熊本県,天草下島のソレアイト質玄武岩と高マグネシア安山岩のK-Ar年代と岩石学的特徴
1992 年 87 巻 7 号 p. 283-290
詳細
1992 年 87 巻 7 号 p. 283-290
One andesite from Kameura and two basalts from Gesujima and Gongenyama in Amakusa-Shimoshima, Kumamoto Prefecture were dated by the K-Ar method. The obtained K-Ar ages are 14.24±0.34 Ma for the Kameura andesite, 10.61±0.94 and 9.36±0.52 Ma for the Gesujima basalt, and 6.99±0.29 and 6.79±0.27Ma for the Gongenyama basalt.
The Kameura andesite has high MgO (8.08%), SiO2 (56.64%), Ni (137 ppm) and Cr(681 ppm) contents corresponding to high-magnesian andesite. The mantle-normalized incompatible element pattern for the Kameura andesite is similar to that of the western Setouchi high-magnesian andesite. The Gesujima and Gongenyama basalts are primitive with low FeO*/MgO and high Ni and Cr concentration and belong to tholeiite series. The mantle-normalized patterns for the Gesujima and Gongenyama basalts are parallel to those of Shimabara Peninsula basalts showing the within-plate basalt signature.
Generation of the Kameura andesite might be related to subduction of the Philippine Sea plate like those of the Setouchi volcanic rocks. On the other hand, the Gesujima and Gongenyama basalts might be derived from the fertile mantle or mantle diapir.
