鄂西古生代硅质岩的地球化学特征及沉积环境

鄂西地区的上奥陶统五峰组、下志留统龙马溪组下部、下二叠统孤峰组和上二叠统大隆组中发育有薄层硅质岩。二叠系栖霞组、茅口组和吴家坪组灰岩中发育有结核状或似层状燧石。Fe Al Mn三角图 ,Cu、Ni、Co、Cr、Zr的关系 ,U Th判别图 ,Ba、As、Sb元素和REE配分模式等地球化学指标表明 :五峰组和龙马溪组层状硅质岩属于正常海水中生物化学和化学沉积 ;孤峰组和大隆组层状硅质岩、茅口组灰岩中燧石结核不属于典型的热水沉积相 ,但受热水作用的影响 ,而孤峰组硅质岩受热水作用明显。鄂西五峰组和龙马溪组硅质岩的沉积环境是四周被古陆或台地环绕的半封闭深水相滞流盆地。鄂西孤峰组和大隆组硅质岩的沉积环境是由裂陷作用形成的碳酸盐台地内部的盆地(或台沟 )滞流还原环境。CaO/(Fe +CaO)值和 (MgO/Al2 O3 )× 10 0值表明 :五峰组、龙马溪组和大隆组硅质岩沉积环境海水略有淡化。栖霞组、茅口组和吴家坪组含结核状或似层状燧石的生物灰岩 ,形成于陆棚上快速海侵和上升洋流形成的还原环境。

Geochemistry and sedimentary environments of the Palaeozoic siliceous rocks in western Hubei

The thin-bedded siliceous rocks are developed in the Upper Ordovician Wufeng Formation, the lower part of the Lower Silurian Longmaxi Formation, Lower Permian Gufeng Formation and Upper Permian Dalong Formation in western Hubei. The nodular and/or stratoid cherts are observed in the limestones of the Permian Qixia, Maokou and Wujiaping Formations. The geochemical signatures based on the Fe-Al-Mn triangular diagram, U-Th discriminant diagram, Cu, Ni, Co, Cr and Zr correlation diagram, the elements Ba, As and Sb and REE distribution patterns all suggest that the bedded siliceous rocks in the Wufeng and Longmaxi Formations may be assigned to the biochemical and chemical sediments deposited from normal sea water. Although the bedded siliceous rocks in the Gufeng and Dalong Formations and chert nodulars in the Maokou Formation are not characteristic of hydrothermal sediments, they are affected by the hydrothermal deposition. This case is especially noticeable for the siliceous rocks in the Gufeng Formation. The sedimentary environments of the siliceous rocks in the Wufeng and Longmaxi Formations are represented by relatively stagnant and deep-water basins surrounded by ancient continents, island chains and carbonate platforms. The siliceous rocks in the Gufeng and Dalong Formations, western Hubei were deposited in the reduced environments of the basins and/or platform trenches within carbonate platforms created by faulting and rifting. The ratios of CaO/(Fe+CaO) and (MgO/Al₂O₃)×100 show that the sea water during the deposition of the siliceous rocks in the Wufeng, Longmaxi and Dalong Formations might be slightly freshened. The bioclastic limestones with nodular or stratoid cherts in the Qixia, Maokou and Wujiaping Formations lay down in the reduced environments caused by rapid transgression and upwelling currents.