Petrogenesis of Late Cretaceous Jiangla'angzong I-Type Granite in Central Lhasa Terrane, Tibet, China: Constraints from Whole-Rock Geochemistry, Zircon U-Pb Geochronology, and Sr-Nd-Pb-Hf Isotopes

The Jiangla'angzong granite in the northern part of the Central Lhasa Terrane is composed of syenogranite and adamellite. LA-ICP-MS zircon U-Pb analyses suggest that syenogranite has a weighted mean Pb-206/U-238 age of 86 +/- 1 Ma (mean square weighted deviation= 0.37), which is in accordance with the muscovite Ar-Ar age (85 +/- 1 Ma) of Cu-Au ore-bearing skarns and the zircon U-Pb age (84 +/- 1 Ma) of adamellite. This suggests that the Jiangla'angzong magmatism and Cu-Au mineralization events took place during the Late Cretaceous. The granite contains hornblende, biotite, and pyroxene, and does not contain Al-bearing minerals, such as muscovite, cordierite, and garnet. It has high contents of SiO2 (65.10 -70.91wt%), K2O (3.44-5.17wt%), and total K2O+ Na2O (7.13-8.15wt%), and moderate contents of Al2O3 (14.14-16.45wt%) and CaO (2.33-4.11wt%), with a Reitman index (sigma(43)) of 2.18 to 2.33, and A/CNK values of 0.88 to 1.02. The P2O5 contents show a negative correlation with SiO2, whereas Pb contents show a positive correlation with SiO2. Th and Y contents are relatively low and show a negative correlation with the Rb contents. These characteristics suggest that the Jiangla'angzong granite is a high K calc-alkaline metaluminous I-type granite. It is enriched in light rare earth elements (LREE) and large ion lithofile elements (LILE), and depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE), with LREE/HREE ratios of 11.7 to 18.1. The granite has negative Eu anomalies of 0.58 to 0.94 without obvious Ce anomalies (delta Ce= 1.00-1.04). The relatively low initial Sr-87/Sr-86 ratios of 0.7106 to 0.7179, positive epsilon(Hf)(t) values of 1.0 to 4.1, and two-stage Hf model ages (T-DM2) ranging from 889 Ma to 1082 Ma, These geochemical features indicate that the granite derived from a juvenile crust. The (Nd-143/Nd-144)(t) values from the Jiangla'angzong granite range from 0.5121 to 0.5123, its epsilon(Nd)(t) values range from -10.17 to -6.10, its (Pb-206 /Pb-204)(t) values range from 18.683 to 18.746, its (Pb-207 /Pb-204)(t) values range from 15.695 to 15.700, and its (Pb-208 /Pb-204)(t) values range from 39.012 to 39.071. These data indicate that the granite was formed by melting of the upper crust with the addition of some mantle materials. We propose that the Jiangla'angzong granite was formed during the post-collision extension of the Qiangtang and Lhasa terranes.