华北克拉通燕辽裂谷长城纪碱性岩锆石U-Pb年代厘定和Hf-O同位素特征  

Zircon U-Pb dating and Hf-O isotope characteristics of Changchengian alkaline rocks from the Yanliao Rift in the North China Craton

作　　者：张健[1,2,3] 李怀坤[1,2,3] 田辉[1,2,3] 刘欢 周红英[1,2,3] 刘文刚 ZHANG Jian;LI HuaiKun;TIAN Hui;LIU Huan;ZHOU HongYing;LIU WenGang(Tianjin Centre,China Geological Survey Tianjin 300170,China;Precambrain Geological Research Centre,China Geological Survey,Tianjin 300170,China;North China Center for Geoscience Innovation,China Geological Survey,Tianjin 300170,China)

机构地区：[1]中国地质调查局天津地质调查中心,天津300170 [2]中国地质调查局前寒武纪地质研究中心,天津300170 [3]中国地质调查局华北科技创新中心,天津300170

出　　处：《岩石学报》2021年第1期231-252,共22页Acta Petrologica Sinica

基　　金：中国地质调查局项目(DD20190359、DD20190009)资助.

摘　　要：基性的碱性岩通常形成于伸展环境,具有富碱和不相容元素富集等地球化学特征,它们来源于深部富集地幔,是探索地幔交代和深部地球动力学的"岩石探针"。华北克拉通北缘燕辽裂谷内发育团山子组和大红峪组钾质火山岩以及侵入串岭沟组的钠质岩脉,它们共同组成了长城纪碱性岩系列。本文利用SHRIMP锆石U-Pb同位素方法对平谷-蓟县地区钾质火山岩进行年代学测试,获得误差范围内一致的年龄:1613±11Ma、1634±18Ma(团山子组)和1605±19Ma、1630±10Ma(大红峪组),说明钾质火山岩喷发的持续时间短,且与侵入串岭沟组的钠质岩脉同期(~1625Ma)。钾质火山岩和钠质岩脉的锆石Hf-O同位素组成相似:εHf(t)=-2~+4(正态分布峰值+0.6),δ^18O=4.5‰~7.7‰明显高于正常地幔锆石的值,钾质火山岩和钠质岩脉的母岩浆源自相同的富集地幔,源区受到高δ^18O物质的交代作用而富含钾质矿物,如角闪石或金云母。华北克拉通中-新元古代岩浆事件是伸展环境的产物,其锆石Hf同位素组成在~1.32Ga发生突变,由富集到亏损,暗示岩石圈地幔经历强烈的交代改造作用,可能成为检验地幔柱或者板块深俯冲驱动超大陆裂解机制的窗口。Mafic alkaline rocks are geochemically enriched in alkali and incompatible elements and genetically related to extensional settings.They are derived from enriched mantle,and thus can decipher the mantle metasomatism and deep geodynamics.The Changchengian alkaline rocks,outcropped in the Yanliao Rift at northern margin of the North China Craton,include the potassium volcanic rocks of the Tuanshanzi and Dahongyu formations along with the sodium dike intruding into the Chuanlinggou Formation.SHRIMP zircon U-Pb isotope chronological dating of the volcanic rocks from Pinggu-Jixian area yield concordant ages of 1613±11Ma,1634±18Ma(the Tuanshanzi Formation)and 1605±19Ma,1630±10Ma(the Dahongyu Formation),respectively.All these consistent ages within analytical error indicate the potassic volcanics were short-lived eruption,broadly coeval with the ca.1625Ma sodic dikes.The potassic volcanic rocks and sodic dikes display similar zircon Hf-O isotope compositions,i.e.,εHf(t)ranging from-2 to+4 with a Gaussian distribution pattern peaking at+0.6,whileδ^18O ranges from 4.5‰to 7.7‰mostly higher than normal mantle zircon value.The potassic volcanic rocks and sodic dikes,composed of K-rich minerals such as hornblende or phlogopite,are originated from the common enrichment mantle which was previously metasomatized by elevatedδ^18O materials.The North China Craton underwent multi-rifting events during the Meso-and Neo-proterozoic,their zircon Hf isotopes abruptly shift from enrichment to depletion at ca.1.32Ga,suggesting that the lithospheric mantle underwent strong metasomatism.The Mesoproterozoic mafic rocks in North China Craton can play a key role for testing the mechanism of supercontinental breakup driven by mantle plumes or/and deep plate subduction.

关 键 词：锆石 U-PB年代学 HF-O同位素 碱性岩 燕辽裂谷 长城纪 

分 类 号：P588.15[天文地球—岩石学] [天文地球—地质学] P597.3[天文地球—地球化学]