Bryan, Scott E., Ferrari, Luca, Reiners, Peter W., Allen, Charlotte M., Petrone, Chiara, Ramos-Rosique, Aldo and Campbell, Ian H. (2007) New insights into crustal contributions to large volume rhyolite generation at the mid-Tertiary Sierra Madre Occidental Province, Mexico, revealed by U-Pb geochronology. Journal of Petrology, 49(1), pp. 47-77. ISSN (print) 0022-3530

Abstract

Voluminous (≥3.9 x 105 km3), prolonged (~18 Myr) explosive silicic volcanism makes the mid-Tertiary Sierra Madre Occidental of Mexico one of the largest intact silicic volcanic provinces known. The rhyolites are generally considered to have formed by closed-system 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 inferred to represent the crustal contaminants and coeval Sierra Madre Occidental 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 and 40Ar/39Ar ages from these areas, but show significant age discrepancies of 1-4 Myrs and greater than errors attributable to calibration and analytical uncertainty. Zircon xenocrysts with new overgrowths in the Late Eocene to earliest Oligocene 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 (i.e. Eocene or older). Instead, these Early Miocene ignimbrites are dominated by antecrystic zircons, representing >33 to ~100 % of the dated population, and most antecrysts range in age between ~20 and 32 Ma. A sub-population of the antecrystic zircons is chemically distinct in terms of their high-U (>1000 ppm to 1.3 wt%) and HREE contents, and are not present in the Oligocene ignimbrites in the northeastern sector of the SMO. The combination of antecryst zircon U-Pb ages and chemistry suggest that much of the zircon in the youngest rhyolites was derived by remelting of partially molten to solidified igneous rocks formed during preceding phases of Sierra Madre Occidental volcanism. Strong Zr-undersaturation, and estimations for very rapid dissolution rates of entrained zircons preclude coeval mafic magmas being parental to the rhyolite magmas by a process of lower crustal assimilation followed by closed-system crystal fractionation (AFC) as interpreted in previous studies of the Sierra Madre Occidental rhyolites. Mafic magmas were more likely important in providing a long-lived heat and material flux into the crust resulting in the remelting and recycling of newly formed igneous materials.

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