Genesis And REE Occurrence Of Dachuan Alkali-Feldspar Granite In Tonghua Area,Jilin Province

Located in the eastern section of the northern margin of the North China Craton,the southern part of Jilin Province is a geological structure area that has experienced the long geological history of Archaean,Proterozoic,Paleozoic,Mesozoic and Cenozoic.The Mesozoic magmatic activity is the most intense and related hydrothermal gold,lead-zinc and other deposits have been formed.In this paper,the late Mesozoic Dachchuan alkali-feldspar granite mass enriched with rare earth elements was selected to carry out field geological survey,petrography,rock mass,trace and rare earth element geochemistry studies,combined with LA-ICPMS in situ microzone analysis and other work,to explore the petrogenesis and tectonic background of the rock mass,and preliminarily establish the occurrence state of rare earth elements.The following key insights have been gained:(1)The Dachuan granitic body is mainly composed of coarse-grained biotite alkali-feldspar granite,medium-grained biotite alkali-feldspar granite,fine-grained biotite alkali-feldspar granite and granite-pegmatite facies.The major elements are high Si,low Al,quasi-aluminum-strong peraluminous,belonging to the high potassium calc-alkaline series.The total rare earth content is high(ΣREE=88.53 ppm ～839.59ppm),(La/Yb)N=2.30～41.24,light rare earth elements are enriched,while heavy rare earth elements are depleted,rare earth partition pattern is right-leaning type,europium negative anomaly is obvious,δEu value is 0.05～0.60.The differentiation index(DI=87.4～97.24)was high,and the consolidation index(SI=0.16～1.13)was low.The results show that the different degrees of separation and crystallization of plagioclase,biotite,apatite and other minerals occurred in the process of magmatic evolution.Combined with the petrographic characteristics,the preliminary conclusion is that the Dachhuan alkali-feldspar granite is a highly differentiated granite formed in the extensibility environment.(2)LA-ICP-MS U-Pb chronology and Hf isotope system indicate that the zircon U-Pb isotopic age of the coarse-grained biotite alkali feldspar granite is 120.8±1.6Ma,indicating that the granite was formed in the Early Cretaceous.The εHf(t)values of zircons are all negative(-5.2 ～-10.7),corresponding to the second-order model ages of2188 ～ 2587 Ma,and the f Lu/Hf(-0.95 ～-0.97)range is small,indicating that the magma is homogeneous,indicating that the magma originated from the Neoarchean-Paleoproterozoic crust.(3)Rock identification and whole rock trace show that with the evolution of magma,the quartz content gradually increased,the biotite content gradually decreased,and the total rare earth content also gradually decreased,and the rare earth is mainly enriched in the coarse-grained biotite alkali feldspar facies.La-icpms in situ microanalysis of single minerals shows that rare earth elements such as La,Nd,Ce and Y occur mainly in apatite and zircon in a homogenous manner.Apatite and zircon are the main mineral carriers of rare earth elements.The content of rare earth in biotite and granite-pegmatite magnetite is slightly higher,and the content of rare metals such as Rb,Nb and Ta is very high,but the content of rare metals in rocks is very low.However,the content of rare earth elements in orthoclase and plagioclase is very low,which is the main mineral carrier of non-rare earth elements.