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.
FIGURE 1 (A) Comparison of exposed bedrock U–Pb zircon ages with displaced detrital zircon U–Pb LA-ICPMS ages from the Sierra Nevada Batholith (California, USA). Both data sets temporally defi ne the beginning (~250 Ma) and cessation (~85 Ma) of Mesozoic magmatism plus timing of three magmatic flare-ups and four lulls. (B) Depth comparison of bedrock Sierran U–Pb igneous ages, with ages separated into surface volcanic, shallow plutons (emplaced above 6 kbar), and deep plutons (>6 kbar emplacement). Timing of flare-ups and lulls appears depth independent, although volcanism may peak slightly earlier than plutonism in the Jurassic and Cretaceous flare-ups. (C) Calculated magma addition rates, measured in km3 for 10 My age bins for both plutonic and volcanic materials in a 110 km wide corridor across the central Sierra Nevada. Plutonic curves show volume estimates for the top 30 km and 70 km of crustal sections. These curves show huge (100 to 1000) increases of magma added during flare-ups versus lulls resulting in plutonic/volcanic ratios of ~30/1 in this arc.
Elements April 2015 DOI: 10.2113/gselements.11.2.91