西藏冈底斯南缘火山-岩浆弧带中桑日群adakite的发现及其意义

Discovery and geological significance of the adakite in Gangdise island arc belt,Xizang(Tibet).

冈底斯南缘火山-岩浆弧带,位于雅鲁藏布江缝合带与冈底斯陆块之间。近年来的研究表明,它是新特提斯洋多次俯冲消减的产物。洋壳大致经历了早(中侏罗世)、晚(晚侏罗—早白垩世)两次俯冲作用,由此形成的火山岛弧带在空间上呈北、南两条平行展布。本文,通过对晚期俯冲形成的南带岛弧火山岩(桑日群)的研究,并与新生代(＜25Ma)环太平洋adakite的对比研究表明,桑日群火山岩具有与adakite相似的特征:缺乏玄武岩,以安山岩-英安岩-流纹岩组合为主;地球化学特征表现为SiO_2平均为61．57%(≥56%)、Al_2O_3平均为16．36%(≥15%)、MgO平均为2．63%(＜3%),Na/K高(3．05);具有低的Yb(1．39×10^(-6),≤1．9×10^(-6))、Y(13．57×10^(-6),≤18×10^(-6))和高的Sr(646×10^(-6),＞400×10^(-6));REE配分型式为亏损重稀土,Eu异常不明显;Sr同位素组成与大洋板片熔融形成的adakite非常相近。这意味着,该弧南带(桑日群)的弧火山岩(＞25Ma)是新特提斯洋壳经俯冲部分熔融形成的;同时也暗示,它的形成可能是一种特殊动力学机制的产物——新特提斯主洋有关的小洋盆快速俯冲。而其成矿意义在于:它是超大型斑岩铜矿和中低温热液矿床成矿的标志之一。

Gangdise island arc belt is located between Yarlung Zangbo suture zone and Gangdise Plate. It is recently research shown that Gangdise island arc was the result of New Tethys time after time subduction, and approximately had come through early（ J2 ） and late（ J3-K1 ） twice subduction, which had produced North and South island arc distributed parallelly each other. On the basis of studies on arc-lava in the South island arc belt（ Sangri group）, and contrast it with Cenozoic（ 〈 25Ma） Circle-Pacific Ocean adakite, the author recognize that arc-lava in this arc has the same feature as adakite, namely ： arc-volcanics is composed of mainly andesite-daciterhyolite, but lack of basalt. It＇s geochemical is characteristics by Al-rich, Mg-rich and high Na/K; low Yb,Y, high Sr; REE pattern has HREE depletion and no obvious negative Eu anomaly; （^87 Sr/^86Sr）1 is similar to the adakite originated by partial melting of a subducted ocean slab, which predicate that arc-lava （ Sangri group） in this arc was originated by partial melting of a subducted New Tethys ocean crust, simultaneity, it suggest that it may be small ocean basin associated with New Tethys speedy subduction. It is ore-forming meaning is one of the symbol for seeking superlarge porphyry Cu and middle-low thermal ore deposits.