Mineral–Geochemical and Geotectonic Features of the Lotmvara-II Ultrabasic Sill, Serpentinite Belt (Kola Peninsula)

—In this paper, we present a description of the characteristics of the Lotmvara-II sill, which is a representative of the Serpentinite Belt (SB) composed of a series of shallowly emplaced ultrabasic intrusive bodies. The Paleoproterozoic SB complexes were derived from a large-scale mantle plume of komatiitic melt. The sill consists predominantly of fine-grained (locally nearly micrograined) harzburgites with subordinate zones of dunites and orthopyroxenites, located in the central and marginal parts, respectively. It formed from an Al-undepleted komatiitic magma of extremely high Mg content and may represent a near-surface laccolithic “ridge.” In general, the sill is comparatively homogeneous and does not have distinct zoning in the distribution of Mg# values in rock compositions (Mg# = 84.2–88.9, average 86.7). Detailed studies show that olivine, chromian spinel, and ilmenite are the most strongly magnesian in the central part of the body. The maximum values of Mg# equal to 90.7–91.4 in the compositions of olivine at the center of the sill are interpreted as “centers of initial crystallization”. The low values of Mg# equal to 73.4–76.4 are attributed to manifestations of the recurrent generation of olivine. The values of Mg# of orthopyroxene in the sill are within the range 84.6 to 92.3. Orthopyroxene grains in a porphyritic texture are surrounded by a rim of calcic amphibole (autometasomatic in origin); they do not differ compositionally from normal grains. The Zn content of the chromian spinel generally decreases toward the marginal parts of the sill. There is an insignificant degree of magmatic differentiation in the sill with respect to the principal components, but incompatible elements (REE and HFSE) locally show increased levels of their relative enrichment, which is reflected in the nature of the mineral associations described. Thus, the sill has a cryptic zonal structure, which is consistent with its overall crystallization from the center to the edges. The data gathered suggest the presence and significant development of volatile components, halogens, CO2, and especially magmatic H2O, which are capable of strongly lowering the liquidus and reducing the density and viscosity of the high-magnesium melt, thereby improving its mobility during ascent from the mantle to the near-surface level of the crust. An increase in fO2 is observed during in situ subvolcanic crystallization of the sill, as noted earlier in the related complexes of the belt. The relatively small volume of the komatiitic magma in the sill crystallized fairly quickly, resulting in unusual mineral intergrowths. Thus, the Lotmvara-II sill is a novel member in the Serpentinite Belt–Tulppio Belt (SB–TB) in the Paleoproterozoic SB–TB megastructure of the Fennoscandian Shield.