ABSTRACT

In the San'in district (back-arc side of the Southwest Japan arc), violent subalkalic magmatism took place related to the opening of the Japan Sea during the Miocene. In the northeastern part of Shimane Peninsula, thick middle Miocene submarine volcanigenic strata crop out. This middle Miocene volcanism took place along with intense subsidence probably related to rifting in a mature arc. In the late Miocene, this volcanism and subsidence were followed by eruption of subaerial alkalic basalt (Matsue basalt). Thirty seven samples of these igneous rocks have been analyzed by XRF and INAA. Middle Miocene igneous activity is divided into three stratigraphic stages, and each stage shows a wide variation in chemistry. Intra-stage chemical variations result from crystal differentiation, whereas variations between Stages II and III rocks are interpreted in terms of various degrees of melting from a common source mantle. Stage I rocks were generated from a more depleted mantle. Most middle Miocene rocks have an arc signature, which resulted from the melting of subarc lithospheric mantle which had been metasomatized by a subduction-related fluid. Associated felsic rocks were formed by magmatic fractionation of basaltic magma.