Mineralogical control on the magnetic anisotropy of lavas and ignimbrites: a case study in the Caviahue-Copahue field (Argentina)

MONCINHATTO, THIAGO R; HAAG, MAURÍCIO B; HARTMANN, GELVAM A; SAVIAN, JAIRO F; POLETTI, WILBOR; SOMMER, CARLOS A; CASELLI, ALBERTO T; TRINDADE, RICARDO I F

GEOPHYSICAL JOURNAL INTERNATIONAL

WILEY-BLACKWELL PUBLISHING, INC

Año: 2020 vol. 220 p. 821 - 838

0956-540X

Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment ofminerals at the site-scale, the imbrication between the magnetic foliation and the emplacementsurface being an indicator of flow direction. However, despite numerous studies examining theflow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy andthe domain state of ferromagnetic phases on the magnetic fabric remains poorly understood.This paper describes the magnetic mineralogy and its influence on the magnetic fabric ofPlio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex inthe Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magneticsusceptibility and anhysteretic remanent magnetization and petrographic observations wereperformed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclasticrocks present varied magnetic mineralogy, formed in different stages of the magmatic evolution. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibilitydata in volcanic rocks and associated ignimbrites, providing ?scattered? fabrics when late Tirich titanomagnetite phases dominate the fabric, and ?inverse? or ?intermediate? fabrics whensingle-domain grains are present. ?Normal? fabrics are typically found when early crystallizedpure magnetite is present. Our results highlight the complexity in the interpretation of magneticanisotropy data in volcanic rocks and ignimbrites.