新疆博格达山西南麓石炭—二叠纪碳酸盐岩的碳氧同位素与古环境研究

Study of the carbon and oxygen isotopes and paleoenvironment of the Carboniferous and Permian carbonate rocks at the southwestern foot of the Bogoda Mountain, Xinjiang

新疆乌鲁木齐西南自柳树沟—祁家沟—塔什库拉沟—井井子沟—郭家沟出露厚达80000m的石炭—二叠纪沉积,早二叠世以前大都为海相,其后为陆相,很多学者进行详细的工作,在沉积方面存在以下争议。一种是中石炭世—早二叠世该地区为一套海退相序的沉积,即由浅海—潮坪相或海陆过渡相—三角洲相。

No previous reports have been read on the carbon and oxygen isotopes and palaeoenvi-ronment at the southwestern foot of the Bogoda Mountain. Based on the analysis of δ13C (PDB) and δ18O (PDB) of carbonate rocks of different periods and types, the present paper reveals that these indices are lower in fresh water limestone of Late Permian age,-4.2‰ and -13.7‰respectively, while the δ18O of contemporous dolostone is 1.5‰ higher than that of limestone, however, the δ13C is as high as 8.6‰, reflecting the lagoon environment that formed by salinization of fresh water. From Middle Carboniferous to Early Permian, δ13C ranges from 0.1 to 3.9‰and δ18O - 7.3 to -13.6‰, apparently showing the features of shallow sea. The change in marine salinity and depth can be reflected by the corresponding isotopic values (Table 1), as a whole, the study area exhibited a gradually regressive process developed on the basis of stable shallow sea. The δ13C value averages between 1.7 and 1.8‰, but the Permian marineδ18O averages -11.3‰ which is 0.9‰ lower than that of Middle Carboniferous age, suggesting the influence of fresh water. Moreover, the author considers that the δ18O of ancient carboniate rocks is mainly controlled by palaeoenvironment rather than the isotopic exchange in diagenetic process. Thus, it still can be used to calculate palaeosalinity and analyse palaeoenvironment (Table 2). A great deal of statistical data proved that for the division of marine and terristrial facies, the calculation proposed by Jean and Webe (1964) by using δ13C and δ18O values of limestone is suitable for the Meso-zoic younger rocks when the Z value is selected to be 120, but as for Palaeozoic rocks, this value seems to be lower, by the combination of diagenetic influence, the author suggests that taking 123 as Z value would be proper for the division of marine and terristrial facies of this period.