2.2 ～2.0Ga magmatism widely occurs in the Trans-North China Orogen (TNCO),which is vital for unveiling the Paleoproterozoic tectonic evolution on the North China Craton (NCC).This paper presents the whole rock geochemical data,zircon UPb ages and Hf isotopes of Paleoproterozoic Dawaliang,Wangjiahui and Lianhuashan granitoids in Wutai area.SHRIMP U-Pb zircon dating of Dawaliang porphyritic granite sample yields an age of 2170 ± 17Ma.One sample from Wangjiahui monzonitic granitic gneiss obtains a LA-ICPMS zircon U-Pb age of 2101-6Ma,and 3 samples from Lianhuashan granite gets zircon U-Pb ages of 2117 ±6Ma,2110 ±4Ma and 2143-± 15Ma,respectively.Dawaliang granite is high in SiO2 and K2O + Na2O,low in CaO,and enriched in REEs (except Eu),Zr,Nb,Ga and Y,and depleted in Sr.Meanwhile,all samples of Dawaliang granite have high Ga/A1 values and Zr +Nb + Y + Ce contents with whole rock zircon saturation temperature as high as 876 ～ 986℃,which is identical with the features of the A-type granite.Wangjiahui and Lianhuashan granites enrich in SiO2,K2O and Na2O,and deplete in MgO and CaO,on the other hand,the granites show minor to intensive differentiation on REEs with obviously negative Eu abnormity.Three plutons (Dawaliang,Wangjiahui and Lianhuashan granites) have high Nb and Y concentrations,so they show the features of post-collision and within plate granite.Zircon εHf(t) values in all samples are lower than that in depleted mantle,and meanwhile,one and two stage Hf model ages are 2.4 ～ 2.6Ga and 2.45 ～ 2.75Ga,respectively,which are much older than the zircon U-Pb ages of the granites.Combining petrology,geochemistry and isotope in the samples,we propose that the Paleoproterozoic granites are derived from the partial melting by the Late Archean TTG gneisses under the extensional environment.Considering the 2.2 ～ 2.0Ga magmatism along the TNCO exhibits two stages of 2.2 ～ 2.1Ga and 2.1 ～ 2.0Ga in different regions,in which 2.2 ～ 2.1Ga magma represent the early stage and some granites at this stage show the features of A-type granite and form the bimodal magmatic assemblages with the coeval mafie rocks.Therefore, we deduce that the middle of Paleoproterozoic magmatism is related with the intracontinental rift in the NCC.%华北克拉通中部造山带内古元古代中期(2.2 ～2.0Ga)岩浆活动较为广泛,对探讨华北古元古代地质演化过程具有十分重要的意义.本文选择了五台地区古元古代大洼梁、王家会和莲花山花岗岩进行了地球化学、锆石U-Pb年代学和Hf同位素研究.大洼梁似斑状花岗岩SHRIMP锆石U-Pb年龄为2170±17Ma,王家会二长花岗质片麻岩LA-ICPMS锆石U-Pb年龄结果为2101±6Ma,3个莲花山花岗岩的LA-ICPMS锆石U-Pb年龄结果分别为2117±6Ma、2110±4Ma和2143±15Ma.大洼梁花岗岩高SiO2、富K2O+ Na2O,低CaO,稀土元素含量较高(Eu除外),Zr、Nb、Ga和Y等高场强元素含量也较高,而Sr含量极低;同时Ga/A1比值和Zr+Nb+Y+Ce总量高,全岩Zr饱和温度高达876 ～ 968℃.该花岗岩体具有A型花岗岩的特征.王家会花岗岩和莲花山花岗岩样品也具有高硅、富碱和低镁钙特征,稀土元素具有弱到较强烈的分异,Eu负异常明显.3个花岗岩岩体微量元素都具有较高的Nb和Y值,因此具有后碰撞到板内花岗岩特征.所有花岗岩样品中锆石的εHf(t)值均远低于同期亏损地幔值,同时单阶段和两阶段模式年龄为2.4 ～2.6Ga和2.45 ～ 2.75Ga,明显大于花岗岩的成岩时代.这些花岗岩与五台地区新太古代晚期TTG质片麻岩具有相似的锆石Hf模式年龄.因此,结合岩石学、地球化学和同位素特征,本文倾向于认为古元古代花岗岩为新太古代TTG质片麻岩在伸展条件下部分熔融形成的.综合资料发现,华北克拉通中部带内2.2 ～2.0Ga岩浆事件不同地区表现有分带性.2.2～2.1Ga岩浆活动代表较早的阶段,其中部分花岗质岩石具有A型花岗岩特征,与同期的基性岩形成双峰式组合,推断其与华北克拉通古元古代中期陆内裂谷有关.
展开▼
