青藏高原东北部西宁盆地晚渐新世-早中新世沉积物岩石磁学特征及其古环境意义

文章通过对西宁盆地谢家剖面上部(620~819m)厚度190m,有精确年代控制且无明显相变的晚渐新世-早中新世的湖相沉积物样品的磁化率系统测量,建立了该区晚渐新世-早中新世高分辨率的磁化率变化序列,揭示高、低频磁化率在约25Ma和21Ma存在两次明显的台阶式长期增大趋势,并在约23.8Ma,21.6Ma和19.8Ma叠加短期峰值,在时间上可能分别对应全球此时的长期变冷趋势及全球性Mi-1.1事件、Mi-1a事件和Mi-1aa事件短暂快速变冷事件,而频率磁化率记录几乎没有变化。代表性样品的系列岩石磁学分析,揭示出沉积物的载磁矿物主要为假单畴(PSD)的磁铁矿,同时可能还存在赤铁矿和针铁矿; 磁化率增大主要由假单畴磁铁矿的含量增多引起,而不是通常情况下的成壤作用形成的超顺磁颗粒的增多引起,推测青藏高原东北部隆升导致的侵蚀增强,或干旱化,或两者兼并,可能是导致西宁盆地上述磁化率增强的原因。

ROCK MAGNETIC CHARACTERISTICS OF THE LATE OLIGOCENE TO EARLY MIOCENE SEDIMENTS IN THE XINING BASIN,NORTHEASTERN TIBETAN PLATEAU

Xiejia section is the most representative cross-section of the Xining basin,which is located in the southeast of Xining City,Qinghai Province about 15km(36°31'N,101°52'E),famous for stratigraphic continuous and production of standard Xiejia mammal fauna.The section thickness is about 819m,which is unconformable on the Cretaceous strata.The magnetostratigraphy reveals that the Xiejia section is formed during about 52~17Ma,while we study the target that is consist of the top of the Xiejia section,which has the upper Xiejia Formation(about 30~22Ma),Chetougou Formation(about 23~18Ma)and Xianshuihe Formation(about 18~15Ma).Therefore,the study is mainly concentrated in the Late Oligocene and Early Miocene about 25.5Ma to 17Ma.And we select the 190 meters brown mudstones and siltstones,which have basically similar sedimentary facies and lithology on the top of the Xiejia section(620~819m),consist of the upper Xiejia Formation,Chetougou Formation and Xianshuihe Formation.The sediments have 20~30cm intervals,which are collected more than 800 bulk samples,measured the magnetic susceptibility and rock magnetic indicators indoors. The low-frequency magnetic susceptibility mainly change between 2.5×10-8m3／kg and 30×10-8m3/kg,and the high-frequency magnetic susceptibility generally change in 2.5×10-8~25×10-8m3/kg in the upper Xiajie section.Both the low-frequency and high-frequency magnetic susceptibility have two stepwise increasing trends at about 25Ma and 21Ma,which is superimposed on the fluctuations of the magnetic susceptibility also presents three significant peak values separately at about 23.8Ma,21.6Ma and 19.8Ma,which can respectively be correlated in dating error ranges with the global long-term cooling and the transient rapid cooling events of Mi-1.1,Mi-1a and Mi-1aa events in the Early Miocene.However,the frequency magnetic susceptibility nearly shows no obvious change trend. The X-T curves of the representative samples in the upper Xiejia section show:All the X-T curves are irreversible,the cooling curves are obviously located above the heating curves.The ratio of the volume magnetic susceptibility reduces at about 120~150℃,some reduces to zero in the vicinity of 580℃ or have a dramatic increase fast then reduce to zero(the Hopkins effect),small amount of the samples show significant decline,and some samples reduce to zero in the vicinity of 680℃,which shows that the magnetic mineral upload is mainly magnetite,and there may also have some goethite and hematite.The vast majority Hysteresis Loop of the samples saturate about 100mT basic,and form the closed curves about in 450mT.The closed curves which are very narrow and relatively steep,have the higher value of saturation magnetization intensity that basically about changes at 0.68×10-2~1.07×10-2Am2/kg.The saturated residual magnetization intensity is also high,which changes about at 0.12×10-2~0.23×10-2Am2/kg.While the coercive force and the remanent coercive force are relatively low,which are change respectively at 15~33mT and 52~83mT.That shows the samples are mainly consist of the low coercivity of the soft magnetic minerals. The increased contents of the PSD magnetite are the main cause for the magnetic susceptibility enhancement,which are very different from the content of superparamagnetic grains that are the major cause for the susceptibility enhancement as we see from pedogenesis and Chinese loess-paleosol sequences.So we propose that the increased erosion by the enhanced tectonic uplift of the NE Tibetan Plateau,or the aridification,or both,may be the main geologic causes for the increased PSD magnetite grains and thus the enhanced magnetic susceptibility in the Xining basin.