(1. 中南大学 有色金属成矿预测教育部重点实验室，长沙 410083；
2. 中南大学 地球科学与信息物理学院，长沙 410083)
摘 要: 对滇西南澜沧江带晚三叠世富钾火山岩的主量元素、微量元素、稀土元素以及Sr-Nd同位素进行测试。结果表明：岩石以低硅(SiO2含量为50.47%~52.99%)、富钾高碱(K2O+Na2O为5.66%~9.39%，K2O的平均值为2.8%)为特征，属于高钾钙碱性玄武岩−钾玄岩系列。岩石富集大离子亲石元素、轻稀土元素、亏损高场强元素，无Eu或弱的负Eu异常(δ(Eu)=0.85~1.09)。87Sr/86Sr和143Nd/144Nd比值变化范围分别为0.707 01~0.710 56和0.512 511~ 0.512 569，所有样品的ε(Nd)均为负值，ε(Nd)介于−2.48~−1.27之间(平均值为−1.94)。岩石的微量元素特征及Nd和Sr同位素组成与EMⅡ富集地幔来源一致，表明源区具有壳幔混源性质。EMⅡ型地幔的形成与古特提斯澜沧江洋向东俯冲消减相关，也与该区火山岩起源于陆缘弧的构造背景相一致。利用微量元素地球化学原理，推断富钾火山岩应来源于受古特提期俯冲带流体交代富集的金云母相尖晶石方辉橄榄岩的部分熔融。
关键字: 南澜沧江带；三叠世；富钾火山岩；Sr-Nd同位素；滇西
volcanic rocks and their origins in southern Lancangjiang belt,
western Yunnan Province, China
（1. Key Laboratory of Metallogenic Prediction of Nonferrous Metals, Ministry of Education,
Central South University, Changsha 410083, China;
2. School of Geosciences and Info-Physics, Central South University, Changsha 410083, China）
Abstract:The late Triassic potash-rich volcanic rocks in the southern Lancangjiang belt are characterized by low-Si (SiO2 50.47%−52.99%), high-alkali (K2O+Na2O 5.66%−9.39%) and high-K (K2O 2.8% on average), and belong to potassium-high calc-alkali basalt-shoshonite series. The rocks are also enriched in large ion lithophile element (LILE), light rare earth elements (LREE), and depleted in high field strength elements (HFES), non or slightly negative Eu anomaly (δ(Eu)=0.85−1.09) in the abundance. The results show that the rocks have relatively low 143Nd/144Nd ratios (0.512 511−0.512 569) and relatively high initial 87Sr/86Sr ratios (0.707 01−0.710 56), as well as negative ε(Nd) values of −2.48−−1.27 (−1.94 on average). Their characteristics of trace elements and Sr-Nd isotopic composition are consistent with EMⅡ, indicating that the source consists of crust-mantle mixed material, which is inevitably related to the subduction eastward of Tethyan Lancangjiang oceanic plate. Based on the geochemistry, it is suggested that the primary magma was likely generated from partial melting of spinel harzburgitic source metasomatized by Paleo-Tethyan subduction-related fluids.
Key words: southern Langcangjiang belt; Triassic; potash-rich volcanic rocks; Sr-Nd isotopes; western Yunnan
