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Geochronologie am Sudrand des Damara-Orogens, S.W.A./Namibia: Hydrothermale Beeinflussungen von Isotopensystemen und Abkuhlalter in prakambrischen Basementgesteinen.
Abstract
Using K/Ar, Rb/Sr and fission-track methods, results of a geochronological study on different basement intrusive rocks in the area between Great Gamsberg and Gollschau Farm at the S margin of Damara Orogen, S.W. Africa/Namibia are presented. Rb-Sr whole-rock system yielded disturbed results. However, some samples from the Gamsberg granite provide a possible Rb-Sr reference-line of 1190 + or - 23 m.y. with an initial Sr ratio of 0.7026 + or - 0.0012. Aplites gave a value of 926 + or - 21 m.y. with Sr ratio 0.7215 + or - 0.0054. Geochemical evidence goes to indicate the possible magmatic differentiation of Gollschau granite gneiss from Gamsberg granite. It is considered probable that the S margin of Damara orogen was strongly effected by hydrothermal fluids between approx 495 and = or <360 m.y. Two preliminary zircon fission-track ages of 359 and 372 m.y. tend to support such a post-Damara event.-W.S.
... Because of similar petrographic characteristics and field relationships, these granitoids were combined as the Weener Igneous Suite by De Waal (1966). However, Rb-Sr age determinations by Reid et al. (1988), Seifert (1986) and one U-Pb zircon analysis (one fraction) by Ziegler and Stoessel (1991) on different bodies yielded considerably varying ages of ca. 1200 Ma, ca. ...
... Since the variation of the Rb/Sr-ratio is low in the WIC (0.14-0.35), increased errors are inherent a prior to the regression analysis. To improve the significance of the calculated age, previously published analyses from the same locality (Seifert, 1986) have been included. Due to their large deviation from the regression line, two samples have been excluded (KAW 2287, PR18). ...
... A compilation of biotite and muscovite isotopic ages (Fig. 12) reveals that K/Ar and Rb/Sr mineral ages of muscovite scatter within a comparatively narrow range between 475 and 525 Ma, whilst the biotite ages were rejuvenated up to 350 Ma. This rejuvenation has been attributed to a hydrothermal event which is recorded by the observed full age spectrum in samples taken only meters apart at Gamsberg Mountain (Seifert, 1986). ...
The Mesoproterozoic domain of the Rehoboth Basement Inlier (RBI) in central Namibia is composed of sedimentary and volcanic rocks which have been intruded by coeval granites of batholithic dimension as well as numerous quartz-feldspar porphyries and mafic dykes. Review of the geochemical, petrographical and geochronological data in combination with field observations and the analysis of high-resolution geophysical magnetic and radiometric data suggests Mesoproterozoic evolution of this area in three stages. The development of a first basin is recorded by clastic fluvial and limnic sedimentary rocks of the Billstein formation with only minor if any felsic pyroclastic activity. Intrusive porphyry dykes constrain a minimum age of 1210 +/− 7 Ma. The second stage (Nückopf, Langberg formations, Kartatsaus member), lasting from at least 1.23 Ga to 1.09 Ga is marked by major felsic igneous activity and strong vertical tectonics: ignimbrite deposits as well as rhyolitic flows occur throughout the RBI and suggest several volcanic eruption centres. The calcalkaline Gamsberg Granite Suite represents the plutonic equivalent to this volcanism; geochemically it shows affinity to arc-related igneous rocks. Contemporaneous sediments range from polymict unsorted debris flow deposits to arkoses and cross-bedded feldspathic sandstones deposited in grabens. An intra- or backarc region of an active continental margin is suggested as possible plate-tectonic setting for stage two. The initiation of the third stage (Opdam formation, Kupferberg, Idaberg members) is marked by regional outpouring of basalts deposited subaerial or in shallow water. High Ti-content and flat REE-patterns in the tholeiites suggest their emplacement in an extensional setting, whereas high Th/Ta and La/Nb ratios, low Ce/Pb values and negative anomalies for Nb-Ta require a subduction-style enrichment of the mantle source. These contrasting features are best explained by extensional collapse related to continent-collision further north: Docking of continents led to slab detachment, allowing the interaction between the asthenospheric mantle and the mantle wedge enriched during the subduction process. At regional scale, the mafic rocks are considered to be part of the Umkondo igneous province.
Tectono-metamorphism of the Mesoproterozoic domain took place only during the Pan-African Damara orogeny. It is marked by intense south-eastward directed thrusting within an anastomizing network of shear zones which constitute the South Damara shear zone. Open to tight southeasterly verging folds form second order deformation structures. The metamorphism varies from medium grade in the north to very low grade in the south.
Two main Mesoproterozoic provinces occur in southern Namibia: (1) The high-grade Namaqua Metamorphic Complex (NMC) composed of a supracrustal sedimentary succession and interpreted as a passive margin sequence in the west of the Kalahari craton; (2) The Sinclair Group and its northeastern correlatives, composed of two main magmatic and metamorphic units, reflecting northeast-directed subduction, which started before 1.37 Ga and lasted until about 1.1 Ga. These two units were tectonically juxtaposed during the 1.1–1.03 Ga Namaqua orogenic event.
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