Journal of Petrology Advance Access originally published online on March 7, 2006
Journal of Petrology 2006 47(6):1095-1118; doi:10.1093/petrology/egl004
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Correlations between U, Th Content and Metamorphic Grade in the Western Namaqualand Belt, South Africa, with Implications for Radioactive Heating of the Crust
1 SCHOOL OF GEOSCIENCES, UNIVERSITY OF THE WITWATERSRAND, PRIVATE BAG 3, WITS 2050, JOHANNESBURG, SOUTH AFRICA
2 ITHEMBA LABS (GAUTENG), P. BAG 3, WITS 2050, JOHANNESBURG, SOUTH AFRICA
3 COUNCIL FOR GEOSCIENCES, P.O. BOX 112, PRETORIA 0001, SOUTH AFRICA
RECEIVED JULY 1, 2004; ACCEPTED JANUARY 13, 2006
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.
KEY WORDS: charnockite; granulite; Namaqualand; thorium; uranium; radioactive heating; metamorphism
* Corresponding author. Present address: Necsa (South African Nuclear Energy Corporation), P.O. Box 582, Pretoria 0001, South Africa. Telephone: +27 (12) 305-6419. Fax: +27 (12) 305-6797. E-mail: marco{at}necsa.co.za
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