中部九州における鮮新世以降の火山活動 : 豊肥火山地域と島原火山地域の比較  [in Japanese] Pliocene and Quaternary volcanism in central Kyushu, Japan : Hohi Volcanic Zone vs. Shimabara Volcanic Zone  [in Japanese]

中部九州の鮮新世以降の火山活動は, 主に, 豊肥火山地域と島原火山地域でおこった。両者はいずれも火山構造性地溝であり, ともに別府-島原地溝を構成するが, それぞれの, 発達様式や火山岩類の組成的特徴は大きく異なる。豊肥火山地域が約5 Maから活動を開始し, 島原火山地域がそれと同時かやや遅れて始まった。島原火山地域では中新世からの北西九州玄武岩の活動が引き続いている。玄武岩類の組成からは, 島原火山地域では海洋島型マントルが地下に存在し続けたが, 豊肥火山地域では, 沈み込むフィリピン海プレートから由来する物質でマントルの汚染が続いたと考えられる。豊肥と島原火山地域は, それぞれ, 火山フロントと背弧海盆に相当すると考えられる。

Pliocene and Quaternary volcanic rocks in central Kyushu are distributed both in the Hohi Volcanic Zone (HVZ) and in the Shimabara Volcanic Zone (SVZ). In HVZ, volcanic activity started around 5 Ma with the formation of lava plateau, and changed into central-vented eruption of large-scale pyroclastic flows and lava domes. Calc-alkalic andesite and dacite are dominant in the early stage, while high K_2O volcanic rocks of both calc-alkalic and tholeiitic series, ranging from basalt to rhyolite, appeared in the later stage. High-magnesian basaltic andesite, which is transitional between ocean-island and island-arc types, erupted in the early stage, and high-alumina basalt of island-arc type in the later-stage. In SVZ, activity of calc-alkalic andesite and dacite started before or around 4 Ma with contemporaneous activities of alkalic and tholeiitic basalts of ocean-island type. Basalts formed lava plateaus, while andesites/dacites formed lava domes, thick lava flows, and volcanic lastic rocks including sedimentary rocks. The erupted products show bimodal frequency distribution of SiO_2 missing basic andesite. Basaltic magma had not erupted since about O.8 Ma. Temporal, chemical change of basaltic rocks in HVZ should reflect the change of source mantle under central Kyushu. The change can be explained by the progressive contamination model (Nakada and Kamata, 1991) in which mantle wedge continues to be contaminated by slab-derived componentas subduction goes on. The activitiesin both HVZ and SVZ probably related to subduction of Philippine Sea plate (PHSp) that had occurred since about 6 Ma. HVZ represents a volcanic front, whereas SVZ a back-arc basin. Subduction of PHSp induced a large amount of partial melt of slightly contaminated (but still fertile) mantle-wedge and fusion of crustal material. The degree of melting reduced with time, resulting in the smaller eruption rate in the later stage of HVZ. Upwelling of fertile mantle under SVZ, which might be introduced by subduction, may also have caused relatively large amount of partial melting of the mantle and fusion of crustal material.