川东地区二叠系结核状、条带状及团块状硅岩成因

川东地区二叠系结核状、条带状和团块状硅岩发育,主要分布于茅口组上段及吴家坪组,夹于石灰岩层间或层内。通过野外露头观察、室内薄片鉴定以及主量元素、微量元素和稀土元素分析,对广元长江沟、旺苍双汇、宣汉立石河和巫溪尖山的二叠系硅岩进行了岩石学及地球化学研究,重点分析了硅质来源,并对其成因机制进行了探讨。结果表明,研究区硅岩形态多样、分布不均,硅岩中见残余的石灰岩;硅质成分主要为玉髓及微晶石英、灰质硅岩中玉髓"侵入"方解石、相邻石灰岩中少量生屑被硅化。硅岩的化学成分以SiO2为主(52.79%～99.21%,平均为85.18%),Fe/Ti值、(Fe+Mn)/Ti值及Al/(Al+Fe+Mn)值反映其形成与热水活动有关;硅岩的部分微量元素含量与地壳中该元素的平均含量差异明显,Co/Ni值与Th/U值较低;稀土元素总量低,Ce负异常,LREE/HREE值低,δEu值相对较高。综合分析认为研究区二叠系硅岩为交代成因,硅质以热水来源为主,玄武岩淋滤及上升流提供了部分硅质。复合来源的硅质通过同生断裂、表层淋滤及上升流进入碳酸盐沉积物中,在不同的成岩阶段交代碳酸盐沉积物(岩),形成了顺层面或层内分布的结核状、条带状及团块状硅岩。

Origin of the Permian nodular, striped and lump siliceous rock in eastern Sichuan Province

The Permian nodular,striped and lump siliceous rock widely distribute in the upper of Maokou Formation and the whole Wujiaping Formation,intercalate between or in the limestone succession,in the eastern Sichuan Province.The authors research the mechanism of origin and the resource of silicon for the siliceous rock by the lithological and geochemical characteristics from the outcrop,rock thin section,major element,trace element and REE,based on the Guangyuan,Wangcang,Xuanhan and Wuxi Sections.The results indicate that shape of siliceous rock is diversified,distribute heterogeneously,and the lump siliceous rock contain limestone cakes.The silicon of the siliceous rock is primary chalcedony and microcrystal quartz,the chalcedony of calcareous-siliceous intrude into the calcite,and several bioclast silicificated.Major element analysis indicates SiO2 is the main chemical composition(52.79%～99.21%,avg 85.18%),some major element ratios of Fe/Ti,(Fe+Mn)/Ti,Al/(Al+Fe+Mn)reflect the formation of siliceous rock is associated with the hydrotherm activities.Trace element suggests different content from the crust,some trace element ratios including Co/Ni,Th/U are relatively lower.The ΣREE is lower,Ce is negative anomalies,LREE/HREE is lower and the δEu is relatively high.All these features above suggest that the origin of this siliceous rock we investigate is metasomatosis,the hydrotherm is primary source of silicon,the basalt leaching and upwelling is secondary.Multiple sources following with the carbonate sediments deposit through contemporaneous faults,surface leaching and upwelling,and then metasomatose the carbonate sediments during the diagenesis,finally form the nodular,striped and lump siliceous rock between or in the surrounding rocks.