Oxidation processes and their effects on the magnetic remanence of Early Cretaceous subaerial basalts from Sierra Chica de Córdoba, Argentina

GEUNA, S. E.; LAGORIO, S.; VIZÁN, H.

GEOLOGICAL SOCIETY OF LONDON SPECIAL PUBLICATION

Geological Society of London

Año: 2015 vol. 396 p. 239 - 263

0305-8719

We carried out magnetic analyses on a sequence of Cretaceous, alkaline to transitionalsubaerial basalts of Córdoba Province, Argentina, which have high-Ti magnetite as the mainopaque phase. Three different groups are distinguished based on the degree of high-temperature oxidation during the lava extrusion, combined with superimposed maghemitization and hematization. In the first group titanomagnetites are optically homogeneous or they exhibit coarse intergrowths with ilmenite. The magnetic susceptibility and its variation with temperature and magnetic field point to Ti-poorer compositions than those indicated by electron microprobe, which is interpreted as due to low-temperature oxidation with subsolvus microexsolution. The second group of basalts suffered moderate high-temperature oxidation, with crowded exsolved ilmenite laths within a Ti-poor magnetitess host, followed by maghemitization and hematite replacement. The third group shows a strongly advanced degree of low-temperature alteration, with the virtual disappearance of magnetite. Based on magnetic properties and field tests applied to the magnetic remanence, we interpret that maghemitization and hematization must have been responsible for the acquisition of a stable magnetic remanence, in the presence of hydrothermal fluids coeval with volcanism. The most advanced degree of alteration, typical of highly porous, amygdaloidal lava flows and volcanic breccias, occurred later, likely due to weathering.