Petrogenesis and Geochemistry of Meso-Cenozoic Granitic Rocks and Implication of Crustal Structure Changes in Shannan Area, Southern Tibet

In the southern part of the Lhasa block in southern Tibet, large-scale zonal distribution of granitic rocks is developed, where the important information on the late subduction of the Neo-Tethys oceanic crust and subsequent collisions and post-collision processes in the Indian-Eurasia continent is recorded. In this paper, LA-ICP-MS zircon U-Pb geochronology, Hf isotope and total trace elements and Sr-Nd isotope geochemical analysis of six granitic rock samples collected in the southern Shannan area of the Lhasa block were obtained. The three-stage zircon ages of ~90 Ma, 65 Ma and 23 Ma show that three phases of magmatic activity occurred in the area. The rock samples of the three eras are all subalkalic series, with similar adakite characteristics, enriched with high field strength elements and depleted with large ion lithophile elements. The distribution of rare earth elements is right-dip, geochemical with the feature of arc magma. The zircon εHf(t) of all samples in this paper are positive (+5.6-+14.6), suggesting that they may be derived from the partial melting of the new lower crust. Based on the previous data, the La/Yb ratio of granitic rocks is used to quantitatively show the evolution of crustal thickness in the Shannan area for 100 Ma. From the late Mesozoic, the thickness of the crust in the area was thinned from thick to thin in the Early Cenozoic, and gradually thickened thereafter. This is in line with the changes in the crustal structure caused by the subduction of the Neo-Tethys and the Indian-Eurasia collision-post-collision process since Mesozoic to Cenozoic. © 2019, Editorial Department of Earth Science. All right reserved.