Abstract:
The digital image of airborne radiometric data across South Africa reveals that the largest anomaly, ?100?nGy/h, is caused by the granulite-facies rocks of the Namaquan metamorphic complex, whereas most of the country is <60?nGy/h. This observation is consistent with geochemical data that show that the ?1900 ± 100?Ma greenschist-facies Richtersveld Terrane near Namibia (max. U = 3·4?ppm; Th = 20·1?ppm) and the adjacent, 1100 ± 100?Ma, amphibolite-facies Aggeneys/Steinkopf Terranes (max. U ≈ 10?ppm; Th ≈ 52?ppm) are the least enriched in U, Th and K. In contrast, the lower-T granulite-facies Okiep Terrane near Springbok hosts more enriched granites (max. U ≈ 17?ppm; Th ≈ 66?ppm) and noritic intrusions (max. U = 14?ppm; Th = 83?ppm). The most enriched rocks are found in the 1030?Ma higher-T granulite-facies core of the Namaquan belt and include quartzo-feldspathic gneisses (max. U = 46?ppm; Th = 90?ppm) and charnockites (max. U = 52?ppm; Th = 400?ppm). Our findings contradict the notion that granulite-facies terrains are characteristically depleted in U and Th. In this study we modeled the heat production in the core of the Namaquan complex, where the granulites have had a very unusual metamorphic history, and show that ultra-high-T (?1000°C, P ? 10?kbar) metamorphic conditions could have been achieved by radiogenic heating without invoking external heat sources. However, monazite-rich veins of charnockite and patches of granulites mark the passage of CO2-dominated melts and fluids derived from fractionated noritic intrusions.
