Diachronous Pan-African granulite-facies metamorphism (650 Ma and 550 Ma) in the Kaoko belt, NW Namibia

The Pan-African Kaoko belt of NW-Namibia is an orogenic belt along the sutu re of the proto South-Atlantic. It experienced a Pan-African greenschist to granulite-facies metamorphism that overprinted the Eastern, Central and We stern Zone of the belt at westward increasing conditions. Structural, litho logical, and petrological data on orthogneisses and metabasites from the Ce ntral and Eastern Zones demonstrate that these rocks experienced a pre-Pan- African high-grade tectono-metamorphic evolution. U-Pb zircon ages from ort hogneisses of the Central and Eastern Zone yield ages at cn. 1930 and cn. 2 620 Ma, respectively. In the Western Zone of the Kaoko belt, high-grade Pan -African metamorphism has wiped out evidence of the original relations betw een the orthogneisses and their wall-rocks. These orthogneisses are structu rally and metamorphically similar to those of the Central and Eastern Zones and, therefore, were thought to represent pre-Pan-African basement. Howeve r, U-Pb chronology on zircon and monazite for these orthogneisses yields Pa n-African ages at 553.6 +/- 1.2 Ma and 558.6 +/- 7.3 Ma (2 sigma). These ag es are considered to represent the time of granulite-facies overprint of th ese rocks. A minimum age for the end of the Pan-African metamorphic overpri nt is given by the Pb-207/U-235 monazite age at 553.6 +/- 1.4 Ma (2 sigma) of a post-metamorphic and post-kinematic granite. An earlier Pan-African gr anulite-facies event is dated with U-Pb at 645.0 +/- 3.5 Ma (2 sigma) in th e western part of the Western Zone. Combined structural, metamorphic, and g eochronologic data demonstrate: (1) structural homology and comparable meta morphic history within the same orogenic belt may have been acquired at dif ferent times and may in extreme cases not be related at all. (2) The Wester n Kaoko belt is characterized by two distinct Pan-African granulite-facies metamorphic events of similar structural style and peak metamorphic conditi ons at 550-560 and 650 Ma. If both events happened in the same geotectonic environment, the 650 Ma granulite-facies metamorphism occurred in the lower crust while sedimentation of Pan-African lithologies took place. Such a sc enario, inferred for many other granulite-facies terranes, could reflect lo wer crustal extension. Alternatively, the 650 Ma granulites may be unrelate d with the early evolution of the Kaoko belt, but originate from the contin uation of the Damara belt in South America, the Rio Paraiba do Sul shear be lt, which displays a Pan-African age range of 550-650 Ma for granitoids and high-grade metamorphites.