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

1. 我们于1989年和1990年在三音地区中部进行了约20天的野外调查，采集了60多个露头的岩石样品，进行了化学分析、古地磁方向测量和K-Ar年龄测定。大多数样品的成分为中间到碱性。在偏光显微镜下观察样品的薄片，以检查改变样品化学性质的程度，并重新设定K-Ar年龄和古磁方向。3 .古地磁研究表明，河合组赤纬值主要向东偏移，而大森组和松江组赤纬值主要向北偏移。Kawai组、Omori组和Matsue组的平均K-Ar年龄分别为16.1 Ma、14.3 Ma和11.3 Ma。古地磁方向和K-Ar年龄表明，在16ma至14ma之间，日本西南部在2 myr内快速顺时针旋转了约40度。川内群、川合组和大森组火山岩具有相似的球粒正态稀土元素模式，轻稀土元素略富集。另一方面，松江组具有强轻稀土元素富集格局。这些结果表明，在日本西南部大自转期间和之前，火山岩的化学特征并没有发生变化，而是在大自转之后的3个月里发生了变化。由于大森组样品中既有玄武岩，也有安山岩，松江组样品均为玄武岩，因此两组样品的差异表明，三仁地区地幔的富集时间为日本海开闭后14 ~ 11 Ma。

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.