北京南口长城系高于庄组硅质臼齿状构造的发现及成因

Discovery of Siliceous Molar Tooth Structure and Its Genesis in Gaoyuzhuang Formation at Nankou,Beijing

臼齿构造主要发育于中新元古代地层中,目前所报道的臼齿构造均由纯净的、基本等粒的微晶方解石(或白云石)构成.野外地质考察中,在北京南口长城系高于庄组第3段泥晶灰岩中发现了形态多样的臼齿状构造,野外观察和室内显微镜下分析,发现臼齿状构造与宿主岩石边界多呈圆弧状或港湾状,界线清晰.扫描电镜(SEM)和能谱分析(EDX)表明,"臼齿"成分主要以硅质为主,矿物成分为玉髓和自生石英.从臼齿状构造的形态和充填序列分析,发现南口臼齿状构造形成经历了裂缝形成、裂缝充填和后期破坏再充填等过程.认为臼齿状构造可能是多成因的结果,其中在半固结的条件下由地质事件引起的微裂隙系统可能是臼齿状构造形成的重要因素之一.

The molar tooth structure(MTS),a special kind of microstructure consisting of pure equigranular micritic calcite or micritic dolomite,mainly developed in Meso-Neoproterozoic strata.During the field geologic investigation,the molar tooth structures were observed in the micritic limestone in the third member of Gaoyuzhuang Formation(Changcheng System)in Nankou,Beijing.Through detailed field observations and microscopic studies,a variety of MTSs were found.The boundaries between host rock and MTS show obvious distinction with a circular arc shape or a gulf-like shape.It is found by scanning electron microscopy(SEM)and energy disperse X-ray microanalysis(EDX)that the MTS consists of siliceous component,such as chalcedony and authigenic quartz.According to the morphology and the filling sequence,the MTS’s formation is characterized by multi-genesis and multi-stage,and may be subdivided into four periods,such as crack forming period,crack filling period,crack transforming period,and crack refilling period.It’s a key factor of the MTS’s formation that the fissure systems are produced by the tectonic movement in semi-consolidated condition.