东天山雅满苏地区安山岩年代学、地球化学特征及其构造意义

Geochronology,Geochemical Characteristics and Tectonic Implications of Andesites in the Yamansu Area,Eastern Tianshan

东天山的构造演化过程至今仍是争论热点,前人提出了多种俯冲模型来解释雅满苏火山弧带的形成。文章报道了雅满苏火山弧带内安山岩的全岩主微量元素、Sr-Nd同位素组成、锆石U-Pb原位定年和Hf同位素,来进一步了解东天山的构造演化历史。这些安山岩具有中等的SiO2含量,高的Al2O3,Fe2OT3和全碱(Na2O+K2O)含量,低MgO含量,属于钙碱性和低钾拉斑系列。在标准化的蛛网图中,它们显示富集大离子亲石元素(LILEs,如Rb、K、Sr),亏损高场强元素(HFSEs,如Nb、Ta),表现出与弧岩浆相似的特征。24颗锆石具有相对一致的206Pb/238U年龄,其加权平均年龄为337.7±0.9 Ma(MSWD=4.9),表明这些安山岩侵位于早石炭世。此外,这些安山岩具有较高的εNd(t)和εHf(t)值,表明其为地幔来源。地球化学和同位素组成指示,本区安山岩起源于受俯冲流体交代富集地幔楔5~10的部分熔融,未表现出明显的地壳污染和分离结晶作用。在构造判别图解上,这些安山岩落入活动大陆边缘的范围内。结合丰富的地质证据,文章认为,雅满苏火山弧带为早石炭世康格尔洋向南俯冲而形成的陆源弧。

Tectonic evolution of the Eastern Tianshan is still under hotly-debated,with varied subduction model proposed to interpret the formation of the Yamansu Arc Belt.In this study,we report the whole rock major and trace elements,Sr-Nd isotopic compositions,in-situ zircon U-Pb dating as well as Hf isotopes of andesites in the Yamansu Arc Belt,in order to further understand the tectonic evolution of the Eastern Tianshan.The andesites have moderate SiO2,high Al2O3 and Fe2OT3,and low MgO compositions,showing calc-alkaline to low-K tholeiitic features.On normalized spider diagrams,they are enriched in large ion lithophile elements(LILEs,e.g.,Rb,K,Sr)and depleted in high field-strength elements(HFSEs,e.g.,Nb,Ta),displaying an affinity to arc magma.24 zircon grains have consistent 206Pb/238U ages and give a weighted mean age of 337.7±0.9 Ma(MSWD=4.9),suggesting that they were emplaced during the early Carboniferous.In addition,these andesites have highεNd(t)andεHf(t)values,indicating that they were derived from mantle source.Geochemical and isotopic compositions indicate that the andesites were derived from 5~10 partial melting of enriched mantle wedge metasomatized by slab-derived fluids,with insignificant crustal contamination and fractional crystallization.On the tectonic discrimination diagrams,the andesites plot in active continental arc field.Combined with abundant geological evidence,we believed that the southward subduction of the Kangguer Ocean led to the Yamansu continental arc during the early Carboniferous.