K-Ar and 40 Ar/ 39 Ar dating of cleavage formed during very low-grade metamorphism: a review

Abstract

Summary Dating cleavage formed during very low-grade (anchizonal) metamorphism of pelitic rocks by K-Ar and/or 40 Ar/ 39 Ar whole-rock analysis is complicated bypolymineralogical sample characteristics and potential argon inheritance within detrital grains. Complex mineralogical effects may be resolved by careful resolution of both K/Ca and 40 Ar/ 39 Ar age spectra. However, detrital influence can be maintained even within penetratively cleaved metapelite samples (whole-rock and constituent white-mica-rich size fractions down to 0.4–0.63 μm) until metamorphic conditions of the anchizone/epizone boundary. The 40 Ar/ 39 and K-Ar whole-rock dating of cleaved metatuffs interbedded with anchizonal metapelites has yielded results consistent with biostratigrapic constraints on the age of cleavage formation, indicating a lack of detritai influence. Constituent grains within white-mica-rich size fractions separated from upper anchizone metatuffs are characterized by well-defined margins and record mutually concordant K-Ar and 40 Ar/ 39 Ar total-gas ages which display no systematic variations with grain size. Middle anchizone metatuff size fractions are composed of white mica grains with poorly defined edges. These consistently record 40 Ar/ 39 Ar total-gas ages which are 10–15% older than corresponding K-Ar dates (whole-rock and size fractions). This discordance is attributed to recoil-loss of 39 Ar during irradiation, and implies that morphology of grain edges (affecting surface/volume ratios) can be a major factor controlling 39 Ar recoil.