On Continent‐Continent Point‐Collision and Ultrahigh‐Pressure Metamorphism

Abstract  Up to now it is known that almost all ultrahigh‐pressure (UHP) metamorphism of non‐impact origin occurred in continent‐continent collisional orogenic belt, as has been evidenced by many outcrops in the eastern hemisphere. UHP metamorphic rocks are represented by coesite‐ and diamond‐bearing eclogites and eclogite facies metamorphic rocks formed at 650–800°C and 2.6–3.5 GPa, and most of the protoliths of UHP rocks are volcanic‐sedimentary sequences of continental crust. From these it may be deduced that deep subduction of continental crust may have occurred. However, UHP rocks are exposed on the surface or occur near the surface now, which implies that they have been exhumed from great depths. The mechanism of deep subduction of continental crust and subsequent exhumation has been a hot topic of the research on continental dynamics, but there are divergent views. The focus of the dispute is how deep continental crust is subducted so that UHP rocks can be formed and what mechanism causes it to be subducted to great depths and again exhumed to the shallow surface. Through an analysis of the continental process and mechanical boundary conditions of the Dabie collisional belt—an UHP metamorphic belt where the largest area of UHP rocks in the world is exposed, this paper discusses the variations of viscous stresses and average pressure in the viscous fluid caused by tectonism with rock physical properties and the contribution of the tectonic stresses to production of UHP. Calculation indicates that the anomalous stress state on the irregular boundary of a continental block may give rise to stress concentration and accumulation at local places (where the compressional stress may be 5–9 times higher than those in their surroundings). The tectonic stresses may account for 20–35% of the total UHP. So we may infer that the HP (high‐pressure)‐UHP rocks in the Dabie Mountains were formed at depths of 60–80 km. Thus the authors propose a new genetic model of UHP rocks—the point‐collision model. This model conforms to the basic principles of the mechanics and also to the geologic records and process in the Dabie orogenic belt. It can explain why UHP rocks do not exist along the entire length of the collisional orogen but occur in some particular positions. The authors also propose that the eastern and western corners of the Himalaya collision zone are typical point‐collision areas and that almost all UHP metamorphism of continental crustal rocks occurred in the two particular positions.