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A Late Mesoproterozoic 40 Ar/ 39 Ar age for a melt breccia from the Keurusselkä impact structure, Finland
Author(s) -
Schmieder Martin,
Jourdan Fred,
Moilanen Jarmo,
Buchner Elmar,
Öhman Teemu
Publication year - 2016
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12594
Subject(s) - breccia , geology , impact structure , geochemistry , petrography , impact crater , metamorphic rock , basement , archaeology , physics , astronomy , history
Field investigations in the eroded central uplift of the ≤30 km Keurusselkä 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 ~8–10 GP a. The mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia. 40 Ar/ 39 Ar dating of dark and clast‐poor whole‐rock chips yielded five concordant Late Mesoproterozoic miniplateau ages and one plateau age of 1151 ± 10 Ma [± 11 Ma] (2σ; MSWD = 0.11; P = 0.98), considered here as the statistically most robust age for the rock. The new 40 Ar/ 39 Ar age is incompatible with ~1.88 Ga Svecofennian tectonism and magmatism in south‐central Finland and probably reflects the Keurusselkä 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 40 Ar/ 39 Ar age of ~1150 Ma should be treated as a minimum age for the impact. The new 40 Ar/ 39 Ar results are consistent with paleomagnetic results that suggested a similar age for Keurusselkä, which is shown to be one of the oldest impact structures currently known in Europe and worldwide.