浙江江山石炭-二叠系碳酸盐岩碳氧同位素特征研究

Carbon and oxygen isotopic characteristics of carbonate rocks of Carboniferous-Permian in Jiangshan,Zhejiang Province

对浙江江山石头山剖面船山组、栖霞组碳酸盐岩进行了C、O同位素测定。Mn/Sr比值、δ18O值和δ18O-δ13C散点图检验表明,所得同位素数据反映了碳酸盐岩的原始沉积特征。δ13C和δ18O值变化范围分别为-5.4‰～4.4‰和-12.2‰～-5.6‰,平均值分别为0.5‰和-8.4‰。在石炭纪-二叠纪分界线附近没有发生明显的同位素漂移现象。本区碳同位素地层曲线与根据沉积微相推断的海平面变化曲线十分吻合,表现为δ13C高值区海水变深。海水在剖面上的变化规律是浅—中等—较深—中等,相应的δ13C演化曲线表现为波谷段—具次级起伏的相对平缓段—波峰段—平缓段。栖霞组台坪相δ13C值大于船山组台坪相,而且曲线较平直,反映海水较深且环境较稳定。研究区地层记录中的海平面从早到晚逐渐增高,与全球海平面变化曲线不一致,可能与浙皖海盆发生过整体沉降有关。这一区域性差异也在碳同位素记录中得到印证,本区晚石炭世晚期—早二叠世早期δ13C值从老到新慢慢增大,最后再略为下降,演化趋势与Veizeretal.的碳同位素年代演化曲线相反,说明碳同位素演化在一定程度上能反映区域地壳演变状况。

The studied carbonate rocks were collected from Chuanshan Formation and Qixia Formation at the Shitoushan section in Jiangshan, Zhejiang Province. T he results of carbon and oxygen isotopic compositions are credible, because of a ll the samples with Mn/Sr < 10, most of the samples with ?18O >-10‰and no cor relation between ?18O and ?13C. Variation range and average of ?13C and ?18O are-5.4‰～4.4‰,-12.2‰～-5.6‰and 0.5‰,-8.4‰, respectively. No obvio us shift of ?13C and ?18O is found near the borderline between Carboniferous a nd Permian. The carbon isotope curve of the strata coincides with the variation curve of sea level inferred from sedimentary facies. The deepening of sea water conforms to the peak of ?13C value. Variation rule of sea water in the section is from shallow, middle, deep to middle, accordingly the evolution curve of ?1 3C is from trough, relatively flat with sub fluctuation, wave peak to flat. ?13 C of platform facies in Qixia Formation is higher than that in Chuanshan Formati on, moreover, the curve is more flat in Qixia Formation than in Chuanshan Format ion, which is an indication of deepening of sea water and stable environment. Th e sea level evolution curve inferred from strata record in the studied area rise s gradually and doesnt coincide with the global change curve of sea level, whi ch is probably the result of settlement of Zhejiang Anhui sea basin. The phenom enon is proved by record of carbon isotope. Value of ?13C rose gradually, and t hen decreased slightly in the last from the late of Late Carboniferous to the ea rly of Early Permian. The evolution curve of carbon isotopes doesnt coincide w ith that from Veizer et al. (1986). Therefore, the evolution curve can reflect t he evolution of regional crust in some degree.