Geochemical Characteristics And Magmatic Evolution Of Liuhe Cenozoic Alkali-rich Intrusive Rocks In Western Yunnan

The Sanjiang region in western Yunnan has experienced a long and complicated geological evolution history.The Jinshajiang-Ailaoshan fault zone,as an important part of the Sanjiang tectonic belt,has been a research hotspot for scholars at home and abroad.The Cenozoic alkali-rich igneous rock belt in Jinshajiang-Ailaoshan zone,with many large-or super-large gold and copper deposits,shows a north-west whole trend.The Liuhe alkali-rich intrusive rocks in the Yunnan Heqing area belong to the Beiya-Liuhe group,and main includes syenite porphyry,monzonite porphyry,granite porphyry and so on.The syenite porphyry has attracted much attention because of its large number of dark and deep-source enclaves.The diabase and lamprophyre dykes associated or symbiotic with syenite porphyry found in the Liuhe area,this article provides another strong evidence,except for enclaves,for studying the formation mechanisms and evolution processes of Cenozoic alkali-rich magmas in western Yunnan.The four alkali-rich intrusive rocks in Liuhe are studied as the research focus,through comparative studies in petrography,mineralogy and isotope geochemistry,combined with in-situ Sr isotope analysis of single minerals,zircon U-Pb dating and oxygen fugacity estimation,this article further discussed the cause of Liuhe alkali-rich intrusive rocks,analyzed and speculated that the formation and migration evolution process of the corresponding alkali-rich magmas.The study show that:（1）The alkali-rich intrusive rocks in Liuhe area are divided into four types:syenite porphyry,granite,lamprophyre and diabase,of which granite exists in syenite porphyry in the form of enclaves,lamprophyre and diabase are in the form of independent small dykes.Based on the analysis of the major and trace elements in rocks,the syenite porphyry,lamprophyre and diabase are of the potassic rock series,granite mainly belongs to the high-K calc-alkaline series.Syenite porphyry and lamprophyre are rich in potassium and low in sodium,while granite enclaves and diabase are rich in sodium and low in potassium.Lamprophyre and diabase have similar alkaline basalt characteristics and are different from other basic dykes in western Yunnan.Liuhe lamprophyre is also different from other calc-alkaline lamprophyre in western Yunnan.These rocks have uniform LREE enrichment with no or weakly negative Eu anomalies.The T-N-T anomaly feature shows that the source had the characteristics of an ancient island arc and was subjected to fluid metasomatism.This feature is consistent with the overall characteristics of the Cenozoic alkali-rich porphyry in western Yunnan.（2）The zircon U-Pb dating study shows that the zircon age of the Liuhe syenite porphyry is 34.3±1.3Ma～38.4±1.2Ma（the peak value is 36.52±0.74Ma）,and the zircon age of granite enclaves is 35.81±0.48 Ma～38.32±0.38Ma（the peak value is 37.21±0.67Ma）.There is no obvious precedence relationship between them.The zircon ages of associated lamprophyre and diabase are relatively young,ranging from 35.15±0.96Ma to 36.20±0.56（the peak value is 35.66±0.67Ma）and 33.16±1.1Ma to 37.8±2.1Ma（the peak value is 35.63±0.84Ma）,respectively.（3）The zircon Ti thermometer and Ce^Ⅳ/Ce^Ⅲratio analysis show that the mutations of temperature and oxygen fugacity occurred during the diagenesis process,indicating that different types of alkali-rich magmas were mixed with each other,it reflect the supplement of mantle-derived magma in the later period.The diopside phenocrysts in the syenite porphyry develop a positive zone structure,with an obvious evolution difference from the core（diopside）to the side（hedenbergite）.The zircon Ti temperature oscillation increased and the oxygen fugacity oscillation decreased.The in-situ Sr isotope analysis of pyroxene phenocrysts found that there were coexisting positive and reverse zone structure.They all indicate that the formation of syenite porphyry underwent magmatic evolution processes of crust-derived material mixing and mantle-derived magma supplementation.（4）The isotopic composition of alkali-rich intrusive rocks is similar to that of the main rocks in western Yunnan,indicating that the alkali-rich magma originated from the heterogeneous remelting of the metasomatic enriched mantle and related crust-mantle magma mixing and assimilation.Liuhe lamprophyre and diabase have similar(⁸⁷Sr/⁸⁶Sr)i values（0.70661 and 0.70678,respectively）,which not only shows that their sources are almost the same,but also indicates that the diagenetic magma of lamprophyre directly originates from the melting of the fluid-rich part in the metasomatic enriched mantle,while the diagenetic magma of diabase comes from the melting and uprise of the relatively barren fluid enriched mantle.The deep magma from the enriched mantle continues to undergo large-scale underplating,triggering crust-mantle mixing and assimilation,which is the reason for the formation of initial magma of syenite porphyry and granite（0.70767 and 0.70765,respectively）with relatively high(⁸⁷Sr/⁸⁶Sr)i value.The granite with low(⁸⁷Sr/⁸⁶Sr)i value（0.70650）may be related to the magma chamber with low(⁸⁷Sr/⁸⁶Sr)i value,which formed by the heterogeneity mantle end-member underplating of the crust-mantle mixed magma chamber.However,the late diagenetic ages of lamprophyre and diabase do not indicate that the deep magmatic activity is lagging,which just reflects that the Cenozoic mantle-derived magmatic activity in western Yunnan has a longer duration.Based on the above analysis,the Liuhe four types of alkali-rich intrusive rocks have the common characteristics,evolution relationships,magma sources and processes in terms of spatial location,geochemical characteristics,diagenetic age and other aspects,which reflect the upwelling of Cenozoic enriched mantle controlled by crust-mantle dynamics caused by India-Eurasia plate collision mechanisms.The delamination caused by subduction and collision and the strike-slip at the mid-shallow structural level provide the necessary conditions for the formation and uplift of mantle-derived magma,which further show the complex evolution of the Jinshajiang-Ailaoshan alkali-rich intrusive rock belt.