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The first in situ Hf and U-Pb isotope analyses of zircon separates from Mesozoic granites in southern Tibet identify a significant, previously unknown stage of magmatism. Igneous zircons (n = 34) from a granite within the Gangdese batholith show a weighted mean ²⁰⁶Pb/²³⁸U age of 188.1 ± 1.4 Ma and ε_Hf(T) (the parts in 10⁴ deviation of initial Hf isotope ratios between the zircon sample and the chondritic reservoir) values between +10.4 and +16.8, suggesting predominantly Early Jurassic intrusive activity with a juvenile mantle contribution. Of 40 inherited zircons from two Cretaceous S-type granites in the northern magmatic belt, 23 delineate a slightly older ²⁰⁶Pb/²³⁸U age cluster between 188 and 210 Ma. These zircons have ε_Hf(T) values from −3.9 to −13.7, yielding crustal Hf model ages from ca. 1.4 to 2.1 Ga, suggesting a major episode of crustal growth in Proterozoic time and remelting of this crust in …