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Geological Magazine; March 2009; v. 146; no. 2; p. 173-186; DOI: 10.1017/S0016756808005499
© 2009 Cambridge University Press (CUP)
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Dehydration, melting and related garnet growth in the deep root of the Amalaoulaou Neoproterozoic magmatic arc (Gourma, NE Mali)

JULIEN BERGER*,{dagger},{ddagger},§, RENAUD CABY, JEAN-PAUL LIÉGEOIS*, JEAN-CLAUDE C. MERCIER{ddagger},#, and DANIEL DEMAIFFE{dagger}

* Musée royal de l'Afrique centrale, Section de géologie isotopique, Leuvensteenweg 13, B-3080 Tervuren, Belgique
{dagger} Université libre de Bruxelles (U.L.B.), Géochimie isotopique et géodynamique chimique, CP 160/02, av. F. Roosevelt 50, B-1050 Bruxelles, Belgique
{ddagger} Université de la Rochelle, UMR CNRS 6250 ‘LIENSs’, ILE, 2 rue Olympe de Gouges F-17042 La Rochelle-cedex 1, France
Université de Montpellier 2, Laboratoire de Tectonophysique, Place E. Bataillon, F-34095 Montpellier-cedex, France
# Université de Nantes, UMR CNRS 6112 ‘LPGN’, BP 92205, 2 rue de la Houssinière, F-44322 Nantes, France

§ Author for correspondence: julien.berger{at}africamuseum.be

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.

Key Words: Gourma • dehydration-melting • island arc • Neoproterozoic • granulite






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