Geochemical characteristics, age and tectonic significance of Miocene aegirine-augite syenite on the north slope of the Tanggula Mountains, China.

Abstract: Aegirine-augite syenite in the Adoi area is alkaline intrusive rock distributed on the north slope of the Tanggula Mountains. It contains a great deal of alkaline minerals such as aegirine-augite, leucite and nepheline. The intrusion shows distinct zoning. The leucite-aegirine-augite syenite in the center is typical alkaline rock, while the aegirine-augite syenite-porphyry occurring at edges or as dikes is more acid aegirine-augite syenite. The rock has the K-rich, high-Si and low-Fe and -Ma features and shows LREE-enriched distribution patterns, without Eu anomalies. Aegirine-augite syenite is intruded into the Early-Mid Jurassic Yanshiping Group and overlain by the Pliocene Quguo Formation. Orthoclase and biotite from the biotite aegirine-augite syenite-porphyry yielded Ar39/Ar40 plateau ages of 10.71±0.08 and 10.26±0.16 Ma respectively, indicating a Miocene emplacement age. Analysis of the tectonic environment suggests that the Qinghai-Tibet Plateau was in a period of transition of the tectonic regime at ~10 Ma. These alkaline rocks are the product of collapse of the orogenic belt in the early stage.