Abstract
White mica bearing fractions ranging in grain size from 0.4 μm to 6.3–20 μm were separated from metapelites and intercalated metatuffs of the eastern Rheinisches Schiefergebirge (FRG). The stratigraphic age of these rocks is Middle Devonian (Eifelian), and they contain detrital material of northwestern provenance (Old Red Continent, probably mainly derived from the Caledonian Orogen). Folding in the Carboniferous was associated with cleavage formation and an apparently synkinematic anchizonal metamorphism. Apparent K-Ar ages of metapelite fractions display a marked positive correlation with grain size illustrating the detrital influence which is diminished with decreasing grain size and increasing metamorphism (determined by illite “crystallinity”). Contrasting grain morphologies observed by SEM enable the interpretation of apparent age/ grain size relationship for coarse fractions. The anticipated lack of detrital mica in metatuffs is confirmed by the fairly consistent apparent K-Ar ages determined for the coarser than 0.63 μm size fractions which date the anchizonal metamorphism at ca. 330 Ma. Comparison of metatuff and metapelite apparent ages suggests that the extent of rejuvenation in the latter was largely dependent on grain size. Rejuvenation was also somewhat controlled by the degree of anchizonal metamorphism as suggested by differences in K-Ar results of metapelites which were metamorphosed at variable anchizonal conditions. Fractions <0.63 μm from upper anchizonal metapelites record ca. 330 Ma ages similar to those of the 0.63–20 μm sizes in metatuffs. Together those results confirm the limited applicability of conventional K-Ar dating on bulk clay fractions (<2 μm) of very-low grade (anchizonal) metamorphic rocks in dating metamorphic events and concomitant cleavage formation.
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Reuter, A. Implications of K-Ar ages of whole-rock and grain-size fractions of metapelites and intercalated metatuffs within an anchizonal terrane. Contr. Mineral. and Petrol. 97, 105–115 (1987). https://doi.org/10.1007/BF00375218
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DOI: https://doi.org/10.1007/BF00375218