PETROGRAPHY AND GEOCHRONOLOGY OF EXHUMATION-RELATED RETROGRADE METAMORPHIC ROCKS IN THE DULAN REGION, NORTH QAIDAM ULTRAHIGH-PRESSURE 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 (including minor marble and calc-silicate rocks), minor ultramafic rock, and eclogite lenses enclosed within gneiss. D119 is an extremely garnet (Grt) rich rock (x-ray maps reveal tiny garnet rims are slightly Fe-richer), with clinozoisite (Czo), quartz (Qtz), and large titanite (Ttn) grains containing small rutile (Rt) inclusions, suggesting that titanite replaced rutile during decompression. D130-A & -B are banded calc-silicate gneisses from the same outcrop; D130-A is carbonate-rich and D130-B is mafic silicate-rich. D130-A is garnet-poor (midsize garnet rims are slightly Ca-richer), with calcite, clinozoisite, biotite, muscovite, quartz, and small titanite grains with inclusions of zircon and clinozoisite, plus reaction rims containing rutile, biotite, calcite, and quartz. The titanite-rutile relationships vary in each compositional band, suggesting a complex exhumation history or variable H₂O vs. CO₂. D130-B has large nearly euhedral garnet porphyroblasts (large garnet rims are slightly Ca-richer and both rims and large cores are slightly Mn-richer) in fine-grained Hbl-Cpx-Pl symplectite, calcite, biotite, chlorite, quartz, and midsized titanite grains with few small rutile inclusions. The sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) titanite U-Pb age for D119 is 404.8 ± 8.5 Ma. Titanite ages are interpreted to represent the decompression reaction Grt + Rt + Qtz + H₂O = Czo + Ttn. SHRIMP-RG rare-earth element (REE) measurements in D119 show a small negative Eu anomaly (suggesting presence of plagioclase) and a steep negative heavy-REE (HREE) slope (indicating presence of garnet). The titanite age for D130-A is 391 ± 13 Ma. The REE data are more scattered, with varying Eu anomalies and a shallower negative HREE slope. Compositional layering of D130-A & -B may have contributed to age and trace element data scattering. The difference between the sample ages is not statistically significant but the ages are close to the previously reported Ar muscovite age of 402 Ma.