青藏高原拉萨地块中部上三叠统多布日组沉积岩古地磁研究及其构造意义  

Paleomagnetism of the Upper Triassic Duoburi Formation sedimentary rocks on the middle Lhasa terrane in the Tibetan Plateau and its tectonic significance

作　　者：姚勇 边伟伟 沈中山 邓成龙[1,2] YAO Yong;BIAN WeiWei;SHEN ZhongShan;DENG ChengLong(State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;University of Chinese Academy of Sciences,Beijing 100049,China;State Key Laboratory of Biogeology and Environmental Geology,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029 [2]中国科学院大学,北京100049 [3]中国地质大学(北京)科学研究院生物地质与环境地质国家重点实验室,北京100083

出　　处：《地球物理学报》2023年第12期5086-5106,共21页Chinese Journal of Geophysics

基　　金：国家自然科学基金项目(41888101)资助。

摘　　要：拉萨地块晚古生代至早新生代期间的演化历史对理解青藏高原的形成和演化至关重要.本文报道了青藏高原中部申扎地区上三叠统多布日组沉积岩的古地磁学、岩石磁学和岩相学分析结果.岩石磁学结果显示红层、灰岩和灰绿色砂岩分别以赤铁矿,赤铁矿与磁铁矿,以及磁铁矿为主要载磁矿物.岩相学结果表明红层、灰岩和灰绿色砂岩中的磁性矿物主要为自生成因.对60个采点(602块样品)进行系统热退磁实验,结果揭示出多布日组沉积岩的特征剩磁方向在地理坐标系下为D_(g)=13.9°,I_(g)=38.9°(α_(95g)=0.9°),在地层坐标系下为D_(s)=16.7°,I_(s)=8.4°(α_(95s)=1.5°).进一步对其特征剩磁方向进行逐步褶皱展平检验,结果显示精度参数k的极大值在9.9%展平状态下最大,对应的特征剩磁方向为D=14.4°,I=35.9°(α_(95)=0.9°),古地磁极为72.9°N,216.2°E(dp=0.6°,dm=1°).这一古地磁极与拉萨地块54~47 Ma的古地磁极接近,表明其重磁化时间可能发生在54~47 Ma期间.结合前人获得的拉萨地块新生代早期古地磁数据以及同期的欧亚大陆古纬度数据,我们推断拉萨地块在56~44 Ma时相对欧亚大陆发生了1676±577 km的南北向构造缩短.The evolution of the Lhasa terrane during the late Paleozoic to early Cenozoic is critical to understanding the formation and evolution of the Tibetan Plateau.Detailed analyses of paleomagnetism,rock magnetism,and petrography have been carried out on the sedimentary rocks of the Upper Triassic Duoburi Formation in the Xainza area of the central Tibetan Plateau.Rock magnetic results reveal that the dominant carriers of remanence are hematite,hematite and magnetite,and magnetite in red beds,limestones,and grey sandstones,respectively.Petrographic results show that magnetic minerals of red beds,limestones,and grey sandstones are of authigenic origin.A total of 602 specimens from 60 sites were subjected to stepwise thermal demagnetization.The site-mean direction is D_(g)=13.9°,I_(g)=38.9°(α_(95g)=0.9°)in situ and D_(s)=16.7°,I_(s)=8.4°(α_(95s)=1.5°)after tilt-correction.The precision parameter(k)has a maximum grouping at 9.9%,which provides the site-mean direction of D=14.4°,I=35.9°(k=389.06,α95=0.9°)corresponding to a paleopole at 72.9°N,216.2°E(dp=0.6°,dm=1°).This paleopole is similar to the Eocene(54~47 Ma)paleopole of the Lhasa terrane,indicating that the remagnetization may have occurred during the early Eocene.The combination of our new paleomagnetic data and previously published reliable early Cenozoic paleomagnetic data shows that a latitudinal convergence of 1676±577 km has taken place between the Lhasa terrane and Eurasia during the period of 56~44 Ma.

关 键 词：拉萨地块 古地磁 重磁化 三叠纪 多布日组 

分 类 号：P318[天文地球—固体地球物理学]