再论鄂西地区见天坝生物礁白云岩的成因  

Re-discussion on the genesis of bioherm dolomites in the Jiantianba area of western Hubei province,China

作　　者：陶艳忠[1,2] 蒋裕强[1,2] 强子同[1,2] 范嘉松[3] 朱讯[1,2] 王珏博[1,2] 

机构地区：[1]西南石油大学地球科学与技术学院 [2]西南石油大学天然气地质四川省重点实验室 [3]中国科学院地质与地球物理研究所

出　　处：《天然气工业》2014年第7期19-26,共8页Natural Gas Industry

基　　金：国家自然科学基金项目"川东鄂西地区晚二叠世生物礁系统演化环境生态动力学研究"(编号:41272135)

摘　　要：川东—鄂西地区上二叠统长兴组发育碳酸盐台地沉积体系,对该区生物礁白云岩的成因认识还存在争议。为此,以该区见天坝生物礁白云岩为例,采用岩石学、地球化学等综合分析方法,探讨了生物礁白云岩的成因。岩石学特征研究结果表明,在白云岩与石灰岩的过渡带上,一些白云石晶体沿缝合线分布或覆盖在缝合线上。通过测定礁白云岩、溶孔中鞍状白云石、石灰岩的微量元素与稳定同位素含量、包裹体温度,发现白云岩的稳定同位素氧比海水的稳定同位素氧数值明显地偏负,白云岩中的Mn和Fe含量比被交代石灰岩中的Mn和Fe含量要高,而Sr和Na含量则相对更低。结论认为：①该区礁灰岩的白云石化作用是在埋藏条件下发生的,具有埋藏白云石化的特征;②鞍状白云石两相流体包裹体的均一化温度（Th）介于190~210℃,高于长兴组的地层温度,这一异常现象说明白云石化作用与下伏深盆地低温热液有关,见天坝生物礁白云岩可能是热液白云岩。Carbonate platform sedimentary systems are well developed in the Upper Permian Changqing Fm in the area from eastern Sichuan to western Hubei province. However, it is still controversial about the genesis of bioherm dolomites in this area. Taking the bioherm dolomites in the Jiantianba area as an example, we studied the genesis of bioherm dolomites by using petrological and geochemical methods. The study of the petrological features reveals that dolomite crystals grow along fissures or overlay the fissures in the transitional zone from dolomites to limestones. Measurements of trace elements and stable isotope content as well as inclusion temperatures of reef-dolomites, saddle dolomites in dissolution pores and limestones show that the dolomites have a significantly lower stable isotope of oxygen than sea water, and a higher content of Mn and Fe but a lower content of Sr and Na than replacement limestones. Two conclusions were obtained. （1） The dolomitization of reef limestones occurred under burial conditions, thus show the features of burial dolomitization. （2） The homogenization temperature （Th） of two-phase fluid inclusions in the saddle dolomites is in the range of 190 - 210 ℃, higher than the formation temperature of Changxing Fro, an abnormal phenomenon indicating that the dolomitization is related with the low temperature thermal fluid in the underlying deep basin. Therefore, it is believed that the bioherm dolomites in the study area are probably of thermal fluid dolomites in nature.

关 键 词：川东—鄂西地区 晚二叠世 白云石化 生物礁 成岩作用 稳定同位素 微量元素 包裹体测温 热液作用 

分 类 号：P618.13[天文地球—矿床学]