Home – Home – Jornals – Russian Geology and Geophysics 2009 number 12

Here we combine petrological-geochemical and thermomechanical modeling techniques to explain origin of primary magmas of both Maimecha-Kotui meimechites and the Gudchikhinskaya basalts of Norilsk region, which represent, respectively, the end and the beginning of flood magmatism in the Siberian Trap Province.
We have analyzed the least altered samples of meimechites, their olivine phenocrysts, and melt inclusions in olivines, as well as samples of dunites and their olivines, from boreholes G-1 and G-3 within the Guli volcanoplutonic complex in the Maimecha-Kotui igneous province of the northern Siberian platform. The Mn/Fe and Ni/MgO ratios in olivines indicate a mantle peridotite source of meimechites. Parental meimechite magma that rose to shallow depths was rich in alkalies and highly magnesian (24 wt.% MgO), largely degassed, undersaturated in sulfide liquid, and oxidized. At greater depths, it was, likely, high in CO₂ (6 wt.%) and H₂O (2 wt.%) and resulted from partial melting of initially highly depleted and later metasomatized harzburgite about 200 km below the surface. Trace-element abundances in primary meimechite magma suggest the presence of garnet and K-clinopyroxene in the mantle source and evidence a genetic relation to the sources of the early Siberian flood basalts (Gudchikhinskaya suite) and kimberlites. The analyzed dunite samples from the Guli complex have chemistry and mineralogy indicating their close relation to meimechites.
We have also computed a thermomechanical model of interaction of a hot mantle plume with the shield lithosphere of variable thickness, using realistic temperature- and stress-dependent visco-elasto-plastic rocks rheology and advanced finite-element solution technique.
Based on our experimental and modeling results, we propose that a Permian-Triassic plume with a potential temperature of about 1650 oC transported a large amount of recycled ancient oceanic crust (up to 15%) as SiO₂-supersaturated carbonated eclogite. Low-degree partial melting of eclogite at depths of 250-300 km produced carbonate-silicate melt that metasomatized the lithospheric roots of the Siberian shield. Further rise of the plume under relatively attenuated lithosphere (Norilsk area) led to the progressive melting of eclogite and formation of reaction pyroxenite, which then melted at depths of 130-180 km. Consequently, a large volume of melt (Gudchikhinskaya suite) penetrated into the lithosphere and caused its destabilization and delamination. Delaminated lithosphere that included fragments of locally metasomatized depleted harzburgite subsided into the plume and was heated to the temperatures of the plume interior with subsequent generation of meimechite magma. Meimechites showed up at the surface only under the thicker part of the lithosphere aside from major melting zone above, otherwise they would have been mixed up in more voluminous flood basalts. We further suggest that meimechites, uncontaminated Siberian flood basalts, and kimberlites all share the same source of strongly incompatible elements, the carbonated recycled oceanic crust carried up by hot mantle plume.