Whole rock major and trace element abundances in aluminous garnet-spinel
websterite, sapphirine-bearing Mg-Al granulite and hibonite-bearing
Ca-Al granulite xenoliths from the Chyulu Hills volcanic field, Kenya,
suggest that the samples represent a meta-igneous suite linked by
fractionation. The incompatible major element contents increase from the
websterites to the Mg-Al granulites and further to the Ca-Al granulites.
High bulk rock Mg#s and very low concentrations of most incompatible
trace elements indicate that the rocks are cumulates rather than
crystallized melts. Elevated Ni abundances, impoverishment in Cr and
HFSE and high contents of normative plagioclase and olivine in the
granulites indicate that their protoliths were similar to troctolite.
The textures and metamorphic reaction paths recorded in the granulites
suggest igneous emplacement in the crust and cooling from igneous to
ambient crustal temperatures accompanied or followed by compression. For
the websterite xenoliths, there is an apparent contradiction between the
results of P-T calculations that suggest high P and T of crystallization
of early generation pyroxenes and elevated P-T conditions during final
equilibration (1.4-2.2 GPa/740-980 degrees C) on the one hand and the
positive Eu anomaly that suggests shallow-level plagioclase accumulation
on the other hand. This contradiction can be reconciled by a model of
compression of a plagioclase-bearing (gabbroic) protolith to mantle
depths where it recrystallized to an ultramafic assemblage, which
requires foundering of dense lower crustal material into the mantle.