西藏东部江达构造带演化与成矿

【作者】 张宪依；

【导师】 施泽进；

【作者基本信息】 成都理工大学 ， 构造地质学， 2005， 博士

【摘要】 江达构造带是三江地区一条重要成矿带。 长期以来,人们对江达构造带的认识,局限为:金沙江洋关闭、碰撞形成火山岩浆带（西藏地质志）或岛弧带或陆缘弧-岛弧带:自中三叠世末碰撞造山以来,便进入所谓陆内会聚期,随后叠加了喜马拉雅期叠缩与走滑。 上述观点虽然在一定程度上反映了江达构造带,但与许多地质事实是矛盾的:①经查阅大量区调报告剖面资料和野外考察,昌都盆地晚三叠世至古新世地层,实际不存在区域性不整合;昌都地块两侧的沉积相也不存在造山应有的同期磨拉建造,断裂无压陷特征。②对江达构造带及邻近昌都地区的晚三叠世至白垩纪沉积相和沉积厚度进行仔细对比,发现,晚三叠世以来古地貌一直为伸展的盆岭型。③晚三叠世,在江达构造带发育的火山—地堑带中,不仅沉积厚度巨大,而且在其中的车所地堑中出现洋脊准洋脊型拉斑玄武岩,等等。 显然,这些观点存在一定问题,不利于江达构造带的资源勘查和资源潜力评价,也不利于对整个三江构造带的科学认识。 本着求实、创新的科学态度,以演化和成矿为题,以地体—板块构造理论和构造成矿学理论为指导,本着不同构造体制具有不同地质—成矿作用,多种构造体制演绎导致多元（个）成矿体系的广义同位复合构成多元复合成矿系统的思想,以《西藏藏东江达火山岛弧带北段铜银多金属矿评价》、《西南三江北段成矿规律和找矿方向综合研究》等项目为依托,在充分吸收消化前人成果、大量野外考察工作的基础上,通过沉积建造、古地理、岩石地球化学、构造岩石地层、不同尺度构造解析、古构造应力场分析、构造-成岩-成矿定年等多角度分析,对江达构造带的构造演化及构造—成矿耦合关系,进行了较为深入的研究,获得如下初步的新认识: （1） 江达构造带作为昌都加里东年轻陆壳地体东部边缘的重要构造带,肇始于古生代金沙江—哀牢山洋的发育。 （2） 形成金沙江—哀牢山洋的裂谷作用开始于泥盆纪,洋盆出现于早石炭纪,石炭纪末,局部洋壳向昌都陆块下的俯冲开始,早二叠世末,发生全面的洋陆俯冲。中三叠世末洋盆关闭,昌都陆块与中咱陆块碰撞焊合,但碰撞造山的强度是不大的（在一定程度上,是由于中咱陆块规模有限,区域伸展机制未发生根本改变）。 （3） 江达构造带先后经历了晚古生代（D₃～C₂⁶）被动陆缘,晚古生代—中三叠世（C₂⁷～T₂³）的洋陆俯冲机制下的活动陆缘,晚三叠世—新生代早期的陆内裂谷（T₃～E₁）,新生代（E₂～Q）由印藏碰撞引发的陆内逆冲—推覆—走滑造山等多阶段演化。 （4） 江达构造带陆内裂谷经历了晚三叠世～早侏罗世火山—地堑、中侏罗世—晚白垩世陆内坳陷、古新世—中始新世走滑堑垒系、始—渐新世裂谷封闭造山、磨拉石盆地发育等阶段。 （5） 从地层接触关系、变形样式及变形的应力场,同/后造山磨拉石建造时代,区域变形、变质及成矿年代学,岩浆活动特征等方面论证,江达构造带,中三叠世末的碰撞造山的强度不大,晚三叠世以来主变形变质于喜马拉雅期,是喜马拉雅褶皱造山带,具逆冲—推覆—走滑特点。 （6） 江达构造带四个演化阶段不同体制下都有成矿作用发生,构成四个各具特色的成矿作用体系:被动陆缘成矿体系仅在江达构造带残存的变质地体中见到一些铜金矿遗迹。活动陆缘（岩浆弧）成矿体系（280Ma,U-Pb;291.33±2.54Ma,Ar-Ar）的成矿作用主要表更多还原

【Abstract】 Jiangda tectonic zone is an important metallogenic zone in the area of SanJiang.It is limited to some original concepts from scholar’s understanding about Jiangda tectonic zone in a long time. The primary acquaintance with the Jiangda tectonic zone is that the collision between the Yangzi plate and the Changdu terrain with the close of JinSha river formes volcanic magma zone or volcanic island arc zone, pericontinental arc-island arc zone, and it came into the intracontinent convergent period from the collision of fold belt in middle Triassic Epoch, and overprinted the fold shrink and sliding in the Himalayan period.The viewpoints above-mentioned reflect some facts about the tectonic zone of Jiangda, but there are some inconsistency with much geological truth: (1)By consulting lots of the geological profile data and the reports of regional geological survey and investigating, we find the fact that the basin of Changdu was not being the regional discordance from Late Triassic stratum to Palaeocene stratum, the sedimentary facies were not being the orogenetic synchronous molasses formation in the both sides of the Changdu basin, and the faults not being the characters of press and sink. (2) Detailed comparing the sedimentary facies and sedimentary thickness of the Jiangda tectonic zone with the adjacent area of Changdu from Late Triassic epoch to Cretaceous, which discovered that the ancient geomorphy was basin-mountain type since Late Triassic epoch. (3)In the Late Triassic epoch, the volcano-graben zone of the Jiangda tectonic zone, not only had huge sedimentary thickness, but also appeared the ocean ridge and alike ocean ridge tholeiite, and so on.By all appearances, these viewpoints had some problems, which not only gone against resource-survey and the estimate of resource potential, but also gone against scientific understanding of the whole Sanjiang tectonic zone.Guided by terrain-plate tectonics theory and the ore-forming tectonics and supported by the research projects of "The evaluation of the northern Jiangda volcano-island arc Cu-Ag polymetallic deposits in Easter Tibet" and "The synthesis study of the metallogenic rule and ore-finding aspect in the northern of the southwest Sanjiang Area" etc, author fully studied the tectonic evolvement and couple relation between tectonic and ore-forming in Jiangda tectonic zone according to different tectonic system having different ore-forming action. Based on the fully absorbing data of predecessor and investigating, analyzing sedimentary construct ,ancient geography, rock geochemistry, tectonic rock-stratum, different scale tectonic parse, ancienttectonic stress field, tectonic-diagenism-metallogenetic dating and so on , some new understanding were achieved as follows:(1) The Jiada tectonic zone was very important as Caledon young crust terrain eastern margin caused by the development of the Jiashajiang-Ainaoshan ocean in Palaeozonic.(2) The rifting which formed the Jiashajiang-Ainaoshan ocean started in Devonian. In Early Carbonic the ocean basin come into being. And partial ocean crust started to dive under into Changdu Land Block in the later Carbonic. Full-scale ocean-land dive occurred in early Permian last stage .The ocean basin closed in the middle Trias last stage which led to the seaming of Changdu Land Block and Zhongzan Land Block. But collision mountain-building force was not great. (To some extent, because the Zhongzan Land Block was limit in scale, regional stretch system didn’t happen ultimately change.)(3) The Jiada tectonic zone undergone passive continental margin in Neopaleozoic, and inland central valley from Late Triassic epoch to Kainozoin era early stage, and inland dive-nappe-fault-strike slip etc multistage development.(4) The Jiada tectonic zone inland central valley undergone the volcano-graben in Late Triassic epoch-Early Jurassic, inland depressed in Middle Jurassic-Later Cretaceous, fault-strike slip moat-horst in Palaeocene-Middle Eccene, valley closed moutain-building in Eccene-Oligocene, Molasse basin development stage and so on.(5) From the relations of stratum contact, deformation fashion and deformative stress field, similar/rear orogenetic molasse building epoch, regional deformation, metamorphism and mineralogenetic chronology, magma action feature etc, the intensity of collasion orogenic in middle Triassic Epoch at JiangDa tectonic zone is not strong. The main deformations deteriorate in Himalayan period since late Triassic Epoch that is Himalaya fold orogen and it is with the feature of thrust-nappe-slide.(6) It constitutes the four different distinctive metallogenesis system because Jiangda tectonic zone’s metallogenesis happens across the different system of four evolutionary processes. Passive epicontinental metallogenic system only be seen with some gold ore traces in metamorphic terrain of JiangDa tectonic zone. The metallogenesis of active epicontinental (Magma Arc) metallogenic system (280Ma, U-Pb; 291.33 ± 2.54Ma, Ar-Ar) mainly represents the type of contact replacement Cu mineralized and maybe exit the type of volcano hydrotherm Cu, Pb-Zn mineralized; The metallogenesis of intracontinental rift mineralogenetic is Fe, Cu, Pb, Zn, Ag, Au, Ca etc metallogenesis of contact replacement pattern caused by the invasion of acid magma in Yanshanian and SEDEX type Fe,Pb,Zn,Ag metallogenesis in the stage of late Triassic Epoch volcano graben; The metallogenesis of intracontinental orogenic metallogenic system is mainly composed of three parts: (D intensive thrust-nappe allocating the ore deposit again; ? structure-magma-hydrothem activating and re-enriching the phantom ore deposit again; G> structure-magma-hydrothem shaping new ore family and new ore deposit on adaptable position.(7) The four metallogenesis metallogenesis systemes mentioned above happen in succession in the same tectonic zone. It appears the features of succession, development and modification certainly which will result in the low display of passive更多还原