The Kerguelen archipelago: evidences of a refractory mantle percolated by basaltic melts and of the formation of granulites in an oceanic setting

Michel GREGOIRE (National Key Centre for Geochemical Evolution and Metallogeny of Continents, School of Earth Sciences, Macquarie University, North Ryde, 2109 NSW, Australia), Jean Yves COTTIN (Département de Géologie, Université Jean Monnet-CNRS URA10, 23 rue du Dr P. Michelon, F-42023 Saint Etienne, France), André GIRET (Département de Géologie, Université Jean Monnet-CNRS URA10, 23 rue du Dr P. Michelon, F-42023 Saint Etienne, France), Nadine MATTIELLI (Laboratoire de Pétrologie et géodynamique chimique, CP 160/02, Av. F.D. Roosevelt 50, 1050 Bruxelles, Belgium) and Dominique WEIS (Laboratoire de Pétrologie et géodynamique chimique, CP 160/02, Av. F.D. Roosevelt 50, 1050 Bruxelles, Belgium).

Two main types of UB-B xenoliths have been uplifted by the alkaline lavas of the Kerguelen islands. Type I xenoliths consist in refractory mantle spinel bearing harzburgites and dunites. Type II corresponding to metamagmatic rocks is subdivided into three mineralogical groups. The subtype IIa consists in two pyroxenes+sp-bearing rocks containing sometimes ol, pl, ga and sa. The subtype IIb consists in cpx+ilm+sp±ga bearing rocks and the subtype IIc is made of ilm+ga metagabbros.

The harzburgites are divided into protogranular harzburgites containing Cr-diopside and poikilitic harzburgites characterized by a Cr-Na-rich Mg-augite. They bear the imprint of two main processes, 1) high degree of partial melting and 2) reaction with basaltic magmas. Partial melting event is well established by the high refractory indexes of both whole rocks and minerals (Fo up to 92, Cr-rich spinel and low CaO, Al2O3, Na2O bulk rock contents). The U-shaped REE patterns in both types of harzburgite and the presence of the Mg-augite in the poikilitic harzburgites are evidences of the reaction with basaltic melts. The dunites are wall rocks of magmatic dykes of tholeiitic or alkaline affinity. They show the same textural, evolution of mg number (minerals and bulk rocks) and REE patterns than the poikilitic harzburgites. These features are consistent with a model whereby the dunites were formed by reaction between depleted harzburgites and basaltic melts propagating within open cracks.

The type II xenoliths are deep basaltic cumulates of tholeiitic-transitional (type IIa and IIc) or alkaline (type IIb) basaltic magmas. Moreover, all the type II plagioclase-bearing rocks have been reequilibrated in the granulite facies, i.e. in the P-T conditions of the Kerguelen oceanic lower crust and upper mantle. These results are in agreement with the existence of a crust thickening evidenced yet by the seismic studies (14-20 km). In that way the calculated Vp in basic granulites are consistent with those observed in the low-velocity region. Such a thickness growth of the oceanic crust may be generated by intrusions of basalts at different levels of the lithosphere and it may be responsible of the granulitic metamorphism in an oceanic setting.

The Kerguelen basaltic traps are intruded by many volcano-plutonic complexes with mantle isotopic signatures. These bodies contain intermediate and acid rocks as monzonites, syenites, trachytes and phonolites. Their volume, > 15 % of the archipelago, added to the volume of the deep basic granulitic rocks evidence a density decrease of the Kerguelen lithosphere regarding its oceanic environment. Therefore we propose that the archipelago represents an oceanic lithospheric bud which low density prevents its further sinking at deep depths or in an hypothetic subduction process. That leads to propose a model in which the oceanic Kerguelen islands and surrounding plateau represent the result of a possible continental nucleation process in a pure oceanic context.