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 episode. These mineral ages range from 510 to 655 Ma and indicate that peak metamorphic conditions were diachronous in the different granulite domains. U-Pb titanite and rutile ages define integrated cooling rates of 2-5 degreesC/Ma for all investigated granulite areas, and suggest a common process for the post-metamorphic histories of the different granulite areas. Prolonged slow cooling-rates are consistent with near-isobaric cooling in the deep crust after the metamorphic peak. The process responsible for crustal thickening during heating did not produce isostatic instability and fast erosion-driven or tectonic exhumation. The thermal history determined in this study is not consistent with the collision of East- and West-Gondwana as the cause of granulite facies metamorphism. Palaeomagnetic data have shown that this collision did not occur until 550 Ma, when the Pan-African granulites in Tanzania had already cooled below 500 degreesC. The high pressure granulites of eastern Tanzania are thus interpreted as having attained their metamorphic peak prior to the final amalgamation of Gondwana, probably in an active continental margin setting. (C) 2000 Elsevier Science B.V. All rights reserved.