CHRONOLOGY OF ULTRAHIGH-PRESSURE METAMORPHISM IN THE DABIE MOUNTAINS AND SU-LU TERRANE .1. SM-ND ISOTOPE SYSTEM

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