Search

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]

Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve understanding of plate tectonic processes and the history of continental growth. Several studies of the intra-oceanic subduction of the Meso-Tethys Ocean have been published in recent years, particularly relating to the Bangong–Nujiang suture zone (BNSZ) and the Shiquanhe–Nam Tso mélange zone (SNMZ) in central Tibet. Here, we report newly discovered occurrences of Jurassic diorite (161.2–160.6 Ma) and plagiogranite (161.0 Ma) from the western part of the central SNMZ. The diorite samples have moderate SiO 2 and high MgO contents and Mg# values; high Ba/Th and Ba/La ratios; low K 2 O, TiO 2 , and REE contents; low (La/Yb) N , (La/Sm) N , and Th/Yb ratios; and positive ε Hf (t) values, similar to high-Mg diorite formed by partial melting of a subduction-modified mantle wedge. The plagiogranite intrudes gabbro of the ophiolite suite and has high SiO 2 and Na 2 O contents; low Al 2 O 3 , K 2 O, P 2 O 5 , TiO 2 , and total REE contents; and clear negative Eu anomalies, similar to shear-related oceanic plagiogranite, which can be formed by partial melting process of gabbro in the high temperature shear zone near the expansion center. The formation processes of Asa high-Mg diorite and shear-related plagiogranite are consistent with initial subduction within the Meso-Tethys ocean. Based on these data and those for sedimentary and Jurassic magmatic rocks of the region, we propose that, during subduction, the Northern Lhasa terrane was formed from a collage of oceanic island arcs, oceanic plateaus, and accretionary complexes. • Intermediate-acid magmatism in the SNMZ, central Tibet. • High-Mg diorites and shear-type plagiogranite with Late Jurassic ages (161–160 Ma). • The Jurassic intra-Meso-Tethys oceanic subduction event (165–160 Ma). • The generation of the Northern Lhasa terrane by accretion of intra-oceanic arcs. [ABSTRACT FROM AUTHOR]