The discovery of Late Mesoproterozoic alkali hornblende-aegirite syenite in E'shan area of central Yunnan and its constraints on Greenwell orogeny

Alkaline rocks are usually formed in the lithosphere stretching environment, which is derived from the partial melting of deep mantle material.As a record of deep geodynamic processes in the earth's crust, it is a good exploration of the deep matter composition, dynamic process and physical and chemical environment of the earth. In the 1:50000 regional geological survey, a syenite rock mass was newly discovered in the Dianzhong area of E'shan County, and its lithology is alkali homblende-aegirite-syenite.Systematic studies show that alkali hornblende-aegirite syenite rich in silicon (SiO₂=67.40%~67.79%) and alkali (Na₂O+K₂O=12.50%~12.73%), depleted in magnesium (MgO=0.10%~0.28%) and K₂O/Na₂O=0.75~0.80, is a set of sodium alkaline rock series (Atlantic type).It is rich in large ion lithophile elements such as K and Rb, and high field strength elements such as Ta, Zr, and Hf, while depleted in high field strength elements such as Ti, P and U as well as large ion lithophile element, such as Sr, Ba.The rare earth distribution curve is also special, the light rare earth element is steeply inclined to the right, (La/Sm) _N=3.61~4.79, and the heavy rare earth element is gently inclined to the left, (Ga/Yb) _N=0.53~0.62.The moderately negative Eu anomaly and weak negative Ce anomaly (δEu=0.41~0.43, δCe=0.74~0.88) suggest that the magma may be formed in a higher oxygen fugacity environment and generally has the characteristics of depleted middle rare earth elements.LA-ICP-MS zircon U-Pb age is 1007±10 Ma (MSWD=2.3, n=21), which belongs to the end of the Middle Proterozoic.It is the earliest alkaline pluton formed on the Yangtze plate, which defines the time limit for the end of the Greenville Orogenic Movement in Central Yunnan Province.On the Th/Hf-Ta/Hf diagram, the samples fell into the "Mantle Hot Pillar Basalt" area, suggesting that the pluton might be the cause of the mantle plume, indicating that the magma is basically not contaminated by the continental crust, and the pluton might be the product of low partial melting of lower mantle garnet peridotite.During magma rise, it might be assimilated and mixed with the upper mantle spinel peridotite, and the content of heavy rare earth elements increases rapidly.The characteristics of depleted middle rare earth elements and enriched Nb compositions suggest that the mantle spinel peridotite might have experienced the metasomatism of subduction fluid and moderate partial melting.The alkali hornblende-aegirite syenite found here is the product of the conversion from Greenville Orogenic compression to extension.