engleski

K-Ar ages versus grain size and illite Kübler index: a case study of parametamorphic rocks from eastern Mt.Papuk, Croatia

One of the frequent and most important problems in K-Ar dating is the contamination with detrital materials which can results in hard to interpret mixed ages. The coarse fraction isolated from samples is most often heterogeneous i.e. composed partially of detrital minerals and of more recent authigenic minerals which grew as a result of post-depositional thermal processes. Variable age mostly depends on the proportions of detrital and authigenic phases, both of which are considered to have a fixed K-Ar age. On the contrary, it is generally assumed that small grain-size fractions (e.g. <2 μm fraction) are mostly devoid of the detrital mica, while their KAr age represents the “mix age” of different generations of illitic minerals present within it. A special attention here was paid to the evaluation of the influence of detrital minerals by observation of variation in KAr ages and illite Kübler index (KI) obtained on different grainsize fractions. Samples of high temperature anchizonal to epizonal altered parametamorphic rocks, representing various lithologies with different petrographic characteristics and stratigraphic age were used for this purpose. Some trends can be observed. The decrease of K-Ar ages with decreasing grain-size can be attributed to an increase of diffusion of radiogenic 40Ar resulting in increasing reset of the K-Ar system with decreasing grain size and to a reduction of detrital minerals contamination effect which becomes less and less pronounced as grain size decreases. Furthermore, KI of the finer fractions increases with decreasing grain size showing negative correlation with decreasing K-Ar ages. This effect is closely connected with the prolonged formation of illitic minerals and their wide temperature stability.

K-Ar ages; grain size; illite; Kübler index; parametamorphic rocks; Papuk; Croatia

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