INCREASING THE SPATIAL RESOLUTION OF AGES AND PEAK PRESSURE-TEMPERATURE CONDITIONS OF ULTRAHIGH-PRESSURE ECLOGITES AND HIGH-PRESSURE GRANULITES IN DULAN UHP TERRANE, NORTHWESTERN CHINA

The North Qaidam ultrahigh-pressure (UHP) terrane in northwestern China contains the ~140 km² Dulan UHP region, with UHP eclogites outcropping in the east and high-pressure (HP) granulites outcropping in the west. Previous Fe²⁺ - Mg exchange thermometry suggests temperatures ranging from ~620 – 800 °C and pressures between 27 and 32 kbars in the eclogites and 750 - 950 °C and pressures of ~14 kbars in the granulites; this large range of temperatures and uncertainty prevented a detailed understanding of P-T histories and exhumation paths. Zirconium-in-Rutile (Zr-in-Rt) thermometry of 5 representative eclogites provides a temperature range of ~670 - 710 ± 10 °C, which suggests a westward-increasing thermal gradient of 3 - 5 °C/km. Rutile inclusions from two samples show temperatures ~40 - 50°C lower than the matrix rutiles, which suggests increasing temperatures during prograde and peak metamorphism. Rutiles from one representative granulite show two much higher temperature populations: 1) ~775 ± 10 °C and 2) ~740 ± 10 °C. The lower temperature population occurs along an amphibolite facies retrogressed fracture, and likely records exhumation conditions. The ~775 °C granulite temperatures are likely underestimated; current pseudosection and past P-T work on the granulite sample suggests peak temperatures of ~810 - 870 °C. Zircon U-Pb SHRIMP-RG geochronology of the 5 eclogites used for Zr-in-Rt thermometry give ages of 432.6 ± 4.0 Ma, 433.9 ± 2.5 Ma, 436 ± 3.0 Ma and 432.4 ± 2.4 Ma. These ages are the same within error. REE analyses of dated zircons show depleted HREE due to concurrent garnet growth and no negative Eu anomaly due to a lack of plagioclase in the system, which suggests that these ages record eclogite facies. These data show that the temperatures recorded by Zr-in-Rt thermometry are a “snapshot” of the P-T conditions rather than an artifact of crystallization times. The temperature discontinuity between the eclogites and granulites supports the presence of an unmapped fault, which could suggest tectonic juxtaposition after peak metamorphic conditions, after which the granulites and eclogites underwent amphibolite facies retrogression concurrently.