华南老堡组硅质岩中草莓状黄铁矿--埃迪卡拉纪末期深海缺氧的证据

Framboidal pyrites in cherts of the Laobao Formation, South China: Evidence for anoxic deep ocean in the terminal Ediacaran.

为恢复埃迪卡拉纪末期深海的氧化还原状态,我们对华南桂北泗里口剖面老堡组(大约550～540Ma)硅质岩中草莓状黄铁矿的粒径和分布进行了测量、统计和研究。老堡组硅质岩样品中普遍存在着草莓状黄铁矿,呈星散状分布,未见自形晶的黄铁矿颗粒和明显后期充填的草莓状黄铁矿集合体。硅质岩中草莓状黄铁矿颗粒的统计表明,它们具有很窄的变化范围,最大粒径小于18μm,大多数样品中草莓状黄铁矿的平均粒径和中间粒径均小于5μm。根据这些硅质岩中原生黄铁矿的粒径和分布,我们判定埃迪卡拉纪末期的深部海水是缺氧的。泗里口剖面老堡组硅质岩中草莓状黄铁矿的平均粒径、中间粒径和最大直径沿剖面向上逐渐增加,这意味着埃迪卡拉纪末期华南的深部海水有逐步被氧化的趋势。

A detailed study of the size distribution of framboidal pyrites in cherts from the Laobao Formation (about 550 similar to 540Ma) at Silikou section in South China was conducted to evaluate the redox state of the terminal Ediacaran deep ocean. Framboidal pyrites are common in the cherts, but infilled framboids and euhedral grains are sparse. Framboids are dispersed in the cherts and show a narrow size range with maximum diameters range from 7.7 to 18 mu m. The mean values of framboidal pyrites diameters in Silikou cherts are less than 5 mu m for the lower seven samples and around 6.5 mu m for the upper three samples. In these chert samples, more than 85% of framboids of the lower seven samples are less than 7 mu m, and around 65% of framboidal pyrites are less than 7 mu m for the upper three samples. In addition, few framboids (fewer than 4%) in most of the chert samples arc > 10 mu m. The small diameters and narrow size range of framboidal pyrites in Silikou cherts are consistent with that formed in euxinic sediments and imply that the terminal Ediacaran deep ocean was mainly anoxic. It is clear that mean values, median values and maximum diameter of framboidal pyrites in Silikou cherts are gradually increased from the lower to the upper section, which may indicate that the terminal Ediacaran deep ocean was gradually oxidized.