重庆石柱二叠纪栖霞组地球化学特征及其环境意义

在对重庆市石柱县二叠纪栖霞组地层野外露头沉积特征研究的基础上,结合室内薄片观察,根据碳酸盐岩样品的元素含量、比值及相关性,探讨了栖霞组沉积地球化学特征与古环境意义.用于古环境分析的10件栖霞组样品SiO₂、MgO含量低,且两者具有较好的正相关性,Sr含量高,平均为1 751×10-6,Mn/Sr<0.1,Fe/Sr<1,δ18O_PDB>-10‰,表明成岩过程中样品几乎不受硅化及白云岩化作用的影响,未发生化学性质上的改变,可作为古环境分析的可靠对象.栖霞组碳酸盐岩CaO含量高,平均为53.81%,接近纯灰岩的理论化学组成,陆源元素Al₂O₃、TiO₂含量很低,平均值分别为0.17%与0.012%,表明其形成于陆源碎屑影响微弱的稳定古海洋环境.古气候指标MgO/CaO比值极低,平均为0.026,古盐度指标100×(MgO/Al₂O₃)比值高,平均为1 241,古水深判别指标Sr/Ba、1000×(Sr/Ca)比值高,平均值分别为286和4.54,指示了栖霞组碳酸盐岩为潮湿气候下的陆表海(远岸)沉积.古氧相地球化学指标U/Al、V/Al、Mo/Al、Cr/Al、Co/Al、V/(V+Ni)和U/Th特征一致表明栖霞组碳酸盐岩沉积于贫氧—缺氧环境.垂向上,陆源元素∑(Al₂O₃+TiO₂)含量逐渐降低,古水深指标Sr/Ba、1000×(Sr/Ca)比值逐渐增大,表明栖霞组沉积过程中离岸渐远、陆源供给逐步减少、水体持续加深.

Geochemical Characteristics and Its Paleoenvironmental Implication of Permian Qixia Formation in Shizhu,Chongqing

Observations from outcrop, thin sections and geochemical analyses of limestone samples collected from Middle Permian Qixia Formation at Jinlingxiang section in Shizhu, Chongqing City, make it possible to reveal characteristics of geochemistry and paleoenvironment. The 10 samples have low contents in SiO₂ (<5%), MgO(<2.5%), low ratios of Mn/Sr(<0.1), Fe/Sr(<1), high contents in Sr(average: 1 751×10-6), δ18O_PDB(>-10‰), meanwhile, SiO₂ are strong positive correlated with MgO, which imply that samples have not experienced intense dolomitization and silicification implying no alteration on the chemical nature during diagenetic processes. Thus, they could be used as reliable objects to reconstruct paleoenvironment. Samples are usually high in CaO (average:53.81%) content, low in Na₂O(average: 0.051%), Al₂O₃(average: 0.17%), TiO₂(average: 0.012%)contents, which indicate stable marine paleoenvironment barely with terrigenous input. Low ratios of MgO/CaO(average:0.026), high ratios of 100×(MgO/Al₂O₃) (average:1241), Sr/Ba(average:286) integrated with 1 000×(Sr/Ca) (average:4.54) hint that Qixia Formation deposited in continental shelf (far shore) under humid climate. Geochemical indices for palaeoredox conditions, for example, U/Al, V/Al, Mo/Al, Cr/Al, Co/Al, V/(V+Ni) and U/Th, conform that the carbonate successions in the study area were accumulated in a dysaerobic-anaerobic environment. Terrestrial elements Σ(Al₂O₃+TiO₂) contents decrease gradually, seawater depth indices: ratios of Sr/Ba and 1 000×(Sr/Ca) increase steadily from bottom to top, which reflect the coast were further away, terrigenous supply reduced gradually and sea level continued rising during deposition.