K-Ar age and geochemical characteristics of the quartz-porphyry at Shimama, southern Tanegashima, and K-Ar age of a lamprophyre from northern Tanegashima: implications for Miocene igneous activities in the Outer Zone of Southwest Japan

Abstract

Tanegashima, an island situated south of the Kyushu island, in the Outer Zone of Southwest Japan, contains small amounts of Miocene igneous rocks which intruded into the Kumage Formation of the Shimanto Supergroup. The quartz-porphyry occurs near Shimama in the southern part of the island, and is peraluminous and rich in K₂O, Rb and Sn. The geochemical characteristics indicate that the quartz-porphyry is derived from S-type magma. The lamprophyre is present in the northern part of the island. K-Ar ages of the Shimama quartz-porphyry and the lamprophyre are 15.6±0.8 Ma and 18.2±0.9 Ma, respectively. The ages indicate two episodes of igneous activity on the island. The age of the Shimama quartz-porphyry is similar to the ages of a quartz-porphyry and a small granitoid body in the northeastern part of Yakushima, an island situated 20 km west of Tanegashima, but is slightly older than the main body of the Yakushima granite. The ages and geochemical characteristics of quartz-porphyries from the two islands suggest that they could be derived from a common magmatic event. The magmatic activity is considered to be part of Miocene acidic magmatism of the Outer Zone of Southwest Japan.