The stability of arc lower crust : insights from the Talkeetna Arc section, south-central Alaska and the seismic structure of modern arcs

Abstract

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. Using rock compositions from the Talkeetna section and the free energy minimization program Perple_X, we calculated equilibrium mineral assemblages for a range of gabbroic and ultramafic compositions at P, T, oxygen fugacity (fO2), and H2O contents appropriate for arc lower crust. The quartz-olivine-garnet-free mineral assemblage found in the Talkeetna gabbronorites (and in the similar Kohistan section in Pakistan) defines a narrow range of fO2 centered on NNO+2 (±1 log unit). Predicted mineral assemblages calculated under these conditions were used to estimate the density and seismic structure of the arc lower crust. We find that the missing gabbroic and ultramafic rocks from the Talkeetna section were likely denser than the underlying mantle, while the gabbronorites that remain are either neutrally or slightly positively buoyant. Generalizing, we show that lower crustal Vp > 7.4 km/s in modern arcs is indicative of lower crust that is convectively unstable relative to the underlying mantle. However, most lower crust in modern arcs is observed to have Vp < 7.4 km/s, implying that gravitationally unstable material must founder rapidly on geologic time-scales, or high Vp plutonic rocks crystallize beneath the Moho.