Nicolás Velilla

Prograde and retrograde diagenetic and metamorphic evolution in metapelitic rocks of Sierra Espuña (Spain)

Abstract An unusually complete sequence of pelitic rocks ranging from diagenetic to greenschist-facies metamorphic grades occurs in southern Sierra Espunã, Spain. Prograde and retrograde reactions have been studied by X-ray diffraction and electron microscopy (SEM, TEM and AEM). The prograde reaction series, with reactions facilitated by tectonic stress, includes: (1) R4 interstratified illite-smectite in the diagenetic Malaguide Complex that preserves the variable orientation of original smectite packets, and has 1Md polytypism; (2) chemically heterogeneous illite and Na-K dioctahedral white micas that progressively evolve toward chemical and textural equilibrium in the anchizonal Intermediate Units; and (3) thick, defect-free packets of phengite, paragonite and clinochlore which have a typical metamorphic texture, in the Alpujarride Complex. Two superimposed retrograde episodes produced: (a) sudoite at near-peak metamorphic conditions and (b) dioctahedral smectite during low-temperature retrograde diagenesis.

Illitic substitution in micas of very low-grade metamorphic clastic rocks

The chemical compositions of micas from six well-characterised sequences have been jointly evaluated in order to analyse the behaviour of illitic substitution (SiAl -1 K -1 ) in relation to diagenetic/metamorphic grade. Data from ~265 crystals, corresponding to 48 samples, were obtained through EDX analyses in SEM and TEM and the influence of analytical artefacts, such as contamination and alkali loss, was critically evaluated. From diagenesis to low epizone, the compositions of the micas are highly heterogeneous at the sample level, do not show systematic differences between grades, and include a significant illitic component, ranging from interlayer cations = 0.6 to 1.0 a.f.u. At high epizone (Kubler Index < 0.2 °2 ), a very significant textural change is accompanied by homogenisation of the chemical composition and a reduction of the illitic component to very low values. Our data suggest continuity in the composition between illite and muscovite and do not allow us to confirm the existence of a miscibility gap in low-T environment natural samples. Some of the mica analyses reaching Si contents of around 3.5 a.f.u. correspond to interlayer- cation population contents in the range of 0.6-0.7 a.f.u., indicating illitic substitution. Therefore, thermobarometry of low- temperature phengites based only on the Si-content, without taking into account the illitic component, would result in pressure overestimates. Resumen: Se evaluan de manera conjunta los datos qu´ omicos de micas correspondientes a seis secuencias bien caracterizadas previamente, con el objeto de analizar el comportamiento de la sustitucion il´ otica (SiAl-1 K-1), en relacion al grado diagenetico/ metamorfico. Se han empleado los datos qu´ omicos obtenidos mediante SEM (EDX) y TEM (AEM) de ~265 cristales pertenecientes a 48 muestras. Se ha considerado cr´oticamente la influencia de artefactos anal´ oticos tales como la contaminacion y la volatilizacion de alcalinos. Desde la diagenesis hasta la epizona debil, las muestras se caracterizan por mostrar micas composicionalmente muy heterogeneas, sin diferencias sistematicas entre grados y con un componente il´ otico significativo, con valores de interlaminares de 0.6 a 1.0 a.f.u. En la epizona intensa (´ de Kubler < 0.2 °2 ) un cambio textural importante acompana a la homogeneizacion de la composicion qu´ omica de las micas con la consiguiente reduccion del componente il´ otico a valores muy bajos. Nuestros datos sugieren continuidad en la composicion entre ilitas y moscovitas y no permiten confirmar la existencia de un hueco de miscibilidad en ambientes naturales de baja temperatura. Algunos analisis de las micas alcanzan valores de Si de 3.5 a.f.u. junto a una poblacion de interlaminares en el rango de 0.6 a 0.7 a.f.u. que indican claramente la sustitucion il´ otica. Por tanto, la termobarometr´ oa de micas feng´ oticas de baja temperatura basada solo en el contenido en Si sin tener en cuenta el componente il´ otico, podr´ oa producir una sobrevaloracion de la presion.

DEFORMATION PROMOTED DEFECTS AND RETROGRADE CHLORITIZATION OF BIOTITE IN SLATES FROM A SHEAR ZONE, SOUTHERN IBERIAN MASSIF, SE SPAIN

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 1Md). 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.

Two-stage formation of kaolinite in shear-zone slates, southern Iberian Massif, SE Spain

Abstract Kaolinite aggregates are the major alteration product in the strongly deformed areas of a shear zone developed in contact-metamorphosed slates from the Southern Iberian Massif near Jaen (SE Spain). Muscovite and Fe-rich chlorite in strain shadows of broken andalusite crystals were observed in slates from less deformed areas. The presence of phyllosilicate packets nearly perpendicular to each other and veinlets parallel to {110} of andalusite reveal the importance of cleavage-controlled fracturing of andalusite to the mineral reactions. The compositional similarity between kaolinite and andalusite suggests a direct transformation of andalusite to kaolinite in the strongly altered area. The absence of clear preferred orientation of phyllosilicates relative to andalusite, and the compositional differences of andalusite with muscovite and chlorite from the less altered samples, indicate andalusite dissolution and chlorite-muscovite precipitation during a first mineral-growth stage. Muscovite-kaolinite and chlorite-kaolinite intergrowths indicate a later retrograde replacement stage.

Deformation-enhanced mineralogical changes in igneous quartz-feldspar rocks from a semi-brittle shear zone, Southern Iberian Massif, SE Spain

Two stages of crystallization at greenschist and sub-greenschist facies were identifi ed in leucocratic dikes and granodioritic rocks in a shear zone on the southern border of the Santa Elena intrusion in the southern Iberian Massif near Jaén (SE Spain). Protoliths containing quartz+plagioclase+K-feldspar±muscovite±biotite experienced intense fracturing, fl uid infi ltration and metasomatism replacing the primary assemblage by muscovite and biotite interlayered with kaolinite, K-feldspar and albite, partly developing a myrmekitic texture. The mineral paragenesis and microcline-low-albite solvus thermometry suggest upper greenschist facies temperatures during a prograde process comprising an initial synkinematic stage and a postkinematic metasomatic replacement. Deformation of porphyroclasts indicates a combination of brittle to plastic shearing which is consistent with this temperature. The synkinematic mica formation caused strain softening. A fi nal retrograde postkinematical stage of shear zone development at high amounts of fl uid at lower temperature is refl ected by the presence of kaolinite formed mainly by the replacement of mica and plagioclase in veins.