柴北缘西段小赛什腾山黑云母二长花岗岩和石英闪长岩岩石成因及其动力学背景:锆石U-Pb年代学、地球化学和Hf同位素约束

Petrogenesis and geodynamic setting of the Xiaosaishiteng Mountain biotite monzonitic granite and quartz diorite in the western part of the northern margin of the Qaidam basin:Constraints from zircon U-Pb geochronology,geochemistry and Hf isotope

位于柴北缘西段的小赛什腾山分布大量海西晚期中酸性岩浆岩,其形成时代和成因类型对揭露区域构造演化具有十分重要的意义。本研究以小赛什腾山二叠纪黑云母二长花岗岩和石英闪长岩作为研究对象,对其进行岩相学、锆石U-Pb年代学、岩石地球化学及Lu-Hf同位素研究。结果表明,黑云母二长花岗岩和石英闪长岩的成岩年龄分别为272±3 Ma和273±2 Ma,指示两者均形成于早—中二叠世。地球化学结果显示,黑云母二长花岗岩属弱过铝质钾玄岩系列I型花岗岩,石英闪长岩为准铝质钙碱性系列I型花岗岩,两者均不同程度富集大离子亲石元素Rb、Th、K等,亏损Nb、Ta、Ti等高场强元素,具有典型的弧岩浆地球化学特征。锆石Lu-Hf同位素表明黑云母二长花岗岩具正的εHf(t)值(2.04~8.16)和较年轻的二阶段Hf模式年龄(t_(DM2)=0.77~1.16 Ga)。结合前人研究成果,认为小赛什腾山早—中二叠世黑云母二长花岗岩和石英闪长岩均为宗务隆洋俯冲消减的产物,石英闪长岩为玄武质洋壳板片发生部分熔融产生的熔体遭受地幔橄榄岩混染而成,而黑云母二长花岗岩为洋壳熔融产生的岩浆底侵加热由新生地壳和古老基底地壳构成的混合地壳,在角闪岩相条件下部分熔融形成。

There are a large number of late Hercynian intermediate-acid magmatic rocks in the Xiaosaishiteng Mountain,located in the western part of the northern margin of Qaidam basin.Their formation age and genetic types are of great significance to reveal the regional tectonic evolution.In this study,the Permian biotite monzonitic granite and quartz diorite in the Xiaosaishiteng Mountain are studied by petrology,zircon U-Pb chronology,petrogeochemistry and Lu-Hf isotopes.The results show that the ages of biotite monzonitic granite and quartz diorite are 272±3 Ma and 273±2 Ma,respectively,indicating that they were formed in Early-Middle Permian.Geochemical results show that biotite monzonitic granite belongs to I-type granite of weakly peraluminous potassium basalt series,and quartz diorite belongs to I-type granite of quasi-aluminous calc-alkaline series.Both of them are enriched in large ion lithophile elements Rb,Th,K to varying degrees,and depleted in high field strength elements such as Nb,Ta,Ti,showing typical arc magma characteristics.Lu-Hf isotopic compositions indicate that biotite monzonitic granite has a positiveεHf(t)value(2.04~8.16),and younger two-stage Hf model age(t_(DM2)=0.77~1.16 Ga).Combined with previous research results,it is considered that the Early-Middle Permian biotite monzonitic granite and quartz diorite in the Xiaosaishiteng Mountain were the products of subduction of the Zongwulong Ocean.The quartz diorite was formed by the melt contaminated by mantle peridotite caused by the partial melting of the basaltic oceanic crust plate,and the biotite monzonitic granite was formed by partial melting of the mixed crust composed of the juvenile crust and the ancient basement crust under the condition of amphibolite facies,which is caused by the magma underplating generated by oceanic crust melting.