The widespread Sanrafaelic remagnetization in the Río de la Plata Craton: magnetic study on carbonates from the Neoproterozoic Sierras Bayas Group

FAZZITO, S.; RAPALINI, A.E.; GEUNA, S.E.; POIRÉ, D.

Congreso; XXI Congreso Geológico Argentino; 2022

A preliminary rock-magnetic research on specimens of limestones and dolostones from the Neoproterozoic Sierras Bayas Group (Poiré 2004, 2008) exposed along the Tandilia system in the Buenos Aires province (Argentina) was performed in order to shed light on the nature of a widespread remagnetization event which affected very large areas of the Río de la Plata craton involving carbonates from the Villa Mónica (Cryogenian, Gómez Peral et al. 2014) and the Loma Negra (Ediacarian) formations, as well as several Ediacaran to Cambrian clastic and carbonatic units exposed in Uruguay (Rapalini and Sánchez Bettucci 2008). The remagnetization process is expressed on a dual-polarity post-tectonic secondary remanence carried by both hematite and magnetite (Rapalini et al. 2013), although some clastic units (claystones and marls), were apparently not affected by the magnetic overprint (Rapalini et al. 2013). The evaluation of a possible connection of this phenomenon to the Sanrafaelic secondary imprint found along the Sierra de la Ventana and other areas of central and western Argentina (Rapalini and Tarling 1993, Truco and Rapalini 1996, Rapalini et al. 2000, Rapalini and Astini 2005, Font et al. 2012, Fazzito and Rapalini 2016, Fazzito et al. 2017) is one of the objectives of this study.The initial experiments, focused on samples from the Villa Mónica and Loma Negra Formations which were previously collected by Rapalini et al. (2013), include several methodologies such as magnetization hysteresis loops, variation of bulk magnetic susceptibility with field, dependence of bulk magnetic susceptibility with frequency and anisotropy of magnetic susceptibility at two different frequencies. Magnetization loops were determined from -1.8 to 1.8 T in one sample per site with a Lake Shore 7404 vibrating sample magnetometer. Magnetic susceptibility (Hpeak=200 Am-1; freq: 976 Hz) and magnetic fabrics (Hpeak=200 Am-1; freq: 976 Hz and 15616 Hz) studies were accomplished by means of an AGICO Multifunction Kappabridge (MFK1-FA). The low-field anisotropy of magnetic susceptibility (AMS) was determined by following a 3-position protocol (spinning specimen method with rotator).Characteristic worldwide magnetic fingerprints of remagnetized carbonates, as high values of frequency dependence parameter and wasp-waisted hysteresis loops, were recognized. Dependence of bulk magnetic susceptibility and of anisotropy of magnetic susceptibility with frequency support the presence of super-paramagnetic (SP) magnetite grains. AMS fabrics are in general inverse and reveal magnetic ellipsoids that suggest a tectonic overprint of the depositional fabric. Discrepancies between AMS ellipsoids at different frequencies indicate superimposition of sub-fabrics originated on ferrimagnetic grain populations of diverse size including very fine-grained magnetite.The rock-magnetic study of samples from the Villa Mónica and the Loma Negra formations (Sierras Bayas Group) will be complemented in the future with thermomagnetic susceptibility curves at different frequencies, anisotropy of anhysteretic magnetic remanence (AARM), petrographic analysis of thin sections by transmitted and reflected light optical microscopy, X-ray diffraction analysis, scanning electronic microscopy (SEM) images and energy dispersive spectra (EDS). Characterization of clay minerals by XDR will help to determine chemical alterations and creation of secondary magnetite. Electronic microscopy will be used to understand the origin of the ferromagnetic minerals.