中新元古界原生白云岩: 以中国典型台地区为例*

基于对中国塔里木盆地新元古界上震旦统奇格布拉克组、四川盆地新元古界上震旦统灯影组以及华北盆地中元古界蓟县系雾迷山组野外露头及钻井岩心白云岩的系统观察,发现中国典型台地区中新元古界中厚层至块状白云岩异常发育,其中泥(粉)晶白云岩占岩层总厚度86%~97%,且大多为厚层至块状。这些中厚层至块状泥(粉)晶白云岩,无论是否含藻类与菌类等微生物,次生交代作用基本都不发育。沉积古地理特征分析结果表明: (1)中新元古界的白云岩几乎覆盖整个海盆,广泛沉积于潮上带、潮间带以及潮下的开阔或局限台地环境;(2)白云岩的亚类在平面上受古基底隆凹格局控制,低凹部位以泥晶白云岩为主,高部位以颗粒泥晶白云岩或丘状叠层石泥晶白云岩为主;(3)白云岩台地中发育较深水台盆。地震剖面显示,这些台盆的形成受控于中新元古代的同生深大断裂活动,大断裂可将深部的Mg²⁺通过热液输入到碳酸盐岩台地中,使得海水中的Mg²⁺浓度增加。地球化学分析结果与古气候特征指示,不同晶粒结构的白云岩,即泥晶白云岩与少量的粉晶及细晶白云岩,其碳氧同位素比值没有明显分异,整体上与全球中新元古代海水背景值吻合,表明粉晶及粉细晶白云岩与泥晶白云岩同为沉积成因,其结构加粗乃原生泥晶白云石晶粒重结晶自生加大而成,并非次生交代成因,即这些少量发育的粉晶—细晶白云岩与主体的泥晶白云岩为同一成因机制。整合岩石学、沉积环境及地球化学等分析结果,认为中国典型台地区中新元古界的白云岩乃较典型的原生沉积成因。

Primary dolostones of the Meso-Neoproterozoic:Cases on typical platforms in China

Based on the exploration of dolostones in outcrops and drilling cores of the Wumishan Formation in the Mesoproterozoic Jixian System in the North China Basin, the Qigbulak Formation in the Neoproterozoic Sinian Series in the Tarim Basin, and the Dengying Formation in the Neoproterozoic Sinian Series in the Sichuan Basin of China, the Meso-Neoproterozoic dolostones show abnormally well-developed medium-thick layered to massive structures. These dolostones contain 86%-97% of dolomicrites, most of which are thick layered to massive. The secondary metasomatism of these dolostones is undeveloped, no matter whether they contain algae, fungi and other microorganisms or not. Sedimentary palaeogeography analyses show: (1) the dolostones in the Meso-Neoproterozoic nearly covered the entire basin, widely deposited in supra- to intertidal zones and open to limited platform environments; (2) Different types of the dolostones were controlled by pre-depositional basement geomorphology. Dolomicrites were mainly developed in depression areas, whereas grainy dolomicrites and domal stromatolitic dolomicrites were mainly developed in uplift areas. (3) Deep-water basins were developed in the carbonate platform, and the seismic profile shows that these basins in the platforms were formed by syngenetic deep faults in the Meso-Neoproterozoic. These faults also led to concentration of magnesium ions in seawater through hydrothermal fluid into carbonate platforms. Geochemical and ancient climate index of dolostones with different sedimentary structures show that their carbon and oxygen isotope ratios have no obvious differentiation, and the ratios are similar to that of the Meso-Neoproterozoic global seawaters, suggesting the fine crystalline dolostones have the same sedimentary origin as the dolomicrites. The fine crystalline dolostones might have resulted from authigenic recrystallization of dolomicrites, rather than secondary metasomatism. All the petrological, sedimentary environmental and geochemical data collectively suggest that the dolostones covered almost the entire Meso-Neoproterozoic typical platform areas of China are of typical primary sedimentary origin.