Lithos Petrology And Geochronology Of Granites From The Chongshan Metamorphic Belt In Simao Block,Western Yunnan

Paleotethys Ocean began to open since the early Carboniferous,and developed the Jinshajiang-Ailaoshan Ocean and the Changning-Menglian Ocean.The NeoTethys Ocean closed in the Cenozoic and began a collision.Then the Indian plate subducted into the Asian plate,which had a profound impact on the geomorphology and tectonic and mineralization of the Asian continent.In this paper,the granites in the Chongshan metamorphic belt of the Simao block in western Yunnan are studied,and combined with previous research,petrology,whole-rock geochemistry,zircon U-Pb isotopic chronology and Hf,Nd isotopes to clarify the magma stage and lithology combination of the area,and to find its genesis and structural indications.Zircon U-Pb geochronology studies show that the granites in the Chongshan metamorphic belt are mainly formed by the Mesozoic Indosinian?233-221 Ma?and the Cenozoic?²1 Ma?.Indosinian rock combination is quartz diorite,monzonitic granite and granodiorite,and the Cenozoic rock combination is quartz diorite and monzonitic granite.The Mesozoic granites in the Chongshan metamorphic belt are Na₂O?3.62-4.73%?and K₂O?0.59-6.50%?,and the K₂O/Na₂O ratio?0.89-4.55?is also relatively high,CaO?1.03-5.63%?,Fe₂O₃^T?0.80-6.97%?,MgO?0.18-2.99%?,TiO₂?0.09-1.01%?and P₂O₅?0.01-0.11%?are relatively low in content.A/CNK is0.78-1.04,which is quasi-aluminum and weakly aluminous.Ba,Nb,Ta are depleted,relatively enriched with light rare earth elements,LREE/HREE values of 1.79-13.46;?La/Yb?_N ratio is 3.83-17.60,with an average of 9.27,reflecting significant fractionation of light and heavy rare earth elements,?Hf?t?values of 0-12.6,the T^C_DMM values of 544-1294Ma,the initial Sr ratio?87Sr/86Sr?_i values of 0.70802-0.71626, and the???Nd?t?values of-10.53⁰.95 is characterized by the fact that the mantle is involved in the crust.The Cenozoic granites have high Na₂O?1.50-2.95%?,K₂O?2.65-6.85%?and K₂O/Na₂O ratio?0.90-4.55?are relatively high,and CaO ?2.12-3.41%?is medium.The contents of Fe₂O₃^T?0.66-6.32%?,MgO?0.27-2.15%?,TiO₂?0.15-0.76%?and P₂O₅?0.07-0.12%?are relatively low.A/CNK is 1.05-1.21 and belongs to aluminum.Ba,Nb,Ta are depleted,relatively rich in light rare earth elements,and depleted heavy rare earths??La/Yb?_N>9.17?.LREE/HREE values of1.79-13.46,which reflects the significant fractionation of light and heavy rare earth elements.The?Hf?t?values of-7.5-5.2,the T^C_DM values of 849-1596Ma,and the initial Sr ratio?87Sr/86Sr?_i values of 0.71523-0.71626,???Nd?t?values of-10.5^-4.7 is the result of fluxed melting of muscovite.In combination with the previous research results and the analysis of Hf and Nd isotopes,the Jinshajiang-Ailaoshan Ocean experienced continental rift,mid-ocean ridge expansion,continental collision and the stage of the post-collision,so we believe that the Jinshajiang-Ailaoshan ocean is expanding in the form of ocean.The Chongshan metamorphic belt Cenozoic granite is similar to the leucogranites which is caused by fluxed melting of muscovite in the Qinghai-Tibet Plateau.According to the age of the melt by the anatexis effect accompanying the strike-slip process,we suggest that the age of the slipping of the Chongshan metamorphic belt was earlier than 21 Ma.