Abstract We carried out magnetic analyses on a sequence of Cretaceous alkaline–transitional subaerial basalts of Córdoba Province, Argentina, which have high-Ti magnetite as the main opaque phase. Three different groups are identified 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 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 magnetite ss 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, probably due to weathering.

The Achala batholith is a post-orogenic intrusion of probable Silurian age exposed in the Sierra de Cordoba in the eastern Sierras Pampeanas of central Argentina (31°6′S to 32°2′S; 64°29′W to 65°7′W). The batholith was emplaced into late Precambrian amphibolite- and granulite-facies metamorphic rocks. Five comagmatic granitoid facies, all monzogranite in composition, have been recognized. Geochemical data suggest these granitoids formed by a magmatic differentiation process that began with porphyritic and fine-grained facies B, C, and D, and ended with leucogranite facies A. Biotite-apatite enclaves that are interpreted as cumulates and tonalitic nodules occur in facies B. Less abundant rock types are quartz-muscovite greisens, feldspathic episyenites, pegmatites, and aplites. The granitoids have S-type chemical affinities and are consistent with a within-plate granite tectonic setting.

Abstract Metamorphic and plutonic basement rocks and cover sequences of the Eastern Sierras Pampeanas, Argentina, have undergone multiple episodes of fault reactivation. Faults take advantage of mid- to late Cambrian, NW-SE-striking, steeply east-dipping foliations in Vendian-aged accretionary prism metasedimentary rocks. Foliations in peraluminous schists, paragneisses and migmatites are deflected into late Cambrian amphibolite-grade high-strain zones. Greenschist-grade mylonite zones and thick retrogressed ultramylonite zones with mainly NNW strikes, easterly dips, and east-over-west movement, affect the metasedimentary rocks and Ordovician-aged intrusive rocks and are presumably related to early Devonian accretion of terranes to the west of Gondwana. pseudotachylyte veins occur in nearly all mylonite zones. Brittle deformation during Carboniferous to Triassic time produced major pull-apart basins located above terrane boundaries. Outcrop patterns of Triassic to Cretaceous sedimentary rocks are consistent with transtensional pull-apart basins followed by Andean transpressional deformation. The theoretical basis for fault reactivation and production of ‘short cuts’ is examined in the context of Tertiary to Recent basin inversion faults. The inversion faults follow the Palaeozoic trends and produce the present-day NNW-oriented, deep sedimentary basins and intervening ranges of basement rocks.