Ua. Glasmacher et al., K-Ar dating of magmatic sericite crystallites for determination of coolingpaths of metamorphic overprints, CHEM GEOL, 175(3-4), 2001, pp. 673-687
A combined optical and X-ray characterisation of fine-grained micas in gran
odioritic to tonalitic dikes and intrusions of the Eastardennes-Eifel nappe
(Linksrheinisches Schiefergebirge, Germany-Belgium) was completed with K-A
r dating of separated size fractions from these rocks. In each sample, the
K-Ar ages decrease with decreasing size fraction, but the same size fractio
ns of the different samples yield different ages. Application of the closur
e temperature concept to the K-Ar ages of the various size fractions indica
tes that the crystallite size expressed as CIS-FWHMser001ad and coherent sc
attering domain (CSD) thickness acts as the effective radius of Ar diffusio
n.
A linear model may describe the correlation between K-Ar ages and crystalli
te thickness. The ages younger than the model prediction are caused by the
occurrence of microcline as an accessory K-bearing mineral phase. The obtai
ned K-Ar ages were interpreted as representative of the cooling path of the
magmatic components in the Eastardennes-Eifel nappe below 330 degreesC. Th
e 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 develo
pment. 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 tem
perature of this mineral at about 140 degreesC. (C) 2001 Elsevier Science B
.V. All rights reserved.