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 H-CT/H-C values. Stepwise thermal and alternating field demagnetization shows that secondary components are completely removed below 20 mT or 400 degrees C. Thereafter, the characteristic component is isolated with small within-site dispersion of mean direction (alpha(95) < 10 degrees 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. (c) 2005 Elsevier B.V. All rights reserved.
J. Volcanol. Geotherm. Res.ISI Document Delivery No.: 973KQTimes Cited: 0Cited Reference Count: 35Cited References:BAER EM, 1997, J GEOPHYS RES-SOL EA, V102, P22565BUDDINGTON AF, 1964, J PETROL, V5, P310CANONTAPIA E, 2000, J VOLCANOL GEOTH RES, V98, P219DAY R, 1977, PHYS EARTH PLANET IN, V13, P260ELLWOOD BB, 1982, EARTH PLANET SC LETT, V59, P303FISHER RV, 1984, PYROCLASTIC ROCKSGILBERT CM, 1985, GEOFISICA INT, V24, P169GOGUITCHAICHVILI A, 2002, PHYS EARTH PLANET IN, V130, P175GRUBENSKY MJ, 1998, J VOLCANOL GEOTH RES, V83, P93HAGGERTY SE, 1976, OXIDE MINERALS, V3HENRY B, 1980, TRAV LAB TECTONOPHYS, V8007, P1HENRY B, 2003, J VOLCANOL GEOTH RES, V127, P153HILLHOUSE JW, 1991, J GEOPHYS RES-SOLID, V96, P12443HOBLITT RP, 1985, GEOLOGY, V13, P242IWAKI H, 2003, ISL ARC, V12, P46JELINEK V, 1981, TECTONOPHYSICS, V79, P63KNIGHT MD, 1986, J VOLCANOL GEOTHERM, V56, P205LEPENNEC JL, 1998, EARTH PLANET SC LETT, V157, P105MACHIDA H, 1992, ATLAS TEPHRA JAPANMAHOOD GA, 1980, J VOLCANOL GEOTH RES, V8, P199ORT MH, 1993, J VOLCANOL GEOTH RES, V56, P221ORT MH, 2003, B VOLCANOL, V65, P55PALMER HC, 1996, B VOLCANOL, V58, P101PALMER HC, 1999, TECTONOPHYSICS, V307, P207PREVOT M, 1983, J GEOPHYS RES, V88, P2316ROSASELGUERA J, 1997, INT GEOL REV, V39, P125ROSENBAUM JG, 1986, J GEOPHYS RES, V91, P12817SEAMAN SJ, 1991, B VOLCANOL, V53, P460TAUXE L, 2002, GEOCHEM GEOPHY GEOSY, V3, P1URRUTIAFUCUGAUC.J, 1983, GEOFIS INT, V22, P277URRUTIAFUCUGAUC.J, 1988, PHYS EARTH PLANET IN, V52, P320WATKINS ND, 1971, GEOL SOC AM BULL, V82, P1955WILLIAMS W, 1995, J GEOPHYS RES-SOL EA, V100, P3859ZANELLA E, 1999, J VOLCANOL GEOTH RES, V93, P217ZANELLA E, 2001, J VOLCANOL GEOTH RES, V107, P71