西藏南羌塘坳陷昂达尔错地区侏罗系布曲组碳酸盐岩阴极发光特征与成岩阶段划分

Cathodoluminescence features and diagenetic stage division of the carbonate rocks in Buqu Formation of Jurassic, Southern Qiangtang Depression,Tibet

通过岩石学特征研究和阴极发光分析,结合Fe、Mn、Sr元素特征及氧同位素资料,研究西藏南羌塘坳陷昂达尔错地区侏罗系布曲组碳酸盐岩阴极发光特征并进行成岩阶段划分。结果表明:(1)布曲组碳酸盐岩阴极发光强度总体较低,这与其较低的Fe、Mn含量有关,说明沉积期后非海源流体对碳酸盐岩的影响较小,海源流体在成岩过程中起主导作用;(2)伴随着成岩作用增强,不同岩石类型中的Mn含量逐渐增加、Sr含量逐渐降低,氧同位素值总体上向高负值方向偏移,对应的阴极发光强度逐步增加;(3)泥-微晶白云岩具有最强的阴极发光性和最高的Mn含量,在白云岩化过程中可能与近地表弱还原环境有关,蒸发泵和渗透回流作用可能是这些岩石白云石化作用的主要机理;而结晶白云岩则具有较弱的阴极发光性和较低的Mn含量,孔隙流体可能远离大气淡水,包裹体均一温度显示其白云岩化作用是在深埋藏环境中发生的;(4)布曲组碳酸盐岩经历了准同生-同生阶段的海水成岩环境→早成岩阶段的近地表-浅埋藏弱还原成岩环境→中-深成岩阶段的中-深埋藏深循环温压水和高盐度地层水成岩环境的连续改造→晚白垩世末期—古近系早期构造热事件热液改造阶段→表生成岩阶段的演化过程。

Based on petrographic characteristics and cathodoluminescence analysis, combined with the corresponding Fe-,Mn-,the Sr and oxygen isotope data,this article studies the cathodoluminescence features and diagenetic stage division in Buqu Formation of Jurassic carbonate rocks in Angdaercuo area of Southern Qiangtang Depression,Tibet. The results show that：（ 1） Carbonate rocks of Buqu Formation generally have dull cathodoluminescence,which results from lower Fe and Mn contents,indicating the limited influence of non-marine fluid during post-sedimentation and the diagenesis of the carbonates should have been governed by the marine fluid.（ 2） Different types of rocks with the increase of the strength of diagenesis,the content of Mn elements increased gradually,the content of Sr elements decreased gradually,the oxygen isotope values generally show the evolution characteristics of high negative direction migration,the corresponding cathodoluminescence intensity is gradually enhanced.（ 3） The mud-microcrystalline dolomite has the strongest cathodoluminescence and the highest Mn content. It may be related to the weakly reducing environment near the surface during dolomization. Evaporation pump and permeate reflux may be the main mechanism of the dolomization of these rocks. The crystalline dolomite has weak cathodoluminescence and low Mn content. The pore fluid may be far away from atmospheric freshwater,and the homogenization temperature of the inclusion shows that the dolomitization occurs in a deep-buried environment.（ 4） The Buqu Formation carbonate rock experienced a contemporaneous-penecontemporaneous stage of seawater diagenetic environment→Early diagenetic stage of near-surface-shallow burial diagenesis weak reduction environment→Middle-deep diagenetic stage of mid-deep burial of deep-cycle water and high-salinity formation water environment → Late-Cretaceous-Paleogene early tectonic thermal events hydrothermal reform stage → The exodiagenesis stage.