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Western branch of the East African rift system, Uganda

The western branch (Uganda) of the East African rift system is noted for its abundance of potassic alkaline rocks. From north to south the province consists of carbonatite lavas at Fort Portal, ultrapotassic mafic rocks in the central Katwe-Kikorongo and Bunyaruguru fields, and potassic mafic-felsic lava flows and plugs in the Bufumbira field. These unique rocks are the focus of a collaborative mineralogical (chemical studies by electron microprobe) and geochemical ( major element and XRF and INAA trace element rock chemistry) study. The participants are G. N. Eby, University of Massachusetts Lowell, F. E. Lloyd, University of Reading, F. Stoppa, Gabriele d'Annunzio University, and A. R. Woolley, Natural History Museum. Publications resulting from the present study and pictures of the field area are found below.

Publications:

Eby, G. N., Lloyd, F.E., Woolley, A.R. (2009) Geochemistry and petrogenesis of the Fort Portal, Uganda, extrusive carbonatite. Lithos 113, 785-800.

Bailey, K., Lloyd, F., Kearns, S., Stoppa, F., Eby, N., and Woolley, A. (2005) Melilitite at Fort Portal, Uganda: Another dimension to the carbonate volcanism. Lithos 85, 15-25.

Eby, G. N., Lloyd, F. E., Woolley, A. R., Stoppa, F., and Weaver, S. D. (2003). Geochemistry and mantle source(s) for carbonatitic and potassic lavas, western branch of the East-African rift system, SW Uganda. Geolines 15, 15-19.

Eby, G. N., Lloyd, F. E., Woolley, A. R., and Stoppa, F. (2002). Geochemistry and mantle source(s) of carbonatitic and potassic lavas from SW Uganda. In Ulrych, J., Cajz, V., Adamovic, J. and Bosák, P. (eds.) HIBSCH 2002 Symposium, Excursion Guide Abstracts. Czech Geological Survey, Prague, Czech Republic, p. 73.

Lloyd, F.E., Woolley, A.R., Stoppa, F., and Eby, G.N. (2002) Phlogopite-biotite parageneses from the K-mafic-carbonatite effusive magmatic association of Katwe-Kikorongo, SW Uganda. Mineralogy and Petrology 74, 299-322.

Stoppa, F., Woolley, A. R., Lloyd, F. E., and Eby, N. (2000) Carbonatite lapilli-bearing tuff and a dolomite carbonatite bomb from Murumuli crater, Katwe volcanic field, Uganda. Mineralogical Magazine 64, 155-164.

Lloyd, F. E., Woolley, A. R., Stoppa, F., and Eby, G. N. (1999) Rift Valley magmatism - is there evidence for laterally variable alkali clinopyroxenite mantle? GeoLines 9, 76-83.

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Petrogenesis of the Fort Portal, Uganda, Extrusive Carbonatite

Eby, G. N., Lloyd, F. E. and Woolley, A. R.

The Quaternary Fort Portal volcanic field occurs at the northern end of the Western Rift in Uganda. The eruptive phases consist of (1) early carbonatite tuff cones followed by (2) a blanket carbonatite tuff (the major unit of the field) and finally (3) a small volume of calciocarbonatite lava. Mantle and crustal xenoliths are found in all eruptive phases and silicate (melilitite) lapilli are associated with the blanket tuffs. Major and trace element geochemistry was done on 37 samples from all three eruptive phases plus 10 crustal xenoliths. The silicate lapilli contain carbonate ocelli and abundant lithic fragments. Major and trace element geochemistry was determined for the melilitite lapilli matrix and carbonate ocelli, plus inter-lapilli carbonates, using a variety of microbeam techniques.

The chemistry of the tuffs was modeled using a 5 component system – carbonatite lava, melilitite lapilli matrix, low REE and HFSE carbonate associated with the silicate lapilli, and felsic and mafic crustal xenoliths. Mixing calculations for individual tuff samples show that the major components are carbonatite lava and xenolithic material. The melilitite lapilli component varies from 0% (cone tuffs) to 27% in the blanket tuffs. The low REE and HFSE carbonate component is generally of minor importance.

The carbonate phases found in the tuffs and lapilli are formed by a variety of processes. The carbonate ocelli in the silicate lapilli, on the basis of major element chemistry and texture, could represent an immiscible liquid separated from melilitite melt. However, the trace element data strongly support a model involving the separation of a late stage fluid phase from the crystallizing melilitite. The inter-lapilli carbonate is the result of deuteric processes. Both major element and trace element data support an origin of the carbonatite lava as an immiscible liquid separated from a carbonated melilitite melt at high pressure (>0.6GPa).

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Melilitite at Fort Portal, Uganda: Another dimension to the carbonate volcanism

Bailey, K., Lloyd, F., Kearns, S., Stoppa, F., Eby, N., and Woolley, A.

Because the calciocarbonatite lavas at Fort Portal were the first ever described they have received great attention, with the pyroclastic rocks being relatively neglected. Volumetrically the lavas are minute, and the major deposit is a 2 m thick blanket of "flaggy" tuffs, long regarded as carbonatite tuff with crustal debris. Fresh examination shows these tuffs to contain melilitite previously unreported from Fort Portal. The rock is a mix of crust and mantle debris with near-isotropic lapilli, set in a matrix composed predominantly of carbonatite. The low birefringence parts of the lapilli are devitrifed melilitite glass. Compound lapilli are abundant, containing aggregates of globules, together with xenolithic/crystic fragments. In some, there are concentric zones of more carbonate rich material alternating with melilitite; tangential phlogopite flakes mark the outer zones, in marked contrast to their planar distribution through the enclosing rock matrix. Euhedral titano-magnetite (10-15%) is the most obvious cognate mineral. Devitrified melilitite contains abundant small crystals and microlites of melilite, apatite, magnetite, and carbonates, mostly formed during disequilibrium quench crystallisation. Because of this, and widespread fine grained accidental debris, a precise bulk melt composition is hard to obtain, but the average is close to melilitite with high P₂O₅. Mantle debris is largely disaggregated magnetite-phlogopite clinopyroxenite, which could give a bulk composition close to the melt. Low Mg and high Mg calcite are present in the melilitite lapilli, and in the enclosing carbonate rich matrix. Previously, high Mg calcite was reported only as cement in lapilli tuffs, while the lavas contain only low Mg calcite in the assemblage calcite-periclase (consistent with low pressure carbonate melt crystallisation). Carbonatite-melilitite magma left the mantle carrying restite debris. Melt fragmentation took place in the deep crust, with rapidly quenched droplets enclosing crust debris. Chemical covariations within the flaggy tuffs are uniform and explicable as carbonatite-melilitite plus a thoroughly mixed combination of crust and mantle debris. New links are indicated with the alkaline ultramafic-carbonate volcanism to the south, in Uganda, and parallel with that in Italy.

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Geochemistry and Mantle Source(s) for Carbonatitic and Potassic Lavas, Western Branch of the East-African Rift System, SW Uganda

Eby, G. N., Lloyd, F. E., Woolley, A. R., Stoppa, F., and Weaver, S. D.

Samples from the various volcanic fields in the Uganda portion of the western branch of the East African rift system were analyzed for major and trace elements. The northernmost Fort Portal field consists of extrusive carbonatite tuffs and lavas. All these samples are mixtures of carbonatite, basement rock fragments and peridotite xenoliths. The central fields, Katwe-Kikorongo and Bunyaraguru, and Kasenyi Crater, are ultrapotassic, but detailed sampling indicates that the degree of K enrichment, with respect to Na, varies geographically with Bunyaraguru and Kasenyi Crater showing the greatest enrichment. The southern field, Bufumbira, while also potassic shows a much lower degree of K enrichment. In terms of a variety of trace elements, and trace element ratios, various mantle domains can be identified that gave rise to the magmas in each of the volcanic areas. The data indicate that the subcontinental mantle under this portion of Uganda has undergone variable degrees of metasomatism. A complete description of the character of the primary magmas requires consideration of both the degree of metasomatism and the degree of melting of the garnet-bearing source.

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Geochemistry and Mantle Source(s) of Carbonatitic and Potassic Lavas from SW Uganda

Eby, G. N., Lloyd, F. E., Woolley, A. R., and Stoppa, F.

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Phlogopite-biotite Parageneses from the K-mafic-carbonatite Effusive Magmatic Association of Katwe-Kikorongo, SW Uganda

Lloyd, F. E., Woolley, A. R., Stoppa, F., and Eby, G. N.

Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic micas in kamafugites and carbonatites are aluminous (>12 wt% Al₂O₃), typically contain significant levels of Cr (up to 1.1 wt% Cr₂O₃), and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al₂O₃ and TiO₂ compositional field of the xenolith mica, and on the basis of Mg# and high P, T experimental evidence they probably crystallised at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values found in primary and metasomatic phlogopites in kimberlite-hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite-clinopyroxenite mantle. Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and mica in Katwe-Kikorongo mixed melilitite-carbonate tephra. Higher Al₂O₃ distinguishes Fort Portal carbonatite xencorysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of the mixed carboantite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite.

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Carbonatite Lapili-bearing Tuff and a Dolomite Carbonatite Bomb from Murumuli Crater, Katwe Volcanic Field, Uganda

Stoppa, F., Woolley, A. R., Lloyd, F. E., and Eby, N.

A group of carbonate-rich tuffs are described from the Murumuli crater, Katwe-Kikorongo volcanic field, SW Uganda which contain abundant carbonatite pelletal lapilli, together with melilitite lapilli and a range of xenocrysts and lithic fragments including clinopyroxenites considered to be of mantle origin. The carbonatite lapilli consist essentially of Sr-bearing calcite and Mg-calcite which form quench-textured laths. The lapilli contain microphenocrysts of Ti-magnetite, perovskite, apatite, clinopyroxene, sanidine and altered prisms of melilite. A 7 cm long dolomite carbonatite bomb is described which displays a form typically assumed by lava clots erupted in a molten state. Chemical analyses of a tuff, the bomb and a range of minerals are presented. Carbonatite clearly played an important role in the Katwe-Kikorongo magmatism and it is suggested that carbonatite magma evolved from carbonate-bearing melilitite.

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Rift Valley Magmatism - Is There Evidence for Laterally Variable Alkali Clinopyroxenite Mantle?

Lloyd, F. E., Woolley, A. R., Stoppa, F, and Eby, G. N.

Alkali pyroxenite xenoliths from three volcanic fields in Uganda are largely composed of clinopyroxene (cpx) and phlogopite-biotite (together >70% of mode). Inter-field compositional variation in these minerals, shown by 749 cpx analyses and 237 mica analyses from 34 xenoliths, indicates bulk-chemical lateral variation in the xenolith source. The ubiquitous presence of alkali clinopyroxenite xenoliths in all the fields suggests this lithology is widespread beneath Uganda's Western Rift. Nd-Sm and Pb isotope systematics indicate that the xenoliths are not cumulate from their host kamafugites, while P-T experiments indicate that the kamafugites were in equilibrium with clinopyroxenite at ~>60 km depth. It is argued therefore that the xenoliths are fragments of a laterally variable clinopyroxenite layer in Uganda's Western Rift deep crust-mantle.

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Photographs of the field area in western Uganda

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