鄂尔多斯盆地马家沟组白云岩地球化学特征及白云岩化机制分析

摘要

The types and genetic mechanism of the Majiagou dolomite in Ordos basin are investigated using petrological analysis and geochemical methods. The results show that dolostones in Majigou Formation mainly consist of dolomicrite and crystalline dolostone. Dolomicrite often associates with gypsum or halite, with medium jacinth light in cathoduluinescence observation, lower degree of order, and high content of Sr and Na. It provides similar δ13C and REE distribution pattern with micrite. The value of Sr isotope is close to that of sea water in the same period, and fluid-inclusion homogenization tempetrature approximate to earths surface. These suggest that dolomitization fluid is high salinity fluid that closely related with sea water, considered as penecontemporaneous dolomitization which can be explained by evaporation pump mode. Crystalline dolostone contains little associated minerals, only pyrite observed occasionally. This crystal structure is usually xenomorphic curve. Crystalline dolostone shows medium-weak jacinth light in cathoduluinescence, whith high degree of order and low content of Sr, while Na and Mn content is close to that of dolomicrite. REE distribution pattern is similar with dolomicrite and close to micrite. The distribution of δI3C is also corresponding to micrite, and fluid-inclusion homogenization tempetrature is hight. These indicate that dolomitization fluid of crystalline dolostone is similar with that of dolomicrite. Crystalline dolostone is buried dolomitisation, where continuous compaction from overlying deposits urges the sealed brine to flow laterally, thus supplies unfailing Mg2 + for dolomitisation.%由岩石学和地球化学研究表明:马家沟组白云岩主要为泥微晶白云岩和晶粒白云岩两种.泥微晶白云岩常与石膏或岩盐伴生,阴极发光呈中等橘红色光,有序度低,具较高的Sr、Na含量,与泥晶灰岩具相似的δ13C分布区间及稀土元素配分模式,Sr同位素值接近同期海水,包裹体均一温度近于地表温度,表明白云岩化流体为与海水密切相关的高盐度流体,为准同生白云岩化,可用蒸发泵模式解释其成因.晶粒白云岩伴生矿物少,偶见黄铁矿,晶体结构多曲面他形,阴极发光为中-弱橘红色光,有序度高,Sr含量较低,Na、Mn含量与泥微晶白云岩相近,稀土元素配分模式与泥微晶白云岩相似且与泥微晶灰岩相近,δ13C分布区间与泥微晶灰岩趋于一致,包裹体均一温度较高,说明晶粒白云岩与泥徼晶白云岩具相似的白云岩化流体,该种白云岩为埋藏白云岩化,上覆沉积物不断压实封存卤水使其侧向流动为白云岩化提供了源源不断的Mg离子.