Temporal Variation of Volcanic Rocks in Southwest Japan During Opening of the Japan Sea

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1. We carried out field survey in the central part of the San'in area for about 20 days in 1989 and 1990, and collected rock samples from more than 60 outcrops for chemical analyses, measurements of paleomagnetic direction and K-Ar age deternimations. Most of the samples are intermediate to basic in composition.2. Thin sections of the samples were observed under a polarized microscope to examine degree of alteration which might change chemical characters of the samples and reset K-Ar ages and paleomagnetic directions.3. The paleomagnetic study implies that, while the Kawai Formation has declination values largely deflected eastward, the Omori and Matsue Formations show northward directions.4. Average K-Ar ages of the Kawai, Omori, and Matsue Formations are 16.1 Ma, 14.3 Ma and 11.3 Ma, respectivery. The paleomagnetic directions and K-Ar ages imply that Southwest Japan rapidly rotated through about 40 degees clockwisely in 2 myrs between 16 Ma and 14 Ma.5. The volcanic rocks of the Kawauchi Group, Kawai Formation and Omori Formation have similar chondrite normalized rare earth elements patterns in which light rare earth elements are slightly enriched. On the other hand, the Matsue Formation has strongly light rare earth elements enriched pattern. These indicate that chemical feature of the volcanic rocks did not changed during and before the large rotation of Southwest Japan, but in 3 mys just after the rotation. Because the samples from the Omori Formation include basalt as well as andesite and all samples from the Matsue Formation are basalts, the difference between the two formations suggests that the mantle beneath the San'in area enriched in the period between 14 Ma and 11 Ma after the opening of the Japan Sea.