浙东沿海富碱花岗岩地球化学特征及其构造环境探讨

在浙东沿海地区存在一条北北东向线型展布的、规模可观的富碱侵入岩带,岩带由碱长花岗岩和碱性花岗岩组成。其岩石地球化学特点是高硅(SiO2>76%)、富碱(N_2O K_2O>8.2%)、低铝(Al_2O_3<12.5%)、贫钙镁(CaO<0.4%、MgO<0.2%)、富集Rb、Th(U)、Nb(Ta)、Zr、Zn和Ga等元素;稀土元素模式曲线具缓右倾“V”字形,铕亏损强烈。上述地球化学特征与福建魁歧碱性花岗岩基本一致并类似澳大利亚东南Lachlan褶皱带A型花岗岩。研究表明,该富碱花岗岩带是燕山晚期岩浆演化后期产物,受区域深断裂控制,即可能是处于活动大陆边缘的深断裂在拉张条件下由深源富碱岩浆上侵而成。

A Tentative Discussion on the Geochemical Characters and Tectonic Setting of Alkali-rich Granites in the Coastal Area of East Zhejiang

There is a large-scale alkali-richintrusive rock zone, with an NNE striking in linear distribution, in the coastal area of east Zhejiang. It is composed of alkali-feldspar granites and alkaline granites. The alkali-rich intrusive rocks are characterized by Si-high (SiO_2>76%), alkali-rich(Na_2O K_2O>8.2%), Al-low (Al_2O_3<12.5%), Ca-and Mg-poor (CaO<0.4%,MgO<0.20%) and concentrations of Rb, Th(U),Nb (Ta), Zr, Zn,Ga, etc. The chondrite-normalized REE patterns exhibit a gently right-dipping V-type curve with a severe depletion in Eu. The characters arc essentially identical with those of alkaline granite in Kuiqi, Fujian and similar to those of the A-type granite in the achlan folded zone of southeast Australia. The, above studies indicate that the alkali-rich granite zone was formed as a result of the magmatic evolution at the late stage of the Yanshanian movement, controlled by a regional deep fracture, that is to say, its formation may be attributed to the upward intrusion of deep-source alkali-rich magma during the tension of the deep fracture on active continental margin.