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《Acta Geological Sinica》 2007-01
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Geochemical Characteristics of the Mafic Dyke Swarms in South Tibet

JIANG Sihong,NIE Fengjun,HU Peng,LIU Yan,LAI Xinrong Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing,100037  
Widely distributed in Gyangzê-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 ε_ 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.
【Fund】: 国家重点基础研究发展规划项目(973)(编号2002CB412606)资助的成果
【CateGory Index】: P588.1
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