ILLITIC SUBSTITUTION IN MICAS OF VERY LOW-GRADE METAMORPHIC CLASTIC ROCKS

Abstract

The chemical compositions of micas from six well-characterised sequences have been jointly evaluated in order to analyse the behaviour of illitic substitution (SiA1-1 □ K-1) in relation to diagenetic/metamorphic grade. Data from ~265 crystals, corresponding to 48 samples, were obtained through EDX analyses in SEM and TEM and the influence of analytical artefacts, such as contamination and alkali loss, was critically evaluated. From diagenesis to low epizone, the compositions of the micas are highly heterogeneous at the sample level, do not show systematic differences between grades, and include a significant illitic component, ranging from ∑interlayer cations = 0.6 to 1.0 a.f.u. At high epizone (Kübler Index < 0.2 Δ °2θ), a very significant textural change is accompanied by homogenisation of the chemical composition and a reduction of the illitic component to very low values. Our data suggest continuity in the composition between illite and muscovite and do not allow us to confirm the existence of a miscibility gap in low-T environment natural samples. Some of the mica analyses reaching Si contents of around 3.5 a.f.u. correspond to interlayer-cation population contents in the range of 0.6-0.7 a.f.u., indicating illitic substitution. Therefore, thermobarometry of low-temperature phengites based only on the Si-content, without taking into account the illitic component, would result in pressure overestimates. © 2006 E. Schweizerbart'sche Verlagsbuchhandlung.