Discovery of high-pressure granulite-facies metamorphism in northern Vietnam: Constraints on the Permo-Triassic Indochinese continental collision tectonics |
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| Institution: | 1. Division of Evolution of Earth Environments, Faculty of Social and Cultural Studies, Kyushu University, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka 810-8560, Japan;2. Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan;3. Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan;4. Division of Earth Sciences, Graduate school of Science and Technology, Yamaguchi University, Yamaguchi 753–8511, Japan |
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| Abstract: | High-pressure mafic granulites containing granoblastic garnet, quartz, and minor hornblende have been found from the Song Ma Suture zone in northern Vietnam, regarded as a microcontinental boundary between the South China and Indochina blocks. Fine-grained symplectite formed during the decompression stage is developed in the granulite and is divided into orthopyroxene + plagioclase and orthopyroxene + clinopyroxene + plagioclase ± hornblende. The former replaces garnet and the latter is regarded as a breakdown of sodic clinopyroxene. Detailed observation and careful data selection revealed that the high-Mg and low-Ca garnet should be in equilibrium with the precursor sodic clinopyroxene, and the pair indicates high-temperature and -pressure conditions (910–930 °C at 1.9–2.0 GPa). Although we could not obtain quantitative age data from the high-pressure granulite, the U–Th–Pb age (233 ± 5 Ma) of pelitic gneiss strongly suggests a Middle to Early Triassic metamorphic event. If the age indicates the timing of the high-pressure granulite-facies metamorphism, it might be related to a continental collision setting by following crustal subduction. According to the metamorphic signatures, north to central Vietnam may be regarded as an orogenic belt formed by the micro-continental collision between the South China and Indochina cratons. |
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