Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg-Al and Ca-Al
granulite xenoliths. Their protoliths are interpreted as troctolitic
cumulates; however, the original mineral assemblages were almost
completely transformed by subsolidus reactions. Mg-Al granulites contain
the minerals spinel, sapphirine, sillimanite, plagioclase, corundum,
clinopyroxene, orthopyroxene and garnet, whereas Ca-Al granulites are
characterized by hibonite, spinel, sapphirine, mullite, sillimanite,
plagioclase, quartz, clinopyroxene, corundum, and garnet. In the Mg-Al
granulites, the first generation of orthopyroxene and some spinel may be
of igneous origin. In the Ca-Al granulites, hibonite (and possibly some
spinel) are the earliest, possibly igneous, minerals in the
crystallization sequence. Most pyroxene, spinel and corundum in Mg-Al
and Ca-Al granulites formed by subsolidus reactions. The qualitative P-T
path derived from metamorphic reactions corresponds to subsolidus
cooling, probably accompanied, or followed by, compression. Final
equilibration was achieved at T approximate to 600-740 degrees C and P <
8 kbar, in the stability field of sillimanite. The early coexistence of
corundum and pyroxenes (+/- spinel), as well as the association of
sillimanite and sapphirine with clinopyroxene and the presence of
hibonite, makes both types of granulite rare. The Ca-Al hibonite-bearing
granulites are unique. Both types enlarge the spectrum of known
Ca-Al-Mg-rich granulites worldwide.