Isotopic disequilibrium in ultrahigh-pressure and retrograde metamorphism of eclogite and gneiss from the Chinese Continental Scientific Drilling in the Sulu orogen, China: evidence from mineral Nd-Sr-O isotopic composition

摘要

Diffusion rates of Sr and O in minerals are often comparable while Nd has a lower diffusion rate during thermal overprint(s); thus, the O isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of Rb-Sr and Sm-Nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism. This study presents a combination of investigation on Sm-Nd, Rb-Sr, and O isotopic compositions of minerals separated from ultra-high-pressure eclogite and gneiss that were collected from the main hole of the Chinese Continental Scientific Drilling project located in the Sulu orogen, eastern China. Oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620-740 and 460-590℃, representing diffusion cessation of isotopic exchange during the eclogite-facies recrystallization and later amphibolite-facies retrograde overprint. Rb-Sr mineral regressions of two eclogite samples give consistent Triassic ages of 244 Ma, corresponding to eclogite-facies metamorphism, while the same minerals do not yield meaningful Sm-Nd isochron ages. This phenomenon likely suggests that Rb-Sr isotopic equilibrium was achieved during eclogite-facies metamorphism and preserved during late amphibolite-facies retrogression. In contrast, Sm-Nd isotopic equilibrium between the minerals of eclogite was not achieved under UHP metamorphic conditions. Regressions of epidote and biotite of one gneiss sample give a Triassic Sm-Nd age of 243 ± 34 Ma, corresponding to the time of the eclogite-facies metamorphism, and a Jurassic mineral Rb-Sr age of 187.5 ±1.8 Ma. These results imply that fluids have played an important role to achievement of the Sm-Nd isotopic equilibrium during eclogite-facies metamorphism and re-equilibration of the Rb-Sr isotopic system during later retrograde overprint.