老挝沙耶武里B.Talan地区玄武岩锆石U-Pb定年及岩石地球化学特征

Zircon U-Pb dating and geochemical characteristics of basalt from the Xaignabouli B. Talan area,Laos

老挝沙耶武里B.Talan地区主要出露枕状、杏仁状玄武岩、中基性(含安山质)熔结火山角砾岩(含集块岩)。采用LA-(MC)-ICP-MS锆石U-Pb同位素定年技术,获得了该玄武岩的结晶年龄为260.8±4.7 Ma(MSWD=0.005 5),其形成时代为中二叠世晚期。该区玄武岩具低SiO_2(45.82%~47.77%)和富钠贫钾(Na_2O/K_2O>1)特征;富集大离子亲石元素(Rb、Ba、U、K和Th),明显亏损Nb、Ta、P、Ti等高场强元素;稀土总量ΣREE为(41.94~63.97)×10^(-6),δEu=0.96~1.08,具弱的正、负铕异常,总体上表现出LREE富集的右倾型配分模式;Nb/Ta值为3.89~11.56(平均7.25)。岩石地球化学证据表明,B.Talan地区玄武岩的形成构造背景为岛弧环境;岩浆源区来自于岛弧下的俯冲板片熔融体,其成因与交代的地幔楔相关。

The basalt of B. Talan region in Laos is chiefly composed of pillow,amygdaloidal basalt and intermediate-basic weldedly volcanic breccia which consists of agglomerates. LA-（ MC）-ICP-MS zircon U-Pb analyses suggest that the crystallization age of the terrace is 260. 8 ± 4. 7 Ma（ MSWD = 0. 005 5）,implying it is the product of magmatic activity in Middle Permian. The basalts exhibit low and variable SiO_2（ 45. 82%-47. 77%）,high Na_2O/K_2O（ Na_2O/K_2O 1） ratios with large ion lithophile elements（ Rb,Ba,U,K and Th）,depletion of high-field strength elements（ Nb,Ta and Ti） and relatively low ΣREE（（ 41. 94-63. 97） × 10^（-6）） as well as a stable variation with δEu of 0. 96-1. 08,showing a right type distribution patterns of LREE enrichment. In addition,Nb/Ta value 3. 89-11. 56 is an average of 7. 25. Based on the integrating of geological and geochemical evidences,the tectonic setting of the basalt in B. Talan region,Xaignabouli should be the island arc environment. The magma source is derived from the subduction plate melt below the island arc,and its origin is related to the metasomatic mantle wedge.