SEM AND TEM EVIDENCES OF ILLITE/SMECTITE MIXED-LAYERS FORMED BY DISSOLUTION-CRYSTALLIZATION PROCESSES IN CONTINENTAL PALEOGENE SEQUENCES IN NORTHWESTERN ARGENTINA

DO CAMPO, M.; BAULUZ, BLANCA; FERNANDO NIETO,; DEL PAPA, C.; HONGN, F.

CLAY MINERALS (PRINT)

MINERALOGICAL SOC

Lugar: Middlesex; Año: 2016 vol. 51 p. 723 - 740

0009-8558

In the northernmost Calchaquí Valley (Salta, Argentina), the Paleogene continental sediments show an evolution from smectite, at the top, to R3 I/S through R1 I/S, in contrast to the other 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). XRD shows that samples at the bottom contain R3 I/S (> 90% illite) and upper samples, R1 I/S (65-80% illite). SEM demonstrates dissolution of primary phases (feldspars, micas and quartz) and crystallization of illite, I/S and kaolinite. As this alteration is not pervasive, intermediate fluid/rock ratio could be inferred. In lattice-fringe images, the upper samples show abundant I1-rich areas and in the lower sample, illite packets and I3 I/S coexist and compositions evolve toward muscovite (tetrahedral-charge increase, principally compensated by Mg by Al substitution in octhaedral sites and slight decrease in Ca in interlayer sites). As burial temperatures should be similar in all the samples, depth was not responsible for the illite formation at the bottom. TEM textures suggest that illitization mainly proceeded by dissolution-crystallization. The active faults close to the northern Calchaquí Valley likely promoted the circulation of hot, deep fluids, favouring illitization.