内蒙古满都拉地区索伦缝合带多期变形年代学

Structural and kinematic analysis of the Solonker suture zone will provide essential constraints on the Palaeo-Asian oceanic crust subduction, subsequent collision and intracontinental tectonism. The Mandula area is located within the Solonker suture zone and crops out chaotic assemblage, turbidite unit, volcanic rock unit and marine-terrigenous sediment unit. They experienced a polyphase deformation during the Late Paleozoic, but insufficient studied by now. Three phases of deformation are recognized by field observations, measurement of foliation and lineation and analysis of superimposed structures. D1 is mainly preserved in the turbidite unit is characterized by a series of isoclinal folds with fold axes striking SW-NE. D2 is developed in all litho-tectonic units and is characterized by ESE-WNW striking thrust faults and a series of fault-related folds. D3 is characterized by the development of regional NE striking strike-slip faults, which crosscut the previous two generations of structures. Each phase of deformation is in need of precise age constraints. A further probe into the relationship between the polyphase deformation and geodynamics is necessary. This study will focus on the geochronology of each phase of structural deformation. Mylonitic rock and mica schist from the shear zone related to each tectonic event will be collected for 40Ar/39Ar dating, with care taken to ensure that the mineral grains to be dated grew during the specific deformation event being analyzed. The deformed/undeformed granitic rock cut by/cutting a certain fault or shear zone will be taken for zircon U-Pb dating to determine the age of fault activity. The structural and geochronologic data will construct the relative deformation order in the Mandula area, reveal the Late Paleozoic geodynamic scenario in the study area, and give a few insights for the tectonic evolutional processes about accretionary orogens.

索伦缝合带是古亚洲洋构造演化的重要界线，其构造变形特征能为理解洋盆俯冲、碰撞拼合和陆内调整过程提供必要约束。索伦缝合带在满都拉地区出露杂乱堆积（无序）、浊积岩、火山岩和海陆交互相沉积岩等岩石构造单元，记录了多期次叠加变形作用。野外露头观察、构造要素测量和构造叠加样式显示3期构造变形：D1期变形主要发育在浊积岩单元中，以轴向SW-NE同斜紧闭褶皱为特征；D2期发育近ESE-WNW走向逆冲推覆构造，并伴随一系列断层相关褶皱；D3期形成区域上NE走向左行走滑断层，切割前两期变形。本研究拟采用云母40Ar/39Ar和锆石LA-ICP-MS U-Pb年代学方法，研究韧性剪切带内糜棱岩和云母片岩，及与构造带存在切割关系的变形和未变形花岗岩年代，结合构造变形几何学和运动学分析，探讨多期变形与造山带构造运动的关系，恢复古亚洲洋晚古生代构造演化动力学过程。

我国内蒙古地区中亚造山带的构造演化存在长期争议，争议的焦点集中在索伦缝合带的成因和时代。代表性的观点主要有：1）索伦缝合带代表古亚洲洋最终闭合的位置，时代为二叠纪末-三叠纪；2）古亚洲洋在早古生代就完成闭合，之后转变为陆内裂谷阶段，形成类似红海的有限洋盆。满都拉地区和甘其毛都地区是索伦缝合带所在地。本研究主要从构造变形学和年代学方面研究索伦缝合带的变形、时代和地球动力学成因。通过野外观察和年代学分析认为，索伦缝合带经历了2期构造变形：第一期以NNE-SSW向的缩短为特征，发育向NNE的逆冲断层和不对称褶皱；第二期以NW-SE向缩短为特征，发育向NW或向SE的逆冲断层和不对称褶皱。这两期近正交的挤压缩短，形成典型的蘑菇状（mushroom）和箭头状（arrowhead-shape）干涉褶皱。第一期变形有关的构造片理和节理中，有较多的石英脉侵入。石英脉中含有热液锆石或热液改造的继承锆石，U-Pb年龄分析获得早-中三叠世年龄，暗示第一期变形的时代为中三叠世或稍早。第二期变形卷入的最新地层为侏罗系，并且被白垩系地层不整合覆盖，说明第二期变形的时代为侏罗纪晚期。通过区域对比，古亚洲洋在二叠纪末-三叠纪期间的南向俯冲和闭合可以导致研究区大规模的NNE-SSW向的挤压缩短和NNE向的逆冲。如果古亚洲洋在早古生代完成闭合，晚古生代新形成的有限洋盆的闭合，也可能形成NNE-SSW向的挤压缩短。但是有限洋盆似乎无法形成持续、稳定的俯冲体系，有限洋盆的闭合需要古亚洲洋体系外部的其它地球动力学解释，如三叠纪期间的秦岭-大别山造山作用，以远程效应导致有限洋盆的闭合，然而这种可能需要更多的证据支持。目前而言，研究区中三叠世的NNE-SSW向的挤压缩短以古亚洲洋在二叠纪末-三叠纪期间的南向俯冲和闭合来解释似乎更合理些。第二期变形与区域上华北克拉通北缘的阴山-燕山构造带、西缘的贺兰山构造带的晚侏罗世的NW-SE向挤压构造相对应。该期构造可能与古太平洋侏罗纪期间向西俯冲和华北克拉通的向西楔入的远程构造效应有关。