A l’occasion du prochain Géoch’Pot, Vendredi 27 mai à 11h salle des séminaires, Emily Mullen nous parlera de l’évolution spatiale et temporelle du magmatisme dans l’arc des Cascades à travers des datations U-Pb sur zircon. Emily est actuellement en post-doc entre le LMV et l’University of British Columbia, et travaille avec Jean-Louis Paquette et Dominique Weis.
Tracking the temporal and spatial evolution of magmatism during the development and demise of the Cascade Arc (western North America) through U-Pb zircon dating
A number of recent studies have demonstrated the episodic nature of magmatic activity in subduction zones. However, most of these investigations have been in extinct arc systems because a long record of magmatism is seldom exposed in young arcs. The Cascade Arc offers an opportunity to examine nearly the entire lifetime of an active subduction system because its subducting plate is now almost completely consumed. In addition, rapid uplift and deep glaciations have combined to expose a near-continuous record of volcanic and plutonic activity since arc initiation at ~40 Ma. Although several large metropolitan centers (Seattle, Vancouver, Portland) are located near major volcanoes, relatively little is known about the arc’s evolution. Previous dating studies relied upon techniques with relatively large uncertainties (K-Ar, fission track) and therefore lack the resolution to resolve magmatic tempos. I will present the first comprehensive geochronologic investigation of Cascade Arc magmatism.
This study focuses on the currently active Mt. Baker volcanic field and its predecessor, the ~1000 km2 Chilliwack batholith of southernwestern Canada and northern Washington, which together preserve the longest and most continuously exposed magmatic record in the Cascades. Thirty U-Pb zircon dates have been measured by LA-ICP-MS and the results indicate that Chilliwack magmatism commenced at ~36 Ma. The most voluminous plutons cluster at ~34-29 Ma, documenting an early flare-up. North of the Chilliwack, the oldest plutons become progressively younger as the edge of the subducting slab is approached, possibly reflecting northerly migration of the slab edge. Chilliwack magmatism continued to ~22 Ma, which marked the beginning of a 10 Myr lull. Intermittent magmatism resumed at ~12 Ma with magmatic centers defining a pattern of southwesterly migration over ~55 km, ending at the currently active Mt. Baker. The migration rate and direction are explained by the combination of slab rollback and forearc rotation that characterize the modern arc, implying that the same tectonic regime has persisted for the past 12 Myr. Large silicic magma chambers formed at regular intervals of ~1.3 Myr and several culminated in major caldera-forming events. Since the most recent explosive eruption was at 1.15 Ma, another major eruption at the site of the currently active Mt. Baker must be considered a possibility. The high-precision dates obtained in this study establish a framework for future examination of the compositional evolution of Cascade Arc magmas.
