Textural and chemical changes in slate-forming phyllosilicates across the external-internal zones transition in the low-grade metamorphic belt of the NW Iberian Variscan Chain

Abstract

Differences in the chemistry and texture of slate-forming phyllosilicates representative of foreland and hinterland rocks of the Iberian Variscan Orogenic Belt have been determined by scanning and transmission electron microscopy. Based on a previous X-ray diffraction study, representative samples of both internal and external domains were selected revealing clearly different characteristics. The two samples corresponding to the foreland rocks have similar texture, showing a mixture of sedimentary and metamorphic features at the backscattered electron scale, independently of their respective epizone and anchizone illite crystallinity indices. At the crystalline lattice level, mica and chlorite packets form subparallel low-angle intergrowths and show strain features, more obvious in chlorite than in mica. The only difference justifying their different crystallinity indices concerns quantitative characteristics such as crystallite size and frequency of defects. The two hinterland samples are very similar and are formed by perfect, defect-free micrometer-size phyllosilicates in a typically metamorphic parallel orientation. The foreland samples are chemically heterogeneous, with each individual analysis being affected by variable degrees of illitic, phengitic and ferrimuscovitic substitutions. In contrast, the hinterland samples have evolved to more homogeneous compositions approaching the end-member muscovite. The effect of tectonic strain was fundamental both in the development of a metamorphic texture and the approach to chemical equilibrium.