High-Volume Magmatic Events in Subduction Systems

Subduction-related magmatic arcs represent immense accumulations of intermediate, calc-alkaline igneous rocks and are considered the main factories of continental crust on Earth. Estimating the pace at which these giant volumes of magmatic rock were added to the crust and the mechanisms responsible for magmatic addition are fundamental issues for the disciplines of igneous petrology and continental tectonics. For decades, geologists have recognized that the evolution of arcs, and in particular those formed onto continental lithosphere, is not steady state but is punctuated by high-volume magmatic, pulses termed “fl are-ups,” occurring against a background of lowervolume activity (Armstrong 1988). These magmatic fl areups do not appear to simply represent a bias in sampling: they dominate the magmatic input for arcs and must refl ect some fundamental cyclical behavior of subduction systems. However, quantifying true magmatic volumes is diffi cult unless one has a large number of reliable ages and a good understanding of how such ages are distributed among the volcanic rocks, the deep crustal plutons, and the cumulates. A further complicating factor is that arcs laterally migrate over time. Therefore, the task of quantifying a given magmatic addition rate (MAR) at a convergent margin is as diffi cult as it is important for understanding arc magmatism and convergent margins tectonics in general. The purpose of this paper is twofold. First, to inform the general geoscience reader how the fl uctuations in magmatic volume over time can be quantifi ed through gathering easy-to-understand yet diffi cult-to-obtain volumetric and temporal geologic data. Second, to provide the more expert reader with a summary of recent developments in quantifying changes of magmatic addition rates (MARs) through time and possible triggers for high-fl ux magmatic events at several better-known locations. We also review four questions to be answered when quantifying magmatic volumes in subduction environments that include continental lithosphere. These questions are as follows: (1) How much surfi cial versus intrusive igneous materials exist in arcs? (2) What is the spatial footprint of long-lived arcs in subduction settings? (3) What is the vertical crustal column of arcs and what are the changes in composition with depth? (4) How can ages of a magmatic arc be tracked? We then summarize the temporal evolution of a few major Phanerozoic arcs, pointing out their high-MAR events and discuss their possible driving mechanisms. No consensus exists in the literature as to which mechanism, if any, is the main driver of high-MAR events at convergent margins. Finally, we briefl y address the possibility that certain arc fl are-up events may be of global signifi cance. We focus primarily on frontal Andean arcs (see below), which complements the articles in this issue that discuss MAR events in island arcs (Jicha et al. 2015 this issue) and continental interiors of compressive margins, such as the Altiplano–Puna plateau of the modern Andes (de Silva et al. 2015 this issue).