2016 年 111 巻 3 号 p. 196-210
We report first finding of low–temperature eclogite–facies metamorphism from the Red River shear zone in northern Vietnam that has been known as the Paleogene high–temperature metamorphic belt. The rock is extremely aluminous (50.13 wt% Al2O3) and oxidized [Fe3+/(Fe3+ + Fe2+) = 0.89] emery–type rock. Its composition is also characterized by very low SiO2, CaO, and Na2O with high concentrations in high field strength elements, suggesting that the protolith is lateritic bauxite. The analyzed rock is composed mainly of porphyroblastic titanomaghemite, kyanite, allanite, and chloritoid with abundant fine–grained corundum in the matrix. The mineral paragenesis and phase equilibria modeling suggest that the aluminous rock underwent a clockwise pressure–temperature trajectory with the peak metamorphic condition corresponding to low–temperature eclogite–facies approximately 520–550 °C at 1.2–1.5 GPa. Monazite U–Th–Pb dating constrains the timing of the metamorphism to be older than 214 Ma. Our results require two geological events; 1) tropical weathering and bauxitization on the erosional surface, and 2) pre–Late Triassic subduction of supracrustal materials most likely during the Permo–Triassic collisional events in East and Southeast Asia. Our study provides the first evidence for the subduction of supra–continental crustal materials up to 40–50 km depth during the collision between the Indochina and South China blocks.
