川北旺苍地区茅口组地球化学特征及古环境记录

摘要

The palaeo-environment revealed by carbonate rocks of middle Permian Maokou Formation in Wangcang region is discussed on the basis of analysis of the geochemical characteristics.It shows that theδ13 C andδ18 O of 32 samples are stable without any linear relationship.These parameters all meetδ18 O>-6‰,Mn/Sr≤0.6,Fe/Sr<3,MgO/CaO<0.12,and are unlikely influenced by late diagenetic alteration,so they can be used as reliable targets for paleo-environmental analysis.The contents of terrigenous elements,Al2 O3 ,TiO2 ,are low and positively correlated,and the degree of REE differentiation is obviously weak.There is a strong positive correlation between ∑ REE and∑(Al2 O3 +TiO2 ).Vertically,∑ (Al2 O3 + TiO2 )and ∑ REE have same variation tendency,but variation of the ancient water depth index Sr/Ba, 1000×(Sr/Ca) has opposite trend. The characteristic ratios of MgO/CaO,Sr/Ba,1000×(Sr/Ca),U/Al,V/Al,Cr/Al,Co/Al,U/Th and V/(V+Ni)indicate that the terrigenous materials are insufficient during the depositional period and the REE content is mainly influenced by terrigenous supply.Analysis reveals that paleo-oceanic temperature is between 14.98～26.55℃,and the salinity of ancient seawater is in the range of 132.3～135.2.It conforms that the carbonate successions in the study area are deposited in an dysaerobic-anaerobic environment, characteristic of relatively high sea level, small range of water depth variation,and three transgression and regression cycles.The trace element evolution curves and the corresponding lithofacies assemblage can be used as the basis of stratigraphic division.%通过对川北旺苍地区鹿渡坝剖面中二叠统茅口组碳酸盐岩地球化学特征的研究,探讨古沉积环境意义.32件样品δ13 C和δ18 O值整体稳定,两者不存在线性关系,均满足δ18 O>-6‰、w Mn/wSr≤0.6、wFe/wSr<3、w MgO/wCaO<0.125,不受后期成岩蚀变的影响,可作为古环境分析的对象.陆源的Al2 O3、TiO2含量低且强烈正相关,稀土元素分异程度整体较弱,∑REE与∑(Al2 O3+TiO2)有较强烈的正相关,纵向上,∑(Al2 O3+TiO2)和∑REE的变化趋势相同,而与古水深指标wSr/wBa、1000×(wSr/wCa)值的变化趋势相反.结合wSr/wBa、1000×(wSr/wCa)、wU/wAl、wV/wAl、wCr/wAl、wCo/wAl、wU/wTh及wV/wV+Ni比值特征,认为茅口组沉积期间陆源物质供给很少,稀土元素含量主要受陆源供给的影响;古海洋温度为14.98～26.55℃,古海水盐度(Z值)为132.3～135.2;茅口组沉积于陆表海缺氧-贫氧环境,海平面相对较高,水深在小范围内呈震荡式变化,经历了3次海侵-海退旋回,发育对应的岩相组合,其痕量元素演化曲线可作为地层划分的依据.