Tectono-metamorphic setting and evolution of granitic sheets in the goanikontes area, Namibia

Abstract

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The Goanikontes area lies within the Central Zone of the northwest-trending branch of the Damara orogen. The cover succession of the area, which comprises Etusis to Chuos Formation meta-sediments, is in tectonic contact with older pre-Damaran basement rocks. The area can be divided into four structural domains and is characterised by a northeast-plunging anticline, which is truncated by a high strain zone approximately l km wide to the east, which is adjacent to the basement rocks. This high strain zone (HSZ) has not been identified before and the lithologies within the HSZ have previously been misidentified

D2 deformation accompanied by F2 folding, has resulted in a prominent layer-parallel S2 metamorphic, gneissic banding within the Damaran meta-sediments. M1 metamorphism at 4kbar and ~700°C is inferred. D3 deformation produced the prominent dome structures and northeast-trending lineation seen on a regional scale and at Goanikontes the most obvious evidence of this phase of deformation is the spectacular L-tectonites within the HSZ. The L-tectonites plot in the constrictional field of a Flinn diagram with X:Y:Z ratios of 10:2:1 and indicate high strain over the full width of the HSZ. L-tectonites are also found in the cores of F3 folds outside the HSZ, and by inference, it is the particular mechanism of folding that occurred which is responsible for HSZ formation. Recrystallisation and high temperature symplectites provide evidence for a thermal overprint termed M2, which has been temporally associated with igneous intrusions into the basement rocks. Evidence from the symplectites and amphibole-plagioclase geothermometiy indicates that this M2 metamorphism reached temperatures in excess of 800°C at pressures 3kbar

Granitoid intrusions are extremely well exposed in the area, and these are dominated by leucogranite sheets, locally termed 'alaskites', which intrude both basement and Damara Sequence. Red and grey granite emplacement was followed by these sheeted leucogranite intrusions (SLG's) which are preferentially located within the HSZ. A detailed description and classification of the SLG's by a combined field, geochemical and fluid-based study, has divided them into 6 different types. Modal proportions of minerals and major element data show a compositional range for the SLG's from granodiorite to alkali-feldspar granite, confirming that they are definitely not all alaskites. All the intrusive rocks at Goanikontes exhibit magmatic characteristics which are not altered by bulk redistribution of elements

Rb/Sr and trace element geochemistry show that the 6 types of sheeted leucogranite may form two separate fractionating groups, and suggest that each of the two groups has originated from a different source area. Previous work has considered that the sheeted leucogranites are derived from in-situ partial melting of Damaran cover rocks, however this thesis shows that this is extremely unlikely, and that they are more easily derived from quartzo-feldspathic pre-Damaran basement. The other intrusives within the Goanikontes area have also been eliminated as parental magmas to the SLG's. The HSZ is in fact a magma pathway which contains granites from different sources emplaced in an overlapping sequence

Primary magmatic uranium enrichment is restricted to two SLG types and is confined to the HSZ. Unlike the nearby Rossing deposit it is not confined to the interface between the Rossing and Khan Formations. Mineralisation of the SLG's at Goanikontes has been compared with work on other uranium anomalies which shows that this type of uranium mineralisation is characterised by Orogenic belts exhibiting low-pressure, high-temperature metamorphism and occurs within late-stage, K-rich magmas