Variation and evolution of vietnamese metamorphic rocks

Trans Vietnam Orogenic Belt distributed in north to central Vietnam includes various types of metamorphic rocks. Most metamorphic rocks show the Permian-Triassic ages and are characterized by decompressional evolution with various metamorphic thermal gradients (8- 25 °C/km). In contrast, some metamorphic rocks having the Ordovician-Silurian age show isobaric heating prograde stage at low-pressure condition (25-30 °C/km). Granitic rocks also show the Permian-Triassic or Ordovician-Silurian ages. The older granitoids have volcanic arc chemical signature, some of which were metamorphosed at the Permian-Triassic. These results suggest that island arc tectonic setting at the Ordovician-Silurian, where arc magmatism heated the arc crust and caused the low-pressure (with isobaric heating) metamorphism at this period. The Permian-Triassic metamorphic rocks and granitoids were formed by subsequent continental collision and subduction of the Indochina craton together with the past Ordovician-Silurian island arc beneath the South China craton.