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The Wadi Haimur-Abu Swayel gneiss belt is composed of biotite-gneiss, biotite-hornblende-gneiss and garnet-gneiss. Amphibole and muscovite compositions indicate that these gneisses belong to low- to intermediate-pressure metamorphic facies. Temperature values calculated using the hornblende-plagioclase geothermometer are in the range of 495-550 degreesC. The retrograde part of the metamorphic evolution for these rocks is documented by the presence of epidote and chloritization of biotite. The chlorite geothermometer indicates that chloritization occurred around 260-300 degreesC. The sedimentary origin of the gneisses is indicated by the high values of normative quartz and corundum. It is suggested that the sedimentary protoliths of the Wadi Haimur-Abu Swayel gneisses evolved in a back-arc basin, Compression of this basin led to intense deformation, metamorphism and granite magmatism. K/Ar ages have been determined on hornblende and biotite. The hornblende ages approximate to 600 Ma and the approximate to 585 Ma of coexisting biotite from the gneisses are interpreted as cooling ages following the peak of metamorphism. These ages are within the range of granitic emplacement in the study region (570-675 Ma), which indicates that the peak of metamorphism could have been contemporaneous with the granitic magmatism in the Wadi Haimur-Abu Swayel gneisses. (C) 2002 Elsevier Science B.V. All rights reserved.