辽源地体辽吉西段碰撞造山期变形规律与时限研究

It remains greatly controversial about final closure time of the Solonker Ocean, which is crucial for understanding the evolution of the Paleo-Asian Ocean and Xingmeng Orogen. The Liaoyuan Terrane, located between the northern boundary fault of the North China Craton and the Solonker Suture, is a Paleozoic continent marginal accretion belt and preserves volumes of orogenic deformation records, which is thought to be an extraordinary study angle for solving the puzzles about closure time of the Solonker Ocean. In this project, the orogenic structures from western Liao-Ji part of the Liaoyuan Terrane will be chosen for studies by detailed field investigating and indoor researching to ascertain their geometry and kinematics features as well as deformation environment and methods, so that we can summarize structural deformation types and spatial variation characteristics. 40Ar/39Ar dating of minerals from the orogenic ductile deformed rocks and zircon U-Pb dating on plutons and dykes in the deformation belt will be conducted for constraining orogenic deformation time. On the bases of the above studies, syn-collisional deformation mechanisms and dynamics in this study area will be analyzed to find out their corresponding relation with the final closure of the Solonker Ocean, which is helpful to reveal the facts about its final closure time as well as convergence approach.

索伦克洋的最终闭合时间，是认识古亚洲洋及兴蒙造山带演化的关键所在，然而对于该问题的认识长期以来争议较大。辽源地体是位于华北克拉通北界断裂与索伦克缝合线之间的一条古生代陆缘增生带，保存了大量碰撞造山过程的构造变形记录，是解决索伦克洋闭合时限问题的极佳对象。本项目将选择辽源地体辽吉西段为研究区，以碰撞造山期构造为研究对象，通过详细的野外调查和室内研究，查明其几何学、运动学特征及变形环境与方式，以此总结构造变形类型及空间变化规律。通过碰撞造山期韧性变形岩石中单矿物的40Ar/39Ar定年以及变形带中岩体和岩脉的锆石U-Pb定年，来限定碰撞造山期变形的时限。在此基础上，分析研究区同碰撞变形机制与动力学，确定它们与索伦克洋最终闭合的成因联系，进而揭示索伦克洋东段的最终闭合时间以及汇聚方式。

辽源地体辽吉西段记录了古亚洲洋东段最终闭合及随后的构造体制转换、叠加与演化等事件的关键信息，然而长期以来前人研究结果存在较大争议。本项目通过对吉林中部地区开展详细的野外调查，重点对辽源地体北缘发育的变质混杂岩和韧性构造变形带进行构造填图与采样工作，并进一步开展了锆石U-Pb定年、单矿物40Ar/39Ar定年、显微构造和石英c轴组构测量等研究。研究表明，辽源地体北缘沿长春-延吉缝合线出露了三处变质混杂岩，自西向东分别是景台变质杂岩、烟筒山变质杂岩和呼兰变质杂岩。景台杂岩变质级稍高，可达高角闪岩相，局部出现混合化作用。烟筒山杂岩变质温度可达绿片岩相，变质压力稍高。呼兰杂岩经历了绿片岩-角闪岩相变质作用。锆石U-Pb定年结果显示三处变质杂岩的原岩主要由二叠纪-中三叠世初的侵入岩、火山岩和沉积岩构成。其中，景台杂岩还具有较老的早古生代和前寒武纪岩石组分，烟筒山杂岩具有最年轻的中三叠世沉积岩组分。三处变质杂岩分别获得了240-250 Ma、~239 Ma和233-243 Ma的变质锆石年龄。呼兰杂岩单矿物40Ar/39Ar定年获得了220-224 Ma冷却年龄。对变质杂岩中未变质的岩体和岩脉锆石U-Pb定年获得了221-229 Ma结晶年龄。对变质杂岩上覆的未变质磨拉石型粗碎屑岩锆石U-Pb定年获得了209 Ma的最大沉积年龄。综合全部定年结果，本项目研究认为246-250 Ma代表了索伦克洋东段的最终闭合时间，随后区域上进入造山带增厚的下地壳局部熔融以及后造山伸展岩浆发育阶段。野外实测表明，三处变质杂岩线理产状以近E-W向和NW-SE向为主，面理总体平缓，但呈现出近E-W向褶皱特征，显示出后期构造应力叠加作用结果。宏观和微观运动学指示标志均指示上盘向东运动。由此表明，索伦克洋东段向西斜向俯冲于辽源地体之下，并于早三叠世最终闭合。