A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselka impact structure, Finland

Field investigations in the eroded central uplift of the 30km Keurusselka impact structure, Finland, revealed a thin, dark melt vein that intersects the autochthonous shatter cone-bearing target rocks near the homestead of Kirkkoranta, close to the center of the impact structure. The petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding similar to 8-10 GPa. The mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia. 40Ar/39Ar dating of dark and clast-poor whole-rock chips yielded five concordant Late Mesoproterozoic miniplateau ages and one plateau age of 1151 +/- 10Ma [+/- 11Ma] (2 sigma; MSWD = 0.11; P=0.98), considered here as the statistically most robust age for the rock. The new 40Ar/39Ar age is incompatible with similar to 1.88Ga Svecofennian tectonism and magmatism in south-central Finland and probably reflects the Keurusselka impact, followed by impact-induced hydrothermal chloritization of the crater basement. In keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration, the Late Mesoproterozoic 40Ar/39Ar age of similar to 1150Ma should be treated as a minimum age for the impact. The new 40Ar/39Ar results are consistent with paleomagnetic results that suggested a similar age for Keurusselka, which is shown to be one of the oldest impact structures currently known in Europe and worldwide.