四川盆地南部寒武系白云石地球化学特征与形成机制

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

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。

Based on geochemical analysis, including the degree o f order, carbon and oxygen isotopes, rare earth elementsand inclusions, the types and generation mechanism o f Cambrian dolostone samples from outcrops at Fandian, Yankong andSanhuichang in the south o f 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 o f low order degree, and similar to 8180 values o f the normalmarine carbonate rocks in the same period. Their REE distribution patterns are approximate consistent, which is characterizedby weak accumulation o f HREE and weak loss o f LREE. The dolomitization fluid that mostly originates from high-salinityseawater is considered as the product o f the evaporation pump dolomitization and the seepage reflux dolomitization. Theorder degree for fine-crystal line dolomite is relatively high and the rare earth elements display depletion and migration withdecreasing Z REE values. It is considered as the result o f the burial dolomitization. The saddle dolomite is characterized byhigh order degree, low 8180 value, enrichment o f REE, Eu positive and negative, extremely high homogenization temperatureo f inclusions and a double range distribution, which indicates that the dolomitization fluid maybe derived deep hydrothermalfluid, or as the product o f the hydrothermal dolomitization.