Keywords: 
• Middle Ordovician Majiagou Group /
• pressolution stylolite /
• &ldquo /
• wheat-shaped&rdquo /
• dolomite

Abstract: 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.