Hf and Nd Isotopic Constraints on Pre‐ and Syn‐collisional Crustal Thickness of Southern Tibet

In Southern Tibet, voluminous granitoids emplaced between 225‐20 Ma provide a spatiotemporal window into the geochemical and tectonic evolution of the crust. Hf and O isotope geochemistry of whole rocks and constituent zircons together with whole‐rock chemistry reveal a coherent magmatic history of Gangdese granitoids, and by extension, crustal thickening history of S. Tibet. We observe a spatial isotopic gradient with N‐S distance from the Indus‐Tsangpo Suture (ITS), with younger, more ε Hf ‐positive granitoids adjacent the ITS. Zircons range from ε Hf = ‐13 to +11 in a broadly systematic fashion from north to south, generally independent of 206 Pb/ 238 U age. Adjacent to the ITS, syncollisional (<50 Ma) rocks have generally more heterogeneous ε Hf than precollisional (>70 Ma) and early syncollisional (50‐70 Ma) granitoids, likely reflecting increased assimilation of crustal material in syncollisional magmas as the crust thickened. Zircon δ 18 O ranges between +4 and +8‰; syncollisional samples have exclusively mantle‐like values (+5.5 to +6‰), with greater heterogeneity in precollisional samples. Zircon and whole‐rock ε Hf data reported here are consistent with previous Nd‐based thermoisotopic models indicating that the Lhasa block maintained a wedge‐shaped crustal geometry from the early Jurassic until the onset of collision. Given evidence of minimal post‐50 Ma upper‐crustal shortening, these results support earlier findings that the Tibetan crust reached its present ~75 km thickness via a roughly equal mixture of upper plate accretion and juvenile magmatic inflation on top of the ~30 km‐thick of Indian crust underthrust beneath the Lhasa block.