HR: 14:45h
AN: V23C-04    [Abstracts]
TI: New insights into large volume rhyolite generation at the mid-Tertiary Sierra Madre Occidental Province, Mexico, revealed by U-Pb geochronology
AU: Bryan, S
EM: S.Bryan@kingston.ac.uk
AF: Department of Geology & Geophysics, Yale Unviersity, New Haven, CT 06520-8109, United States
AU: * Ferrari, L
EM: luca@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, Campus Juriquilla, Queretaro, Qro 76230, Mexico
AU: Reisner, P
EM: reiners@email.arizona.edu
AF: Department of Geosciences, University of Arizona, Tucson, AZ 85721, United States
AU: Allen, C M
EM: charlotte.allen@anu.edu au
AF: Research School of Earth Sciences, Australia National University, Canberra, ACT 0200, Australia
AU: Campbell, I H
EM: ian.campbell@anu.edu.au
AF: Research School of Earth Sciences, Australia National University, Canberra, ACT 0200, Australia
AB: The voluminous (~390,000 km3) and prolonged (~18 myr) explosive silicic volcanism of the mid Tertiary Sierra Madre Occidental of Mexico is generally considered to have formed by fractional crystallisation from crustally contaminated andesitic parental magmas (AFC), with <20% crustal contributions. Evidence for larger crustal contributions has been constrained by the lack of isotopic variation among the lower crustal xenoliths and coeval SMO rhyolite and basaltic andesite to andesite volcanic rocks. Here, we use zircon age populations as probes to assess crustal involvement in Sierra Madre Occidental silicic magmatism. Laser ablation ICP-MS analyses of zircons from rhyolitic ignimbrites located at the northeastern and southwestern sectors of the province yield U-Pb ages that are generally consistent with previously obtained K/Ar ages from these areas. However, zircon xenocrysts with new overgrowths in some of the oldest rhyolite ignimbrites from the northeastern sector provide direct evidence for some involvement of Proterozoic crustal materials, and potentially of more importance, the derivation of zircon from Mesozoic and Eocene age, and isotopically primitive subduction related igneous basement. The youngest rhyolitic ignimbrites from the southwestern sector show even stronger evidence for inheritance in the age spectra but lack old inherited zircon (ie. Eocene or older). Instead, inherited grain ages range between ~20-32 Ma in the southern and youngest Sierra Madre Occidental ignimbrites that have eruptive ages of ~18 and 25 Ma; these inherited zircon ages suggest much of the zircon in the youngest rhyolites was derived by remelting of igneous rocks formed during earlier phases of Sierra Madre Occidental volcanism. The incorporation of zircons derived from earlier phases of rhyolite generation may indicate that the crustal source regions had become overwhelmed by coeval igneous underplate and/or the locus of crustal melting had migrated to shallower crustal levels by the terminal stages of magmatism. The evidence for involvement of young and isotopically primitive crustal materials via the Late Mesozoic to Oligocene age zircon xenocrysts have important implications for how we interpret mantle-like isotopic compositions in rhyolites, as generally observed for the Sierra Madre Occidental. Strong zircon undersaturation, and estimations for very rapid dissolution rates of entrained zircons preclude the coeval mafic magmas as being parental to the rhyolite magmas via assimilation and crystal fractionation (AFC) processes. A greater role for crustal anatexis is indicated such that long-lived basaltic fluxes into the crust may result in the early onset of the recycling of newly formed igneous crustal materials.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 8428 Explosive volcanism
DE: 8440 Calderas
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly