Iron- and LREE-enriched mantle source for early Proterozoic intraplate magmatism as exemplified by the Pechenga ferropicrites, Kola Peninsula, Russia

Abstract

Ferropicritic magmatism has produced a variety of volcanic and subvolcanic formations in the upper part of the early Proterozoic Pechenga Group, including sills, tuffs, pillowed and massive lavas, and thick, differentiated lava flows that possess spinifex-textured upper parts and lower cumulate parts enriched in olivine. Various olivine and pyroxene spinifex types can be distinguished. The occurrence of the spinifex texture in the Pechenga ferropicrites demonstrates that this texture is not restricted to rocks of komatiitic affinity. The existence of kaersutitic amphibole in spinifex-textured ferropicrites is discussed as a paleodepth indicator of the volcanism. The parental ferropicritic magma contained about 15 wt.% MgO. Other geochemical characteristics include high FeO tot and TiO 2 and low Al 2O 3 contents and high abundances of LREE and HFSE. Many incompatible element ratios correspond to those found in modern ocean island basalts. A notable exception are ratios involving phssphorous due to the relatively low P 2O 5 content of ferropicrites. The Al 2O 3 vs. MgO + K D × FeO diagram is used to show that ferropicritic magma is not simultaneously saturated in olivine and garnet at high pressures and hence, it was segregated from a garnet-free mantle residue. This implies that the LREE-enriched nature of the magma was inherited from the mantle source region. The positive initial ϵ Nd value (+ 1.4 ± 0.4) suggests a long time-integrated depletion and therefore, the inferred enrichment event in the mantle source must have taken place relatively shortly prior to melt segregation. Geochemical and isotopic data are compatible with a mantle-plume related origin of the Pechenga ferropicrites but the application of this model is limited by the present uncertainty on the duration of the volcanism.