Zircon U-Pb geochronology and Hf isotopes of granitic rocks and river sands in the Nyingchi region, Tibet: Constraints on evolution of the deep crust beneath the southeast Lhasa terrane

The Nyingchi region is the best location to investigate the nature and evolution of the deep crust beneath the Lhasa terrane, as the middle and lower crust rocks are well exposed due to the long-term uplift and erosion of the Namche Barwa syntaxis. A systematic study of whole rock geochemistry and zircon U-Pb age and Hf isotope was conducted on granitic rocks exposed in the area. These data, combined with U-Pb ages and Hf isotopes of detrital zircons from river sands, were utilized to reveal the nature of the deep crust. The granitic intrusions in the area can be divided into two main stages, i.e., the latest Cretaceous to Eocene (67-44 Ma) and the Late Oligocene (28-23 Ma). The earlier stage of granitic rocks has SiO2 contents of 67.65-75.22 wt% and exhibits highly variable zircon elif(t) values ranging from - 15.5 +/- 1.3 to 9.2 +/- 0.8, suggesting that they were derived from a remarkably heterogeneous source. The late stage of granitic rocks, with Si02 of 64.37-75.86 wt%, has zircon eHf(t) values ranging from 1.6 +/- 1.3 to 3.9 +/- 0.8, indicating that they were derived from a relatively homogenous and slightly depleted source. In combination with literature data, it is proposed that the Late Oligocene magmatism may prevail in the Lhasa terrane and this extensive magmatic pulse can be attributed to coeval delamination of the subducted Indian continent.