桂西南柳桥地区上二叠统大隆组层状硅质岩成因和沉积环境

The Petrogenesis and Sedimentary Environment of the Bedded Cherts from Upper Permian Dalong Formation,Southwest Guangxi

对桂西南上二叠统大隆组层状硅质岩地球化学的研究发现:陆源主量元素Al、Ti等含量较高,并呈很好的相关性(R>0.90);热液来源的主量元素Mn和Fe等含量偏低,并具有较小的负相关关系(R=-0.30);陆源元素(Al、Ti、Hf、Zr、Th等)与总稀土元素含量具有较高正相关性(R为0.70～0.83);Al-Fe-Mn三角图解指示研究区的硅质岩为非热液成因。这些说明陆源物质是硅质岩形成的重要物源,结合硅质岩中含有大量的硅质生物(放射虫和海绵骨针等)的事实,我们认为研究区硅质岩是在生物作用为主,并有大量物源物质和少量热液物质(可能与大断裂导致的玄武岩喷发有关)和火山物质混入的条件下形成的。Ce/Ce*、(La/Yb)Shale、(La/Ce)Shale和∑REE与细粒沉积物沉积环境的关系以及(La/Ce)Shale—Al2O3/(Al2O3+Fe2O3)图解等说明研究区硅质岩沉积环境为大陆边缘的中下部。Th/U和Ce指示了硅质岩形成于氧化环境。

The geochemical researches have been conducted. And we have the on the upper Permian cherts of the following findings： the chert is rich Dalong Formation in the southwest Guangxi in the terrestrial elements （for example Al, Ti etc. ）, and poor in hydrothermal elements （for example Mn, Fe etc. ）, with high correlation between the terrestrial elements （R 〉0. 90） and low correlation between the hydrothermal elements （R = -0. 30） ; The correlation between the terrestrial elements （Al, Ti, Hf, Zr, Th etc. ） and ∑REE is high （R =0.70 -0.83） ; Al-Fe-Mn diagram shows that the cherts is non-hydrothermal type. The characteristics above confirm that terrestrial debris is one of the most important provenances. Considering the truth, the high content of the siliceous bio-debris in the cherts, we prove that the cherts were formed by strong bioor bio-chemical sedimentation, with lets of terrestrial debris and little hydrothermal and volcanic material interfused. Ce/Ce^＋, （La/Yb）shale, （La,/Ce）shale, Th/U, Ce ∑REE and （La/Ce）shar -Al2O3/ （Al2O3 ＋ Fe2O3 ） diagram make sure that the cherts are deposited on middle-outer shelf in oxic conditions.