The Magmatic Evolution Of The Hezhong Cenozoic Ultrapotassic Lavas In The Western Yunnan Province

The western Yunnan region is located at the western margin of the Yangtze Plate.There are extensive occurrence of Cenozoic ultrapotassic magmatic rocks adjacent to the Ailaoshan-Red River fault.This paper studied the mineralogical,petrological,geochemical characteristics of the shoshonite rocks dominated by high-K trachytic basalts and latite outcropped in the Hezhong village.Combined the Fe-Mg diffusion modelling in olivine,magmatic evolution and petrogenesis had been discussed.Moreover,the crystal size distribution?CSD?indicates significant magma mixing during the formation of the sample.The cores of olivine phenocrysts have high Fo values?0.89-0.93?and NiO contents?0.2-0.5 wt%?,and low CaO values?mostly less than 0.15 wt%?),But its high degree of automorphic crystal and the absence of torsional bands,as well as the inclusion of spinel and molten inclusions strongly indicate that they are crystallized from magma rather than trapped from mantle.According to the chemical composition,the cpx can be divided into three types:pyroxene without ring structure,pyroxene with anti-ring structure and pyroxene with shock ring structure.The core of undeveloped zonal pyroxene was significantly higher in Mg^#?0.86⁰.88?and lower in Al₂O₃?0.84%².67%?and TiO₂?0.15%⁰.81%?.The core of pyroxenite in the anticyclic zone is relatively low(Mg^#?0.77⁰.84?,and the core of mantle and eutectic zone is similar and has a relatively narrow range of components,with high Mg^#?0.86⁰.90?,low TiO₂?0.15%⁰.39%?,and Al₂O₃?0.70%².32%?,indicating that there is a process of basic magma supplementation and magma mixing in the magma reservoir.The pyroxene composition of the oscillating ring structure changes frequently between the positive and negative bands,which may indicate that this process occurs many times.Moreover,the value of Mg^#of the undeveloped ring zone remained basically unchanged,which also indicated that the magma had been in a stable state for a long time and extensively formed crystalline,which dissociated when the magma supplemented and mixed,and then formed fragments of crystallina ejected from the surface.According to clinopyroxene-melting temperature and pressure calculation result,western Yunnan area there are three different depth of magma reservoirs,deep in the earth's crust?about 29km?have taken place in the magma chamber of the partial melting of the rock,through both the band structure at home and abroad of green nuclear pyroxene composition contrast,the author speculates that the deep magma reservoir olivine crystallize low calcium green-core pyroxene which is from the magma of low calcium and high-Mg.The supplementary magma is roughened magma with high calcium and low magnesium.In the middle layer?about 18km?of the crust and the shallow?about 8km?open magma chamber,the two are mixed as trachyte-andesite basaltic magma.