The thermodynamic conditions, character and degree of fractionation of basic and ultrabasic volcanites of the greenstone belts in the basement of the East European Platform is examined by reference to rare-element distributions. The various volcanites in a given greenstone structure may be related by fractionation and/or may have arisen from different primary magmas. A single volcanic succession may have arisen from reservoirs at various depths, which is dependent on the individuality of each greenstone structure. A zone of greenstone structures has been identified for which deep-seated sites of basite-ultrabasite magma fractionation are characteristic. -Journal summary