滇东北乌蒙山地区峨眉地幔柱活动与火山-沉积盆地的响应关系

为加深滇东北乌蒙山地区峨眉地幔柱演化的认识,对该区的峨眉山玄武岩及下伏的栖霞组—茅口组开展了详细地质调查工作,并对峨眉山玄武岩进行了岩石学、同位素年代学研究,结果显示:峨眉地幔柱活动导致的地壳抬升形成了研究区栖霞组—茅口组西薄东厚、峨眉山玄武岩西厚东薄的地质特征,地壳开始隆升的时限为(273.1±3.1) Ma,地壳抬升约为512 m,但该过程是缓慢进行的,并未造成茅口组顶部灰岩的差异风化剥蚀,茅口组与峨眉山玄武岩基本上为连续沉积;乌蒙山地区的峨眉山玄武岩经历4次大规模的喷发,分别对应4个亚旋回,每一亚旋回以沉积夹层开始,后期为大规模的溢流相玄武岩,4次大规模喷发时限分别为(261.9±2.0) Ma、(261.8±3.7) Ma、(261.6±1.9) Ma、(261.5±2.1) Ma,主喷发期历时约0.4 Ma,地幔柱活动可能持续到(258.86±0.71) Ma;在同位素年代学数据基础上,依据栖霞组—茅口组厚度差异、峨眉山玄武岩各亚旋回横向及纵向变化特征、各亚旋回及沉积夹层的岩性和岩相特征,将峨眉地幔柱在乌蒙山地区的火山-沉积盆地演化关系划分为地幔柱隆升阶段(273.1~262.0 Ma)、玄武岩幕式喷发阶段(261.9~261.5 Ma)和地幔柱消亡阶段(261.5~258.86 Ma)。

Relation Between the Emeishan Mantle Plume Activity and Wumengshan Volcanic-Sedimentary Basin in Northeastern Yunnan

To deepen the understanding on the evolution of the Emeishan mantle plume in Wumengshan area of northeastern Yunnan,detailed geological investigations of the Emeishan flood basalt and the underlying Qixia-Maokou Formation in the area, as well as petrological and isotopic chronological studies of the Emeishan flood basalt have been carried out. The results show that the crustal uplift caused by the Emeishan mantle plume activity formed the geological characteristics of the Qixia-Maokou Formation, which is thin in the west but thick in the east, and the Emeishan flood basalt, which is thick in the west but thin in the east, and the crustal uplift started at (273.1±3.1) Ma, and the uplift was about 512 m. However, this process was slow and did not cause the differential weathering and denudation of the limestone at the top of the Maokou Formation. Besides, the Emeishan flood basalt and Maokou Formation were basically deposited continuously. The Emeishan flood basalt in the Wumengshan area may have undergone four large-scale eruptions, corresponding to four sub-cycles. Each sub-cycle started with a sedimentary interbed, covered by a large number of overflow basalt facies. The eruption timing of the four subcycles was determined to be (261.9±2.0) Ma, (261.8±3.7) Ma, (261.6±1.9) Ma, (261.5±2.1) Ma, respectively. The main eruption phase lasted for about 0.4 My,and the mantle plume activity may last until (258.86±0.71) Ma. Based on the isotope chronology data, according to the thickness difference of Qixia-Maokou Formation, the horizontal and vertical variation characteristics of each subcycle of Emeishan flood basalt, the lithology and lithofacies characteristics of each subcycle and sedimentary interbed, the volcanic-sedimentary basinal evolution of Emeishan mantle plume in Wumengshan can be divided into the mantle plume upwelling (273.1-262.0 Ma), the basalt eruption(261.9-261.5 Ma ), and the mantle plume extinction stages (261.5-258.86 Ma ).