北祁连造山带晚奥陶世-泥盆纪构造演化: 碎屑锆石年代学证据

北祁连造山带晚奥陶世-泥盆纪处于同造山的构造背景.上奥陶统-泥盆系沿造山带不对称分布.上奥陶统-泥盆系碎屑锆石年代学特征显示, 造山带东段武威一带上奥陶统底部沉积物主要来自北祁连岛弧, 南部中祁连地块和北部华北板块的沉积物在上奥陶统上部才出现, 根据同沉积锆石年龄将中祁连地块和华北板块在东段初始碰撞的时间限定在470~450 Ma之间; 中祁连地块和华北板块的物质在造山带西段肃南一带被保存在下志留统, 地层中也有大量来自早古生代北祁连岛弧和同碰撞花岗岩的物质, 暗示造山带西段的碰撞时间在早志留世.而造山带东段下志留统中却仅有来自中祁连地块和华北板块的物质, 缺乏代表北祁连岛弧的早古生代碎屑锆石年龄, 对比上奥陶统-下志留统岩相分布和碎屑锆石年代学特征, 北祁连造山带的碰撞具有"东早西晚"的"斜向碰撞、不规则边缘碰撞"的特征, 而这种碰撞方式导致中祁连地块在造山带东段仰冲到北祁连岛弧之上, 阻止北祁连岛弧为盆地提供沉积物; 泥盆纪早期, 北祁连岛弧年龄在东段下、中泥盆统中重新出现, 结合志留系和泥盆系在造山带东、西两段的分布和变形特征推断, 泥盆纪早期北祁连造山带具有"东强西弱"的不均一隆升特征, 这种差异隆升特征是由"东早西晚"的"斜向碰撞、不规则边缘碰撞"引起的, 它导致了北祁连岛弧在造山带东段被重新剥露出地表, 同时来自早期中、上志留统以及同碰撞花岗岩的物质也被汇入盆地.河西走廊盆地性质经历了弧后盆地-弧后残留洋盆-前陆盆地的转换过程. 

Tectonic Evolution of the North Qilian Orogenic Belt from the Late Ordovician to Devonian: Evidence from Detrital Zircon Geochronology

The Upper Ordovician-Devonian strata are distributed asymmetrically in the North Qilian orogenic belt (NQOB) and are record of the collision process between Central Qilian block and North China plate. Age spectra of detrital zircons from the Upper Ordovician strata in the eastern of belt suggest that sediments in the basement of Upper Ordovician originate from the North Qilian arc and detritus in the upper part of Ordovician come from Central Qilian block and the North Qilian arc to the South and North China plate to the north. This change of source, integrated with ages of synsedimentary zircons, implies that the time of initial collision between Central Qilian block and North China plate is between 470 and 450 Ma. The Lower Silurian strata in the western part of belt preserve a large number of grains from Central Qilian block and North China plate, with the infill of sediments from the North Qilian arc and synorogenic granites, which imply the time of collision in the western part of belt. Compared to Ordovician lithofacies and the time of collision in the eastern and western parts, the asynchronous collision between Central Qilian block and North China plate suggest a "diachronous, unequal margin collision" model. This collisional model resulted in the Central Qilian block thrust onto the North Qilian arc and missed from sediments in the Lower Silurian in the eastern part of belt. Sediments form the North Qilian arc re-appeared in the Lower-Mid Devonian. Combined the distribution and deformation of Silurian and Devonian in the whole belt, the orogeny of NQOB is diachronous in the trending direction due to the "diachronous, unequal margin collision". The eastern sector had stronger tectonic intensity compared to the western sector, which resulted in the re-exposure of the North Qilian arc covered by Central Qilian block during the Early Silurian, infilling of synorogenic granites and recycle of the Mid-Upper Silurian. The Hexi-Corridor basin completed the transition from retroarc basin to remnant retroarc basin, to foreland basin during the Late Ordovician-Devonian.