Mineralogical Characterization of Olivine and Coexisting Phases as a Key to Reconstructing Crystallization and Deformation Processes in the Monchepluton Massif

This paper presents a detailed physicochemical reconstruction of the evolution of the Proterozoic layered intrusion of Monchepluton (Kola Peninsula), based on the crystal chemistry of olivine and the coexisting silicate and ore phases. To link magmatic processes with the subsequent tectonothermal history, precise microchemical analyses in microscale (EDS, EMPA), whole-rock geochemical profiling (XRF, ICP-MS), and integration of the obtained results with an external rare earth element (REE) database was performed. Linear EDS, EMPA core-rim transections of olivine grains revealed the presence of broad, homogeneous central zones of high magnesium (Fo62–91) and stable nickel content. At the outermost crystal edges (<15 μm), a diffuse decrease in Mg and Ni in favor of Fe and Mn was recorded, which, based on modeling of the entire Gaussian error equation, was interpreted as a transition from stationary adcumulate crystallization to a closed in situ regime, controlled by the effect of trapped residual melt (trapped liquid shift). The high Ni content in the silicate cores excludes early magma saturation with sulfur. Our results demonstrate a primary, magmatic genesis of plagioharzburgites, in which olivine constitutes the early phase, and plagioclase crystallizes after reaching the plagioclase-in point, which correlates with a prominent positive europium anomaly EuEu* (=1.53) identified in the norite facies. The geochemical purity of the Dunite Block (SiO2 = 37.47–38.92 wt. %, K/Ti ≤ 0.04) confirms its position as a deep, isolated basal domain. Contrasting with this are the picrite veins, which represent much younger, diachronous injections of extremely evolved dike swarms (Cr = 0.008%, Sr = 369 ppm), documenting multi-stage reactivation of deep rifts. A unique pattern was identified in the tectonic deformation zones (mylonites and breccias): fresh, unaltered olivine cores survived in the fine-grained matrix due to cataclasis. These grains were likely isolated by a fine-crystalline matrix and the high degree of rock compaction, which protected them from alteration during hydrothermal processes under specific conditions (low infiltration rate and decreased water activity). The strong chalcophile anomaly (Zn-Pb-Cu-S) accompanying the shear zones and the jump in the K/Ti ratio to 0.38 prove that these dislocations acted as structural conduits facilitating fluid migration.