塔里木盆地北部上震旦统葡萄状白云岩的发现及成因探讨*

“皮壳—葡萄状”白云岩是一种非常特殊结构的白云岩。文中报道了塔里木盆地西北缘东二沟及塔北星火101井上震旦统奇格布拉克组中皮壳—葡萄状白云岩特征,并对其成因进行了探讨。皮壳状—葡萄状白云石中发育明、暗纤状白云石或纹层条带的含少量细晶的纤状—细粒状白云石以及肾状和葡萄状粉细晶白云石;由球形、杆形、孢子形等蓝细菌构成泡沫状结构;明暗、环带状橙红色光或中等橙红色光、核部发光较暗或不发光;与基质白云石相比,葡萄—皮壳状白云石δ13CPDB相似;但δ18OPDB正偏, ⁸⁷Sr/⁸⁶Sr 相对高(0.70887~0.70939)、但与震旦纪海水(0.7087~0.7094)相似,从皮壳边缘、暗色(蓝细菌)至核部,微区δ18OPDB、δ13CPDB显示了环带内变化较小、环带外的强烈负漂移;云化程度的增加,δ18OPDB、δ13CPDB负偏明显, ⁸⁷Sr/⁸⁶Sr 增加。皮壳—葡萄状白云岩中Al₂O₃ 、Fe₂O₃ 、MnO、LREE/HREE值均低于基质泥粉晶云岩对应值、稀土总量介于基质与粉细晶云岩之间,而Na₂O+K₂O、P₂O₅、 Sr、Hg、Cu、Sr/Ba、Sr/Mn、δCe值均高于基质泥粉晶云岩对应值;且随着云化程度的提高,总体呈现出Mn含量增加,Al₂O₃ 、Fe₂O₃ 、Na₂O+K₂O、P₂O₅、Sr/Ba、Fe/Mn等值递减的趋势;由此判断皮壳状—葡萄状白云石可能是弱还原、保存较好的海水中形成,或在成岩早期或浅埋藏孔隙海水为主的流体中形成、部分经历了较强的大气水作用改造。

Discovery and discussion on origin of botryoidal dolostone in the Upper Sinian in North Tarim Basin

The botryoidal dolostone of a unusual textures features in Qigebulake Formation,in the Upper Sinian developed at Dong'ergou section and Well XH 101 of northern Tarim Basin,has been firstly reported. Constraining its origin based on lithology and geochemical data,the shell-comb or botryoidal dolostone,which consists of shrub fibrous cement,bladed radial as well as granular dolomite crystal with alternately dark and luminescent and zonation,the average of botryoidal dolostone of δ13CPDB is 2.50‰(1.80‰~3.16‰),δ18OPDB is-3.30‰(-6.40‰~0.86‰), ⁸⁷Sr/⁸⁶Sr is 0.7091(0.7089~0.7096),compared with the values of matrix dolostone. The botryoidal dolostone have rather high positive of δ18OPDB and ⁸⁷Sr/⁸⁶Sr(within the change values of coeval normal sea-water: 0.7087~0.7094)with a similar δ13CPDB. The δ¹³C and δ¹⁸O of zonation varies from the dark rimming to the luminescent centre varies slightly decreasing or increasing,while the strong negative deplete of δ¹³C and δ¹⁸O in outer of zonation,shows the tendency of negative deplete of δ¹³C and δ¹⁸O and increasing of ⁸⁷Sr/⁸⁶Sr and Mn,decresing of the contents of Al₂O₃ 、Fe₂O₃、Na₂O+K₂O、P₂O₅、Sr/Ba、Fe/Mn. Coupled with a higher dolomitization. The contents of Al₂O₃、Fe₂O₃ 、MnO are relatively lower,and Na₂O+K₂O,P₂O₅,Sr,Hg,Cu,and the ratios of Sr/Ba,Sr/Mn are higher in comparison with the values of matrix dolostone. The contents of REE(10.21×10^-6~15.75×10^-6)with the “Left-leaning”distribution pattern of rare earth elements with LREE depletion,heavy REE enrichment,strong negative CE anomaly,indicated that the botryoidal dolostone was formed either in a well-preserved,weak reduction and oversaturated sea water environment with an increasing of pH value and free oxygen contents in sea waters. Or in a residual sea water of pore fluids in the early stage of diagenesis or shallow buried environment,later partly superimposed by a cyclic subaerial-exposed fresh water.