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The mid-Cretaceous Langshan Formation, which crops out on the north Lhasa subterrane, central Tibet, is composed of a thick carbonate sequence with local volcanic rock and sandstone interlayers. In this study, we present data from the Asuo area, including fossil assemblages within carbonates, detrital zircon U–Pb ages of sandstone interlayers, zircon U–Pb ages and whole-rock geochemistry of andesite interlayers within the Langshan Formation to provide new constraints on the tectonic evolution of north Lhasa subterrane. The fossil assemblage indicates that the Langshan Formation was deposited on a carbonate platform at water depths of approximately 0–50 m during the late Albian to early Cenomanian period. The volcanic rock interlayers (111–109 Ma) primarily consist of high-K calc-alkaline andesites/dacites (SiO2: 56.48–67.03 wt. %; Mg#: 42.5–58.9). These andesites exhibit limited evidence of significant modification by crustal contamination. The high Mg# values and arc-like trace element signatures suggest that the andesites may have resulted from partial melting of the mantle wedge induced by fluids derived from a subducted slab. Furthermore, the lithic fragments within the sandstone interlayers predominantly exhibit characteristics of "recycled orogen" and "arc orogen," indicating that the sandstone was derived from recycled ancient strata and a few magmatic arc materials. Considering the regional paleogeographic context, the carbonate accumulation of the Langshan Formation appears to have occurred nearly simultaneously with a period of high-flux magmatism, long-term slow tectonic subsidence, and the closure of the Meso-Tethys Ocean. The arc–arc "soft" collision provides a plausible mechanism to explain the widespread and rapid accumulation of thick carbonates in collision settings. It also accounts for the presence of magmatic rocks with enhanced mantle contributions on both sides of the north Lhasa subterrane. [ABSTRACT FROM AUTHOR]

The timing and mechanisms of collapse of the middle and western segments of the Lhasa–Qiangtang orogenic belt in central Tibet are poorly constrained. Here, we report whole-rock geochemical, and zircon U–Pb age and Hf-isotopic data for the Asa-intrusive rocks located at the southern edge of the northern Lhasa subterrane. The Asa-intrusive rocks include the Namujeler granite porphyry (NGP), the Gernicamdro granodiorite porphyry (GGDP), and the Neeze granodiorite porphyry (NGDP). The NGP and GGDP have adakitic geochemical characteristics, such as high Sr (317–511 ppm), Sr/Y (57.8–96.8), and (La/Yb)N (13.1–16.0), and low Y (4.68–5.49 ppm) and heavy rare-earth element contents (e.g., 0.44 ≤ Yb ≤ 0.57 ppm). In situ zircon U–Pb dating of three samples yielded Late Cretaceous ages (NGP = 88.7 Ma; GGDP = 89.7 Ma; NGDP = 90.1 Ma). Zircon εHf(t) values vary over a wide range (NGP = − 11.5 to + 9.3; GGDP = + 4.6 to + 7.6; NGDP = − 21.2 to + 7.6). Our results suggest that the Asa adakitic rocks (NGP and GGDP) are most likely generated by partial melting of thickened mafic lower crust under a garnet-bearing amphibolite facies. The presence of the Asa adakitic rocks indicates that the crust beneath the Lhasa–Qiangtang collision zone had experienced thickening and the crustal thickness remains quite large (> 40 km) at ca.90 Ma. On the basis of evidence from the Asa-intrusive suite and coeval igneous rocks, along with some stratigraphic and tectonic constraints, we proposed that the middle and western segments of the Lhasa–Qiangtang orogenic belt was collapsed by lithospheric delamination during the early Late Cretaceous (ca. 94–82 Ma), and the thickened lithospheric keel did not delaminate as a wholly, it delaminated piece by piece. From east to west, the time of the lithospheric delamination is getting younger. [ABSTRACT FROM AUTHOR]