下扬子中二叠统孤峰组热液硅质岩地球化学约束及其意义

Geochemical Constraints on the Hydrothermal Chert of the Kuhfeng Formation in the Middle Permian in the Lower Yangtze and Its Significance

【目的】下扬子地区中二叠世沉积了一套富含有机质的孤峰组黑色硅质岩,热液硅质岩的分析是该区硅质岩与异常高有机质富集关系研究的关键,进而为该区页岩气勘探提供理论依据。【方法】利用主量、微量和稀土元素地球化学结合岩相学特征分析,安徽铜陵叶山和巢湖平顶山地区孤峰组硅质岩的成因及其构造背景。【结果】巢湖地区孤峰组黑色薄层硅质岩中富含硅质海绵骨针以及放射虫,而铜陵地区孤峰组青灰色硅质岩中几乎不含硅质生物,由微晶石英组成。热液活动指标Al/(Al+Fe+Mn)、Eu/Eu^(*)、Al-Fe-Mn三角图、Lu_(N)/La_(N)比值指示铜陵地区孤峰组青灰色硅质岩为热液成因,巢湖地区孤峰组黑色薄层硅质岩为生物成因。微量和稀土元素La-Th-Sc三角图以及La/Sc与Ti/Zr交会图指示铜陵地区孤峰组沉积期为洋岛弧构造背景,而巢湖地区为陆岛弧和主动大陆边缘背景。【结论】铜陵地区孤峰组硅质岩为原生热液硅质沉积,是斜坡带活跃的构造裂陷活动带来的热液硅酸沉积而成。下扬子地区孤峰组黑色层状硅质岩虽然为生物成因,但其硅的来源与斜坡带的热液活动有关。下扬子地区孤峰组富有机质沉积可能与热液活动带来大量的营养物质造成浮游生物的繁盛有关。

[Objective]The organic-enriched black bedded cherts in the Kuhfeng Formation were deposited in the Lower Yangtze during the Middle Permian.The hydrothermal cherts may be the key part for the relationship analysis between chert deposits and the extra-ordinarily high organic matter enrichment.This would be the basic theories for the shale gas exploration in this area.[Methods]This work uses major,minor,and rare-earth elements,combined with the petrology,to analyze the origin of chert and their tectonic setting for the Kuhfeng Formation in the Yeshan of Tongling and Pingdingshan of Chaohu,Anhui province.[Results]The black bedded cherts in the Chaohu area contain abundant siliceous sponge spicules and radiolarians.The cinerous cherts in the Kuhfeng Formation in the Tongling area contain extremely rare siliceous fossils and consist of micro-quartz.Hydrothermal proxies,such as Al/(Al+Fe+Mn),Eu/Eu^(*),Al-Fe-Mn diagram,and Lu_(N)/La_(N),indicate a hydrothermal and biotic origin for the chert in the Kuhfeng Formation in the Tongling and Chaohu areas,respectively.The La-Th-Sc diagram and crossplot between La/Sc and Ti/Zr indicate oceanic island arc and active continental margin origins for the Tongling and Chaohu areas.[Conclusions]The chert in the Kuhfeng Formation in the Tongling area was primary deposited by hydrothermal activity.The silicic acid was brought by a hydrothermal vent near the slope environments.The black bedded cherts in the Lower Yangtze area are biotic in origin,but the source of silicon was related to the hydrothermal activity.The organic matter enrichment in the Kuhfeng Formation in the Lower Yangtze area may be related to the phytoplankton flourishment induced by the nutrient element input from hydrothermal activity.