In the Susa area, western San-in district, Southwest Japan, marine sedimentary rocks (Susa Group), volcanic rocks (Yamashima) and gabbros (Koyama) of Teritary age occur in close relation. The former two rocks have partly undergone contact metamorphism by the Koyama gabbro complex. The Yamashima volcanic rocks, presumed to lie on the Susa Group, consist mainly of submarine massive lavas, hyaloclastites, pseudopillow and feeder dykes of basaltic andesites and their reworked deposits. The Koyama gabbro complex is mainly composed of olivine gabbro, quartz gabbro and quartz diorite. The representative specimens from the Yamashima volcanic rocks and related rocks were dated with the K-Ar method. The groundmass separates from Yamashima volcanic rocks give ages of 13.4 and 14.2 Ma, while biotite from the Koyama gabbro gives an age of 14.1 Ma. The pelitic hornfels of the Susa Group is dated to be 14.8 Ma for biotite, and a porphyrite dyke to be 12.1 Ma for groundmass. The age of the Maeubuyu quartz diorite, 50 km northeast of the Koyama gabbro complex, is 13.7 Ma for the whole rock.
On the basis of newly obtained and reported geochronological and geochemical data and field evidence of igneous rocks in the Susa area, it is argued that the Yamashima-Koyama igneous rocks are a submarine volcano-plutonic complex produced in relation to the opening of Japan Sea in the Middle Miocene.
