Geochemistry of the alkalic dolerites of the nemuro peninsula,Japan |
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| Authors: | K Yagi |
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| Institution: | 1. Department of Geology and Mineralogy, Hokkaido University, Sapporo, Japan
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| Abstract: | Many sheets of alkalic dolerites found in the Cretaceous formations in the Nemuro peninsula can be divided into the following two types. Differentiated sheets, more than 100 m in thickness. Gravitational differentiationin situ is distinguished, producing rock types such as picritic dolerite, dolerite, monzonite and syenite. Undifferentiated sheets, less than 30 m in thickness. Differentiationin situ is not observed, and dolerite is the only rock type. Distinguished pillow structures are commonly observed. Petrochemistry of these rocks indicates that K.O contents are high, especially in the middle and later stages, predominating over Na2O and that MgO content is rather high as compared with iron oxides. These features place the suite in a characteristic position clearly distinguished from the other alkalic rock suites in Japan. Fourteen trace elements were determined spectroscopically and their distribution during the differentiation is discussed. The parent magma of the Nemuro rocks is estimated to be shonkinitic in composition, rather rich in potash. Its possible genesis by the partial melting of the phlogopite-peridotite in the upper mantle under the continent is discussed. |
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