Unusual cooling of the Middle Miocene Ichifusayama Granodiorite, Kyushu, Japan

九州中部,中期中新世市房山花崗閃緑岩の冷却史

及川 輝樹; 梅田 浩司; 金沢 淳; 松崎 達二*

Oikawa, Teruki; Umeda, Koji; Kanazawa, Sunao; Matsuzaki, Tatsuji*

南部九州の西南日本外帯に貫入した市房山花崗閃緑岩から黒雲母K-Ar,ジルコンFT,アパタイトFT年代を求めその岩体の冷却史を明らかにした。その結果、市房山花崗閃緑岩は約10-13Maに300-100C冷えた。この花崗岩体の約13Maの急冷は、九州外帯がフィリピン海プレートへオブダクションしたことが原因と考えられる。

The authors report biotite K-Ar, zircon FT, and apatite FT ages from four sites in the Ichifusayama Granodiorite in the Outer Zone acidic rocks of Southwest Japan, Southern Kyushu. All the K-Ar ages and FT ages are concentrated on 13-10Ma. The difference in the apatite FT ages was caused by an influence of paleotopography such as a deep valley. The rapid cooling of the granodiorite in ca. 13 Ma occurred by uplift throughout the Outer Zone of Southwest Japan in Kyushu. It is considered that obduction to the Philippine Sea Plate of the Outer Zone of Kyushu also occurred in ca. 13 Ma. It is considered that obduction to the Philippine Sea Plate of the Outer Zone of Kyushu also occurred in ca. 13 Ma.