Geochemistry And Geochronology Of Cenozoic Alkali-rich Porphyries In Nilang,Yunnan Province

“Sanjiang”area locates on the southeastern of Tibet Plateau.It spans the west of Yunnan,western Sichuan,southern Qinghai,and the east of Tibet.The name of Sanjiang area comes from three important rivers that are Jinshajiang,Lancangjiang,and Nujiang.At 45-50 Ma,due to the impact of the collision between the Indian and Eurasian,many alkali-rich intrusive rock masses appeared on both sides of the Ailaoshan-Red River fault zone and nearby.The researching area is Yujiaping and Sifang village in Nilang.In this paper,three methods are mainly used that are geochemistry,geochronology,and isotope geochemistry to research the alkali-rich porphyry in the research area.This research project aims to explore the nature of the source regions,tectonic setting,and the petrogenesis of the Cenozoic alkali-rich porphyry.Lithologic characters of Alkali-rich porphyries that in the study area are syenite,biotite syenite porphyry,and biotite monzonitic porphyry.Alkali-rich porphyries are characterized by high alkali and high potassium rich aluminum.Individual rock masses have the adakitic characteristics,but they are not typical adakites.Alkali-rich porphyries in the research area mainly belong to the shoshonitic and exhibit metaluminous-peraluminous features(A/CNK=0.80~1.08).All samples that from the research area are enriched in LILE,and depleted in HFSE,and display the“TNT”anomalies.The rocks are characterized by enrichment in light rare earth elements(LREE),depleted in heavy rare earth elements(HREE),obvious fractionation of light and heavy rare earth elements((La/Yb)_N=54.4~65.5),and moderate Eu anomalies(?Eu=0.92~0.98).The samples of alkali-rich porphyry have similar geological characteristics with adakitic rocks in Sifang Village.They are rich in aluminum,depleted magnesium,lean Y and Yb,rich in Sr,and no significant negative Eu anomalies.By zircon U-Pb dating show that syenite is 33.51±0.58Ma,biotite syenite porphyry is 34.9±1.2Ma,34.31±0.67Ma,33.67±0.65Ma,and biotite monzonitic porphyry is 33.89±0.51.Ma,34.36±0.62 Ma,consistent with the time of Cenozoic magmatism in Yunnan.Due to the collision between India and Eurasia,the crust and lithosphere substantially shorten and thicken.The delamination of the thickened crust and lithospheric caused the upwelling of the asthenosphere,so that the lithospheric mantle and the K-rich crust became partial melting and formed the“Sanjiang”area large-scale alkali-rich porphyry belt.