The Geochronology,Geochemistry Characteristics And Petrogenesis Of The Alkali-rich Porphyry In Yao'an,West Yunnan Province

Yao'an is located in the western margin of Yangtze craton,mid-west of the Chuxiong basin in Jinshajiang-Ailaoshan fault zone.There is a suite of alkali-rich porphyry,that with complex lithological combination,and an Pb-Ag deposit associated with the potassic magmatism has developed by the effect of the Cenozoic collision of India-Asian continent in the region.It's an important part of alkali-rich porphyry metallogenic belt in western Yunnan.In this paper,we discussed the formation age,source composition and petrogenesis of the alkali-rich porphyry of Yao'an Pb-Ag deposit by the analysis research of petrology,petrography,geochemistry,zircon U-Pb isotope and Hf isotope,whole rock Sr,Nd-Pb isotope.Several conclusions were drawn as below:?1?The results of U-Pb isotope analysis showed that the alkali-rich porphyry of Yao'an is the products of the magmatism in the post-collision of India-Asian continent,the period of the magmatism is about 31³5 Ma,the trachyte and Leucite porphyry were developed throughout the period,and orthophyre developed relatively late,mainly in 31³3 Ma..?2?The analysis result of major elements suggests the alkali-rich porphyry of Yao'an is belong to intermediate-acid,shoshonite and peraluminous rock.For orthophyre,the SiO₂ content is 66.52-68.66 wt%,total alkali content?K₂O+Na₂O?is 10.51¹2.61wt%,Al₂O₃ content is14.80¹6.00wt%,C/ANK=0.98¹.29,I_?Sr?=0.7088⁰.7100,?Nd?t?=-16.18^-9.04,and?Hf?t?=-11.5^-7.53,T_DMC=1.6¹.8Ga;For trachyte,the SiO₂ content is 63.50⁶9.44wt%,the total alkali content is 10.07¹1.65wt%,Al₂O₃ content is 13.96¹6.26wt%,A/CNK is 1.23¹.30,I_?Sr?=0.7095⁰.7098,?Nd?t?=-11.83^-9.67;For Leucite porphyry,the Si O₂ content is54.04⁶8.69wt%,total alkali content is 10.51¹2.61wt%,and Al₂O₃ content is 14.02-16.65 wt%,A/CNK=1.02¹.32,and I_?Sr?=0.7095⁰.7098,?Nd?t?=-11.83^-9.67.?3?There is a similar component characteristic of trace elements among the three kinds of rock,which is similar as volcanics,shoshonite that derived by the metasomatized lithospheric mantle?EMII?.It's enriched of large ion lithophile elements and light rare earth elements,lacked of high field strength elements and heavy rare earth elements,an with no or weak Eu and Ce anomaly.The higher Y and Yb content of the alkali-rich porphyry of Yao'an indicates they has no adakite properties,and the relationship between la-la/Yb shows that diagenesis was mainly controlled by partial melting.?4?The trace elements component and the characteristic of high I_?Sr?,?Nd?t?of the alkali-rich porphyry of Yao'an all indicated that the magma source is the metasomatized lithospheric mantle?EMII?.And the Pb isotope composition and higher Al₂O₃ content,La/Nb,lower?Hf?t?of the rock indicated there is important material contribution made by that the ancient crust during the diagenesis.But the stable I_?Sr?of the rock suggested no obvious crustal contamination was occured in the process of magmatic emplacement.Therefore,the alkali-rich porphyry of Yao'an may derived from a heterogeneous metasomatized lithospheric mantle which contains some ancient crustal material that has not been completely assimilation.?5?The metasomatized lithospheric mantle was partial melted by the upwelling of asthenosphere which caused by the continental collision,and the ancient crust in the mantle was remelted accordingly.The magma which was characterized by commixture of metasomatized lithospheric mantle and ancient crustal was formed in the process,and provides material conditions for mineralization.