中文摘要：

利用在南海西南海盆南缘首次发现的花岗闪长岩样品,通过等离子质谱仪、岩相显微镜和电子探针等设备对其地球化学组成进行了分析;并对其组分中黑云母矿物进行了40Ar/39Ar年代学研究。40Ar/39Ar等时线年龄显示岩石的结晶年龄为（109.5±1.5）Ma,是早白垩世岩浆入侵的产物。地球化学测试表明：样品以富SiO2、K2O,高Al2O3、贫TiO2为特征。地球化学-构造判别图解显示,样品属于火山弧花岗岩,为造山类花岗岩。样品可能是海盆早期张裂阶段裂谷系组成的重要岩石类型,而裂谷拉张的过程可能与现代红海的演化相似,经历了裂谷沉陷过程,当裂谷宽度、深度达到一定规模时,洋壳随之而产生。该样品的发现,揭示其产出位置可能是南海西南海盆的南部边界,而且是洋盆扩张初期断裂系统的组成部分,为探讨南海的张裂、形成与演化历史提供了新的线索。

英文摘要：

The Southwestern Basin is the key region to the understanding of the origin and the evolvement of the South China Sea.Some critical geological problems remain in a dilemma so far concerning the evolution of the South China Sea,such as the formation of the sea floor and the boundaries of the basin because its complexity in regional tectonic background,active sedimentation and lack of rock samples.Granodiorite samples have been collected recently from the southern margin of the southwest basin of the South China Sea（SCS）.The samples were analyzed for major elements,trace elements and rare earth trace elements（REE） with ICP-MS.Mineralogy was studied using the petrographic microscope and electron microprobe.The 40Ar/39Ar laser probe was used to date the biotite in the samples.The 40Ar-39Ar ages of the biotite of the samples are in the range of 109.5±1.5Ma,indicating that they are products of magmatic intrusion in Early Cretaceous.Their geochemistry shows that the granodiorites are characterized by high SiO2,K2O,and Al2O3,but low TiO2.Tectonic discrimination diagrams suggest an extension-related magmatism,either in an arc or forearc setting,mainly belonging to the syncollision type and is hardly related to orogeny.They may be the main rocks formed in a rift system in the early tensional rifting stage.The process of tensional rifting is somewhat similar with the evolutionary process of the Red Sea,which has experienced rifting and sagging.The oceanic crust was come into being when the rift valley reached certain scale of width and depth.The granodiorite samples have also provided important evidence to define the boundary of the Southwestern Basin and the clues to study the expanding model.