地幔包体金云母^(40)Ar/^(39)Ar年龄对玄武岩浆喷发结束时间的约束  

Determining the ending time of basaltic volcanic activity by ^(40)Ar/^(39)Ar age of phlogopites from mantle-derived xenolith

作　　者：杜世俊[1] 徐兴旺[1] 杨列坤[1] 符超[1] 苏捷[1] 崔敏利[1] 

机构地区：[1]中国科学院矿产资源重点实验室中国科学院地质与地球物理研究所,北京100029

出　　处：《岩石学报》2009年第12期3251-3258,共8页Acta Petrologica Sinica

基　　金：中国科学院知识创新工程主要方向项目(KZCXZ-YW-107);"十一五"国家科技支撑计划重点项目(2006BAB07B01-03)联合资助

摘　　要：山东临朐-昌乐地区新生代岩浆活动强烈,以形成含大量地幔包体的玄武岩为特征。作者在考察与研究昌乐北岩古火山口玄武岩中地幔包体的过程中,发现一些包体中发育有地幔钾交代成因的金云母细脉。鉴于金云母的封闭温度远低于玄武岩浆啧发时的温度以及金云母可以记录喷发的玄武岩浆冷却通过金云母封闭温度的时间,本文尝试通过金云母所记录的^(40)Ar/^(39)Ar年龄来推断玄武岩浆喷溢的结束时间。该火山口玄武岩不同部位的三个地幔岩包体其金云母的^(40)Ar/^(39)Ar坪年龄分别为18.42±0.21 Ma、18.65±0.27Ma和18.39±0.36Ma,年龄结果具有很好的一致性,充分说明了该定年手段的有效性因此可以确定该火山口玄武岩浆喷溢活动约在18.5Ma前结束。由此推测山旺盆地中不整合发育在源于该火山口喷溢的玄武岩之上的化石群的形成时代下限约为18.5Ma。用玄武岩地幔包体金云母的^(40)Ar/^(39)Ar年龄来确定玄武岩年龄是玄武岩定年一个有效的新方法。Cenozoic basalts in the Linqu-Changle area,Shandong Province,are characterized by abundant mantle-derived xenoliths. Recently,we discovered that some xenoliths in the Beiyan paleovolcanic crater contain some fine phlogopite veins related to mantle K-metasomatism.Since the closure temperature of phlogopite is much lower than the eruptive temperature of basaltic magma and the phlogopite has potential to record the time when hot magma cool down through the closure temperature of phlogopite,here we try to use the ^(40)Ar/^(39)Ar age of phlogopites to determine the ending time of eruption of basaltic magmas and age of basalt in the Beiyan paleovolcanic crater.Three phlogopite samples from three mantle-derived xenoliths from different positions in the Beiyan paleovocanic crater yielded identical ^(40)Ar/^(39)Ar plateau ages of 18.42±0.21 Ma,18.65±0.27 Ma and 18.03±0.63 Ma respectively.It is concluded that the basaltic volcanic activities in the Beiyan paleovolcanic crater ended before about 18.5 Ma.It further suggests that the lacustrine sedimentary rocks containing abundant fossils in the Shanwang Basin unconformably overlying the basaltic rocks from the Beiyan paleovolcanic crater formed later after 18.5Ma.^(40)Ar/^(39)Ar dating of phlogopites is a new effective method for the dating of basalts containing phlogopite-bearing xenoliths.

关 键 词：地幔包体 金云母 岩浆喷发 结束时间 约束 VOLCANIC activity time 玄武岩浆 closure temperature 火山口 VOLCANIC activities sedimentary rocks 封闭温度 不整合发育 three effective different 岩浆冷却 岩浆活动 形成时代 

分 类 号：P58[天文地球—岩石学]