A Study Of Migmatite In The Dabie-Sulu Orogenic Belt

During the subduction processes at the plate convergent boundary, the continental crust and oceanic crust, due to their remarkable differences in composition, show striking different dynamic characters. We already well understand that the subduction of oceanic crust induces partial melting of the mantle wedge and produces island arc magmatic activities. However, many important facts of the continental subduction processes still remain unknown.In the 1980s, continental crust was first proved to have subducted to the depth of more than 80 km and thereafter come back rapidly to the surface, since the discoveries of coesite in supracrustal rocks in the West alpines and West Norway. Up to now, geologists have identified ultra-high pressure (UHP) index minerals, such as coesite and diamond, in 22 metamorphic belts on the global earth. Continental deep subduction and related ultra-high pressure metamorphism has been the highlight field in the earth science research in the recent two decades. The Dabie-Sulu orogenic belt, formed by the collision of the Yangtze craton and the North China craton in the late Triassic, has been know as the largest and best-preserved UHP terrane all over the world. Therefore, it becomes the ideal area for the UHP metamorphic research.Numerous studies show that most parts of the continental crust are "dry" compared to the oceanic crust. No large scale fluid activities occurred during the continental deep subduction and hence island arc magmatism similar to the oceanic subduction could not be generated. Therefore, lack of syn-collisional magmatism is one of the most important characters of the continental deep subduction. However, more and more evidences have been identified showing post-UHP metamorphic fluid activities and contemporary magmatism related to the exhumation process. It has become an active topic in the continental deep subduction research to disclose whether it exist partial melting or even magma activities during the retrogression and exhumation processes of the UHP metamorphic rocks.In the Dabie-Sulu UHP metamorphic belt, post orogenic migmatite (130 Ma) is widely distributed. It is commonly understand that these migmatite, which are most reside in the Northern Dabie terrane, have no direct tectonic connection with the continental deep subduction. However, we identified certain migmatite showing partial melting characters in the Weihai and Rongcheng area, Northern Sulu UHP metamorphic terrane, and in the Bixiling area, Central Dabie UHP terrane. Evidences indicate that they have close temporal-spatial relationship with the UHP metamorphism.In this study, we choose two typical migmatite in the Bixiling area and Rongcheng area to carry out detailed field investigation, mineral chemistry, isotope chemistry and isotopic dating, to explore their geological age, origin, as well as their relations to the UHP metamorphism. We further discuss whether partial melting has ever occurred in the UHP metamorphosed supracrustal rocks, and try to understand the significance to the evolution of post-collisional structure and composition of the continental deep subduction belt.The migmatite to the Northeast of the Bixiling complex, Dabie terrane is composed of amphibolite and trondhjemite. Both of the rocks develop two types of amphibole - typeâ… is actinolite, and typeâ…¡is hastingsite. Despite the striking difference of the amphibolite and trondhjemite in bulk chemical composition, the typeâ… amphibole in both rocks is similar and the typeâ…¡amphibole in both rocks has nearly same composition. The compositions of these amphiboles are far from those in the MgAl-riched fresh eclogite and the retrograde FeTi-riched eclogite. Combined with the earlier isotopic and geochemical research work of Li H.Y., we further demonstrate that the migmatite in Bixiling represents the products of a 780 Ma partial melting event.In the Rongcheng area, Sulu UHP terrane, migmatite are composed of pale grey gneiss and pink gneiss. Zircon SHRIMP U-Pb dating shows that the protolith of the two types of gneiss were formed in ca. 780 Ma. Analysis of Hf isotopic data indicates that they obtain consistent Hf isotopic character at that time. This implies the two types of gneiss might be originated from a same magmatic source. The zircon dating also shows a 242 Ma event, which represents the peak UHP metamorphism, and a 220 event, which represents the partial melting. The later attributed to the pink gneiss with the rheological property and formed the migmatitic structure.In summary, according to the research work on the migmatite in the Bixiling area, Dabie UHP terrane and Rongcheng area, Sulu UHP terrane, partial melting events might have occurred in two stages. The earlier stage was around 780 Ma, i.e. the time of the protolith of the UHP metamorphic rocks; the later stage was around 220 Ma, which coincident with the exhumation of the UHP metamorphic rocks.