老挝南部帕莱通双峰式火山岩锆石UPb定年及岩石地球化学特征

摘要

老挝南部帕莱通（Phlaythong）矿区出露了一套双峰式火山岩组合，其基性端元为致密块状玄武岩，酸性端元为流纹岩。流纹岩的锆石 SHRIMP U Pb 年龄为（229．0±2．0）Ma，属中晚三叠世，代表双峰式火山岩形成时代。地球化学数据显示：玄武岩 SiO2质量分数均值为50．70％，富 TiO2、MgO、CaO 和TFeO，具较低的全碱和 P2 O5，富集 LILE（Sr、Rb、Ba、Th）和轻微亏损 HFSE，轻稀土略微富集，Eu 负异常不明显；流纹岩具有高的 SiO2（平均质量分数为76．33％）和全碱，富钾（w （K2 O）＞w （Na2 O）），极低的TiO2、MgO、CaO 和 TFeO，富 Th、U 和 Zr，明显亏损 Ta、Nb、Ba、Sr 和 Ti，轻稀土富集，显示明显的 Eu 负异常（δEu＝0．31～0．82）特征。初步研究表明，玄武岩和流纹岩来自不同的岩浆源区，玄武岩由亏损的软流圈地幔受陆壳物质混染作用形成，流纹岩为壳源物质受幔源岩浆底侵加热之后熔融的产物。结合南海印支地块构造演化特征，认为该双峰式火山岩形成于大陆板内裂谷环境。%SHRIMP U Pb zircon ages are reported for rhyolites in the bimodal volcanic rocks from the Phlaythong area at the Southern of Laos.The zircons with well-defined oscillatory zoning from rhyolite sample yielded mean 206 Pb/238 U age of (229.0±2.0)Ma,which represents the eruption age of the volcanic rocks during Middle to Upper Triassic (T2 3 ).Geochemically,the basalt shows an average SiO2 content of 50.70%,enrichment of TiO2 ,MgO,CaO,TFeO,LILE(Sr,Rb,Ba,Th)and LREE,and depletion of alkali,P2 O5 and HFSE.Rhyolites are generally characterized by rich SiO2 (76.33%),alkali (w(K2 O)>w(Na2 O)),and poor TiO2 ,MgO,CaO and TFeO,they have relatively high Th,U and Zr and rather low Ta,Nb,Ba,Sr and Ti,and display obvious negative anomaly of Ba,Sr and Ti,obvious enrichment of LREE,noticeable negative Eu anomalies (δEu=0.31 0.82).According to this research, the basalt was originated from depleted asthenosphere mantle (DM ) material upwelling and contaminating a little part of crustal materials,and rhyolite resulted from melting crustal materials heated by basaltic magma and felsic magma.The bimodal volcanic rocks are the products of partial extension in an intracontinental rift.