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

Fil: Moncinhatto, Thiago R. Universidade de Sao Paulo. Instituto de Astronomia, Geofısica e Ciências Atmosféricas. São Paulo, Brazil. Fil: Haag, Maurício B. Universidade Federal do Rio Grande do Sul. Instituto de Geociências. Porto Alegre, Brazil. Fil: Hartmann, Gelvam A. Universidade Estadual de Campinas. Instituto de Geociências.Campinas. Brazil. Fil: Savian, Jairo F. Universidade Federal do Rio Grande do Sul. Instituto de Geociências. Porto Alegre, Brazil. Fil: Poletti, Wilbor. Universidade Federal dos Vales do Jequitinhonha e Mucuri. Instituto de Ciência e Tecnologia. Diamantina, Brazil. Fil: Sommer, Carlos A. Universidade Federal do Rio Grande do Sul. Instituto de Geociências. Porto Alegre, Brazil. Fil: Caselli, Alberto Tomás Universidad Nacional de Río Negro. Laboratorio de Estudio y Seguimiento de Volcanes Activos. Río Negro, Argentina. Fil: Trindade, Ricardo I. Universidade de Sao Paulo. Instituto de Astronomia, Geofısica e Ciências Atmosféricas. São Paulo, Brazil. Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the 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 of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolu- tion. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing ‘scattered’ fabrics when late Ti- rich titanomagnetite phases dominate the fabric, and ‘inverse’ or ‘intermediate’ fabrics when single-domain grains are present. ‘Normal’ fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites. Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the 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 of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolu- tion. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing ‘scattered’ fabrics when late Ti- rich titanomagnetite phases dominate the fabric, and ‘inverse’ or ‘intermediate’ fabrics when single-domain grains are present. ‘Normal’ fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites.