Previous studies have deeply examined the Neoproterozoic Ⅰ-type and S-type granites on the western margin of the Yangtze Plate,whereas very insufficient attention has been paid to A-type granites,and hence their petrogenetic nature and tectonic implications are still controversial.In this paper,the authors report the results of a detailed study of the newly discovered Lugu Atype granites in the Yangtze Plate,using LA-ICP-MS zircon U-Pb geochronology,petrology,and whole-rock geochemistry,with emphasis placed on the petrogenesis and tectonic setting of the A-type granites.The Lugu granitoid consists of alkali feldspar granite and monzonitic granite that yielded zircon U-Pb ages of (806±5) Ma and (815±5) Ma by LA-ICP-MS,respectively.The Lugu granitoid is deduced as the Neoproterozoic magmatic product.The Lugu granitoid is characterized by high SiO2 (71.2％-76.1％),high total alkali values (K2O +Na2O values from 6.73％ to 8.95％),high K2O/Na2O ratios (1.89-3.60)and peraluminous features (A/CNK=l.07-1.49).Both [FeOT/(FeOT+MgO)] versus SiO2 and [(Na2O+K2O)-CaO] versus SiO2 plots indicate ferroan and alkali-calcic signatures typical of A-type granitoids.The content of rare earth elements of alkali feldspar granite and monzonitic granite is high (∑ REE=221 × 10-6-387× 10-6),and both rocks show enrichment of LREE rather than HREE with a right-inclined REE patterns for (La/Yb)N ratios from 3.42 to 9.69.Both type rocks have obvious negative Eu anomaly (δEu=0.10-0.34).Meanwhile,the Lugu granitoid has relative high content of Zr,Nb,Ce and Y with high ratios of Ga/A1,Y/Nb,Yb/Ta and Ce/Nb,showing that it is a typical A2-type granite.Combined with the data on geochemistry and regional geology,the authors hold that the Lugu Neoproterozoic A2-type granite was derived from the partial melting ofpelitic rock in early Neoproterozoic subduction orogeny.%前人对扬子地块西缘新元古代Ⅰ型和S型花岗岩开展了深入的研究工作,但有关A型花岗岩的研究则鲜有报道,其岩石成因及大地构造意义仍存较大争议.文章选择最新发现的泸沽A型花岗岩体为研究对象,开展系统的LA-ICP-MS锆石U-Pb年代学、岩石学和地球化学研究,探讨其成因机制和地质意义.泸沽花岗岩由钾长花岗岩和二长花岗岩组成,LA-ICP-MS锆石U-Pb年代学分析显示其结晶年龄分别为(806士5)Ma和(815±5)Ma,属新元古代岩浆活动产物.岩石具有高硅(SiO2=71.2％～ 76.1％)、高碱(K2O+Na2O=6.73％-8.95％)、高K2O/Na2O比值(1.89～3.60)和强过铝质(A/CNK=1.07～ 1.49)的特征.[FeOT/(FeOT+MgO)]-SiO2和[(Na2O+K2O)-CaO]-SiO2图解均指示泸沽花岗岩为铁质碱钙性A型花岗岩.泸沽花岗岩具有较高的稀土元素含量(∑REE=221×10-6～ 387×10-6),具有轻稀土富集而重稀土亏损的特征((La/Yb)N=3.42 ～ 9.69),具有明显Eu的负异常(δEu=0.10 ～ 0.34).泸沽花岗岩具有较高的Zr、Nb、Ce和Y含量,具有较高的Ga/A1、Y/Nb、Yb/Ta和Ce/Nb比值,显示出典型的A2型花岗岩特征.综合元素地球化学及区域地质资料,文章提出泸沽新元古代铝质A2型花岗岩起源于新元古代早期俯冲造山过程中的变泥质岩熔融.
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