Collaborative Research:Processing Magma and Constructing Plutons in the Upper Crust:Insights on Magma Chambers from Top-to-Bottom Views of Plutons and Associated Volcanic Sequences

San Jose State University Foundation, San Jose CA

Investigators

Abstract

Miller EAR-0107119 Understanding how magma is processed on its journey from source to final rocky resting place is vital for understanding crustal evolution, the mechanics of magma ascent and emplacement and their relation to tectonics, and eruptive processes. Storage in upper crustal magma chambers has been, and still is, central to most views of this journey. However, concepts of the magma chamber, the processes that occur within it, and its relationship to plutons that are ultimately constructed have been intensely reevaluated in recent years. Many widely held views (e.g. that magma chambers are mostly crystal-poor bodies of magma throughout most of their "lifespan") have been called into question. We propose to investigate the evolution of exceptionally exposed plutons and probable equivalent volcanic sequences in the Eldorado Mountains and Highland Range, Nevada, and to test hypotheses about the mechanical and chemical processing of magma as it enters, resides in, and is extracted from magma chambers. Existing data that include general mapping and a sizable data set of 40Ar/39Ar ages and whole-rock elemental and isotopic analyses provide a sound initial basis for this study. We will add to the existing data base, but our main emphasis will be on a combination of updated traditional and new methods (very detailed mapping of internal pluton structures; detailed petrography using compositional imaging to emphasize zoning, reaction textures, and accessory mineral features; crystal size distribution analysis; in situ Pb and Sr isotope analysis of feldspars; ion probe dating of zircon). This methodology will clarify the evolution of these magma chambers, including their filling and replenishment; disruption and rearrangement by internal and external processes such as convection, mass wasting, roof collapse, and eruption; and, ultimately, pluton construction. It will also permit us to evaluate the filtering effect of magma chambers by comparing input magmas, outputs (erupted magmas), and retained materials (plutons), and to constrain eruptive triggers and controls of eruptive styles.