Geochemical Characteristics And Geodynamic Implications Of Komatiite In Barberton Greenstone Belt,South Africa

Komatiites originating from the mantle sources are mafic-ultramafic volcanic lavas,which is an important component of the early Precambrian greenstone belts.The study of the original composition of komatiites can trace the melting temperature and the depth of the mantle sources to deduce the mechanism of the formation,and provide important basis for the study of petrogenesis,the interpretation of the properties of the mantle and the discussion of the Precambrian geodynamic setting.We conduct petrological and geochemical analyses on the komatiites,komatiitic basalts and basalts from the Barberton greenstone belt in South Africa,to determine the rock types,magma sources and evolution characteristics of these mafic-ultramafic rocks,coupled with the previous studies on the komatiites of the Barberton greenstone belt,and further to determine the original magmatic composition and tectonic setting of these mafic-ultramafic rocks.The mafic-ultramafic volcanic lava samples have the typical spinifex structure.The main mineral components of the samples are pyroxene and olivine observed from hand specimens and electron microscope,of which most olivine experienced serpentinization.The precisely quantitative analyses of minerals by TIMA and backscatter image imaging indicates that the spinifex structure of the samples is mainly consisted of pyroxene.The results of major elemental analyses show that the content of Si O₂ is 40.15 wt%～51.94 wt%,Fe O_tot content ranges from 9.83 wt%to 13.50wt%,Ti O₂ content is 0.15 wt%～0.49 wt%,Al₂O₃ content is 4.18 wt%to 14.47 wt%,the content of Mg O ranges from 6.04 wt%to 31.92 wt%.According to the rock classification diagram,the mafic-ultramafic rocks from the Barberton greenstone belt in South Africa can be classified as komatiites,komatiitic basalts and tholeiites,and the komatiites are the Al-undepleted type.The komatiite samples all have La/Ta less than25,with an average of 2.24 and Th/Ta of 0.23-0.45,much lower than the Th/Ta ratio（=10）of the upper crust;Nb/U=71.58-166.25,indicating that the komatiite is not mixed by the crust.Basalt samples have La/Ta=10.53 to 13.98,Th/Ta ratios of 0.65 to 1.8and Nb/U=2.12 to 3.23,indicating that the associated basalt has undergone minor crustal mixing.The compatibility of the Ci and Nb elements with respect to olivine suggests that the komatiite underwent olivine segregation crystallization while the basalt mainly underwent pyroxene segregation crystallization.The mantle potential temperatures（Tp=1388～1778℃）calculated by recovering the original magmatic composition of komatiites and associated basalts indicate an anomalously high mantle melting temperature,much higher than that of the subduction zone setting（1200℃）,suggesting that the Barberton area of South Africa was in a mantle plume setting during the Mesoarchean.