Presentation Information
[T1-P-22]P–T evolution and tectonic implication of metamorphic rocks in Nam Dinh, southeasternmost part of the Day Nui Con Voi metamorphic complex, northern Vietnam
*Ippei KITANO1, Vuong Thi Sinh Bui1 (1. The Hokkaido University Museum, Hokkaido University)
Keywords:
Day Nui Con Voi metamorphic complex,Red River Shear Zone,East Sea (South China Sea),Pressure–temperature path,Nam Dinh
The Day Nui Con Voi high-grade metamorphic complex (DNCV) in northern Vietnam has considered as a southeastern part of the Red River Shear Zone (RRSZ), a major tectonic boundary between the South China Block and Indochina Block. This study investigated minor outcrops in Nam Dinh, located at the southeasternmost part of the complex, which are deficient in petrological research. It aims to establish the tectonic evolution based on the petrographic and structural features of pelitic schists and gneisses. Apart from sandstones found from the northern part, the petrographic observations reveal the metamorphic zonation of chlorite zone, biotite–muscovite zone and sillimanite zone progressing eastward with increasing metamorphic grade. The prominent subhorizontal mylonitic foliation (S2) folded by later open fold composes doming structure in study area and pre-S2 microstructures of S0 and S1 and post-S2 myrmekite are also preserved. Applying geothermobarometry for a pelitic schist in the chlorite zone and a gneiss in the sillimanite zone indicates a clockwise pressure–temperature path with the metamorphic geothermal gradients of 20–25 ℃/km at prograde stage, 30 ℃/km at peak granulite-facies metamorphism and ca. 60 ℃/km at retrograde condition with the myrmekite formation. The petrological and structural characteristics are well comparable with the main body of the DNCV and a contemporaneous metamorphic body of the Bu Khang Dome (BKD). The combining thermal histories of the DNCV and the East Sea opening suggest the following regional tectonics during Eocene to Oligocene. The Himalayan orogeny led the DNCV and BKD to the crustal thickening regime on the prograde stage. The initial extensional magmatism and crustal thinning due to the onset of the East Sea opening in the back-arc setting caused the granulite-facies metamorphism with extensive crustal anataxis that produced numerous syn-metamorphic leucogranites in the lower crust. The subsequent East Sea opening resulted in the extensional ductile deformation in the DNCV and BKD as well as the RRSZ. Crustal strike-slip shearing took place along the thinning and weakened anatectic crust and uplifted their metamorphic rocks with doming structure toward greenschist-facies upper crust. This possibly correlated with the southward ridge jump of the East Sea.