U-Pb dating of metamorphic minerals: Pan-African metamorphism and prolonged slow cooling of high pressure granulites in Tanzania, East Africa

U-Pb monazite and zircon ages reveal that the high pressure granulites from eastern Tanzania were metamorphosed during a Pan-African tectonothermal ep isode. These mineral ages range from 510 to 655 Ma and indicate that peak m etamorphic conditions were diachronous in the different granulite domains. U-Pb titanite and rutile ages define integrated cooling rates of 2-5 degree sC/Ma for all investigated granulite areas, and suggest a common process fo r the post-metamorphic histories of the different granulite areas. Prolonge d slow cooling-rates are consistent with near-isobaric cooling in the deep crust after the metamorphic peak. The process responsible for crustal thick ening during heating did not produce isostatic instability and fast erosion -driven or tectonic exhumation. The thermal history determined in this stud y is not consistent with the collision of East- and West-Gondwana as the ca use of granulite facies metamorphism. Palaeomagnetic data have shown that t his collision did not occur until 550 Ma, when the Pan-African granulites i n Tanzania had already cooled below 500 degreesC. The high pressure granuli tes of eastern Tanzania are thus interpreted as having attained their metam orphic peak prior to the final amalgamation of Gondwana, probably in an act ive continental margin setting. (C) 2000 Elsevier Science B.V. All rights r eserved.