Identification of Jurassic pure sediment-derived granites in the Central Lhasa Terrane, Tibetan Plateau: Implications for continental crustal reworking during Mesozoic Tethyan subduction

Pure sediment-derived granite, a recently recognized crustal geochemical end-member, is relatively rare but provide primary access to probe the process of continental crustal reworking. This paper presents geochemical and geochronological data for granites emplaced in the Asuo area within the central Lhasa Terrane of the Tibetan Plateau at ca. 158 Ma (zircon U - Pb isotopic dating). These granites are two mica-bearing and contain high SiO2 (75.40-79.11 wt%), high A/CNK (1.06-1.33), heterogeneous (Sr-87/Sr-86); (0.705519-0.710891), and relatively uniform and low epsilon(Nd)(t) (-13.18 to -14.81) values, which are analogous to typical S-type granites. The low epsilon(Hf)(t) (-13.98 to -12.08) and high delta O-18 (8.01 parts per thousand-8.85 parts per thousand) values of magmatic zircons, the similar of epsilon(Nd)(t) values between the Asuo granites and regional Jurassic slates, and the absence of mafic enclaves, verify that these granites were derived from a pure sedimentary source. The trondhjemitic features, low CaO/Na2O (<0.1) and molar CaO/(MgO + FeOT) (0.06-0.58), relatively constant and low Rb/Sr (0.04-0.32), as well as the heterogeneous (Sr-87/Sr-86); ratios, indicate that the Asuo granites formed via fluid-present muscovite-dehydration melting of metapelitic protoliths, with mica as the main residue. Combined with a synthesis of existing tectonomagmatic data on the Lhasa Terrane, we propose that the Jurassic Asuo granites were formed in a continental extensional setting associated with the northward subduction of the Neo-Tethyan lithosphere. The present study highlights that the inboard region of an active subduction zone is probably an important site of crustal reworking, and provides a geochemical end-member of pure sediment-derived melts for the central Lhasa Terrane.