MAKING CONTINENTAL CRUST REQUIRES THICK ARCS AND ARCLOGITES, NOT ISLAND ARCS

It is widely thought that continental crust, at least since the Proterozoic, was formed through magmatism associated with subduction. There are two endmember flavors of subduction zone magmatism: island arc and continental arc magmatism. In island arcs, where much of our understanding of subduction zones was inspired, extension in the upper plate results in the generation of a steady state thin crust and a fixed arc magmatic front relative to the trench. Island arc magmas are mostly basaltic and initial differentiation of the magmas leads to iron enrichment. In continental arcs, the upper plate is in compression, which results in a thick upper plate, deep crustal magmatic differentiation, and a gradual rearward migration of the arc front with time, culminating in termination of magmatism. Continental arc magmas are mostly andesitic and differ fundamentally from island arcs in that magmatic differentiation leads to early iron depletion. High elevations associated with thick crust in continental arcs promotes rapid erosion and production of sediment in the trench. Finally, the back-arc region of island arcs is characterized by oceanic rifting, whereas in continental arcs, compression leads to retro-arc underthrusting of continental material. Volatile budgets of arc magmas come primarily from the slab in island arcs, but in continental arcs, a considerable fraction comes also from the upper plate. While the simplicity of island arc systems make them useful for interrogating the nature of subducting slabs and the mantle wedge, island arcs may not be that important for understanding how continents form. Continental crust is iron depleted and thicker than any island arc. Here, we review the geochemical, petrological and tectonic differences between island and continental arcs. We show that magmatic thickening to the point of garnet fractionation is key to driving the iron depleted nature of continental arc magmas and the continental crust in general. These garnet-rich arc cumulates, termed arclogites, delaminate and leave behind a felsic crust. These arclogite reservoirs are reduced, rutile- and sulfide-bearing, and enriched in high field strength and chalcophile elements, explaining the Nb-Ta and chalcophile element depleted nature of continental crust.