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