Mineral chemistry of clinopyroxene from the Hongshan syenite complex in Wu'an, Hebei Province: Implications for magma evolution

Hongshan syenite complex has fairly abundant clinopyroxene with core-rim (core-mantle-rim) structure. By studying the changes in the composition of clinopyroxene,information on the genesis and evolution of rocks can be obtained. Based on detailed field geological survey,the authors used zircon U-Pb chronology and mineralogical studies to obtain the biotite pyroxene syenite zircon U-Pb dating data of Hongshan syentite complex,with the age being(126.9±1.2)Ma,suggesting a product of the thinning peak period of the North China Craton lithosphere. The Mg^# values of clinopyroxene from pyroxene syenite and biotite pyroxene syenite in the Hongshan syenite complex are 39.4-72.5 and 55.4-81.7,with characteristics of high FeO,Na₂O and CaO content and low Al₂O₃,MgO and TiO₂ content. The clinopyroxene generally had the evolutionary trend of diopside and aegirine,and reached an equilibrium state with the melt. The clinopyroxene had a Fe²⁺ to Mg²⁺ substitution relationship during the initial evolution. With the evolution,the magma became richer in sodium and iron,which suggests that the magma system had the characteristics of high temperature,medium oxygen fugacity and richness in alkali. Combined with the clinopyroxene core-rim (core-mantle-rim),the authors hold that there existed a clear contact relationship and discontinuous chemical composition. It is shown that,after the formation of the Hongshan syenite complex,it also underwent the transformation of sodium-rich and iron-rich fluids,causing the clinopyroxene to form a sodium-rich and iron-rich rim. The fluid may have modified the complex from west to east or from the inside of the Hongshan syenite complex to the outside.