塔里木盆地玉北地区蓬莱坝组白云岩类型及成因

The types and origin of the Penglaiba Formation dolomite in the Yubei area,Tarim Basin

塔里木盆地玉北地区蓬莱坝组发育粉-细晶自形白云岩、中晶自形-半自形白云岩、粗晶自形-他形白云岩三种白云岩类型。粉-细晶白云岩具纹层状构造,稀土元素配分模式、总稀土含量与同期灰岩相似,氧同位素组成与同期海水成因的白云岩相近,表明白云岩化流体为同期海水,较高的盐度指数指示了相对浓缩的海水条件,较低的有序度值反应出快速的白云岩化过程,为准同生白云岩化的产物,白云石晶间均匀充填富Fe、富Si及高^(87)Sr/^(86)Sr比值的陆源物质,表明玉北地区在蓬莱坝组粉-细晶白云岩沉积期由于相对海平面较低从而受到陆源物质混入的影响。中晶白云岩和粗晶白云岩的稀土元素配分模式、总稀土含量、^(87)Sr/^(86)Sr比值等与同期海水特征相似,表明白云岩化流体为海水,可见残余颗粒结构,成岩温度较低,埋藏较浅,为埋藏的海水以及沿原始颗粒灰岩的粒间孔及可能存在的裂缝等通道向下运移的海水提供的Mg^(2+)导致的白云岩化,后期重结晶作用破坏了粗晶白云岩中的残余颗粒结构并导致其Fe、Mn含量及成岩温度、有序度值高于中晶白云岩,Sr含量及δ^(18)O值低于中晶白云岩。部分中-粗晶白云石边缘可见加大边,表明后期存在少量他源流体导致的次生加大作用。

The Penglaiba Formation dolomites in the Yubei area can be divided into three types, which are laminated micritic-fine euhedral dolomite, medium euhedral-subhedral dolomite, coarse euhedral-anhedral dolomite respectively. The lamina of penecontemporaneous micritic-fine dolomite can be observed from both the core and the thin section. The REE normalized pattern and ∑REE of micritic-fine dolomite resemble the characters of contemporaneous limestone. The δ18O shows similarity with the dolomite precipitated from the contemporaneous seawater. Those characters indicate that the dolomitization fluid of micritic-fine dolomite is contemporaneous seawater. The high salinity index shows a concentrated fluid character and in such an environment the rapid crystallization process results in less ordered dolomite. The high contents of Fe, Si and high 87Sr/86Sr in the intercrystal pores of dolostone demonstrate a relative low sea level and the mix of terrigenous material. The seepage reflux originated medium dolomite and coarse dolomite also have similar REE normalized pattern, ∑REE and 87Sr/86Sr with the contemporaneous seawater, which show the seawater is still responsible for the dolomitization. The residual particles structure and the low diagenesis temperature calculated from the δ18O show that the reflux of seawater along the intergranular pores and fractures results in the dolomitization. The coarse dolomite experiences stronger recrystallization process than the medium dolomite during which the residual particles are destroyed and the value of Fe, Mn, δ18O are higher while Sr are expelled. The secondary outgrowth can also be recognized from the cathode-luminescence image from which we can figure out an obvious brighter margin in some dolostones.