ISOTOPIC DATING OF LOW-GRADE METAMORPHIC SHALES IN NORTHERN NAMIBIA (SOUTH WEST-AFRICA) AND IMPLICATIONS FOR THE OROGENIC EVOLUTION OF THE PAN-AFRICAN DAMARA BELT

The fine mineral fractions (< 2 μm) from pelitic sediments of the molasse-type Mulden Group in the Pan-African Damara belt of Namibia were dated by the Rb-Sr and K-Ar methods. The sediments contain two major parageneses of metamorphic origin which can be related to two separate low-grade regional tectono-thermal events of anchizonal intensity at about 535 (545) Ma* * All ages reported and cited in this paper are based on the decay constant λ 87Rb = 1.42 · 10-11 a-1 and errors are quoted at the 2-σ level. For easy comparison with previously published ages from southern African the values based on λ 87Rb = 1.39 · 10-11 a-1 are given in brackets. and 455 (465) Ma respectively. A depositional model age for the top of the Mulden Group is about 550-560 (565-570) Ma and it is possible that the Precambrian-Cambrian boundary in northern Namibia may be placed at the base of the Mulden sequence. The above metamorphic events are reflected by mineral and whole-rock ages throughout the Damara belt and are therefore of regional significance. They are part of Clifford's (1967) Damaran Episode which is now more complex than previously suggested and which postdates an earlier tectonic event of preMulden age, provisionally dated at about 650 (665) Ma. We propose to abandon the use of the terms Damaran and Katangan episodes in their original connotation for two chronologically and regionally distinct Pan African events as defined by Clifford (1967) and suggest to restrict these names to the tectono-thermal domains of the Damara and Katanga belts whose orogenic development was broadly coeval. © 1979.