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青海玉树县南洞卡组粗面安山岩的SiO_2含量为55.92%~57.85%,高Na_2O、MgO和Al_2O_3,低K_2O、TiO_2和P_2O_5,铝饱和指数A/CNK=0.98~1.08,具有准铝-弱过铝质的特点。岩石K_2O含量低,Na_2O/K_2O=12.60~15.97,显示出钠质粗面安山岩特征。SiO_2-K_2O图解和里特曼指数(2.89~3.25),说明岩石属于钙碱性系列。稀土配分模式为轻稀土弱富集右倾型,铕弱负异常(σEu=0.90~0.96)。微量元素蛛网图显示,岩石富集Rb、Th和U等元素,亏损高场强元素Nb、Ta和大离子亲石元素Ba,样品Ce/Pb、Nb/U并未随着MgO含量的升高而升高,样品的La/Nb、Zr/Nb、Ba/Nb比值均表现出良好的稳定性,表明岩浆在上升过程中遭受地壳物质混染的可能性不大,可能是其源区特征的反映。研究表明岩浆经历了橄榄石和单斜辉石的结晶分异,且受到俯冲板片熔体对源区的改造作用。结合234 Ma的形成年龄,说明该粗面安山岩是金沙江洋盆向西俯冲消减和斜向碰撞的地质记录,其形成时代和地球化学特征也为江达-维西火山弧的形成演化及成矿作用研究提供依据。
The content of SiO_2 in the Nandongkan group of Yushu County, Qinghai Province is 55.92% -57.85%. The high Na_2O, MgO and Al_2O_3, low K_2O, TiO_2 and P_2O_5, the aluminum saturation index A / CNK = 0.98 ~ 1.08, Aluminum characteristics. Rock K_2O content is low, Na_2O / K_2O = 12.60 ~ 15.97, showing the characteristics of the Na mammian andesite. SiO 2 -K 2 O diagram and the Lettman index (2.89 ~ 3.25), indicating that the rock belongs to the series of calc-alkaline. The REE pattern is characterized by weak enrichment of light rare earths and weak negative anomalies of europium (σEu = 0.90 ~ 0.96). Trace elemental cobweb images showed that rocks were enriched in elements such as Rb, Th and U, depleted in high field strength elements Nb and Ta and large ion lithophile elements Ba, and samples Ce / Pb and Nb / U did not increase with MgO content However, the La / Nb, Zr / Nb, Ba / Nb ratios of the samples show good stability, indicating that the magma is unlikely to be contaminated by the crustal material during the ascending process and may be characterized by its source region reflect. The study shows that the magma undergoes the crystal differentiation of olivine and clinopyroxene and is remodeled by subducting slab melt on the source region. Combined with the formation ages of 234 Ma, it shows that this trachyte andesite is a geological record of the subduction and diagonal collision of the Jinshajiang basin to the west. The formation age and geochemical characteristics are also the formation and evolution of the Jiangda-Wiexi volcano arc. Provide a basis for the research of ore effect.