Eruptive Productivity of the Ceboruco-San Pedro Volcanic Field, Nayarit, Mexico

High-precision ⁴⁰Ar/³⁹Ar geochronology coupled with GIS spatial analysis provides constraints on magma eruption rates over the past 1 Myr of the Ceboruco-San Pedro volcanic field (1870 km²), located in the Tepic-Zacoalco rift in western Mexico. The volcanic field is part of the Trans Mexican Volcanic arc and is dominated by the andesitic-dacitic stratocone of Volcan Ceboruco and includes peripheral fissure-fed flows, domes, and monogenetic cinder cones. The ages of these volcanic features were determined using ⁴⁰Ar/³⁹Ar laser step-heating techniques on groundmass or mineral separates, with 78% of the 52 analyses yielding plateau ages with a 2 sigma error < 50 kyrs. The volumes were determined using high resolution (1:50,000) digital elevation models, orthophotos, and GIS software, which allowed for the delineation of individual volcanic features, reconstruction of the pre-eruptive topography, and volume calculations by linear interpolation. The relative proportions of the 80 km³ erupted over the past 1 Myr are 14.5% basaltic andesite, 64.5% andesite, 20% dacite, and 1% rhyolite, demonstrating the dominance of intermediate magma types (in terms of silica content). Overall, there appears to be no systematic progression in the eruption of different magma types (e.g., basalt, andesite, dacite, etc.) with time. However, more than 75% of the total volume of lava within the Ceboruco-San Pedro volcanic field erupted in the last 100 kyrs. This reflects the youthfulness of Volcan Ceboruco, which was constructed during the last 50 kyrs and has a present day volume of 50 +/- 2.5 km³, accounting for 81% of the andesite and 50% of the dacite within the volcanic field. Eleven cinder cones, ranging from the Holocene to 0.37 Ma, display a narrow compositional range, with 52-58 wt% SiO₂, 3-5.5 wt% MgO, and relatively high TiO₂ concentrations (0.9-1.8 wt%). The total volume of the cinder cones is 0.83 km³. No lavas with < 51 wt% SiO₂ have erupted in the past 1 Myr. Peripheral andesites to dacite domes, totaling about 14 km³, were dated from 0.4 to 0.6 Ma. The eruptive productivity of the Ceboruco-San Pedro volcanic field over the past 1 Myr is 43 m³ per km² per year, which corresponds to a lava accumulation rate of 43 m/Myr. This rate is less than 1/6 of the lava accumulation rate of 268 +/- 71 m/Myr at the Mt. Adams volcanic field in the Cascade arc (Hildreth and Lanphere, 1994). However, if only the last 100 kyrs are considered (which includes the Ceboruco cone-building episode), the resulting eruptive rate of 323 m/kyr is comparable to the 160-500 m/kyr cited for cone-building episodes at Mt. Adams. The non-focal or peripheral magmatism in the Ceboruco-San Pedro volcanic field is predominantly comprised of phenocryst-poor andesites and dacites that erupted in a 200 kyr interval. This is in marked contrast to the Mt. Adams volcanic field, in which non-focal eruptions are dominantly basaltic. Given that the continental crust is 30-35 km thick beneath Ceboruco and 40-45 km thick beneath Mt. Adams, there is not a positive correlation between crustal thickness and more evolved magma types. These results underscore the importance of studying multiple volcanic fields to better understand the interaction of arc volcanoes and peripheral lavas and their evolution through time.