扬子地块西缘新元古代泸沽A型花岗岩成因与变泥质岩熔融

Genesis and geological implications of the Neoproterozoic A-type granite from the Lugu area, western Yangtze block

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±5)Ma和(815±5)Ma,属新元古代岩浆活动产物。岩石具有高硅(SiO_2=71.2%~76.1%)、高碱(K_2O+Na_2O=6.73%~8.95%)、高K_2O/Na_2O比值(1.89~3.60)和强过铝质(A/CNK=1.07~1.49)的特征。[FeO^T/(FeO^T+Mg O)]-SiO_2和[(Na_2O+K_2O)-CaO]-SiO_2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/Al、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。

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 from6.73%to8.95%),high K2O/Na2O ratios(1.89-3.60)and peraluminous features(A/CNK=1.07-1.49).Both[FeOT/(FeOT+MgO)]versus SiO2and[(Na2O+K2O)?CaO]versus SiO2plots 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(ΣREE=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 from3.42to9.69.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/Al,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 NeoproterozoicA2-type granite was derived from the partial melting of pelitic rock in early Neoproterozoic subduction orogeny.