藏南侏罗纪残留洋弧的地球化学特征及其大地构造意义  

Remnant Jurassic intra-oceanic arc system in Southern Tibet:Geochemistry and tectonic implications

作　　者：王莉[1] 曾令森[1] 高利娥[1] 唐索寒[1] 胡古月[1] 

机构地区：[1]大陆构造与动力学国家重点实验室,中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2012年第6期1741-1754,共14页Acta Petrologica Sinica

基　　金：国家自然科学基金(41073024;40872048);中国地壳探测项目(SinoProbe-2-6)联合资助

摘　　要：沿雅鲁藏布江缝合带残留一系列中生代洋弧,厘定这些洋弧的形成时代和地球化学性质对于理解新特提斯洋的消减过程、确定南拉萨地体的组成和限定印度-欧亚板块的碰撞时限等都具有重要的意义。泽当微地体位于雅鲁藏布江缝合带东段,主要由英云闪长岩、花岗闪长岩和角闪岩组成。SHRIMP锆石U-Pb定年结果表明,位于泽当西部的花岗闪长岩(简称泽当花岗闪长岩)形成于157.5±1.4Ma,与东部的英云闪长岩形成时代相近。全岩元素和同位素(Sr和Nd)地球化学分析结果表明泽当花岗闪长岩具有以下地球化学特征:(1)较高的SiO2(64.4%～68.4%)和Al2O3(16.9%～18.4%);(2)较高的Na2O/K2O比值(>2.1),显示富钠的特征;(3)富集LREE,亏损HREE,从Ho到Lu稀土分布样式上翘((Ho/Yb)N=0.69～0.90)和明显的Eu负异常;(4)较低的Y(<7.19×10-6)和Yb(<0.88×10-6),较高的Sr/Y>119.7和La/Yb>22.4,在Sr/Y-Y和La/Yb-Yb图解中,泽当花岗闪长岩都落入埃达克岩区域内;(5)87Sr/86Sr(t)(0.704065～0.704439)值较低,εNd(t)(+5.1～+6.1)值较高,显示其来自亏损地幔的特征;(6)亏损Zr、Hf、Ti和Y等高场强元素,富集大离子亲石元素,显示了岛弧岩浆岩的典型特征。上述数据表明,泽当花岗闪长岩不仅具有明显的岛弧岩浆岩的特征,而且具有显著的埃达克质特征,可能是在来自地幔楔部分熔融体的板底垫托作用下,新生基性下地壳重熔的产物。泽当微地体是一个残留的晚侏罗纪洋弧系统,是中生代新特提斯洋洋内俯冲系统的残留。A remnant Jurassic intra-oceanic arc systems was preserved in the Zedong subterrane within the Yanglung-Tsangpo suture. This terrane was bounded by the Yanglung-Tsangpo suture to the south and the Gangdese thrust to the north. Characterizing the geochemical nature and formation age of this intra-oceanic arc system is critical to understand the northward subduction of Neo-Tethyan Ocean and would provide valuable constraints on the timing of India-Asia colhsion and the composition and structure of southern Lhasa terrane. SHRIMP zircon U-Pb dating indicates that the granodiorite formed at 157.5±1.4Ma, similar to that of tonalite to the east. Whole rock geochemical data and isotope （ Sr and Nd） data of the Zedong granodiorite show the following characteristics ： （ 1 ） high SiO2 （ 64. 4% - 68. 4% ）, Al2 O3 （ 16. 9% - 18.4% ）, low FeO^T （ 〈 0. 77 % ） and MgO （ 〈 1.16 % ） ; （2） relatively high Na2 O/K2O （ 〉 2. 1 ）, indicating that they are of granite rich in sodium; （3） enriched in LREE, depleted in HREE, slightly upward deflected HREE patterns for elements from Ho to Yb with （Ho/Yb）N = 0. 69 -0. 90, and pronounced negative Eu anomalies with Eu/Eu^＊ = 0. 1 -0.6; （4） relatively high Sr and LREE, but low Y （ 〈7.19×10^-6） and Yb （ 〈0.88×10^-6）, which leads to high Sr/Y （ 〉 119. 7） and La/Yb （ 〉22. 4） in these granodiorites; in addition, they are plotted within the adakite field in Sr/Y-Y as well as La/Yb-Yb diagrams; （5） relatively low initial Sr （^87Sr/^86Sr（t） =0.7041 -0.7042） and high εNd values （εNd（t） = ＋5.01- ＋6. 09）, indicating that they were mainly derived from depleted mantle （DM） or from rocks with a dominant DM signature; and （6） depleted in the HFSE （e. g. , Zr, Hf, Ti and Y）, indicating that they are of island-arc magmatic rocks. Our data indicate that the granitic rocks within the Zedong terrane represent the re-melting products of juvenile marie lower crust derived from the mantle wedge. The

关 键 词：泽当花岗闪长岩 俯冲作用 埃达克岩 残留洋弧 雅鲁藏布江缝合带 西藏南部 

分 类 号：P588.122[天文地球—岩石学] P597.3[天文地球—地质学]