Late Triassic basaltic andesites unveil inception of Neo-Tethys Ocean in Gyaca area of Tethyan Himalaya

The formation and evolution of the Neo-Tethys Ocean profoundly influenced the pre-collisional configuration of the Tibetan Plateau before the India-Asia collision. However, the timing of the Neo-Tethys Ocean's opening and the resulting magmatism remain subjects of ongoing debate. Here we present an integrated investigation of a suite of basaltic andesites exposed in the Gyaca area, eastern Tethyan Himalaya, southern Tibetan Plateau. Using zircon U-Pb-Hf isotopes, bulk rock geochemical data, and whole-rock Sr-Nd-Hf isotopic data, we attempt to temporally and petrogenetically constrain the early stages of magmatism associated with the opening of the Neo-Tethys Ocean. The Gyaca basaltic andesites were formed in the Late Triassic (ca. 217 Ma). They exhibit geochemical features resembling those of arc magmatic rocks, characterized by moderate light/heavy rare earth element fractionation ((La/Yb)_N = 5.16–6.57), enrichment in large ion lithophile elements, and depletion in high field strength elements. They also show variable whole-rock SrNd (⁸⁷Sr/⁸⁶Sr_i = 0.709848–0.712233; ε_Nd(t) = −1.12 to +0.19) and zircon Hf (ε_Hf(t) = −6.2 to +3.2) isotope compositions, alongside depleted whole-rock Hf isotopes (ε_Hf(t) = +2.83 – +7.42). Compared to coeval arc magmatism in the southern Lhasa Terrane, southern Tibetan Plateau, the Gyaca basaltic andesites show higher incompatible element contents and more enriched NdHf isotope compositions, ruling out their origin as products of northward subduction of the Neo-Tethys oceanic plate. The negative correlation between the Mg# of these basaltic andesites and ε_Nd(t) suggests that more primitive magmas have more enriched Nd isotopes, likely due to assimilation with sediments during turbulent magma ascent under high thermal conditions. Combining existing petrological and sedimentological evidence, we propose that the Gyaca basaltic andesites likely document the early interaction between the upwelling asthenosphere and the overlying sediments during the initial spreading of the Neo-Tethys seafloor. Consequently, the opening of the Neo-Tethys Ocean in the eastern Himalaya would not postdate the Late Triassic.