Asko Käpyaho

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

Abstract 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 MgO, Ni and Cr contents. The MgO, 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. Supplementary material: Table detailing oxygen-isotope analyses from this study is available at https://doi.org/10.6084/m9.figshare.c.3466575