兴蒙地区晚二叠世林西组灰岩微量元素与碳、氧同位素特征及沉积环境讨论

兴蒙地区晚二叠世林西组沉积环境争议较大,多数学者认为其为陆相沉积,少数学者则认为早中期为封闭海盆,晚期转化为陆相湖盆。在对林西官地林西组剖面实测时,于其上部发现大量灰岩透镜体,内含钙藻和苔藓虫等海相化石。对系统采集的灰岩透镜体进行微量元素及碳、氧同位素组成分析,结果显示灰岩中轻稀土略亏损,Eu正异常明显,La、Ce具有轻微的正异常,Gd、Y/Ho基本无异常;Si O2含量较高、且Zr与REE具有良好正相关性,表明其沉积环境为海相近岸带,水体可能有正常海水、河水或大气降水和热液三个来源。剖面沉积特征、化石组合及灰岩中碳、氧同位素组成也为近岸带环境提供了进一步支持。这说明晚二叠世兴蒙—吉林一带仍存在狭长海盆,海盆内部仍存在热液活动,在近岸地区,海水循环可能受阻,使得短期内以蒸发作用或河水注入占主要地位,造成碳同位素大幅漂移,而氧同位素漂移主要由成岩作用引起。

The Late Permian Sedimentary Environments of Linxi Formation in Xingmeng Area: Constraints from Carbon and Oxygen Isotopes and Trace Elements

The Late Permian sedimentary environment in Xingmeng area has long been a topic of controversy. Most researchers believe it is continental deposit, while a few people hold that it had been a closed marine basin at the early stage, and then changed into a lacustrine basin. In order to settle down this dispute, the authors measured the section of Guandi in Linxi area and discovered large quantities of limestone lenses containing calcium algae, bryozoans and some other marine fossils. Limestone lenses were collected systemically, and their elements and carbon and oxygen isotopic compositions were analyzed. The results show that the limestone lenses are characterized by depletion of LREE relative to MREE and HREE, obvious positive Eu anomalies, minor positive La, Ce anomalies and lack of Gd and Y/Ho anomalies. SiO2 values are high, and Zr and REE have obvious positive correlation, suggesting a near-coastal deposition environment. The depositional water was derived from three sources, i.e., normal seawater, river or atmospheric precipitation and hydrothermal solution. The sedimentary sequence, the assemblages of fossils in the section and the composition of carbon and oxygen isotope in limestone also provide further support for the near-shore environment. In Late Permian, there was still a narrow marine basin with hydrothermal activity in the Xingmeng-Jilin area. The sea water circulation was probably blocked in the coastal area, which caused the domination of evaporation or river discharge in a short period, thus resulting in a substantial shift of carbon isotopes. However, the oxygen isotope excursion was caused mainly by diagenesis.