中祁连东段上莫吉岩体地球化学特征及年代学意义

Geochemical characteristics and geochronological significance of the rock mass of Shangmoji in the eastern part of the middle Qilian mountains

为了研究中祁连东段上莫吉岩体的地球化学特征及年代学意义,通过LA-ICP-MS锆石U-Pb测年测得上莫吉二长花岗岩同位素年龄为(922.3±5.3)Ma(MSWD=0.037);岩石中SiO_(2)含量为70.92%~75.00%,Na_(2) O+K_(2) O含量为5.18%~7.66%,山德指数(A/CNK)为1.049~1.346,平均为1.25,碱度率AR为1.79~3.23;稀土元素配分模式图为富集型模式,曲线向右倾斜,(La/Yb)n为2.71~22.84,具有明显的负Eu异常;微量元素蛛网图中,具Rb、Th的强烈正异常,Nb、Zr不同程度的负异常,类似于同碰撞花岗岩。因此,中祁连东段上莫吉侵入体的为CPG(含白云母过铝质花岗岩)型花岗岩,该岩体的侵位时代为新元古代,具有富钠、富钾的特征,其岩体由古老的硅铝质大陆壳经重熔再结晶而成。稀土及微量元素特征显示上莫吉二长花岗岩形成于大陆碰撞的构造环境。

In order to study the geochemical characteristics and geochronological significance of the rock mass of Shangmoji in the eastern part of the middle Qilian mountains,the isotopic age of Shangmoji monzogranite is(922.3±5.3)MA(MSWD=0.037)by using the method of LA-ICP-MS zircon U-Pb dating;the content of SiO_(2) is 70.92%-75.00%,Na_(2)O+K_(2)O is 5.18%-7.66%,Sander index(A/CNK)is 1.049-1.346 with an average of 1.25 and alkalinity ratio is 1.79-3.23;the distribution model of rare earth elements is shown as the enrichment model and the value of(La/Yb)n is 2.71-22.84.Its curve slopes to the right,which shows an obvious negative Eu anomaly.There are strong positive anomalies of Rb and Th,and negative anomalies of Nb and Zr in different degrees,which are similar to syn collisional granites.Therefore,the intrusive body of Shangmoji in the eastern part of the middle Qilian Mountains is CPG(muscovite with peraluminous granite)granite.The emplacement age of the intrusive body is Neoproterozoic,which is rich in sodium and potassium.Besides,it is formed by remelting and recrystallization of the ancient sialic continental crust.The characteristics of rare earth elements and trace elements show that Shangmoji monzogranite is formed in the tectonic environment of continental collision.