Abstract:

Two types of Liuchapo Formation cherts, bedded cherts and mounded cherts, were deposited in Bahuang section of Tongren in Guizhou Province and Ganziping section of Zhangjiajie in Hunan Province. The origin of the cherts was discussed by analyzing the characteristics of major, trace elements and REE compositions and Ge/Si ratios. It was revealed that the content of SiO₂ in bedded cherts was range from 96.06% to 99.61%, and the content of SiO₂ in mounded cherts was range from 98.62% to 99.56%, averaging 99.13%, and the contents of other chemical components were very low. Thus, two types of cherts are pure cherts. Moreover, the ΣREE of the bedded cherts in Bahuang section is ranged from 20.14 to 248.56 μg/g (averaging 100.62 μg/g), and no obvious abnormality in Eu/Eu* which is ranged from 0.90 to 1.10 (averaging 1.06), and the ratio of Ge/Si is 0.13−0.98 μmol/mol (averaging is 0.50 μmol/mol), indicating that the origin of bedded cherts affected terrigenous input. Whereas, the ΣREE of the mounded cherts in Ganziping district is ranged from 3.75 to 7.24 μg/g (averaging 5.73 μg/g), the ratio of Ce/Ce* is 0.46−0.66 (averaging 0.57) which shows a negative abnormal, Eu/Eu* is ranged from 2.28 to 11.07 (averaging 4.60) which shows a positive abnormal, and Ge/Si ratio is 1.09−1.43 μmol/mol, (averaging 1.25 μmol/mol) all of which reflect a hydrothermal origin of the mounded cherts. Otherwise, the relationship between Al₂O₃ and ΣREE in the bedded cherts has an excellent correlation, while in the mounded cherts has a poor correlation. It also illustrates that continental substance plays an important role in controlling the formation of bedded cherts, and the mounded cherts is originated from submarine volcanic or hydrothermal activity. The relationship between Al₂O₃ and Ge/Si also reflect the origin of cherts. Combined with the paleogeographic environment, it can conclude that the mounded cherts were deposited in syngenetic fault at the margin of the basin, originated from submarine hydrothermal activity, while the bedded cherts were deposit in the deep basin, mainly influenced by terrigenous input. What’s more, using 1 μmol/mol as the critical value of Ge/Si may provide a new way to trace the material sources of cherts.

Key words: Liuchapo Formation, bedded cherts, mounded cherts, Ge/Si, hydrothermal

摘要：

对埃迪卡拉纪?寒武纪之交的贵州铜仁坝黄剖面留茶坡组层状硅质岩和湖南张家界柑子坪剖面留茶坡组穹隆状硅质岩的主量、微量元素和稀土元素以及Ge/Si特征进行分析, 探讨两种硅质岩的成因。层状硅质岩的SiO₂含量为96.06%~99.61%, 穹窿状硅质岩的SiO₂含量为98.62%~99.56%, 平均值为99.13%; 其他元素含量很低, 两者均为纯硅质岩。坝黄层状硅质岩的ΣREE为20.14~248.56 μg/g (平均100.62 μg/g), Eu/Eu*值为0.90~1.10 (平均1.06), 无明显异常, Ge/Si值为0.13~0.98 μmol/mol (平均0.50 μmol/mol), Al₂O₃与ΣREE明显正相关, 表明硅质岩成分明显受陆源输入的控制。柑子坪穹隆状硅质岩的ΣREE值低, 为3.75~7.24 μg/g (平均5.73 μg/g), Ce/Ce*值为0.46~0.66 (平均0.57), 具负异常, Eu/Eu*值为2.28~11.07 (平均4.60), 具明显正异常, Ge/Si值为1.09~1.43 μmol/mol (平均1.25 μmol/mol), Al₂O₃与ΣREE的相关性较差, 表明硅质岩为海底热液成因。Al₂O₃与Ge/Si的相关性强弱也可以反映硅质岩的来源。结合古地理环境, 推断层状硅质岩可能形成于盆地内部, 而穹隆状硅质岩可能发育在台缘同生断裂处, 因海底热液喷流而形成。以1 μmol/mol 作为Ge/Si的临界值可以为示踪硅质岩的物质来源提供新的思路。

关键词: 留茶坡组, 层状硅质岩, 穹隆状硅质岩, Ge/Si, 热液