An embryonic intra-oceanic subduction in the Bangong−Nujiang Meso-Tethys: insights from Late Jurassic NdHf isotopic decoupling adakitic granodiorites in the Dongco ophiolite of central Tibet

Subduction of the Bangong−Nujiang Meso-Tethys not only bred world-class Cu − Au polymetallic deposits in central Tibet but also created the initial elevation of the Tibetan Plateau during the Late Cretaceous following its final closure. Consequently, the Meso-Tethys carry critical codes for the evolution of Tethyan realm and the development of the Tibetan Plateau but some fundamental aspects still remain poorly explored. Here we proposed an embryonic intra-oceanic subduction event in the Bangong−Nujiang Meso-Tethys during the Late Jurassic based on investigations of adakitic granodiorites intruded within the Dongco ophiolite suite, central Tibet. These granodiorites were dated at 150–156 Ma by zircon UPb isotopes and geochemically display an affinity with high-silica adakite (Sr/Y = 24–47; La/Yb = 6–16), specifically marked by high SiO₂, Al₂O₃ and Na₂O contents but low MgO, CaO + Na₂O contents as well as extremely low Yb and Y contents. They have (⁸⁷Sr/⁸⁶Sr)_i ratios varying from 0.7061 to 0.7069, ε_Nd(t) values of −0.83 to 0.50, and ε_Hf(t) values of 3.3 to 8.9 (with one exception at −5.0), showing a distinct NdHf isotopic decoupling (Δε_Hf(t) = 2.1–7.6; Δε_Hf(t) = ε_Hf(t)–1.55 × ε_Nd(t)–1.21). Geochemical modeling indicates that they were derived from partial melting of subducted Meso-Tethyan oceanic crust and moderate amounts of sediments. Residual plagioclases in the source region likely resulted in relatively lower Sr and La contents than typical high-silica adakite. Combined with previous studies, we suggest that the Dongco granodiorites formed in the early stage of an intra-oceanic subduction zone in the Bangong−Nujiang Meso-Tethys where subsidence of colder lithosphere inductively commenced along an active transform fault bounded by younger lithosphere during the Late Jurassic. This embryonic intra-oceanic subduction possibly occurred locally and was very short-lived.