鄂尔多斯盆地奥陶系马家沟组豹皮灰岩特征及其成因

Characteristics and genesis of leopard limestone of the Ordovician Majiagou Formation in Ordos Basin

在野外剖面观察与勘测的基础上,应用岩石薄片鉴定、地球化学分析和包裹体测温等方法,开展了鄂尔多斯盆地马家沟组沉积与成岩序列、碳酸盐岩岩相及地球化学特征研究,并分析和探讨了马四段和马六段豹皮灰岩的特征及其成因。鄂尔多斯盆地南部金粟山剖面马六段B亚段可划分为3个准层序组,其中下部2个准层序组以豹皮灰岩为主,且自下向上云斑的连续性及白云化程度呈现增强的趋势。云斑由有序度较高的自形—半自形白云石构成,具有相对较高的δ13C值和相对较低的δ18O及87Sr/86Sr值,贫Fe、Mn等元素,包裹体均一温度及盐度较高。研究结果表明,鄂尔多斯盆地马四段和马六段的豹皮灰岩主要形成于埋藏阶段,是低海平面期渗入生物钻孔的蒸发海水在埋深和地温增加过程中白云化作用的结果。

Based on the outcrops observation and survey, thin sections identification, geochemistry analysis and temperature measuring of fluid inclusions, we studied the lithofacies, geochemical character- istics and structural feature of the leopard limestone from the Members 4 and 6 of Ordovician Majiagou Formation （M4 ＆ M6 intervals） in Ordos Basin. In the Jinsushan section of southern 0rdos Basin, three pa- rasequence sets are divided by the eustasy in the M6B interval. The two parasequence sets in the lower part are mainly composed of leopard limestone, in which the continuity and ordering degree of the dolomite mottle are enhanced in ascending order. The dolomite mottle is comprised of euhedral to semihedral dolo- mites, with the relatively high value of δ^13C, relatively low value of δ^18C and ^87Sr/Sr^86, lower concentra- tion of Fe and Mn, and higher salinity ratio and inclusion homogenization temperature. The results show that the leopard limestone of M4 ＆ M6 intervals was mainly formed at the burial stage during which with the increasing buried depth and formation temperature, the dolomitization occurred by the evaprative sea- water in the boriny pore refluxed at low sea-level period.