Correlations between U, Th Content and Metamorphic Grade in the Western Namaqualand Belt, South Africa, with Implications for Radioactive Heating of the Crust

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 AE 100 Ma greenschist-facies Richtersveld Terrane near Namibia (max. U 34 ppm; Th 201 ppm) and the adjacent, 1100 AE 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 CO 2 -dominated melts and fluids derived from fractionated noritic intrusions.