Scientific publications

Olivine gabbronorites of Belomorye (~2,4 Ga), belonging to the widespread natural group of “partially eclogitised” gabbros, were investigated across a series of objects within a unified complex that reflects progressively intensified metamorphic transformations occurring under subsolidus conditions. A strong correlation between magmatic and metamorphic processes has been established. In the most extensively metamorphosed samples, three genetically distinct generations of Omp ± Aeg-Aug minerals have been recognized: magmatic generation – Omp20-25 ± Aeg-Aug20-22; metamorphic generation related to Cpx and Opx breakdown textures – Omp20-34 ± Aeg-Aug20-23; and another metamorphic generation resulting from Px-Pl reactions – Omp20-73 ± Aeg-Aug20-24, associated with plagioclase (An content ranging from 30–35 to 4–5%). The Px-Pl reactions evolved cyclically through two sequential stages: first, a Na-alkaline stage (Omp + Pl) characterized by leaching of primary elements (Fe, Mg, Ca) under rising activities of H₂O and Na₂O but diminishing SiO₂ activity, followed by a second stage marked by precipitation of secondary minerals (Opx, Grt) under elevated SiO₂ activity levels (in line with a compensatory mechanism). These reactions occurred in an autoclave-like autometasomatic regime, maintaining constant volume and preserving chemical balance, driven by transformed magmatic fluids and rapid rates of mineral crystallization, which may exceed those optimal for diffusive crystal growth. As a result, this led to substantial variations in mineral chemistry and pronounced disequilibria among coexisting mineral phases. Variability in Na-Cpx – Omp compositions observed within individual thin sections clearly indicates differing degrees of Na₂O activity despite otherwise consistent pressure-temperature conditions. Moreover, Omp and Grt minerals, formed at disparate times and under divergent geochemical settings, are unlikely to be paragenetically linked. The unique alkaline nature of the studied subsolidus metamorphism, characterized by increased H₂O and Na₂O activities concurrent with decreased SiO₂ activity, is highly unusual even among the broader category of “partially eclogitised” gabbros.