福岡県行橋市および周辺部に分布する白亜紀花こう岩類中のジルコンの結晶形態

Abstract

The morphology of zircon crystals from some Cretaceous granitoids in northeastern Kyushu is described. They are Hirao granodiorite, Kawara granodiorite, Masaki granite, Katsuyama granite, and Kaho granite in geologic ascending order. The Kawara granodiorite and the Kaho granite, distributed on the western side of the Kokura-Tagawa Tectonic Line, contain the 100 dominant prismatic zircon. On the eastern side, the Hirao granodiorite and the Katsuyama granite are of the intermediate type (prismatic to long-prismatic), and the Masaki granite (long-prismatic) is of the 110 dominant type. The difference in the prism index can be explained that the western granitoids were formed at a little higher temperature than the eastern ones. Similarly, judging from the difference in the elongation index, the western Kawara granodiorite is considered to have intruded first, followed by the Hirao granodiorite and the Masaki granite on the eastern side. The Katsuyama granite of S type (Chapell and White, 1974) contains many acidental zircon crystals of the 110 type which exhibit developed cracks.