宁夏香山群徐家圈组顶部薄层石灰岩锶同位素与局限海盆地分析  

Strontium isotope and restricted marine basin analysis from thin-bedded limestone at the top of Xujiajuan Formation,Xiangshan Group in Ningxia,China

作　　者：李向东[1,2] 魏泽昳 何幼斌[3] 钟军伟[1] LI Xiangdong;WEI Zeyi;HE Youbin;ZHONG Junwei(School of Land Resource Engineering,Kunming University of Science and Technology,Kunming,Yunnan 650093,China;Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institude of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Geoscience,Yangtze University,Wuhan,Hubei 430000,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]中国地质科学院地质研究所,自然资源部深地动力学重点实验室,北京100037 [3]长江大学地球科学学院,湖北武汉430000

出　　处：《地质学报》2024年第4期1229-1243,共15页Acta Geologica Sinica

基　　金：自然资源部深地动力学重点实验室开放课题基金(编号J1901-16);国家自然科学基金项目(编号41272119)联合资助的成果。

摘　　要：宁夏香山群主体为一套深水浊流沉积的灰绿色细砂岩夹泥岩,在鄂尔多斯盆地西缘及河西走廊地区下古生界占有重要的位置,其时代归属对于北祁连地区大地构造演化、宁夏区域地质和鄂尔多斯盆地西缘海相深水油气勘探均具有重要意义。本文采用沉积学和沉积地球化学的方法,着重分析了香山群徐家圈组顶部薄层石灰岩锶同位素的特征,结果表明:①薄层石灰岩受成岩蚀变程度较小,^(87)Sr/^(86)Sr值基本可代表沉积时海水的特征;②^(87)Sr/^(86)Sr值在0.71057~0.71356之间,平均值为0.71189,远大于寒武纪和奥陶纪海水^(87)Sr/^(86)Sr值;③^(87)Sr/^(86)Sr值和盐度呈较好的负线性相关,和温度及Mn/Sr比值呈较好的正线性相关;④^(87)Sr/^(86)Sr值和Si含量略具负线性相关趋势,和Al含量及Mg含量具有较弱的负线性相关性。结合鄂尔多斯盆地西缘早古生代大地构造演化和香山群相关研究成果认为:徐家圈组顶部薄层石灰岩可能沉积于受古陆或水下隆起阻隔的局限海盆深水环境,沉积时海水主要受和硅铝质岩石进行充分锶同位素交换的海底热液及流经喀斯特风化壳的地表径流影响,与同期大洋海水相比具有高的^(87)Sr/^(86)Sr值.Xiangshan Group in Ningxia is mainly composed of grayish-green,fine-grained sandstones interbedded with mudstones induced by deep-water turbidity currents.Determining the geological age of the Xiangshan Group is crucial for understanding the tectonic evolution in the northern Qilian area,regional geology in Ningxia,and marine deep-water petroleum exploration in the western Ordos basin.Here we focus on the characteristics of strontium isotope in thin-bedded limestones at the top of the Xujiajuan Formation.We employ sedimentology and sedimentary geochemistry methods to investigate this.The results reveal that:①the weak degree of diagenetic alteration in the thin-bedded limestones suggests that the measured^(87)Sr/^(86)Sr values can represent the coeval seawater Sr isotope composition;②the^(87)Sr/^(86)Sr ratios range from 0.71057 to 0.71356,with an average value of 0.71189.These ratios are significantly higher than those observed in Cambrian and Ordovician ocean water;③there is an average negative linear correlation between the^(87)Sr/^(86)Sr ratios and sea water salinities(Z values caculated with carbon and oxygen isotopes).Additionally,a strong and average positive linear correlation is observed between the^(87)Sr/^(86)Sr ratios and sea water temperatures(oxygen isotope temperature)and Mn/Sr ratios,respectively;④the^(87)Sr/^(86)Sr ratios also display a negative nonlinear correlation with silicon contents and a weak negative linear correlation with aluminium and magnesium contents.Considering the Early Paleozoic tectonic evolution in the western Ordos basin and some previous research on the Xiangshan Group,it is plausible to suggest that the thin-bedded limestones at the top of the Xujiajuan Formation were possibly deposited in a deep-water environment within a restricted marine basin.This basin may have been confined by a small paleocontinent or underwater uplift from the open sea.The influence of coeval confined seawater in the basin could have included submarine hydrothermal solutions and surface runoff.

关 键 词：薄层石灰岩 锶同位素 局限海盆地 香山群 鄂尔多斯盆地西缘 

分 类 号：P618.13[天文地球—矿床学]