Closure of the Paleo-Tethys Ocean: Insights from zircon U-Pb ages and geochemistry of the Late Triassic granite in the Amugangri area, central Qiangtang, North Xizang Plateau

Objective The Longmu Co−Shuanghu plate suture zone is located in the hinterland of the Qinghai−Xizang Plateau, which plays a key role in understanding the tectonic evolution of the Paleo−Tethys Ocean. It is still unclear for the geodynamic mechanism and close timing of this ocean. Igneous rocks are widely distributed on both sides of the Longmu Co−Shuanghu suture zone, which provides key information for exploring tectonic−magmatic process with the closure of the Paleo−Tethys Ocean.

Methods In this study, zircon LA−ICP−MS U−Pb dating and Lu−Hf isotope, as well as whole−rock geochemistry and Sr−Nd isotope analyses were carried out on granite in the Amugangri area in the eastern part of the suture zone.

Results Zircon dating yielded ages of 213~212 Ma, representing crystallizing age of the granite. This age is slightly later than that of the peak metamorphism of the high−pressure metamorphic zone in this area. All rocks have high SiO₂ (56.88%~70.35%) and Al₂O₃ (14.45%~16.82%) contents, with variable the K₂O (1.84%~5.47%) and all−alkali contents (K₂O+Na₂O=5.80%~8.08%). The negative P₂O₅−SiO₂ correlation and occurrence of hornblende indicate an I−type granitoid affinity. The chondrite−normalized rare earth element diagram shows an obvious right−dipping type with Eu−negative anomaly, indicating that there is a certain degree of plagioclase separation or residue in its source. The trace element partition curves are basically consistent, with strong enrichment in large ionic lithophilic elements (Rb, Th, U, K and La, etc.), and depleted in high−field−strength elements (Nb, Ta, Ti and P, etc.). In addition, their high I_Sr (0.713530~0.715744) and negative zircon ɛ_Hf(t) values (–19.2~–7.1) and ɛ_Nd(t) values (–9.5~–8.6) characteristics suggest that these rocks probably originated from partial melting of ancient crustal material. Mixed simulation indicates that about 10%~20% mantle−derived basic magma was inputted into its source.

Conclusions Combined with regional geologic data, we suggest that the Amugangri granite was probably formed in a post−collision setting. After the closure of the ocean basin and oceanic slab breakoff, upwelling mantle magma induced wide Late Triassic magmatism along the suture zone in the central Qiangtang area. They are petrological records for the closure of the Paleo−Tethys Ocean.