汉南—米仓山地区东部寒武系仙女洞组斑块白云岩特征及形成机制

Characteristics and formation mechanism of patchy dolostone of the Cambrian Xiannüdong Formation in eastern Hannan-Micangshan area

斑块白云岩具有非均质性强、内部结构复杂等特点,发育特征和成因亟待理清。文中以汉南—米仓山地区东部杨家沟剖面为例,在系统野外调查工作的基础上,利用岩相学、沉积学和地球化学等方法,在寒武系仙女洞组共识别出13个岩石微相类型以及5种白云岩类型,并对不同组构碳、氧同位素和微量元素组成特征进行了分析。研究发现,斑块白云岩的分布与垂向构造破碎带及其横向延伸带的位置有关,斑块白云岩及其与围岩之间的构造缝中发育大量鞍形白云石;斑块白云岩中氧同位素组成(δ^(18)O:-8.17‰~-6.58‰)明显低于当时的海水,原位微区技术揭示白云石晶体具有高Mn/Sr值(Mn/Sr_(均值)=14.8)、Eu正异常(Eu/Eu^(*)_(均值)=1.3)以及页岩化后的中稀土元素富集配型。在排除其他可能的成因后,推测研究区斑块白云岩的形成与区域断层控制的热液流体活动有关。相关工作的开展对于查明汉南—米仓山周缘地区寒武系仙女洞组白云岩的分布和发育规律、理清斑块白云岩的特征和成因具有重要作用,也可为四川盆地北部白云岩储集层勘探提供参考。

Patchy dolostone,a common type of dolostone,is distinguished by its heterogeneity and complex internal structure,requiring a comprehensive investigation of its origins and formation mechanisms.In this study,the Yangjiagou section in the eastern Hannan-Micangshan area is selected as a representative example.Through comprehensive field observations and various methods including petrography,sedimentology and geochemistry,13 types of lithofacies and five dolomite types were identified in the Xiannüdong Formation.Geochemical methods were employed to analyze the carbon and oxygen isotopes as well as trace element compositions of these dolostone.Our findings indicate that the occurrence of patchy dolostone is strongly associated with the location of vertical tectonic fractures and transverse extension zones.The fractures within the patchy dolostone and surrounding limestones are filled by coarse-grained saddle dolomite.Notably,theδ^(18)O values of the patchy dolomites(δ^(18)O:-8.17‰to-6.58‰)are lower than those of the contemporaneous seawater-derived dolomites.Additionally,in-situ trace element measurements reveal that dolomite crystals exhibit high Mn/Sr ratios(mean of 14.8),a positive Eu anomalies(mean of 1.3),and a middle rare earth elements-enriched pattern when normalized to shale.After excluding other possibilities,it is inferred that the formation of patchy dolostone in the study area can be attributed to the influence of regional fault-controlled hydrothermal fluids.The significance of this study lies in its ability to identify the distribution and development of Cambrian dolomites within the Xiannüdong Formation in the surrounding area,as well as to clarify the identification markers and formation processes of hydrothermal dolomites.This study may provide valuable insights for oil and gas exploration in the northern Sichuan Basin.