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Petrogenetic significance of some new xenolithic alkaline rocks from East Africa

Published online by Cambridge University Press:  05 July 2018

N. M. S. Rock
N. M. S. Rock*
Affiliation:
Department of Mineralogy and Petrology, University, Cambridge, CB2 3EW
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Summary

New occurrences of xenolithic alkaline rocks from three East African volcanoes are described. Micro-ijolites and alkali pyroxenites occur in the carbonate-rich tufts and lavas of Mount Elgon; kaersutite-rich pyroxenites and a suite of fenitized basement rocks in the caldera flows of Mount Meru, and apparently unique melanocratic hortonolite nepheline syenites in the central foyaite plug of Mount Kenya. The 87Sr/86Sr ratios indicate mantle origin, and all are enriched in trace elements such as Sr, Nb, and Zr. The xenoliths from Mount Kenya are cumulate equivalents of the glassy kenyte lavas of the mountain; the pyroxenites of Mount Elgon are also cumulates whose predominant titanaugite is of identical composition to that of the host lavas. The contrast between the 'dry' Elgon pyroxenites and the 'wet' amphibole-rich types from Mount Meru is believed to reflect different CO2/H2O ratios in the volatile phase accompanying the alkaline vulcanicity. Strongly alkaline, carbonate-rich suites such as that of Mount Elgon derive from fairly dry alkali-basaltic magma under high CO2 activity by dominant pyroxene fractionation, whereas the mildly alkaline lineage seen in some areas of Mount Meru forms under more hydrous conditions in which amphibole fractionation is important. Mount Meru in addition shows a transition to rock-types typical of the strongly alkaline lineage, and this is reflected in the occurrence of relatively amphibole-poor pyroxenites with the amphibole-rich types.

Type
Research Article
Information
Mineralogical Magazine , Volume 40 , Issue 314 , June 1976 , pp. 611 - 625
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1976

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