ジルコンの結晶形態からみた中部九州の新第三紀火山活動  [in Japanese] Neogene volcanic activity in central Kyushu in relation to zircon morphology  [in Japanese]

中部九州の新第三紀火山岩類について, 今回提唱したジルコン指数(ZI=zircon index)を用いて, 岩石成因論的な問題点を検討した。瀬戸内火山系列の中期中新世大野火山岩類のジルコンは100卓越型で, 輝石安山岩溶岩の代三五山安山岩と流紋岩溶結凝灰岩の三宅山流紋岩のジルコンの結晶形態はほとんど同じである。グリーンタフ火山系列の山国累層と後期中新世北坂本累層のジルコンは基本的に110卓越型で, 末期には長柱状のジルコンが認められる。豊肥火山系列の鮮新世英彦山火山岩類のジルコンは, 100卓越型で, 110卓越型の異質ジルコンを常に含んでいる。柱面指数はマグマ温度に比例するので, 中部九州の新第三紀火山活動は, 高温型の大野火山岩類に始まり, ついで低温型のグリーンタフ火山岩類に変化し, 再び高温型の豊肥火山岩類を噴出した考えられる。

The crystal morphology of zircon separated from Neogene volcanic rocks in central Kyushu is described using the method formed HAYASHI (1990). For a simpler description, the zircon index (ZI) is proposed: the prism, pyramid, elongation and flatness indexes are shown in this order as a single four-digit number, with each digit representing the value of one index multiplied by ten and rounded to the nearest number. The average crystal width in μm is shown in parenthses, right after the zircon index. The zircon in the middle Miocene Ono volcanic rocks is of the 100 dominant type, ZI-8448 (60) and 8448 (50). In contrast, the late Miocene Green-Tuff volcanic rocks (Yamakuni and Kitasakamoto formations) are characterized by zircon of the 110 dominant type, ZI=2548 (80), 3548 (100) and 2549 (80): it becomes long-prismatic at later stages. The Pliocene Hikosan volcanic rocks contain again the 100 dominant type, ZI=7549 (80), 7439 (80), 7449 (80) and 7449 (80), always being accompanied with accidental zircon of the 110 dominant type. Since the prism index is proportional to magma temperature, the middle Miocene Ono volcanic rocks and the Pliocene Hikosan volcanic rocks are of high-temperature type, while the late Miocene Green-Tuff volcanic rocks are of low-temperature type.