Sr Isotope and Trace Element Studies of the Great Dyke and Bushveld Mafic Phase and their Relation to Early Proterozoic Magma Genesis in Southern Africa |
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| Authors: | HAMILTON JO |
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| Institution: | Dept. of Geology and Mineralogy Parks Road, Oxford, OXI 3 PR England |
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| Abstract: | New Rb-Sr and trace element data are reported for the GreatDyke and Bushveld Mafic Phase layered intrusions. It is arguedthat geochemical characteristics, such as 87Sr/86Sr ratios andR.E.E. distribution patterns have been little modified by crustalcontamination. Rb-Sr data for whole-rocks of the Great Dyke yield an age of2514±16 m.y. and an initial 87Sr/86Sr ratio of 0.70261±4.Mineral data are consistent with these results. The low errorson the results indicate no significant variation of 87Sr/86Srratios of successive magmatic influxes emplaced in differentmagma chambers. Earlier Great Dyke magmas were highly Mg-richand represent extensive partial melts of the source material.One such influx is shown to have a high Rb/Sr ratio ( 0.25) anda fractionated R.E.E. pattern (CeN/YBN 12). These ratios areconsidered to approximate those of the source region. The Bushveld Mafic Phase has been dated accurately for the firsttime and has a Rb-Sr age of 2095±24 m.y. Initial 87Sr/86Srratios increase in a stepwise manner upwards in the intrusionfrom 0.70563±2 to 0.70769±6. Each increase isabrupt and occurs at a horizon also characterized by a suddenirregularity in cryptic variation. The Mafic Phase was emplacedas a succession of magmatic influxes each of which had higher87Sr/86Sr ratio than its predecessor. The first magma was both Mg-rich (MgO  21.5 per cent) and SiO2-rich(50–55 per cent SiO2) and was derived by extensive partialmelting of a shallow level upper mantle source. This sourcewas characterized by trace element abundance ratios (e.g. Rb/Sr 0.25; K/Rb 90; CeN/YbN 11), similar to those of kimberlitesand some potassic lavas and comparable with those deduced forthe Great Dyke source region. It is postulated that when the Rhodesian and Kaapvaal cratonsstabilized, underlying refractory mantle became fixed theretoto form a proto-lithosphere. Shortly afterwards, at about 2800m.y. ago, this proto-lithospheric mantle was enriched by passagethrough it of fluids with kimberlitic trace element chemistry.This sub-cratonic mantle thereafter evolved with a relativelyhigh Rb/Sr ratio. Magmas derived from it have anomalous chemicalcharacteristics with respect to those of ocean-floor basalts,reflecting major differences in the evolution of their respectivesource regions. |
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