东秦岭丹凤地区钾长花岗岩年龄、地球化学特征及其地质意义

Zircon U-Pb Age,Geochemical Characteristics and Their Geological Significance of Moyite From Danfeng Area in the Eastern Qinling Mountains

东秦岭丹凤地区古生代构造岩浆活动十分活跃。为进一步约束早古生代后碰撞阶段的开始时间,通过LA-ICP-MS锆石U-Pb年龄、主量元素和微量元素测定,对马莲滩一带产出的钾长花岗岩进行了岩相学、地球化学和同位素年代学研究。结果表明,砖红色钾长花岗岩是高硅(71.79%~72.99%)、过钾质(K2O/Na2O 3.02~4.32)、偏碱(AI值0.80~0.89)的过铝质(A/CNK值1.03~1.16)S型花岗岩,它相对富集大离子亲石元素Rb、Th、U、Pb,明显亏损Ba、Nb、Ta、Sr、Ti等元素,其轻稀土元素分馏较强、重稀土元素分馏较弱且铕负异常。其岩浆锆石在CL图像上颜色较深,协和年龄为(430.8±2.6)Ma。该钾长花岗岩是后碰撞构造环境下由富含黏土质或泥质岩石低度部分熔融形成的岩浆岩,它约束了东秦岭地区古生代碰撞造山的结束时间,标志着早古生代碰撞造山作用进入后碰撞阶段(第二期第四阶段)。

The tectonic magmatism is very active in the Danfeng area from the eastern Qingling mountains in Paleozoic.In order to further constrain the starting time of post collision,the petrology,petrography,geochemistry and isotopic chronology of the brick-red moyite distributed at the Maliantan area were investigated through the analysis of LA-ICP-MS zircon U-Pb age,major and trace elements.The results show that the moyite is high silicon(71.79%~72.99%),potassium(K2O/Na2O 3.02~4.32),alkali(AI 0.80~0.89)and peraluminous(A/CNK 1.03~1.16)S type granite.It is characterized by enrichment in LILEs(e.g.,Rb,Th,U and Pb)and LREEs,and depletion in Ba,Nb,Ta,Sr,Ti and HREEs,with Eu negative anomalies.Magmatic zircons from moyite are darker in CL images,with concordant age of(430.8±2.6)Ma.The moyite was formed from low degree part-melting of clayey or muddy rocks under the post-collision tectonic environment,which restricts the ending time of strongly collision of the Paleozoic orogeny,and it means the eastern Qinling mountains entered into post collision stage(the fourth stage of the second period).