南秦岭板板山二长花岗岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及其地质意义

LA-ICP-MS zircon U-Pb ages and geochemical characteristics of the Banbanshan monzogranite in Southern Qinling orogenic belt and their geological significance

南秦岭北缘山阳县板板山复式岩体存在3个阶段的岩浆侵位,分别为早期阶段的二长花岗岩、中期阶段的钾长花岗岩及晚期阶段的辉绿岩。针对早期阶段的二长花岗岩进行了岩石地球化学、LA-ICP-MS锆石U-Pb年龄研究,结果表明:二长花岗岩Si O2含量为73.35%~77.49%,全碱(K2O+Na2O)含量为3.15%~7.19%,里特曼指数为0.30~1.66,A/CNK值为1.18~2.48,属中高钾钙碱性的过铝质S型花岗岩;稀土元素总量变化较大,轻稀土元素相对富集,重稀土元素相对亏损,具弱的负铕异常(δEu=0.70~0.89),大离子亲石元素Rb、K、Th、Ba相对富集,高场强元素Nb、Ta、Ti、P相对亏损,具岛弧岩浆岩地球化学特征。反映了板板山二长花岗岩形成于同碰撞构造环境,且携带火山岩弧环境的地球化学特征,暗示岩浆来源于上地壳岩石的部分熔融,且受残留的俯冲洋壳脱水形成的流体交代。2件样品的LA-ICP-MS锆石U-Pb年龄分别为743.2±6.1 Ma和743.7±5.9 Ma,表明板板山二长花岗岩形成于新元古代。结合前人研究成果,认为板板山一带在743 Ma之前为古洋壳俯冲阶段,743 Ma之后古洋盆闭合,Rodinia超大陆开始聚合,发生陆陆碰撞。

There exist three stages of magmatic emplacement in the Banbanshan complex massif,Shanyang County,on the northern margin of the South Qinling Mountains.They are the early monzogranite,the middle K-feldspar granite and the late diabase.In this paper,the authors studied the geochemistry and LA-ICP-MS zircon U-Pb age of the early monzogranite.The results show that the Si O2 content of monzogranite is between 73.35%and 77.49%,the content of total alkali(K2 O+Na2 O)is 3.15%~7.19%,the Rittman index is 0.30~1.66,and the A/CNK ratio is 1.18~2.48,suggesting that they belong to peraluminous S type granite with medium and high potassium calc-alkaline nature.The total amount of rare earth elements varies greatly.Light rare earth elements are relatively enriched and heavy rare earth elements are relatively deficient.The rocks have a weak negative europium anomaly(δEu=0.70~0.89).Large ion lithophile elements Rb,K,Th,Ba are relatively enriched,and high field strength elements Nb,Ta,Ti,P are relatively deficient.It is shown that the Banbanshan monzogranite was formed in the same collisional tectonic environment and carried the geochemical characteristics of the volcanic arc environment,suggesting that the magma originated from the partial melting of the upper crust rocks and was metasomatized by the residual fluid formed by the dehydration of the subducted oceanic crust.The zircon U-Pb ages of the two samples are 743.2±6.1 Ma and 743.7±5.9 Ma,respectively,indicating that the Banbanshan monzogranite was formed in Neoproterozoic.Combined with the previous research results,it is considered that the Banbanshan area had been in the stage of paleooceanic subduction before 743 Ma,and after 743 Ma,the paleooceanic basin was closed,Rodinia supercontinent began to converge,and land collision occurred.