湘南多金属矿集区燕山期成矿花岗岩的主元素地球化学特征和成因探讨

湘南地区燕山期成矿花岗岩的主元素地球化学特征可划分为3种类型,不同成矿花岗岩形成的岩浆演化机理有明显差异:(1)成矿花岗岩的K_2O/Na_2O比值较高,均显示高钾钙碱性-钾玄岩系列特征。(2)MC型与CM型早期次单元花岗岩相对贫硅、碱.富钙、镁、铁,铝质指数(A/KNC)较低,碱度指数(KN/A)都不高,属镁质-铁质准铝质的高钾钙碱性系列花岗岩类,总体显示出I型花岗岩的特征。C型和CM型晚期次单元花岗岩相对富硅碱、贫镁钙,属铁质弱过铝质-过铝质钾玄岩系列-高钾钙碱性系列花岗岩类:岩石的FeO~T/MgO值明显高于一般I型和M型花岗岩.较高的FeO~T值又与高分异的I型花岗岩相区别,总体显示出S型花岗岩的特征。(3)成矿花岗岩的F或Cl含量高.岩浆向过铝质方向或过碱性方向演化,晚期岩浆中的高场强元素浓度增大,导致MC型与CM型的花岗岩的早期次单元多有Cu、Pb、Zn、Sb等多金属化,C型和CM型的晚期次单元花岗岩则常有大型Sn、W、Pb、Zn、Nb、Ta和稀土等矿化。(4)成矿花岗岩的形成与壳幔岩浆混合作用有关.形成MC型和CM型早期次单元花岗岩的岩浆演化主要是岩浆混合作用.而CM型花岗岩晚期次的花岗岩类和C型花岗岩类的岩浆演化可能还存在分离结晶作用。

Major element geochemistry and genesis of Yanshanian mineralized granites in the polymetallic deposit concentration region, southern Hunan

There are three types of Yanshanian mineralized granite distributed in Hunan.Major element geochemical analyses show the following:(1)Mineralized granites have a higher ratio of K_2O/Na_2O and display the features of the high-K,calc-alkaline shoshonite series.(2)The MC-type and early CM-type granites are poor in Si and alkalis(K Na)and rich in Ca,Mg and Fe with a lower aluminum index(A/KNC)and a lower alkalinity ratio of KN/A,which indicates that those granites have the same characteristics as magnesian and ferruginous,para-aluminous,high-K calc-alkaline rocks,such as I-type granite.The 1C-type and late CM-type granites are relatively rich in Si and alkalis and poor in Ca and Mg,belonging to the ferruginous,slightly peraluminous-peraluminous shoshonite series.Their ratios of FeO~T/MgO are notably higher than those of I-and M-type granites and their higher FeOT values indicate that these granites are apparently distinguished from highly differentiated I-type granites,but generally,they show the features of S-type granites.(3)The higher percentages of F and C1 in mineralized granites indicate that the magma of Yanshanian mineralized granites evolved toward the peraluminous or peralkaline one.The increase in concentrations of the HFSE in the late-stage magma led to mineralization of multiple metals such as Cu,Pb,Zn and Sb in the early-stage MC-and CM-type granites and large-scale mineralization of Sn,W,Pb,Zn,Nb,Ta and LREE in the late-stage aluminous C-type and CM- type granites.(4)The formation of the mineralized granites is related with crust-mantle magma intermingling.The magma generating the MC-type and early-stage CM-type granites was sourced from magma intermingling, while the magmatic evolution of C-type and late-stage CM-type granites might also involve fractional crystallization.