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

Abstract

SUMMARY 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 evolution. 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.

Magnetic fabrics of rhyolite ignimbrites reveal complex emplacement dynamics of pyroclastic density currents, an example from the Altenberg–Teplice Caldera, Bohemian Massif

“…9a and 10), which corresponds to the fabric imbrication with respect to the basal flow plane of the deposit (e.g., Fisher et al 1993;Le Pennec et al 1998;Giordano et al 2008;LaBerge et al 2009;Cas et al 2011;Gountié Dedzo et al 2011;Lerner et al 2022). In previous studies, the shape of the AMS ellipsoids from ignimbrites is, in most cases, oblate (e.g., Ellwood 1982;Knight et al 1986;Agrò et al 2015;Ort et al 2015;Moncinhatto et al 2019). A normal fabric has a magnetic lineation parallel or near parallel to the flow direction.…”

AMS and rock magnetism in the Caviahue-Copahue Volcanic Complex (Southern Andes): emission center, flow dynamics, and implications to the emplacement of non-welded PDCs

“…In this context, the CCVC composes one of the most active volcanic centers in the Andean belt, with several eruptive events in the last century (Caselli et al, 2016;Tassi et al, 2016). The particular setting of the CCVC attracted several studies in the recent decades, with a broad range of topics including geomorphology (e.g., Díaz, 2003;Báez et al, 2020a), geochemistry (e.g., Mazzoni and Licitra, 2000;Melnick et al, 2006;Varekamp et al, 2006), geochronology (e.g., Pesce, 1989;Melnicik et al, 2006) structural (e.g., Melnick et al, 2006;Velez et al, 2011;Folguera et al, 2016), geothermal (e.g., Barcelona et al, 2019), and AMS and paleomagnetism (e.g., Ort et al, 2014;Moncinhatto et al, 2019Moncinhatto et al, , 2020.…”

“…Hysteresis data, including the ratio of saturation remanence to saturation magnetization (M rs /M s ) and the coercivity of remanence to coercive force (H cr /H c ), can be used in the Day plot, a diagram that can help discriminate between single domain (SD), pseudo-single domain (PSD) and multidomain (MD) particles (Day et al, 1977). This differentiation is important because it can have effects on the behavior of the magnetic particles (e.g., Moncinhatto et al, 2020). The RBI samples (Table 1) lie within the pseudo-PSD and MD of the Day plot (Fig.…”

“…Recent studies have shown that AMS fabric of pyroclastic materials is strongly dependent on microscopic factors, such as composition, mineral magnetic interactions, and domain structure of the Fe-Ti oxides (e.g., Cañón-Tapia and Mendoza-Borunda, 2014;Moncinhatto et al, 2020 (Mazzoni and Licitra, 2000;Varekamp et al, 2006). On average, K PARA ranges from 2 x 10 -10 to 1.4 x 10 -9 SI, revealing an insignificant paramagnetic contribution to K m and suggesting a main ferromagnetic origin for the AMS in our samples.…”

“…Manuscript submitted to Journal of Volcanology and Geothermal Research have been attributed to several causes, including flow dynamics (Ort et al, 1999;LaBerge et al, 2009;Agrò et al, 2014;Ort et al, 2014Ort et al, , 2015, and mineralogy (e.g., Rochette et al, 1992;Moncinhatto et al, 2020).…”