Mg–Al Sapphirine- and Ca–Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya

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Abstract

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 ≈ 600–740°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.

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