冈底斯西段萨果花岗岩地球化学和年代学特征及其地质意义

Geochemistry and Chronological Characteristics of Saguo Granitoids in Western Gangdise belt,Tibet,China and Its Geological Significance

萨果花岗岩位于冈底斯西段中、南亚带交界位置,主要岩石类型有二长花岗岩和花岗闪长斑岩,岩石总体上富Si,K,贫Mg,Ca,w(Si O2)=64.84%~70.1%,w(K2O)/w(Na2O)=1.21~1.75,A/CNK=0.99~1.2,w(Al2O3)介于14.48%~15.52%,属过铝质岩石,稀土总量(ΣREE)变化范围为256.1×10^(-6)~428.8×10^(-6),LREE/HREE>10,轻稀土富集,δEu=0.77~0.85,弱的铕负异常。Rb、Th和K等大离子亲石元素富集,而Ba、U和Ta等高场强元素亏损。岩石学特征表明岩石总体上具岛弧花岗岩特征,仅少数岩石显示碰撞花岗岩的特点,结合区域花岗岩浆演化和构造环境等地质背景,推断萨果花岗岩体可能形成于俯冲向同碰撞的过渡环境。二长花岗岩的LA-ICP-MS锆石U-Pb年龄为(88.6±1.5)Ma。萨果花岗岩是新特提斯洋向北俯冲的产物,表明印亚大陆碰撞起始时间晚于88.6 Ma。

On the intersection between the middle of Gangdese western belt and south subzone, monzonitic granite and granodiorite-porphyry dominate in Saguo, which is often rich in Si and K and poor in Mg and Ca, with w（SiO2） ranging from 64.84% to 70.1%, w（K2O）/w（Na20）= 1.21- 1.75, A/CNK = 0.99 - 1.2, w（AL2o3） between 14.48% -15.52%, and indicated as peraluminous rock. Total content of rare earth varies from 256. 1 × 10-6 to 428.8 × 10-6, with LREE/HREE 〉 10, showing an enrichment of LREE.The ~Eu = 0.77 -0. 85 indicates a weak negative abnormity of Eu. The rock is characterized by an enrichment of large ion lithophile element such as Rb, Th, and K, and depletion of high field strength elements as Ba, U, and Ta. Generally, the rock presents island arc characteristics in petrology and geochemistry, and only few parts show the collisional features in granite of geochemistry, so with the evolution of granitic magma in this zone and tectonic environment, we can conclude that the granite in Saguo possibly formed in transitional environment of island arc to collision.The LA-ICP-MS zircon U-Pb age of monzonitic granite is （88.6±1.5） Ma, and Saguo granitoid is the product of Neotethvs ocean subducting towards north, so we can get a conclusion that the collisionbetween Asian and Indian continent is after 88.6 Ma.