华北秦皇岛地区柳江盆地马家沟组碳酸盐岩多期白云化作用

摘要

华北秦皇岛地区的中奥陶统马家沟组以广泛发育海相白云质灰岩、灰质白云岩和白云岩为特征.在石门寨奥陶系亮甲山剖面,马家沟组自下而上识别出四类碳酸盐岩:含白云石泥晶灰岩(类型Ⅰ)、细-粉晶白云岩(类型Ⅱ)、"麦粒状"细-粉晶白云岩(类型Ⅲ)和钙质泥晶白云岩(类型IV).类型I主要由泥晶方解石构成,含三叶虫和介形类生物碎片,少量自形的粉晶白云石呈"漂浮状"分布于压溶缝合线内,基质中少见.岩相学和地球化学特征表明此类白云石形成于埋藏成岩期压溶作用之后,压溶缝为云化流体提供通道,压溶缝内泥质组分的成岩转化可能为白云化作用提供了部分镁离子来源;类型II白云岩主要由自形、半自形不等粒粉晶-细晶白云石构成,白云石普遍具有"雾心亮边",在背散射和阴极发光照片中白云石可见清晰的多圈亮、暗相间环带.环带和带间主量元素的差异表明白云石经历了埋藏成岩期多期成岩流体的改造;类型Ⅲ白云岩中白云石呈单向延伸的"米粒"或"麦粒"状,粉晶为主,晶体长轴方向具有垂直结晶轴c的特点,白云石具富铁、贫锰、锶的特点,长、短对角线上钙、镁离子的微小差异以及阴极发光特征表明此类白云石也经历了埋藏成岩期的改造,成岩流体使白云石发生微溶作用可能是导致白云石晶体单向延伸且光学性质固定取向的主要原因;类型Ⅳ为钙质泥晶球粒白云岩,含石膏假晶,白云石多为微晶和微亮晶,球粒也多由微晶白云石构成,溶孔发育,但全被亮晶贫铁方解石充填,此类岩石的白云化作用发生得很早,可能形成于潮上带澙湖或潮坪环境.综上所述,研究区马家沟组碳酸盐岩具有经历了不同类型及多期白云化作用的特点.自剖面底部向顶部,白云石的有序度由0.8降至0.47,而去云化作用则呈现逐渐增强的趋势.%Middle Ordovician Majiagou Group, widely distributed in North China, was characterized by layers of marine dolomitic limestones, calcareous dolostones and dolostones in Liujiang Basin, Qinhuangdao area, North China.Four types of carbonate rocks in Majiagou Group were recognized in Liangjiashan Section, Shimenzhai area.From bottom to the top, they are type Ⅰ-dolomitic wackstones;type Ⅱ-fine to medium grained dolostones;type Ⅲ-"wheat shaped" fine to medium grained dolostones;and type IV-calcareous microcrystalline pellets dolostones.Type Ⅰ rock is mainly composed of microcrystalline calcite with fragments of trilobites and ostracods.Some fine-grained euhedral dolomites are "floating" in the pressolution stylolites and seams, rare in matrix.Petrological analysis and geochemical features of type Ⅰ rocks indicate that the dolomitization occurred after the pressolution.The pressolution seams and stylolites provided pathways for dolomitizing fluids.Meanwhile, the conversion of clay minerals within the stylolites could provide part of magnesium for the dolomitization;Type Ⅱ dolostone consists largely of euhedral to subhedral inequigranular (fine to medium grained) dolomites with "cloudy center and clear rim" structure.Photomicrographs of backscattered electron and cathodeluminescence of type Ⅱ dolostones show that most dolomites have clear zonal structures.The variations of major elements, especially Fe ions between the rings and the intervals in a single dolomite crystal suggest that type Ⅱ dolomites were formed by multi-stage dolomitizations during the burial history.The dolomite in type Ⅲ dolostone reveals the feature of unidirectional extending, named as "wheat-shaped" crystals.The long axis orientation of the dolomite is fixed and perpendicular to crystal c axis.The EPMA analysis indicates that type Ⅲ dolomite is Fe rich and poor with Mn and Sr.The content of MgO along the long axes is slightly higer than that along the short axes, while CaO content shows the opposite tendency.Both the geochemical and cathodeluminescence features indicate that the "wheat-shaped" dolomite was also the result of burial dolomitization.The diagenetic fluids may play an important role in the formation of the unidirectional extending dolomite crystals;Type Ⅳ is calcareous microcrystalline pellets dolostones, with pseudomorphisms of gypsum.This type is mainly composed of microcrystalline to microsparry dolomite, including pellets and algae.The dissolution pores are common in type Ⅳ rocks, but had already been filled by poor-Fe sparry calcites totally.The petrology of type IV dolostones indicates that the dolomitization occurred very early, probably formed in lagoon or tidal flat of supratidal zones.In summary, these four types of dolomite in research area are the products of different dolomitizations, varied from very early to burial dolomitizations.From the bottom to the top of this section, the dolomite degree of order declined from 0.8 to 0.47, while the dedolomitization in four types of rocks appears the reverse tendency.