敦煌地块南缘石炭纪埃达克岩的发现及其地质意义

Discovery of adakitic rocks in south margin of Dunhuang block and its geological significance

侵位于敦煌杂岩中的青石沟黑云母石英闪长岩,锆石LA ICP-MS U-Pb同位素测年研究结果表明其形成年龄为335±2Ma。岩石地球化学分析显示其具有高SiO2、高Al2O3和低MgO的主量元素地球化学特征,强烈富集Sr、亏损Yb和Y,具有较高的Sr/Y比值等特征,与埃达克质岩石具有一致的地球化学特征。青石沟黑云母石英闪长岩的Nd和Hf同位素值(εNd(t)=-9.39^-11.03,εHf(t)=-7.6^-16.6)显示其具有明显的壳源特征;Nd及Hf模式年龄值(Nd模式年龄t DM=1.83~1.98Ga,Hf二阶段模式年龄t DM2=1.53~1.95Ga)均表明其源区可能与元古代敦煌杂岩相关;结合较高的Mg#,低Cr和Ni含量,无Eu异常等特征,暗示该岩体为加厚下地壳部分熔融的产物。综合区域地质特征,初步认为敦煌地区早石炭世处于由陆陆俯冲(地壳加厚)作用向陆内伸展体制转化的构造环境。

The Qingshigou biotitequartz-diorite intruded into the Dunhuang complexes,zircon LA ICP-MS U-Pb data reveal the crystallization age of 335 ± 2Ma. The diorite is featured by high contents of SiO2,Al2O3,and low content of MgO; the trace element display adakitic affinity,which are characaterized by strong enrichment of Sr,depletion of Yb and Y,and high ratio of Sr / Y; moreover,the whole rock shows a high Mg#,low Cr,Ni contents,and normal Eu value. Nd and Hf isotopic compositions（ ε Nd（ t） =- 9. 39 ^- 11. 03,ε Hf（ t） =-7. 6 ^-16. 6） suggest that the rock originated from the typical continental crust. Nd isochronal age of the whole rock（ 1. 83~1. 98Ga） and Hf two-stage model age（ 1. 53~1. 95Ga） demonstrate that the Qingshigou biotitequartz-diorite is likely derived from the source where materials are related to the Proterozoic Dunhuang complexes. Therefore,we suggest that the Qingshigou biotitequartz-diorite may have originated from the partial melted thickened lower crust. Integrating with regional geology,it can be concluded that the Dunhuang area was in a transition tectonic setting between continent-continent subduction（ crust thickened） and intracontinental extension during the Early Carboniferous.