Geochemistry and petrology of 2.40-2.45 Ga magmatic rocks in the north-western Belomorian Belt, Fennoscandian Shield, Russia

Widespread mafic and ultramafic rocks (drusites) and minor granites of 2.40-2.45 Ga age occur within the Archaean gneisses of the Belomorian fold belt. Several of these mafic-ultramafic and acid intrusions in the north-western part of that belt have been studied in detail. The mafic and ultramafic rocks are rich in Si and Mg, and also show LREE enrichment. They have epsilon(Nd)(245) values varying between +0.2 and -1.8. Assimilation-fractional crystallization (AFC) and mixing models in terms of epsilon(Nd)(T)-La/Sm ratios have been evaluated for the origin of the mafic rocks. The results show that an AFC model is preferable. It involves 20-30% crustal rock assimilation by undepleted picrite magma with fractional crystallization. The granites are similar in composition to A-type (within-plate) granites. The Tupaya Bay granite pluton is characterized by geochemical layering which supports an anorogenic origin. The granites and mafic-ultramafic rocks (drusites) in the Belomorian Belt are similar in composition to coeval granites, acid and mafic volcanic rocks, mafic dykes and mafic layered intrusions within the Kola and the Karelian terrains, and can be interpreted as part of the Palaeoproterozoic igneous province connected with ascent of a mantle plume. The small sizes, corona textures and evidence for ductile deformation in the drusites are connected with their deeper level of emplacement (P=8-12 kbar) than that of the mafic layered intrusions (2-7 kbar). (C) 1998 Elsevier Science B.V. All rights reserved.