Massive Burial Dolomitization: The Jurassic Vajont Oolite of Northeastern Italy

大块替代白云岩的成因一直是沉积学研究中未解决的主要问题之一。目前对块状白云岩起源的解释主要集中在涉及地表水或近地表水的“早期”低温过程，而诸如“晚期”埋藏白云化等其他过程，除了少数工作者外，都被忽视了。沉积盆地地下水流动系统的理论模型和计算机模型的最新发展，使得对埋藏白云化进行新认识的时机成熟。流体流动是我们在解决白云岩问题的努力中需要加入的一个具有重大意义的新因素。本文对意大利北部威尼斯阿尔卑斯山脉侏罗纪瓦扬特鲕粒白云岩进行了研究，初步的野外和地球化学工作表明，白云化发生在埋藏条件下的高温下。该方法将整合野外工作、岩石学、同位素和流体包裹体数据以及水文建模。Vajont Oolite有几个特别的特点，值得进行这样的研究：(1)它是大规模的白云化，但不是完全的白云化，因此可以绘制白云化前沿，为白云化流体的地下路径提供直接线索（见Newton， Hardie和Phillips， 1990）；(2)浅水鲕粒在蚀变前经斜坡作用在深水中再沉积，排除浅水白云石化作用；(3)岩浆岩及其粒间胶结物保存较好，可详细记录其白云化前岩石学和同位素地球化学特征；因此，可以识别和测量白云化产生的变化；(4)巷道中部分但大量白云化的Vajont的暴露表明，白云化流体通道受晚第三纪（阿尔卑斯造山）断裂和断面控制，明确了白云化是“晚”埋藏成因。这项研究的结果可以揭示深层循环的地下流体在块状白云岩起源中的作用，特别是那些作为石油、天然气和热液矿石储集层的白云岩。

The origin of massive replacement dolomite remains one of the major unresolved problems of sedimentology. Presently favored interpretations for the origin of massive dolomites have centered on "early" low temperature processes involving surface or near- surface waters, and alternative processes such as "late" burial dolomitization have been overlooked by all but a few workers. The recent development of theoretical and computer models of groundwater flow systems in sedimentary basins makes the time ripe for a new look at burial dolomitization. Fluid flow is the one new factor of major significance that needs to be added to our attempts to resolve the dolomite problem. This is a study of the dolomite of the Jurassic Vajont Oolite of the Venetian Alps in northern Italy where preliminary field and geochemical work indicates that dolomitization occurred at elevated temperatures under burial conditions. The approach will be to integrate fieldwork, petrography, isotope and fluid inclusion data, and hydrologic modeling. The Vajont Oolite offers several special features that make such a study worthwhile: (1) it is massively but not completely dolomitized so that dolomitization fronts that give direct clues to the subsurface pathways of the dolomitizing fluids can be mapped (see Newton, Hardie and Phillips, 1990); (2) it consists of shallow water ooids that were resedimented in deep water by slope processes before alteration, thus shallow water dolomitization processes are ruled out; (3) the ooids and their intergranular cements are well preserved in the undolomitized parts of the Vajont sot hat it is possible to document in detail their pre-dolomitization petrography and isotope geochemistry; thus, the changes produced by dolomitization can be identified and measured; (4) exposures of partly but massively dolomitized Vajont in roadcuts reveal that dolomitization fluid pathways were controlled by fracture and fault planes of Late Tertiary age (Alpine orogeny), indication unequivocally that the dolomitization was of "late" burial origin. The results of the study could shed new light on the role of deeply circulated subsurface fluids in the origin of massive dolomites, particularly those that act as reservoirs to oil, gas, and hydrothermal ores.