辽北法库韧性剪切带变形及其区域构造意义

Deformation and regional tectonic significance of Faku ductile shear zone in northern Liaoning Province

晚古生代—早中生代古亚洲洋板块俯冲华北板块在辽北法库地区形成大型产状近水平的韧性剪切带。剪切带发展伴随着多期幔源及壳源的岩浆侵入,侵入岩在韧性剪切作用下发生韧性变形,记录了韧性剪切带变形历史。详细的野外地质调查结合岩石的宏观变形、显微构造及石英c轴组构特征分析,揭示了法库韧性剪切带内五龙山杂岩、高丽沟杂岩、早期十间房超单元变形处于上盘向北的剪切作用之下,晚期十间房超单元岩石变形指示构造运动转变为上盘向南剪切。LA-ICP-MS锆石U-Pb分析结果显示高丽沟杂岩变形时间为264.7±3.6 Ma(MSWD=1.3),早期十间房超单变形时间为253.9±4.3 Ma(MSWD=3.3),晚期十间房超单元244.0±3.0 Ma(MSWD=1.9)。岩石地球化学特征表明早期十间房超单元及小房申岩体中部分岩石来源于部分熔融的岩石圈地幔。结合前人研究成果,我们认为华北板块北缘东段(辽北地区)晚二叠世(264.7~253.9 Ma)处于古亚洲洋板块俯冲华北板块作用之下;晚二叠世至早三叠世(253.9~244.0 Ma)构造体制转变为碰撞后伸展且逐渐停止,244.0±3.0 Ma碰撞后伸展逐渐结束;构造体制转变并逐渐结束的过程是壳幔共同作用的结果,暗示了古亚洲洋板块的俯冲板片断离、重力失衡,这标志着辽北地区古亚洲洋构造域演化结束。

Objectives:Faku ductile sheer zone located in north Liaoning is genetically related to the subduction of PaleoAsian Ocean plate during Late Paleozoic to early Mesozoic.The study of Faku ductile sheer zone deformation will help to understand the evolution of Paleo-Asian Ocean plate.Methods:Detailed field surveys,macro and micro structure analysis on intrusive rocks,quartz c-axis fabric study and LA-ICP-MS zircon U-Pb dating.Results:The deformation of Wulongshan intrusions,Gaoligou intrusions and early Shijianfang intrusions were controlled by Faku ductile shearing,whose upper plate moved northward.When it turn to later Shijianfang intrusions and Xiaofangshen intrusions,the deformation were controled by the ductile shearing,whose upper plate moved southward.The age of Gaoligou intrusions is 264.7±3.6 Ma(MSWD=1.3),the age of early Shijianfang intrusions is 253.9±4.3 Ma(MSWD=3.3)and the age of later Shijianfang intrusions is 244.0±3.0 Ma(MSWD=1.9).Conclusions:The subduction of the Paleo-Asian ocean plate happened in the east section of the northern margin of the North China Plate(northern Liaoning Province)during Late Permian(264.7~253.9 Ma),while it has translated into postcollision extension form Late Permian to Early Triassic(253.9~244.0 Ma).It is a response of subduction slab felled off from the Paleo-Asian ocean plate,and it also marks the end of the evolution of the Paleo-Asian oceanic tectonic domain.When it comes to Late Triassic Epoch the northern margin of the North China Plate(northern Liaoning Province)was under another tectonic system,and ductile shearing has nothing to do with the evolution of the Paleo-Asian Ocean Plate.