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Intracrustal radioactivity as an important heat source for Neoarchean metamorphism in the Central Zone of the Limpopo Complex

By
Marco A.G. Andreoli
Marco A.G. Andreoli
South African Nuclear Energy Corporation, P.O. Box 582, Pretoria 0001, South Africa and School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa
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Günther Brandl
Günther Brandl
Council for Geoscience, Private Bag X112, Pretoria 0001, South Africa
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Henk Coetzee
Henk Coetzee
Council for Geoscience, Private Bag X112, Pretoria 0001, South Africa
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Jan D. Kramers
Jan D. Kramers
School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa, and Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, Johannesburg, South Africa
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Hassina Mouri
Hassina Mouri
Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, Johannesburg, South Africa
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Published:
February 01, 2011

The major periods of metamorphism in the Central Zone (CZ) of the Limpopo Complex occurred at 2.0 Ga and in the time range between ca. 2.7 and ca. 2.55 Ga. We investigate intracrustal radioactivity as a possible heat source for the earlier of these episodes. Available airborne radiometric surveys that cover the South African part of the CZ, combined with rock analyses, yield 2.15 μg/g U, 12.3 μg/g Th, and 12,650 μg/g K as a weighted regional average. The corresponding heat production rate at 2.65 Ga is 2.6 μW × m−3. A steady-state geotherm, calculated assuming uniform [U], [Th], and [K] throughout the crustal column and its thickening to 45 km during the ca. 2.65 Ga event (both arguable on the basis of peak metamorphic pressure-temperature [P-T] data), surpasses temperatures of the peak metamorphism at middle and lower crustal levels, which cluster around the fluid-absent biotite dehydration solidus. Intracrustal radioactivity thus provided a sufficient heat source to account for the metamorphism at ca. 2.65 Ga, and partial melting acted as a lower crustal thermostat. After crustal thickening, up to more than 100 m.y. (dependent on U, Th, and K concentrations) would be needed to approach a new steady state. Predicted regional variations thus account for the long duration of the ca. 2.65 Ga metamorphism. Lower crustal partial melting could have led to diapirism, yielding the steep structures in the CZ, which are not aligned to a regional fabric. Metamorphism ceased after crustal thinning to a normal 30 km. The metamorphic event at 2.0 Ga cannot be explained by this type of process.

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GSA Memoirs

Origin and Evolution of Precambrian High-Grade Gneiss Terranes, with Special Emphasis on the Limpopo Complex of Southern Africa

Dirk D. van Reenen
Dirk D. van Reenen
Department of Geology, University of Johannesburg, Auckland Park, South Africa
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Jan D. Kramers
Jan D. Kramers
Department of Geology, University of Johannesburg, and School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
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Stephen McCourt
Stephen McCourt
School of Geological Sciences, University of KwaZulu Natal, Durban, South Africa
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Leonid L. Perchuk
Leonid L. Perchuk
Department of Petrology, Moscow State University, and the Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow District, Russia
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Geological Society of America
Volume
207
ISBN print:
9780813712079
Publication date:
February 01, 2011

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