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Upper-mantle metasomatism beneath a continental rift: clinopyroxenes in alkali mafic lavas and nodules from Sruth West Uganda

Published online by Cambridge University Press:  05 July 2018

F. E. Lloyd
F. E. Lloyd*
Affiliation:
Department of Geology, University of Reading
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Abstract

Clinopyroxenes are dominant in highly potassic, silica undersaturated mafic volcanics occurring on the western rim of the uplifted, rifted East African craton. A kimbcrlite style of eruption provides nodules of alkali clinopyroxenite (clinopyroxene + titaniferous phlogopite+titanomagnetite, apatite, sphene, and rare corroded olivine) which have similar bulk chemistry to the feldspathoid-bearing lavas. Many nodules display metasomatic textures supporting a formation from the alteration of pre-existing material; clinopyroxene growth is characterized by complex, non-oscillatory colour zoning. Comparison of natural clinopyroxene chemistry with published data for elinopyroxenes crystallized from synthetic potassium-rich mafic material, suggests that a significant proportion of the nodules crystallized at upper-mantle pressures. Neither garnet- nor orthopyroxene-bearing nodules have ever been recorded from south-west Uganda, suggesting that metasomatism of the local mantle has proceeded far enough to obliterate all recognizable remnants of four-phase lherzolite.

Type
Research Article
Information
Mineralogical Magazine , Volume 44 , Issue 335 , September 1981 , pp. 315 - 323
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1981

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