The stability of arc lower crust : insights from the Talkeetna Arc section, south-central Alaska and the seismic structure of modern arcs
Citable URI
https://hdl.handle.net/1912/1369As published
https://doi.org/10.1029/2006JB004327DOI
10.1029/2006JB004327Abstract
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.
Description
Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 111 (2006): B11207, doi:10.1029/2006JB004327.
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