云南保山普里道利统沉积特征及其对冈瓦纳北缘古海洋环境的指示

Sedimentary characteristics of the Pridolian Series in Baoshan,Yunnan Province and their implications for paleomarine environment

保山地区志留纪晚期位于冈瓦纳古陆北缘的滇缅马苏地块,具有异于同时期华南陆块、塔里木陆块和华北陆块的古气候和古海洋环境。保山地区志留系栗柴坝组第三段隶属普里道利统,在对其海相碳酸盐岩沉积特征研究的基础上,结合区域地质背景、古生物资料、沉积相特征、元素组分含量及相关元素的比值特征和碳氧同位素数据等,对该区普里道利统的古海洋环境进行了探讨。研究表明:①参考Wilson的标准微相沉积模式,结合岩性特征,可以识别出RMF8、RMF10、RMF11、RMF16、RMF20和RMF23共6种微相类型;②碳同位素共表现出3次明显的正偏移现象,分别在卢德福德中晚期、普里道利世晚期及普里道利世末期,即志留系与泥盆系界线处,3次偏移均具有全球可对比性,分别对应于欧美地区的"Lau"事件、普里道利世晚期的δ13 C正偏移事件以及"Klonk"事件,但在不同地区δ13 C值偏移程度具有差异性;③依据碳氧同位素,重建了古海水盐度和温度演化曲线,表明古海水的盐度指数Z值在120~130之间,温度分布在17.03~28.85℃之间,且多数集中在20℃以上,说明研究区该时期为温暖或炎热的亚热带海洋气候;④通过元素地球化学分析,重点利用B、Ni、V和Ti等元素及Mn/Fe、V/(V+Ni)和MgO/CaO等比值信息,对沉积时期的物源供给情况和氧化还原条件等作了综合探讨,结果表明:陆源物质供给充足的阶段分别对应于后两次碳同位素正偏事件,这一现象解释为造山运动使古陆面积逐渐增大,为海洋提供了更多的物源供给;V/(V+Ni)比值大多处于0.57~0.83之间,代表了水体流通较差的还原环境,与沉积相特征具有一致性;⑤综合推断3次碳同位素正偏移事件是对志留纪末期加里东造山运动背景下海平面大规模下降的响应,为海退背景下的碳同位素正异常;碳酸盐岩岩相特征和沉积地球化学指标表明该时期的古海洋环境与构造事件具有较好的耦合性。

Baoshan area,located in the northern margin of Gondwanaland in the late Silurian,exhibits different paleoclimate and paleomarine environment characteristics from those of the South China,Tarim and North China blocks.The 3 rd Member of the Silurian Lichaiba Formation is of Pridoli age.Based on the study of sedimentary characteristics of carbonate rocks,supplemented by regional geological background,paleontological data,sedimentary facies,element content and ratios,carbon and oxygen isotope data,the paleomarine environment is discussed.The results show that:(1)by referring to Wilson’s standard microfacies sedimentary model and combining it with lithologic characteristics,six microfacies types,i.e.RMF8,RMF10,RMF11,RMF16,RMF20 and RMF23,can be identified;(2)the carbon isotopes show three significant positive excursions,one in the middle-late Ludford,second during the late Pridorian,and the last in the Silurian-Devonian boundary,all of which are globally comparable,corresponding respectively to the"Lau"Event,the late Pridolian positive excursion Event and the"Klonk"Event in Europe or America.However,the degree of excursions is different in different regions;(3)based on the analysis of carbon and oxygen isotopes,the variation in salinity and temperature of the ancient seawater are reconstructed,showing that the salinity index value Zrange from 120 to 130,the temperatures between17.03~28.85℃,and most of them are above 20℃,indicating a warm or hot subtropical marine climate for the studied area;(4)an element geochemical analysis,mainly using the information from B,Ni,V and Ti and the ratios of Mn/Fe,V/(V+Ni)and MgO/Cao,was conducted.The results show that two obvious increases of material supply from land coincide with the time intervals after two carbon isotope excursions.This phenomenon is due to the increasing exposure of ancient land under the influence of orogeny,which supplies more terrestrial material to the ocean.The V/(V+Ni)ratios in the studied interval,mostly within the range of 0.57~0.83,represent a reduction environment with sluggish water circulation,and are consistent with the lithological and sedimentary faciescharacteristics;(5)It is concluded that the three positive carbon isotope excursion events were a response to the large-scale regression against the background of Caledonian orogeny in the late Silurian.The petrological,microfacies and geochemical characteristics of carbonate rocks show that the paleomarine environment is associated with tectonic events.