The comparison of the composition variations of the tholeite-type basalts of the oceanic rifts TOR of the Mid-Atlantic Ridge MAR and of the East Pacific Rise EPR, using the results of more than 12,000 analyses of quench glasses, showed the following results. In all of these three regions, basalt magmatism developed under the conditions of two different geodynamic environments, spreading and plume development, with the formation of the respective TOR rock associations. The spreading TOR rock associations of all regions are enriched in Na2O and TiO2 and experienced polybaric fractionation during their cotectic crystallization. The TOR plume rock associations are enriched in SiO2 and CaO. This differentiation took place under the conditions of the transition of polybaric fractionation to intrachamber one. This rock association also includes the local TOR rocks enriched in potassium. The growing spreading rate enhanced the magmatism productivity, enlarged the interval of the PT conditions of cotectic crystallization, the variation range of the TOR composition, and the contribution of the spreading-type rocks relative to those of the plume origin. Where the spreading rate was low, the plume development seem to have operated as an independent process, superposed over the spreading. In these cases the plume and spreading rock associations are spaced far from one another. The areas of high spreading rates do not show any boundary between the plume- and spreading-type rock associations. The interval of the mantle rise and the depth of the mantle melting zone do not show any association with the spreading rate.
basalt magmatism, velocity of spreading, Mid-Atlantic Ridge, East Pacific Rise.
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