华北地台东部奥陶系蠕虫状灰岩的结构特征及成因探讨

Characteristics and origin of Ordovician vermicular limestone in the eastern North China Platform

蠕虫状灰岩是一种岩石表面具有"蠕虫体"的含云或白云质灰岩,其"蠕虫体"的成因一直存在多种解释。以华北奥陶系蠕虫状灰岩为例,通过观测野外露头剖面、分析典型样品的岩石矿物和地球化学组成,刻画蠕虫状灰岩的空间展布和蠕虫体的个体形态,对比蠕虫体和基质的主元素、微量元素、稀土元素、同位素(δ^(13)C、δ^(18)O、^(87)Sr/^(86)Sr、δ^(26)Mg)等特征。结果表明:(1)蠕虫状灰岩与白云岩地层密切相关,往往下伏于同层系白云岩地层;(2)外形上,"蠕虫体"有渗透型、充填型和浸染型三种类型。渗透型蠕虫状灰岩白云石沿裂缝、粒间缝等发育,连通性好,充填型受生物潜穴和疏松的生物碎片影响,浸染型主要与上覆地层载荷作用有关;(3)蠕虫体和基质均有较低的Sr、Fe、Mn等微量元素含量。相对基质,蠕虫体具有较高的白云石、石英、Sr同位素组成和更为分散的稀土元素特征。蠕虫体的Al_2O_3、Fe_2O_3等含量与Sr同位素呈正相关关系,与Sr含量呈负相关关系;(4)蠕虫体的δ^(26)Mg与原生或准同生成因白云石较为一致,如混合水成因白云石、湖相白云石,而与生物成因白云石δ^(26)Mg差异较大。宏观和微观证据指示,蠕虫状灰岩白云石质蠕虫体以渗透-回流为主:富镁流体渗透流经多孔灰泥质灰岩层,并在前期存在的生物钻孔、微裂缝、未固结的塑性空间内浓缩沉淀形成白云石组分,后期抬升出地表后经差异风化作用形成蠕虫体。

The origin of vermicular limestone,which is characterized by a vermicular structure and dolomite composition,has been debated by many models,which have proposed chemical coagulation,the biogenic progresses,mechanical transportation,etc.Taking the Ordovician vermicular limestone in the eastern North China platform as an example,this study investigated the spatial distribution of vermicular limestone in strata as well as the structure and elemental composition,including the major,trace,and the rare earth elements and isotopes(δ13C,δ18O,87Sr/86Sr,δ26Mg).The results indicated the following:(1)Vermicular limestone,which frequently laid below dolomite strata,was deeply influenced by dolomite strata.(2)Vermicular limestone could be classified into three types,namely permeation,filling,and disseminated,according to its structure.Dolomite of Permeation was distributed along intergranular pores,fractures,chinks etc.,and that of Filling was influenced by biological burrows and bioclasts.Meanwhile,the load of the overlying strata influenced the disseminated dolomitization.(3)Vermicular and matrix limestone held similar major and trace elements,and had low contents of Sr,Fe,and Mn.In comparison with the matrix,vermicular limestone held completely different contents of quartz,dolomite,and isotopes,such as higher contents of quartz,dolomite,and 87Sr/86Sr,and more dispersed rare earth elements.In relation to the 87Sr/86Sr composition was positively correlated with Al2O3 and Fe2O3,and was negatively correlated with Sr.(4)Component ofδ26Mg for vermicular limestone was similar to contem-poraneous and penecon-temporaneous dolomitization,such as mixed water dolomitization,while it was signifi-cantly different from biogenic dolomitization.This evidence showed that the infiltration and circumfluent dolomitization controlled the dolomitization of vermicular limestone.The mechanic of dolomitization,in which Mg-rich fluid infiltrated and refluxed through porous lime mud mounds and partly consolidated limestone through the channels of fractures,intergranular pores,biological burrows,loose bioclasts,etc.,and different weathering between dolomite and calcite brought about vermicular formation.