09:45 〜 10:00
[SMP09-03] ベトナム北部・レッドリバー剪断帯に分布する変成ボーキサイト
Aluminous metamorphic rocks from the Red River Shear zone in northern Vietnam were investigated in this study. The shear zone has abundant pelitic rocks that have been formed under sillimanite-grade metamorphic condition at the Paleogene period. The aluminous rocks are mainly divided into high-grade (garnet-sillimanite-corundum, garnet-sillimanite-spinel, garnet-corundum-spinel rocks) and low-grade (kyanite-corundum-hematite-phengite rock) rocks and both types show similar whole rock chemical composition with basaltic to andesitic bauxites. Detailed petrographical observations, the high-grade metamorphosed bauxites have experienced prograde stage more than 2.0 GPa at 800 C (based on the assemblage of staurolite + kyanite + rutile + siderite in garnet). The low-grade bauxite preserves the peak condition at 500 C and pressure more than 1.2 GPa (based on the assemblage of hematite + corundum + rutile + phengite + allanite + chloritoid + kyanite in the matrix). Because bauxite commonly formed at the surface on the continent, such high-pressure metamorphic conditions from the metamorphosed bauxites suggest continent-continent collision and its subduction.
Zircon grains in garnet-sillimanite-corundum rock show several U-Pb ages from 265 Ma to 36 Ma may due to the loss of Pb during shearing and/or thermal event at the Paleogene time. However, dark luminescent zircon cores show concordia age of 257 +/- 8 Ma. The zircon cores contain CO2 rich fluid inclusions and the density is similar to other CO2 rich fluid inclusions trapped in garnet, corundum, and staurolite. Therefore, we conclude that the high-pressure metamorphism has occurred at the Late Permian that should have strong relation to continental collision between the Indochina and South China cratons.
Zircon grains in garnet-sillimanite-corundum rock show several U-Pb ages from 265 Ma to 36 Ma may due to the loss of Pb during shearing and/or thermal event at the Paleogene time. However, dark luminescent zircon cores show concordia age of 257 +/- 8 Ma. The zircon cores contain CO2 rich fluid inclusions and the density is similar to other CO2 rich fluid inclusions trapped in garnet, corundum, and staurolite. Therefore, we conclude that the high-pressure metamorphism has occurred at the Late Permian that should have strong relation to continental collision between the Indochina and South China cratons.