Origin of dolomites in the Cambrian (upper 3rd‐Furongian) formation, south‐eastern Sichuan Basin, China
Origin of dolomites in the Cambrian (upper 3rd‐Furongian) formation, south‐eastern Sichuan Basin, China
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DOI：
10.1111/gfl.12193
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发表时间：
2016-12
期刊：
影响因子：
1.7
通讯作者：
Mingcai Hou;W. Jiang;F. Xing;S. Xu;X. C. Liu;C. Xiao
中科院分区：
文献类型：
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作者：
Mingcai Hou;W. Jiang;F. Xing;S. Xu;X. C. Liu;C. Xiao
大型古白云岩的成因是沉积学研究的热点问题之一。四川盆地东南缘上三叠统芙蓉统娄山关群中含有大量的碳酸盐岩，虽然它们可能是良好的油气储集层，但其成因尚未见报道。本文通过岩石学、流体包裹体、碳氧同位素、微量元素和稀土元素等方面的系统分析，对该矿床的成因提供了一些独特的认识。研究区白云岩主要有四种类型：微晶白云岩、组构保持型鲕粒白云岩、组构闭塞型白云岩和鞍状白云岩胶结物。在微晶硅灰岩和组构保持型鲕粒白云岩中，高Sr含量（相对于组构闭塞型白云岩）和缺乏两相水包体表明它们在低温（<50-60°C）下形成于浅至中等埋藏深度。碳、氧同位素和类海水REE+Y特征表明，成矿流体为蒸发海水或轻微变质海水。原始沉积组构的消失和相对较低的δ 18 O和Sr值表明，组构闭塞白云岩形成于中深埋藏成岩作用。其化学成分接近纯白云岩，稀土元素和钇元素特征与组构固溶白云岩相似，表明组构闭塞白云岩是由组构固溶白云岩在埋藏深度和埋藏温度升高时重结晶而形成的。高均一温度（115-150°C）、低δ 18 O值、非平面α晶体和海水型REE+Y特征表明鞍状白云岩胶结物是由白云岩再沉淀形成的，与海水驱动和深埋藏流体有关。在研究区，微晶灰岩和组构保持型的鲕粒白云岩可能是在早期成岩作用中由准同生或渗流回流石化作用形成的。随后，在递进埋藏过程中，大部分的组构-保持白云岩转化为组构-闭塞白云岩的重结晶。这项研究证实，组构-闭塞白云岩是主要的白云岩类型，虽然深埋，这些寒武纪的白云岩最有可能保存同时代的海水地球化学信号。
The origin of large-scale ancient dolomite is one of the most hotly debated topics in sedimentology. The Loushanguan group of the upper 3rd-Furongian Cambrian series on the south-eastern margin of the Sichuan Basin consists of numerous dolomites, and the origins of these dolomites have never been reported previously although they are probably good hydrocarbon reservoirs. Based on a systematic analysis of petrology, fluid inclusions, carbon and oxygen isotopes, trace elements and rare earth elements (REEs), this study provides some unique insights into the origins of the dolomites. Four dolomite types have been identified in the study area: dolomicrite, fabric-retentive oolitic dolomite, fabric-obliterative dolomite and saddle dolomite cement. In the dolomicrite and fabric-retentive oolitic dolomite, high Sr contents (with respect to the fabric-obliterative dolomite) and the lack of two-phase aqueous inclusions suggest that they formed at shallow-to-intermediate burial depths at low temperatures (<50–60°C). Carbon and oxygen isotopes and seawater-like REE+Y characteristics of the dolomicrite and fabric-retentive oolitic dolomite indicate that the dolomitizing fluids were evaporated seawater or slightly modified seawater. The obliteration of the original sedimentary fabric and relatively low δ18O and Sr values compared to the fabric-retentive dolomite indicate that fabric-obliterative dolomite formed at intermediate-to-deep burial diagenesis. The chemical composition approaches pure dolomite and the REE+Y characteristics are similar to those of the fabric-retentive dolomite, indicating that the fabric-obliterative dolomite was formed due to the recrystallization of the previously formed fabric-retentive dolomite at elevated burial depths and temperatures. High fluid inclusion homogenization temperatures (115–150°C), low δ18O values, nonplanar-a crystals and seawater-like REE+Y characteristics suggest that saddle dolomite cement formed by reprecipitation of dolomite that related to seawater-driven and deep burial fluid. In the study area, dolomicrite and fabric-retentive oolitic dolomite may have been formed by penecontemporaneous or seepage-reflux dolomitization during early-stage diagenesis. Subsequently, during progressive burial, most of the fabric-retentive dolomite was converted into fabric-obliterative dolomite by recrystallization. This study confirms that fabric-obliterative dolomite was the main dolomite type, and although deeply buried, these Cambrian dolomites most likely have preserved coeval seawater geochemical signals.