Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya |
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| Authors: | ULIANOV A; KALT A |
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| Institution: | UNIVERSITY OF NEUCHÂTEL, INSTITUTE OF GEOLOGY, RUE EMILE-ARGAND 11, CH-2007 NEUCHÂTEL, SWITZERLAND |
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| Abstract: | Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg–Aland Ca–Al granulite xenoliths. Their protoliths are interpretedas troctolitic cumulates; however, the original mineral assemblageswere almost completely transformed by subsolidus reactions.Mg–Al granulites contain the minerals spinel, sapphirine,sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxeneand garnet, whereas Ca–Al granulites are characterizedby hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase,quartz, clinopyroxene, corundum, and garnet. In the Mg–Algranulites, the first generation of orthopyroxene and some spinelmay 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, spineland corundum in Mg–Al and Ca–Al granulites formedby subsolidus reactions. The qualitative P–T path derivedfrom metamorphic reactions corresponds to subsolidus cooling,probably accompanied, or followed by, compression. Final equilibrationwas achieved at T  600–740°C and P <8 kbar, inthe stability field of sillimanite. The early coexistence ofcorundum and pyroxenes (± spinel), as well as the associationof sillimanite and sapphirine with clinopyroxene and the presenceof hibonite, makes both types of granulite rare. The Ca–Alhibonite-bearing granulites are unique. Both types enlarge thespectrum of known Ca–Al–Mg-rich granulites worldwide. KEY WORDS: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift |
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| Keywords: | : granulite xenoliths corundum sapphirine hibonite Kenya Rift |
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