Extending the geochronological record of intrusive rocks of the Siberian Traps Large Igneous Province

Abstract

A large volume of intrusive sills of the Siberian Traps Large Igneous Province (STLIP) is found in the Tunguska Basin, Siberia. These rocks intrude evaporite, carbonate and siliciclastic lithologies, which are sometimes rich in organic material and bear massive coal layers. The interaction of hot basaltic magma with the cold country rocks is considered to have released large volumes of thermogenic volatiles that altered atmosphere composition and had a serious impact on climate and biodiversity (e.g., Svensen et al. 2019). Previous high precision isotope dilution thermal ionisation mass spectrometry (ID–TIMS) U-Pb dates obtained on zircon, baddeleyite and perovskite suggest that the intrusive activity lasted from ca. 251.9 Ma until 251.0 Ma (Burgess et al., 2017). However, the total duration of the intrusive magmatic history, as well as its spatial extent, are not well explored in terms of high precision geochronology, since zircon from a mere 17 sills have been dated at highest precision by ID-TIMS so far.To enhance the temporal control on the STLIP intrusive history, we extracted accessory minerals from sill samples of 5 borehole sites that have been characterised geochemically (low-Ti basalt, Callegaro et al., 2021), to perform U-Pb ID-TIMS dating. These boreholes are situated between Bratsk and Tura, in an area for which no ID-TIMS geochronology is available so far. These boreholes will help to fill the gap on the western part of the STLIP dataset presented by Burgess et al. (2017). For three boreholes, we were able to extract baddeleyite and/or zircon from three/four stratigraphic levels. First U-Pb data of baddeleyite indicate predominantly young U-Pb dates, in the range of 251.5 Ma down to 250.0 Ma, with a tendency to minor discordance and a range in dates that exceeds expectation for a single growth population. Curiously, the geochemical characteristics of samples analysed from the same boreholes (Callegaro et al., 2021) are interpreted as potentially earliest stage of the STLIP, a mismatch with our preliminary U-Pb dating results. Given the doubts about the robustness of baddeleyite against Pb-loss during secondary processes, we hope to directly compare baddeleyite and zircon U-Pb dates in selected samples. Should the accuracy of baddeleyite U-Pb dates be confirmed through similarly young zircon U-Pb, this would mean that STLIP intrusive activity and related volatile injection (including Hg) into the atmosphere significantly post-dated the Permo-Triassic Boundary and extended into the Lower Triassic (Griesbachian-Dienerian). This fact would question the assumption that Hg spikes in the sedimentary record of South China are necessarily synchronous and can be used as time markers for the biological crisis at around the PTB.