Abstract
Upper Cretaceous to Paleogene felsic and intermediate intrusive rocks are widely distributed in the San-in Belt, Southwest Japan. They are geochronologically divided into the late Cretaceous-Eocene Inbi-stage intrusive rocks and Eocene-Oligocene Tamagawa-stage intrusive rocks. The Taimasan plutonic complex in the western part of Hamada city, Shimane Prefecture, is a zoned pluton composed mainly of quartz diorite, granodiorite, granite and aplitic granite. The granodiorite intruded into the quartz diorite, and was in turn intruded by the aplitic granite. The granodiorite grades into the granite though transitional zones. All rocks have high magnetic susceptibility corresponding to the magnetite-series. The quartz diorite gives a K-Ar biotite age of 71.9±1.6Ma, and the granodiorite gives K-Ar hornblende ages of 68.5±3.5Ma and 69.8±3.6Ma. These age data indicate that the Taimasan plutonic complex was formed in the late Cretaceous, different from the previous concept that the complex intruded in the middle Paleogene. Considering from the ages and high magnetic susceptibility, the complex is considered to be one of the Inbi-stage intrusive rocks located in the westernmost part of the San-in Belt. The Taimasan plutonic complex was intruded by Paleogene small stocks with K-Ar ages of 46 to 34Ma. Based on chemical compositions of the rocks and minerals, fractional crystallization model calculations (GENMIX) was examined for the genesis of the complex. The results are consistent with a conception that the Taimasan plutonic complex was formed by fractional crystallization of a single magma. Plagioclase and clinopyroxene at the earlier stages of crystallization, and plagioclase and hornblende at the later stages were mainly fractionated from the magma. Plagioclase is an important fractional mineral throughout the fractional crystallization process of the magna. This is consistent with linearly decreasing trends of Sr and CaO contents on differentiation diagrams and Eu negative anormaly
