K---Ar hornblende dating of late Pan-African metamorphism in the furua granulite complex of Southern Tanzania

K---Ar analyses are reported for six hornblendes from the Furua granulitic complex in southern Tanzania. The M1 granulite-facies metamorphism has locally been followed by an M2 amphibolite-facies retrogradation to varying degrees. Three of the hornblendes (olive-green and orange—brown) come from granulites not showing any M2 retrogradation. They were produced as a stable phase during M1 and are concordant at approximately 630 Ma. Of the other hornblendes (Bluish-green), two come from completely M2 retrograded rocks and one from a post-M1 metadiorite. Two of them, one M2 hornblende and the metadiorite hornblende, are concordant with the M1 hornblendes, the third is somewhat older. The age of approximately 630 Ma is related to the closure of the K---Ar hornblende systems following the termination of the M2 amphibolite-facies conditions. Taking also into account an earlier U-Pb zircon investigation and U-Pb zircon data reported from the Wami River granulite complex to the northeast, the M1 granulite-facies metamorphism is dated at approximately 715 Ma and the termination of the M2 amphibolite-facies retrogradation at approximately 650 Ma. It is argued that a prolonged period of high crustal temperature prevailed after M1, with a slow cooling rate from approximately 800–825°C during M1 approximately 715 Ma ago to 490–550°C approximately 630 Ma ago, shortly after M2. This thermal regime may be related to a continent—continent collision model for the evolution of the Mozambique belt.