四川盆地南部寒武系白云石地球化学特征与形成机制

以四川盆地南部范店、岩孔、三汇场三个野外露头剖面的寒武系白云岩样品为研究对象,通过对白云石有序度、碳氧同位素、稀土元素、包裹体等地球化学特征的分析,认为研究区主要发育同生期微晶白云石(岩)、细-粉晶白云石、浅埋藏细晶白云石、以及鞍状白云石等四种类型,其中:微晶白云石(岩)、细-粉晶白云石的有序度较低,具有与同时期正常海相碳酸盐岩相近似的δ18O值,稀土元素配分模式近似一致,表现为具有LREE弱富集、HREE弱亏损的特征,这揭示了白云石化流体主要来源于高盐度海水,为蒸发泵白云石化、渗透回流白云石化作用的产物;而细晶白云石的有序度相对较高,稀土元素出现贫化迁移现象,∑REE值有所减少,推断为埋藏白云石化作用的产物;鞍状白云石的有序度高、δ18O值低,稀土元素富集,Eu具有正、负异常,包裹体均一化温度呈异常高值,呈双区间分布,表征白云石化流体来源于深部热液流体,可能为热液白云石化作用的产物。

Geochemical Characteristics and Generation Mechanism of Cambrian Dolomite in the South of Sichuan Basin

Based on geochemical analysis, including the degree of order, carbon and oxygen isotopes, rare earth elements and inclusions, the types and generation mechanism of Cambrian dolostone samples from outcrops at Fandian, Yankong and Sanhuichang in the south of Sichuan Basin are investigated. It is shown that contemporaneous microcrystalline dolomite, fine-crystalline dolomite, shallow-buried fine-crystalline dolomite and saddle dolomite mainly develop in the study area. The microcrystalline dolomite and fine-crystalline dolomite are of low order degree, and similar to δ18O values of the normal marine carbonate rocks in the same period. Their REE distribution pat erns are approximate consistent, which is characterized by weak accumulation of HREE and weak loss of LREE. The dolomitization fluid that mostly originates from high-salinity seawater is considered as the product of the evaporation pump dolomitization and the seepage reflux dolomitization. The order degree for fine-crystalline dolomite is relatively high and the rare earth elements display depletion and migration with decreasing ΣREE values. It is considered as the result of the burial dolomitization. The saddle dolomite is characterized by high order degree, lowδ18O value, enrichment of REE, Eu positive and negative, extremely high homogenization temperature of inclusions and a double range distribution, which indicates that the dolomitization fluid maybe derived deep hydrothermal fluid, or as the product of the hydrothermal dolomitization.