鄂尔多斯盆地中东部马四段白云岩地球化学特征及成因模式

在岩石学分析的基础上,结合痕量元素、稀土元素、碳氧同位素以及锶同位素的研究,探讨鄂尔多斯盆地中东部奥陶系马家沟组第四段(简称“马四段”)白云岩的成因。结果表明:马四段白云岩主要由泥微晶白云岩、粉-细晶白云岩、(残余)砂砾屑白云岩及斑状白云岩组成。不同类型白云岩的稀土配分模式均与泥晶灰岩相似,且Sr同位素值及碳氧同位值均位于同期海水范围内,表明白云石化流体与海水密切相关。其中泥微晶白云岩在镜下可见晶型较好的硬石膏与之伴生,同时δ¹³C_PDB与δ¹⁸O_PDB值较泥晶灰岩偏高,显示其高盐度沉积环境。粉-细晶白云岩及(残余)砂砾屑白云岩Fe、Mn含量相对较高,Sr含量较低,δ¹⁸O_PDB值比泥微晶白云岩低,说明受到了一定埋藏作用的影响;而斑状白云岩虽具有相对较低的Fe、Mn含量,但较泥晶灰岩相对偏大,在镜下可见雾心亮边结构,表明其后期也遭受了一定的埋藏作用,同时,在岩心表面见生物扰动构造和生物钻孔,表明其在前期受到了生物活动的影响,并为流体提供了通道。综合岩石学特征、地球化学指标以及古地理背景,认为鄂尔多斯盆地中东部马四段泥微晶白云岩具有蒸发泵成因模式的特征,(残余)砂砾屑白云岩与粉-细晶白云岩是渗透回流白云石化作用叠加埋藏效应的结果,而斑状白云岩则属于典型的生物扰动型成因。

Geochemical characteristics and genetic model of Ordovician Ma-4 Member dolomite in central eastern Ordos Basin,China

On the basis of petrological analysis,combined with the study of trace elements,rare earth elements,carbon and oxygen isotopes and strontium isotopes,this paper discusses the origin of dolomite in the fourth member of Ordovician Majiagou Formation(Ma-4)in the central-eastern Ordos Basin.The results show that the Ma-4 Member dolomite is mainly composed of dolomicrite,powder-microcrystalline dolomite,dolarenite and spotty dolomite.The REE distribution patterns of different types of dolomites are similar to those of micritic limestones,and the values of ⁸⁷Sr/⁸⁶Sr,δ¹³C_PDB andδ¹⁸O_PDB are all within the range of seawater at the same time,indicating that dolomitic fluids are closely related to seawater.The fact that the dolomicrite is associated with anhydrite with better crystal shapes,and the values ofδ¹³C_(PDB( andδ¹⁸O_PDB are higher than those of micritic limestone indicates a high salinity sedimentary environment.And the fact that the Fe and Mn contents in powder-microcrystalline dolomite and dolarenite are relatively high,the Sr content is relatively low and theδ¹⁸OPDB values of powder-microcrystalline dolomite and dolarenite are lower than those of micritic dolomite indicates that they have been affected by burial.Although spotty dolomite has relatively low Fe and Mn content,it is relatively larger than micritic limestone.It also suffered certain burial effect in the later stage,evidenced by the bright rim structure of fog core observed under the microscope.At the same time,bioturbation structures and biological boreholes are observed on the drilling core surface,indicating that the rock is influenced by biological activity in the early stage,which provides channel for later fluid migration.Based on the petrological characteristics,geochemical index and paleogeographic background,it is considered that the Ma-4 Member micritic dolomite in Ordos Basin has the characteristics of the genetic model of evaporation pump,the powder-microcrystalline dolomite and dolarenite are resulted from the infiltration refluxing dolomitization superimposed with burial effect,while spotty dolomite is resulted from the typical biological disturbance.