The meta-igneous granulite xenoliths from Kerguelen Archipelago: evidence of a continent nucleation in an oceanic setting |
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Authors: | Michel Grégoire Jean Yves Cottin André Giret Nadine Mattielli Dominique Weis |
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Institution: | (1) National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, North Ryde 2109, Australia; E-mail: mgregoir@laurel.ocs.mq.edu.au, AU;(2) Département de Géologie-Pétrologie-Géochimie, CNRS-UMR6524, Université Jean Monnet, 23 Dr P. Michelon Street, F-42023 Saint Etienne, France, FR;(3) Département des Sciences de la Terre et de l'Environnement, Université Libre de Bruxelles, CP160/02, F.D. Roosevelt Av. 50, B-1050 Brussels, Belgium, BE |
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Abstract: | Xenoliths of meta-igneous origin occur as one of the two main types of ultramafic and mafic xenoliths entrained by alkaline
lavas of the Kerguelen islands. These are designated type II xenoliths and are subdivided into three mineralogical groups.
Subtype IIa and IIc xenoliths are interpreted as crystallisation products of basaltic melts that were emplaced near the crust-mantle
boundary during the early tholeiitic–transitional magmatic activity of the Kerguelen islands. Younger magmatism became more
alkaline and subtype IIb xenoliths were formed as high-pressure alkaline cumulates related to the last alkaline volcanic stage.
Subsequently, the plagioclase-bearing type II rocks have been re-equilibrated under granulite facies conditions. This addition
of mafic material around the crust-mantle boundary is consistent with seismic evidence for crustal thickening to 14–20 km.
Calculated compressional seismic velocities (Vp) for the basic granulites are consistent with the range of observed Vp in
the low-velocity region underlying the oceanic crust. Such growth in the thickness of the oceanic crust may be caused by intrusion
of basalts at different levels in the lithosphere and may provide the heat responsible for granulitic metamorphism in the
oceanic setting. This study suggests that basic granulites can account for the observed seismic characteristics of oceanic
plateaux and can be important components of Kerguelen oceanic lithosphere where there has been large-scale magma production.
Moreover we speculate that the Kerguelen islands and perhaps the surrounding plateau represent a continental nucleation process.
Received: 30 September 1997 / Accepted: 17 June 1998 |
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