白云岩成岩相与地球化学特征——以鄂尔多斯盆地中东部马家沟组马五5—马五1亚段为例

摘要

白云岩成岩相研究对恢复其成岩环境具有重要意义.以鄂尔多斯盆地中东部马家沟组马五5—马五1亚段为例,利用岩芯及岩石薄片研究了白云岩的成岩相类型及岩相特征,并根据氧、碳稳定同位素、锶同位素以及主、微量元素等分析了各类成岩相的地球化学特征.结果显示,鄂尔多斯盆地中东部马五5—马五1亚段白云岩成岩相主要包括近地表环境下的膏盐溶蚀角砾相、活跃回流渗透云化相和隐伏回流渗透云化相,以及表生淡水环境下的风化岩溶角砾相等四类,其中受淡水或混合水影响的近地表环境下的膏盐溶蚀角砾相、活跃回流渗透云化相及隐伏回流渗透云化相白云岩样品具有相对奥陶纪海水"负偏"的δ18 O和δ13 C值,相对高Mn或低Fe含量,中等—强阴极发光,高于同期海水的87 Sr/86 Sr值以及相对较低的Al和Ti含量;而表生淡水环境下的风化岩溶角砾相白云岩样品因受表生期淡水以及大量陆源碎屑物的影响,呈现出低于同期海水的δ18 O和δ13 C值,相对高Mn和过高Fe含量,弱或无阴极发光,明显高于同期海水的87 Sr/86 Sr值以及明显较高Al和Ti含量.研究结果表明,氧、碳稳定同位素、锶同位素及主、微量元素等地化数据是识别白云岩成岩相的有效指标.%It is important to study on the diagenetic facies of dolomite for the restoration of its diagenetic environment. The cores and thin sections as well as the whole rock major and trace element geochemistry, O, C and Sr isotopic compositions were analyzed to study the diagenetic facies types and lithofacies of the dolomite as well as the geochem-istry of different diagenetic facies of Ma55-Ma51 sub-members of Majiagou Formation in the central-eastern Ordos Ba-sin, respectively. The results show that the diagenetic facies of dolomite of Ma55-Ma51 sub-members in central-eastern Ordos Basin mainly include the gypsum and salt dissolution, active reflow seepage dolomitization and implicit reflux seepage dolomitization diagenetic facies in a near-surface diagenetic tract, and weathering karstific breccia diagenetic facies in an epidiagenetic tract. The former dolomite samples display lowerδ18 O andδ13 C values than those of the Or-dovician seawater, higher Mn and lower Fe contents, medium-strong cathodoluminescence, higher 87 Sr/ 86 Sr ratios than the coeval seawater as well as their high Al and Ti contents may be caused by the effects of fresh water or mixed water, while the latter dolomite samples may be affected by the epigenetic coeval seawater shows relatively high Mn and significant high Fe contents, weak or non-cathodoluminescence, obvious higher 87 Sr/ 86 Sr values than those of the coeval seawater, significant high Al and Ti contents. The results implicate that theδ18 O ,δ13 C, 87 Sr/ 86 Sr, major and trace elements are the effective indicators for identifying the diagenetic facies of dolomite.