川西坳陷彭州地区雷四上亚段白云岩形成机理

Genetic mechanism of the dolostones from the upper part of the fourth member of the Leikoupo Formation in the Pengzhou region, western Sichuan depression

关键词:  川西坳陷  雷四上亚段  潮坪相  白云化作用  成因机理

Key words:  western Sichuan depression  upper part of the fourth member of the Leikoupo Formation  tidal-flat facies  dolomitization  genetic mechanism

摘要:

根据岩心观察、普通薄片及铸体薄片鉴定、阴极发光显微镜观察、扫描电镜、碳氧稳定同位素测定、锶同位素测定、全岩分析、微量元素分析、X衍射分析等手段,对川西坳陷彭州地区雷口坡组四段上亚段白云岩类型及成因机理进行了详细的研究。研究区雷四上亚段白云岩晶粒细,主要由微晶-粉晶组成,细晶白云岩少见,白云石呈半自形-自形晶;δ¹³C同位素在-4.8‰~3‰之间,平均值为1.92‰,呈现低正值的特征;δ¹⁸O同位素在-8.1‰~0.6‰之间,平均值为-5.04‰,呈现中负值的特征;锶同位素比值较低,在0.7077~0.7087之间,二价铁离子含量较高、锰离子含量较低,具有较低的有序度和低的Mg/Ca值的特点。各种资料表明,这套展布稳定的白云岩主要为沉积成因,为潮坪相潮间-潮上带的同生-准同生白云岩,其次为埋藏白云石化成因。

Abstract:

The types and genetic mechanism are examined in details for the dolostones from the upper part of the fourth member of the Leikoupo Formation in the Pengzhou region, western Sichuan depression on the basis of core observation, thin section examination, cathodoluminescence images, scanning electronic microscope, carbon, oxygen and strontium isotopic measurement, whole-rock analysis, trace element analysis and X-ray diffraction analysis. The deeply buried dolostones in the study area mainly consist of micritic and silty dolostones with fine grains, and euhedral and subhedral crystals. The finely crystalline dolostones are rarely observed. The δ¹³C values range between -4.8‰ and 3‰, with an average of 1.92‰, indicating low positive values. The δ¹³O values vary between -8.1‰ and 0.6‰, with an average of -5.04‰, indicating moderately negative values. The ⁸⁷Sr/⁸⁶Sr ratios vary from 0.7077 to 0.7087. The dolostones in the study area are characterized by higher Fe²⁺ contents, lower Mn²⁺ contents, lower ordering and Mg/Ca ratios. They have firstly a sedimentary origin and secondly a burial origin, representing the intertidal-supertidal syngenetic and parasyngenetic dolostones.