Petrogenesis and tectonic implications of Manbing granitoids, southern Lancangjiang zone: Constraints by geochemistry, zircon U-Pb chronology and Hf isotope

The Changning-Menglian suture is the main remnant of Paleo-Tethys in Sanjiang area, SW China, whereas the subduction process is still a mystery. The Manbing arc-like granitoids in the southern Lancangjiang zone provide a new clue to explore the subduction process. In this paper, zircon U-Pb geochronology, Hf isotopes and whole-rock trace element geochemistry of the gneissic biotite granodiorites and plagioclase amphibolites in Manbing granitic body were studied. The LA-ICP-MS zircon U-Pb ages of those samples are 251.5 +/- 2.7Ma, 253.8 +/- 1.1Ma and 241.8 +/- 0.8Ma, 259.5 +/- 1.9Ma, respectively, indicating the related to Late Permian-Early Triassic magma activity. Menbing granodiorites belong to calc-alkaline to high-K calc-alkaline series, which are enriched in light rare earth elements and large ionic lithophile elements, with depleted in Nb, Ta, Ba, Sr and high field strength elements, and have negative Eu anomalies (Eu = 0. 45 similar to 0. 69) . The mean values of Hf-176/Hf-177 for granodiorite and anorthosite amphibolite are 0.2829035 and 0.2829762, respectively. The mean values of zircon epsilon(Hf)(t) were 11.0 and 12.5. The weighted mean crustal model ages of t(DM)(C) were 578.4Ma and 469.3Ma, which were similar to arc-like granitoids within the juvenile lower crust. The low Ce/Pb (3.89 similar to 6.57) , Th/La (0.32 similar to 0.58) and Sm/La (0.11 similar to 0.15) of Menbing granitoids indicate the juvenile lower arc crust was replaced by fluids from pelagic sediments. Combined with the previous research results, it is believed that the Manbing arc-like granitoids were formed in the tectonic environment transition from subduction to continental collision, indicating that the closure of the main oceanic basin in the Sanjiang area may not be completed until the Early Triassic.