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Timing of the Traps and the Permo-Triassic boundary
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The Siberian Traps - Radiometric Dating

Published 40Ar/39Ar ages (whole-rock, biotite and plagioclase separates) exist for basalts and gabbros from Noril'sk, Putorana, Maymecha-Kotuy, West Siberian Basin, Taimyr, Kuznetsk Basin and the polar Urals. Normalised to a common standard, they all lie within error of each other, and indicate a magmatic crystallization ages of between 248-250 Ma (Figure 1). The differences between the youngest and oldest age obtained on the earliest Noril’sk Ivakinsky suite rocks (248.5±1.9 and 250.1±1.9 Ma; Renne and Basu, 1991; Venkatesan et al., 1997) and a recently obtained age on the uppermost Kumginsky suite (250.3±1.1 Ma; Reichow et al., 2009) are 1.8±2.2 Ma or 0.2±2.2 Ma, respectively. These ages confirm previous estimates of the short duration of magmatism not only in the Noril’sk area but also for the larger Siberian Traps province. 40Ar/39Ar measurements of biotite from intrusive rocks tend to give slightly older ages, possibly related to the presence of excess argon.

Zircons and baddeleyites from intrusions at Noril'sk, together with perovskite, zircon and baddeleyite from Maymecha-Kotuy, give U/Pb concordia ages that are slightly but significantly older than the 40Ar/39Ar ages. Whilst there are fewer U/Pb ages, they do tend to be very precise. Similarly, the U/Pb (zircon) age of the Meishan and Shangsi tuffs is slightly older than the equivalent 40Ar/39Ar sanidine age.

Figure Caption. Published radiometric ages, coded according to technique and sample type, for the Siberian Traps and contiguous regions. Modified after Figure 2 in Reichow et al. (2002) , with the addition of more recent determinations. Data sources, 40Ar/39Ar dates: West Siberian Basin (WSB) - Reichow et al. (2002, 2009) and Pringle et al. (1995); Noril'sk region - Renne and Basu (1991), Dalrymple et al. (1995), Renne et al. (1995), Venkatesan et al. (1997); Putorana region - Renne and Basu (1991); Maymecha-Kotuy region -Basu et al. (1995); P-Tr boundary: Renne et al. (1995), Reichow et al. (2009) . U/Pb dates: Noril'sk - Kamo et al. (1996); Maymecha-Kotuy - Kamo et al. (2003); P-Tr boundary (left to right): Claoué-Long et al. (1991) and Mundil et al. (2001, 2004). 40Ar/39Ar age of ash Bed 25 (Renne et al., 1995) with error bars in grey delineates the peak of the end-Permian extinction at its base. All Ar-Ar dates are given relative to FC at 28.02 Ma (Renne et al., 1998).

Outliers of basalt throughout western Siberia, and extensive subcrops within the West Siberian Basin, are often associated with the 250 Ma Siberian Traps volcanism. Borehole samples located south of the city of Chelyabinsk gave ages of 243.3 ± 0.6 Ma and 242.2 ± 0.6 Ma clearly demonstrating that volcanism to the southeast of the Urals is ~7 Ma younger than the main activity on the Siberian craton and within the WSB (Reichow et al., 2009). Sills with Triassic ages were also reported (Ivanov et al., 2005) in the Kansk-Taseevskaya basin along the southern border of the Siberian craton and for extrusive rocks in the Semeitau area, Kazakhstan (Lyons et al., 2002). These results indicate that volcanism in Siberia occurred in at least two stages.