论文部分内容阅读
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δ18 O值在-16.0‰～-1.9‰(VPDB标准,下同)之间,平均为-8.5‰;δ13 C值在-16.1‰～4.83‰之间,平均为-0.90‰。激光显微采样样品δ18 O值在-15.8‰～-0.8‰之间,平均为-7.7‰;δ13 C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δ18 O最大值均为5.8‰,代表原始碳酸盐岩的δ18 O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δ18 O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δ18 O值。原始沉积的白云石或白云岩δ18 O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δ18 O值。碳酸盐岩δ13C最大值明显偏高,与全球海水的δ13 C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。
The Middle Ordovician Majiagou Formation in the Ordos Basin is deposited on confined, high salinity land surface sea carbonate platforms, dominated by dolomite, dolomitic limestone and evaporite. Some of the rocks were transformed into karst breccias by karstification. The δ18O value of the whole rock or hand sample of carbonate rocks in Majiagou Formation ranges from -16.0 ‰ to -1.9 ‰ (VPDB standard, the same below) with an average of -8.5 ‰; the δ13C value ranges from -16.1 ‰ ~ 4.83 ‰, with an average of -0.90 ‰. The δ18O value of the laser microdissected sample was between -15.8 ‰ and -0.8 ‰, with an average of -7.7 ‰; the δ13C value was between -12.4 ‰ and 5.77 ‰ (VPDB standard and the same below) with an average of -0.15 ‰ . Overall, the results of the two sampling methods are not significantly different, the latter more accurately reflect the characteristics of different components. The δ18O values of the original sedimentary calcite and limestone are both 5.8 ‰, representing the δ18O values of the original carbonate rocks, which accords with the results of most of the scholars’ global isotopic stratigraphy. Due to the freshwater leaching and burial, the minimum and average δ18 O values of the primary sedimentary calcite and limestone are lower. The calcite filled in the pores is affected by freshwater leaching and high temperature burial, and usually has a low δ18O value. The δ18O values of the dolomite or dolomite originally deposited were significantly higher than those of the original sedimentary calcite and limestone. Dolomite was deposited under the high salinity and confined conditions and formed by the microbial dolomite mechanism. Dolomite filling the pores has a lower δ18O value because of the late formation. The δ13C maximum value of carbonate rocks is obviously higher than that of the global seawater, which is related to the increase of δ13C value in the Late Ordovician in the world. The δ13C value of seawater in the Ordos Basin has risen in the late Middle Ordovician. Affected by organic carbon, some samples have low δ13C values.