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Mantle Plume Geodynamic Significances Of The Emeishan Large Igneous Province: Evidence From Mafic Dykes, Geochemistry And Stratigraphic Records

Posted on:2013-02-02Degree:DoctorType:Dissertation
Country:ChinaCandidate:H B LiFull Text:PDF
GTID:1110330371482253Subject:Mineralogy Petrology Gitology
Abstract/Summary:PDF Full Text Request
Since the geodynamic model of Emeishan mantle plume was been addressed,many scholars have done a lot of work to solve some controversial issue about thismodel and made a great progress. However, some key problem are still not clear up.Are there giant mafic dyke swarms in Emeishan Large Igneous Provinces (ELIP)?Was the crust uplifted preceding volcanism in ELIP? Where is the centre of theEmeishan mantle plume? Based on the observation in field, mafic dyke swarms,Qixia and Maokou formations have been studied and the geodynamic model ofEmeishan mantle plume has been considered. New results and progress have beenobtained.The giant radiating mafic dyke swarms in ELIP and its distribution featureswere identified firstly, which provide a key evidence for the geodynamic model ofEmeishan mantle plume. The pattern of the dyke swarms is radiating from a centre,which is composed of six giant dyke swarms with radiate angle ~200°. The centre islocated at the Yongren area, Yunnan province, which likely locates the center ofmantle plume head which was responsible for the Permian Emeishan flood basaltevent. The new results from the dyke swarms are consistent with the location of themaximum crust uplift inferred by previous study from stratigraphic study.Geochronology, petrology and geochemistry were performed systematically onmafic dykes in Mianning, Sichuan province and Fumin, Yunnan province. Maficdykes in Mianning were homologous with Emeishan basalts and were derived frommantle plume with little contamination. The LA-ICP-MS U-Pb zircon data of maficdyke yields a concordant age, 256.7±4.3Ma, which may represents the rock masscrystallization time. It shows that the Mianming mafic dykes were formed in the lateperiod of Emeishan mantle plume magmatism. In addition, the dating implies thatdykes experienced a geological events occurred about 244Ma, which was caused bythermal erosion of magma and triggered by the Indosinian movement.Mafic dykes in Fumin belong to tholeiitic basalt series in lithology andPangxiejing dyke is alkaline monzonlite. These dykes and Emeishan basalts werederived from the same source and formed magmatic evolution sequence, in whichthe fractional crystallization of plagioclase was dominant. Three kinds of rocksformed the'|mode of Triplet assemblage'. Mafic dykes were also derived frommantle plume with little contamination. The result of isotope geochronology is256.7±4.3Ma and indicate that mafic dykes were in the late of ELIP magmatism.The geochronology and emplacement mechanism of mafic dykes in Fumin wasconsistent with the classic theory of mantle plume and then support the geodynamicmodel of Emeishan mantle plume.New constrains are given for plume-induced uplift preceding volcanism in ELIP.Isopach maps indicate that uplift caused by Emeishan mantle plume has started inQixianian. Disconformities were distributed broadly on the top of Maokou formation,which demonstrate that Maokou formation has been uplifted over the sea level anddenudated differentially. This has been predicated in Campbell and Griffiths's classicmantle plume model. There is a blank area of Permian stratum inYongren-Dayao-Chuxiong-Shiping and Yiliang-Qujing, which probably was the maximum uplift area. The maximum uplift of ELIP was 1500m induced throughlaboratory model, which was agreement with the result of sedimentary records. Thecenter of mantle plume implied by differential erosion was consistent of the focus ofradiating basic dyke swarms. Hydromagmatic deposits and submarine basalticextrusions were the result of the rapid subsidence followed the uplift and weren'tenough to deny the mantle plume dynamic model of ELIP.This paper discusses the tail of Emeishan mantle plume and suggests that it locatesMiyi-Yongren. On the basis of the classic mantle plume theory, the calculateddiameter of Emeishan plume tail was 65~105km when it reached the upper mantle.Picrite distributions and sedimentary responses showed that the center zone ofEmeishan plume was ca. 260~300km wide. Eruption and outcropping of floodbasalts were constrained by deep-seated faults and paleogeomorphology, whichresulted in the concentration of picrite in the west of Emeishan large igneousprovince.On the basis of the predecessors'work and research results of this paper, thegeodynamic model of Emeishan mantle plume has been advanced. The magmatismof Emeishan mantle plume had three stages, that is early stage, extensive volcanismand late stage. The activities and magmatism of Emeishan mantle plume have beenconstrained in every stage.
Keywords/Search Tags:Emeishan large igneous province, Mantle plume, Mafic dyke swarms, Geochemistry, Isopach map
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