Abstract

Relamination, delamination and tadpole zones: Formation of lower continental crust by density sorting of buoyant arc lavas and plutons

Peter Kelemen, Mark Behn & Brad Hacker

Kelemen & Behn, Nature Geosci 2016: Volcanic arc magmas generated above subduction zones have geochemical compositions that are similar to continental crust, implying that arc magmatic processes played a central role in generating continental crust. Yet the deep crust within volcanic arcs has a very different composition from crust at similar depths beneath the continents. It is therefore unclear how arc crust is transformed into continental crust. The densest parts of arc lower crust may delaminate and become recycled into the underlying mantle. However, even after delamination, arc lower crust still has significantly different trace element contents from continental lower crust. We suggest that it is not delamination that determines the composition of continental crust, but relamination. In our conceptual model, buoyant magmatic rocks generated at arcs are subducted. Then, upon heating at depth, they ascend and are relaminated at the base of the overlying crust. A review of the average compositions of buoyant magmatic rocks — lavas and plutons — sampled from the Aleutians, Izu–Bonin–Marianas, Kohistan and Talkeetna arcs reveals that they fall within the range of estimated major and trace elements in lower continental crust. Relamination may thus provide an efficient process for generating lower continental crust.

Kelemen, Behn & Hacker, in progress: A hybrid process incorporating aspects of both relamination and delamination may also play an important role. During “subduction erosion”, forearc crust may be thrust to Moho depths beneath arcs, where it undergoes undergoes density sorting once it is heated to temperatures where viscous, gravitational instabilities become possible on geologically relevant time scales.