Early Cretaceous arc granitoids from the central Lhasa subterrane: Production of the northward subduction of Yarlung Zangbo Neo‐Tethyan Ocean?

Abstract

There is ongoing debate as to the genesis of the Lhasa Terrane during the Mesozoic (northward subduction of Yanglu‐Zangbo Tethyan Ocean [YZTO] or southward subduction of Bangong–Nujiang Tethyan Ocean [BNTO]). In this paper, we report a dataset of geochronology, geochemistry, and Sr–Nd–Hf isotopes for the Azhale granitoids in order to elucidate the dominant geodynamic processes of the central subterrane. These rocks are dated at 142.2 ± 1.0 and 136.6 ± 0.6 Ma by zircon LA–ICP–MS method, indicating that these rocks were contemporaneous. In situ zircon Hf isotopic analyses yielded εHf(t) values of −4.2 to −0.1. Geochemically, these samples mostly belong to calc‐alkaline series and show strong enrichments in large‐ion lithophile elements (LILE;e.g., Rb, Ba, and Th) and depletions in high‐field‐strength elements (HFSE;e.g., Nb and Ta), indicating the Azhale granitoids are typically I‐type granite. In terms of isotopic composition, these rocks also have low εNd(t) values (−6.51 to −6.80) and high initial 87Sr/86Sr ratios (0.7078 to 0.7084) relative to melts derived from partial melting of ancient mafic lower crust. We considered that these rocks should be interpreted to have resulted from melting, assimilation, storage, and homogenization (MASH process) above subduction zones. In conclusion, combined with the published and our new data, we argued for the petrogenesis of Early Cretaceous arc‐related granitoids were due to the YZTO lithosphere subducted northward beneath the Lhasa Terrane.