Arc Magmatic Tempos: Gathering the Evidence

In this issue of Elements we explore the characteristics, potential causes, and implications of episodic magmatism in arcs. A comparison of U–Pb bedrock and detrital zircon ages in arcs with independent calculations of volumetric magma addition rates (MARs) indicates that the former nicely track the episodic temporal histories of arc magmatism but not MARs. MAR estimates indicate that 100–1000 times more magmatism is added to continental arcs during flare-ups than during lulls and result in plutonic/volcanic ratios of >30/1. Episodic arc magmatism may result from external forcing on arc systems caused by events outside the arc and/or from internal cyclic processes driven by feedback between linked tectonic and magmatic processes within the arc. Along and across arc strike, changes and asymmetries in magmatic, tectonic, and geochemical histories provide important constraints for evaluating these poorly understood driving mechanisms. * Apparent flux : Rate of magma input (plutonic) or output (volcanic) inferred from areal size of mapped igneous units. Units measured in km2 My−1 ([Table 1][1]). Arc residue : Materials located in the lower crust of arcs. They comprise the low-silica cumulates after fractional crystallization and/or restites after partial melting of intermediate melts (e.g. tonalites, granodiorites). They are rich in ortho- and clinopyroxenes, amphiboles, and depending on the depth below surface, also contain plagioclase and/or garnet as major minerals. Arc roots : The lower part of magmatic arcs (crust and mantle lithosphere) considered to be dominated by magmatic residual (restitic from partial melt and cumulate materials) mixed with new additions from the mantle. Arc roots have an average silica composition lower than basalt. Arc tempo : A measure of cadence or rhythm of processes operating within magmatic arcs. In cases where these tempos can be defined by a wave-like pattern, then the following terms help describe the wave pattern and thus processes: Back arc : A secondary line of magmatic products, sometimes present at subduction margins, more scattered on the upper plate than the frontal arc products. Batholith : Areas of plutonic rock larger than 100 km2 that represent the magmatic portion of arcs. The term is used both for single solidified magma bodies (plutons) and areas of multiple, closely spaced plutonic bodies (e.g. Sierra Nevada Batholith). Bedrock zircon ages : U–Pb ages of zircon grains obtained from bedrock units. Benioff zone : Planar zone of deep (~35–700 km) earthquakes corresponding to location of the subducted slab in the mantle. Continental arc : Subduction-related magmatic arc in which the lower plate is oceanic lithosphere whereas the upper plate is continental lithosphere. Note that some upper plates, while made of continental lithosphere, may be under extension and thus form at low elevations and can sometimes be submarine. Crust production rates : The rate at which arc crust is produced per unit of time. Long-term, time-averaged, crust production rates for arcs are typically determined by estimating an existing volume of crust and dividing by its age. These rates are expressed in units of volume (km3) per km of arc length per unit of time, which is typically expressed in millions of years (km3 km−1 My−1). Cyclic processes : Processes that repeat in a regular fashion due to internal feedback mechanisms. Detrital zircon ages : U–Pb zircon ages from sedimentary rocks. These zircon grains formed in igneous units elsewhere and were deposited at the sampling location. Episodic processes : Series of loosely connected parts or events occurring at repeated (can be regular or irregular) intervals. It does not assume that there are internal feedback mechanisms driving the repeated events. External forcing : Events external to systems that change boundary forces and thus potentially drive a change of behavior within the system. Flare-up: : Period where the volume of magmatism added to the crust is much greater than average amounts. Frontal arc : The first line of volcanoes and underlying batholiths away from the subduction trench; always present at subduction margins and parallel to the trench. Ignimbrite flare-up : Period of volcanic activity dominated by eruption of ignimbrites, with volume at least an order of magnitude greater than steady-state volcanic activity. Lithospheric delamination : The removal and sinking of a portion of the lowermost lithosphere into the mantle below. Lower plate : a subducting oceanic plate and all rock units below this plate Lull : Period where the volume of magmatism added to the crust is much less than average. Magma addition rate (MAR) : Amount of plutonic material added to the examined area measured in km3 My−1 ([Table 1][1]). Magma production rates : The total volume of magma produced for a given arc or ridge segment per unit of time. Magma production rate estimates take into account the volume of the arc crust produced and lost since inception. Crust can be lost via any number of processes including rifting, subduction erosion, delamination, and surficial erosion. These rates, like crust production rates, are expressed in units of volume (km3) per km of arc length per unit of time, which is typically expressed in millions of years (km3 km−1 My−1) ([Table 1][1]). Mantle-power input : Thermal-energy and volatile influx to the base of the crust by basalt intrusion from the mantle. Mixed arc : Long-lived, continental arcs that incorporate coeval island arcs that collided and accreted to the continental upper plate. Oceanic arc : Subduction-related magmatic arc in which both the lower and upper plates are oceanic (made of oceanic crust and oceanic mantle lithosphere). Paleo- and Neo-Tethys : Ancient Paleozoic oceans located between the continents of Gondwana and Laurasia. Regional “nodes” : Concentrations or foci of magmatism or volcanism within a broader area. Seismic anisotropy : A phenomenon that occurs when a polarized shear-wave travels through an anisotropic medium. The incident shear-wave splits into two separately polarized shear waves. The two waves travel at different speeds and the delay time difference between the two arrivals provides an estimate of the amount of anisotropy in the material. The orientation of the faster shear-wave records the orientation of the anisotropic fabric and can be used to infer mantle flow. Slab rollback : The process where sinking oceanic lithosphere (or slab) subducts into the mantle and sweeps backwards causing the hinge or trench location to migrate away from the arc in the direction of the subducting plate. Slab window : A gap or tear in the subducted oceanic lithosphere. Tertiary pulses : Third-order temporal peaks of volcanic activity; part of a hierarchy of temporal peaks. Upper plate : All rock units above a subducting oceanic plate. Volume-normalized U–Pb age data : Number of U–Pb analyses weighted by estimated volume of the geologic unit analyzed. [1]: #T1