The Sea of Japan is one of the back arc basins distributed around the western rim of the Pacific. Nature of the plume for back arc basin (= BAB) opening is unknown partly because petrological characteristics of the BAB mantle are not well known compared with mid ocean ridge (= MOR) mantle.Peridotite xenoliths in an alkali basalt dredged from the Takeshima seamount (tentative name), ca. 230 km to the north of Oki islands, are examined to clarify the petrological feature of the Japan Sea mantle. They are spinel bearing lherzolite to harzburgite with equigranular to porphyroclastic textures. They can be classified into two types (Type 1 and Type 2 peridotites) in terms of REE (= rare earth elements) patterns of clinopyroxene (= cpx). Type 1 peridotites (Cr# of spinels, 0.4 0.5) are similar to abyssal peridotites in terms of major element chemistry and middle to heavy REE concentrations of cpx. The cpx in the former shows slight enrichment of LREE (= light REE), which is apparently due to metasomatism by slab derived fluids/melts. Type 2 peridotites have high REE cpx with flat to slightly LREE enriched patterns, and are quite similar to subcontinental peridotites obtained as xenoliths from China. The southern part of the Sea of Japan was formed by mainly thinning and rifting of the continental lithosphere. Type 2 peridotites are a remnant of the continental lithospheric mantle.Chemical characteristics of BAB basalts and Type 1 peridotites indicate that open system melting of peridotite affected by a flux from the downgoing slab was involved in the Japan Sea opening.

Recent developments in instrumentation for in situ trace element analysis of peridotite minerals give us valuable data set of petrological and geochemical insights of the upper mantle. Here, we summarize petrographical, petrological and geochemical characteristics of mantle xenoliths from the Northeastern and Southwestern Japan arcs. They have clear correlations between microtexture and mineral compositions in terms of both major and trace elements. That suggests that influx assisted melt extraction occurs simultaneously with deformation/recrystalization in the upper mantle. Their characteristics of trace elements in clinopyroxene are distinguished from those of abyssal peridotite and peridotite xenoliths from continental regions. The geochemistry of Japan arcs clinopyroxenes have characteristics of arc type mantle source; i.e., low light rare earth element (LREE) and high field strength element (HFSE; Ti, Zr and so on), rather high heavy rare earth (HREE) relative to LREE elements and Sr concentrations. They also have a rather constant Ti/Zr ratio, variable REE patterns, and relatively low LREE/HREE ratios. The simple melt extraction model can not explain these characteristics. These features are due to metasomatism, which is different from carbonatite metasomatism.

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Zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry LA-ICP-MS and zircon fission-track FT dating were performed on granitic rocks from the Nojima Fault and its vicinity on northern Awaji Island, Hyogo Prefecture, Japan. The analyses yielded zircon FT ages of-Ma, which are in agreement with previously reported zircon FT ages. Zircon U-Pb U-Pb ages of-Ma were also determined. By combining these results with existing radiometric age data, we estimated the entire cooling history of the granitic basement on northern Awaji Island, spanning the period from the time of intrusion to the present. U-Pb ages were determined from zircons that were prepared for FT dating. Moreover, U-Pb ages were corrected based on analyses of Fish Canyon Tuff zircons as a standard material. As a result, a precise and accurate age of. . Ma was obtained for the Buluk Member Tuff. This result demonstrates that the effect of common Pb is minimal even when analyzing zircons that are chemically etched, and that the Fish Canyon Tuff, which is a well-known age standard for FT dating, can be used as a standard for U-Pb dating. Many previous studies have reported on zircon FT analyses performed throughout the world. The method applied in the present study should pave the way for adding information on the crystallization age of the studied rocks by analyzing the zircons prepared previously for FT dating.