Correlations between U, Th content and metamorphic grade in the western Namaqualand belt, South Africa, with implications for radioactive heating of the crust

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 similar to 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 approximate to 10 ppm; Th approximate to 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 approximate to 17 ppm; Th approximate to 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 (similar to 1000 degrees C, P similar to 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.