No abstract

No abstract

No abstract

SUMMAR YThe Setouchi volcanic belt in SW Japan is characterized by the occurrence of andesites with an unusually high Mg concentration (high-Mg andesites, HMAs). Such HMAs may have formed under unusual tectonic settings, as basaltic magmas are produced dominantly in the mantle wedge of modern subduction zones. A total of 50 new K±Ar ages for the Setouchi rocks con®rmed that intermediate to felsic magmatism including the eruption of HMAs took place in this volcanic belt within a short period of 13.2t0.4 Ma. This is synchronous with the clockwise rotation of the SW Japan arc sliver that was associated with the opening of the Japan Sea back-arc basin. Such rotation of an arc sliver forced the initiation of subduction and partial melting of a young and hot lithosphere of the Shikoku Basin beneath the arc. Interaction between slab melts and mantle wedge peridotites may be a possible generation process of unusual HMA magmas. When the SW Japan arc sliver ceased to drift, the Setouchi magmatism was switched off by the cessation of subduction. The eruption of HMA magmas may also be attributed to an extensional stress ®eld in the Setouchi region, in contrast to the compressional stress regime documented for other regions of the SW Japan arc.

Gneiss samples were collected from the Shandong Peninsula, eastern China, in order to carry out Rb-Sr dating using whole rock powder and biotite mineral separates. Middle Proterozoic whole rock age (1808 k 71 Ma) and Middle to Late Proterozoic biotite ages (1312 and 729 Ma) were obtained for country gneisses in the northwestern part of the peninsula. Late Proterozoic whole rock age (707 k 6 1 Ma) and Mesozoic biotite ages (172 and 222 Ma) were obtained for orthogneisses in the southeastern part of the peninsula. These new age data, as well as recent petrological results and published radiometric data, support the subdivision of the Shandong Peninsula into two geological districts: (i) a northwestern area of Proterozoic age represented by a low-and middle-pressure metamorphic sequence belonging to the Sino-Korean craton; and (ii) a southeastern area of Late Proterozoic and Mesozoic age accompanied by ultra-high pressure eclogite.