Hydrogen and oxygen isotope geochemistry of high to ultrahigh pressure metamorphic rocks from Southeast Dabie

Abstract

The hydrogen and oxygen isotope compositions of single minerals and whole-rock were measured for coesitebearing eclogite at Shima and quartz-bearing eclogite at Huangzhen in southeastern Dabie. The δ 18O values of the eclogites vary from 1.0 to 7.4‰ at Shima but from -2.1 to 1.3‰ at Huangzhen; the δD values of micas from the eclogites are -76 to -61‰ at Shima and -89 to -77‰ at Huangzhen. Hydrogen and oxygen isotope fractionations between the minerals are in both equilibrium and disequilibrium, indicating that the eclogites have not only inherited the 18O-depletion feature of their protoliths, but also experienced strong retrograde alteration during exhumation. Oxygen isotope temperatures of the eclogite mineral-pairs are cumulated at two peak values of 600 to 640°C and 380 to 430 °C, respectively, corresponding to retrograde reequilibration at greenschist-facie and high amphibolite-facies conditions; the peak eclogite-facies temperatures of 750 to 800°C cannot be retrieved due to the strong retrograde alteration. The high and ultrahigh pressure rocks in southeastern Dabie are geologically separated by the quartz-bearing eclogite and granitic gneiss, but the oxygen isotope analyses indicate an in situ relationship between the two rocks.