The tectonomagmatic significance of Neoarchaean variably alkali-enriched gabbro and diorite intrusions of the western Karelia Province

This study describes a group of Neoarchaean alkali enriched gabbros and diorites from the western Karelia Province of the Fennoscandian Shield. We provide new field observations, petrography, whole-rock chemical data and additional whole-rock Sm–Nd and O-isotope data from these Neoarchean rocks. Compositionally, the rocks can be classified as shoshonitic with elevated rare earth element, K₂O, Ba and Sr contents together with variable M_gO, Ni and Cr contents. The M_gO, Ni and Cr depletion observed especially in one of the intrusions could have been caused by fractionation of olivine and clinopyroxene from the system. Zircon O-isotope values from one of the intrusions (δ¹⁸O = 7.34 ± 0.10‰) indicate the involvement of material that had undergone low-temperature fractionation of oxygen in the genesis. Samarium–Nd data imply contribution from older material in the petrogenesis of these rocks. The above-mentioned characteristics can be explained with a magma source in the mantle that was heterogeneous owing to the variable degrees of metasomatism. The alkali-enriched gabbros and diorites provide additional evidence for magmatism derived from heterogeneously enriched mantle during the Neoarchaean in the Karelia Province and associated with the cratonization of the area.