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扬子板块西北缘热水沉积硅质岩稀土元素配分模式的影响因素
投稿时间:2017-07-11  修订日期:2017-09-19  点此下载全文
引用本文:张岩,漆富成,陈文,叶会寿.2018.扬子板块西北缘热水沉积硅质岩稀土元素配分模式的影响因素[J].地质学报,92(2):378-394
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作者单位E-mail
张岩 1) 中国地质科学院地质研究所北京100037 ZhYan2018@163.com 
漆富成 2) 核工业北京地质研究院北京100029  
陈文 1) 中国地质科学院地质研究所北京100037  
叶会寿 3) 中国地质科学院矿产资源研究所北京100037  
基金项目:本文为中国黑色岩系非常规铀资源矿集区成矿条件分析及资源预测评价(编号201493)、中国地质科学院基本科研业务费项目(编号YYWF201707)共同资助的成果。
中文摘要:本文对四川省若尔盖县北部-甘肃省迭部县中-下志留统(兰多维列统(Llandovery) -温洛克统(Wenlock))富有机质热水沉积硅质岩及下伏震旦系火山岩进行了以稀土元素地球化学为主的研究,目的是探讨热水沉积硅质岩稀土元素来源及其配分模式的主要控制因素,并建立相应模式图。主要对1个钻孔、2个剖面共28件硅质岩的岩相学、扫描电镜、稀土元素(重点)特征,1个剖面的14件火山岩的主量元素、稀土元素(重点)特征进行了研究。黑色热水沉积硅质岩的岩相学、扫描电镜分析显示,其具有典型的沉积构造特征,后期弱变质;下伏震旦系火山岩样品的TAS图显示其主要为酸性火山岩,少量为基性火山岩。结合前人研究成果,对下伏震旦系火山岩与上覆地层中早-中志留世热水沉积硅质岩的稀土元素对比分析后发现:①张性构造环境是产生热水沉积硅质岩的动力学因素,而热水沉积硅质岩稀土元素主要来源于热水流体、古海水(混有陆源物质)、下伏地层中的稀土元素。②热水沉积硅质岩的稀土元素的相对丰度中存在一个阈值(或区间):在这个阈值之下,轻稀土元素(LREEs)配分模式及含量与热水流体一致,而重稀土元素(HREEs)配分模式与下伏火山岩的重稀土元素配分模式一致;在这个阈值之上,轻稀土元素(LREEs)配分模式则主要显示了与热水流体混合的古海水(混入有陆源物质)的特征,此时的热水沉积硅质岩的轻稀土元素配分模式及含量与平均上地壳的轻稀土元素配分模式一致,而重稀土元素(HREEs)配分模式仍然与下伏火山岩的重稀土元素配分模式一致。因此,在一定条件下热水沉积硅质岩可以指示古沉积构造环境。
中文关键词:热水沉积硅质岩  稀土元素  来源  影响因素  古构造环境
 
Factors Influencing the REE Distribution Patterns of Hydrothermal Sedimentary Cherts in the Northwestern Margin of Yangtze Block
NameInstitution
ZHANG Yan1) Institute of Geology, Chinese Academy of Geological Sciences, 100037, Beijing, China
QI Fucheng2) Beijing Research Institute of Uranium Geology, 100029, Beijing, China
CHEN Wen1) Institute of Geology, Chinese Academy of Geological Sciences, 100037, Beijing, China
YE Huishou3) Institute of Mineral Resources, Chinese Academy of Geological Sciences, 100037, Beijing, China
Abstract:In this study,we carried out an REE geochemical study of lower middle Silurian organic rich bedded cherts and underlying Sinian volcanic rocks in the regions of northern Ruoergai (Zoigê) County in Sichuan and Diebu (Têwo) County in Gansu, central China. The aim of this study was to understand the origin of rare earth elements in hydrothermal cherts and factors affecting REE patterns, and establish a corresponding model. 28 samples of cherts from one drilling core and two profiles were studied using petrographic analysis, scanning electron microscopy (SEM) analysis, major and rare earth element analyses. 14 samples of volcanic rocks from one profile were investigated using major elements and REE analyses. Petrographic and SEM analyses of black hydrothermal cherts show that the black cherts have typical features of deposition structure and weak late stage metamorphism. The TAS diagram of lower Sinian volcanic rocks suggests that they are dominantly felsic volcanic rock, with minor being mafic volcanic rocks. Combined with previous results, comparison analysis of underlying Sinian volcanic rocks and overlying middle early Silurian hydrothermal siliceous rock draw the following conclusions. (1) Tensional tectonic setting is an essential dynamic factor for formation of hydrothermal chert, REEs of which mainly derived from hydrothermal fluids, ancient seawater (mixed in continental matters). (2) Relative abundances of the REEs in hydrothermal cherts have a threshold, below which LREEs’ distribution pattern is consistent with that of hydrothermal fluids while HREEs’ distribution patterns consistent with that of underlying volcanic rocks; and above which LREEs’ distribution presents the characters of mixing of hydrothermal fluid with ancient water (terrestrial material). The LREE patterns of hydrothermal cherts are consistent with that of upper crust, while HREE patterns are consistent with that of underlying volcanic rocks. Therefore, hydrothermal chert deposited in a certain condition can indicate paleodepositional structure environment.
keywords:Hydrothermal chert  REE  Sources  Influence factors  Paleodeposition setting
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