Continental crust is thought to be formed as a result of are magmatism, but
many of the lavas produced in these settings are basaltic, while those tha
t are silicic are commonly evolved, with lower Mg #s than the continental c
rust. The bulk composition of continental crust can be produced by mixing o
f end-member basaltic and silicic compositions, via magma mixing or in mech
anical, tectonic juxtaposition, but some process is required to remove the
cumulates and residues formed during generation of the silicic, "granitic"
end-member. We consider convective instability of dense mafic and ultramafi
c lower crust as a means to remove mafic residues of basalt differentiation
in order to produce end-member compositions that can mix to form the bulk
composition of the continental crust. Using a range of lower crustal and ma
ntle bulk compositions, ranging from mafic and ultramafic cumulates to prim
ary liquid compositions, we calculated the subsolidus phase assemblage and
resulting density. The results show that densities of likely lower crustal
lithologies can exceed those of the mantle (by similar to 50-250 kg m(-3))
I but the density contrast is a strong function of composition, temperature
, and pressure. For a "cold" geotherm with a Moho temperature of 300 degree
sC, relevant to cratonic settings, densities of all of the lower crustal co
mpositions that we considered, except granulite, exceed the density of the
underlying mantle at pressures as low as 0.8 GPa. For a "hot" geotherm with
a Moho temperature in the range of 800-1000 degreesC, the density of the l
ower crust is much more variable, with gabbroic and granulite compositions
having lower densities than the mantle, while "arc gabbronorite" and ultram
afic cumulate compositions having higher densities than the mantle at press
ures similar to that for the cold geotherm. Instability times calculated fo
r a two-dimensional Rayleigh-Taylor convective instability, where a dense l
ower crustal layer sinks into a lower-density mantle, show that high temper
atures (>700 degreesC, or >500 degreesC with a background strain rate) are
required for this process to occur on a timescale of 10 Myr with theologica
l parameters expected for the crust and mantle. The high temperature requir
ed for dense lower crustal mafic-ultramafic cumulates to sink into the mant
le suggests that this process is restricted to arcs, volcanic rifted margin
s, and continental regions that are undergoing extension, are underlain by
a mantle plume, or have had part of the conductive upper mantle removed.