Late Triassic Granites From the Quxu Batholith Shedding a New Light on the Evolution of the Gangdese Belt in Southern Tibet

The Gangdese magmatic belt formed during Late Triassic to Neogene in the southernmost Lhasa terrane of the Tibetan plateau. It is interpreted as a major component of a continental margin related to the northward subduction of the Neo-Tethys oceanic slab beneath Eurasia and it is the key in understanding the tectonic framework of southern Tibet prior to the India-Eurasia collision. It is widely accepted that northward subduction of the Neo-Tethys oceanic crust formed the Gangdese magmatic belt, but the occurrence of Late Triassic magmatism and the detailed tectonic evolution of southern Tibet are still debated. This work presents new zircon U-Pb-Hf isotope data and whole-rock geochemical compositions of a mylonitic granite pluton in the central Gangdese belt, southern Tibet. Zircon U-Pb dating from two representative samples yields consistent ages of 225.3~=1.8 Ma and 229.9~1.5 Ma, respectively, indicating that the granite pluton was formed during the early phase of Late Triassic instead of Early Eocene （47-52 Ma） as previously suggested. Geochemically, the mylonitic granite pluton has a sub-alkaline composition and low-medium K calc-alkaline affinities and it can be defined as an I-type granite with metaluminous features （A/CNK〈I.1）. The analyzed samples are characterized by strong enrichments of LREE and pronounced depletions of Nb, Ta and Ti, suggesting that the granite was generated in an island-arc setting. However, the use of tectonic discrimination diagrams indicates a continental arc setting. Zircon Lu-Hf isotopes indicate that the granite has highly positive till（t） values ranging from ＋13.91 to ＋15.54 （mean value ＋14.79）, reflecting the input of depleted mantle material during its magmatic evolution, consistent with Mg# numbers. Additionally, the studied samples also reveal relatively young Hf two-stage model ages ranging from 238 Ma to 342 Ma （mean value 292 Ma）, suggesting that the pluton was derived from partial melting of juvenile crust. Geochemical discrimination diagrams also suggest that the granite was derived from partial melting of the mafic lower crust. Taking into account both the spatial and temporal distribution of the mylonitic granite, its geochemical fingerprints as well as previous studies, we propose that the northward subduction of the Neo-Tethys oceanic slab beneath the Lhasa terrane had already commenced in Late Triassic （-230 Ma）, and that the Late Triassic magmatic events were formed in an active continental margin that subsequently evolved into the numerous sub- terranes, paleo-island-arcs and multiple collision phases that form the present southern Tibet.

Petrogenesis and Tectonic Implications of the Latest Cretaceous Intrusive Rocks from the Eastern Gangdese Belt,Southeast Tibet

The latest Cretaceous magmatic activity in the eastern segment of the Lhasa terrane provides important insights for tracking the magma source and geodynamic setting of the eastern Gangdese batholith,eastward of eastern Himalayan Syntaxis.Detailed petrological,geochemical and geochronological studies of the intrusive rocks(monzodiorites and granodiorites)of the eastern Gangdese batholith are presented with monzodiorites and granodiorites giving zircon U-Pb crystallization dates of 70-66 Ma and 71-66 Ma withεHf(t)values of−4.8 to+6.2 and−1.9 to+5.3,respectively.These rocks are metaluminous to weakly peraluminous I-type granites showing geochemically arc-related features of enrichment in LREEs and some LILEs,e.g.,Rb,Th,and U,and depletion in HREEs and some HFSEs,e.g.,Nb,Ta,and Ti.The rocks are interpreted to be derived from partial melting of mantle material and juvenile crust,respectively,which are proposed to be triggered by Neo-Tethyan slab rollback during northward subduction,with both experiencing ancient crustal contamination.The studied intrusive rocks formed in a transitional geodynamic setting caused by Neo-Tethyan oceanic flat subduction to slab rollback beneath the eastern Gangdese belt during the latest Cretaceous.

Mantle Input to the Crust in Southern Gangdese, Tibet, during the Cenozoic: Zircon Hf Isotopic Evidence

The Quxu （曲水） complex is a typical intrusive among the Gangdese batholiths. Two sets of samples collected from the Mianjiang （棉将） and Niedang （聂当） villages in Quxu County, including gabbro, mafic micro-enclaves （MME）, and granodiorites in each set, were well dated in a previous SHRIMP zircon U-Pb analysis （47-51 Ma）. In this article, the same zircons of the 6 samples were applied for LA ICP-MS Hf isotopic analysis. The total of 6 samples yields 176Hf/177Hf ratio ranging from 0.282 921 to 0.283 159, corresponding to εHf（t） values of 6.3-14.7. Their Hf depleted-mantle modal ages （TDM） are in the range of 137-555 Ma, and the zircon Hf isotope crustal model ages （TDMC） range from 178 to 718 Ma. The mantle-like high and positive Era（t） values in these samples suggest a mantledominated input of the juvenile source regions from which the batholith originated. The large variations in εHf（t） values, up to 5-ε unit among zircons within a single rock and up to 15-ε unit among zircons from the 6 samples, further suggest the presence of a magma mixing event during the time of magma generation. We suggest that the crustal end-member involved in the magma mixing is likely from the ancient basement within the Lhasa terrane itself. The zircon Hf isotopic compositions further suggest that magma mixing and magma underplating at about 50 Ma may have played an important role in creating the crust of the southern Tibetan plateau.