PALEOMAGNETISM OF EAST SIBERIAN TRAPS AND KIMBERLITES: TWO NEW POLES AND PALAEOGEOGRAPHIC RECONSTRUCTIONS AT ABOUT 360 AND 250 MA

Abstract

We studied basalt sills and dykes and kimberlite pipes from the eastern part of the Siberian platform. These are spread out over a few hundred kilometres and are divided into two age groups: Late Permian–Early Triassic (Mir, Aikhal and Olenyek regions) and Late Devonian–Early Carboniferous (Viluy and Markha basins, Aikhal region). The palaeomagnetic poles determined for each sill or dyke are statistically different from each other, but averaging all corresponding directions from these sills and dykes with those from kimberlites of different generations yields results with acceptable statistics. This is probably due to the averaging out of secular variation and/or changes in magnetic polarity. The resulting palaeomagnetic poles allow one to reconstruct the palaeopositions of Siberia during the two large flood basalt events at about 360 and 250 Ma. A significant rotation of the Siberian platform during the Devonian and Carboniferous is suggested. The new data are in general agreement with some earlier palaeomagnetic poles for the Late Permian–Early Triassic traps of the Tungusska basin, and significantly reduce uncertainties for previous Late Devonian–Early Carboniferous Siberian poles, which were very scattered. Together with geological and absolute age data, the palaeomagnetic data support the hypothesis that kimberlite magmatism and trap intrusions were both rapid and relatively close in time. The 360 Ma event is emerging as a magmatic event with considerable extent over the Russian and Siberian platforms, related to incipient continental break-up and triple junction formation, possibly at or near the time of the Frasnian–Famennian mass extinction.