The meta-igneous granulite xenoliths from Kerguelen Archipelago: evidence of a continent nucleation in an oceanic setting

Xenoliths of meta-igneous origin occur as one of the two main types of ultr amafic and mafic xenoliths entrained by alkaline lavas of the Kerguelen isl ands. These are designated type II xenoliths and are subdivided into three mineralogical groups. Subtype IIa and IIc xenoliths are interpreted as crys tallisation products of basaltic melts that were emplaced near the crust-ma ntle 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 th e last alkaline volcanic stage. Subsequently, the plagioclase-bearing type II rocks have been reequilibrated under granulite facies conditions. This a ddition of mafic material around the crust-mantle boundary is consistent wi th seismic evidence for crustal thickening to 14-20 km. Calculated compress ional 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 i ntrusion of basalts at different levels in the lithosphere and may provide the heat responsible for granulitic metamorphism in the oceanic setting. Th is study suggests that basic granulites can account for the observed seismi c characteristics of oceanic plateaux and can be important components of Ke rguelen oceanic lithosphere where there has been large-scale magma producti on. Moreover we speculate that the Kerguelen islands and perhaps the surrou nding plateau represent a, continental nucleation process.