华北平原中部第四纪沉积物的岩石磁学性质

摘要

岩石磁学是古地磁学和环境磁学研究的基础.本文对华北平原中部SK1和G4两个第四系钻孔岩芯代表性样品进行详细的岩石磁学和系统退磁实验研究,结果表明,SK1和G4孔沉积物中磁性矿物以赤铁矿和磁铁矿为主,赤铁矿含量较高.SK1孔中高矫顽力磁性矿物以赤铁矿和针铁矿为主,而G4孔则以赤铁矿为主;两孔沉积物的磁化率主要来自磁铁矿的贡献,SK1孔中磁铁矿含量较高,其平均磁化率较高,而G4孔中多赤铁矿,其平均磁化率较低;SK1孔沉积物的特征剩磁载体主要是磁铁矿,而G4孔沉积物的特征剩磁载体主要是赤铁矿.赤铁矿主要有两个来源:一是河流侵蚀再搬运上游沉积物中的赤铁矿,为碎屑成因;二是同沉积过程中,风化作用将磁铁矿氧化为赤铁矿,这类赤铁矿为次生成因.%Rock magnetism can provide insights into paleomagnetic and enviromagnetic studies.In this study,detailed rock magnetic investigations and stepwise thermal demagnetization were carried out on the sediments from boreholes SK1 and G4 of the Quaternary in the central North China Plain.Borehole SK1 (38°12′17″N,115°31′03″E;27m a.s.l) is located in Jizhong depression,and has a length of 480m.Borehole G4 (38°02′15″N,117°38′51″E;6m a.s.l) is located in Chengning uplift,and has a length of 400m.SK1-1～ 4 specimens from borehole SK1 and G4-1～4 specimens from borehole G4 were subjected to the rock magnetic measurements including temperature-dependent magnetic susceptibilities (x-T curves),hysteresis loops,and isothermal remanent magnetization (IRM) acquisition curves and its back-field demagnetization curves.SK1-5～8 specimens from borehole SK1 and G4-5～8 specimens from borehole G4 were subjected to progressive thermal demagnetization up to a maximum temperature of 690℃,with intervals of 10～25℃.The magnetic minerals in the sediments of boreholes SK1 and G4 consist of magnetite,maghemite,hematite and goethite.Magnetic susceptibilities of the sediments of both boreholes are dominantly contributed by magnetite.Magnetite,hematite and goethite were identified as the carriers for the natural remanent magnetizations (NRMs).Magnetite serves as the dominant carrier of the characteristic remanent magnetizations (ChRMs) of the SK1 borehole sediments;and hematite,of the G4 borehole sediments.Hematite grains in the Quaternary sediments in the central North China Plain come from two ways:one is of secondary origin and was formed by chemical weathering;and the other is of detrital origin and originated from the sediments eroded and transported by rivers.