THE EMPLACEMENT PRESSURE OF SYENITE ESTIMATED FROM THE STABILITY FIELD OF AMPHIBOLE FROM THE YAMATO MOUNTAINS, EAST ANTARCTICA

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T. OBA and K. SHIRAISHI (Proc. NIPR Symp. Antarct. Geosci., 6,72,1993) attempted to estimate the pressure of amphibole-bearing syenites based on the stability field of amphibole in melting experiments. The present study shows improvement over our previous paper. Four hornblende clinopyroxene quartz syenites, a hornblende clinopyroxene mela-syenite and a hornblende two-pyroxene syenite from the Yamato Mountains were determined under 0.05-0.3 GPa P_<H_2O> in the temperature range 650-900℃, and at the oxygen fugacity of the FMQ buffer. The stability limit curves of amphibole from four syenites (Y556,Y557,Y904 and Y34A) intersect with their solidi at lower pressure than 0.3 GPa. In all syenites except for a mela-syenite (Y405), quartz disappears first, then plagioclase and amphibole in that order with increasing temperature at 0.1 GPa. K-feldspar, in experiments on sample Y405,disappears at lower temperature than amphibole. In experiments on Y405,Y556 and Y904,albite contents in the synthesized K-feldspar slightly increase with increasing temperature below the solidus temperature, whereas the chemical compositions of K-feldspar are constant and around Ab_<45>Or_<55> above the solidus temperature. The chemical compositions of liquids formed by partial melting of syenites are quartz and corundum normative and show the granite minimum composition at 0.1 GPa. They are similar to the chemical compositions of aplitic granites from the southern Yamato Mountains, suggesting that the aplitic granites are low pressure derivatives from the syenites.
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THE EMPLACEMENT PRESSURE OF SYENITE ESTIMATED FROM THE STABILITY FIELD OF AMPHIBOLE FROM THE YAMATO MOUNTAINS, EAST ANTARCTICA

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