SYSTEMATIC CHANGES IN METAMORPHIC STYLES ALONG THE DABIE–HONGSEONG AND HIMALAYAN COLLISION BELTS, AND THEIR TECTONIC IMPLICATIONS

The Gyeonggi massif located in the middle part of Korean Peninsula is one of the important basements in Korea. From the Hongseong area in the southwest Gyeonggi Massif, Triassic (ca. 230 Ma) eclogites were found indicating that the Dabie-Sulu collision belt between the Sino-Korean block (North China block) and South China block is extended into the Hongseong area in Korea. The eclogites formed at 16.5–20.0kb and 775–850°C and was overprinted by granulite facies metamorphism (at 11.0–15.6kb and 760–825°C). Late Permian (ca. 257 Ma) mangerites that intruded the Odesan area in the eastern part of the Gyeonggi Massif show geochemical characteristics of post-collision tectonic settings, implying that the Hongseong collision belt extends to the Odesan area. In the Odesan area, gneisses near mangerite intrusion underwent UHT metamorphism (9.0~10.6kb, 914-1157°C) at around 245 Ma. The Hongseong-Odesan collision belt in Korea may continue into the Paleozoic subduction complexes in the southwest Japan and further to the Yanji belt, a Carboniferous and Permian subduction complex along the northeastern boundary of the Sino-Korean block. These data indicate that Phanerozoic subduction along the margin of the Sino-Korean block and the collision between the Sino-Korean and South China blocks contributed to formation of the Dabie-Sulu-Hongseong-Odesan-Southwest Japan-Yanji belt (Dabie-Hongseong collision belt). The metamorphic ages of peak metamorphism along the belt decrease from east (late Permian in the Odesan area) to west (Triassic-early Jurassic in the Sulu and Dabie areas) suggesting that collision had started from Korea in late Permian and propagated towards east until early Jurassic due to the clockwise rotation of the South China block. P-T estimations along the Dabie-Hongseong collision belt reveal that geothermal gradient and first-stage retrograde metamorphic grade decrease from the collision starting area to the east, resulting UHT metamorphism in the Odesan area (collision starting area), HP metamorphism overprinted by granulite facies in the Hongseong area and UHP metamorphism overprinted by amphibolite facies (~8kb and 500–650°C) in the Dabie areas. In the Himalayan collision belt, opposite trend was recognized; the collision started from west (~ 55 Ma) and propagated towards east (~ 35 Ma) and the geothermal gradient and the retrograde metamorphic grade increase from the collision starting area to the west resulting UHP eclogite (22~24kb and 610–700°C) overprinted by amphibolite facies metamorphism (10~13kb and 610–580-600°C) in the western Himalaya and HP eclogite overprinted by granulite facies metamorphism (7~10kb and 750–790°C) in the eastern Himalaya. These differences are related to the distance between two blocks before collision and the time of delamination of oceanic slab. In the Dabie-Hongseong collision belt, the collision started from east where the distance between Sino-Korean and South China blocks was narrow and the amount of subducted oceanic slab was not enough to pull down continental crust to the UHP condition depth. The distance between the Sino-Korean and South China blocks before collision increased toward west, which increased the amount of subducted oceanic slab towards west. As a result, the largest amount of oceanic slab subducted in the Dabie and Sulu areas, causing enough pulling force which subducted the South China block to UHP condition depths. The main delamination of oceanic slab along the Dabie and Hongseong belt started after reaching of the continental crust in the Dabie area to UHP condition depth. The buoyancy force of subducted continental crust after delamination increases as the subduction depth increases, and stronger buoyancy causes faster uplift. As a result, the uplifting time span increased to the east causing longer thermal relaxation and higher geothermal gradients during the retrograde metamorphic stage towards east. While in the Himalayan collision belt, there was a wide ocean between the Asian and Indian blocks before collision and the amount of subducted oceanic slab was enough to pull down continental crust to the UHP condition in the west Himalaya where collision started. The process of oceanic slab break-off most likely occurred from ~ 44-48 Ma, Oceanic slab break-off likely coincided with exhumation of UHP terranes in the western Himalaya and led to the initiation of low angle subduction and S-type granite generation along the belt, particularly in the east. As a result, the UHP eclogite occurred only in the west and HP eclogite occurred in the east. Due to the difference of buoyancy, the UHP eclogite uplifted fast and HP eclogite uplifted relatively slow, which caused more thermal relaxation and higher geothermal gradient towards east. As a result, UHP eclogite in the western Himalaya was overprinted by amphibolite facies metamorphism and the eastern Himalaya eclogite by granulite facies metamorphism. The results of this study indicate that the systematic change of metamorphic conditions along collision belt can be used to interpret the tectonic process during collision.