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斑状白云岩在碳酸盐岩地层中较为常见,其成因存在不同的解释.四川广元中二叠世栖霞组中部发育一套厚层斑状白云岩,形成于浅滩相沉积环境.白云岩主要由等粒自形晶白云石和不等粒镶嵌状白云石构成,含少量的鞍形白云石.阴极发光显示白云石具有4期环带结构:(Ⅰ)中心不发光的雾心;(Ⅱ)中间发橙色光的窄亮边;(Ⅲ)次外围不发光的环带;(Ⅳ)最外围发暗红色光的环带.白云岩和宿主灰岩的δ13C分别分布在+2.17‰～+3.19‰PDB和+1.39‰～+2.58‰PDB;δ18O分别分布在-1.7‰～-5.45‰PDB和-3.66‰～-6.38‰PDB.白云岩和灰岩的δ13C呈正相关,表明白云岩继承了灰岩的碳同位素值,而δ18O变化趋势不一致表明白云岩的氧同位素受到流体改造作用较强.综合研究表明,该套白云岩的形成大致经历了4个阶段:(1)准同生期富镁流体回流渗透交代灰泥基质成核期,形成了环带(Ⅰ);(2)浅埋藏阶段,形成了环带(Ⅱ)和(Ⅲ);(3)热液流体的注入,形成环带(Ⅳ),同时在裂隙中形成了鞍形白云石胶结物;(4)白云石粒间孔隙的充填和改造.
Porphyry dolomite is more common in carbonate rocks and has different explanation for its genesis: A thick porphyritic dolomite developed in the central Permian Qixia Formation in Guangyuan, Sichuan, and formed in a shallow facies depositional environment. The dolomite dolomite is composed of dolomite with irregular dolomite and a small amount of saddle-shaped dolomite.The cathodoluminescence shows that dolomite has four stages of zonal structure: (Ⅰ) fog center without luminescence; (Ⅱ) intermediate hair (Ⅲ) the sub-peripheral glow zone; (Ⅳ) the outermost peripheral zone of dark red light.The δ13C of dolomite and host limestone are distributed in the range of + 2.17 ‰ to + 3.19 ‰ PDB And + 1.39 ‰ ~ + 2.58 ‰ PDB, respectively, and δ18O distributed in PDB ranging from -1.7 ‰ to -5.45 ‰ and PDB ranging from -3.66 ‰ to -6.38 ‰, respectively.The δ13C values of dolomite and limestone are positively correlated, indicating that dolomite inherits limestone Carbon isotope values, while the δ18O variation tendency shows that the oxygen isotope of dolomite is strongly modified by fluid.Comprehensive studies show that the formation of dolomite generally goes through four stages: (1) quasi-syngenetic magnesium-rich fluid backflow Infiltration through the limestone matrix nucleation stage, the formation of the zoning (Ⅰ); (2) shallow burial stage, (2) and (III); (3) hydrothermal fluid injection to form annulus (IV) while forming saddle-shaped dolomite cements in fractures; (4) dolomite intergranular pores Filling and remodeling.