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

Abstract

The Pan-African Kaoko belt of NW-Namibia is an orogenic belt along the suture of the proto South-Atlantic. It experienced a Pan-African greenschist to granulite-facies metamorphism that overprinted the Eastern, Central and Western Zone of the belt at westward increasing conditions. Structural, lithological, and petrological data on orthogneisses and metabasites from the Central and Eastern Zones demonstrate that these rocks experienced a pre-Pan-African high-grade tectono-metamorphic evolution. U-Pb zircon ages from orthogneisses of the Central and Eastern Zone yield ages at cn. 1930 and cn. 2620 Ma, respectively. In the Western Zone of the Kaoko belt, high-grade Pan-African metamorphism has wiped out evidence of the original relations between the orthogneisses and their wall-rocks. These orthogneisses are structurally and metamorphically similar to those of the Central and Eastern Zones and, therefore, were thought to represent pre-Pan-African basement. However, U-Pb chronology on zircon and monazite for these orthogneisses yields Pan-African ages at 553.6 +/- 1.2 Ma and 558.6 +/- 7.3 Ma (2 sigma). These ages are considered to represent the time of granulite-facies overprint of these rocks. A minimum age for the end of the Pan-African metamorphic overprint 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 granulite-facies event is dated with U-Pb at 645.0 +/- 3.5 Ma (2 sigma) in the western part of the Western Zone. Combined structural, metamorphic, and geochronologic data demonstrate: (1) structural homology and comparable metamorphic history within the same orogenic belt may have been acquired at different times and may in extreme cases not be related at all. (2) The Western Kaoko belt is characterized by two distinct Pan-African granulite-facies metamorphic events of similar structural style and peak metamorphic conditions 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 scenario, inferred for many other granulite-facies terranes, could reflect lower crustal extension. Alternatively, the 650 Ma granulites may be unrelated with the early evolution of the Kaoko belt, but originate from the continuation of the Damara belt in South America, the Rio Paraiba do Sul shear belt, which displays a Pan-African age range of 550-650 Ma for granitoids and high-grade metamorphites.