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[SCG47-P10] Petrological study of granitic rocks from Omishima, Ehime Prefecture.
Keywords:Omishima, granitic rocks
Various types of granitoids are distributed in the Omishima Island, Ehime Prefecture, which locates in transitional zone between Sanyo Belt and Ryoke Belt of southwest Japan. Although geological and petrographic studies have been performed for granitoids in the Omishima Island (e.g., Momoi et al., 1991; Ochi, 1991), geochemical data such as whole-rock and mineral compositions have not been reported. In this study, we report new petrographic and geochemical data of granitoids in this area.
The granitoids distributed in Omishima Island can be divided into two groups (biotite granite and hornblende biotite granite). The biotite granites are dominant in Omishima and consist of quartz, plagioclase, K-feldspar, biotite, opaque minerals and zircon. The hornblende biotite granites are subdominant and distributed in south of the Island. They consist of quartz, plagioclase, K-feldspar, biotite, hornblende, opaque minerals and zircon. Whole-rock SiO2 contents of biotite granite ranges from 75 to 79 (wt %), whereas those of hornblende biotite granites are ~71 (wt %). Based on the norm An-Ab-Or granite classification diagram, all granitoids in Omishima Island have composition of granite.
Based on mass balance calculation for the major elements, compositions of the biotite granites are modeled by fractional crystallization from potential parent magma with ~75 wt % SiO2, suggestive of fractional crystallization process for the genesis of biotite granites. On the other hand, chemical compositions of biotite granites can not be reproduced by fractional crystallization from hornblende biotite granite magma with ~71 wt % SiO2 implying that biotite granite and hornblende biotite granite have no genetic relationship.
The granitoids distributed in Omishima Island can be divided into two groups (biotite granite and hornblende biotite granite). The biotite granites are dominant in Omishima and consist of quartz, plagioclase, K-feldspar, biotite, opaque minerals and zircon. The hornblende biotite granites are subdominant and distributed in south of the Island. They consist of quartz, plagioclase, K-feldspar, biotite, hornblende, opaque minerals and zircon. Whole-rock SiO2 contents of biotite granite ranges from 75 to 79 (wt %), whereas those of hornblende biotite granites are ~71 (wt %). Based on the norm An-Ab-Or granite classification diagram, all granitoids in Omishima Island have composition of granite.
Based on mass balance calculation for the major elements, compositions of the biotite granites are modeled by fractional crystallization from potential parent magma with ~75 wt % SiO2, suggestive of fractional crystallization process for the genesis of biotite granites. On the other hand, chemical compositions of biotite granites can not be reproduced by fractional crystallization from hornblende biotite granite magma with ~71 wt % SiO2 implying that biotite granite and hornblende biotite granite have no genetic relationship.