摘要： 以详细的岩石学研究为基础,综合利用碳、氧、锶同位素等地球化学资料,深入分析了塔里木盆地中央隆起区上寒武统—下奥陶统白云石化流体演化规律以及白云岩成因机制。结果表明,上寒武统白云岩主要由泥晶—粉晶白云岩、微生物白云岩和(残余)颗粒白云岩等原始结构保留较好的白云岩构成,其C、Sr同位素与同期海水相近,O同位素值偏正,属于同生/准同生期与轻微蒸发海水有关的白云石化的产物;下奥陶统白云岩以细晶自形—半自形白云石为主,原始结构保留差,其C、Sr同位素与同期海水近似,但O同位素值略微偏负,主要为浅埋藏期白云石化的产物。部分早期白云岩在中—深埋藏过程中受埋藏重结晶和构造—热液白云石化的影响,形成细晶—粗晶他形白云岩和缝洞鞍形白云石充填物,该阶段白云石化流体主要来自于地层内封存的海源流体、深部热液以及蒸发岩层间热卤水,多期多源流体的共同作用导致该类白云岩具有较宽的Sr同位素组成和明显负偏的O同位素值。总体上,研究区白云岩具有早期形成(近地表到浅埋藏期大规模交代)、中期加强(中—深埋藏期部分重结晶)、晚期改造(热液局部调整)的整体演化趋势。

关键词: 塔里木盆地, 上寒武统—, 下奥陶统, 白云岩, 地球化学, 白云石化流体

Abstract: Based on the detailed petrology study as well as isotope data (δ¹³C,δ¹⁸O and ⁸⁷Sr/⁸⁶Sr),the evolution of dolomitizing fluids and genetic mechanism of the Upper Cambrian-Lower Ordovician dolostones are investigated in the Central Uplift,Tarim Basin.The results indicate that:The Upper Cambrian dolostones are composed of micritic dolostones,microbial dolostones and(relict)grainy dolostones with well-preserved precursor lithologic fabric. These dolostones have similar δ¹³C values and ⁸⁷Sr/⁸⁶Sr ratios to the coeval seawater and slightly higher δ¹⁸O values,suggesting that the dolomitization occurred during the penecontemporaneous period and the dolomitizing fluid was associated with the slightly evaporitic seawater. The Lower Ordovician dolostones are mainly comprised of fine-crystalline,planar-euhedral to subhedral dolomites. Poor-preserved precursor texture,similar δ¹³C values and ⁸⁷Sr/⁸⁶Sr ratios to the coeval seawater as well as slightly depleted δ¹⁸O values of the dolostone indicated the dolomitization occurred during shallow burial. Some of the dolostones formed in early stage could be altered to fine-coarse crystalline,nonplanar-anhedral dolostones and rift-cavity-filling saddle dolostone cements through recrystallization and tectonically controlled hydrothermal dolomitization during medium to deep burial. The dolomitizing fluids of the burial dolomitization were derived from remnant seawater preserved in the formations,deep mantle hydrothermal fluids and hot brine water in the underlying Middle Cambrian evaporate rocks. These dolostones formed during burial have wide range of ⁸⁷Sr/⁸⁶Sr ratios and obviously more negative δ¹⁸O values due to the combined impact of multi-stage and multi-source diagenetic fluids. Generally,these dolostones in the study area formed in multiple stages of dolomitization with the tendency of forming in early period(large scale replacement during near-surface and shallow burial),being mature in middle diagenetic stage (partly recrystallization during medium-deep burial)and modified in late diagenetic stage(partial adjustment of hydrothermal fluid).

Key words: Tarim Basin, Upper Cambrian-Lower Ordovician, dolostone, geochemistry, dolomitizing fluids

中图分类号: 

• P588.24+5