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 2M 1 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 twolayer polytype (probably 2M 1 ). 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.

Retrograde Assemblages of Gneisses at Arigidi and Erusu Areas of Southwestern Nigeria

“…Also, Boesse and Ocan [39] in a study of high grade terrains of southwestern Nigeria have recognized two phase of deformation in southwestern Nigeria. Biotite chloritization at the shear zones have been reported in different parts of the world [7,[40][41][42]. The geological map (Figure 2b) and field photographs are evidences that the rocks of Arigidi and Erusu (Figures 1b, 1c & 1d) have been subjected to shearing.…”

“…Several researches have over the years been carried out on the chloritization of biotite [1][2][3][4][5][6][7][8].…”

“…The breakdown of biotite has been reported to begin with the diffusion and exchange of K + for H + within biotite interlayers [4]. Biotite chloritization can take place in both hydrothermal [8,[13][14][15] and metamorphic [7,[16][17][18] environments. Chloritization of biotite can also result from the weathering process of rocks [19][20][21].…”

Colour Transformation and Textural Change in Biotite: Some Remarks for the Interpretation of Firing Technology in Greyware Pottery Thin-Sections

“…This has been a topic of special interest to soil scientists for a long time, and much of the research on mica weathering is found in soil science literature [27][28][29][30][31] rather than in geochemical literature [32,33]. Despite this, the mechanisms of biotite breakdown under pressure and heat conditions-which are closer to those in pottery fabrication than mere weathering reactions-have been widely explored within the domain of metamorphic petrology [34][35][36][37]. Shock-induced pressure-dependent transformations concomitant to impact metamorphism can lead to the formation of new minerals such as garnet or vesiculated glass [38], but the most common affectations of this mineral are those related to thermal [39][40][41][42] or hydrothermal alteration [43,44].…”

Stress-Induced Alteration of Sudoite: Structural and Chemical Modifications

“…Vernon, 1977;Knipe, 1981;Bell and Wilson, 1981;Amouric, 1987) the structural and chemical modifications associated with natural deformations have been investigated to a lesser extent (e.g. Olives et al, 1983;Sánchez-Navas and Galindo-Zaldívar, 1993;Ooteman et al, 2003;Jiménez-Millán et al, 2007), and devoted in most cases to biotite chloritization. An illustrative example of the structural and compositional modifications associated with progressive deformation of chlorite (sudoite) present in quartz-rich veins from the Internal Zone of the Rif range is presented here.…”