川西北剑阁地区栖霞组地球化学特征及古环境意义

Geochemical characteristics and palaeoenvironmental significance of Qixia Formation in Jiange area,Sichuan,China

研究川西北剑阁地区马儿岩剖面中二叠统栖霞组碳酸盐岩的地球化学特征,探讨其古沉积环境意义,为川西北地区中二叠世古环境演化的研究提供信息。通过野外剖面实测和室内薄片鉴定,结合主元素、痕量元素测试分析等方法对马儿岩剖面栖霞组沉积时期的沉积环境进行讨论。陆源Al2O3和TiO2含量较低,两者具强烈正相关;ΣREE与陆源Σ(Al2O3+TiO2)具强烈正相关,指示栖霞组沉积水体较为稳定且不受陆源碎屑影响;古水深指标Sr/Ba、1000×(Sr/Ca)的平均值分别为6.89、8.40,指示栖霞组为离岸较近的海相沉积环境;古气候指标MgO/CaO平均值为0.06,指示气候环境较为潮湿;古氧相指标V/(V+Ni)为0.35~0.79(平均值为0.52),V/Cr平均值为0.95,Ni/Co比值为5.51~7.42(平均值为6.47),U/Th比值为0.33~2.57(平均值为0.93),指示整体处于贫氧-富氧沉积环境;平均古水温为28.2℃。研究区栖霞组碳酸盐岩沉积于温暖潮湿气候条件下的贫氧-富氧环境,从栖霞组到茅口组水体呈明显上升趋势,古水温、盐度和含氧量呈下降趋势。

Geochemical characteristics of middle Permian Qixia Formation carbonate rocks in the Maeryan section of Jiange area,northwest Sichuan is studied by means of field cross section observation and thin section identification,combined with the determination and analysis of major elements and trace elements so as to discuss the palaeosedimentary environment and provide information for the evolution of palaeoenvironment of middle Permian in the northwest Sichuan.Geochemical analysis reveals that the content of terrestrial elements Al2O3 and TiO2 is low with a strong positive correlation between them.Strong positive correlation of REE and terrestrial element(Al2O3+TiO2)indicates that the sedimentary water body of Qixia Epoch is relatively stable and not affected by terrigenous detrital.The average values of paleo-water depth indicators Sr/Ba and 1000×(Sr/Ca)are 6.89 and 8.40,respectively,representing a marine sedimentary environment near the coast,while the paleoclimate indices Mg/Ca and MgO/CaO are 0.048 and 0.06 respectively,indicating a humid climate environment during the middle Permian.Paleooxygen indices V/(V+Ni)is in the range of 0.35~0.79,with an average value of 0.52,V/Cr is 0.95,Ni/Co ratio ranges from 5.51 to 7.42,with an average of 6.47,and the U/Th ratio ranges from 0.33 to 2.57(average value 0.93).These geometrical analytical values show that,during the middle Permian,the northwest Sichuan area is an oxygen-poor and oxygen-rich depositional environment with an average paleo-water temperature of 28.2℃.The carbonate rocks of Qixia Formation in the study area are deposited in an oxygen-rich environment with a warm and humid climate.The water bodies show a significant upward trend,while the paleo-water temperature,salinity and oxygen content show a downward trend from Qixia Epoch to Maokou Epoch.