山东昌乐刚玉中锆石包裹体的地球化学特征

摘要

玄武岩中刚玉巨晶形成过程与壳幔相互作用有关,因此对其研究具有极高的科学价值.虽然多年来众多学者对新生代碱性玄武岩中刚玉在包裹体、地球化学等方面做了大量工作,但是到目前为止大家尚未就刚玉巨晶成因达成统一认识.本文通过扫描电镜对压碎后的刚玉进行观察,发现刚玉中含有锆石包裹体.为了进一步探究山东昌乐刚玉巨晶的成因,本文对包裹在刚玉巨晶中的锆石进行了氧同位素、微量元素、Lu-Hf同位素的分析.因为刚玉具有致密的晶格,锆石包裹体不会在碱性玄武岩浆的高温作用下Pb丢失,所以锆石包裹体的U-Pb年龄可以代表刚玉的结晶年龄.本次得到的年龄数据为(15.5±0.4) Ma,与宿主玄武岩的喷发年龄一致,并且与寄主玄武岩有一致Hf、O同位素,说明刚玉形成与岩浆作用有关.与此同时,锆石包裹体与同产区的锆石巨晶具有一致的Hf同位素和微量元素特征说明刚玉巨晶与锆石巨晶在形成上关系密切.虽然部分锆石微量元素具有壳源特征,综合本次试验所有结果(δ18OVSMOW=4.31‰～6.10‰,εHf(t)=5.9～11.1,∑REE=53～1854 μg/g)山东昌乐刚玉中锆石包裹体以地幔特征为主.%It is of high scientific value to research the corundum found in basalt,because the formation of corundum is related to crust-mantle interactions.Although many scholars have worked extensively on Cenozoic alkaline basalt corundum over the years,they have not agreed on the origin of corundum.Zircon inclusions were found through scanning electron microscopy in crushed Changle corundum.In order to further explore the cause of the Changle corundum,the zircon inclusions were analyzed for oxygen isotopes,U-Pb isotopes,trace elements,and Lu-Hf isotopes.Because of the corundum dense lattice,the zircon inclusion would not lose Pb under geological temperatures.Therefore,the U-Pb age of the zircon inclusion can represent the crystallization age of corundum.The age ((15.5 ± 0.4) Ma) is consistent with the host basalt eruption age;therefore,the formation of corundum is related to magmatism.At the same time,zircon inclusions and megacrystal zircon are consistent with similar Hf isotopes and trace elements.Therefore,there may be some links in the origin of both.Although some zircons have crustal characteristics in trace elements,most zircon inclusions in Changle corundum given mantle feather of δ8OVSMOW =4.31.‰-6.10‰,εHf(t) =5.9-11.1,∑REE =53-1854 μg/g.