Chronology of ultrahigh-pressure metamorphism in the Dabie Mountains and Su-Lu terrane: I. Sm-Nd isotope system

<正> If the protolith of coesite-bearing eclogite was gabbro, it could undergo ultrahigh-pressure metamorphism under dry condition. In this case, the Hocking temperatures of those minerals could be higher, so that Nd isotope disequilibrium between minerals could be observed. If the protolith of coesite-bearing edogite was metabasalt, amphiboles in the metabasalt were decomposed during ultrahigh-pressure metamorphism and released water. Thus Nd isotope compositions between the minerals were in equilibrium because the blocking temperatures of those minerals could he lower. The secondary alteration and fluid-rock interaction in high-pressure are major problems for Sm-Nd dating of eclogite. The reliable Sm-Nd isotopic ages of coesite-bearing eclogite from the Dabie Mountains and Su-Lu terrane range from 221 to 232 Ma. They are slightly lower, but very close to the peak metamorphic ages of the eclogites. The cold eclogite from the Sujiahe complex could be oceanic subduction origin in the Paleozoic.