Abstract
Pelitic schists from Murchison Falls, an area within the Late-Archean Congo Craton in NW Uganda, were studied with the aim to deduce the P-T path and the age of metamorphism. These rocks possibly represent a northern extension of a Post-Archean SW-NE trending metasedimentary series (Igisi Group), which separates the sediments in the Albert Rift from the assumed Archean Granulite Group, where it is superimposed on. We here present for the first time constraints for the metamorphic evolution and the metamorphic age of these metasediments, which demonstrate the significance of Pan-African orogenic events in the cratonic domains of central Africa. The metapelites of Murchison Falls contain the peak metamorphic assemblage kyanite+garnet+biotite+muscovite+quartz+plagioclase ±K-feldspar+monazite. The metamorphic conditions were obtained from zoned garnet porphyroblasts, relic plagioclase inclusions in garnet which have higher anorthite contents in the inner parts of the garnet compared to those included in the outer zones and to matrix plagioclase. Conventional thermobarometry using the anorthite-quartz-grossular-kyanite barometer and Fe-Mg garnet-biotite exchange equilibria yield peak conditions within the kyanite stability field at about 0.8 GPa and 650–680 °C. These conditions are in accordance with pseudosection calculations for a particular sample, which suggests temperatures in excess of 650 °C during formation of the equilibrium assemblage ms+ky+grt+bt+qtz. To determine the age of metamorphism we used U-Th-total Pb chemical dating of metamorphic monazite with the electron microprobe. Monazites show a one-stage growth history. U-Th-total Pb dating of two samples yield Pan-African ages of 621 ± 26 and 633 ± 27 Ma. No indications for older monazite relics were found. These data imply that after the Archean/Early Proterozoic granulite-facies metamorphism of the basement rocks, sediments were deposited within epicontinental basins. During the Pan-African orogeny these sediments experienced a single stage metamorphism at amphibolite facies conditions induced by a crustal thickening event.
