柴达木盆地南缘晚奥陶世万宝沟花岗岩：锆石SHRIMP U-Pb年龄、Hf同位素和元素地球化学

Late Ordovician Wanbaogou granitoid pluton from the southern margin of the Qaidam basin:Zircon SHRIMP U-Pb age,Hf isotope and geochemistry

柴达木盆地南缘万宝沟花岗岩体主要由似斑状黑云母石英二长岩、环斑结构黑云母二长花岗岩和中粒黑云母二长花岗岩组成,岩体中发育岩浆暗色包体。环斑结构黑云母二长花岗的锆石SHRIMP U-Pb定年为441±5Ma,表明其形成于晚奥陶世。该花岗岩的SiO2含量变化较大(62.20%～75.32%),高钾(3.58%～5.15%)和碱(K2O+Na2O>7%),A/CNK为0.98～1.09,属弱过铝质高钾钙碱性系列;K2O/Na2O(>1)、FeOT/MgO(3.4～6.5)和Ga/Al(2.3～3.1)比值较高,亏损不相容元素Ba、Sr、Nb、P和Ti,相对富集Ta、Hf和Zr,具有A-型花岗岩的特征。黑云母二长花岗岩的εHf(t)=-1.1～10.5,tDM2=744～1490Ma,变化范围较大,表明其物质来源具有多源性,但以壳源为主。较高的εHf(t)(达10)值和较年轻的tDM2(仅为744Ma)暗示,源区中有年轻组分的参与。结合区域地质特征分析,认为该花岗岩形成于后造山的拉张环境。万宝沟花岗岩在结构和地球化学特征上与典型环斑花岗岩有相似之处也有差异,与秦岭中生代环斑结构花岗岩基本相似,表明中央造山带存在古生代和中生代两期环斑结构花岗岩。这将对中央造山带构造演化的进一步研究具有重要的科学意义。

The Wanbaogou pluton,located in the southern margin of the Qaidam basin,contains porphyro-texture quartz monzonite,rapakivi-textured monzogranite and medium grained monzogranite as well as mafic magmatic enclaves.Zircon SHRIMP U-Pb dating for the rapakivi-textured monzogranite yields 441±5Ma.The granitoids are high K calc-alkalic series and metaluminous with large variation in SiO2（62.20%~75.32%）,high in K2O（3.58%~5.15%） and Na2O＋K2O（〉7%）,and A/CNK of 0.98~1.09.They have high ratios in K2O/Na2O（〉1）,FeOT/MgO（3.4~6.5） and Ga/Al（2.3~3.1）,deplete in Ba,Sr,Nb,P and Ti but rich in Ta,Hf and Zr,showing some features of A-type granite.The εHf（t） and tDM2 of the granite change from-1.1 to 10.5 and 744 Ma to 1490Ma,respectively,suggesting mult-sources for the granitoids.The large εHf（t）（ca.10） values and young tDM2（744Ma） indicate juvenile component involved in the formation of the granitoids.All above features,together with the regional geology,suggest that the granitoids are in an extensional setting of post collision.The granitoids are similar to the Mesozoic rapakivi-textured granitoids of the Qinling orogen in texture and geological setting,showing that there two periods rapakivi-textured granitoids occurring in the central orogen belt in China.