川东北地区长兴组-飞仙关组碳酸盐岩同位素地球化学响应及其地质意义

碳、氧同位素是碳酸盐岩研究的重要地化指标之一,在反映全球气候变化、海平面升降变化、成岩作用影响及进行地层对比等方面发挥着重要作用。基于川东北地区野外剖面考察、岩石薄片鉴定和微量元素分析,结合长兴组-飞仙关组241件碳酸盐岩样品的碳、氧同位素组成,建立同位素演化曲线,重点对碳、氧同位素地球化学响应特征进行区域性对比分析,进一步对其地质意义进行试验性探讨。研究结果表明:川东北地区长兴期碳同位素演化曲线与海平面变化呈明显的正相关关系,并可与层序内部体系域进行良好对应,该时期氧同位素组成呈相对稳定趋势;在二叠系/三叠系界线附近碳、氧同位素值均发生强烈负漂移,达到最低值;进入飞仙关期后碳同位素值进入低幅波动的恢复阶段,在飞四段内部达到极大值后降低,氧同位素组成在飞仙关期也较为稳定。碳、氧同位素值在二叠系/三叠系界线处发生强烈负漂移可能与甲烷释放及火山喷发事件有密切关系。

Isotope geochemical responses and their geological significance of Changxing-Feixianguan Formation carbonates,northeastern Sichuan Basin

The carbon and oxygen isotopes are important geochemical indexes for the study of carbonate rocks and have been widely used in revealing global climate changes and sea level fluctuation as well as diagenesis and stratigraphic correlations. This study starts with outcrop observation, thin-section identification and trace element analysis, continues with establishing isotope evolution curves with emphasis on the regional comparative analysis of the geochemical response characteristics of carbon and oxygen isotopes based on analyses of carbon and oxygen isotope compositions of 241 carbonate samples from the Changxing-Feixianguan Formations in northeastern Sichuan Basin, and ends with a discussion on the geological significance of these characteristics. The results show that the carbon isotope evolution curves positively correlate with sea level changes in the Changxing period and correspond well with the system tracts in the sequence. The oxygen isotope composition shows a relatively stable trend during this period. Strong negative drift of carbon and oxygen isotope values occurs near the Permian/Triassic boundary while reaching their lowest values. After entering the Feixianguan period, the carbon isotope value keeps rising with a low-amplitude fluctuation until reaching the maximum value inside the fourth member of Feixianguan Formation. The oxygen isotope composition is stable during the Feixianguan period. It is also suggested that the strong negative drift of carbon and oxygen isotopes at the Permian/Triassic boundary may be closely related to methane release and volcanic eruption events.