Detrital zircon U–Pb and Hf isotopic data from the Xigaze fore-arc basin: Constraints on Transhimalayan magmatic evolution in southern Tibet

Before the continental collision of India with Asia, northward subduction of the Tethyan oceanic lithosphere beneath South Asia resulted in widespread arc magmatism in the Lhasa terrane of southern Tibet from Early Jurassic to Eocene time. The detailed magmatic history of this Transhimalayan arc system, however, remains unclear. Here we report in- situ detrital zircon U–Pb and Hf isotopic analyses of sedimentary rocks from the Xigaze fore-arc basin, which developed along the southern margin of the Lhasa terrane. The zircon U–Pb results allow us to better constrain the depositional age of the Xigaze Group, which consists from bottom to top of the Chongdui, Ngamring, Padana and Qubeiya formations deposited between ca. 116 and 65 Ma, with the main stage (i.e., the Ngamring Formation) being deposited between ca. 107 and 84 Ma. The majority of these zircons are characterized by high 176Hf/ 177Hf isotopic ratios and positive ε Hf( t) values that are similar to those of magmatic zircons from the Gangdese batholith, suggesting the latter was a predominant source provenance of the fore-arc sediments. In the younger sequences, i.e., the upper Ngamring and Padana/Qubeiya formations, zircons that record either pre-Mesozoic U–Pb ages or negative ε Hf( t) values become more abundant, implying additional sources from the northern Lhasa and/or Qiangtang terranes due to northward development of the fluvial systems. The overall zircon U–Pb and Hf isotopic data furthermore suggest that during fore-arc deposition, the exposed Gangdese arc was dominated by igneous rocks formed between 130 and 80 Ma, associated with lesser amounts zircon with ages between 190 and 150 Ma, thus recording two important stages of arc magmatism that are only sporadically preserved in modern outcrops as a result of extensive erosion related to uplift during Mesozoic–Cenozoic time.