Geochemistry of deep Tunguska Basin sills, Siberian Traps: correlations and potential implications for the end-Permian environmental crisis

S Callegaro,S Planke,A G Polozov,D A Jerram,O V Shiganova,N V Melnikov,E R Neumann,N A Ivanova,F M Deegan,H H Svensen

doi:10.1007/s00410-021-01807-3

Abstract

A vast portion of the plumbing system of the Siberian Traps Large Igneous Province (STLIP) is emplaced in the Tunguska Basin, where borehole data reveal ubiquitous and abundant sills with great lateral extension. These intrusions intersect Cambrian–Ordovician evaporite, carbonate and siliciclastic series, and locally coal-bearing Permian host rocks, with a high potential for thermogenic gas generation. Here we present new geochemical data from 71 magmatic and 4 sedimentary rock samples from the Tunguska Basin center and periphery, recovered from 15 deep sills intercepted by boreholes. The studied samples are all low-Ti basalt and basaltic andesites, confirming absence of high-Ti and alkaline STLIP magmatism in the Tunguska Basin. The sills derive from picritic parental melts produced by extensive melting of a mantle source with recycled crustal components below a thinned lithosphere (50–60 km), within the spinel stability field. The mantle source was dominantly peridotitic, with enriched pyroxenitic domains formed by recycled lower crust, in agreement with previous models for the main tholeiitic STLIP phase. Limited amounts (up to 5%) of highly radiogenic granitoids or moderately radiogenic metapelites were assimilated in upper crustal magma reservoirs. After emplacement, sills intruded in Cambrian evaporites assimilated marlstones and interacted with the evaporitic host rocks, probably via fluids and brines. This is the first time that such process is described in subvolcanic rocks from all across the volcanic basin. The sills are correlated geochemically with the established chemostratigraphy for the on-craton STLIP lava piles and intrusions (Norilsk region). Sills correlated with the Morongovsky–Mokulaevsky Fm. and the Norilsk-type intrusions are the most voluminous, present all across the central Tunguska Basin, and bear the strongest evidence of interaction with evaporites. Massive discharge of thermogenic volatiles is suggested by explosive pipes and hydrothermal vent structures throughout the Tunguska Basin. We propose that this voluminous pulse of magmatism is a good candidate for the hitherto unidentified early intrusive phase of the STLIP, and may link the deep Tunguska basin sills to the end-Permian environmental crisis.

Highlights

The Siberian Traps Large Igneous Province (STLIP; ca. 251–252 Ma; Fig. 1), synchronous with the end-Permian mass extinction (Burgess et al 2017; Dal Corso et al 2020) and home to world-class Cu–nickel oxide (Ni)–PGE ore deposits (Ryabov et al 2014), is one of the most intensely studied LIPs
Derive from a depleted asthenosphere (DMM-like, with the isotopic flavor of the Lower Sequence rocks; Sharma 1997; Model D), acquiring their crustal-like trace elements only by assimilation en-route to the surface. We find this interpretation less likely because (a) it would not explain the high percentage of pyroxenite-derived melts calculated by Sobolev et al (2011); (b) in such case one would expect to see STLIP products with compositions in between the more isotopically depleted Lower Sequence and the more enriched Middle and Upper Sequences; and (c) this scenario would require higher percentages of assimilated crust, which are not favored by the moderately radiogenic 187Os/188Os signatures found in the STLIP main tholeiitic series (Horan et al 1995)
Deep sills intruded into the Tunguska Basin are low-Ti, tholeiitic dolerites geochemically similar to the Upper Series lavas of the Siberian Traps LIP

Summary

Introduction

The Siberian Traps Large Igneous Province (STLIP; ca. 251–252 Ma; Fig. 1), synchronous with the end-Permian mass extinction (Burgess et al 2017; Dal Corso et al 2020) and home to world-class Cu–Ni–PGE ore deposits (Ryabov et al 2014), is one of the most intensely studied LIPs. For the atmospheric impact of a LIP, the tempo of volcanism is a crucial factor, perhaps more so than a cumulative appraisal of the volumes of volcanics and volatiles involved (Sprain et al 2019). As much as they can be considered geologically brief events, LIPs usually last much longer than the related extinction intervals (< 100 kyr). The start of intrusive activity is not constrained, it is possibile that the extinction-triggering part of the Siberian Traps is a yet undetected or unsampled early intrusive batch

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Discussion

Conclusion