Stability of arc lower crust: Insights from the Talkeetna arc section, south central Alaska, and the seismic structure of modern arcs

摘要

One process for the formation of continental crust is the accretion of arc terranes at continental margins. A longstanding problem with this model is that although the composition of the continental crust is andesitic, the majority of arc lavas are basaltic. Moreover, those arc lavas that are andesitic tend to be evolved (lower Mg #) compared to the continental crust. Continental crust can be produced through mixing of basaltic and silicic arc lava compositions, assuming that mafic cumulates formed during generation of the silicic component are removed. If these cumulates are denser than the underlying mantle, removal can occur via foundering of lower arc crust. Indeed, field observations of the Talkeetna arc section in south central Alaska, combined with modeling of fractionation in primitive arc magmas, suggest that large amounts of primitive gabbronorite and pyroxenite are missing from the lower crust.