UNUSUALLY LOW DELTA-O-18 ULTRA-HIGH-PRESSURE METAMORPHIC ROCKS FROM THE SULU TERRAIN, EASTERN CHINA

Eclogite and quartzite interlayers, containing ultra-high-pressure (UHP) metamorphic mineral assemblages, occur at Qinglongshan in the Sulu terrain, eastern China. These rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks. The delta(18)O value of minerals in eclogite ranges from -10.4 to -9.0 parts per thousand and in quartzite ranges from -10.2 to -7.3 parts per thousand. Near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the UHP metamorphism, through near surface meteoric water-rock interactions. The rocks were then subducted to a great depth during continent-continent collision, forming UHP minerals with concomitant local oxygen isotope redistribution. It is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle.