Propiedades magnéticas y petrofábrica de unidades volcánicas terciarias de Ia región de Arandas-Atotonilco, Jalisco, México

Abstract

Detailed paleomagnetic, rock-magnetic, ore-microscopy, scanning electron microscopy (SEM) and anisotropy of magnetic susceptibility (AMS) studies of volcanic units from Arandas-Atotonilco, NW Jalisco State, Mexico are reported. Units are basaltic and basaltic-andesite flows of Miocene age (dated by K-Ar). The magnetic mineralogy and domain states are derived from measurements of low-field susceptibility, intensity and direction of natural remanent magnetization (NRM), Q-coefficients, NRM coercivity and unblocking temperature spectra, thermomagnetic curves (Curie points), susceptibility variations with low-temperatures (down to liquid nitrogen temperature), isothermal remanent magnetization (IRM) acquisition, alternating field (AF) demagnetization of saturation IRM, anhysteretic remanent magnetization (ARM) acquisition, AF demagnetization of saturation ARM, Lowrie-Fuller test of domain state, and viscous remanent magnetization (VRM) tests. The petrofabrics and lava flow characteristics are determined from the AMS results. The minimum principal susceptibility axes correlate with the lava flow directions (estimated from field observations). The NRM intensities closely follow a lognormal distribution. The ore-microscopy, SEM, rock magnetics and AMS data document a complex assemblage of magnetic minerals and grain sizes. Within-and between-lava flow differences and similitudes are discussed in terms of various magnetic parameters and domain state indicators. Paleointensity experiments are discussed in terms of domain states, particularly with respect to Curie temperatures and blocking temperature spectra and the susceptibility variation with low temperatures (down to liquid nitrogen temperature).