Abstract:

Based on comprehensive analysis of field work, petrological and geochemical characteristics, the authors study the sources of dolomitizing fluids and the models of dolomitization of the Lower Ordovician Penglaiba Formation in Tongguzibulong Outcrop, the northwestern margin of Tarim Basin, China. Four types of
dolomite are recognized: euhedral-subhedral powder crystallized dolomite, euhedral-subhedral fine crystallized dolomite, subhedral-xenotopic medium crystallized dolomite and subhedral-xenotopic coarse crystallized dolomite. Powder-fine crystallized dolomite is distributed in the lower part of Penglaiba Formation, and has cloudy center surrounded by clear rim. With residual sand texture, inter-crystal pores and inter-partical pores, medium-coarse crystallized dolomite is distributed in the upper part of Penglaiba Formation. The REE patterns of dolomite and contemporaneous limestone rich in LREE and deplete in HREE, present a trait of unobvious Ce anomaly and Eu negative anomaly, the values of C-O isotope locate in the scope of contemporaneous marine dolomite, and Eu negative anomaly. All these denote that the dolomitizing fluid is normal or slightly concentrated seawater. Besides, Powder-fine crystallized dolomite present a low value of Fe, Mn and a high value of Sr, Ba, formed by reflux seepage dolomitization in penesaline seawater. Cloudy center surrounded by clear rim texture and multi-rimmed texture are the results of over-dolomitization. Medium-coarse crystallized dolomite with residual grain texture, interbedded with sand limestone, are controlled by high frequency sea level change. With a higher value of Fe, Mn and a lower value of Sr, Ba when compared with powder-fine crystallized dolomite, medium-coarse dolomite formed by the early reflux seepage dolomitization and intensified by the subsequent burial recrystallization.

Key words: Tarim Basin, Tongguzibulong Outcrop, Penglaiba Formation, Mg-rich-fluids, dolomitization

摘要：

通过剖面观测、岩石学特征对比和地球化学分析, 对塔里木盆地西北缘通古孜布隆剖面蓬莱坝组白云岩的白云石化流体来源和白云岩成因进行探讨。观测结果表明: 白云岩的主要类型有粉晶自形?半自形白云岩、细晶自形?半自形白云岩、中晶半自形?它形白云岩和粗晶半自形?它形白云岩; 粉?细晶白云岩主要分布于蓬莱坝组下段, 粉晶白云岩质地均一, 细晶白云岩普遍发育雾心亮边; 中?粗晶白云岩主要分布于蓬莱坝组上段, 并与砂屑灰岩高频次互层, 发育残余颗粒结构和少量晶间孔、残余粒间孔。地球化学分析结果表明: 各类白云岩稀土元素配分模式与同期灰岩相似, 具有LREE富集、HREE亏损、Ce异常不明显、Eu负异常的特征, 碳、氧同位素值位于同期海相白云石范围内, 指示白云石化流体为同期正常或偏咸海水; 粉?细晶白云岩具有低Fe, Mn和高Sr, Ba的特征, 为近地表中等盐度海水渗透回流作用的产物, 多期次流体作用使细晶白云岩发生过度白云石化, 形成雾心亮边或环带; 中?粗晶白云岩在近地表浅埋藏阶段就被半局限台地环境下蒸发作用产生的偏咸海水交代, 高频海平面变化导致周期性的白云石化作用, 从而导致砂屑灰岩与残余颗粒白云岩周期性叠置, 阴极发光显微镜下可见次生加大边, 与粉?细晶白云岩相比, 具有Fe, Mn含量增高而Sr, Ba含量降低的特征, 表明在后期受埋藏作用的影响发生重结晶, 具有早期渗透回流形成、中期埋藏重结晶加强的成因特征。

关键词: 塔里木盆地, 通古孜布隆剖面, 蓬莱坝组, 白云石化流体, 白云岩成因