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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 SiO 2 (75.40–79.11 wt%), high A/CNK (1.06–1.33), heterogeneous (87Sr/86Sr) i (0.705519–0.710891), and relatively uniform and low ε Nd (t) (−13.18 to −14.81) values, which are analogous to typical S-type granites. The low ε Hf (t) (−13.98 to −12.08) and high δ18O (8.01‰–8.85‰) values of magmatic zircons, the similar of ε 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/Na 2 O (<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 (87Sr/86Sr) i 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. • Jurassic two-mica granites are discovered in the Asuo Area of central Lhasa Terrane; • The Asuo granites were derived from a pure sediment protolith of pelite; • The rear-arc region of a subduction zone is an important site for crustal reworking. [ABSTRACT FROM AUTHOR]

[Display omitted] • Late Cretaceous gabbronorites were identified in southwestern Lhasa Terrane. • The subducted Neo-Tethyan sediments were melted under hot and dry conditions. • The Late Cretaceous magmatic flare-up was caused by Neo-Tethyan slab roll-back. The geodynamic processes responsible for the Late Cretaceous magmatic flare-up in the southern Lhasa Terrane, Tibetan Plateau, have been a matter of debate. In this study, we have reported on newly identified Late Cretaceous gabbronorites in the Dajiacuo Area of the southwestern Lhasa Terrane. These rocks show ophitic textures comprising plagioclase, orthopyroxene, and clinopyroxene, while hydrous minerals, such as amphibole and biotite, are not observed. They display geochemical affinities for high-Mg andesites (HMAs) given their SiO 2 (53.5–59.7 wt. %), Mg# (56.8–59.9), and Cr (up to 178 ppm). Combined with their high Th/Yb and low Ba/La ratios and similar Sr–Nd–Hf isotopes to those of the Neo-Tethyan sediments, it is likely that the Dajiacuo gabbronorites were formed by the interaction between sediment-derived melts and the mantle. The Dajiacuo gabbronorites have similar Zr/Hf ratios to the Neo-Tethyan sediments and present no negative Zr–Hf anomalies. This indicates that the zircons in the subducted sediments may have been fully destroyed. This requires specific dynamic conditions to provide a hot subduction geothermal gradient. This is consistent with the high-temperature affinity of the parent magma inferred from Ti-in-zircon thermometry and the presence of orthopyroxene. Based on this, coupled with the synthesis of existing tectonomagmatic data for the southern Tibetan Plateau, we propose that the Late Cretaceous magmatic flare-up in the southern Lhasa Terrane was likely triggered by the roll-back of the northwards subducted Neo-Tethyan lithosphere. [ABSTRACT FROM AUTHOR]

Clarifying the mechanism of recycling of pre-existing continental crustal materials into the source of mantle-derived magma is a challenging efort that can be of great value to improving our understanding of mantle processes and continental crust growth. This study presents an integrated investigation of whole-rock and mineral geochemical and Nd-Hf-O-Pb isotopic data for dolerites and diorites intruded in the central Lhasa Terrane of Tibetan Plateau at ca. 120 Ma (zircon U-Pb ages). These intrusions have similar distributions of trace elements that are characterized by depletion in Nb-Ta relative to Th, Ba, and U, and moderately negative whole-rock εNd(t) (–5.0 to –1.7) values. Magmatic zircon shows dramatically variable εHf(t) values (from –5.0 to +13.7 in the same rock, including up to 12 epsilon unit variability in single grains). On the other hand, the zircon δ18O values are relatively uniform (+6.0‰ to +7.7‰). The constant 208Pb/206Pb values of clinopyroxene crystallized at ca. 500–900 MPa suggest no contamination with lower continental crust. The lack of covariation between Hf and O isotopes from the same grains, and the lack of relationship between Hf isotopes and trace elements (e.g., Hf, Th/U, and Yb/Gd) in the magmatic zircons, together with the absence of ancient zircon xenocrysts, imply limited upper crustal contamination. In combination with high-whole-rock Th/La (>0.29) ratios, we interpret the zircon Hf isotope heterogeneity as inherited from a depleted asthenospheric mantle with the addition of 1–4% Hf from isotopically heterogeneous sediments. Our study therefore emphasizes the need for caution when using complex Hf isotopic zonation in zircon as an argument for intracrustal hybridization of two end-member magmas derived from distinct reservoirs. In addition, the high-Zr/Y ratios and no negative Zr-Hf anomalies of the Aruo intrusions imply a high surface temperature of the down going slab that was able to fully dissolve zircons in the subducted sediments. This requires a special geodynamic condition that was most likely related to the steepening of flatly subducted Neo-Tethyan lithosphere at ca. 120 Ma according to a synthesis of regional tectonic-magmatic-sedimentary records. [ABSTRACT FROM AUTHOR]

The timing of raising the northern Lhasa crust to its present-day value remains controversial. Here, we address this issue through the geochemical makeup of Eocene (ca. 37 Ma; zircon U Pb dating) volcanic rocks erupted in the Gerze Region, northwestern Lhasa Terrane. The Gerze volcanic rocks (GVRs) possess uniform whole-rock (87Sr/86Sr) i (0.7045–0.7059) and ε Nd (t) (0.78–1.50), and zircon ε Hf (t) (5.25–9.83) and δ18O (6.74‰–7.81‰) values. However, based on the differences in major and trace elements, the GVRs are classified into two groups. The Group I rocks show adakitic affinities, as evidenced by their high SiO 2 (67.62–70.66 wt%), Sr (399–519 ppm), low Y (8.2–10.8 ppm), and Yb (0.78–1.15 ppm) concentrations, and absence of negative Eu anomalies. In conjunction with the intra-block setting of the study area during the Eocene, the lack of coeval mafic rocks, and the low Mg# (<41) and compatible element concentrations (e.g., Ni <35 ppm), we propose that the Group I rocks were formed by partially molten thickened lower continental crust with garnet and minor amphibole in the melting residue. The Group II rocks, in comparison, have higher SiO 2 (70.08–77.48 wt%), lower Sr (155–244 ppm) values, and distinctly negative Eu anomalies, which, coupled with their similar Sr-Nd-Hf-O isotopic makeup with that of the Group I rocks, suggest they were most likely evolved from the pristine lower crust-derived magmas through removal of feldspar, biotite, and accessory minerals. Quantitative estimates of the paleo-crustal thickness (70.2 ± 12.7 km) and elevation (5.7 ± 0.6 km) through the whole-rock (La/Yb) N and Sr/Y ratios suggest that the northern Lhasa Terrane, at least part of its western segment, had achieved its present-day thickness and elevation by the Mid-Eocene, as a result of horizontally tectonic shortening of continental crust according to a synthesis of our new and the regionally tectonmagamtic data. • Cenozoic adakitic rocks are identified in northern Lhasa Terrane. • The northern Lhasa Terrane might have reached the present-day thickness and elevation by the Eocene. • Crustal shortening was the main mechanism of raising the central Tibet. [ABSTRACT FROM AUTHOR]