摘要
泥盆纪时期各板块的古地理重建缺乏可靠古地磁数据,以至于古特提斯洋在泥盆纪的演化过程存在争议,而高质量的古地磁数据定量限定板块的古纬度位置是解决上述问题的关键.本文对北羌塘地块早泥盆世平沙沟组灰岩进行了古地磁学研究,提供了一个较为可靠的古地磁数据.镜下观察及岩石磁学实验结果表明平沙沟组灰岩样品中的主要载磁矿物为磁铁矿和赤铁矿,并且没有重磁化的特征.系统热退磁与混合退磁实验结果表明,样品的天然剩磁强度较低,从54块样品中获得了较为集中的特征剩磁分量,在样品水平下的平均方向为 Dg=358.5.,Ig=22.8°,kg=17.7,α95=4.6°,Ds=8.8°,Is=-0.9°,ks=18.1,α95=4.5°,通过了 C级倒转检验,对应的古地磁极为λp=54.8°N,φp=251.2°E,(dp/dm=2.3°/4.5°),这一极位置与石炭纪以来可靠的古地磁极位置存在差异,很可能代表了早泥盆世的原生剩磁方向.结合古生物学和地质证据,本文提出北羌塘地块早泥盆世位于古赤道附近(古纬度为0.5°S),此时北羌塘地块与华南板块位置接近,古特提斯洋可能已经存在.
Abstract
The paleogeographic reconstructions of the plates lack of reliable paleomagnetic data,so that the evolution of the Paleo-Tethys Ocean in the Devonian is controversial,and high-quality paleomagnetic data quantitatively qualifying the paleolatitudinal position of the plates is the key to solving the above problem.In this paper,we conducted a paleomagnetic study on the limestone from the Lower Devonian Pingshagou Formation of the North Qiangtang Terrane to provide a relative reliable paleomagnetic data.The results of microscopic observation and rock magnetic experiments show that the main magnetic minerals in samples are magnetite and hematite,and there is no remagnetization characteristic.The results of systematic and alternating field demagnetization curves show that the Natural Remanent Magnetization(NRM)intensity of the samples is low.This stable Characteristic Remanent Magnetization(ChRM)is separated from 54 samples and passes the C-class reversal test at the sample level.The tilt-corrected mean direction for the Early Devonian rocks from North Qiangtang Terrane is Ds=8.8°,Is=-0.9°,ks=18.1,α95=4.5°,n=54,which yields a corresponding paleomagnetic pole at 54.8° N,251.2° E(dp/dm=2.3°/4.5°).This paleopole does not resemble to younger paleopoles from the North Qiangtang Terrane,indicating that the ChRM is very likely to be the primary remanence.Our results,together with the evidence of paleontology,imply that the North Qiangtang Terrane was located near the paleo equator(0.5°S)during the Early Devonian,in which,the North Qiangtang Terrane may be closely related to the South China Block.The Paleo-Tethy Ocean may have existed.
基金项目
国家自然科学基金项目特提斯地球动力系统重大研究计划(91855211)
国家自然科学基金面上项目(41774073)
维普
万方数据