Isabel Abad

Amazonian Mesoproterozoic basement in the core of the Ibero-Armorican Arc: 40Ar/39Ar detrital mica ages complement the zircon's tale

The 40 Ar/ 39 Ar age data on single detrital muscovite grains complement U-Pb zircon ages in provenance studies, as micas are mostly derived from proximal sources and record low-temperature processes. Ediacaran and Cambrian sedimentary rocks from northwest Iberia contain unmetamorphosed detrital micas whose 40 Ar/ 39 Ar age spectra suggest an Amazonian‐Middle American provenance. The Ediacaran sample contained only Neoproterozoic micas (590‐783 Ma), whereas the Cambrian sample contained three age groups: Neoproterozoic (550‐640 Ma, Avalonian‐Cadomian‐Pan African), MesoproterozoicNeoproterozoic boundary (ca. 920‐1060 Ma, Grenvillian-Sunsas), and late Paleoproterozoic (ca. 1580‐1780 Ma, Rio Negro). Comparison of 40 Ar/ 39 Ar muscovite ages with published detrital zircon age data from the same formations supports the hypothesis that the Neoproterozoic basins of northwest Iberia were located in a peri-Amazonian realm, where the sedimentary input was dominated by local periarc sources. Tectonic slivering and strike-slip transport along the northern Gondwanan margin affected both the basins and fragments of basement that were transferred from Amazonian to northern African realms during the latest Neoproterozoic‐earliest Cambrian. Exhumation and erosion of these basement sources caused shedding of detritus to the Cambrian basins, in addition to detritus sourced in the continental mainland. The apparent dominance of Rio Negro‐aged micas in the Cambrian sandstone suggests the presence of unexposed basement of that age beneath the core of the Ibero-Armorican Arc.

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.

The structure and the phyllosilicates (chemistry, crystallinity and texture) of Talas Ala-Tau (Tien Shan, Kyrgyz Republic): comparison with more recent subduction complexes

Abstract Geological mapping and structural analysis of the Talas Ala Tau (Tien Shan, Kyrgyz Republic) have revealed a complex structure composed of folds with axial-plane cleavage and thrust faults verging towards the NE. The main structures of the range correspond to minor Tertiary and Carboniferous–Permian deformation superimposed on the main deformation event that took place during the Baikalian orogeny. The pervasive axial-plane cleavage diminishes in penetrativity from the hinterland to the foreland in both the Uzunakhmat and Karagoin sheets. The main thrusts developed phyllonitic shear-related rocks on the hangingwall immediately above the thrust planes. A crystal-chemical study of the phyllosilicates growth during the Baikalian deformation event along a cross-section revealed changes in the crystallinity, composition and lattice parameters of them. The phyllosilicates present in the Talas Ala Tau rocks were crystallized in very low-grade metamorphic conditions, that is below 300 °C, as indicated by their Kubler Index (KI), which decreases from SW towards the NE. Detailed TEM study of the phyllosilicates reveals a clear textural difference at the lattice level between samples with higher or lower KI parameters. There is also a clear difference in crystal-chemical parameters (KI and b) and composition between the phyllosilicates growth in relation to the axial-plane cleavage and the ones belonging to the thrust-related phyllonites. The first ones are more affected by the ferrimuscovitic vector than the phyllosilicates of phyllonites, closer to the theoretical phengitic component. Huge ranges of values of phengitic content of micas at sample level are interpreted as the result of a decompression path from at least 8 kbar. We propose a subduction geodynamic environment for the regional deformation and the origin of the phyllosilicates, as they are similar to those obtained in more recent accretionary complexes.

The role of H3O in the crystal structure of illite

In spite of decades of research on the subject, the crystal structure of illite is still poorly understood. The purpose of this study was to address this problem by investigating the nature of the interlayer content in illite IMt-2 from Silver Hill, Montana, using analytical transmission electron microscopy (ATEM), thermogravimetry (TG), and X-ray powder diffraction (XRPD) analyses. The ATEM data, together with literature and TG results, yielded the formula K0.70a0.01(H2O)0.42 (Al1.53Fe2+0.06Fe3+0.19Mg0.28)Σ−2.06(Si3.44Al0.56)O10(OH)2 or, assuming the presence of H3O+, K0.69Na0.01(H3O)+0.28(Al1.47Fe2+0.06Fe3+0.19Mg0.28)Σ−1.99(Si3.40Al0.60)O10(OH)2. The first formula indicates surplus interlayer and octahedral species, whereas the second shows no excess. The XRPD data were refined by Rietveld techniques, down to an Rp factor of 10.48–13.8%. The mineral composition consists largely of illite-2M1, illite-1M, and minor quartz. Although the refinement accuracy is limited by the intrinsic poor quality diffraction of the illites, the partially refined model is consistent with the chemical composition; in particular, attempts to introduce octahedral cations in excess of 2 were fruitless. All the results support the simple structural model, by which the illite structure strictly corresponds to a dioctahedral mica with H3O+ replacing K. As a consequence, the crystalchemical formula of illites should be calculated on the basis of six tetrahedral plus octahedral cations.

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.

Diagenesis to metamorphism transition in an episutural basin: the late Paleozoic St. Mary’s Basin, Nova Scotia, Canada

The Late Devonian - Early Carboniferous St. Marys Basin in the Canadian Appalachians consists of Horton Group fluviatile and lacustrine clastic rocks. The basin occurs along the boundary between the Avalon and Meguma ter- ranes and developed during coeval dextral shear along that boundary. X-ray diffraction reveals that the rocks contain ubiq- uitous quartz, K-white mica, and albite; illite-smectite mixed layers and chlorite are very common and Na-K mica, kaolinite, chlorite-smectite mixed layers, K-feldspar, berthierine, and rutile occur in some samples. Crystal-chemical pa- rameters of white mica indicate the pressure and temperature of mineral growth and discriminate between diagenetic, an- chizone, and low-grade metamorphic processes. Kubler index values measured in the 5 A u peak and the presence of chlorite-mica stacks are indicative of high-anchizone-epizone grades, with a crystallinity (crystal size and number of de- fects) that increases towards the Chedabucto Fault, which defines the northern margin of the basin. Kubler index values measured in the 10 A u peak indicate that a late fluid-rich event could have produced the observed illite-smectite mixed layers. The overall clay-mineral content and the b-cell dimension of the K-white micas are typical of postdepositional evo- lution in extensional sedimentary basins with high heat flow (>35 8C/km). Taken together, our data record two superposed events related to deformation along the basin margins and coeval regional fluid flow, in which retrograde reactions at tem- perature T 300 8C). Regional syntheses indicate that this fluid flow may have occurred during epi- sodes of Late Carboniferous dextral shear along the Avalon-Meguma terrane boundary.

Prograde epizonal clay mineral assemblages and retrograde alteration in tectonic basins controlled by major strike-slip zones (W Iberian Variscan chain)

Abstract We have carried out optical microscopy, X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM) studies of phyllosilicates from black slates of very low to low-grade metamorphism. Such slates belong to a Middle/Late Devonian basin and an Early Carboniferous basin associated with the Porto-Tomar-Ferreira do Alentejo strike-slip shear zone (Ossa-Morena Zone, Portuguese Iberian Variscan Massif). These black slates are imbricated in an Upper Proterozoic substratum of higher metamorphic grade. Kübler Index values of white micas and mineral assemblages deduced from the XRD, SEM and TEM data (muscovite, chlorite and pyrophyllite) indicate high anchizonal and epizonal metamorphic conditions for slates from these basins. The b parameter and the phengitic contents of mica suggest the occurrence of low pressures (1-2 kbar) related to an extensional geotectonic setting. The dense fracture network shown by SEM images and the high density of crystal defects revealed by the TEM study in the eastern basin, adjacent to faults produced by shearing, suggest that their epizonal phyllosilicates were more affected during deformation than those belonging to the western basin, favouring the development of a retrograde association (siderite, kaolin group minerals and Al-smectite) on the epizonal paragenesis. Microcavities formed along phyllosilicate cleavage acted as channels for fluid transport favouring alteration under low-temperature conditions.

ANOMALOUS REVERSE ZONING OF SAPONITE AND CORRENSITE CAUSED BY CONTACT METAMORPHISM AND HYDROTHERMAL ALTERATION OF MARLY ROCKS ASSOCIATED WITH SUBVOLCANIC BODIES

The effects of a subvolcanic intrusion on its host rocks in the Priego de Córdoba area (SE Spain) was studied by optical microscopy, X-ray diffraction and electron microscopy. The intrusion of a laccolith of stratiform dolerite in partially consolidated marly sediments at quite shallow depths below the ocean floor took place during the intracontinental rifting phase of the Subbetic zone of the Betic Cordilleras. In the first stage, contact metamorphism caused the crystallization of calc-silicate consisting of grossular andradite garnet, diopside, vesuvianite and titanite for which the upper temperature limit was 500°C. Phyllosilicates are found in a network of veins cross-cutting the carbonate and calc-silicate matrix, indicating their formation during a process of hydrothermal alteration superimposed on the contact aureole. In the area closest to the subvolcanic rocks, saponite is the main phyllosilicate although some dispersed chlorite is found. In the zone furthest from the contact, corrensite together with saponite and beidellitic smectites were observed. The presence of low-temperature phases (saponite) in the area closest to the laccolith reveals the evolution of the hydrothermal system toward cooler conditions. In the early stage, the circulation of a hot hydrothermal fluid caused the crystallization of chlorite in the area surrounding the subvolcanic body and corrensite in the more distal area, which might have begun even during the contact metamorphism stage. A cooling phase followed, resulting in the crystallization of saponite in the host rocks, and the crystallization of dioctahedral and trioctahedral smectites inside the subvolcanic body.

Contrasting alteration processes in hydrothermally altered dolerites from the Betic Cordillera, Spain

Abstract Dolerites from the Sierra de San Pedro (Betic Cordillera, southern Spain) develop three main types of microsystems during hydrothermal alteration: (1) centimetre-sized veins crosscutting the dolerites; (2) microfractures in feldspar and diopside grains; and (3) alterations involving primary-igneous mafic phyllosilicate grains. The vein and microfracture sites developed alteration assemblages of randomly oriented smectites (saponite and beidellite) and halloysite. At these sites, the alteration mechanism was governed by complete dissolution of the parent material with subsequent crystallization of the products, with no parent-mineral crystallographic control. Pseudomorphed mafic phyllosilicate sites are characterized by oriented complex mineral assemblages made of chlorite, chlorite/smectite mixed layers, corrensite, saponite and relicts of biotite. These assemblages formed during the initial high-temperature stage of the hydrothermal process. In these microsites, the alteration sequence was controlled by the parent-mineral structure and chemistry, with products determined by structural relations with the parent phyllosilicate. Alteration of one phyllosilicate to the next most stable proceeds via interstratification of the parent and product phyllosilicates.

Influence of phyllosilicates and fluid–rock interaction on the deformation style and mechanical behaviour of quartz-rich rocks in the Carboneras and Palomares fault areas (SE Spain)

Abstract Deformed quartzitic rocks from the Carboneras and Palomares fault areas (SE Spain) are enriched in phyllosilicates compared to their respective protoliths. Deformation is mainly localized in highly foliated chlorite-rich bands. Quartz-rich bands show brittle deformation developing dolomite-rich cross-cutting veins re-cementing microcataclasite areas. Undamaged lenses within the cataclastic rocks contain patches of phyllosilicates with randomly oriented chlorite and mica. Mg, Fe, water, As and Zn enrichment of the damaged rocks suggests a process of hydrothermal chloritization associated with the Cabo de Gata volcanism. Petrographic characteristics indicate that hydrothermal alteration that produced chlorite and mica-enrichment occurred before faulting. Phyllosilicates provided lubricating properties to the quartzitic rocks, favouring the predominance of creep over seismic stick-slip and reducing the possibility of large seismogenic events. Dolomite cementation as a consequence of fluid-rock interaction processes would have a limited effect, due to the presence of weak phyllosilicate surfaces.