Study On Characteristics Of Melt Inclusions In The Alkali-rich Porphyries Of Xiaoqiaotou In West Yunnan

The Sanjiang area of western Yunnan experienced a long geological tectonic evolution history.Since the closure of the Paleo-Tethys Ocean,a series of magmatic activities had been developed,which were caused by ocean basin closure,intra-continental orogeny and strike-slip extension.In particular,it is famous for the Cenozoic alkali-rich igneous rocks,deep-derived xenoliths and related magmatic-hydrothermal deposits produced along the Jinshajiang-Ailaoshan paleo-suture zone and nearby areas.The Xiaoqiaotou rock mass is located in the Shitou township in the northwest of the southern section of the paleo-suture zone,belonging to the Jianchuan-Shitou rock group.In this study,the H₂O-rich and CO₂containing crystalline melt inclusions along growth zoning are found in quartz phenocrysts of the Xiaoqiaotou rock mass,which are the key research objects.In combination with petrology,mineralogy,elements and isotopical geochemistry,and zircon U-Pb dating,this paper deeply analyzes and discusses the origin,migration,and evolution of the Cenozoic alkali-rich magmas in western Yunnan,typical of the Xiaoqiaotou rock mass.The conclusions and understanding will help reveal the deep geological processes and mantle fluids action in the formation and evolution of the Cenozoic alkali-rich igneous belt in the Jinshajiang-Ailaoshan area of western Yunnan,and provide important magmatic inclusions geochemistry evidences for the Cenozoic magmatic activity and tectonic setting in the whole Sanjiang area.The main knowledge gained in this paper is as follows:?1?The Xiaoqiaotou alkali-rich porphyry mainly consists of quartz monzonite porphyry,which intrusive into the red sandstone of Oligocene in the form of stocks and apophyses.There are various types of inclusions?or xenoliths?in the rock mass,including dark and deep-derived inclusions.Alkaline metasomatism and silicification are widely developed in both host rock and xenoliths.The petrogeochemistry characteristics indicate that the Cenozoic alkali-rich magma sources in different regions,ages and lithologies in western Yunnan are of the same nature,and the magmatic evolution process is similar;the formation of source area is constrained by the unified deep geological process.?2?The study of petrography shows that the magmatic inclusions are mainly deposited in quartz phenocrysts,which are crystalline melt inclusions.The crystalline melt inclusions are identified by Raman spectroscopy as albite and OH-unknown minerals,and there is independent H₂O in partial melt inclusions.The melt inclusions can coexist with the CO₂-rich fluid inclusions,and there are primary high salinity fluid inclusions in the outer growth zoning of quartz phenocrysts.It indicates that the Xiaoqiaotou alkali-rich porphyry magma is rich in H₂O.?3?The melt inclusions Microthermometry research of quartz phenocrysts has shown that there is a homogenization temperature difference of inclusions in different growth zoning,the core of phenocrysts is 1060¹250?,concentrated on1125¹150?;The central is 960¹060?,and is concentrated on 1025¹060?;The rim is 1040¹060?.The homogenization temperature difference on the same growth zoning is related to the immiscibility of melt-fluid,the explosion of inclusions,main crystal melting and the diffusion of H.The higher homogenization temperature in the rim of phenocryst indicates the existence of magma mixing.The bubble disappeared temperature of high salinity fluid inclusions in phenocryst outer growth zoning is158⁶50?,and the halite daughter crystal melting temperature is 291⁵70?.According to the Tm>Th of fluid inclusions,estimated pressure is 270MPa?depth of9.45km?,and from the isothermal pressure diagram of the whole rock CIPW standardized of quartz-albite-orthoclase,estimated pressure of alkali-rich porphyry magma is greater than 1000MPa?depth is greater than 35Km?.All of these characteristics together reflect the decompression and pressure fluctuation in the process of magmatic migration,and which also causes the immiscibility of melt-fluid.?4?The insitu EMPA and LA-ICP-MS analysis of crystalline melt inclusions show that in the core of phenocryst,SiO₂ content is 64.34%⁷9.96%?mean 70.71%?,the total alkali?Na₂O+K₂O?content is 6.86%¹1.99%?mean 9.69%?,and it is alkali-rich,the K₂O/Na₂O is 0.65³.62?mean 1.43?.In the central to rim of the phenocryst,SiO₂ content is 65.10%⁷1.01%?mean 67.39%?,the total alkali?Na₂O+K₂O?content is 11.66%¹4.14%?mean 12.83%?and the K₂O/Na₂O is 1.35⁴.23?mean 2.58?.The inclusions in different growth zoning have similar characteristics in high LILE?such as Cs,Rb?and depleted transition elements?such as Co,Cr,Ni?.This implies that the Xiaoqiaotou alkali-rich porphyry magma has experienced at least two major melt action processes,the component of the early melt is unorganized and the late melt has obvious component evolution relationship.At present,it is believed that the initial magma is a silicon-rich magma with low content of alkali.The silicon alkali content differences of different inclusions in the same growth zoning implies that the Xiaoqiaotou alkali-rich porphyry magma source area is not homogeneous or uniform,and it is a immiscible magma that is coupled by the molten magmas with different geological components,the magma chamber itself is also"non-uniform"or there are multiple magma chamber.?5?Comprehensive study indicates that the Xiaoqiaotou alkali-rich porphyry is an important part of the Jianchuan-Ailaoshan Cenozoic alkali-rich porphyry belt of Sanjiang area in western Yunnan,the diagenetic age is 35.40±0.37 Ma and the magmatic activity lasted 1.5Ma.The diagenetic age is synchronized with the Cenozoic alkali-rich magma in the Sanjiang area of western Yunnan,and its formation is constrained by uniform homogenous metasomatism and enriched mantle fluids.There exists a widespread magma mixing in the process of formation and evolution of alkali-rich magma,and it is synchronized with the immiscibility of melt and fluid.The main factor that keeps the immiscibility continuous is the decrease of pressure and the rapid rise of magma.The advance of mantle fluid activities has inoculated the alkali-rich magma source region.Then the hot mantle source region material partial melting under the background of extension and strike-slip generates the alkali-rich magma.Because of the differences of source region lithology,metasomatism degree and partial melting degree and so on,on the basis of multiple magma chamber,alkali-rich magma and immiscible mantle melt-fluids are involved in the formation of a series of alkali-rich porphyries with different lithology combinations in the process of coupling and decoupling with magmatic systems and crust-mantle mixing.