Margarita Do Campo

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.

Geothermobarometry of very low-grade metamorphic pelites of the Vendian–Early Cambrian Puncoviscana Formation (NW Argentina)

The Vendian–Early Cambrian Puncoviscana Formation is a pelite-greywacke turbidite sequence affected by polyphase deformation cropping out extensively in the Cordillera Oriental of northwestern Argentina. Previous X-ray diffraction and analytical high-resolution TEM studies on southern locations found anchizonal grade and proposed medium–high pressure metamorphism followed by higher thermal conditions. We have determined the chemical composition of mica and chlorite with EDX on small uncontaminated areas selected using SEM in backscattered electron mode. The application of mica-chlorite geothermobarometry to mineral grains defining two different foliations has produced consistent pressure/temperature results based on the intersection of six reactions: 9 kbar/250 °C peak conditions were followed by an isothermal decompression to 3 kbar and a successive increase of temperature to nearly 350 °C at 1.5 kbar. The shales from the overlying Meson and Santa Victoria Cambro–Ordovician groups do not contain chlorite, thus chlorite-mica geothermobarometry could not be applied. According to the range of KICIS obtained (0.41–0.68Δ°2θ) and their mineralogical assemblage (illite ± kaolinite ± corrensite + Qtz + Pl ± Kfs), these rocks have not surpassed diagenetic conditions, which implies a minimum temperature difference with the Puncoviscana Formation of 100 °C, consistent with the sharp angular unconformity between the two units.

SEM and TEM evidence of mixed-layer illite-smectite formed by dissolution-crystallization processes in continental Paleogene sequences in northwestern Argentina

Abstract In the northernmost Calchaquí Valley (Salta, Argentina), the Paleogene continental sediments show a transition from smectite, at the top, to R3 I-S (>90% illite) through R1 I-S (65-80% illite), in contrast to the remaining sectors, containing smectite up to the bottom. Samples at the base of the succession were characterized by high-quality step-scan X-ray diffraction (XRD), scanning electron microscopy (SEM) and analytical high-resolution transmission electron microscopy (HRTEM). Analysis by SEM demonstrated dissolution of primary phases (feldspars, micas and quartz) and crystallization of illite, I-S and kaolinite. As this alteration is not pervasive, an intermediate fluid/rock ratio could be inferred. The lattice-fringe images of the samples from upper parts of the sequence show abundant I1-rich areas, whereas in the lower parts of the sequence, illite packets and I3 I-S coexist and compositions evolve towards muscovite (tetrahedral-charge increase, principally compensated by Mg-by- Al substitution in octahedral sites and by a slight decrease in Ca in interlayer sites). As burial temperatures were probably similar in all the samples, depth was not responsible for the illite formation at the bottom. The TEM textures suggest that illitization proceeded mainly by dissolution-crystallization. The active faults close to the northern Calchaquí Valley probably promoted the circulation of hot, deep fluids, favouring illitization.