MIDDLE JURASSIC CORDILLERAN ARC FLARE-UP AND BAJOCIAN ENVIRONMENTAL IMPACT (Invited Presentation)

Rhyolite-dominated volcanic outpourings with eruptive volumes comparable to continental flood basalts and characterized by short duration high flux magmatic events are referred to as silicic large igneous provinces (SLIPs). The circumstances that trigger such events is still poorly understood but have been related to mantle plumes or shallower-sourced melting anomalies related to plate tectonics. Here, the ~2500-km-long Jurassic continental margin magmatic arc segment extending from eastern California to east-central Mexico is recognized as a subduction-related SLIP on the basis of its aerial extent, thick rhyolite ignimbrite-dominated volcanic/sedimentary sections, rapid emplacement during an arc magmatic flare-up at ~171-166 Ma, and similarity in terms of petrology and tectonic setting to other major Phanerozoic SLIPs including the mid-Tertiary Sierra Madre Occidental of Mexico and the Jurassic Chon Aike province of South America and the Antarctic Peninsula. While the timing of the Middle Jurassic North American rhyolite flare-up is demonstrated on a regional basis via a compilation of zircon U-Pb ages, the duration of peak volcanic flux here is perhaps best reflected by higher precision ⁴⁰Ar/³⁹Ar sanidine ages determined from ash fall tuff deposits in shallow marine foreland basin deposits in southwestern Utah (Kowallis et al., 2001). The North American Middle Jurassic flare-up is synchronous with the second of the three main volcanic episodes documented from the Chon Aike province which has been related to rifting across the paleo-Pacific margin of western Gondwana. The coincidence of these two widely separated Cordilleran rhyolite flare-ups and indeed the widespread record of subduction-related Middle Jurassic throughout the Americas at this time correlates with an abrupt acceleration in opening of the Central Atlantic Ocean and increased subduction rates of oceanic lithosphere beneath the western edge of Pangea. The inception of explosive Cordilleran silicic volcanism correlates with Bajocian (170-168 Ma) global cooling, oceanic anoxia, and a positive δ¹³C carbon isotope excursion. Following Cather et al. (2009) ocean iron fertilization by volcanic ash is suggested as a key forcing mechanism driving these changes.