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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 εHf‐positive granitoids adjacent the ITS. Zircons range from εHf= ‐13 to +11 in a broadly systematic fashion from north to south, generally independent of 206Pb/238U age. Adjacent to the ITS, syncollisional (<50 Ma) rocks have generally more heterogeneous εHfthan 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 δ18O 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 εHfdata 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. Granites in what is now the Tibetan Plateau have formed continuously over the past 200 million years. The chemistry of these rocks, and the individual minerals inside them, contain information about the environment in which they formed. Zircon, an accessory mineral that is present in many types of igneous rocks, including these granites, is a time capsule of information about its formation: it contains radioactive elements that allow us to date the age of formation of the host rock as well as stable isotopes that can be used as proxies for the proportions of crustal and mantle parent material that the granites are made of. There are also trace elements in the whole rock that may provide information about the depth at which the granite formed. Our findings suggest that the southern margin of Tibet was thin from >200 million years ago until after the onset of the India‐Asia continental collision approximately 50 million years ago, and the crust was thicker to the north well before the onset of collision. These findings provide a boundary the possible ways that the Lhasa block of Tibet may have accommodated the deformation induced by continental collision. Geochemistry of Tibetan granitoids may serve as indirect proxies for crustal thickening processes throughout the India‐Asia collisionWhole‐rock and zircon Lu‐Hf isotopes are well‐correlated in samples spanning ~180 Ma to ~20 MaGeochemical proxies for depth of melt generation suggest a persistently thin crust at the southern margin of the Lhasa block until <50 Ma

Carbon cycling in subduction zones remains poorly constrained due to the lack of relevant geological records. Here we report magnesium isotope data (δ26MgDSM3) from calcium‐silicate rocks (rodingites) from the Xigaze ophiolite, southern Tibet, which is thought to represent remnants of Neo‐Tethyan oceanic lithosphere. Behaviors of immobile trace elements in rodingites resemble those of their mafic dike protoliths, showing subduction‐related signatures. The majority of rodingites exhibits low δ26Mg values of −0.72‰ to −0.33‰ with a weighted average of −0.47 ± 0.11‰ (2 SD), significantly lighter than that of their protoliths (−0.31 ± 0.03‰). This difference likely reflects the interaction of the protolith with isotopically light carbonate fluids. Modeling indicates that this hypothesis requires the input of 5 to 15 wt % carbonates during rodingitization. Our study suggests that rodingite may represent a previously unrecognized reservoir of dissolved Ca from subducted carbonates. The majority of rodingites exhibits low δ26MgDSM3values significantly lighter than that of their protolithsReflects the interaction of their protoliths with isotopically light carbonate fluidsRodingite may represent a previously unrecognized reservoir of dissolved Ca from subducted carbonates