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

1.我们于1989年和1990年在山阴地区中部进行了20天左右的野外调查，在60多个露头上采集了岩石样品，进行了化学分析、古地磁方向测定和K-Ar年龄测定。大多数样品的成分是中间到碱性的。在偏光显微镜下观察样品的薄片，研究可能改变样品化学性质和重置K-Ar年龄及古地磁方向的蚀变程度.古地磁研究表明，河合组的地磁值大部分向东偏移，而大森组和松江组的地磁值则向北偏移。河合组、大森组和松江组的K-Ar年龄分别为16.1Ma、14.3Ma和11.3Ma。古地磁方向和K-Ar年龄表明，在16 Ma至14 Ma之间，日本西南部在2万年内迅速顺时针旋转了约40度。5。川内群、川井组和大森组火山岩具有相似的球粒陨石标准化稀土元素模式，轻稀土元素略有富集。另一方面，松江组具有较强的轻稀土元素富集模式。这表明，火山岩的化学特征在日本西南部的大旋转期间和之前没有发生变化，但在旋转后的3百万年内发生了变化。由于大森组的样品包括玄武岩和安山岩，而松江组的样品都是玄武岩，因此两个组之间的差异表明，在日本海开放后的14 Ma和11 Ma之间，山阴地区下方的地幔富集。

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.