鄂尔多斯盆地苏里格地区奥陶系马家沟组马五段白云岩的地球化学特征

Geochemical Characteristics of Dolomite from 5th Member of the Ordovician Majiagou Formation in Sulige Area,Ordos Basin

苏里格地区奥陶系马家沟组马五段白云岩是重要的油气储集层。多年来,马家沟组白云岩的成因一直是地质研究者讨论的热点话题。对苏里格地区马家沟组马五段白云岩的微量元素及C、O、Sr同位素地球化学资料的分析表明,微量元素表现为高铁、低锰、低锶、低钠;白云石、白云质角砾和孔洞充填白云岩的δ13C和δ18O值较为近似,且白云岩的δ18O值比奥陶纪海水δ18O值要偏负0.81‰～5.68‰(平均为3.442‰);87Sr/86Sr比值比奥陶纪海水比值要高。从微量元素分析,马五段白云岩为埋藏条件下形成;从C、O同位素数据上看,形成白云岩和沉淀于孔洞中的白云石的流体为同源流体;造成白云岩的值偏负的原因主要是埋藏条件的温度效应;87Sr/86Sr比值偏高可能是交代流体来自或流经了富含放射成因的铝硅酸盐基底或硅质碎屑岩。这些特征表明马五段白云岩可能形成于埋藏环境。

The dolomite in the 5th Member of the Ordovician Majiagou Formation is a significant reservoir in Sulige area.The origin of the Ordovician Majiagou Formation has been a hot topic for many years.The trace element and carbon,oxygen,strontium isotope compositions of dolomite developed in this deposit were determined in this study.The results show that the content of manganese,sodium and strontium is low.Meanwhile, the values of isotopes ofδ 13 C andδ 18 O in secondary void-filling dolomite are much similar to those of the dolomite and dolomitic breccia.Theδ1 8O values of the dolomite are in the range of 0.81‰~5.68‰（averagely 3.442‰）,lower than those of the Ordovician seawater.The 87 Sr/ 86 Sr ratio is higher than that of the Ordovician seawater.An analysis shows that the fluids that generated dolomite and void-filling dolomite might have come from the same fluid source.The negative values ofδ1 8O of dolomite result probably from temperature effect of the burial process.The high values of 87 Sr/ 86 Sr may be explained by the hypothesis that metasomatic fluids came from or flew though aluminosilicate basal rock or siliciclastic rock,which had rich radioactive strontium.Those characteristics of dolomite from 5th Member of Majiagou Formation were probably formed in a burial environment.