[S-MP21] Oceanic and Continental Subduction Processes: petrologic and geochemical perspective

[SMP21-P02] Metamorphic evolution of a high-pressure gneiss from the eastern Ngoc Linh Complex of the Kon Tum Massif, Vietnam: Implications for the Indochina-South China collision

*Tadashi Usuki¹, Duc Nam Nguyen¹, Jian-Wei Lin², Yoshiyuki Iizuka², Hao-Yang Lee², Kwan-Nang Pang², Ching-Hua Lo¹ (1.Department of Geosciences, National Taiwan University, 2.Institute of Earth Sciences, Academia Sinica)

Keywords:Indochina, continental collision, Kon Tum Massif, Zr-in-rutile, kyanite, subduction polarity

Northern to central Vietnam is considered the continental collision zone between Indochina and South China blocks during the Late Permian to the Middle Triassic. Several high-pressure rocks have been reported from the Song Ma suture zone in the northern part and the Kon Tun Massif in the southern part of the collision zone. So far, in the Kon Tum Massif, the evidence for high-pressure metamorphism has been found only in a lesser extent area in the western margin of the Ngoc Linh Complex. Here, we report a recent discovery of a high-pressure felsic gneiss from the eastern part of the Ngoc Linh complex, which was regarded as a low-pressure amphibolite facies (up to ~5 kbar and ~700 °C) area in previous studies. This gneiss mainly comprises garnet + kyanite + white mica + biotite + K-feldspar + quartz + rutile. The rock underwent a moderately retrograde metamorphism, with some kyanites replaced by sillimanite, and the internal fractures and margins of garnet replaced by biotite, chlorite, and white mica. Most rutile grains are also wholly or partially replaced by ilmenite, and their Zr content is affected by retrogressive metamorphism in varying degrees. However, rutile inclusions isolated from fractures within garnet exhibit the highest Zr content (1470 ppm) and retain the temperature conditions during peak metamorphism. The Zr-in-rutile thermometer in combination with the P-T pseudosection shows that peak P-T conditions are ~12 kbar and ~810 °C. Zircons separated from this high-pressure gneiss were found to have developed radial sector zoning or fir-tree sector zoning characteristic of highly metamorphic rocks. The ²⁰⁶Pb/²³⁸U ages for these zircons using LA-ICPMS displays an average age of 253.0±1.7 Ma (n=33), which is interpreted to indicate the timing of the peak metamorphism. This suggests that the impact of the Permo-Triassic collision metamorphism is recognized not only in the western margin but also in the eastern part of the Ngoc Linh Complex. Compared with the P-T path of the high-pressure (20-30 kbar) and ultrahigh-temperature metamorphic rocks (900-1000 °C) from the western margin, the studied gneiss was exhumed from a shallower depth along a cooler path. Many studies concerning Late Carboniferous to Late Triassic arc magmatic rocks in the Truong Son belt and Song Ma suture zone suggested that the Paleotethys was subducted southwestward beneath the Indochina Block. The difference in the maximum depths of high-pressure metamorphic rocks between the eastern and western of the Ngoc Linh Complex may reflect the subduction polarity.