Chemical and textural characterisation of diagenetic to low-grade metamorphic phyllosilicates in turbidite sandstones of the South Portuguese Zone: A comparison between metapelites and sandstones

Abstract

The intercalation at the meter scale of shales and sandstones in the incipient metamorphic sequence of the South Portuguese Zone allows to compare the evolution from diagenesis to low metamorphic grade in both of these lithologies. Data from X-ray diffraction, optical, scanning and transmission electron microscopy as well as wholerock chemical analysis from the sandstones have been obtained and are compared with previously published data from the shales. Obvious textural differences are present from the outcrop scale to the low-magnification TEM scale; phyllosilicate packets in sandstones are clearly randomly oriented, with high-angle boundaries contrasting with the usual sub-parallel orientation in shales. But no differences have been found between the phyllosilicates of the two rocks types when comparing lattice-fringe images, polytypes, chemical composition or crystalline-domain size. Consequently, illite crystallinity and traditional crystal-chemical parameters, such as d001 and b cell dimension of K-white mica, produce equivalent and inter-changeable information between the sandstones and shales. The chemical compositions of the phyllosilicates are characterised by a general lack of homogeneity at the sample level. The phengitic vector is mainly responsible for the scattering of compositions, with a minor influence from the illitic vector. The grain size of very low-grade metamorphic turbidite sequences such as these should not be a limiting criterion in the sampling, because samples with intermediate characteristics between metapelites and sandstones may also provide valid results.