Whole-Rock Geochemistry,Zircon U-Pb Geochronology And Prospecting Significance Of Nepheline Syenite In Gejiu,Yunnan

Gejiu nepheline syenite has trace and rare earth element metallogenic potential.This paper uses zircon U-Pb chronology,major trace element geochemistry,Sr-Nd-Pb-Hf isotope geochemistry and mathematical statistical analysis and other research methods and technical means to systematically and comprehensively study it.The results are as follows:(1)LA-ICP-MS zircon U-Pb dating results indicate that the Gejiu nepheline syenite has ages ranging from 84.07±0.52Ma to 80.20±0.41Ma.They are all the products of large-scale tectonomagmatism during the Late Cretaceous.(2)The nepheline syenite has low contents of Si O₂(50.4%?55.92%)and high contents of K₂O(6.77%?8.32%),and total alkali(with K₂O+Na₂O content of 11.79%?15.38%).Moreover,these rocks are enriched in U,Th,Hf,Zr,and light rare earth elements,and depleted in Sr,Ba,Ta,Ti,P and heavy rare earth elements.(3)The nepheline syenite has a low(⁸⁷Sr/⁸⁶Sr)_i ratios(0.70915?0.71117),low(¹⁴³Nd/¹⁴⁴Nd)_i ratios(0.512096?0.512165)with?Nd(t)values varying from-7.1 to-8.6and high Pb isotopic ratios(²⁰⁶Pb/²⁰⁴Pb=18.442?8.659,²⁰⁷Pb/²⁰⁴Pb=15.670?15.752,²⁰⁸Pb/²⁰⁴Pb=38.906?39.445).The ratios of ¹⁷⁶Hf/¹⁷⁷Hf range from 0.2825 to 0.2827,with?Hf(t)values varying from-9.00 to+0.39.(4)Compared with the alkaline syenite in eastern China,the Gejiu nepheline syenite is significantly enriched in Th(k=6.15),enriched in U,Nb,La,Lu,Yb,Ce,Tm,and depleted in Nd,Sm,Eu.The results of metallogenic analyses show that the elements Th,U,Nb have high metallogenic potentiality.(5)The Sr-Nd-Pb-Hf isotopic and whole-rock geochemical characteristics indicate that the Gejiu nepheline syenite was derived from an EMII source with low degree of partial melting and high degree of fractional crystallization during the diagenesis process in an intraplate extensional environmen.