Age determinations in the Rehoboth Basement Inlier, SWA/Namibia

Abstract

Abstract by author:

The aim of the present study is to elucidate the geological history of the southernmost basement inlier within the Damara Orogen, the so-called Rehoboth Basement Inlier. This inlier covers an area of approximately 15 000 km; the northernmost outcrops of it are situated about 70 km south of Windhoek, the capital of Namibia. The inlier is composed of a succession of metavolcanosedimentary acid and basic rock formations which are frequently intruded by granitoid as well as basic magmas. Sm-Nd and U-Pb analyses of granitoids, amphibolites and basic dykes during the present study have shown that the major part of the Rehoboth crust must have formed during the early Proterozoic between 2. 6 and 1. 8 Ga as accretionary belt extending the Archean nucleii of the Kalahari craton in a northwestern direction as an equivalent of the younger parts of the South African and Zimbabwean Limpopo Province. During the early stages of this crustal evolution the freshly formed crust must also have been readily reworked by various stages of erosional, metamorphic and anatectical processes in order to be present today as metamorphosed volcano-sedimentary sequence including the lithologies from the Mooirivier Complex at its base to the Billstein Formation at its top. This theory of an early reworking of the originally formed crust is supported by the occurrence of granitic pebbles and boulders in various formations of the Rehoboth Sequence. In this place it is worth of noting that the majority of the formations from the Mooirivier Complex to the Billstein Formation originally had most similar lithological compositions, thus leaving doubts about the present stratigraphic classification according to the South African Committee for Stratigraphy (1980) which is, for the most part, based on tectonic and metamorphic considerations. Geochemical and U-Pb studies have shown that the Weener- and Piksteel Intrusive Suites intruded the Rehoboth area during the Eburnian Orogeny, between about 2. 0 Ga and 1. 8 Ga, and that these magmas, which all show calc-alkaline characteristics, probably were generated in a collisional tectonic environment. At about 1. 4 Ga before today the Alberta Mafic Compex and related intrusives were emplaced in the northern parts of the Rehoboth Basement Inlier. This was followed between 1. 25 and 1. 0 Ga by the intrusion of the members of the Gamsberg Granite Suite and related rhyolitic dykes whose related hydrothermal activities led to a strong alteration of the major part of the intruded rock sequence. These magmas could be characterised by the geochemical analyses of this study as being of calc-alkaline composition. The strong hydrothermal alteration of the country rock by the Gamsberg magmatism could be verified by numerous Rb-Sr analyses of samples deriving from the intruded rock sequence; the age of the Gamsberg related rhyolitic dykes was determined by the U-Pb method. The use of the Rb-Sr method and geochemical analyses allowed description of the high potassium rhyolites of the Nueckopf Formation which formed at about 950 Ma during the final stages of Gamsberg magmatism. About 130 Ma later, at approximately 820 Ma, the whole area of the Rehoboth Basement Inlier was intruded by swarms of basic dykes in a diverging tectonic regime in directions paralleling the direction of the Areb shear zone (a major tectonic lineament crosscutting the Rehoboth Basement Inlier in a Sw-Ne direction) as may be deduced from Rb-Sr and geochemical studies on samples of these basic dykes

Numerous K-Ar age determinations on biotites, white mica and sericite carried out during this study have shown that the compressional phase of the Damaran Orogeny to the north of the Rehoboth Basement Inlier led to metamorphic conditions (300 °C) in the northern part of the inlier capable of resetting the K-Ar systems of biotites and white micas; south of 23 55' they were not strong enough to completely reset the K-Ar systems of biotites. The Rehoboth Basement Inlier must therefore have been overridden by nappes of the Damara Orogen as may be deduced from the previously known Damara ages of micas from the Namib Naukluft Nappe Complex. After the Damaran Orogeny the Rehoboth Inlier was affected by the breakup of Gondwana, which resulted in the opening of the South Atlantic and the reactivation of the Areb shear zone. This is indicated by K-Ar analyses conducted in the course of this study on samples of the Mooirivier Complex and the Elim Formation collected along the Areb shear zone, which show the partial rejuvenation of biotite and white mica separates. These samples currently yield apparent ages of 315 Ma to 427 Ma