Occurrence of oceanic plagiogranites in the older tectonic zone,Southwest Japan |
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Authors: | Kyoichi Ishizaka Takeru Yanagi |
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Affiliation: | Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Kyoto Japan;Department of Geology, Faculty of Science, Kyushu University, Fukuoka Japan |
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Abstract: | K, Rb and Sr concentrations and Sr isotopic compositions were determined for the Dai granitic rocks of trondhjemitic composition occurring in a serpentinite mass in the Nagato tectonic zone formed in the Late Paleozoic era, and for the granitic rocks of quartz dioritic composition recently dredged from the seamount of the Kyushu-Palao Ridge. Both granitic rocks are characterized by low abundances of K and Rb, low K2O/Na2O ratios, high K/Rb ratios, low Rb/Sr ratios and low initial87Sr/86Sr ratios. These characteristics suggest that strong similarities may exist between the Dai granitic rocks and the dredged granitic rocks, and that the Dai granitic rocks may be classified as oceanic plagiogranite. These oceanic plagiogranites may plausibly represent single-stage mantle-derived granites, possibly from the suboceanic mantle. |
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