Temporal geochemical trends in Kerguelen Archipelago basalts: evidence for decreasing magma supply from the Kerguelen Plume

The Kerguelen Archipelago, a ∼39 Ma to recent volcanic–plutonic complex, is interpreted to be a manifestation of the Kerguelen Plume. Most, ∼85%, of the surface area is covered by flood basalts ranging in age from ∼29 to 25 Ma. The youngest (∼25 Ma) studied flood basalts are in the Southeast (SE) Province of the archipelago. A composite 460 m section of this southeast flood basalt dominantly consists of evolved (3 to 6% MgO) alkalic basalt and trachybasalt with a few interbedded highly evolved lavas (trachyandesites), a 40–70 m conglomerate which contains lignite beds, and a trachytic breccia/tuff unit. All of the lavas in this composite section have Sr and Nd isotopic ratios that are typical of the Kerguelen Plume; e.g., >80% of the 115 analyzed archipelago lavas with >2.3% MgO have (87Sr/86Sr)i=0.70515±12 and (143Nd/144Nd)i=0.51259±5. These ranges include the southeast flood basalts. Pb isotopes, however, are more variable; these 25 Ma lavas have high 206Pb/204Pb at ∼18.4 to 18.6, relative to other archipelago lavas. The temporal trend of the archipelago flood basalt from older, ∼29 Ma, transitional basalts to younger, ∼25 Ma, alkalic basalt with an increasing proportion of highly evolved lavas and intra-bedded sediments in the relatively young southeast section indicates: (a) a temporal decrease in extent of melting and (b) a decreasing supply of magma from the plume to the crust. These temporal trends are attributed to increasing lithosphere thickness as the plume evolved from a spreading ridge-centered plume at ∼43 Ma to its intraplate setting. Supporting evidence for this interpretation is: (a) the absence of a MORB geochemical signature in these 25 Ma lavas; and (b) the relatively low abundances of heavy rare-earth elements in these southeast lavas which reflect partial melting within the garnet stability field.