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

The Kerguelen Archipelago, a similar to 39 Ma to recent volcanic-plutonic c omplex, is interpreted to be a manifestation of the Kerguelen Plume. Most, similar to 85%, of the surface area is covered by flood basalts ranging in age from similar to 29 to 25 Ma, The youngest (similar to 25 Ma) studied fl ood basalts are in the Southeast (SE) Province of the archipelago. A compos ite 460 m section of this southeast flood basalt dominantly consists of evo lved (3 to 6% MgO) alkalic basalt and trachybasalt with a few interbedded h ighly evolved lavas (trachyandesites), a 40-70 m conglomerate which contain s 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 K erguelen Plume: e.g., > 80% of the 115 analyzed archipelago lavas with > 2. 3% MgO have (Sr-87/Sr-86)(i) = 0.70515 +/- 12 and (Nd-143/Nd-144)(i) = 0.51 259 +/- 5, These ranges include the southeast flood basalts. Pb isotopes, h owever, are more variable; these 25 Ma lavas have high Pb-206/Pb-204 at sim ilar to 18.4 to 18.6, relative to other archipelago lavas. The temporal tre nd of the archipelago flood basalt from older. similar to 29 Ma, transition al basalts to younger, similar to 25 Ma, alkalic basalt with an increasing proportion of highly evolved lavas and intra-bedded sediments in the relati vely young southeast section indicates: (a) a temporal decrease in extent o f melting and (b) a decreasing supply of magma from the plume to the crust. These temporal trends are attributed to increasing lithosphere thickness a s the plume evolved from a spreading ridge-centered plume at similar to 43 Ma to its intraplate setting, Supporting evidence for this interpretation i s: (a) the absence of a MORE geochemical signature in these 25 Ma lavas; an d (b) the relatively low abundances of heavy rare-earth elements in these s outheast lavas which reflect partial melting within the garnet stability fi eld. (C) 2000 Elsevier Science B.V. All rights reserved.