The Kerguelen archipelago : an hypothetic continental mafic protolith

M. Gregoire1, S. Y. O'Reilly1, J.Y. Cottin3 and A. Giret3 1. GEMOC, Macquarie, 2. Dépt de Géologie, Université Jean Monnet, Saint Etienne

The ultramafic and mafic xenoliths from the Kerguelen Islands occur in dykes, lava-flows and breccia pipes of the youngest and more alkaline magmas. They include many types of ultramafic and mafic xenoliths entrained by within-plate basalts in continental and oceanic settings (except eclogites and garnet peridotites). Two main types can be distinguished, corresponding to types I and II xenoliths in the Frey and Prinz classification (1978) [equivalent to Wilshire & Shervais (1975) Cr-diopside and Al-augite series respectively]. The first type consists of mantle spinel-bearing harzburgites and dunites and associated composite xenoliths. The second type comprises a wide variety of metamorphosed igneous rocks ranging from peridotites to basic granulites. Three main mineralogical sub-types have been distinguished for the type II xenoliths. The first (IIa) is the two pyroxenes-spinel series which represents 45 % of the whole collected xenoliths. It comprises a series of rocks showing a regular modal evolution from ultramafic rocks such as wehrlites, clinopyroxene-rich lherzolites, websterites with or without olivine and plagioclase, to mafic rock types such as metagabbros and metagabbronorites with or without garnet and sapphirine. The second subgroup (IIb) is the clinopyroxene + ilmenite + spinel series represented by clinopyroxenites with or without garnet and by garnet bearing metagabbros. The third subgroup (IIc) is made of ilmenite bearing metagabbros. The type II xenoliths are deep crustal and upper mantle segregates from the mantle derived tholeiitic-transitional basaltic magmas (type IIa and Type IIc xenoliths) and the mantle derived alkaline basaltic magmas (type IIb xenoliths) of the archipelago.

Preliminary isotopic data (Mattielli et al., 1996) suggest that some of the type II xenoliths record various degrees of mixing between a depleted MORB-related mantle source and the Kerguelen plume source. Moreover, all the type II plagioclase-bearing rocks have reequilibrated in the granulite facies, i.e. in the P-T conditions of the Kerguelen oceanic lower crust and upper mantle. In particular, the trace-element characteristics of the type II cpx and garnet evidence the importance of the subsolidus reequilibration. These mafic granulites are the first occurrence ever reported in an oceanic environment (Grégoire et al., 1994). The existence of oceanic granulites beneath the Kerguelen islands is consistent with the presence of a thickened crust evidenced by the seismic studies (14-20 km, Charvis, 1984; Recq et al., 1990; Operto & Charvis, 1996). Furthermore the calculated Vp vawes for basic granulites are consistent with those observed in the low-velocity region beneath the oceanic crust (Vp = 7.2-7.5 km s-1). Thus, the first study of the Kerguelen ultramafic and mafic xenoliths supports crustal thickening consistent with previously recorded seismic velocity characteristics. The synergy of the young East-Indian Ridge and of the Kerguelen hot spot is considered to have caused voluminous tholeiitic-transitional magmas with resultant intrusion and formation of associated cumulates in the vicinity of the Moho. The process of crustal thickening by deep intrusions later amplified by the extrusive volcanic sequences related to the longevity of the hot spot activity, may explain the occurrence of basic granulites in an oceanic setting.

It exist a significant volume of basic granulite beneath the Kerguelen archipelago. The existence of "low-velocity" zones beneath oceanic areas have been recognized beneath the Marquesas, the Ontong Java plateau and Broken Ridge, and a progressive change in seismic velocities has also been proposed for the Madagascar ridge and the Crozet plateau. Those results lead us to speculate that basic granulites may explain for the observed seismic characteristics, and then they can be important constituents in those oceanic areas where large-scale magma production happened.

On the other hand, the Kerguelen basaltic traps are intruded by many volcano-plutonic complexes with mantle isotopic signatures. Those intrusive bodies contain abundant intermediate and acid rocks as monzonites, syenites, trachytes and phonolites. Their volume, > 15 % of the archipelago, added to the volume of the deep rocks which have been metamorphized in the granulite facies in the crust-mantle boundary evidence a density decrease of the Kerguelen archipelago lithosphere regarding its oceanic environment. Therefore we propose that the Kerguelen archipelago represents an oceanic lithospheric bud which low density prevents its further sinking at deep depths or in an hypothetic subduction process. The P-T conditions which are calculated from the granulitic assemblages of the basic type II inclusions are similar to those which have been proposed for some lower continental crust involved in thinning processes. That leads to compare the thickened Kerguelen oceanic lithosphere with the thinned continental lithosphere, and consequently to propose a model in which the oceanic Kerguelen islands and surrounding plateau represent the result of a possible continental nucleation process.

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