下扬子地区古生界硅岩地球化学特征及成因

下扬子地区的 3 套区域性烃源岩（下寒武统、中二叠统以及上二叠统）均发育灰黑色层状硅岩，硅岩地球化学特征及成因的研究对认识该区烃源岩的沉积环境具有重要意义。通过对下扬子无为-南陵地区 4 条典型地质剖面（下寒武统青阳青坑剖面、二叠系巢湖平顶山剖面、巢湖马家山剖面以及泾县昌桥剖面）的硅岩主量元素和稀土元素的地球化学特征进行研究，结果发现：下寒武统硅岩为纯硅质岩；二叠系硅岩部分样品含少量泥质成分；下寒武统黄柏岭组、中二叠统孤峰组以及上二叠统大隆组硅岩均存在 Ce 负异常，而这 3 个层段的 Eu 异常明显不同。 根据研究区古生界硅岩的 MnO/TiO₂ 值、Fe₂O₃/TiO₂Al₂O₃/（Al₂O₃+Fe₂O₃）关系以及稀土元素特征，综合判断上二叠统大隆组硅岩沉积背景为大陆边缘浅海，而下寒武统和孤峰组层状硅岩沉积于海盆中，其沉积环境与大隆组硅岩相比，更远离陆源影响。根据研究区 3 个层段的硅岩 Al/（Al+Fe+Mn）值、Al-Fe-Mn 三角散点图以及稀土元素特征，判断研究区下寒武统硅岩为热水沉积成因硅岩，上二叠统硅岩为典型生物成因硅岩，中二叠统孤峰组硅岩为生物和热水混合成因硅岩。

Geochemical characteristics and origin of Palaeozoic siliceous rocks in Lower Yangtze area

As regional source rocks, the Lower Cambrian, Middle Permian and Upper Permian in Lower Yangtze area developed bedded siliceous rocks. The analysis of geochemical characteristics and origin of the Palaeozoic siliceous rocks in Lower Yangtze area is very important for understanding sedimentary setting of source rocks. Based on the analysis of major elements and rare earth elements of the siliceous rocks from the four typical sections （Lower Cambrian Qingkeng section in Qingyang town, Pingdingshan section in Chaohu Lake, Majiashan section in Chaohu Lake and Changqiao section in Jing county） in Lower Yangtze Wuwei-Nanling area, the SiO₂ content of the Lower Cambrian is a little higher than that of the Permian; the siliceous rocks of Lower Cambrian Huangpailing Formation, Middle Permian Gufeng Formation and Upper Permian Dalong Formation have weak negative Ce anomalies, but they have different Eu anomalies. Based on the MnO/TiO₂ ratio of Paleozoic siliceous rock, the relation of Fe₂O₃/TiO₂-Al₂O₃/（Al₂O₃+Fe₂O₃） and the rare earth element characteristics in the study area, it is concluded that the sedimentary setting of the siliceous rocks of Upper Permian Dalong Formation is neritic shelf facies. While the bedded siliceous rock of the Lower Cambrian and Middle Permian Gufeng Formation deposited on the marine basin, of which the depositional environment is more closely to the inner of basin compared to the siliceous rocks of Dalong Formation. According to Al/（Al+Fe+Mn） ratio of the bedded siliceous rocks of three layers, the triangular plots of Al-Fe-Mn and rare earth element characteristics, it is considered that the Lower Cambrian siliceous rock is hydrothermal genesis, the Upper Permian siliceous rock is typically biogenic genesis, and the siliceous rock of Middle Permian Gufeng Formation is biological and hydrothermal mixed genesis.