Identification Of Forearc Sediments In The Milin-Zedong Region And Their Constraints On Tectonomagmatic Evolution Of The Gangdese Arc, Southern Tibet

The Xigaze forearc sediments revealed the part of the tectonomagmatic history of the Gangdese arc that the bedrocks did not record. However, the sediments' development is restricted to the region around and west of Xigaze City. Whether the eastern segment of the arc had a corresponding forearc basin is yet to be resolved. 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. The analytical results revealed the existence of three distinct provenances. The lower sequence is characterized by fine-grained sandstone, interbedded mudstone, and some metamorphic rocks (e.g., gneiss and schist). 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 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 ages and Hf isotope signatures indicate that the middle sequences are Jurassic to Early Cretaceous in age (similar to 200-100 Ma) and show clear affinity with the Gangdese arc rocks, that is, positive epsilon(Hf) (t) values. In contrast, the upper sequences are characterized by Mesozoic detrital zircons (150-100 Ma) and negative epsilon(Hf) (t) values, indicative of derivation from the central Lhasa terrane. 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. It is possible that the forearc equivalents were eroded or destroyed during the later orogenesis. 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.