Exsolution microstructures in minerals of rocks from orogenic belts played an important role in recognition of ultrahigh-pressure (UHP) metamorphism in their host rocks by defining the subduction depth and improving our understanding of the dynamics during the subduction and exhumation of UHP rocks. However, it is a challenging scientific topic to distinguish the 'exsolution microstructures' from the 'non-exsolution microstructures' and decipher their geological implications. This paper describes the subtle differences between the 'exsolution microstructures' and the 'non-exsolution microstructures' and summarizes the progress in studies of exolution microstructures from UHP rocks and mantle rocks of ultra-deep origin. We emphasize distinguishing the 'exsolution microstructures' from the 'non-exsolution microstructures' based on their geometric topotaxy and chemistry. In order to decipher correctly the exsolution microstructures, it is crucial to understand the changes of chemistry and habits of host minerals with pressure and temperature. Therefore, it is important to combine observations of exsolution microstructure in natural rocks with experimental results at high pressure and temperature and results of micro-scale analyses. Such studies will improve our understanding of the UHP metamorphism and cast new lights on solid geoscience issues such as deep subduction of continental crusts and crust-mantle interactions.
LIU LiangYANG JiaXiZHANG JunFengCHEN DanLingWANG ChaoYANG WenQiang
During the past ten years, various types of HP-UHP metamorphic rocks have been dis-covered in the South Altyn Tagh, the North Qaidam and the North Qinling (秦岭) in the West and Middle Central China orogen. The UHP rocks, as lentoid bodies in regional gneisses, include eclogite (garnet-bearing pyroxenite), garnet peridotite and various pelitic or felsic gneisses. There are many re-cords of minerals and microstructures of exsolution indicate the UHP metamorphism, such as coesite (or its pseudomorph), diamond, exsolution of clinopyroxene/amphibole/+rutile or rutile+quartz+apatite in garnet, exsolution of quartz in omphacite and exsolution of kyanite+spinel in precursor stishovite.The discovery of microstructure evidence for the presence of precursor stishovite in typical Al-rich gneiss from the South Altyn Tagh reveals continental subduction and exhumation to and from a depth of more than 350 km. It is the petrological record of the deepest subduction and exhumation of continental rock in the world. The in situ zircon U-Pb dating using LA-ICP-MS or SHRIMP methods shows that the meta-morphic ages of the HP-UHP rocks in the South Altyn Tagh, the North Qaidam and the North Qinling are 475–509, 420–457, and 485–514 Ma, respectively. The metamorphic ages of HP-UHProcks in the North Qaidam are 20–80 Ma younger than those in the South Altyn Tagh and the North Qinling, and the metamorphic ages do not systematically increase or decrease from the South Altyn Tagh through the North Qaidam to the North Qinling. The absence of time transgressive variety of the metamorphism in the three regions does not support the hypothesis that the HP-UHP rocks in these regions form the same HP-UHP metamorphic zone. And the HP-UHP rocks in these regions can not be simply correlated to the collision between the North China plate and the South China plate. At present, the study of the HP-UHP rocks in the West and Middle Central China orogen faces several key issues or challenges, such as: (1) the continental subduction to the mantle d
