13:45 〜 14:00
[SMP24-11] Geological structures and metamorphic evolution of the Ban Rai Basin, U-Thai Thani Province, Thailand
キーワード：Metamorphism, Sibumasu, Granite emplacement, Greenschist
The geological structures and metamorphic evolution within the Khao Chum Thahan area and its vicinity, located in the Huai Khot district of U-Thai Thani Province, western Thailand, were scrutinized to elucidate the tectonic evolution of the Ban Rai basin from the Late Paleotectonic to the Neotectonic episode. Fieldwork and petrographic observations were employed to deduce the main processes that delineated the deformation history and metamorphic conditions. Our findings reveal that the region experienced three distinct deformation stages accompanied by three successive metamorphic phases. The first deformation (D1), which accompanies regional metamorphism (M1), is typified by elongated tight-to-isoclinal folds with fold axial planes moderately inclined to the west. The second deformation (D2) formed the NW-SE cross folds superimposing on the previous folds. This interference folding episode may occur together with, or shortly followed by a granitic intrusion in the Triassic and cause a contact metamorphism (M2). Petrographic analyses unambiguously suggest a progression in metamorphic grades, spanning from the greenschist to the pyroxene-hornfels facies. Minerals such as quartz, muscovite, biotite, and chlorite in the metamorphosed fine-grained sedimentary rocks denote low-temperature conditions ranging from 300oC to 450oC, whereas the presence of diopside, orthopyroxene, garnet, sillimanite, and kyanite in the high-grade metamorphic rocks point to elevated temperature conditions between 500oC and 700oC. There was an upgrade in metamorphism from M1 to M2 within a very short distance towards the exposure of a small pluton of granite. The third deformation (D3) was characterized by sinistral movement along an NW-SE direction, with an overprinting of metamorphism (M3). Asymmetrical folds, hyper-extended pegmatite dykes, asymmetrical boudinage, and quartz recrystallization microstructures in a NW direction were likely a result of this fault movement. Our interpretations suggest that the earliest deformation may have been influenced by east-west compressive stress during the tectonic collision of the Sibumasu Block and the Indochina Block (c. 220 – 230 Ma). This was followed by a significant metamorphic event under high-temperature and low-pressure conditions, induced by Triassic granite intrusions (c. 210 – 200 Ma). The final phase was posited to be connected with Cretaceous events (c. 60 – 50 Ma), where the increased obliquity of the Indian Plate’s subduction towards SE Asia triggered the strike-slip system along NW-SE trending with mineralized pegmatite