早三叠世海水的碳同位素组成是近年来地学研究的热点之一，但全球早三叠世碳同位素组成并没有获得统一的认识。上扬子地区早三叠世晚期普遍发育蒸发岩，地表剖面则因表生成岩作用而形成一套夹盐溶角砾岩及次生灰岩的碳酸盐地层，其地球化学信息不同程度地偏离海水，因而样品成岩蚀变性对古海水碳同位素组成的代表性的评估显得十分重要。本文以中、下三叠统界线清楚的重庆北碚剖面为对象，研究了下三叠统嘉陵江组第三、第四段和中三叠统雷口坡组底部样品的元素组成，碳、氧同位素组成及彼此之间的相互关系，选择 Mn 含量和氧同位素作为条件评价碳酸盐岩的成岩蚀变性及其对海水碳同位素组成的代表性，在此基础上建立了早三叠世晚期海水的碳同位素演化曲线。研究表明：（1）北碚剖面嘉陵江组第三、第四段（嘉三、嘉四段）及雷口坡组底部碳酸盐岩的 Mn、Fe 含量与碳同位素组成之间具有良好的负相关性，氧同位素组成和碳同位素组成之间则具有良好的正相关性，大量样品具有低δ18 O 值、低δ13 C 值以及较高 Mn、Fe 含量的特征，说明其碳同位素组成偏离海水。（2）以δ18 O＞－7．5‰、方解石的 Mn 质量分数＜100×10－6、白云石的 Mn 质量分数＜120×10－6为评价条件，北碚剖面嘉三、嘉四段（含雷口坡组底部）的73个样品中有38个需要在建立海水碳同位素曲线时剔除，占全部样品的52％。其中嘉四段需要剔除的样品占嘉四段样品的78％，而嘉三段的该数值为11％，说明作为原始沉积的灰岩段对海水地球化学信息保存较好，而分布盐溶角砾岩及次生灰岩的层位对海水地球化学信息保存很差。（3）氧同位素作为成岩蚀变评价标准比 Mn 含量更为苛刻，并可基本覆盖 Mn 含量评价条件，说明对于嘉三—嘉四段这样的石灰岩—白云岩—蒸发岩地层的地表剖面的样品来说，选择氧同位素作为评价条件可较好地对碳同位素的原生性进行控制。（4）白云岩对海水地球化学信息总体上保存较好，与相邻石灰岩相比，白云岩具有更高的δ18 O 和δ13 C 值，但其原因仍然需要进一步研究，可能与同位素分馏系数的差别以及白云岩形成于盐度更高的流体有关，因而白云岩所代表的海水地球化学信息可能不完全与伴生石灰岩相同。（5）除绿豆岩附近出现了短时间的变化超过4‰的大幅度波动以外，早三叠世晚期海水的碳同位素演化相对平稳，大致在－0．3‰～－1．8‰的范围内波动，呈现一个低幅的“V”字形，显示早三叠世晚期地球生态的恢复与碳循环系统的重建。%The carbon isotopic composition of seawater during Early Triassic is recently one of the hot topics in the geological research field,but the recognition on the Early Triassic C isotopic composition in the world is not consistent.In the Upper Yangtze region,the upper part of the Lower Triassic sequence is dominated by evaporates.There forms a suit of carbonate rocks strata mixed with secondary limestone and evaporite-solution breccia in the outcrop section during epidiagenesis.The geochemical signatures in the altered rocks deviate from the coeval seawater. Therefore, the evaluation of diagenetic alteration for the representativeness as the C isotopic composition of paleo-seawater is very important.Based on the study of the constitution and the interrelationship of the elements,the carbon and oxygen isotopic compositions of carbonate rocks from the Beibei section of Chongqing,the Mn content and oxygen isotopic composition are selected as the factors to evaluate the diagenetic alteration of the carbonate rocks and the representativeness as primary carbon isotope in order to establish the C isotopic profile of the late stage of Early Triassic.The results are as follow. (1 )There is negative correlation between Mn vs.δ13 C and Fe vs.δ13 C,and positive correlation betweenδ18 O vs.δ13 C in the carbonate rocks in Member 3 & Member 4 of Jialingjiang Formation and the bottom of Leikoupo Formation.This suggests that a number of samples with high content of Mn and Fe and lowδ18 O also have a lowδ13 C value which are obviously deviates from the coeval seawater. (2)Taking the mass fraction of Mn <100 ×10 -6 ~120 ×10 -6 andδ18 O>-7.5‰ as the evaluation factors,there are 38 samples must be winkled out from the total 73 samples with a percent of 52%. Among them,32 samples are winkled out from Member 4 of Jialingjiang Formation with a percent of 78% and 3 samples are winkled out from Member 3 of Jialingjiang Formation with a percent of 11%. This shows that the original seawater geochemical information is preserved better in the primary depositional limestone but worse in the secondary limestone and the evaporite-solution breccia.(3 ) Oxygen isotope as a factor to evaluate the diagenetic alteration is stricter than Mn content, consequently,and the data winkled by oxygen isotope overlap that by Mn content basically.This indicates that taking oxygen isotope as a factor can more rigorously control the primary nature of carbon isotope for the samples from the outcrop section of the limestone-dolomite-evaporate sequence. (4)The preservation of the original seawater geochemical information in dolomite is normally better than that in limestone,since theδ18 O andδ13 C of dolomite are higher than those of limestone.The possible reasons for that include the difference of the isotopic fractionation efficients between dolomite-fluid and calcite-fluid,and/or the difference of seawater salinity between dolomite and limestone formed,which suggests that the dolomite may originate from the seawater with higher salinity than the limestone.(5 )δ13 C trend of the seawater at the end of Early Triassic shows a relatively stable and gradual V-shaped variation with a relatively narrow fluctuation range of -0.3‰~ -1.8‰,except a large δ13 C fluctuation with an amplitude over 4‰ in the samples nearby the mung bean rocks.This indicate an ecosystem recovery and reconstruction.
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