L. Franz et al., Diachronous Pan-African granulite-facies metamorphism (650 Ma and 550 Ma) in the Kaoko belt, NW Namibia, EUR J MINER, 11(1), 1999, pp. 167-180
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.