宁夏香山群徐家圈组顶部石灰岩稀土元素特征与沉积介质分析

doi:10.13278/j.cnki.jjuese.20180178

摘要/Abstract

摘要： 鄂尔多斯盆地西缘宁夏香山群自20世纪40年代发现以来一直存在的时代争议，已经严重地阻碍了该地区及北祁连地区早古生代大地构造、地层和古地理等研究工作，香山群徐家圈组顶部的薄层状石灰岩以其含有的古生物化石已成为解决香山群时代归属的关键地层单元。本文通过对薄层状石灰岩稀土元素及相关微量元素测试结果的分析，探讨其沉积时的海水介质条件。结果表明：依据稀土元素的分布特征，可将薄层状石灰岩分为A、B、C三组；稀土元素具有明显的Eu正异常和Ce负异常，从A组到C组Eu正异常增大，Ce负异常减小；从A组到C组，各地球化学参数呈规律性变化，（La/La^*）_N、（Y/Y^*）_N、（Gd/Gd^*）_N、Y/Ho值逐渐减小，稀土总量和轻重稀土比值逐渐增加，（La/Yb）_N与（La/Sm）_N值由小变大再变小，（Ce/Ce^*）_N值逐渐增大而U/Th值逐渐减小，Sr/Ba值稳定而Zr/Rb值增大。依据地球化学特征可推测沉积时从A组到C组的水介质变化为陆源物质影响增加、海水性质减弱、水深波动、海水由还原到氧化、水动力增强和盐度基本稳定。结合稀土元素Eu异常现象及相关研究成果，推测徐家圈组顶部的薄层状石灰岩沉积于氧化-还原层化海洋靠近分界线的还原带内，热液羽引发的深水水体流动（内波、内潮汐）可能控制着陆源物质的输入和水体的氧化-还原环境，在还原的总体背景上产生水动力氧化现象。

关键词: 稀土元素, 热液羽, 石灰岩, 徐家圈组, 香山群, 鄂尔多斯盆地西缘

Abstract: The thin-bedded limestone,at the top of Xujiajuan Formation of Xiangshan Group in Ningxia, the western of Ordos basin, has become the pivotal stratigraphic unit to solve the age controversy of Xiangshan Group for some critical fossils found in Xujiajuan Formation. There has been an age controversy since the 1940s when the group was discovered, which now seriously blocks the research such as Early Paleozoic tectonics, stratigraphy and paleogeography in the northern Qilian area. The authors discussed the aqueous media from which these thin-bedded limestones were deposited based on the data of rare earth elements (REEs) and other trace elements. The results show that:1) the thin-bedded limestone can be divided into three types of A, B and C according to their REEs characteristics. 2) the positive Eu anomalies and negative Ce anomalies increase and decrease from type A to type C, respectively. 3) the geochemical indices of limestone change regularly from type A to type C, including the decrease of (La/La^*)_N, (Y/Y^*)_N, (Gd/Gd^*)_N, and Y/Ho ratios, the increase of the concentration of total REE and the ratios of light REE to heavy REE, the first increase and then decrease of (La/Yb)_N and (La/Sm)_N values, the increase of (Ce/Ce^*)_N values relative to the decrease of U/Th ratios, and the stability of Sr/Ba ratios relative to the distinct increase of Zr/Rb ratios. These geochemical changes suggest that the water media from type A to type C during deposition were characterized by:1)the attributes of seawater was decreased while the influence of terrigenous origin increased. 2)the depositional hydrodynamic intensity increased distinctively with variation of water depth. 3)the redox condition was from reduction to oxidation, and the water salinity was stable. Based on these interpretations of sedimentary aqueous media (seawater), sedimentary environments, and the significant positive Eu anomalies, the deposition of thin-bedded limestones perhaps occurred in an anoxic zone near the boundary of redox-stratified ocean and the internal-waves and internal-tides influenced by the sea floor hydrothermal plumes, which probably controlled the input of terrigenous material and the redox conditions, causing hydrodynamic oxygenation on the anoxic background.

Key words: rare earth elements, hydrothermal plumes, limestone, Xujiajuan Formation, Xiangshan Group, western of Ordos basin

中图分类号:

P512.2

李向东, 何幼斌. 宁夏香山群徐家圈组顶部石灰岩稀土元素特征与沉积介质分析[J]. 吉林大学学报(地球科学版), 2020, 50(1): 139-157.

Li Xiangdong, He Youbin. REE Geochemistry and Indicators of Sedimentary Media of Limestone at Top of Xujiajuan Formation, Xiangshan Group in Ningxia Autonomous Region, China[J]. Journal of Jilin University(Earth Science Edition), 2020, 50(1): 139-157.

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