Coronitic Associations at Gabrish in the Kovdozero Layered Complex in the Southern Part of the Lapland—Belomorian Belt, Kola Peninsula, Russia

The Paleoproterozoic Kovdozero complex, one of largest in the Fennoscandian Shield, was emplaced in a peripheral region of the SB–TB–LBB (Serpentinite Belt–Tulppio Belt–Lapland–Belomorian Belt) megastructure. Coronitic rocks of ultrabasic–basic compositions, investigated along a cross-section in the Gabrish area, are members of a cryptically layered series. They crystallized from the northern margin inward, as indicated by variations in mineral compositions and geochemical trends. Unsteady conditions of crystallization arose because of uneven cooling of the shallowly emplaced complex. Rapid drops in temperature likely caused the forced deposition of different generations of variously textured pyroxenes and chromian spinel or resulted in the unique development of narrow recurrent rims of orthopyroxene hosted by olivine. The unstable conditions of crystallization are expressed by (1) textural diversity, (2) broad variations in values of Mg#, and (3) virtual presence of double trends of Mg# as a function of distance. The coronitic textures are intimately associated with interstitial grains of plagioclase (An≤65), also present as relics in a rim of calcic amphibole. The coronas are results of (1) rapid cooling leading to unsteady conditions of crystallization, which caused the sudden cessation of olivine crystallization and the development of an orthopyroxene rim on olivine and (2) an intrinsic enrichment in H2O (and essential Cl in scapolite) coupled with a progressive accumulation of Al and alkalis, giving rise to fluid-rich environments in the intercumulus melt at advances stages of crystallization. These processes were followed by deuteric composite rims of calcic amphibole and reaction of fluid with early rims or grains of pyroxenes and late plagioclase. The coronitic sequences Ol → Opx → Cpx → calcic Amp → Pl (plus Qz + Mca) observed at a microscopic scale reproduce, in miniature, the normal order of crystallization in an ultrabasic–basic complex. A composite orthopyroxene + calcic amphibole corona resembles some rocks in complexes of the Serpentinite Belt. The prominence of such coronas may well be characteristic of the crystallization of komatiite-derived melts.