Compositional changes due to hydrothermal alteration and mixing combined with differentiation and accumulation of igneous rocks from northern part of Mt. Shaku-dake, northern Kyushu, southwest Japan

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  • Eshima Keisuke
    Graduate school of Sciences and Technology for Innovation, Yamaguchi University

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  • 北部九州，尺岳北部に産する火成岩類の熱水変質作用と分化・集積作用を伴う混合作用における組成変化
  • ホクブ キュウシュウ,シャクダケ ホクブ ニ サンスル カセイガンルイ ノ ネッスイ ヘンシツ サヨウ ト ブンカ ・ シュウセキ サヨウ オ トモナウ コンゴウ サヨウ ニ オケル ソセイ ヘンカ

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<p>In the northern part of Mt. Shaku-dake (Kitakyushu city, Fukuoka prefecture, SW Japan), porphyritic fine-grained tonalite and clinopyroxene quartz diorite intrude the Cretaceous Wakino sub-group as stocks. Previous geochronological studies have suggested that the tonalite magma formed prior to intrusion of the diorite magma, in advance of the large-scale igneous activity that created the Northern Kyushu granitic batholith. The tonalite is exposed over a wide area extending ~5 km from east to west and 3 km from north to south. The tonalite shows a peperitic texture at the boundary with the host rock (the Wakino sub-group), and the presence of secondary minerals and veins due to hydrothermal alteration in the tonalite suggest that the whole-rock compositions have been modified. Geochemical modeling reveals that owing to hydrothermal alteration, the tonalite gained SiO2, Al2O3, and K2O, and lost CaO and Na2O. The diorite, however, has sharp boundaries where it intrudes the Wakino sub-group and the tonalites. The petrological features of the rocks suggest that the magma evolved through fractional crystallization, crystal accumulation, and mixing in a magma chamber. Evidently, the Cretaceous igneous activity in northern Kyushu began with shallow-level magmatism forming the intrusive porphyritic (or volcanic) rocks represented by the tonalite, followed by hydrothermal activity, and culminating with the emplacement of large-scale batholiths from stock-like plutonic activity.</p>

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