摘要
蓬莱坝剖面位于塔里木盆地北缘巴楚地区,发育有中上寒武统-下奥陶统的白云岩.其中上寒武统白云岩局部可见由易于溶蚀白云岩和难于溶蚀白云岩组成的薄互层,层厚为几十厘米至一米不等.易于溶蚀的白云岩层向内凹入,并在表面见有较多的溶蚀孔洞;难于溶蚀的白云岩层向外凸出,表面光滑.地球化学分析表明,溶蚀程度不同的白云岩具有相似的碳氧同位素组成,与同时期正常沉积碳酸盐岩的同位素组成相当.但二者在锶同位素组成与矿物含量上有着显著差异:易于溶蚀和难于溶蚀白云岩样品的Sr同位素之比,即w(87 Sr)/w(86 Sr)比值范围分别为0.708 814～0.709 351和0.709 091～0.710 202之间,平均值分别为0.709 141和0.709 500;前者不但在总体上低于后者,而且每一个易于溶蚀的白云岩样品的值要低于相邻的较早时期形成的难于溶蚀的白云岩的值;在矿物组成方面,易于溶蚀白云岩中的石英体积分数显著低于难于溶蚀白云岩中的,二者平均值分别为0.90%和4.37%.综合分析认为,易于溶蚀和难于溶蚀的白云岩分别形成于海平面相对较高和相对较低的环境中.在海平面相对较高的环境中形成的白云岩具有较粗的晶体结构组成,并具有较多的原生孔隙,是次生溶蚀易于发生的关键因素.
There are well developed middle-upper Cambrian to lower Ordovician dolomites in Bachu area in the northern bound of Tarim Basin.Some thin interbeds of dolomite with different dissolution degree can be found locally in the middle-upper Cambrian,whose thickness is from dozens of cm to 1m.The easily-dissolved dolomite layers are concave inward,and there are a lot of dissolution pores in the surface of the layers.The difficultly-dissolved dolomite layers are convex outward,and the dissolution pores in the surface are sparse.The carbon and oxygen isotope compositions of the dolomite samples with different dissolution degree are similar,and the carbon and oxygen isotopes are in the same ranges as those the contemporaneous normal marine carbonates.But there is significant difference in strontium isotope composition and mineral content between the easily-dissolved dolomite and the difficultly-dissolved dolomite although they are in the range of the normal marine carbonates.The 87Sr/86Sr of the easily and difficultly-dissolved dolomites are in the range of 0.708 814~0.709 351 and 0.709 091~0.710 202 respectively,and the average values are 0.709 141 and 0.709 500 respectively.Not only are the 87Sr/86Sr values of the easily-dissolved dolomite samples lower than those of the difficultly-dissolved dolomite samples in total,but also the value of each easily-dissolved sample is lower than that of the nearby difficultly-dissolved dolomite sample which was formed earlier.The dolomites of different dissolution degree are composed of similar minerals,namely the dolomite and quartz,but the content of each mineral is different.The quartz content of the easily-dissolved dolomite is notably lower than that of the difficultly-dissolved dolomite,and the average volume fractions of quartz in them are 0.90% and 4.73% respectively.According to integrated analyses,it is held that the easily and difficultly-dissolved dolomites are formed in relatively high and low sea level conditions,respectively.The dolomite formed under high sea level condition is not only relatively coarse in mineral crystal composition but also has many original pores which are the key factor to the secondary dissolution to develop.
出处
《西安石油大学学报（自然科学版）》
CAS
北大核心
2011年第5期1-5,113,共5页
Journal of Xi’an Shiyou University（Natural Science Edition）
基金
国家自然科学基金项目(编号:40872088)
中国博士后科学基金(编号:20100470500)资助
