Magma addition and flux calculations of incrementally constructed magma chambers in continental margin arcs: Combined field, geochronologic, and thermal modeling studies

Incrementally constructed magma systems have been recognized from studies of the resulting plutons for more than three decades. However, magma addition rates, fluxes, growth durations, sizes of increments, and sizes and durations of the resulting magma chambers have been difficult to ascertain, emphasizing the need for a better understanding of how magmatic systems evolve. Our results from studies of plutons and arc sections in the North American Cordillera indicate that a large range exists in all of these values. Although arc sections and individual plutons clearly have dramatic temporal changes in volumetric magma additions, true volumetric flux calculations are particularly difficult to determine. Thus, although subduction beneath arcs may have active durations of hundreds of millions of years, volumetrically most magmatism is emplaced during magmatic flare-ups of similar to 10-30 m.y. duration. Individual plutons and batholiths in these arcs can grow in <0.5 m.y. to 10 m.y. Pulse sizes moving through these magma plumbing systems vary from small dike-like to large diapir-like pulses, both of which may form from earlier amalgamation of poorly defined pulses. Our thermal modeling, using a range of incremental growth scenarios, concludes that focused incremental growth with greater than a certain volumetric flux results in magma chambers that are much larger than individual pulses but less than the size of the final batholith, and with hypersolidus durations of hundreds of thousands to millions of years. The volumetric magma flux and the spatial distribution of volumetric addition rates of magma, rather than size or shape of individual pulses, are the dominant controlling factors on growth scenarios and chamber sizes and durations.