Studies of the Gangdese batholith have revealed its long magmatic history from the Middle Triassic to the Miocene, with peaks in activity at 109-80 Ma and 55-45 Ma. It is widely considered to have been a typical Andean-type convergent continental margin prior to the Paleocene India-Eurasia collision. The Gangdese batholith, which is an important part of the Trans-Himalayan batholith, occurs as a huge plutonic belt (>1500 km) along the southern margin of the Lhasa terrane, which extends from the Kailas in the west to the Namche Barwa in the east. The south Lhasa terrane mainly consists of the juvenile Gangdese magmatic arc, including the Gangdese batholith and Mesozoic-Tertiary volcanic rocks such as the Middle-Lower Jurassic Yeba Formation, the Upper Jurassic-Lower Cretaceous Sangri group, and the Paleocene Linzizong group. The Luobadui-Milashan fault zone in the south and the Shiquan River-Nam Tso ophiolitic mélange belt in the north separate the Lhasa terrane into its southern, central, and northern parts (Figure 1(b)). The Lhasa terrane consists of the locally developed Precambrian metamorphic basement (i.e., the Nyainqentanglha group and Nyingtri group), the Paleozoic to Cenozoic sedimentary strata, and various types of magmatic rocks. There are two possible explanations for this: either forearc sediments did not develop along this section of the arc or they developed but were destroyed during later orogenesis. In contrast, no forearc sediments have been reported in the eastern part of the Gangdese arc (e.g., ). However, only restricted exposures of the forearc sediments have been documented along the huge Gangdese plutonic belt (>1500 km long), that is, mainly outcropping near and to the west of Xigaze. These studies have revealed its tectonic, erosional, and sedimentary evolution and have effectively constrained the magmatic history of the Gangdese arc. Numerous studies of the detrital zircon U-Pb ages and Hf isotopic characteristics of the Xigaze forearc sediments have been conducted. The Xigaze forearc basin, located in the southern margin of the Lhasa terrane, southern Tibet, is one of the best exposed forearc basins in the world (e.g., ). Additionally, the detrital zircons from these forearc sediments indicate that this segment of the Gangdese arc experienced more active and continuous magmatism from the Early Jurassic to Early Cretaceous than its bedrock records indicate.įorearc basins develop along convergent plate margins and receive detritus from the adjacent magmatic arc, and thus, they are important for studying the evolution of arcs. It is possible that the forearc equivalents were eroded or destroyed during the later orogenesis. The overall compositions of the detrital zircon U-Pb ages and Hf isotopes of the middle to upper sequences resemble those of the Xigaze forearc sediments, implying that related forearc sediments may have been developed in the eastern part of the Gangdese arc. In contrast, the upper sequences are characterized by Mesozoic detrital zircons (150–100 Ma) and negative ε Hf t values, indicative of derivation from the central Lhasa terrane. The detrital zircon U-Pb ages and Hf isotope signatures indicate that the middle sequences are Jurassic to Early Cretaceous in age (~200–100 Ma) and show clear affinity with the Gangdese arc rocks, that is, positive ε Hf t values. The middle and upper sequences are predominantly composed of medium- to coarse-grained volcaniclastic/quartzose sandstones, which are generally interbedded with mudstone. The detrital zircon U-Pb age distribution of this sequence is analogous to those of the Carboniferous-Permian strata and metasediments of the Nyingtri group in the Lhasa terrane. The lower sequence is characterized by fine-grained sandstone, interbedded mudstone, and some metamorphic rocks (e.g., gneiss and schist).
The analytical results revealed the existence of three distinct provenances. In this study, a field-based stratigraphic study, detrital zircon U-Pb geochronology (15 samples), and Hf isotopic analyses (11 of the 15 samples) were carried out on four sections in the Milin-Zedong area, southeast Tibet. Whether the eastern segment of the arc had a corresponding forearc basin is yet to be resolved. However, the sediments’ development is restricted to the region around and west of Xigaze City. The Xigaze forearc sediments revealed the part of the tectonomagmatic history of the Gangdese arc that the bedrocks did not record.