15:30 〜 16:45
[SIT10-P02] Petrologic evidence for the low P-T origin of the Minami-Shimabara basalts from northwest Kyushu, southwest Japan
キーワード：intra-plate basalt, multi-stage partial melting, plate tectonics, mantle plume
The origin of hot spot volcanism is important for our understanding of the dynamics and evolution of the Earth. OIB-type basalts, that characterize hot spot volcanism, are distributed even in plate convergent zones such as subduction zones. OIB-type basalts erupted from 4.6 to 0.6 Ma are distributed at Minami-Shimabara, the southern foot of the Unzen volcano in NW Kyushu, SW Japan (Ohtsuka et al., 1995).
Their negative CaO-MgO correlations indicate that olivine fractionation would have occurred in the Minami-Shimabara basalts (MSBs). The normative olivine-cpx-quartz compositions of some Mg-rich samples, however, overlap with those of experimental partial melts of peridotites. The Mg-Fe-Ni compositions of such samples indicate that they could have been in equilibrium with Fe-rich peridotites containing olivine with Fo =85-87. These lines of evidence imply that some of MSBs would have been originally primitive magmas. This interpretation is consistent with the compositional features of the Karatsu-Takashima lava including mantle xenoliths (Arai et al. 2001). The bulk rock compositions of the lava indicate that the magma would have been in equilibrium with Fo =84 olivines at 1.5 GPa, which indicates that the Fe-rich mantle would lie beneath NW Kyushu.
With the Karatsu-Takashima lava, primitive MSBs are plotted along the melting mantle adiabat at the 1300 °C potential temperature on the normative olivine-quartz-[Jd + CaTs] diagram, which reconfirms the hypothesis that the samples would have been primitive magmas. The normative compositions also suggest that the primitive SSBs would have segregated from the source mantle at pressure ranging from 1.5 to 0.5 GPa. The low pressure estimated for the final melt segregation is consistent with the results of the controlled seismic experiments at the Shimabara Peninsula implying the crust thinner than 20 km there (Takahashi et al., 2002).
In conclusion, compositional features of the MSBs indicate that their primitive melts would have been segregated from the source mantle with low potential temperature such as 1300 °C at low pressure such as 1.5 to 0.5 GPa. Upwelling of the mantle with such a low potential temperature would have been induced by shallow mantle processes such as plate tectonics rather than by deep mantle plumes. A possible factor causing such shallow mantle upwelling is transtensional tectonics at NW Kyushu, the junction of the Southwest Japan Arc and the Ryukyu Arc.
Their negative CaO-MgO correlations indicate that olivine fractionation would have occurred in the Minami-Shimabara basalts (MSBs). The normative olivine-cpx-quartz compositions of some Mg-rich samples, however, overlap with those of experimental partial melts of peridotites. The Mg-Fe-Ni compositions of such samples indicate that they could have been in equilibrium with Fe-rich peridotites containing olivine with Fo =85-87. These lines of evidence imply that some of MSBs would have been originally primitive magmas. This interpretation is consistent with the compositional features of the Karatsu-Takashima lava including mantle xenoliths (Arai et al. 2001). The bulk rock compositions of the lava indicate that the magma would have been in equilibrium with Fo =84 olivines at 1.5 GPa, which indicates that the Fe-rich mantle would lie beneath NW Kyushu.
With the Karatsu-Takashima lava, primitive MSBs are plotted along the melting mantle adiabat at the 1300 °C potential temperature on the normative olivine-quartz-[Jd + CaTs] diagram, which reconfirms the hypothesis that the samples would have been primitive magmas. The normative compositions also suggest that the primitive SSBs would have segregated from the source mantle at pressure ranging from 1.5 to 0.5 GPa. The low pressure estimated for the final melt segregation is consistent with the results of the controlled seismic experiments at the Shimabara Peninsula implying the crust thinner than 20 km there (Takahashi et al., 2002).
In conclusion, compositional features of the MSBs indicate that their primitive melts would have been segregated from the source mantle with low potential temperature such as 1300 °C at low pressure such as 1.5 to 0.5 GPa. Upwelling of the mantle with such a low potential temperature would have been induced by shallow mantle processes such as plate tectonics rather than by deep mantle plumes. A possible factor causing such shallow mantle upwelling is transtensional tectonics at NW Kyushu, the junction of the Southwest Japan Arc and the Ryukyu Arc.