Systematic changes in metamorphic styles along the Dabie–Hongseong and Himalayan collision belts, and their tectonic implications

Different continental collision belts show contrasting metamorphic trend along their length, including the distribution of extreme metamorphism; i.e., ultrahigh-pressure (>100 km depth) and ultrahigh-temperature (900–1150 °C) metamorphisms. However, no previous study has succeeded in explaining these trends. The present study investigates the main factors that control the metamorphic trends along collision belts, with reference to the Dabie–Hongseong collision belt between the North and South China blocks and the Himalayan collision belt between the Indian and Asian blocks. In the Dabie–Hongseong collision belt, collision began in the east before 245 Ma and propagated westward until ca. 220 Ma. In the eastern part of the belt, the amount of oceanic slab that subducted before collision was insufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism; however, ultrahigh-pressure metamorphism occurred in the western part of the belt. Slab break-off also migrated from east to west, with a westward increase in the depth of break-off (from ca. 10 kbar in the west to ca. 35 kbar in the east). These lateral trends along the belt resulted in a westward change from ultrahigh-temperature (915–1160 °C, 9.0–10.6 kbar) to high-pressure (835–860 °C, 17.0–20.9 kbar) and finally ultrahigh-pressure metamorphism (680–880 °C, 30–40 kbar). In the Himalayan collision belt, collision started from the west at 50 Ma and propagated eastward. The amount of oceanic slab subducted prior to collision was sufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism in the west, but not in the east. Slab break-off started in the west at ca. 46 Ma and propagated eastward, with an eastward decrease in the depth of slab break-off from 27–29 to 17–18 kbar. Consequently, the metamorphic trend along the belt changes eastward from ultrahigh-pressure (690–750 °C, 27–29 kbar) to high-pressure and finally high-pressure granulite facies metamorphism (890 °C, 17–18 kbar). The differences in metamorphic trend between the Dabie–Hongseong and Himalayan collision belts reflect the amount of oceanic crust subducted prior to collision and the depth and timing of slab break-off along each belt.