High-pressure granulite facies metamorphism in the Pan-African belt of eastern Tanzania: P-T-t evidence against granulite formation by continent collision

To constrain the tectonic history of the Pan-African belt in Tanzania, we have studied the P-T evolution of granulites from northern and eastern Tanzania representative for a large part of the southern Pan-African belt of East Africa (e.g. Part, Usambara, Ukaguru and Uluguru Mountains). Thermobarometry (conventional and multireaction equilibria) on enderbites and metapelites gives 9.5-11 kbar and 810+/-40 degrees C during peak metamorphism at 650-620 R Ia. This is consistent with the occurrence of both sillimanite and kyanite in metapelites and of the high-P granulite facies assemblage garnet-clinopyroxene-quartz in mafic rocks. Peak metamorphic conditions are surprisingly similar over a very large area with N-S and E-W extents of about 700 and 200 km respectively. The prograde metamorphic evolution in the entire area started in the kyanite field but evolved mainly within the sillimanite stability field. The retrograde P-T evolution is characterized by late-stage kyanite in metapelites and garnet-clinopyroxene coronas around orthopyroxene in meta-igneous rocks. This is in agreement with thermobarometric results and isotopic dating, indicating a period of nearly isobaric and slow cooling prior to tectonic uplift. The anticlockwise P-T path could have resulted from magmatic underplating and loading of the lower continental crust which caused heating and thickening of the crust. Substantial postmetamorphic crustal thickening of yet unknown age (presumably after 550 Ma) led subsequently to the exhumation of high-P granulites over a large area. The results are consistent with formation of the Pan-African granulites at an active continental margin where tonalitic intrusions caused crustal growth and heating 70-100 Ma prior to continental collision. The P-T-t path contradicts recent geodynamic models which proposed tectonic crustal thickening due to continental collision between East and West Gondwana as the cause of granulite formation in the southern part of the Pan-African belt.