Why ultrahigh metamorphism (UHP) occurred locally in the Himalayan and dabie-hongseong collision belts
|Location||International Geological Congress,oslo 2008|
|Holding Date||17 September 2008|
In the Himalayan collision belt, 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 towards east resulting UHP eclogite (22-24kb and 610-700íÆC) in the western Himalaya and HP eclogite in the eastern Himalaya. As there was a wide ocean between the Asian and Indian blocks before collision, 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 started in the western part from ~ 44-48 Ma due to very strong buoyancy of deeply subducted continental block, causing an exhumation of UHP terranes. The oceanic slab break-off propagated towards east lowering the subduction angle along the belt. The lowered subduction angle in the eastern part of the Himalayan collision belt prohibited the subduction of continental crust to the depth of UHP metamorphism. 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 resulting amphibolite facies (10-13kb and 580-610íÆC) overprint on UHP eclogite in the western Himalaya and granulite facies (7-10kb and 750-790íÆC) overprint on the HP eclogites in the eastern Himalaya. In the Dabie-Hongseong collision belt (the collision belt between the North and South China blocks), opposite trend was recognized; the collision started from Late Permian in the east (~ 257 Ma) and propagated towards west until Triassic (~ 210-230 Ma). Towards west, the peak metamorphic condition changed from UHT metamorphism (9.0-10.6kb, 914-1157íÆC) in the easternmost part through HP metamorphism (16.5-20.0kb and 775-850íÆC) in the middle part (the Hongseong area, South Korea) to UHP metamorphism (22-41 kb and 600-880íÆC) in the westernmost part (the Dabie-Sulu belt, China) indicating decrease of geothermal gradient towards west. Before collision of the North and South China blocks, the ocean between two blocks was narrow in the east where collision started but wide in the west. As a result, the amount of subducted oceanic slab was enough to pull down the South China block to UHP condition depth in the western end but not enough in the eastern parts. The oceanic slab break-off started from the east. After oceanic slab break-off, the buoyancy force of subducted continental crust increases as the subducted depth increases.
The fastest uplift of UHP rocks in the Dabie and Sulu belt caused amphibolite facies retrograde metamorphism but UHT metamorphism occurred in the Odesan area due to the slowest uplift. The local occurrence of UHP metamorphic rocks is related to the distance between two blocks before collision and the time of oceanic slab break-off.