九州東部大野川層群の花崗岩礫のK-Ar年代(古領家帯と黒瀬川帯の構成要素と改変過程)  [in Japanese] K-Ar ages of the granitic clasts from the Onogawa Group, eastern Kyushu(Geotectonic evolution of the Paleo-Ryoke and Kurosegawa Terranes)  [in Japanese]

大野川層群霊山層中の花崗岩礫と, 朝地変成岩類分布域中の山中花崗閃緑岩のK-Ar年代を測定した.花崗岩礫4試料から得られたホルンブレンドと黒雲母のK-Ar年代は103〜108Maとよく一致した.一方, 山中花崗閃緑岩1試料について得られたK-Ar年代は, ホルンブレンドが103Ma, 黒雲母が104Ma, カリ長石が75.1Maとなった.霊山層中の花崗岩礫と山中花崗閃緑岩は年代的には一致するものの, 微量元素や帯磁率が異なる.しかしながら, 大野川層群の堆積物は北方から供給されたという報告があることから, 霊山層中の花崗岩礫の供給源が朝地地域周辺の花崗岩類から由来した可能性は高い.大野川層群が領家帯の和泉層群の西方延長に相当するのか, あるいは古領家帯の跡倉層-真穴層に対比されるかを明らかにするためには, 今後朝地変成岩類の帰属問題を解決する必要がある.

The K-Ar age determinations were carried out on the granitic clasts from the Ryozen Formation of the Onogawa Group, and the Yamanaka Granodiorite to the north of the Onogawa Group in eastern Kyushu. The hornblende and biotite K-Ar ages from four samples from the Ryozen Formation range from 103 to 108Ma. The Yamanaka Granodiorite, lithologically similar to the dated samples from the Ryozen Formation, gives the K-Ar ages of 103Ma for hornblende, 104Ma for biotite and 75.1Ma for K-feldspar. Although the K-Ar biotite and hornblende ages of the granitic clasts from the Ryozen Formation are the same as those from the Yamanaka Granodiorite, magnetic susceptibility and contents of some major and minor elements are different. Accordingly, the granitic clasts are not identical to the Yamanaka Granodiorite, but they are correlative with the granitic rocks around the Asaji metamorphic region. This correlation can be also supported by the previous paleocurrent data observed in the Group. Further studies are needed, however, to clarify the geotectonic correlation whether the Onogawa Group is correlative with the Izumi Group in the Ryoke Belt or with the Atokura-Maana Formations in the Paleo-Ryoke Belt.