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

• Published in: Journal of geophysical research. Solid earth 2019-11, Vol.124 (11), p.11038-11054
• Main Authors: Alexander, E. W., Wielicki, M. M., Harrison, T. M., DePaolo, D. J., Zhao, Z. D., Zhu, D. C.
• Format: Article
• Language: English
• Subjects: Accretion; Crustal shortening; Crustal thickness; Deformation; Deposition; Geochemistry; geochronology; Geophysics; Granite; Heterogeneity; Igneous rocks; Isotopes; Jurassic; Lead isotopes; Mantle; Minerals; Organic chemistry; Radioisotopes; Stable isotopes; Tectonics; Thickening; Thickness; Tibet; Trace elements; Zircon
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2019JB017696

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 (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 εHf 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. Plain Language Summary 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 w