Trace element abundances, isotopic compositions and ages for the Tertiary acidic igneous rocks from the coastal areas of the Japan Sea.

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Determinations of trace element abundances, Sr isotopic compositions and Rb-Sr ages were performed for the Tertiary acidic as well as related basic to intermediate igneous rocks from the coastal areas of the Japan Sea. The major purpose of this study is to examine the differences in trace element abundances and Sr isotopic compositions between the rocks formed before and after the opening of the Japan Sea which event occurred at about 15 Ma. The summary of the results on the northern Hyogo and Kyoto prefectures is as follows, and the results on Fukui area are consistent with the conclusions below. The initial Sr isotopic ratios for the igneous rocks of ages ca. 62, 43, 17 - 13, 5 - 3 Ma are 0.7073 - 0.7080, 0.7058, 0.7070 - 0.7091, and 0.7058 - 0.7076, respectively. These values are indistinguishable from those of Cretaceous acidic rocks in the surrounding areas (0.707 - 0.708 for Hyogo; 0.7054 - 0.7075 for eastern Tottori; 0.708 - 0.7104 for Kyoto and eastern ones). Moreover, it would … More be important that rather high Sr isotopic ratios of about 0.7060 - 0.7065 for some Quaternary basic rocks were reported. The REE patterns for all of the Tertiary acidic rocks show light REE enrichment, though size of Eu anomaly is variable: some samples seem to have no Eu anomaly. With respect to trace element abundances and Sr isotopic ratios, any clear differences between the acidic rocks formed before and after the opening of the Japan Sea were not found in this study. Moreover, the acidic rocks have almost identical Sr isotopic ratios to those of basic to intermediate ones, though the acidic rocks may tend to have higher Sr isotopic ratios. In conclusion, it was considered that the Sr isotopic compositions of the Tertiary acidic magmas may reflect those of mantle values, though some effect of crustal processes might have changed them in some extent. The author speculated that the uppermost mantle beneath Southwest Japan had been rotated together with the crust of Southwest Japan. In order to clarify the above conclusions, more data, especially Nd isotopic data, is necessary and mantle metasomatic processes or the role of fluid in the mantle would be important for producing those magmas. Less