CONSTRAINTS FROM ARC CRUSTAL SECTIONS ON ARC MAGMATIC PROCESSES

Many published, crustal-scale cartoons of arc magmatic systems are highly misleading when compared to natural systems, and in turn used incorrectly to imply the presence or absence of magmatic and emplacement processes at different crustal levels. Particularly problematic are models showing deep MASH or “hot zones” feeding single dikes that ascend through much of the crust and in turn feed upper crustal plutons or volcanoes, and accompanying inferences that upper crustal chemistries thus largely reflect processes in the deep crustal melt zones.

Numerous arc crustal sections have now been described in the literature and should be used to constrain published cartoons and implied processes. Dikes, large sheets, and large elliptical to irregularly shaped bodies occur at all levels. Both laccolith and piston-shaped bodies are present at shallow levels and ring complexes and funnel-shaped plutons occur down to mid- to deep crust. Vertical and horizontal sheeted complexes are locally present but are not dominant. Single, active magmatic systems change from unfocused to focused with decreasing depths. This focusing of magmatism, the size, shape and orientation of bodies relative to regional strain fields, and accompanying deformation of host rock, do not typically favor simple diking as the dominant ascent mechanism.

Conservative estimates of total percent of plutonic versus host rock in arc crustal sections are large and always increase downwards from ~65% at < 10 km to ~90% at 30 km depths. New plutonic/volcanic volume estimates using these percents indicate ratios of 30/1 or more indicating that the vast majority of arc magma is trapped in the crust. Widespread stoping, recycling, crystal mixing, and fractionation occur at all crustal levels. And average plutonic compositions gradually change from mafic/ultramafic in deep roots to upper crustal granites/granodiorites and must reflect the complex interplay between upward fractionation and mixing.

These broad patterns are fairly consistent in most continental arc sections. They are difficult to explain (and often ignored) in models arguing for simple melting, dike ascent, and dike assembly of upper crustal plutons. We hope future attempts to explain arc magmatism embrace these now well-known characteristics of arc crustal sections.