豫西济源盆地侏罗系泥质岩地球化学特征及其环境和物源示踪

摘要

The sedimentary environments and provenances were investigated on the basis of trace elements and rare earth elements analysis of Jurassic argillaceous rock samples collected from the Jiyuan basin. Chondrite-normalized rare earth elements (REEs ) distribution patterns indicated a right-oriented enrichment of light rare earth elements (LREE),with a moderate negative Eu anomaly (δEu=0.64)and a weak Ce depletion (δCe=0.90).The values of LREE/HREE (9.1)and (La/Yb)N(9.5)suggest a distinct differentiation between LREE and HREE. The characteristics of sedimentary environments and provenances were analyzed using geochemical approaches.The results showed that a weak oxidation-reduction condition dominated the Jurassic period of the Jiyuan basin.The reduction potential decreased from the bottom to the top,along with the decrease of water depth.Belonging to the opening system of the large Ordos basin,the Jiyuan basin was under fresh water environment in the period of Anyao Formation and Yangshuzhuang formation,with its water salinity free from the effect of the gradually drying climate.Because of the Taihang mountain uplift in the period of Maao Formation,the Jiyuan basin was isolated from the large Ordos basin.The dry climate induced the increase of water salinity,resulting in the brackish sedimentary environment.There is a reduced tend towards felsic rocks and growing trend towards basic igneous rocks for Jurassic provenances of Jiyuan basin,from bottom to top.The evolution of Jiyuan Basin in Jurassic was controlled by the uplift and denudation of Taihang anticlinorium,the source of Jiyuan basin during early Jurassic is mainly cycled felsic from sedimentary cover of Taihang anticlinorium, later,as the anticlinorium quickened rise and denudation,Lower Proterozoic-Archaean basic material from core of it become important sources of the basin.At the same time,Qinling orogenic belt contributed part of the source.The Jurassic of Jiyuan basin reveals the process of gradual uplift and denudation of Taihang anticlinorium,it is significant for us to understand the destruction process of North China Craton.%本文通过对济源盆地侏罗系泥质岩样品的微量元素及稀土元素测试，探讨了其形成的环境和物源。稀土元素球粒陨石标准化后表现为轻稀土元素富集的右倾式，δEu 均值0．64，中度负异常，δCe 均值0．90，轻微亏损，LREE/HREE、（La/Yb）N均值分别为9．1和9．5，轻重稀土分异明显。利用地球化学方法分析了该盆地侏罗纪沉积环境及物源特征，结果表明：济源盆地侏罗纪总体处于弱氧化-还原环境，且还原性自下而上依次减弱，水体逐渐变浅；早侏罗世鞍腰组和中侏罗世早期杨树庄组沉积时期，济源盆地处于鄂尔多斯大型盆地开放体系中，气候逐渐干旱并未影响水体盐度，总体为淡水沉积；中侏罗世晚期马凹组沉积时期，太行山复背斜隆升使济源盆地完全封闭，气候干旱造成水体盐度增大，总体为半咸水沉积；济源盆地侏罗系沉积物源自下而上有长英质物质逐步减少、基性火成物质逐步增加的趋势，说明济源盆地在侏罗纪受到太行山复背斜隆升和剥蚀的控制，其早期大量物源来自复背斜上被剥蚀的复旋回长英质盖层，到晚期，随着复背斜的隆升和剥蚀的加强，其核部古元古-太古界基性物质成为盆地重要的沉积物源之一。与此同时，秦岭造山带也向济源盆地侏罗系贡献了部分物源。因此，济源盆地的侏罗系揭示了太行山复背斜逐步隆升-剥蚀的过程，成为我们理解华北克拉通破坏的一个重要窗口。