The Val Gabbro Plutonic Suite: A Sub-volcanic Intrusion Emplaced at the End of Flood Basalt Volcanism on the Kerguelen Archipelago

The age, petrology, major and trace element geochemistry, andSr–Nd–Hf–Pb isotopic geochemistry of basicand felsic rocks from the Val gabbro plutonic suite on the KerguelenArchipelago, Southern Indian Ocean, are used to constrain thetemporal and compositional relationships between sub-volcanicintrusions and flood basalt volcanism during the formation ofa major oceanic island. The 4 km² Val gabbro plutonic suitewas emplaced at 24·25 ± 0·15 Ma (U–Pbzircon) into 25 Ma volcanic rocks of the Southeast Province,locally producing a large zone of overlying basaltic breccia.Cumulate basic–ultrabasic rocks are the dominant lithologyin the intrusion, with horizontally layered peridotites at thebase of the exposed part of the intrusion, overlain by verticallylayered, coarse-grained plagioclase-bearing peridotites, melagabbrosand equigranular gabbros. The intrusion was formed by repeatedinjections of relatively crystal-rich and crystal-poor magmasinto an open-system magma reservoir. Strong geochemical andisotopic similarities between the fine-grained marginal microgabbrosand cross-cutting felsic rocks and the hosting mildly alkalicbasalts and trachytes of the Southeast Province indicate thatthey were derived from similar alkalic basaltic parental magmas,which were dominated by the enriched component of the Kerguelenmantle plume source. At 25 Ma, the change from tholeiitic–transitionalto mildly alkalic basalts marks the terminal stage of floodbasalt volcanism on the Kerguelen Archipelago. This compositionalchange was associated with deeper melting within the Kerguelenplume source, lower extents of melting, a decrease in magmasupply, and the emplacement of high-level intrusions such asthe Val gabbro plutonic suite. KEY WORDS: Kerguelen Archipelago; Val gabbro plutonic suite; oceanic island; gabbros; sub-volcanic intrusion; alkalinity; Sr–Nd–Hf–Pb isotopes