Magma-evaporite interaction in doleritic sills from the Siberian Traps: insights from whole-rock and mineral data

The Tunguska Basin in East Siberia (Russia) hosts an extensive network of thick sills, part of the Siberian Traps Large Igneous Province. High-precision geochronology links the initial phase of sill emplacement to the end-Permian cascade of environmental catastrophes that almost expunged life on Earth (Dal Corso et al., 2022). The end-Permian atmosphere was impacted by a voluminous cocktail of gases, from CO₂ and SO₂ to halocarbons. Multiple lines of evidence suggest that sills emplaced within the evaporitic and coal-rich series of the Tunguska Basin acted as major contributors to this outgassing. Basin-scale observations and thermal modelling provide evidence of thermogenic gas production and release (Svensen et al., 2018). For the Tunguska sills, whole-rock geochemistry (Callegaro et al., 2021) and micro-analyses track multiple processes of magma host-rock interaction occurring at different levels across the plumbing system and the volcanic basin. Whole-rock trace elements and radiogenic isotopes reveal assimilation of variable crustal lithologies, from the crystalline basement to evaporites and carbonates in the Tunguska Basin. Assimilation of anhydrite-dominated evaporites is confirmed by whole-rocks sulfur isotopes. Assimilation of halogen-dominated evaporites is tracked by detailed mineral analyses of dolerite sills. We found widespread evolved late-stage pockets among the larger plagioclase and clinopyroxene crystals in the Tunguska dolerites. These pockets filled with an evolved, volatile-rich minerals, dominated by biotite and quartz, with minor K-feldspar, chloro-apatite, Cl-rich amphibole, sulfides and occasional baddeleyite and zircon. Biotite in the pockets is extremely enriched in Cl, especially at the rims. Plagioclase surrounding the pockets shows highly albitic rims. These compositions are widespread across the Tunguska Basin, where sills intruded halite- and anhydrite-rich evaporites, and suggest extensive mobilization of crustal halogens and sulfur associated with the emplacement of the sills, along with previously demonstrated thermogenic carbon production. Notably, most investigated sills are geochemically correlated with the phase of Siberian Traps magmatism coeval with the main extinction horizon (Callegaro et al., 2021).

 

Callegaro S., Svensen H.H., Neumann E.R., Polozov A.G., Jerram D.A., Deegan F.M. Planke S., Shiganova O.V., Ivanova N.A. & Melnikov N.V., 2021. Geochemistry of deep Tunguska Basin sills, Siberian Traps: correlations and potential implications for the end-Permian environmental crisis. Contrib. Mineral. Petrol., 176, 49.

Dal Corso J., Song H., Callegaro S., Chu D., Sun Y., Hilton J., Grasby S.E., Joachimski M.M. & Wignall P.B. 2022. Environmental crises at the Permian–Triassic mass extinction. Nat. Rev. Earth Environ., 3(3), 197–214.

Svensen H.H., Frolov S., Akhmanov G.G., Polozov A.G., Jerram D.A., Shiganova O.V., Melnikov N.V., Iyer K. & Planke S. 2018. Sills and gas generation in the Siberian Traps. Phil. Trans. R. Soc. A., 376:20170080.