塔里木盆地塔北、塔中地区寒武系—奥陶系碳酸盐岩中鞍形白云石胶结物特征

Characteristics of saddle dolomite cements in the Cambrian-Ordovician carbonates in Tabei and Tazhong area of Tarim Basin

通过对塔里木盆地中、北部地区寒武系—奥陶系碳酸盐岩的研究,发现鞍形白云石胶结物发育比较普遍,常见于孔洞或裂缝之中,乳白色,晶体粗大,晶面弯曲或呈阶梯状,镜下波状消光,晶体内部常见微裂缝,常与热液矿物共生。本文对28个鞍形白云石样品进行了碳、氧、锶同位素测试,结果显示鞍形白云石的δ^(13)C和δ^(18)O值分别介于-2.446‰～0.686‰和-9.101‰～-5.117‰之间,^(87)Sr/^(86)Sr值介于0.708 6～0.710 2之间;流体包裹体测温分析表明,鞍形白云石中气—液两相包裹体的均一温度(T_h)介于121～159.5℃之间,但集中分布在135～145℃之间;根据最后冰融点温度(T_m)求得的白云岩化流体盐度介于21.3%～23.1%之间。这些数据表明,该类型白云石形成于热卤水(盐度是海水的5～8倍)之中。塔里木盆地鞍形白云石与世界范围内其它盆地的鞍形白云石的碳、氧同位素特征基本相似,但其^(87)Sr/^(86)Sr值相对偏低。导致这一现象的原因可能是鞍形白云石形成于来自深部的岩浆热液流体之中,这些流体伴随岩浆侵位或通过切穿基底的深大断裂及其与之相连的次级断裂系统从深部直接进入碳酸盐岩地层中,未经过碎屑岩输导层的长时间运移,所以导致其中形成的鞍形白云石^(87)Sr/^(86)Sr值偏低。

Detailed researches of Cambrian-Ordovician dolostones in Tabei and Tazhong areas of Tarim Basin demonstrated widespread presence of saddle dolomite cements,which commonly occur as cements（or infilling）in vugs and/or fractures. These dolomite crystals,generally 500 μm up to 6 mm in size,are creamy white non-planar crystals with curved or lobate or stepped crystal faces. They are gappy and sweeping extinction under microscope,commonly associated with other hydrothermal minerals. The saddle dolomites have δ^（13）C values from -2.446‰ to 0.686‰ PDB,and δ^（18） O values from -9.101‰ to -5.117‰ PDB,and ^（87）Sr/^（86）Sr ratios from 0.708 6 to 0.710 2. Fluid inclusion micro thermometry data of the saddle dolomites cements indicates homogenization temperatures（T_h） of gas-liquid inclusions is about 121～159.5℃ and salinities of dolomitization fluids is about 18.7% ～23.1%（wt% NaCl equivalent）from which the saddle dolomites precipitated. These data suggest that the saddle dolomites could have formed in thermal brine. Compared with saddle dolomites of other basins all over the world,the saddle dolomites of Tarim Basin have similar δ^（13）C and δ^（18） O values,but lower ^（87）Sr/^（86）Sr ratios. The reason could be that the saddle dolomites were formed in magmatic hydrothermal,which was mantle-derived and reached carbonate formation accompanying igneous intrusion or through discordogenic faults without going through siliciclastic sediments. Thus the saddle dolomites formed from the magmatic hydrothermal have lower ^（87）Sr/^（86）Sr ratios than others.