藏西措勤含铜双峰岩系的发现及其意义

Discovery and singificance of copper-bearing bimodal rock series in Coqin area of Tibet.

青藏高原中-西部措勤县城东南(约100km)的日阿铜矿区发育两种矿化类型,一种是与二长花岗斑岩岩株有关的夕卡岩型铜矿化,另一种是与辉绿玢岩脉有关的细脉浸染型(斑岩型)铜矿化。锆石SHRIMP U-Pb定年发现二者为一套双峰含铜岩系,酸性端元二长花岗斑岩年龄为90．1±1．5Ma,MSWD=1．24;基性端元辉绿玢岩年龄为87．2±1．6 Ma,MSWD= 0．84,形成于中—晚白垩世。岩石化学分析表明这些含铜双峰岩石富K_2O,属钾玄岩和高钾钙碱性岩。基性端元SiO_2在49．56%～54．9%之间;酸性端元SiO_2在67．21%～71．35%之间,戴里间隔为12．31%。微量元素地球化学显示岩石强烈亏损高场强元素Nb、Ta,富集大离子不相容元素Rb、K、Sr、Pb,具有岛弧岩浆作用特征。K的富集和Ba相对亏损说明岩石形成于碰撞后伸展阶段。在碰撞后伸展环境中,富集的大陆岩石圈地幔发生部分熔融形成了具OIB特征的镁铁质岩浆。这些基性岩浆底侵与地壳岩石相互作用形成了措勤含铜双峰岩系。这个岩系产于拉萨地块内部,形成于碰撞后伸展环境,与以前人们提出的新特提斯洋壳在白垩纪沿雅鲁藏布缝合线位置向拉萨地块之下低角度俯冲的构造模式不符。特别是联系到最近在阿索(Asa)盆地中发现的早白垩世弧后双峰火山岩,笔者认为措勤双峰含铜岩系应该与中特提斯洋壳沿班公-怒江缝合带位置向南向拉萨地体之下的俯冲有关,或者中特提斯洋壳可能有多条俯冲带,它们有的向南俯冲,有的向北俯冲。

Ria copper deposit southeast （about 100km） of Coqin county city in western central Tibet plateau includes two mineralization types. One is skarn-type copper mineralization related to adamellite porphyry stocks, and the other is veinlet-disseminated （porphyry-type） copper mineralization related to diabase porphyrite dyke. Zircon SHRIMP U-Pb dating demonstrates that they are copper-bearing bimodal rock series, with the felsic end-member having an age of 90.1 ± 1.5Ma and MSWD = 1.24 and the mafic end-member having an age of 87.2 ± 1.6Ma and MSWD =0.84. Major element analyses show that these copper-bearing bimodal rocks are high in K2O and belong to shoshonite and high-K calc-alkaline rock. The mafic end-member has a SiO2 range of 49.56% - 54.90% and K2 O contents of 2.35% - 3.51%. And the felsie end-menber has a SiO2 range of 67.21% - 71.55% and K2 O contents of 3.48% -4.23%. The Daly gap is 12.31%. Trace element geochemistry displays strong depletion of hight field strength element （HFSE） Nb, Ta and enrichment of large ion lithophile element （ LILE ） Rb, K, Sr, Pb, which are typical of arc magmatism. Their K-enrichment and relative Ba-depletion indicates that these bimodal rocks formed in a post-collisional extensional stage. In such a postcollisional extensional environment enriched lithospheric mantle was subjected to partial melting to generate mafic magma. The mafic magma intruded and interacted with crustal rocks at different depth to form the copper-bearing bimodal rock series. Coqin copperbearing bimodal rock series is located within the Lhasa lerrane and was generated in a post-collisional extensional environment, which conflict with the tectonic model proposed formerly that the Yaluzangbo oceanic crust was subducted in a low degree beneath the Lhasa terrane in Cretaceous period. Especially considering the early Cretaceous back-arc bimodal volcanic rocks found recently in the Asa basin, the authors think that Coqin copper-bearing bimodal rock series was probably related to toward south subduction of Bangong- Nujiang oceanic crust beneath the Lhasa terrane, or the Bangong-Nujiang suture zone with two subduction directinos of toward south and north.