Petrogenesis of Synplutonic High-Mg Porphyritic Dikes from Mafic–Granitoid Plutons of the Voronezh Crystalline Massif, East European Platform

Аннотация

Abstract—The paper reports data on the high-Mg porphyrite dikes (PD) in the ~2.09 Ma mafic to felsic plutons of the Elan Complex (EC). Thin (a few cm thick) synplutonic dikes are characterized by the sharp even contacts without visible chill zones, unlike thicker (up to 119 m thick) dikes with gradual transitions. The dikes are fresh, porphyritic (bronzite, Al-enstatite, and labradorite phenocrysts) with fine-grained mainly quartz–feldspar (+biotite, sulfides, accessories, ±hypersthene) groundmass. Based on chemistry, the PD are intermediate rocks (SiO₂ = 58.9–60.3 wt %) ascribed to the calc-alkaline series with high magnesian (Mg# ~0.7) whole-rock composition and felsic (68.9–70.2 wt %) groundmass (Mg# ~0.5). The porphyrites have fractionated REE distribution pattern without sharp Eu anomalies. Bronzite phenocrysts of variable size show patchy zoning and contain unevenly distributed inclusions of olivine (Mg# ~0.85), clinopyroxene (Mg# ~0.88), phlogopite (Mg# up to 0.94), labradorite, chromite, graphite, and sulfide. Al-enstatite phenocrysts are almost devoid of trace elements and mineral inclusions. Geochemical features, as well as the presence of diffusion zones, reaction rims, and resorbed facets in the orthopyroxene and plagioclase phenocrysts serve as evidence for disequilibrium recrystallization and/or partial dissolution of the crystals and intratelluric nature of dike phenocrysts, the core of which was inherited from derivatives/cumulates of the EC. Mineral thermometry showed that: (1) the initial temperatures of parental magma are 1200–1400°С, (2) the crystallization temperatures of the EC are 1080–1155°С, and (3) the emplacement temperatures of the PD are 910–1070°С. Petrological model suggests that the high-temperature boninite-like EC magmas were generated from the metasomatized upper mantle. The parental melt was contaminated by continental crust rocks. The existence of an transitional evolved magma chamber is inferred in the crust. At the final stage of its evolution, the PD melt intruded yet hot EC plutons.