Paleomagnetic and magnetic fabric studies of the San Gaspar ignimbrite, western Mexico-constraints on emplacement mode and source vents

Paleomagnetic and magnetic fabric data for the 4.8 Ma San Gaspar ignimbrite, one of the largest in western Mexico, are used to investigate the source vents, emplacement mechanism and tectonics. Rock magnetic properties from distant parts of the ignimbrite are similar, suggesting relatively homogeneous mineralogy of the unit. Isothermal remanence and continuous susceptibility-temperature experiments point to low to medium-Ti titanomagnetites as the main magnetic carriers. Hysteresis ratio parameters of most samples fall in the pseudo-single-domain grain size region; wasp-waisted hysteresis loops were identified corresponding to high Hcr / Hc values. Stepwise thermal and alternating field demagnetization shows that secondary components are completely removed below 20 mT or 400 °C. Thereafter, the characteristic component is isolated with small within-site dispersion of mean direction (α95 < 10° at 7 out of 10 sites). The within-site consistency of directional features of magnetic fabric supports that the ignimbrite welded and cooled in situ and was probably emplaced in a NW-SE rift zone from a single eruption center located south or east of a younger Pleistocene caldera in the area. Paleomagnetic, rock magnetic and ore microscopy data support the hypothesis that the extensive and widespread ignimbrite deposits in western Mexico correspond to a major explosive volcanic phase in the Pliocene.