羌北地块早-中志留世古地磁学初步研究结果及其构造意义  

Preliminary paleomagnetic results of the Early-Middle Silurian rocks from North Qiangtang Terrane,and its tectonic implications

作　　者：姜南 吴汉宁[1] 李玉玉[1] 卫弼天 周亚楠[1] 张伟杰 高扬[1,3] 于亮 武佳坤[1,5] 霍斐斐 刘雨纯 王保锋[1] 程鑫 JIANG Nan;WU HanNing;LI YuYu;WEI BiTian;ZHOU YaNan;ZHANG WeiJie;GAO Yang;YU Liang;WU JiaKun;HUO FeiFei;LIU YuChun;WANG BaoFeng;CHENG Xin(Department of Geology/State Key Laboratory of Continental Dynamics,Northwest University,Xi′an 710069,China;Department of Ocean Science and Engineering,Southern University of Science and Technology,Guangdong Shenzhen 518055,China;Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100081,China;Institute of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing 100101,China;College of Earth and Environmental Sciences,Lanzhou University,Lanzhou 730000,China;Zunyi Normal University,Guizhou Zunyi 563006,China)

机构地区：[1]西北大学地质学系/大陆动力学国家重点实验室,西安710069 [2]南方科技大学海洋科学与工程系,广东深圳518055 [3]中国地质科学院地质力学研究所,北京100081 [4]中国科学院青藏高原研究所,北京100101 [5]兰州大学资源环境学院,兰州730000 [6]遵义师范学院,贵州遵义563006

出　　处：《地球物理学报》2022年第3期1057-1070,共14页Chinese Journal of Geophysics

基　　金：国家自然科学基金项目(41774073,91855211,42074075);第二次青藏高原综合科学考察研究(2019QZKK0704)资助。

摘　　要：本文报道羌北地块早-中志留世龙木措组灰岩古地磁研究初步结果,据此探讨羌北地块早-中志留世的古位置及其起源问题.在日土县龙木措北岸(34.4°N,80.3°E)龙木措组中采集了10个采点,共125块古地磁独立定向样品.岩相学与岩石磁学结果表明样品中主要载磁矿物为磁铁矿.系统热退磁与交变退磁显示,退磁曲线具有双分量或单分量特征,单分量方向与双分量中稳定趋向原点的高温/高场分量方向一致,在采点水平下的平均方向为Ds=321.8°,I_(s)=41.7°,k_(s)=31.1,α_(95)=10.1°,N=8,该平均方向在样品水平下可通过C级倒转检验,对应的古地磁极位置(λ_(p)=-55.3°N,φ_(p)=163.3°E(dp/dm=7.6°/12.4°))显著区别于羌北地块晚古生代以来可靠的古地磁极位置,可能代表了岩石形成时的原生剩磁方向.结合地质学证据,本文提出羌北地块早-中志留世位于南半球低纬度地区(古纬度为24.0°S),此时羌北地块可能与华南地块位置接近,位于冈瓦纳大陆的印度板块北侧古大洋中.In order to reconstruct the Early-Middle Silurian paleogeographic position of the North Qiangtang Terrane(NQT)and to further discuss its origin,we present preliminary paleomagnetic data(10 sites,125 samples)from the Early-Middle Silurian Longmu Co Formation sedimentary rocks at the Longmu Co area(34.4°N,80.3°E)of the NQT.Petrography and rock magnetism reveals that magnetite is the dominant magnetic carrier in limestone samples from the Longmu Co Formation.Systematic thermal and alternating field demagnetization curves show a feature of single or two components,while the direction of the single component is consistent with that of the high-temperate/field component that decays toward the origin stably.This stable characteristic remanent magnetization(ChRM)passes the C-class reversal test at the sample level.The tilt-corrected mean direction for the Early-Middle Silurian rocks from the NQT is D_(s)=321.8°,I_(s)=41.7°,k_(s)=31.1,α_(95)=10.1°,N=8,which yields a corresponding paleomagnetic pole at-55.3°N,163.3°E(dp/dm=7.6°/12.4°).This paleopole does not resemble to younger paleopoles from the NQT,indicating that the ChRM is very likely to be the primary remanence.Our results,together with the evidence of geology,imply that the NQT was located in the low latitudes of the Southern Hemisphere(24.0°S)during the Early-Middle Silurian times,in which,the NQT may be closely related to the South China Block and located in the paleo-ocean on the northern side of the India Plate of Gondwana.

关 键 词：羌北地块 早-中志留世 古地磁学 倒转检验 古地磁极 

分 类 号：P318[天文地球—固体地球物理学]