ABSTRACT

The Mesoproterozoic Jixian System (1.6–1.4 Ga) in the Luonan area, southern margin of the North China Craton (NCC), is characterized by complete development, continuous sedimentation and a low degree of structural deformation, resulting in well-preserved palaeontological and paleoenvironmental records. Carbon, oxygen and strontium isotopes of 382 dolostone samples from the Jixian System in Xiong’er Rift, southern margin of the NCC are analysed for the restoration of paleoenvironment and stratigraphic correlation. The results show that the δ¹³C values of these samples mainly concentrate between −1.8‰ and 2.0‰, and the values can be divided into three segments. The δ¹⁸O values are mainly between −6.5‰ and −3.5‰, with frequent fluctuations and weak regularity. The ⁸⁷Sr/⁸⁶Sr ratio ranges from 0.7051 to 0.7139, with an average value of 0.7085. Its fluctuations have certain regularity and are negatively correlated with the carbon isotopes, indicating that the sea-level changes during the Jixian period in the southern margin of the NCC. The carbon isotope curve of the samples from the study section is very similar to those of the Xiong’er Rift and the Yanliao Rift of the NCC. The −3.2‰ δ¹³C negative excursion in the middle and upper parts of the Xunjiansi Formation corresponds to the δ¹³C negative excursion from the top of the Wumishan Formation to the bottom of the Tieling Formation in the Yanliao Rift. Combined with previous research data, the Jixian System that deposited in the Xiong’er Rift and the Yanliao Rift can be approximately correlated with each other. In addition, the carbon isotope data obtained from this study is comparable to global carbon isotope values during the same period. The rise and oscillation of carbon and strontium isotope values during this period may be a response to the break-up of the Colombian supercontinent.

摘要

华北克拉通（NCC）南缘洛南地区中元古代集贤系统（1.6~1.4 Ga）发育完整、沉积连续、构造变形程度低，古生物和古环境保存完好。记录。分析了华北克拉通南缘熊尔裂谷集贤系统382个白云岩样品的碳、氧和锶同位素，以恢复古环境和地层对比。结果表明，这些样品的δ ¹³ C值主要集中在-1.8‰～2.0‰之间，可分为三段。δ ¹⁸ O值主要在-6.5‰～-3.5‰之间，波动频繁，规律性弱。在⁸⁷Sr/ ⁸⁶ Sr 比值范围为 0.7051 至 0.7139，平均值为 0.7085。其波动具有一定规律性，且与碳同位素呈负相关，表明华北克拉通南缘蓟县时期海平面发生变化。研究剖面样品的碳同位素曲线与华北克拉通的熊耳裂谷和燕辽裂谷的碳同位素曲线非常相似。寻间寺组中上部-3.2‰δ ¹³ C负偏移对应δ ¹³燕辽裂谷乌米山组顶部至铁岭组底部的C负偏移。结合前人的研究资料，熊耳裂谷与燕辽裂谷沉积的蓟县系统可以大致相关。此外，本研究获得的碳同位素数据与同期全球碳同位素值相当。这一时期碳和锶同位素值的上升和振荡可能是对哥伦比亚超大陆分裂的反应。