川东天府地区文星场长兴组生物礁剖面碳氧稳定同位素特征及地质意义

Abstract

海相碳酸盐岩碳氧同位素具有显著的古海洋学意义，其组成具有随地质历史时期演变而变化的趋势，沉积环境的变化影响着古海水的碳氧同位素组成的差异。通过碳氧同位素地球化学分析方法，探讨了文星场地区古环境以及碳氧同位素与生物礁之间的关系。分析测试结果表明：样品基本上保持了原始碳、氧同位素组成，且δ (13CPDB)与δ (18OPDB)变化具有明显的周期性，同时因区域不同而呈现不同幅度的波动；此外，礁相δ (13CPDB)与δ (18OPDB)平均值均具高于非礁相δ (18OPDB)和δ (13CPDB)平均值的特点。基于以上对碳氧同位素值的组成特征及与生物礁之间的关系的分析研究认为：① δ (13CPDB)的变化与海平面升降变化呈正比，因而具有很强的周期性，且每一个周期内δ (18OPDB)值也呈现出周期性的波动，表明海水的温度和盐度受海平面变化的影响而出现周期内和周期间的波动；② 文星场地区低洼的古地貌、峨眉运动影响及硅质供源体(热水来源、玄武岩淋滤及上升流等因素)，共同作用造成了礁相δ (13CPDB)与δ (18OPDB)平均值均高于非礁相δ (18OPDB)和δ (13CPDB)平均值。 Carbon and oxygen isotope of marine carbonate rocks had significant meaning of paleoceanography, its composition varied with geological evolution history, but the composition was differed in different depositional environments. By using the method of isotope geochemistry, the paleo- environment of Wenxingchang Area and the relationship between carbon and oxygen isotope and organic reefs were discussed. The results of data analyzing showed that all the samples were marine carbonate and kept the original composition of carbon and oxygen isotope and they were obviously cyclical, the carbon and oxygen isotope was differed in different depositional environments and regions. Therefore the average value of δ (13CPDB) and δ (18OPDB) in organic reef facies was higher than that of δ (18OPDB) and δ (13CPDB). The above analysis indicates that first, the δ (13CPDB) changes with the change of sea level and also presents strong cyclical and the value of δ (18OPDB) also presents cyclical fluctuation, which shows that the temperature and salinity of sea water are affected by the change of sea level so that it appears fluctuations during the cyclical period. Secondly, the low paleogeomorphology of Wenxingchang Area and the Ermei movement and the source body of siliceous including hydrotherm, basalt leaching and upwelling are the reasons that the average value of δ (13CPDB) and δ (18OPDB) in organic reef facies is higher than that of no-reef facies δ (18OPDB) and δ (13CPDB).