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    南岭中段中生代构造-岩浆活动与成矿作用研究进展

    Progress in the study of Mesozoic tectono-magmatism and mineralization in the central segment of the Nanling Mountains-Summary of major achievements of the 1∶250,000 geological survey in southeastern Hunan

    • 摘要: 提要:对南岭中段北部湘东南地区中生代构造-岩浆活动及成矿作用进行了较系统研究,主要提出以下认识。(1)中三叠世后期—中侏罗世初(早中生代)为板内造山阶段。中三叠世后期在区域NWW—SEE向挤压构造体制下发生强烈的陆内俯冲、汇聚作用,形成大量东倾为主的NNE向逆冲断裂与褶皱。其中茶陵—郴州断裂以东隆起区的隔槽式褶皱形成机制为“厚皮式”而非“薄皮式”。NW向基底隐伏断裂产生强烈左旋走滑,并使构造线产生逆时针旋转,形成了安仁“y”字型构造和水口山—香花岭南北向构造。挤压造山使地壳持续大幅增厚、深部地壳温度持续升高。中三叠世末—晚三叠世后期(233~210 Ma)区域挤压应力松弛,被加热的中地壳下部岩石熔融,同时存在幔源基性岩浆底侵,从而于后碰撞环境下产生较大规模的花岗质岩浆活动。晚三叠世末—早侏罗世因同造山上隆伸展作用而形成裂陷盆地,中侏罗世初期在区域NNE向左旋汇聚走滑体制下形成逆冲断裂及山前冲断收缩盆地、NW向右旋走滑断裂等。早中生代板内造山活动的动力机制主要与板块汇聚的远程挤压效应有关。(2)中侏罗世早期—白垩纪(晚中生代)为后造山—陆内裂谷伸展阶段。中侏罗世早期—晚侏罗世(174~135 Ma)因岩石圈拆沉而发生大规模花岗质岩浆活动与成矿作用。岩体的被动就位机制、暗色镁铁质微粒包体的发育、Sr-Nd同位素特征、以(高钾)钙碱性岩类为主的岩石组成、构造环境的岩石地球化学判别、大规模有色金属成矿、区域构造演化背景等,指示该时期为后造山构造环境。白垩纪进入强烈的陆内伸展阶段,形成盆-岭构造和相关的离散走滑断裂,广泛发育各类岩脉,局部形成AA型花岗岩小岩体和基性火山岩。热年代学资料暗示盆-岭构造的演化先后经历了构造剥蚀和风化剥蚀-沉积两个阶段。(3)造成湘东南燕山早期花岗岩成矿能力远高于印支期花岗岩的原因,主要是区域构造环境暨构造体制差异,即燕山早期后造山伸展构造体制下岩体中矿质更易于向周围扩散并沉淀,而印支期后碰撞环境弱挤压体制下矿质则被封闭;其次是花岗岩岩石地球化学特征差异,即构造-岩浆演化历史和深部成矿流体的参与使燕山早期花岗岩具有更好的成矿岩石地球化学条件。(4)燕山早期钨锡多金属和铅锌多金属两类矿床组合的形成可能主要与岩石圈结构(厚度)和深部热扰动强度,以及相应的岩浆作用规模和岩体侵位深度等因素有关。

       

      Abstract: Abstract:Mesozoic tectono-magmatism and mineralization in southeastern Hunan in the central segment of the Nanling Mountains have been systematically studied. The following ideas are put forward. (1) The period from the late Middle Triassic to early Middle Jurassic (early Mesozoic) was an intracontinental orogenic stage. The late Middle Triassic witnessed strong intracontinental subduction-convergence in a regional WNW-ESE compressional regime, which resulted in formation of large numbers of E-dipping, NNE-verging thrusts and folds. The formation mechanism of the trough-like folds in an uplift area east of the Chaling-Chenzhou fault were "thick-skinned" rather than "thin-skinned". NW-trending basement hidden faults underwent strong sinistral strike-slip movement under WNW compression, which led the NNE-directed lineament to rotate counterclockwise, thus forming the Anren “y”-type structure and the N-S-trending Shuikoushan-Xianghualing tectonic zone. The strong compression made the crust thicken substantially and caused the temperature of the deep crust rise continuously. In the terminal Middle Triassic-latest Triassic (233-210 Ma), the regional stress weakened, the heated middle crust melted and mantle-derived basic magma underplated; as a result, large-scale granitic magmatism took place in a post-collisional setting. Down-faulted basins formed by syn-orogenic uplift-extension in the terminal Late Triassic-Early Jurassic. Piedmont thrust-compressional basins and thrusts and NW-trending dextral strike-slip faults occurred in a NNE-trending sinistral convergence and strike-slip regime at the beginning of the Middle Jurassic. The dynamic mechanism of early Mesozoic intracontinental orogeny is mainly related to far-field compression effects of plate convergence. (2) The early Middle Jurassic-Cretaceous (late Mesozoic) was a post-orogenic-intracontinental rifting stage. In the early Middle Jurassic-Late Jurassic (174-135 Ma), there occurred large-scale granitic magmatism and mineralization due to lithospheric delamination. The passive emplacement mechanism of the plutons, occurrence of mafic microgranular enclaves, Sr-Nd isotopic features, predominance of high-K calc-alkaline and calc-alkaline rocks in the plutons, geochemical discriminations of structural environment, large-scale nonferrous metal mineralization and tectonic evolution setting suggest a post-orogenic environment in the stage. The Cretaceous was a strong intracontinental extensional stage, when basin-range tectonics and related divergent strike-slip faults formed, various kinds of dikes were widely developed and small AA-type granite bodies and basic volcanic rocks originated locally. Thermochronological data suggest that the evolution of the basin-range tectonics progressed through the tectonic denudation stage and weathering erosion-sedimentation stage. (3) The cause for the fact that the ore-forming capacity of the early Yanshanian granites in southeastern Hunan was far larger than that of the Indosinian granites is mainly attributed to the differences in tectonic setting and tectonic regime. The ore materials in magma of granites could spread and be precipitated more easily in an Early Yanshanian post-orogenic extensional regime; on the contrary, the ore materials were confined in the pluton in an Indosinian post-collisional weakly compressional setting. The second cause of the difference in ore-formation capacity is due to the difference in geochemistry of granites, i.e. the tectono-magmatic evolution and introduction of deep ore-forming fluids caused the early Yanshanian granites to have better ore-forming geochemical conditions than the Indosinian granites. (4) The formation of the W-Sn-polymetallic and Pb-Zn-polymetallic deposit assemblage is probably mainly related to such factors as the structure (or thickness) of the lithosphere, intensity of thermal perturbations in the deep interior of the Earth and corresponding scale of magmatism and depth of pluton emplacement.

       

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