Geology, petrology and age determination of Archaean-Proterozoic basement rocks of western Uganda
Final Report Abstract
This project aimed to improve our understanding of the generation of volcanic rocks located in fields to the east and southeast of the Rwenzori Mountains and of the ultramafic nodules within them. The volcanics consist of lavas of tuffs of exceptionally rare composition, namely kamafugites and carbonatites, whereas the ultramafic modules comprise assemblages dominated by clinopyroxene and dark mica. The geochemical study was begun prior to fieldwork in Uganda by obtaining samples from museums in England from the original collections of A.D. Combe, based on extensive field campaigns in the 1930s for the Geological Survey of Uganda. These samples were subjected to extensive petrographic studies and initial whole-rock geochemical analyses many years ago, but very little in the way of modern mineral chemical or whole-rock geochemical, and particularly isotope work, has been done on them. This is despite these rocks being the type area for kamafugites, a type of rock that has since been better characterized in other areas of the world. Our project was aimed at redressing this data gap for the Ugandan volcanics. Two field campaigns in 2007 documented that the coverage of outcrops by the Combe and Holmes collection was an adequate representation of the outcrops available. The field campaigns resulted in the collection of more than 150 ultramafic modules, particularly from the Katwe crater in the Katwe-Kikorongo volcanic field. These include several composite nodules (i.e. xenoliths containing vein edges) which have not been described from western Uganda before, although they are known from other areas as key samples documenting multiple ages of ultramafic assemblages formed in the mantle. The work on the ultramafic xenoliths was begun after most of the whole-rock geochemical work, and so is in progress, whereas the geochemical work on the lavas is essentially complete.
Publications
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Volcanoes and volcanic sources in the East African Rift. Proc. Belgian Royal Acad. Sci., pp.43-55
S.F.Foley
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(2007) Geochemical constraints on the origin of silica-poor alkaline volcanic rocks in the Toro-Ankole and Bufumbira regions of western Uganda. AVCOR, Luxembourg, November 2007
A. Rosenthal, K. Link, D.G. Pearson, G.M. Nowell, E. Barafaijo, J. Tiberindwa, S.F. Foley
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(2007) The origin of melts in the rift branches of equatorial Africa. EARS 07 International conference on the East African Rift 2007 (Uganda)
S.F.Foley
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Isotopic constraints on the petrogenesis of the kamafugites of Uganda. EGU Vienna April 2007
K. Link, A. Rosenthal, S.F.Foley, D.G.Pearson, & G. Nowell
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(2008) Origin of kamafugite magmas in the east African Rift of western Uganda. 9th Kimberlite Conference, Frankfurt. (9IKC-A-00210)
A. Rosenthal, S.F. Foley, D.G. Pearson, G.M. Nowell & S. Tappe
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(2008) Pyroxenite- xenoliths from the silica-poor alkaline volcanic rocks in the Toro-Ankole region of western Uganda. EGU Vienna, April 2008
K. Link, E. Barifaijo, J. Tiberindwa, S.F. Foley
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(2008) Veined pyroxenite xenoliths from the kamafugites in the Toro-Ankole region of western Uganda: a window to a rift-related mantle. 9th Kimberlite Conference, Frankfurt. (9IKC-A-00403)
K. Link, E. Barifaijo, J. Tiberindwa & S.F. Foley
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(2008) Volcanics and their mantle xenoliths from western Uganda: a window to recent activity in the sub-rift mantle. RIFTLINK workshop (Neustadt) 2008
K. Link, E. Barifaijo, J. Tiberindwa, A. Rosenthal, D.G. Pearson, S.F. Foley
