阿尔泰额尔齐斯带东段酸性岩墙群地球化学特征及其地质意义

Geochemical characteristics of acidic dike swarm from the eastern segment of the Ertix tectonic belt,Altai orogeny and its geological implications.

野外地质调查发现在阿尔泰南缘额尔齐斯构造带东段、额尔齐斯活动断裂与富蕴-锡泊渡断裂之间发育了一套未变形的酸性岩墙群。岩墙群侵位于海西期片麻岩化花岗岩和上石炭统深变质的额尔齐斯组岩层中,薄片鉴定和岩石地球化学分析确定为具有细晶结构的流纹斑岩。岩石的SiO_2含量为70.9%～75.38%,K_2O+Na_2O含量为7.55%～8.99%;大部分样品Na_2O>K_2O,里特曼指数值为1.8～2.5之间,Al_2O_3=12.80%～14.53%,A/CNK=0.9～1.1,NK/A=0.7～0.9,具有准铝质—弱过铝质、低镁、高钾钠、低钙和锶、高(Fe)_(mol)/(Mg)_(mol)特点,具A型花岗岩类特点,属于亚碱性脉岩。岩石轻稀土富集,大离子亲石元素相对富集,具有明显的中等负铕异常,壳源特征明显。锆石U-Pb二次粒子微探针测年显示岩墙群形成于277～286Ma。推断岩墙是阿尔泰海西期造山运动结束后,在拉张构造环境下的地壳局部熔融产物。

Field investigation revealed series of undeformed acidic dike swarm developed between the Ertix active fault and the Fuyun-Xibodu fault in the eastern segment of the Ertix tectonic belt, southern Altai orogeny, where Hercynian gneissic granites and the Upper Carboniferous Ertix Formation outcropped. The content of SiO2 in the dike is 70.9% - 75.38% , and that of K2O ＋ Na2O is 7.55% -8.99%. For most of samples, the content of Na is larger than that of K. The Rittmann index （B） varies from 1.8 to 2.5. The content of AI203 equals to 12.8% - 14.53%. The value of A/CNK is 0.9 - 1.1, and that of NK/A ranges from 0.7 to 0.9, suggesting that the dike should be attributed to A-type granitoids and sub-alkaline rock with metaluminous-low peraluminous character and lower Magnesium, higher Kalium and Natrium, lower Calcium and Strontium, and higher （Fe/Mg） mal. Analyses of trace and rare earth elements showed that these rocks are rich of light rare earth elements and relatively rich of great ion lithophile elements with obviously middle minus-Eu abnormity, indicating obviously fountainhead of crust resource. The U-Pb dating of zircon by Second Ion Microprobe revealed that the dike formed between 277 -286Ma. Therefore, it was suggested that acidic dike swarm resulted from the partial melting of crustal material in regional extensional surrounding after the end of the Hercynian Altai orogenesis.