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...

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.