Tectonic affinity of the Nyainrong microcontinent: Constraints from zircon U-Pb age and Hf isotopes compositions

Located between Amdo County and Nagqu County, the Nyainrong microcontinent is a EW-trending lenticular body in the middle of the Bangong Co-Nujiang River suture zone. The Nyainrong microcontinent is composed predominately of crystalline basement, Paleozoic strata, and Mesozoic granitoids. To better understand the Neoproterozoic tectono-thermal event in the Nyainrong microcontinent and constrain its tectonic evolution, the authors conducted detailed field observation, precise zircon U-Pb dating and geochemical study of the basement orthogneiss in the Amdo area. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the three gneiss samples indicate that the protolith of the gneisses was intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis show that the zircon has typical characteristics of magmatic zircon. The 206Pb/238U concordant ages of the zircon vary in the range of 820~843Ma, corresponding to the Neoproterozoic, which is the formation age of the protolith. Zircon saturation temperature estimates suggest that the initial temperatures of the magmatic source are 794~856℃, whereas the εHf(t) values are -0.8~0.5, 0.3 on average. The two-stage Hf-isotope model ages (TDM2) range from 1.58 to 1.96Ga. Combined with regional data, the authors hold that the tectonic setting of the Amdo gneiss protolith was closely related to the extensional environment resulting from stretching of the lithosphere. It is preliminarily considered that the Neoproterozoic magmatic events in the microcontinent might have resulted from the breakup of the Rodinia Supercontinent, with the tectonic setting possibly related to the breakup of the Yangtze plate continental margin.