四川盆地震旦系白云岩成因研究

自然界有无原生沉淀白云岩成为白云石成因研究中长期争论的话题.迄今为止,在世界范围内对白云石成因机制尚没有完善的科学答案.有趣的是前寒武纪全球广泛发育白云岩,而寒武纪以来形成的白云石很少.目前很少有文章涉及这未解之谜.本文选择了四川盆地震旦系藻白云岩和非藻白云岩,将宏观全球构造、板块运动观点与微观先进分析方法相结合,对白云石形成的地质构造背景、水介质条件、成因机制进行了深入讨论,提出藻白云岩,微、泥晶白云岩为原生(或准同生)形成,纯细晶白云岩为早期成岩白云石化作用形成,含硅细晶白云岩为埋藏环境白云石化作用形成的认识,并首次应用晶胞参数和结晶有序度判别原生和次生白云石.

Study of Origin of the Sinian Algal and Nonalgal Dolomitites in Sichuan Basin

Algal and nonalgal dolomites develop extensively in Sinian System, Southwest of Sichuan Basin, which arc good natural gas reservoir beds. Some experts have done much basic research on their origin, but they had got dif ferents conclusions. Based on the previous works and the combinations of global and reginal tectonizations with advanced laboratory analyses, the medium conditions and origin of dolomites, have been deeply analysed, it was proposed that Sinian System dolomites consiste of primary dolomites and secondary dolomites. It is the first time that the unit cell parameters and crystalinity have been used for distinguishing primary dolomites from secondary dolomites. The developing of algal dolomites, microlite and micritic dolomites were controlled by the factors such as global tectonic settings, sea level changing, hot palaeoclimate, evaprat tidal flat environment, strong and weak of water dynamics and so on. These dolomites were percipitated in marine tidal enviroment by trapping, cohering, and biochemical processes of blue-green algals. They have lower crystallinity from 0.682 to 0.744 with average of 0.696, higher salinity, higher contents of trace elements (Sr, Ba, Sb, Sn, Cu) , and Mg/Ca ratio gererally lower than 0.5. The a。 and c。 of their unit cells are smaller than those of standard dolomites, its crystall is fine and has a low automorphic degree. Pure fine-crystall dolomites were developed by dolomitization under the environment that salty enviroment have had changed by air-water in early diagenetic stage. They constitute of rhythmic structure, the colour of cathodoluminesence consist of yellowish red and dark red. Fine-crystall quartz contained dolomites with dark red cathodoluminesence colour were fomred in late diagenetic stage of deeply burial enviromnt by dolomitization, residual quartz is wide spreaded in it, crystallinity of this kind of dolomite is higher (0.896-0.992) and its δ~(18)Ois light (-8.64-9.01‰) with the Mg/Ca greater than 0.5, its unit cell a。 is 4.876A and c。 is 15.249A which are similar to those of standard dolomite, and its crystall is croase, possesing a high automorphic degree.