Combined U-Pb, Lu-Hf, Sm-Nd and Ar-Ar multichronometric dating on the Bailang eclogite constrains the closure timing of the Paleo-Tethys Ocean in the Lhasa terrane, Tibet

The Lhasa terrane, the main tectonic component of the Himalayan-Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P-T and U-Pb, Lu-Hf, Sm-Nd and Ar-Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P-T conditions of similar to 2.6 GPa and 465-503 degrees C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet-whole rock-omphacite Lu-Hf and Sm-Nd ages of 238.1 +/- 3.6 Ma and 230.0 +/- 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U-Pb age of 227.4 +/- 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu-Hf and Sm-Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu-Hf system skewing to the age of the garnet core and the Sm-Nd system favoring the rim age. The Sm-Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu-Hf age likely pre-dated the eclogite-facies metamorphism. 40Ar/39Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (similar to 2.6 GPa and similar to 480 degrees C; ca. 230 Ma), the Sumdo eclogite (similar to 3.4 GPa and similar to 650 degrees C; ca. 262 Ma) and Jiang eclogite (similar to 3.6 GPa and similar to 750 degrees C; ca. 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P-T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230 Ma. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.