The Qinglongshan oxygen and hydrogen isotope anomaly near Donghai in Jiangsu Province, China

The Qinglongshan isotope anomaly has unusually low values of both delta(18)O and delta D. Grnets from coesite-bearing eclogite are as low as -11 parts per thousand (VSMOW) and rutiles are -15 parts per thousand. Phengites have delta D of -120 parts per thousand (VSMOW). The low values were acquired in an ancient geothermal system prior to subduction during Triassic continental collision. New data shows that depleted isotope values in different rock types extend over distances of 20 X 40 km demonstrating that the geothermal system retained structural coherence throughout subduction and exhumation. The finding of structural coherence suggests that not only eclogites, with their characteristic ultra-high pressure mineral assemblages, but also gneisses, meta-granites, and meta sediments were all subjected to metamorphism in the coesite-eclogite facies. Oxygen isotope fractionation between quartz and garnet gives estimates of peak metamorphic temperatures at 754-893 degrees C. The spatial extent of oxygen isotopic equilibration between different rock types was limited to tens of centimeters. The persistence of hydrous minerals such as epidote and phengite approaching high temperature oxygen isotope exchange equilibrium with ultrahigh pressure minerals in coesite-bearing rocks shows that mineral-bound water can be subducted into the upper mantle. Copyright (C) 1998 Elsevier Science Ltd.