| 峨眉地幔柱的动力学特征 |
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| 关键词:Emei mantle plume dynamical characteristics magma activities |
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| Dynamical Characteristics of the Emei Mantle Plume |
| The Emei mantle plume originated from nearby equatorial area. The diameter of the plume head was about 800km. It's dynamical characteristics are embodied mainly in four geological events.(1)Basic magma activities. Decompression melting in the plume head induced basalt eruptions voluminously and basic-ultrabasic intrusions in large. The area covered by the Emeishan Basalt is approximately 0. 5Mkm2. Major eruptive stage was between the Early and Late Permian age, but the duration of the activities was probably from Late Hercynian to indosinian epoch. The mean compositions of the basalts belong to the mild alkaline one, they are enriched in Fe.Ti and highly incompatible elements and the light rare earth elements especially. In Hongge and Baima intursions, Nd(t)<1 and is approximately equal to the value of the original mantle. In Panzhihua intrusion, Nd(t) = 3. 3 and may be have some oceanic crust materials.(2)Acid magma activities. The crust remelting produced by the plume thermal anomaly induced acid magma extrusion in large and formed granite belt of 300km long. As acid extrusive products, in addition to the liparite, acid ashes are scattered over the whole South China. It formed various clay rocks, including Tonteins.Boundary Clayrocks "Mungbean" rocks and partial silicalites. Violent acid extrusion was from the Late Permian to the early Middle Triassic and the peak value of the volcanism was in late Changxingian. The acidic intrusion occured for Indosinian -Yanshanian. It possesses the alkali granite (type A), the biotite K-feldspar granite (type I)and the alkali-feldspar granite (type S), they are associated with the lay6red basic intrusions.(3)Pale-geothermal field and reworking metallization. Under the plume activities, a huge geothermal field have occured in the upper Yangtze and it's outlying regions. The geothermal anomaly was from the Late Hercynian to the Yanshanian. The plume thermal anomaly drived the cycle of the hydrothermal fluids, produced reworkmg metallization and formed widespread strata -bound metallic ore deposit. It exerted an significant influence on the evolution of oil and gas in Sichuan basin and the coal metamorphism in Southwest China. The present papr also discuss some concepts about reworking metallization.(4)Uplift, subsidence and fault depression of crust. The crustal movement in sleuth China during the Hercynian-Indooinian was in close relationship with the plume. As the plume head arrived at the base of continental lithosphere overlying lithosphre streching and thinning occurred as a result of heating-up and softening. At the same time the crust sinked and the see level in the Qixiaian rose by a big margin. Then, the pull-apart and fault depression occurred in the Maokouan strongly. Following the rise of the plume head and the basaltic magma in diapire, the crust expanded by heat and the uplift in the Dengwu movement followed by a large-scale basaltic magma extrusion. Later, the crust sinked again and the transgression took place during the Late Permian and Early Triassic, because of the removal of magma from the mantle or the crust and the load of the volcanic material. At the same stress field, the crust pull-aparted intensely; the Youjiang-Nanpanjiang basin inherited prior platform-basin framework; the Garze-Litang ocean basin opened.The plume activities have important effect on the atmosphere, hydrosphere and biosphere. It give rise to biotic mass extinction propably. |
| LU Ji-ren.1996.Dynamical Characteristics of the Emei Mantle Plume[J].Acta Geoscientica Sinica,17(4):424-438. |
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