New insights into crustal contributions to large volume rhyolite generation at the mid-Tertiary Sierra Madre Occidental Province, Mexico, revealed by U-Pb geochronology

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 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