Controlling factors on heavy mineral assemblages in Chinese loess and Red Clay
Controlling factors on heavy mineral assemblages in Chinese loess and Red Clay
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DOI：
10.1016/j.palaeo.2013.04.020
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发表时间：
2013-07
期刊：
影响因子：
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通讯作者：
J. Nie;Wenbin Peng;K. Pfaff;A. Möller;E. Garzanti;S. Andó;T. Stevens;A. Bird;Hong Chang
中科院分区：
文献类型：
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作者：
J. Nie;Wenbin Peng;K. Pfaff;A. Möller;E. Garzanti;S. Andó;T. Stevens;A. Bird;Hong Chang
重矿物分析在砂岩物源判别方面是一项敏感的技术，但在黄土中的应用却很少。在这里，我们报告了重矿物研究的选择样品洛川，西峰和曹县黄土-红粘土剖面在中国黄土高原，基于新的QEMSCAN（定量评价矿物扫描电子显微镜）技术。我们发现，通过过去的500 kyr沉积黄土的重矿物组合是相似的，沉积后的化学溶解不变。相反，在900 ka-3 Ma沉积的样品中，稳定矿物的相对比例呈上升趋势。此外，西峰样品的不稳定铁镁矿物含量始终高于洛川样品。通过光学方法对不同矿物表面结构的详细分析表明，这种成分差异可以归因于洛川剖面更有效的成岩溶解作用，这可以通过在较潮湿的气候条件下更广泛的间隙沃茨渗透来解释。有趣的是，西峰上中新世红粘土（沉积年龄~ 7 Ma）的重矿物组合与500 ka以来的现代黄土沉积相似。这种相似性表明气候和/或当地保存条件阻碍了溶解反应，从而有助于保存在整个考虑的时间段内基本保持不变的原始出处信号。我们的研究表明，气候和时间控制的成岩作用起着关键作用，在确定的组成中所包含的重矿物组合黄土沉积几十万年前。我们还表明，通过使用QEMSCAN和传统的光学技术对相同的样品，我们可以获得基本的互补信息的重矿物组合的正确解释。
Heavy-mineral analysis is a sensitive technique in constraining provenance of sandstone, but has rarely been applied to loess. Here we report a heavy-mineral study of selected samples from the Luochuan, Xifeng and Caoxian loess-Red Clay sections on the Chinese Loess Plateau, based on the novel QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) technique. We found that heavy mineral assemblages of loess deposited through the past 500kyr are similar and unchanged by post-depositional chemical dissolution. In contrast, in samples deposited from 900ka to 3Ma, the relative proportion of stable minerals tends to increase down section. In addition, the Xifeng samples consistently display higher contents of unstable ferromagnesian minerals than the Luochuan samples. Detailed analysis of surface textures displayed by different minerals by optical methods indicates that such a compositional difference can be ascribed to more effective diagenetic dissolution for the Luochuan section, explained by more extensive percolation of interstitial waters in wetter climatic conditions. Interestingly, heavy-mineral assemblages in the underlying upper Miocene Red Clay from Xifeng (deposition age~7Ma) are similar to those of recent loess deposited since 500ka. This similarity indicates that climate and/or local preservation conditions hampered dissolution reactions, thus helping to preserve an original provenance signal that remained largely unchanged throughout the considered time period. Our study demonstrates that climatically- and time-controlled diagenesis plays a key role in determining the composition of heavy-mineral assemblages contained in loess deposited several hundreds of thousands years ago. We also show that by using both QEMSCAN and traditional optical techniques on the same samples we can obtain fundamental complementary information for a correct interpretation of the heavy-mineral assemblage.