Geochemical Characteristics of the Mafic Dyke Swarms in South Tibet
Geochemical Characteristics of the Mafic Dyke Swarms in South Tibet
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发表时间：
2007
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通讯作者：
Lai Xinrong
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作者：
Lai Xinrong
元素地球化学特征表明，这些基性岩脉具有中等LREE富集、无明显Eu异常的特征，岩浆分馏作用弱。不相容的微量元素模式表现为Ta、Ce相对富集和La、Y、Yb槽。206Pb/204Pb和207Pb/204Pb比值变化不大，分别为18.330 ~ 18.717和15.504 ~ 15.674；208Pb/204Pb比值和μ值分别在37.664 ~ 39.156和9.296 ~ 9.931范围内变化较大。初始87Sr/86Sr比值为0.7044 ~ 0.7105,e_ Nd(t)值为-4.49 ~ +6.77，其中大部分为正。元素地球化学和Pb-Sr-Nd同位素研究表明，基性岩脉群可能来源于贫地幔和富地幔岩浆的混合（EMⅡ）。基性岩浆的快速侵位导致分选作用弱，而不同来源岩浆的非均质混合导致HFSE槽不明显，LILE富集程度强，初始87Sr/86Sr比值变化较大。
Widely distributed in Gyangze-Chigu area,south Tibet,the NW-and nearly EW-trending mafic to intermediate dyke swarms are regarded as the product of the large-scale spreading of the late Neo-Tethys ocean.This paper reported the systematic research on the element geochemistry and Pb-Sr-Nd isotopes of the mafic dyke swarms in south Tibet.According to the element geochemistry,these mafic dykes are characterized by moderate LREE enrichment and no obvious Eu anomalies,which means the weak magmatic fractionation.Incompatible trace element patterns exhibit relative Ta and Ce enrichment and La,Y and Yb trough.The 206Pb/204Pb and 207Pb/204Pb ratios vary narrowly,ranging from 18.330 to 18.717 and from 15.504 to 15.674,respectively;whereas the 208Pb/204Pb ratios and μ values demonstrate a wide variation from 37.664 to 39.156 and from 9.296 to 9.931,respectively.The initial 87Sr/86Sr ratios range from 0.7044 to 0.7105 and the e_ Nd(t)values from-4.49 to +6.77,of which most are positive.Based on the research on the element geochemistry and Pb-Sr-Nd isotopes,the mafic dyke swarms may be derived from the mixture of magmas from depleted mantle and enriched mantle(EMⅡ).The rapid emplacement of the mafic magma made the weak fractionation,while the heterogeneous mixing of magmas from different sources resulted in no clear HFSE trough and strong enrichment of LILE,and large variation of the initial 87Sr/86Sr ratios.It shows the complex mantle structure underneath the Qinghai-Tibet Plateau.The data presented in this paper is of great significance to study the mantle evolution of the Qinghai-Tibet Plateau from Mesozoic to Cenozoic.