新疆柴窝堡盆地南缘晚新生代陆源沉积物岩石磁学特征及其古环境意义

亚洲内陆晚新生代干旱化历史及其驱动机制是非常重要的科学问题。选择天山北麓柴窝堡盆地南缘乌鲁木齐河剖面为研究对象，基于磁性地层年代学研究建立的时间标尺，分析了该剖面碎屑沉积物岩石磁学特征及其控制因素，进一步探讨了柴窝堡盆地晚新生代古气候特征。磁性地层年代学研究约束乌鲁木齐河剖面底界年龄为~6.8 Ma，顶界年龄为~3.3 Ma。详细的岩石磁学测量及漫反射光谱分析结果表明，乌鲁木齐河剖面沉积物中磁性矿物主要包括磁铁矿、赤铁矿等，磁学参数χ_lf、χ_ARM、SIRM、S_-100mT、S_-300mT、 χ_ARM/SIRM等在~6.3 Ma、~5.2 Ma存在明显变化，揭示磁性矿物粒度、含量等存在相应变化，如在~6.3 Ma前后，磁性矿物颗粒逐渐变粗、含量降低；在~5.2 Ma前后，磁性矿物颗粒逐渐变细后趋于稳定、含量逐渐增多。基于稀土元素分析、沉积粒度与磁学参数相关性分析，认为沉积物源与沉积物粒度不是导致乌鲁木齐河剖面磁学性质变化的主要因素，自~6.8 Ma以来逐步干旱化的气候条件可能是导致该剖面磁学特征变化的重要原因。基于漫反射光谱分析得到的红度与亮度数据结果也揭示了同样的古气候特征。

Rock Magnetic Properties and Paleoclimate Implications of Late Cenozoic Terrigenous Succession from the Southern Chaiwopu Basin,Xinjiang

The history and mechanism of aridification in the Asian interior during the late Cenozoic have been an important scientific issue.This study presents the terrigenous deposits from the ~303 m-thick Urumqi River section (URS) in the southern Chaiwopu Basin of the northern Chinese Tian Shan foreland.Based on the results of magnetostratigraphic chronology and rock magnetic characteristics, we discussed the magnetic enhancement mechanism of the URS sediments and paleo-climatic implications during the late Cenozoic in this region.The magnetostratigraphic chronology constrains the time spans from about 6.8 Ma to about 3.3 Ma.The results of rock magnetic and diffuse reflection spectrum (DRS) measurements show that the magnetic minerals in the URS sediments are mainly magnetite and hematite, and some magnetic parameters, such as clf, cARM, SIRM, S-100mT, S-300mT, cARM/SIRM, changed obviously at ~6.3 Ma and ~5.2 Ma.These changes reveal that the measured samples contain coarser magnetic grains with less magnetic grains (after ~6.3 Ma).In contrast, the magnetic grains became finer steadily before ~5.2 Ma, with the content increasing.By the analysis of rare earth elements (REEs), we know that the distribution patterns of rare earth elements (REEs) are consistent obviously.The analysis of rare earth elements (REEs) is likely to indicate a stable source area of the URS deposits.The sediment grain size can also be used to explain variations of terrigenous deposits magnetic characteristics.There are significant correlation relationships between the sediment grain size parameters and clf, cARM/clf, the insignificant correlation relationships between the magnetic parameters, such as cARM, SIRM,S-100mT, S-300mT, cARM/SIRM, cfd, and the sediment grain size parameters indicate that the observed variations of magnetic characteristics may not be mainly caused by the sediment grain size parameters.The continuous aridification in the present study area is more likely to be the main cause of the variations of magnetic characteristics.The continuous aridification can contribute to the oxidation of magnetite, which can be used to explain variations of terrigenous deposits magnetic characteristics.In order to verify the results, we make the DRS analysis.The results show that the study region occurred aridification obviously at ~6.3 Ma and ~5.2 Ma.The results of the DRS analysis show the similar paleo-climatic change.