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

Marco A. G. Andreoli, Guenther Brandl, Henk Coetzee, Jan D. Kramers and Hassina Mouri
Intracrustal radioactivity as an important heat source for Neoarchean metamorphism in the Central Zone of the Limpopo Complex (in Origin and evolution of Precambrian high-grade gneiss terranes, with special emphasis on the Limpopo Complex of Southern Africa, Dirk D. van Reenen (editor), Jan D. Kramers (editor), Stephen McCourt (editor) and Leonid L. Perchuk (editor))
Memoir - Geological Society of America (2011) 207: 143-161

Abstract

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 mu g/g U, 12.3 mu g/g Th, and 12,650 mu g/g K as a weighted regional average. The corresponding heat production rate at 2.65 Ga is 2.6 mu W X m (super -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.


ISSN: 0072-1069
Coden: GSAMAQ
Serial Title: Memoir - Geological Society of America
Serial Volume: 207
Title: Intracrustal radioactivity as an important heat source for Neoarchean metamorphism in the Central Zone of the Limpopo Complex
Title: Origin and evolution of Precambrian high-grade gneiss terranes, with special emphasis on the Limpopo Complex of Southern Africa
Author(s): Andreoli, Marco A. G.Brandl, GuentherCoetzee, HenkKramers, Jan D.Mouri, Hassina
Author(s): van Reenen, Dirk D.editor
Author(s): Kramers, Jan D.editor
Author(s): McCourt, Stepheneditor
Author(s): Perchuk, Leonid L.editor
Affiliation: South African Nuclear Energy Corporation, Pretoria, South Africa
Affiliation: University of Johannesburg, Department of Geology, Auckland Park, South Africa
Pages: 143-161
Published: 2011
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 81
Accession Number: 2011-102891
Categories: Igneous and metamorphic petrologySolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch maps
S23°30'00" - S22°00'00", E28°30'00" - E31°30'00"
Secondary Affiliation: University of KwaZulu Natal, ZAF, South AfricaMoscow State University, RUS, Russian FederationCouncil for Science, ZAF, South AfricaUniversity of the Witwatersrand, ZAF, South AfricaUniversity of Johannesburg, ZAF, South Africa
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201152
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