贵州施秉云台山白云岩地球化学特征及成因探讨

贵州省施秉县云台山景区喀斯特地貌主要由寒武纪白云岩组成,其白云石含量多在90%以上,无论是基底中的白云石,还是内碎屑中的白云石均呈自形-半自形晶体。白云岩中CaO的平均含量为30.4%,MgO的平均含量为21.7%,与白云岩的理想含量相当。白云岩Sr的平均含量为72.4×10^-6,Sr/Ba比值大于1,反映其为海相沉积。白云岩的稀土总量低,仅为12.54×10^-6,Eu具较弱负异常,Ce基本无异常,稀土配分模式为轻稀土富集的右倾型。白云岩的形成环境总体表现为局限台地上的泻湖潮下环境,水体较浅,水流不畅,水动力弱-中等。成岩环境以弱还原-还原条件为主,干燥炎热是其古气候条件的特点。在干燥炎热的气候条件下,先期形成的碳酸钙沉积物,经强烈蒸发而形成泻湖卤水,通过向海方向的渗滤回流被交代而形成白云岩,属准同生期-早成岩阶段近地表的交代白云岩。

Geochemistry and origin of the dolostones from the Yuntai Mountain area, Shibing, Guizhou

The karast landforms in the Yuntai Mountain area, Shibing, Guizhou are dominantly made up of the Cambrian dolostones in which the dolomite contents come up to 90%, and display euhedral-subhedral crystals. The dolostones in this area have the contents of 30.4% for CaO, 21.7% for MgO and 72.4×10-6 for the average of Sr contents with the Sr/Ba ratios of more than 1, and are characterized by low ΣREE (12.54×10-6), slightly negative Eu anomaly, and right-leaning LREE-enriched distribution patterns. It is inferred that the dolostones were originated from the lagoon-subtidal environments within the restricted platforms with shallower water depths and weakly to moderate hydrodynamic environments, and xerothermic palaeoclimatic conditions. Influenced by the xerothermic climates, the pre-existing calcium carbonates were highly evaporated to give rise to the lagoon brines, and the seaward infiltration and seepage reflux of the brines finally resulted in the formation of the dolomites. It follows that the dolostones in the study area should be assigned to the near-surface replacement dolostones generated during the penecontemporaneous-early diagenetic stages.