Effect of small-scale heterogeneities on interpretation of crustal compositions exemplified by a layered anorthosite
- Lithos 216-217 February 2015, Pages 298-314, ISSN 0024-4937, http://dx.doi.org/10.1016/j.lithos.2014.12.019.
Abstract
The composition of the lower crust has a significant effect on
geodynamic processes because it influences physical rock properties
such as densities and seismic velocities. Compositional differences in
lower crustal rocks are potentially large and exist on the scales of
centimeters up to kilometers resulting in non-unique seismic and
gravity data. While larger heterogeneities can be detected as
reflections on seismic profiles, irregular small-scale compositional
variations are not likely to be discovered, but will influence the
averaged seismic velocities and densities of an area. The extent and
effects of such small-scale heterogeneities are explored on an exposed
high-grade layered anorthositic body by providing a detailed field
map, petrological descriptions, pycnometry measurements as well as
whole rock and mineral analyses combined with thermodynamic phase
equilibria calculations. To evaluate the results of our thermodynamic
calculations, densities and mineral modes obtained from the modeled
phase equilibria are compared to measured densities and estimated
mineral modes from rock samples. The proportion of mafic to ultramafic
(plagioclase-poor) rocks in the mapped field area amounts to 10-15%
but higher proportions of these rock types in the lower crust are
feasible. To further study the effects of compositional variations, we
have generated mixtures of mafic to ultramafic and
anorthositic/intermediate rocks until the average properties of these
mixtures are comparable to those of mafic granulites (3000-3100 kg/m3;
7.1-7.3 km/s). Mixtures of anorthosite with 40-45% and of tonalite
with 50-60% high-grade mafic to ultramafic rocks yield average
densities and seismic velocities similar to mafic granulites although
they still contain 50-60 vol.% plagioclase. Hence small-scale mixing
of certain rock types may result in the overestimation of the
proportion of mafic (garnet) granulites in the lithologic
interpretation of crustal compositions from seismic data. Since the
transition to eclogite-facies in plagioclase-rich rocks is shifted to
higher pressures and anorthositic/intermediate eclogites yield lower
densities, a lower crust with higher modal amounts of plagioclase may
not always provide the significant densification needed for certain
geodynamic settings (e.g. delamination or subsidence).