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

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,α₉₅=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.

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

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,α₉₅=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.