Petrologiâ

Mafic within-plate magmatism is the main source of information about the geodynamics of processes that lead to the breakup of continental blocks. The article discusses the problem of geodynamics of the split of the Archean supercraton Superia in the Middle Paleoproterozoic. The discussion is based on data on 2.1 Ga magmatism in the Karelian Craton, where basites of this age are represented by tholeiites of two geochemical types: depleted and enriched. Geochemically close to N-MORB, depleted tholeiites were studied in Northern Priladozhye where they form methadoleritic dike swarms at c. 2111 ± 6 Ma (U-Pb, SIMS, zircon) in the Khatunoiya locality, and pillow lavas and sills near Lake Maloye Janisjarvi. Enriched tholeiites were studied in the Lake Tulos locality where they form a large swarm of doleritic dikes at 2118 ± 5 Ma (U-Pb, ID-TIMS, baddeleyite). The results of these studies provide deeper insight into 2.1 Ga mafic magmatism. Depleted tholeiites with N-MORB geochemistry have a spatial distribution in the Karelian Craton and could be formed as a result of decompression melting of a depleted asthenospheric mantle, raising melts along the extension zones, and minimal contamination by the Archean crust. The simultaneous formation of enriched tholeiitic melts probably occurred at differentiation and crustal contamination of depleted tholeiites during melt migration through more rigid Archean crustal blocks. Data on basic magmatism with an age of 2.1 Ga in the Karelian craton, which are difficult to explain within the framework of the mantle plume rise model, are consistent with the model of lithosphere extension due to the retreat of the subduction zone in the northeastern framing of the craton, in the Lapland-Kola Ocean in the interval of 2.0–2.2 Ga years. The maximum thinning, discontinuity of the Archean continental lithosphere, and the opening of an oceanic basin at the western edge of the Karelian craton were probably controlled by the suture zone of the junction of the Neoarchean crust with the Paleoarchean blocks, a chain of which was traced in the west of the Karelian craton. An additional factor that led to breakup of the lithosphere 2.1 Ga ago could be the rise of a deep mantle plume in the Khern craton, which occupied a spatial position close to the Karelian craton in the Archean supercraton Superia.

The Rudnaya dyke of the Imangda ore junction is composed of the weakly differentiated olivine-bearing to olivine gabbrodolerites with sulfide globules and disseminated sulfides of (pentlandite-pyrrhotite)-chalcopyrite-cubanite composition. Along with cogenetic sulfide mineralization, dyke’s gabbrodolerites contain xenoliths of hornfelsed basalts, abundant amygdales and rare grains of zoned Ol-1 Fo_90-47 with 0.5–0.06 wt % NiO that coexist with subhedral olivine Fo_74-36 of the second generation. Modeling in the COMAGMAT and alphaMELTS programs showed that high-Mg olivine 1 with Cr-spinel inclusions could not be crystallized from a Fe-enriched tholeiitic magma that is parental for the dyke with 4.8–7.3 wt % MgO and 11.6–16.7 wt % total Fe₂O₃. The trend of variations and high Ni up to 0.5 wt. % in the cores of xenocrystic olivine Fo_90-76 in contrast to maximum Fo₈₃ and 0.4 wt. % NiO in olivine from the ore-bearing intrusions and picritic basalts of the Norilsk region point toward the presence of picritic cumulates, which magma had not exchanged with sulfide liquid. Platinum group element (PGE) abundances increase (up to 2.2 ppm) with Cu/Ni in the whole rocks as well as with proportions of pentlandite in a sulfide association. A specific chalcophile metal distribution, which is characterized by Ni, Os and Ir minima, elevated Cu/Ni (5–15) and Cu/Pd (3200–10 900) as well as lower both PGE tenor of sulfides (2–65 ppm) and Pd content in pentlandite (<175 ppm) compared to typical of ore-bearing intrusions, suggests that Cu-rich sulfide mineralization was not mechanically captured from highly fractionated sulfide fractions of ore-bearing magmas but is cogenetic with a magma of the dyke. Sulfide saturation, near-simultaneous with fluid saturation and degassing, was achieved due to assimilation of sedimentary sulfur and volatiles from Devonian evaporites in the dyke conduit that is supported by the heavy S isotope composition of dyke’s sulfides with the average δ³⁴S = 14.7 ±1.1‰ (n = 31), close to the values in sulfides from the endocontact zones of the ore-bearing Imangda intrusions hosted by Devonian strata. The initial isotopic characteristics of dyke’s rocks (Sr_i 0.70517–0.70532, ɛ_Nd from –0.4 to 0.8) imply its comagmatic origin with the Norilsk-type intrusions whereas the overall data do not exclude even its spatial connection with an upper crustal conduit system of the ore-bearing magmas.

The problems associated with the use of quartz and zircon as proxy minerals for the reconstruction of δ¹⁸O values in acidic melts are considered. It is shown that the correction values Δ(Qz-R) and Δ(R-Zrn) used for the reconstructions are not strictly constant and depend on the mineral composition of the rock and the closing temperature of the oxygen isotopic system of the proxy mineral (T_q, T_z). For Δ(Qz-R), an equation for the calculation is proposed, taking into account these factors, and it is shown that, under a number of conditions, the approximation of this correction by a constant value gives consistent results. To estimate Δ(R-Zrn), an approach has been proposed that consists in calculating the weighted average fractionation coefficient and estimating T_z using a zirconium thermometer. An analysis of the advantages and limitations of the use of quartz and zircon as proxy minerals has been carried out.