鄂尔多斯盆地中奥陶统马家沟组碳酸盐岩碳、氧稳定同位素特征

鄂尔多斯盆地中奥陶统马家沟组沉积于局限的、高盐度陆表海碳酸盐岩台地，以泥晶白云岩、泥晶石灰岩和蒸发岩为主。部分岩石经岩溶作用改造成岩溶角砾岩。马家沟组碳酸盐岩全岩或手选样品的δ¹⁸O值在-16.0‰~-1.9‰（VPDB标准，下同）之间，平均为-8.5‰；δ¹³C值在-16.1‰~4.83‰之间，平均为-0.90‰。激光显微采样样品δ¹⁸O值在-15.8‰~-0.8‰之间，平均为-7.7‰；δ¹³C值在-12.4‰~5.77‰（VPDB标准，下同）之间，平均为-0.15‰。总的来说，2种采样方法的结果没有大的差别，后者较前者能更准确地反映不同组分特征。原始沉积方解石和石灰岩δ¹⁸O最大值均为5.8‰，代表原始碳酸盐岩的δ¹⁸O值，与大多数学者的全球同位素地层学研究成果吻合。由于淡水淋滤和埋藏作用，原始沉积方解石和石灰岩δ¹⁸O最小值和平均值均较低。充填于孔隙的方解石受淡水淋滤和高温埋藏作用影响，一般有较低的δ¹⁸O值。原始沉积的白云石或白云岩δ¹⁸O最大值较原始沉积方解石和石灰岩相应值明显偏高，白云石沉积于高盐度、局限环境条件下，由微生物白云石机理形成。因形成较晚，充填于孔隙的白云石有较低的δ¹⁸O值。碳酸盐岩δ¹³C最大值明显偏高，与全球海水的δ¹³C值在晚奥陶世升高有关，鄂尔多斯盆地海水δ¹³C值在中奥陶世晚期已升高。受有机碳影响，部分样品具有低的δ¹³C值。

Characteristics of Oxygen and Carbon Stable Isotopes for Middle Ordovician Majiagou Formation Carbonate Rocks in the Ordos Basin

The middle Ordovician Majiagou Formation,which mainly consists of micritic dolomite and limestone,and evaporates,deposited in a restricted epeiric carbonate platform with high salinity in the Ordos basin,China.A part of rocks were reformed into karst breccias by karstification.The δ18O and δ13C values for the whole rocks or hand-picked carbonate rocks in the middle Ordovician Majiagou Formation range from-16.0‰ to-1.9‰(average value =-8.5‰,VPDB) and from-16.1‰ to 4.83‰(average value=-0.90‰,VPDB),respectively.The δ18O and δ13C values for the laser-micro sampled carbonates in the Majiagou Formation range from-15.8‰ to-0.8‰(average value=-7.7‰,VPDB) and from-12.4‰ to 5.77‰(average value=-0.15‰,VPDB),respectively.Although the data measured by two sampling methods do not show the difference,the data measured by the later method give more information about different carbonate components.The originally deposited calcite and limestone have the maximum δ18O value of-5.8‰(VPDB),indicating that it would be δ18O values of original marine carbonates.This result is consistent with the published results of global stable isotope sequence.The means and minimums of the δ18O values for the originally deposited calcite and limestone are low due to meteoric leaching and burial diagenesis.The δ18O values of calcite cements filled in the dissolution pores are generally low under action of meteoric leaching and high temperature burial.The maximum of δ18O values for the originally deposited dolomite formed in the restricted environments with high salinity is obviously higher than that of the originally deposited calcite and limestone.The δ18O values for the dolomite filled in pores are generally low since its formation time was later.The maximum increase of δ13C values for the middle Ordovician marine carbonate in Ordos basin would be related to the increase of δ13C value for global marine water during late Ordovician,since the δ13C values for marine water in middle and later Ordovician onset increase.The low δ13C values for a part of samples are related with the organic carbon mixing.