湘西奥陶系红色碳酸盐岩碳、氧同位素组成特征及古环境意义

Characteristics of carbon and oxygen isotopic compositions and the palaeoenvironmental significance of Ordovician red carbonate rocks in Xiangxi,Hunan

红色碳酸盐岩是华南奥陶系的一种具有紫红—砖红色调的代表性沉积岩,其中可能蕴含特殊的古环境信息。湖南湘西三百洞地区具有一条出露良好、厚层为主的红色碳酸盐岩剖面,主要发育于大湾组和牯牛潭组内,文章对该段地层采集了355件碳、氧同位素和Mn、Sr元素分析样品,以及10件牙形刺化石样品进行研究。根据牙形刺样品约束,大湾组底界可能大致相当于弗洛阶顶界,大湾组和牯牛潭组界线在达瑞威尔阶中下部,牯牛潭组顶界大致相当于达瑞威尔阶顶界。据显微薄片鉴定、δ^(13)C-δ^(18)O相关性分析和Mn-Sr元素评估,推断样品受到成岩作用影响不大。355件同位素测试样品显示三百洞剖面的δ^(13)C和δ^(18)O均为低幅高频振荡,δ^(13)C在-0.38‰~1.67‰之间,δ^(18)O在-9.34‰~-7.21‰之间,其中可与全球对比的同位素事件有大坪阶底部碳同位素负漂事件(BDNICE)和达瑞威尔阶中部碳同位素漂移事件(MDICE),可以间接对比的有达瑞威尔阶下部碳同位素负漂事件(LDNICE),此外还有2次前人未命名的具有潜在对比价值的漂移。总体来看,冈瓦纳大陆北缘沿赤道由东向西具有δ^(13)C波动幅度逐渐增强的趋势,华南的波动幅度普遍偏小,可能由海水循环过程中不同块体的底层海水分层差异造成。另外基于δ^(13)C、δ^(18)O的古盐度计算,红色碳酸盐岩应属于盐度中等到较高的海相碳酸盐岩,并且剖面自下到上盐度增高,大湾组底部可能存在一次氧化事件。结合剖面岩石组成、颜色、古生物丰度以及区域古地理研究,认为红色碳酸盐岩可能是暂时性相对海平面下降叠加氧化事件的产物。

The red carbonate rock is a kind of representative sedimentary rock with purple-red colors of the Ordovician in South China,and may contain special palaeoenvironmental information.There is a well exposed thick red carbonate rock section in the Sanbaidong area of Xiangxi,which is mainly developed in the Dawan Formation and the Guniutan Formation.This paper studied 355 carbon-oxygen isotope,Mn-Sr element samples and 10 conodont samples on it.According to the restriction of conodont samples from these formations,the bottom boundary of the Dawan Formation may be near the top boundary of the Florian Stage,the boundary between the Dawan Formation and the Guniutan Formation may be in the lower to middle part of the Darriwilian Stage,and the top boundary of the Guniutan Formation is near the top boundary of the Darriwilian Stage.Withδ^(13)C-δ^(18)O correlation analysis and Mn-Sr element evaluation,it is considered the samples are not strongly affected by diagenesis.355 samples show that theδ^(13)C andδ^(18)O of the Sanbaidong section are low amplitude and high frequency fluctuations.Theδ^(13)C are between-0.38‰to 1.67‰,while theδ^(18)O are between-9.34‰and-7.21‰.Among them,BDNICE and MDICE are two global isotopic events that can be compared with this section,and LDNICE can be compared indirectly.In addition,there are two unnamedδ^(13)C isotope shifts with potential comparative value.In general,there is an increasing trend ofδ^(13)C from east to west along the equator on the north Gondwana.The fluctuation amplitude in South China is generally smooth,which may have been caused by the difference in bottom seawater stratification in different blocks during the seawater circulation.Based on theδ^(13)C andδ^(18)O calculation,the red carbonate rocks should contain marine characteristics with middle to high palaeosalinity that increases upwards.Combined with the lithology,color,palaeontological quantity of the section and regional palaeogeography,it is believed that there may be an oxidation event at the bottom of the Dawan Formation,and the red carbonate rocks may be the product of the temporary relative sea level decline superimposed oxidation events.