华南埃迪卡拉纪陡山沱组物源风化对全球性成磷事件的启示

华南埃迪卡拉纪陡山沱组沉积于原始地球-生命系统向现代地球-生命系统过渡的关键时期,该套地层记录了地质历史时期一次大规模成磷事件,但关于该次成磷事件中磷元素的来源仍存在诸多争议。浅海区磷元素主要来源于由河流搬运的陆表含磷矿物风化产物及上升流作用所带来的其他海域含磷水体,这两种来源可以通过全岩碎屑元素(Al、Fe、Th等)和稀土元素的含量及其分布特征进行区分。本文通过对上扬子地区乡党坪剖面和四斗坪剖面的陡山沱组主、微量元素特征的综合分析,恢复了该地区的物源体系及其风化作用过程。较高的P₂O₅-Al₂O₃、P₂O₅-Fe₂O₃相关性,表明该时期磷来源于陆地风化而非上升流;较低的Th/Co、La/Sc、Th/Cr等比值,TiO₂-Zr相关性以及Th-Hf-Co、La-Th-Sc三角图分布关系,表明研究区内陡山沱组的物源体系主要为玄武岩;较高的化学蚀变指数(CIA)和较低的Sr/Cu值,表明物源区当时以温暖湿润气候为主,经历了中等程度的化学风化作用。对不同类型火山岩中磷含量的大数据分析结果表明,镁铁质火山岩中的磷含量远高于长英质火山岩,故陡山沱组沉积期间较为强烈的玄武岩风化作用大大提高了浅海陆架区的磷沉积通量以及海水磷浓度水平,为磷块岩的大规模形成提供了丰富磷质来源,并改善了早期海洋的磷循环,促进了全球规模的成磷事件以及大型后生动物的辐射。

Enlightenment of source and weathering of the Ediacaran Doushantuo Formation in South China on the global phosphorus event

The Ediacaran Doushantuo Formation in South China was deposited in a key period of the transition from the original earth-life system to the modern earth-life system.This set of strata recorded the first large-scale phosphorus event in the Earth''s history.However,there are still many controversies about the source of phosphorus.The phosphorus in the shallow sea area mainly comes from the riverine weathering products of phosphate minerals on the land surface and the upwelling deliver of phosphorus-bearing water-masses in other sea areas.These two sources can be distinguished by the content and distribution characteristics of whole rock detrital elements and rare earth elements.Based on the comprehensive analysis of the major and trace element of Doushantuo Formation in Xiangdangping and Sidouping sections in the upper Yangtze region,the provenance system and regional weathering have been recovered in this paper.The high correlation of P₂O₅-Al₂O₃ and P₂O₅-Fe₂O₃ indicates that the source of phosphorus is terrestrial weathering input rather than upwelling.The lower Th/Co,La/Sc,Th/Cr ratios,TiO₂-Zr correlation and ternary plots of (Th-Hf-Co and La-Th-Sc) indicate that the major source rock of Doushantuo Formation is basalt.The higher chemical alteration index (CIA) and the lower Sr/Cu ratio indicate that the provenance area was dominated by warm and humid climate at that time and experienced moderate chemical weathering.The big data analysis of phosphorus content in different types of volcanic rocks shows that the phosphorus content in mafic volcanic rocks is much higher than that of felsic volcanic rocks.Therefore,the relatively strong basalt weathering during the deposition of Doushantuo Formation greatly increased the phosphorus deposition flux and marine concentration of phosphorus in the shallow sea shelf area,which offered mass of phosphorus to widespread phosphorus burial,improved the circulation of phosphorus in early oceans,and promoted the global phosphorus event and the radiation of large metazoans.