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    志留纪昆仑山地区构造古地理环境及其成矿意义

    Silurian tectonic-paleogeographic environment in Kunlun Mountain area and its metallogenic significance

    • 摘要: 提要:青藏高原北部基础地质调查与研究工作中,在昆仑山地区测得志留纪(S),奥陶纪—志留纪(O-S)剖面39条,剖面厚度从169 m到8260 m不等,地层的上下接触关系多为断层和角度不整合。从沉积环境、物质组成等方面,反映出昆仑地区从西向东古地理环境为滨浅海-深海-滨浅海-深海的分布特征。西昆仑温泉沟群(S1W)在英吉莎县西,以砂岩、板岩为主,沉积环境为浅海陆棚;在麻扎一带,主要为砂岩、粉砂岩、板岩及硅质岩等,麻扎附近的石英片岩、砂板岩及灰岩中,含拉斑玄武岩,为活动大陆边缘环境下,深海相(俯冲带海沟沉积环境,麻扎一带),斜坡相浊积岩沉积(麻扎东、西一带),在麻扎一带叠复厚度8260 m,向北西、南东方向变薄;东昆仑在木孜塔格峰北,该群为一套深海-半深海复理石沉积,由灰色、浅灰色浅变质各种岩屑砂岩、粉砂岩、泥岩组成。中晚志留世达坂沟群(S2-3D)在西昆仑为半深海到滨浅海沉积环境,在东昆仑沉积一套碳酸盐岩夹碎屑岩地层,以浅海环境为主。在若羌南的白干湖组(S1b)发育深海浮游生物——笔石,为深海相浊积岩,从下到上浊积扇向海推进,水体逐渐变深,属拉张环境下的深海弧后盆地沉积环境。在格尔木西南志留系赛什腾组(Ss)自下而上由大陆斜坡至陆隆和深海沉积到浅海的沉积环境演化,具有非扇大陆斜坡沉积类型,以远源浊积岩、半远洋沉积为特征,代表古海盆收缩到残留海盆阶段的沉积。布咯达坂峰东,加里东晚期(S3)同碰撞型二长花岗岩体锆石的LA-ICP-MS U-Pb年龄(421±3) Ma、(423±16)Ma。在格尔木东南的纳赤台群(OSN)超镁铁岩-辉绿岩墙-玄武岩-深水硅质岩构成早古生代的洋壳组合,具有蛇绿岩套特征。基性变玄武岩显示为洋中脊或洋岛环境。硅质岩的地球化学指标也显示为洋中脊或大洋盆地的远洋深水环境,超镁铁岩是古洋壳残片的地幔岩部分,变碎屑岩属大陆斜坡相沉积,碳酸盐岩具近岸沉积特点。格尔木东南纳赤台群大洋中脊拉斑玄武岩(MORB)型,锆石SHRIMP 206Pb/238U年龄的加权平均值为(419±5)Ma,为S4。总体上,志留纪昆仑地区应是加里东运动之后在逐渐消失的残留洋,表现出复杂的多岛洋盆体系。这一构造古地理环境的确定,为该地区已经发现的矿床形成机制的研究和下一步的找矿工作提供了重要的依据。

       

      Abstract: Abstract:In the research work conducted in the Kunlun Mountain area, the authors measured 39 Silurian (S) and Ordovician-Silurian (O-S) geological sections, whose thicknesses range from 169 m to 8260 m, mostly with fault and angular unconformities between various strata. Littoral (shallow sea)-abyssal- littoral (shallow sea)-abyssal paleogeographic sedimentary environments characterize the study area, as shown by these sections.In West Kunlun, Wenquangou Group(S1W)is dominated by sandstone and slate, assuming a shallow sea(shelf sea)sedimentary environment, in west Yingjisha, and by silicalite, sandstone, siltstone and slate in Mazha area, with the existence of tholeiite, demonstrating a deep sea sedimentary environment (subduction zone ocean trench) in this area, a clinoform sedimentary environment in the west and east of Mazha, and thinning of strata northwestward and southeastward. In East Kunlun, Wenquangou Group(S1W)is composed of epimetamorphic rock, sandstone and mudstone, assuming abyssal-bathyal flyuzitage Peak. In middle and late Silurian(S2-3), Dabangou Group (S2-3D) was bathyal-littoral (shallow sea) in the west and shallow sea sediments such as carbonate and fragmental rock in the east.In East Kunlun, the Baiganhu Formation(S1b)in south Ruoqiang is characterized by abyssal turbidite aggradation with graptolites as the mark of the deep sea basin environment under the tensile tectonic condition. The Saishiteng Formation (S3)in south Geermu City shows the upward evolution from clinoform through continental rise and shallow sea to abyssal facies, characterized by turbidite and semi-pelagic aggradation in relict sea basin, with the zircon LA-ICP-MS U-Pb ages of monzonitic granite being 421±3 Ma and 423±16 Ma. The Nachitai Group(OSN)strata in southeast Geermu City possess ultramafite-diabase-basalt-deep sea silicalite, derived from ocean crust substance. Ultramafite is a part of ocean crust relic fragments, metamorphic basalt marks midoceanic ridge and ocean island arc, silicalite signifies midoceanic ridge or big ocean basin sediments in geochemical features, metamorphic fragmental rock implies clinoform sediments, and carbonate means littoral sediments. The SHRIMP zircon 206Pb/238age of big midoceanic ridge tholeiite(MORB)in the strata is 419±5Ma, suggesting Upper Silurian(S4). On the whole, the Kunlun Mountain in Silurian was a relict ocean exhibiting a complex ocean system with more islands and basins formed after Caledonian movement. The results obtained by the authors in the study of tectonic-paleogeographic environment provide important evidence for the study of the formation mechanism of the known ore deposits and the arrangement of further ore prospecting work in this area.

       

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