Variaciones de la "cristalinidad" de la mica blanca y otros parámetros cristaloquímicos a través del antiforme del Narcea (Orógeno variso del NO de Iberia)

Résumé

X-ray diffraction analyses were performed on white-micas, formed under strain and low-grade metamorphic conditions, from sandstones to siltstones along a transect through the Narcea Antiform, a large structure that separates the external from the internal zones of the Variscan belt of NW Spain. The results indicate a close spatial relationship between the onset of metamorphism and the cleavage front. The cleavage front is accompanied by an decrease in the "crystallinity" index of the white-mica from very low to low metamorphic grade. This decrease corresponds qualitatively to an increase in penetrativeness of the axial planar cleavage. In the western part of the Narcea Antiform, the major deformation event was the development of kilometer-scale, reverse shear zones. Because of polyphase deformation, these rocks are not suitable for quantitative strain analysis, but qualitatively, finite strain can be observed to be prolate and invariably larger in this region than in the eastern sector. The metamorphic grade is higher in the western part of the Narcea Antiform, with local maxima occurring next to the base of the shear zones. The ¿>0 cell parameter, a semi-quantitative geobarometer measured in the white-mica, is chaotic in the Eastern sector, showing a clear detrital in heritance. In the western sector it is rational due to complete re-equilibration during thermal resetting.