CONSTRAINTS OF THE TECTONO-METAMORPHIC EVOLUTION OF THE DULAN AREA, NORTH QAIDAM UHP TERRANE, CHINA: PRESSURE AND TEMPERATURE SPATIAL DISTRIBUTION, INSIGHTS OF ZR-IN-RUTILE THERMOMETRY AND PHASE EQUILIBRIA MODELLING OF ECLOGITES

HP and UHP gneiss-hosted eclogites from the Dulan area, southeastern part of the North Qaidam UHP terrane, experienced a ~20 Myr UHP event at P-T conditions of ~30 kbar and ~700 °C. A set of 13 spatially distributed eclogites along the Dulan area were analyzed using phase equilibria modelling and Zr-in-rutile thermometry to evaluate the spatial distribution of pressure and temperature conditions in the area.

A kyanite phengite eclogite (D197) shows a mineral assemblage of grt + omp + ph + ky + rt + zo + qz. Garnet exhibits an increase of Mg content from core to rim, and decrease of Ca content from core to rim. Phengite has Si values of ~3.3 apfu. Omphacite display X_Jd content of 0.51-57, and contain quartz pseudomorphs after coesite. Pseudosections and Zr-in-rutile isopleths constructed for this sample indicates that this eclogite reached peak conditions of 685-716 °C at 30 kbar. A phengite eclogite (D205) shows a mineral assemblage of grt + omp + ph + rt + zo + qz. Garnet exhibits an increase of Ca and Mg content from core to rim. Phengite has a Si values of ~3.5 apfu. Omphacite display X_Jd content 0.39-40. Pseudosections and Zr-in-rutile isopleths indicates that this eclogite experienced peak conditions of 652-673°C at 30 kbar.

Zr concentration of the studied samples vary from 63 ppm to 278 ppm, and average temperatures range from 647 ± 14 °C to 700 ± 8 °C. Rutile inclusions in amphibole and garnet give slightly lower temperatures compared with rutiles from matrix. Zr-in-rutile thermometry of the rutiles from matrix are interpreted to represent peak temperatures. Eclogites that show the highest temperatures display weak garnet zoning, whereas eclogites that present lower temperatures display strong garnet zoning. Strongly retrogressed eclogites give scattered temperatures, fresh eclogites tend to give more consistent temperatures. Thermal changes within the set of eclogites are supported by the presence or lack of chemical zoning in garnets, high scatter in calculated temperatures suggest Zr resetting during retrogression. Calculated temperatures suggests that there may not be an evident P-T gradient along the Dulan area, although there are ~60 °C difference in peak temperatures.