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Research project on Basement Geology of Uganda

Staff:	D. Loose V. Schenk P. Appel A. Schumann
Funding:	N.N.
Publications:	Appel, P., Schenk, V. & Schumann, A. (2005). P-T path and metamorphic ages of pelitic schists at Murchison Falls, NW Uganda: evidence for a Pan-African tectonometamorphic event in the Congo Craton. European Journal of Mineralogy 17, 655-664. | Abstract | doi: 10.1127/0935-1221/2005/0017-0655 |

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The four major cycles of crustal growth and orogenic reworking which characterise the geological evolution of the African continent have all been recognized and isotopically dated in Uganda (Leggo, 1974)

During the Archean, the Watian has been formed and experienced its first metamorphism at 2.9 Ga. A second metamorphic overprinting took place at 2.55 Ga (Aruan metamorphism). The Buganda-Toro System has been formed and metamorphosed during the Paleoproterozoic Eburnian orogenic cycle. The Mirian metamorphism locally reworked the Archean basement complex at c. 1 Ga and might be attributed to the Kibaran event which seems to be related to the formation of the Rodinia supercontinent. In addition, the effects of the Pan-African orogeny (650-500 Ma) are seen on rocks of the basement complex in NE Uganda.

The petrological-geochemical project proposed here is aimed to concentrate on the Archean basement complex and the later metamorphic overprinting events that affected it. By reconstructing metamorphic pressure-temperature-time paths of lithological units, it will be possible to decipher their tectonic (pressure) and thermal history and thus the geodynamic causes and evolutions of orogenic events. E.g., during the time of formation and magmatic accretion of a new crustal segment, rocks are expected to become hotter and the crust will grow to normal thickness. As a consequence, rocks will show an anticlockwise P-T path and would cool isobarically after peak metamorphism; metamorphism would be nearly synchroneous with crust formation. In contrast, metamorphism induced by tectonic reworking during continent collision after subduction of an oceanic lithosphere, will be characterised by a clockwise P-T path. Strong erosion of the tectonically thickened crust should be expressed as isothermal uplift after peak metamorphism.

In the context of the Ugandan geology, it is interesting to compare the P-T paths of the early Archean metamorphic events with those that occurred during later orogenic reworking. A second, more general question is, whether the Archean geodynamic processes were similar to those characterising modern Phanerozoic orogenies.

The only metamorphic rocks of Uganda that have been treated with modern petrological methods, including microprobe work and geothermobarometry are the sapphirine-quartz granulites of Labwor Hills (Nixon et al. 1973, Sandiford et al. 1987). But even here, the age of the ultrahigh temperature metamorphism is not yet resolved. It could be Archean or Pan-African in age.

Labwor Hills

The Labwor Hills are a range of mountains in northeast Uganda. They are located approximately one hundred kilometers north of Soroti in the southwestern part of the district Kotido. The surrounding area is also known as Karamoja (after the former name of this district).

The geology of the Labwor Hills is - unlike most parts of the Uganda Basement Complex - relatively good known, because they were mapped in the 1960s by the "Geological survey and mines department" of the Republic of Uganda (Baldock et al., 1969). The reason the Labwor Hills are famous for is the uncommon mineralogy of the rocks. The occurrence of the mineral assemblage sapphirine + quartz shows, that UHT metamorphic conditions (> 950°C) have been reached. These sapphirine bearing granulites were first described by Nixon et al. (1973). Nixon et al. (1984) also described the rare mineral kornerupine from this locality.
Apart from the work of Sandiford et al. (1987) the Labwor Hills granulites were not examined with modern petrological methods.
According to zircon dating (Leggo, 1974) the Labwor Hills granulites represent Archean basement that experienced metamorphic reworking during the Pan-African cycle (635±30 Ma).
Our field work showed that sapphirine-quartz granulites are much more widespread than indicated on the unpublished map of Baldock et al. (1969) and by the study of Nixon et al. (1973). Denny Loose´s diploma and doctorate theses concentrate on the petrology of the sapphirine-quartz granulites.

West Nile

Geologically the West Nile area of NW Uganda belongs to the Uganda Basement Complex, which is part of the Archean Congo craton. This area has been very well mapped by Hepworth (1964) and Macdonald. Their interpretation of the basement evolution in Uganda (Hepworth and Macdonald, 1966) relies mainly on observations made in the West Nile area.
Zircon ages (Leggo, 1974) point to 2880Ma interpreted as the age of the granulite facies metamorphism ("Watian"). "Aruan" reworking took place at 2550Ma under amphibolithe facies conditions. Later orogenic events affected the basement only locally, e. g. along the SW-NE striking "refoliation zone".

In the granulite-facies ("Watian") Mt. Luku area a geological mapping project was carried out by Niels Jöns, Denny Loose, Heike Wegner and Nadine Wittig. Heike performed a petrological-geochemical study on the Watian rocks from the West Nile area to unravel the Archean crust formation and metamorphic reworking.
In addition, monazite CHIME-dating will help to resolve the time framework of the metamorphic history.

References

• Baldock, J.W., Clark, L., Reedman, A.J. and Wren, A.E. (1969): Explanation of the geology of sheet 25 (Labwor Hills). 59p. The Republic of Uganda, Geological Survey and Mines Department.
• Hepworth, J. V. (1964): Explanation of the Geology of sheets 19, 20, 28 and 29 (Southern West Nile). p. 1-128, Geological Survey of Uganda, Entebbe.
• Hepworth, J. V. and Macdonald, R. (1966): Orogenic Belts of the Northern Uganda Basement. Nature, 210, 726-727.
• Leggo, P.J. (1974): A geochronological study of the basement complex of Uganda. Jl. geol. Soc. Lond., 130, 263-277.
• Nixon, P. H., Reedman, A. J. and Burns, L. K. (1973): Sapphirine-bearing granulites from Labwor, Uganda. Min. Mag., 39, 420-428.
• Nixon, P.H., Grew, E.S. and Condliffe, E. (1984): Kornerupine in a sapphirine-spinel granulite from Labwor Hills, Uganda. Min. Mag., 48, 550-552.
• Sandiford, M., Neall, F. B. and Powell, R. (1987): Metamorphic evolution of aluminous granulites from Labwor Hills, Uganda. Contrib. Mineral. Petrol., 95, 217-225.

further information concerning the Uganda project

The Labwor Hills	Preparing the field work.
The small village Pupu in the Labwor Hills.	The Mt. Luku Inselberg in the West Nile area.
Mesoperthite.	Backscattered-electron image of a monazite from the West Nile area.

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