Deformation-promoted defects and retrograde chloritization of biotite in slates from a shear zone, Southern Iberian Massif, SE Spain

Abstract

Naturally deformed biotite in contact-metamorphosed slates affected by a shear zone of the Southern Iberian Massif near Jaén (SE Spain) were studied by X-ray diffraction, optical microscopy, scanning electron microscopy, electron probe microanalysis and high-resolution transmission and analytical electron microscopy. Biotite is found in the contact metamorphism aureole produced by the intrusion of a granodioritic stock, but shear strain caused its deformation. The southern part of the shear band is strongly deformed, containing thick clay gouge zones. The northern part is less deformed and develops weaker planar-linear fabrics. X-ray diffraction data reveal the predominance of the 2M₁ biotite in the undeformed samples whereas the 1M polytype is predominant in the sheared samples. Chemical data and electron images of the biotite from unsheared slates do not show the presence of intercalated phases. This biotite is almost defect-free and electron diffraction patterns have sharp reflections indicating a two-layer polytype (probably 2M₁). Back-scattered electron images from the deformed biotite in the moderate deformation part of the shear zone do not reveal intergrown minerals, but the electron microprobe analyses show some Fe- and Mg-enriched compositions. Transmission electron microscopy indicates that disordered polytype packets are predominant (probably 1M_d). Their electron diffraction patterns have diffuse streaking along c*. These packets have high dislocation densities, microcavities with ∼5 Å latticefringe regions (probably brucite-like sheets) and interlayering of chlorite-berthierine. Kaolinized biotite can be observed in the clay gouges from the strongly deformed south part of the shear zone. The degree of streaking, as an indication of the intensity of deformation, revealed that the disordered polytype packets are more deformed than the two-layer polytype packets. The microcavities of the disordered polytype packets could act as potential channels for transport of fluids during the shearing stage and serve as sites for chloritization of biotite, producing chlorite-berthierine domains within biotite. Berthierine is an intermediate metastable phase replaced by chlorite with along-layer transitions.