宁夏香山群徐家圈组顶部石灰岩地球化学特征及其时代意义  

Geochemical characteristics and their epochal significance of limestones at the top of Xujiajuan Formation,Xiangshan Group in the Ningxia autonomous region,China

作　　者：李向东[1,2] 何幼斌[2] LI Xiang-dong;HE You-bin(School of Land Resource Engineering,Kunming University of Science and Technology,Kunming,650093,China;School of Geoscience,Yangtze University,Wuhan ?430100,China)

机构地区：[1]昆明理工大学国土资源工程学院,云南昆明650093 [2]长江大学地球科学学院,湖北武汉430100

出　　处：《地球化学》2019年第4期325-341,共17页Geochimica

基　　金：国家自然科学基金(41272119)

摘　　要：鄂尔多斯盆地西缘宁夏香山群徐家圈组顶部的薄层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,而香山群自20世纪40年代发现以来的时代争议,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、地层和古地理等项研究工作的开展。本文通过薄层石灰岩碳-氧稳定同位素测温结果、稀土元素及相关微量元素测试结果的分析,对其成因进行探讨。结果表明:(1)碳、氧同位素测温结果显示温度范围分布一般在52.4~61.7℃之间,平均为57.5℃;(2)稀土元素具有明显的Eu正异常(较强)和Ce负异常(较弱);(3)在微量元素U-Th图解中全部落入热水沉积区,在Zn-Ni-Co图解中大部分落入热水沉积区,所有样品均远离水成沉积区。结合沉积环境与沉积特征的相关研究成果,认为徐家圈组顶部的薄层石灰岩具有区域扩散型热水沉积岩特征,其成因可能与远离热液喷溢区的热液羽有关。参照北祁连地区大地构造的相关研究成果,薄层石灰岩最有可能形成于大洋向岛弧的转换时期或弧后扩张时期,即中奥陶世达瑞威尔晚期至晚奥陶世艾家山早期。The thin-bedded limestones at the top of the Xujiajuan Formation of the Xiangshan Group in the Ningxia area, at the western margin of the Ordos Basin are regarded as the pivotal stratigraphic unit in the controversies surrounding the age of Xiangshan Group because of certain rare fossils. Those controversies have existed since the 1940s, when the Xiangshan Group was discovered, and seriously impeded some areas of research, such as early Paleozoic tectonics, stratigraphy, and paleogeography in the northern Qilian area since then. This paper discusses the genesis of these thin-bedded limestones based on carbon and oxygen isotope data, rare earth elements (REE), and other trace elements. The results show that:(1) the depositional water temperature given by the carbon and oxygen isotopes of the limestones is 57.5 °C on average, with a range of 52.4 °C to 61.7 °C;(2) the REE data present significant positive Eu anomalies (slightly strong) and negative Ce anomalies (relatively weak);(3) the samples are all or nearly all restricted to the hydrothermal field in the plots of U-Th and Zn-Ni-Co, respectively, and all samples are separate from the aqueous field. Combining these results with previous studies of sedimentary characteristics and environments of the Xujiajuan Formation, we here suggest that the thin-bedded limestones at the top of the Xujiajuan Formation have some characteristics of regional-diffuse hydrothermal sedimentary rocks, and were perhaps induced by sea-floor hydrothermal plumes that are away from the spray zone of thermal fluids. Combined with recent advances in tectonic studies in the northern Qilian area, the age for the thin-bedded limestones deposits is most likely during the transitional era from ocean to arc or the expansion of the back-arc basin, namely, from the Darriwilian Age of the Middle Ordovician Period to the early Aijiashan Age of the Later Ordovician Period.

关 键 词：薄层石灰岩 稀土元素 热液羽 香山群 

分 类 号：P595[天文地球—地球化学] P597[天文地球—地质学]