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

Abstract

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°C. 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°C.