PRESSURE-TEMPERATURE-TIME CONSTRAINTS FOR EXHUMATION OF ULTRAHIGH-PRESSURE METAMORPHIC ROCKS IN THE DULAN REGION, NORTH QAIDAM TERRANE, WESTERN CHINA

The North Qaidam ultrahigh-pressure (UHP) terrane is a major early-Paleozoic continental suture zone in Western China. The Dulan region is in the southeast portion of the terrane, consisting mainly of granitic gneiss, metasedimentary gneiss and schist enclosing minor marble, calc-silicate rocks, ultramafic rock, and eclogite lenses. D119 is extremely garnet-rich, with minor czo + qtz + chl + ttn; rutile inclusions in titanite suggest that titanite replaced rutile during decompression. The pseudosection created in Perple_X for D119 predicts the observed mineral assemblage at 490 ± 20 °C, with pressures relatively unconstrained. THERMOCALC was then used to constrain pressures associated with this assemblage; the reaction grs + rt + qtz + H2O = czo + ttn intersects reactions involving chlorite at ~470 ± 25 °C and ~18 ± 2 kbar. D130B is a mafic calc-silicate gneiss, with large, nearly euhedral garnet porphyroblasts in a fine-grained hbl-cpx-pl-qtz symplectite that is interpreted as former omphacitic pyroxene. The sample’s foliation is defined by calcite veins and titanite grains. The pseudosection created for the D130B symplectite yields P-T constraints of 640 ± 100 °C and 9.0 ± 2.5 kbar. Titanite is predicted to be stable at P < 11-14 kbar and T < 500-700 °C. The SHRIMP-RG titanite U-Pb age for D119 is 404.8 ± 8.5 Ma. The titanite age for D130B is 391 ± 13 Ma. Titanite ages are interpreted to represent the decompression reaction grs + rt + qtz + H2O = czo + ttn. The sample ages are not statistically different, but the ages are close to the previously reported Ar muscovite age of 402 Ma. These data, combined with previously determined UHP data (youngest UHP age = 423 Ma, P = 27 kbar), produce preliminary average exhumation rates. In the south (D119), the calculated exhumation rate (Dd/Dt) is 1.5 mm/yr. In the north (D130B), the calculated exhumation rate is 1.7 mm/yr. In comparison with other UHP terranes, these rates are slow. D130B is compatible with early, rapid, buoyancy-driven exhumation, with rates slowing at the crust-mantle boundary. D119 has unexpectedly low T-high P conditions that are not compatible with this exhumation path. However, D119 may represent a tectonically separate slice, or the titanite-producing reaction in D119 may have been significantly overstepped due to lack of fluid until late in the exhumation path.