塔中—巴楚地区下奥陶统云灰岩地球化学与成因探讨

Geochemistry and Genesis Research on Lower Ordovician LimestonesDolostones Transitional Rocks in Tazhong-Bachu Uplift,Tarim Basin

塔里木盆地下奥陶统分布着规模巨大的白云岩地层，在这些白云岩地层中已经发现了工业油气藏，勘探潜力巨大。应用岩石学与地球化学方法，对塔中隆起一巴楚隆起内下奥陶统的云灰岩开展成因研究。根据岩石学特征，下奥陶统云灰岩分为低云化程度灰岩、高云化程度白云岩、与中云化程度云灰岩3个端元。地球化学分析结果显示，白云岩与云灰岩δ18O值均比灰岩重约3．5‰，3个端元具有相似的稀土元素配分模式与相似的87Sr／88Sr同位素组成，表明云化流体主要来源于海水或蒸发海水。云灰岩与白云岩端元表现出富Fe、贫u和Mo、负ce异常特征，指示其形成于水体偏浅的氧化环境中；相反，灰岩端元表现出形成水体偏深的还原环境中。利用白云岩一海水与灰岩一海水的氧同位素温度计估算岩石形成温度的结果显示，白云岩形成温度高于灰岩，表明温度可能是克服白云岩化动力学障碍的关键性因素。根据流体源、氧化还原条件、形成温度与水体深度的认识，建立了“层状水温控云灰互层模式”；并认为白云岩化主要发生在水体较浅（0～70m）、温度相对较高（夏季达30℃）的环境中，对应沉积相主要包括半局限台地潮坪亚相与潮下浅滩亚相；灰岩沉积水体相对较深，温度相对较低（～10℃），对应沉积相主要为半局限台地潮下滩或滩间海亚相。

In the Lower Ordovician of Tarim Basin,there is large amount of bedded dolostones,limestones and transitional rocks, in which the industrial hydrocarbon reserves have been discovered with great explo- ration potential. Via petrologic and geochemical methods,the transitional rocks occurring in Lower Ordovi- cian were studied in this paper. Based on the petrologic characteristics, the limestone-dolostone transitional rocks were divided into limestone,transitional rocks and dolostones,representing low,middle and high do- lomitization parts respectively. Geochemical results show that the oxygen isotope values（31s O）in dolostones and transitional rocks are about 3. 5%0 higher than that in limestones, and the three parts share similar PAAS normalized REE patterns and similar 87 Sr/SG Sr ratios of paleo-seawater,indicating that the dolomiti- zation fluids come from seawater or evaporated seawater. The transitional rocks and dolostones demon- strate Fe-enriched, U and Mo-depleted, and negative Ce anomaly characteristics, indicating of shallow water and oxidized environments; whereas limestones might have been formed in relatively deep water and re- duced environment. Moreover, based on oxygen isotope-temperature meter, the formation temperature of dolostones and transitional rocks is higher than that of limestones, suggesting that it is the temperature that controls the dolomitization. Based on the discussion about fluid source, redox environments, tempera- ture and water depth,the ＂interbedded dolostones and limestones controlled by bedded seawater tempera- ture model＂ is proposed. The model demonstrates that dolomitization occurs in shallow water（0-70m）with relatively high temperature（-30~C）environment, such as tidal or shallow shoal facies, whereas limestones commonly occur in relatively deep water with low temperature environment, corresponding to deep shoal facies.