Constraining the deep dynamic process beneath the Bangong-Nujiang suture zone: A case study from the early cretaceous trachytic rocks

The geodynamic processes that occurred beneath the Bangong - Nujiang suture zone during the evolution of the Indus - Yarlung Neo-Tethys and Bangong - Nujiang Meso-Tethys are unclear. We conducted a study of Early Cretaceous andesitic rocks from the Nyima area, north - central Tibet, to investigate these processes. The studied rocks have zircon U - Pb ages of 110-104 Ma and negative to positive epsilon(Hf)(t) values (-12.1 to+10.3). These rocks have affinities with trachytes, and have high total alkali contents and Na2O/K2O ratios (5-125). They also have high Sr (281-1409 ppm), Y (21.0-28.5 ppm), and Yb (2.00-2.65 ppm) contents, and exhibit depletions in Nb, Ta, and Ti. Such features indicate these rocks are not adakites, but are similar to typical arc rocks. The geochemical and isotopic data suggest the studied rocks were formed by partial melting of amphibolite-facies oceanic lithosphere, with the addition of enriched sedimentary materials. Based on the regional crustal evolution, we suggest that the Nyima andesitic rocks formed by melting of Bangong - Nujiang oceanic lithosphere. As such, the Early Cretaceous igneous rocks along the central Bangong - Nujiang suture zone were not associated with northward subduction in the Indus - Yarlung Neo-Tethys.