IRON-ENRICHED AND LREE-ENRICHED MANTLE SOURCE FOR EARLY PROTEROZOIC INTRAPLATE MAGMATISM AS EXEMPLIFIED BY THE PECHENGA FERROPICRITES, KOLA-PENINSULA, RUSSIA

Ferropicritic magmatism has produced a variety of volcanic and subvolc anic formations in the upper pact of the early Proterozoic Pechenga Gr oup, including sills, tuffs, pillowed and massive lavas, and thick, di fferentiated lava flows that possess spinifex-textured upper parts and lower cumulate parts enriched in olivine. Various olivine and pyroxen e spinifex types can be distinguished. The occurrence of the spinifex texture in the Pechenga ferropicrites demonstrates that this texture i s not restricted to rocks of komatiitic affinity. The existence of kae rsutitic amphibole in spinifex-textured ferropicrites is discussed as a paleodepth indicator of the volcanism. The parental ferropicritic ma gma contained about 15 wt.% MgO. Other geochemical characteristics inc lude high FeOtot and TiO2 and low Al2O3 contents and high abundances o f LREE and HFSE. Many incompatible element ratios correspond to those found in modern ocean island basalts. A notable exception are ratios i nvolving phosphorous due to the relatively low P2O5 content of ferropi crites. The Al2O3 vs. MgO+K-D X FeO diagram is used to show that ferro picritic magma is not simultaneously saturated in olivine and garnet a t high pressures and hence, it was segregated from a garnet-free mantl e residue. This implies that the LREE-enriched nature of the magma was inherited from the mantle source region. The positive initial epsilon (Nd),, value (+1.4+/-0.4) suggests a long time-integrated depletion an d therefore, the inferred enrichment event in the mantle source must h ave taken place relatively shortly prior to melt segregation. Geochemi cal and isotopic data are compatible with a mantle-plume related origi n of the Pechenga ferropicrites but the application of this model is l imited by the present uncertainty on the duration of the volcanism.