Collaborative Research: Focused Study of Aleutian Plutons and their Host Rocks: Understanding the building blocks of continental crust
DESCRIPTION: Arc magmatism is the most important process that generates the continental crust today and likely throughout Earth's history. However, average continental crust composition is andesitic and calc-alkaline, while average arc lava composition is basaltic and tholeiitic. The largely unexposed and unsampled plutonic part of the arcs, on the other hand, may be more similar to the continental crust. Therefore, understanding the genesis of plutonic rocks is a key to understanding continental crust formation and evolution via arc magmatism, a key science goal for the GeoPRISMS initiative. The Aleutian arc is uniquely well-suited for such a study, because of the extensive exposures of plutonic rocks, unmatched in any other intra-oceanic arc. In the Aleutian arc, most felsic plutonic rocks have compositions that overlap estimates for the bulk continental crust. Our pilot study found that Eocene-Miocene plutonic rocks and Holocene volcanic rocks show distinctly different elemental and isotopic signatures, which indicate that they were derived from distinct parental magmas. This difference could reflect temporal variation of the mantle under the region, or fundamentally different mechanisms that form plutons and lavas - perhaps strongly calc-alkaline magmas, with high H2O contents, tend to degas in the mid-crust, causing a rapid increase in viscosity and crystallinity, therefore they tent to stall and form plutons; while hotter, drier, tholeiitic basalts have lower viscosity and readily erupt to form lavas. As it is crucial to understand the extent and origin of the compositional difference between central Aleutian lavas and plutons through time and space, this project will map and sample plutonic rocks exposed on the central Aleutians and their coeval volcanic host rocks. A subset of the most promising samples will be measured for major element, trace element and isotopic compositions (Sr, Nd, Hf, Pb). The ages of the plutons and/or the lavas will be constrained using U-Pb zircon geochronology and 40Ar/39Ar geochronology analyses of plagioclase phenocrysts and the groundmass. These preliminary results and the samples acquired in this study will help to answer fundamental questions of continental crust formation, and shed light on the formation mechanisms of plutons and volcanics in arcs.