三江地区澜沧江带南段半坡杂岩体锆石U-Pb年龄、岩石地球化学特征及板块构造环境

摘要

The Banpo Complex is located in the southern Lancangjiang arc terrane,Sanjiang region,SW China.It mainly consists of wehrlite,clinopyroxenite,gabbro and gabbrodiorite.The U-Pb age of zircons from the gabbrodiorite determined by ID-TIMS is（294.9±2.6） Ma.The rock samples are all characterized by slight-moderate enrichment of light rare-earth elements relative to heavy rare-earth elements and depletion of some high field strength elements such as Nb,Ta,Zr,Hf.These characteristics are similar to those of island arc magmas produced by flux melting in the mantle wedge.Whole rock εNd values for the Banpo Complex vary between ＋3.4 and ＋6.6,similar to the values of modern island arc magmas.Positive εNd values coupled by significant negative Nb,Ta,Zr and Hf anomalies for the Banpo Complex can be explained by mixing of magma derived from the mantle wedge with 2%-7% of a partial melt derived from subducted sediments,plus slab fluids from altered oceanic crust.The data from the Banpo Complex provide convincing evidence for Early Permian intra-oceanic subduction in the region.%半坡杂岩体位于滇西澜沧江构造岩浆带南段,岩体主要由橄榄岩、辉石岩、辉长岩和辉长闪长岩等镁铁-超镁铁质岩组成。通过ID-TIMS锆石U-Pb年代学方法对半坡杂岩体中辉长闪长岩进行精确定年,获得其结晶年龄为（294.9±2.6）Ma,表明半坡杂岩体的形成时代为早二叠世。半坡杂岩体中不同类型岩石微量元素、稀土元素和同位素组成相似,均呈现轻稀土元素（LREEs）相对于重稀土元素（HREEs）弱到中等富集,高场强元素Nb、Ta、Zr和Hf相对亏损的特征,εNd（t=295Ma）为正值,在＋3.4～＋6.6,接近亏损地幔值,这些特征与典型的大洋岛弧玄武岩的特征相似。混合计算表明,半坡杂岩体母岩浆为地幔楔部分熔融,加上约2%～7%源于大洋俯冲沉积物的富硅熔浆和蚀变洋壳流体形成的岛弧玄武质岩浆。上述年龄和同位素数据提供了早二叠世早期南澜沧江地区洋内俯冲的可靠证据。