Abstract:

Research of the Late Cretaceous magmatic-tectonic evolution is a focus on the northern and southern margins of the Lhasa terrane. The response to the evolution of the Bangong-Nujiang ocean or Neo-Tethys ocean, and the mantle contribution still lacks precise constraints on the central Lhasa terrane. This study reports the rhyolitic volcanic rock outcrops at Konglong on the southern margin of the central Lhasa subterrane. Results of LA-ICP-MS zircon U-Pb geochronology, whole-rock geochemistry and zircon Hf isotope analyses show that the Konglong rhyolite was formed in the early Late Cretaceous (88.0±1.7 Ma). The rocks have high contents of SiO₂ and Al₂O₃, and low Mg^#, enriched in LREEs and relatively depleted in HREEs. The rocks are distinctly enriched in Rb, Th, and K, but relatively depleted in Ba, Eu, Sr, P, and Ti, which are clearly distinguished from the surrounding adakites. The zircon ε_Hf(t) values range from -10.2 to -5.3, with T_DM2 model age of 1 492-1 804 Ma. This, combining with the average Nb/La (0.21) and Nb/Ta (10.1) values, imply an ancient lower crustal source. The results indicate that the Konglong volcanic rocks were mostly derived directly from ancient lower crustal anatexis without mantle-sourced input or assimilation. The petrogenetic geodynamics may have been the north-dipping Neotethys subduction and the heating from lower crustal delamination in the north. This study provides new constraints on the evolution of the Lhasa terrane prior to the India-Asia continent-continent collision.

Key words: rhyolitic volcanic rock, crustal anatexis, Late Cretaceous, Lhasa terrane, Tibet