A model for continental crust genesis by arc accretion: rare earth element evidence from the Irish Caledonides

The formation of continental crust is a complex problem with a paradox at its center: continental material is believed to form by arc magmatism, a model that does not reconcile the bulk mafic and light rare earth element (LREE)-depleted composition of intra-oceanic arcs with the andesitic, LREE-enriched composition of continents. We present evidence supporting an arc origin for continental crust by demonstrating significant changes in magmatic chemistry during arc–continent collision. We use as an example the Connemara region of the western Irish Caledonides, where metamorphism and orogenesis of Dalradian sedimentary sequences record the collision of the oceanic Lough Nafooey Arc with the passive margin of Laurentia at ∼475 Ma (Arenig). This arc was mafic and LREE-depleted during its oceanic activity. During collision, rhyolites more LREE-enriched than the surrounding continental crust were erupted, while the mid-crust was intruded by similarly LREE-enriched gabbro and diorite. Crystal fractionation caused LREE enrichment to exceed that expected by assimilation of continental crust alone. We propose that subsequent loss of the corresponding lower crustal cumulates into the mantle removed this LREE-depleted material from the net crust added to the continent. The combined processes of crystal fractionation and lower crustal loss during arc–continent collision drive the average bulk arc composition toward that of average continental crust.