义敦岛弧带晚白垩世海子山二长花岗岩成因及其地质意义

Petrogenesis and Geological Significance of the Late Cretaceous Haizishan Monzogranite from the Yidun Island Arc

义敦岛弧是三江特提斯复合造山带的重要组成部分。文章对义敦岛弧海子山花岗岩体进行锆石U-Pb年代学、全岩地球化学及Sr-Nd-Pb同位素地球化学进行分析,探讨其成因与构造意义。海子山花岗岩锆石U-Pb年龄为93.7±1.1 Ma(MSWD=2.1, 2σ),为晚白垩世早期岩浆活动产物,岩石具高硅、富碱的特征,铝饱和指数A/CNK=1.04～1.12,属于弱过铝质岩石。稀土配分曲线呈燕式分布,Eu/Eu~*=0.05～0.32,具有明显的Eu负异常。富集Rb、Th、U、Ta、Pb等元素,明显亏损Ba和Sr,表现出A型花岗岩的特征。岩石的ε_(Nd)(t)=–4.8～-3.4,二阶段模式年龄T_(DM2)=0.91～1.00 Ga,结合岩石的Pb同位素特征及低的CaO/Na_2O比值与高的Al_2O_3/TiO_2比值,说明其应起源于泥质岩石的部分熔融并有少量地幔组分加入。综合地球化学、同位素特征及义敦岛弧地区构造资料,表明海子山花岗岩是造山后伸展背景下形成的A型花岗岩,为地壳拉张、减薄,软流圈地幔上涌引起中上地壳泥质岩部分熔融的产物,具有壳幔物质混合的特征。

The Yidun island arc is a significant component of Sanjiang Tethys composite orogenic belt. This paper presents zircon U-Pb age, whole-rock geochemistry and Sr-Nd-Pb isotopic data for the Haizishan granite from the Yidun island arc, aiming to exploreits petrogenesis and geological significance. Zircon U-Pb dating results show that the Haizishan granite pluton was crystallized at 93.7±1.1 Ma （MSWD=2.1, 2σ）, indicating it was produced during early period of Late Cretaceous. The Haizishan granite is characterized byhigh SiO2 and alkali, with A/CNK values of 1.04-1.12, showing a weakly peraluminous affinity. REEs of the granites are characterizedby significant negative Eu anomalies with Eu/Eu＊ values of 0.05-0.32. The granitic rocks are enriched in Rb, Th, U, Ta, Pb butdepleted in Ba, Sr. The εNd（t） values of the granites range from -4.8 to -3.4, with two-stage model age of 0.91-1.00 Ga. In combinationwith their Pb isotopic characteristics and the chemical signatures of low CaO/Na2O and high Al2O3/TiO2 ratios, it is suggested that theHaizishan granite was originated from partial melting of argillaceous rocks with addition of minor mantle component. Based on asynthesis of their geochemistry, isotope characteristics and geological setting, we suggest that the Haizishan granite can be geneticallygrouped into A-type granites which were generated by partial melting of argillaceous rocks with minor involvement of mantlecomponents in a post-collisional extension tectonic setting induced by upwelling of asthenosphere mantle.