Sr–Nd–Hf isotope evidence from Paleoproterozoic mafic–ultramafic intrusions, Pechenga belt, Russia: Implications for recycled oceanic crust in mantle plume

Abstract

The mafic–ultramafic rocks in the Pechenga belt offer new insights into intraplate magmatic processes associated with mantle plumes within the Fennoscandian Shield. This study presents new major- and trace element data, along with Sr–Nd–Hf isotope compositions for wehrlite, clinopyroxenite, and gabbro rocks from the 1980 Ma ore–bearing Western and Southeastern Pilgujarvi intrusions. These data are used to constrain the nature of the mantle source and tectonic setting of these intrusions. All rocks show enrichment in LREEs, slight enrichment to depletion in Nb and Zr, weak to strongly positive Ti anomalies, and depletion in LILEs (e.g., Ba and Sr). The E–MORB–like wehrlites have positive initial ε_Nd (+0.7 to +2.3) and ε_Hf (+2.4 to +5.5) values in both intrusions and show low ⁸⁷Sr/⁸⁶Sr_(i) ratios (0.70163–0.70353) in the Western intrusion but slightly elevated ones (0.70240–0.70719) in the Southeastern intrusion. The Western clinopyroxenites have positive initial values of ε_Nd (+0.6 to +1.2) and ε_Hf (+1.6 to +3.1) with low ⁸⁷Sr/⁸⁶Sr_(i) ratios (0.70061–0.70256). The OIB–like gabbros in both intrusions are marked by slightly negative to positive initial values of ε_Nd (−0.2 to +2) and ε_Hf (−0.76 to +8.7), with low ⁸⁷Sr/⁸⁶Sr_(i) ratios (0.70109–0.70386). Trace elements ratios (e.g., La/Yb and Dy/Yb) and isotopic compositions indicate that all rocks were formed by partial melting of a garnet–bearing asthenospheric mantle source. This source was previously metasomatized by young altered oceanic crust and subducted components (i.e., fluids and melts), leading to HIMU–like signatures. The contribution of recycled oceanic crust resulted in a hybrid mantle source, dominated by pyroxenite/eclogite rather than peridotite. The parental magma of these rocks underwent variable degrees of closed–system crystallization, and minimal wall–rock contamination (∼1–5 %) during ascent. We infer that both Western and Southeastern Pilgujarvi intrusions were genetically related to an asthenosphere–derived mantle plume during an intra–extensional regime. This model is consistent with geological, geochemical, and isotopic evidence from other Paleoproterozoic mafic–ultramafic rocks across the eastern Fennoscandian Shield.