crust is thicker. The ratio of the clinopyroxene phase separated from the magma to the plagioclase phase should be larger where the level of fractionation is deeper. Thus, magmatic liquid should be enriched in K where K-poor crystal fractions are subtracted at the deeper level. |
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