Low-T eclogite in the Dabie terrane of China: petrological and isotopic constraints on fluid activity and radiometric dating

摘要

While extensive studies have demonstrated fluid release during subduction of oceanic crust, little attention has been paid to fluid activity during subduction and exhumation of continental crust. Abundant occurrence of quartz veins within eclogites in the Dabie-Sulu orogenic belt of China provides us with an opportunity to study the origin and role of vein-forming fluids with respect to heat and mass transfer during ultrahigh pressure (UHP) metamorphism and its relevant processes. This study focuses on kyanite-quartz vein that occurs as polycrystalline aggregates within the low-T eclogite in the Dabie terrane, which are interpreted as pseudomorphs after former porphyroblasts of lawsonite. Coesite pseudomorphs were found for the first time in eclogite garnet, resulting in a revised estimate of peak P–T conditions at 670°C and 3.3 GPa for the eclogite and thus upgrading the high-P unit to an UHP unit. On the basis of the relationship between calculated P–T path and metamorphic reactions as well as the absence of foliation texture, and undulose extinction of quartzes in the vein, we conclude that lawsonite breakdown into kyanite–quartz–zoisite assemblage took place at the onset of exhumation subsequent to peak pressure. Retrograde metamorphism caused O and H isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in stable isotope compositions. Zircon U–Pb dating and whole-rock Nd–Sr isotope analyses indicate that eclogite protolith is the paleoceanic basalt that was derived from the depleted mantle by magmatism at about 1.8 to 1.9 Ga but experienced hydrothermal alteration by surface waters. The altered basalt underwent UHP metamorphism in the Triassic that caused fluid release for zircon growth/overgrowth not only at about 242±3 Ma prior to the onset of peak pressure but also at about 222±4 Ma during decompression dehydration by lawsonite breakdown and hydroxyl exsolution in the low-T/UHP eclogite. Consistent ages of 236.1±4.2 Ma and 230±7 Ma were obtained from mineral Sm–Nd and Rb–Sr isochron dating, respectively, indicating attainment and preservation of Nd and Sr isotope equilibria during the Triassic UHP eclogite-facies metamorphism. Ar–Ar dating on paragonite from the eclogite gave consistent plateau and isochron ages of 241.3±3.1 Ma and 245.5±9.8 Ma, respectively, which are interpreted to date paragonite crystallization during the prograde eclogite-facies metamorphism. The timing of peak UHP metamorphism for the low-T eclogite is constrained at sometime prior to 236.1±4.2 Ma. Thus the termination age of peak UHP metamorphism may be different in different slices of deep-subducted slab.