K-Ar dating of magmatic sericite crystallites for determination of cooling paths of metamorphic overprints

A combined optical and X-ray characterisation of fine-grained micas in granodioritic to tonalitic dikes and intrusions of the Eastardennes-Eifel nappe (Linksrheinisches Schiefergebirge, Germany-Belgium) was completed with K-Ar dating of separated size fractions from these rocks. In each sample, the K-Ar ages decrease with decreasing size fraction, but the same size fractions of the different samples yield different ages. Application of the closure temperature concept to the K-Ar ages of the various size fractions indicates that the crystallite size expressed as CIS-FWHM(ser001ad) and coherent scattering domain (CSD) thickness acts as the effective radius of Ar diffusion. A linear model may describe the correlation between K-Ar ages and crystallite thickness. The ages younger than the model prediction are caused by the occurrence of microcline as an accessory K-bearing mineral phase. The obtained K-Ar ages were interpreted as representative of the cooling path of the magmatic components in the Eastardennes-Eifel nappe below 330 degreesC. The K-Ar age of the largest size fraction of the tonalitic dike #V11 (313.6 +/- 7.2 Ma) is the closest value for the age of the Variscan cleavage development. The oldest age of the hydrothermally altered tonalitic dike (300.2 +/- 8.4 Ma) might represent an episode of hydrothermal Fluid migration after cleavage development; it represents at least a minimum age. The microcline age (242.8 +/- 6.0 Ma) of the Hill intrusion may represent the closure temperature of this mineral at about 140 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.