ROCK-MAGNETIC CHARACTERISTICS OF THE TIANSHAN LOESS AND THE MECHANISM FOR ENHANCING MAGNETIC SUSCEPTIBILITY
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摘要: 天山北坡海拔2 400~2 700 m不同高度的地貌面上分布有大片风成黄土,是详细研究我国西北内陆极端干旱区形成演化过程和中亚内陆黄土岩石磁学性质与物源物质、气候条件相互关系的理想材料。通过对天山北坡沙湾县东湾镇清水河第6级阶地黄土剖面进行采样、高精度的磁化率及其他岩石磁学参数的测量分析,发现天山黄土的磁学性质主要为低矫顽力的亚铁磁性矿物(如磁铁矿和磁赤铁矿)所主导,与黄土高原地区黄土沉积序列明显不同。成壤作用形成的细颗粒磁性矿物对天山黄土磁化率的影响微弱,而来自源区的粗颗粒磁铁矿含量的增加是天山地区磁化率增强的主要影响机制。磁化率和粒度自0.8 Ma以来逐渐增加和变粗的趋势,可能是对亚洲内陆中更新世以来干旱化过程的响应。Abstract: The large area of eolian loess distributed over the northern slope of the middle Tianshan between the altitude of 2 400~2 700 m has provided a good opportunity to study the loss rock magnetic properties of Central Asia and the evolutionary process of the extreme arid areas of NW China. Our research profile is located on the sixth terrace of Qingshui river on the northern slope of Tianshan. We collected 580 of bulk samples for analysis of the rock magnetic susceptibility at a 10~20 cm interval and selected several samples for analyses of other magnetic parameters. Results show that the magnetic properties of the Tianshan Loess are mainly controlled by low-coercivity ferrimagnetic minerals (such as magnetite and maghemite), significantly different from the loess sequence of the Loess Plateau, and the contribution of ultrafine pedogenically-produced magnetic grains to loess magnetic susceptibility is very limited, while the increased coarse ferrimagnetic minerals from the source region play a decisive role in increasing magnetic susceptibility in the Tianshan region. The gradual increasing trend in magnetic susceptibility and the coarsening upwards trend of grain size since 0.8Ma may be the response to the aridification process in the inland Asia since the mid-Pleistocene.
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Key words:
- loess /
- magnetic susceptibility /
- rock magnetism /
- Tianshan
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