早寒武纪小壳化石的地球化学研究

A Geochemical Study of Anabarites Fossils from Lower Cambrian Strata in Yangtze Platform of South China and Their Palaeoceangraphic Implications

寒武纪是生命的开端，化石则是生命开始的直接证据，因寒武纪地层对揭示生命的成因具有重大意义，多年来寒武纪地层都是众多学科领域关注的热点。对寒武纪的研究，在古生物学方面，已经取得了很好的进展；但是对于这些生物所赖以生存的环境的研究还仅停留在定性阶段，仅限于利用环境指标反应环境的变化。而且，经过成岩作用过程，沉积岩可能已经改变了与海洋所达成的化学平衡。

Anabarites fossils belong to Small Shelly Fossils （SSF） and occur widely in Lower Cambrian strata of Yangtze Platform, South China. They are phosphate shells in composition and represent the earliest stages of the Cambrian bioradiation of Bilateria, the so-called “Cambrian Explosion”. In this study, the authors attempted to separate Anabarites fossils from the Lower Cambrian dolostones by 1 N H3PO4 and obtain both the SSF and granular phosphates （GP）. Geochemical analyses were performed on samples of SSF, GP, and matrix dolostone （DH-23）. The results show that the oxygen isotope compositions of phosphate of SSF and GP are identical within the error （δ^18Ophos= 16.8 ±0.2 per mil）. The calculated temperatures for paleo-seawater based on the data and equation of Longinelli and Nuti （1973） are 28.4-29.4℃. δ^30Si values of quartz in SSF show negative values of - 0.6 per mil, which are consistent with the data of siliceous rocks and cherts of submarine hydrothermal origin, probably indicating the existence of late hydrothermal replacement in the SSF-hearing sedimentary rocks. In addition, trace elements data are also obtained. Mn/Sr ratios increase in order of SSF-2〈 GP〈 SSF-1 〈 〈DH-23, in which SSF-2 represents the phosphate parts of SSF and SSF-1 is the carbonate parts of SSF. These data also suggest that the phosphate parts in SSF have received the least change or no change in diagenesis, and therefore can reliably record the paleo-seawater environments during the important interval.