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Dehydration, melting and related garnet growth in the deep root of the Amalaoulaou Neoproterozoic magmatic arc (Gourma, NE Mali)

Published online by Cambridge University Press:  17 September 2008

JULIEN BERGER ,
RENAUD CABY ,
JEAN-PAUL LIÉGEOIS ,
JEAN-CLAUDE C. MERCIER  and
DANIEL DEMAIFFE
JULIEN BERGER*
Affiliation:
Musée royal de l'Afrique centrale, Section de géologie isotopique, Leuvensteenweg 13, B-3080 Tervuren, Belgique Université libre de Bruxelles (U.L.B.), Géochimie isotopique et géodynamique chimique, CP 160/02, av. F. Roosevelt 50, B-1050 Bruxelles, Belgique Université de la Rochelle, UMR CNRS 6250 ‘LIENSs’, ILE, 2 rue Olympe de Gouges F-17042 La Rochelle-cedex 1, France
RENAUD CABY
Affiliation:
Université de Montpellier 2, Laboratoire de Tectonophysique, Place E. Bataillon, F-34095 Montpellier-cedex, France
JEAN-PAUL LIÉGEOIS
Affiliation:
Musée royal de l'Afrique centrale, Section de géologie isotopique, Leuvensteenweg 13, B-3080 Tervuren, Belgique
JEAN-CLAUDE C. MERCIER
Affiliation:
Université de la Rochelle, UMR CNRS 6250 ‘LIENSs’, ILE, 2 rue Olympe de Gouges F-17042 La Rochelle-cedex 1, France Université de Nantes, UMR CNRS 6112 ‘LPGN’, BP 92205, 2 rue de la Houssinière, F-44322 Nantes, France
DANIEL DEMAIFFE
Affiliation:
Université libre de Bruxelles (U.L.B.), Géochimie isotopique et géodynamique chimique, CP 160/02, av. F. Roosevelt 50, B-1050 Bruxelles, Belgique
*
§Author for correspondence: julien.berger@africamuseum.be
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Abstract

The Amalaoulaou Neoproterozoic island-arc massif belongs to the Gourma belt in Mali. The metagabbros and pyroxenites forming the main body of this arc root show the pervasive development of garnet. In the pyroxenites, the latter has grown by reaction between pyroxene and spinel during isobaric cooling. By contrast, in the metagabbros, garnet textures and relations to felsic veins exclude an origin through solid-state reactions only. It is proposed that garnet has grown following dehydration and localized melting of amphibole-bearing gabbros at the base of the arc. The plagioclase-saturated melts represented by anorthositic veins in the metagabbros and by trondhjemites in the upper part of the massif provide evidence for melting in the deep arc crust, which locally generated high-density garnet–clinopyroxene–rutile residues. Garnet growth and melting began around 850 °C at 10 kbar and the tonalitic melts were most probably generated around 1050 °C at P ≥ 10 kbar. This HT granulitic imprint can be related to arc maturation, leading to a P–T increase in the deep arc root and dehydration and/or dehydration-melting of amphibole-bearing gabbros. Observation of such features in the root of this Neoproterozoic island arc has important consequences, as it provides a link to models concerning the early generation of continental crust.

Keywords

Gourmadehydration-meltingisland arcNeoproterozoicgranulite
Type
Original Article
Information
Geological Magazine , Volume 146 , Issue 2 , March 2009 , pp. 173 - 186
Copyright
Copyright © Cambridge University Press 2008

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