山东昌乐新生代碧玄岩中歪长石巨晶特征及成因  

Characteristics and genesis of anorthoclase megacrysts in Cenozoic basanite from Changle area, Shandong Province

作　　者：杜青 孟凡超 王千军[3] 田雨露 韩慧妹 刘浩毅 左耿超 Du Qing;Meng Fanchao;Wang Qianjun;Tian Yulu;Han Huimei;Liu Haoyi;Zuo Gengchao(School of Geosciences,China University of Petroleum,Qingdao,Shandong 266580;Pilot National Laboratory for Marine Science and Technology,Qingdao,Shandong 266061;Exploration and Development Research Institute of Shengli Oilfield Company,Dongying,Shandong 257015)

机构地区：[1]中国石油大学(华东)地球科学与技术学院,山东青岛266580 [2]青岛海洋科学与技术试点国家实验室,山东青岛266061 [3]中国石化胜利油田分公司勘探开发研究院,山东东营257015

出　　处：《地质科学》2022年第3期766-781,共16页Chinese Journal of Geology(Scientia Geologica Sinica)

基　　金：中国石油大学(华东)研究生创新工程资助项目(编号:YCX2021020);青岛海洋科学与技术试点国家实验室山东省专项(编号:2021QNLM020001-1);山东省自然科学基金面上项目(编号:ZR2021MD083)资助。

摘　　要：山东昌乐新生代玄武岩中发育大量长石、辉石和刚玉巨晶。前人的研究多集中在辉石和刚玉上,而长石巨晶的来源仍存在争议。本文对新生代玄武岩岩石学、地球化学,以及长石巨晶矿物特征、包裹体成分和原位主微量元素进行了系统分析,结果显示:长石巨晶主要存在于K_(2)O和Na_(2)O含量较高、SiO_(2)含量较低的碧玄岩中。背散射图像下长石巨晶无环带,存在长石、辉石微晶及玻璃质组成的反应边。巨晶原位主量元素分析表明:长石为歪长石,An值为4.76%~5.62%,Ab值为49.81%~71.89%,巨晶核部Mg、Fe、Na、Al、K等元素及An、Ab的含量处于小范围内的无序波动状态,无明显的变化规律,可能为地幔环境下结晶的高温无序矿物;长石巨晶富集轻稀土元素(LREE),亏损重稀土元素(HREE),具有明显的Eu正异常(δEu=12.37~26.22),Sm、Nd、Gd相对亏损,核部稀土元素总量较低,ΣREE=1.98×10^(-6)~2.46×10^(-6),LREE/HREE=21.71~76.51,(La/Yb)N为18.14~36.70,长石巨晶与碧玄岩的稀土元素总量比值在0.006~0.015之间,暗示两者无演化联系。综合歪长石巨晶以及寄主岩浆的成因特征,本文认为歪长石巨晶结晶于含有洋壳俯冲流体组分的地幔环境,后期被来源于软流圈地幔的玄武质岩浆捕获,带出地表。A large number of feldspars,pyroxene and corundum megacrysts were found in the Cenozoic basalts in Changle,Shandong Province.Previous studies mainly focus on the genetic characteristics of pyroxene and corundum,but the origin of feldspar megacrysts is still controversial.In this study,mineralogical and in situ analyses of major and trace elements of feldspar megacrysts in Cenozoic basalts from Changle were conducted.The results show that feldspar megacrysts mainly exist in basanite with high K_(2)O and Na_(2)O contents and low SiO_(2) contents.The backscatter images show no bands in the feldspar,and the reaction edges are mainly composed of feldspar,pyroxene microcrystals and vitreous.Laser ablation data show that the feldspar megacrysts are anorthoclases with An value of 4.76%~5.62%and Ab value of 49.81%~71.89%.The contents of Mg,Fe,Na,Al,K and An and Ab in the giant crystal nucleus fluctuate in a small range without obvious change rule,indicating that the anorthoclase is a high-temperature disordered mineral crystallized in the mantle source environment.The samples are enriched in light rare earth elements(LREE)and depleted in heavy rare earth elements(HREE),with obvious positive Eu anomaly(δEu=12.37~26.22).The Sm,Nd and Gd contents in anorthoclases are relatively depleted,and the total amount of rare earth elements in the core is low(ΣREE=1.98×10^(-6)~2.46×10^(-6),LREE/HREE=21.71~76.51,(La/Yb)N=18.14~36.70).The ratio of total rare earth elements between anorthoclases and host rock is 0.006~0.015,indicating disequilibrium relationship between the anorthoclase and the host magma.Based on the characteristics of mineralogical petrology and elemental geochemistry of host rocks and anorthoclase,we consider that the anorthoclases are the product of early crystallization under the background of lithospheric mantle metasomatism by subduction fluid of oceanic crust,and were captured by basaltic magma from asthenosphere.Therefore,the basanite magma is likely mixed with melts from lithosphere and asthenosphere.

关 键 词：新生代玄武岩 长石巨晶 碧玄岩成因 昌乐火山 中国东部 

分 类 号：P571[天文地球—矿物学] P588.145[天文地球—地质学]