Kerguelen basic and ultrabasic xenoliths: Evidence for long-lived Kerguelen hotspot activity

Abstract

The xenoliths from the Southeast Province of the Kerguelen Archipelago derived from the lower crust or the upper mantle, can contribute to define the characteristics of the mantle sources below Kerguelen and improve the constraints on the formation of the Kerguelen Islands and plateau. Our petrographic, geochemical and isotopic (Sr, Nd and Pb) study focuses on peridotites ( Type Iα: harzburgite/clinopyroxene-poor Iherzolite and Type Iβ: dunite), 2- pyroxenes-spinel bearing ultrabasic and basic xenoliths [ Type IIa: clinopyroxene-rich Iherzolite, wehrlite, (± olivine ± plagioclase) websterite, (± garnet ± sapphirine) metagabbro and anorthosite] and ilmenite metagabbros (Type IIc). The large ranges of isotopic ratios for the xenoliths reflect different degrees of interaction between a depleted MORB-type component, quite abundant in the Type II xenoliths, and the Kerguelen plume, distinctly predominant in the Type I xenoliths. Type I peridotites are residues of a previous partial melting event of the Kerguelen plume; residues that subsequently interacted with a percolating alkaline melt. 2-pyroxenes-spinel bearing ultrabasic and basic xenoliths (Type IIa) and ilmenite metagabbroic xenoliths (Type IIc) are deep cumulates crystallized from tholeiitic magmas. The isotopic results for the xenoliths strengthen the hypothesis of an oceanic origin for the Kerguelen Islands and refute the existence of pieces of old continental crust beneath the Islands and the northern part of the Kerguelen Plateau. They also confirm the importance of plume-spreading ridge interactions throughout the history of the Kerguelen plume. The isotopic and geochemical characteristics of the Type IIa and IIc xenoliths are consistent with the hypothesis of an Iceland-type setting for the northern part of the Kerguelen Plateau. The results for the Type I xenoliths on the other hand suggest a similarity between the Hawaii-type midplate volcanic structure and that of Kerguelen Islands. The isotopic data suggest that the Kerguelen xenoliths were formed recently (≤ 45 Ma), and thus support the hypothesis of the formation of the Plateau by the arrival of the plume at the base of the lithosphere (~ 115 Ma ago). The Plateau would have grown through several pulses of plume activity (~ 115, ~ 80, ~ 40 Ma), while the geotectonic environment changed with time (from a ridge-centered position to the present intraplate position). The occurrence of deep Type IIa and IIc xenoliths can explain the crustal thickening and provides evidence for the growth of oceanic plateaus by vertical accretion.