40Ar/39Ar dating of intrusive magmatism in the Angara-Taseevskaya syncline and its implication for duration of magmatism of the Siberian traps

Dolerite sills are the dominant type of mafic intrusions within the Siberian traps province. Their total volume is comparable to the volume of effusive magmas. Six large dolerite sills are known on basis of geological survey data within the Angara-Taseevskaya syncline at the southeastern part of the Siberian traps. We obtained 40Ar/39Ar 'plateau' ages of 240.1 ± 1.0/2.5/5.4 Ma and 241.6 ± 1.3/2.6/5.5 Ma for Tulunskii and Padunskii sills, respectively (errors are reported as x/y/z, where x, y and z are analytical, internal and external errors, respectively). The obtained ages are indistinguishable from each other at level of analytical errors and suggest emplacement of the sills during the same Middle Triassic pulse of magmatism. Samples from Tolstomysovskii and Chuna-Biryusinskii sills did not yield statistically acceptable plateau ages, though their 40Ar/39Ar spectra and K/Ca versus apparent age diagrams suggest that they were emplaced in the Middle Triassic too. The dolerites are characterized by uniform trace element and platinum group element patterns typical for low-Ti tholeiites of the Noril'sk area, although the Noril'sk low-Ti tholeiites were erupted at Permo-Triassic boundary about 9 Ma earlier. The low-Ti tholeiites make up the greatest volume of the flood basalts and possess a distinctive chemical signature. Our data suggests that low-Ti tholeiitic magma was emplaced at more than one locality in the Siberian traps province, and in more than one temporal pulse.