Petrogenesis of Middle Triassic andesite in Sayaburi area, Laos: Constraints from whole-rock geochemistry, zircon U-Pb geochronology, and Sr-Nd isotopes

老挝沙耶武里省中三叠世安山岩的成因与构造环境：地球化学、锆石U-Pb年代学和Sr-Nd同位素的约束（英文）

Despite the presence of a large area of andesite in the Sayaburi Province of Laos, it has received very little attention. Based on a combination of detailed field investigations, geochronology and geochemical analysis, this study aims to explore the geochemical, Sr-Nd isotopic, and source rock characteristics, as well as the genesis and tectonic setting of the andesite in this region. In the Sayaburi Province, the andesite zircon U-Pb age is(241.2±1.2) Ma. The andesite rock is classified in the metaluminous-weak peraluminous calc-alkaline series. The light rare-earth elements(LREEs) are enriched and characterized by clear fractionation, whereas heavy rare-earth elements(HREEs) are relatively depleted and have no signs of fractionation. The average δEu is 0.96 with weak-or-no Eu anomalies. It is enriched in large ion lithophile elements such as Rb and K, while depleted in high field-strength elements such as Nb, Ta, P and Ti. For andesites in the Sayaburi Province, the(87Rb/86Sr)t value ranges in 0.702849-0.704687, the εNd(t) value is between 3.53 and 4.77, the tDM(t) value ranges in 633-835 Ma, and the tDM2(t) ranges in 625–724 Ma. The results based on the synthesis of petrology, geochemistry, and regional tectonic background studies show that 1) the andesitic magma source in the study area is an enriched mantle, which is modified by subduction zone fluids;2) the geotectonic background environment of the andesite in Sayaburi area is the continental island arc environment and related to the tectonic evolution of Jinghong–Nan–Uttaradit back-arc basin, which reflects that the magmatic source is enriched with a mantle wedge component modified by a subduction zone fluid(or melt).

老挝沙耶武里省分布有大面积安山岩,该地区地质工作程度极低。本文拟在对该地区安山岩详细野外调查的基础上结合年代学、地球化学数据分析,探讨该套安山岩的地球化学特征、Sr Nd同位素特征、源区特征、岩石成因及构造环境。老挝沙耶武里省安山岩锆石U Pb年龄为(241.2±1.2)Ma。岩石属准铝质-弱过铝质钙碱性系列,LREE富集,分馏明显;而HREE相对亏损,分馏不明显,δEu平均值为0.96,具有弱的Eu异常或无Eu异常,相对富集Rb、K等大离子亲石元素,而Nb、Ta、P、Ti等高场强元素呈现低谷。安山岩的(87Rb/86Sr)t值介于0.702849~0.704687,εNd(t)值介于3.533358~4.774175,tDM(t)值介于633~835 Ma,tDM2(t)值介于625~724 Ma。综合岩石学、地球化学和区域构造背景,研究区安山岩的岩浆源区为遭受了俯冲带流体改造的富集地幔源区,形成的大地构造背景环境为大陆岛弧环境,且与景洪-难河-程逸弧后洋盆构造演化相关。